VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 33 coupe belching soot

My 33 has begun to blow black smoke while idling and under acceleration blows smoke like a diesel. Naturally I am also fouling plugs. Can I adjust the fuel mixture on the carb to get the car to run leaner, or do I simply need to rebuild the carb?

If this a recent occurrence, it could mean something broke. Otherwise it may be the rings. A simple test is to warm the engine up to operating temperature and slowly dribble some transmission fluid down the carburetor while keeping the engine running. You will get an enormous cloud of smoke, so warn the neighbors not to call 911.

Next step is to check the float in the carb and see if it has sunk.

Keep us posted.

Well it may be more serious than I thought. I figured if it was bluish smoke it would be the rings. I will run the test you suggest. The car runs pretty smoothly but seems to be lacking power and compression. I am about to run a compression test. What should I see, about 90lbs per cylinder?

First thing I would check is the float. It may be filling with gasoline and therefore causing a rich condition. Next is the needle and seat. If not sealing properly will overfill the float bowl and cause engine to carbon up.

I just pulled each plug and ran a compression test on each cylinder. All my brand new plugs are black with carbon-the plugs are dry-no oil. Each cylinder did not show a reading. All were 0 lbs. That seems odd because the car starts just fine and runs fine albeit maybe not as powerful and quick to get up to speed as I think it should. Chipper I will check the carb, but now I am concerned I am showing no compression. With the way car runs I should get a reading on my compression gauge. I may have a leak in the gauge. With the condition of the plugs it seems to be the carb. I will check back when I run Chippers tests.

Check to be sure the air cleaner is not plugged. Little air and lots of fuel have the same results.

Your compression gauge may have a problem.

Chipper the flat bowl is half full.

Did you check the float to see if it is full of gas?

The float has no fuel in it, just the float bowl. The float appears to be fine-no cracks or holes. I did notice the air filter is not letting much light in so that definitely was overlooked and needs replacing. So as Steve D suggested can it simply just be the clogged air filter?

Try to run it with the air cleaner on and then off. That will check the air cleaner. Put your thumb over a spark plug hole and push as hard as you can. If your thumb is blown off the plug hole when the engine is cranked over you have decent compression. If the exhaust system is partially plugged it will also cause a rich combustion condition.

Just ran car with air cleaner off. Very dark black smoke when I punch accelerator. I think you are onto something with the exhaust. I have noticed the exhaust has suddenly gotten loud. The car sat for about a year and was not driven much so the muffler could have rusted out from lack of use and gotten clogged. Guess I can piss off the neighbors and run the car without the muffler to see how black the exhaust is. There is likely lots of carbon to be blown out of the engine so the exhaust will be black for a while. I will pull off the muffler and shake it to see if the insides have broken down.

Remove ALL spark plugs, before doing compression test.

hi there,
reading all the posts made me think of the very obvious............is the choke knob puled out and needs to be pushed in?
simple solution but so obvious that no one else posted it?
I had the same problem with my ride on mower, running roughly and I had forgot to move the choke to RUN position....such is life and getting older ..

Peter

Black smoke generally indicates excess fuel, blue smoke indicates oil and white smoke (steam) indicates water (often seen in cold humid weather).

Generally engines are hard to start with less the 60lb compression on at least 4 cylinders. I look for at least 80lb in all and preferably 100lb + with no more than 15lb variation between least to most.
A squirt of oil in each cylinder and retest, big difference indicates rings worn and little difference indicates problem above the head gasket. With this in mind I would think JYD is right, your compression guage is not working correctly.

Tony

Car starts easily and with a loosely fit high performance muffler, still blowing tons of black smoke. Took loose fit pipes and muffler off and inside of pipes are black as coal. Will run compression test next. Car starts easily but has no power.

Still betting on a sinking float (float filled with gasoline) or a needle and seat problem.

Do you have experience with rebuilding the carb ?

Not a carb guy. I might just ask Doyle Stokes to send me a rebuilt exchange. He sent me a rebuilt for my 34 sedan. Ran great. I sure hope it is nothing more serious than the carb.

Thanks much

Let us know what you find when you do your compression test.

Have you added ANYTHING to the gas? No need for additives, substitutes or oil of any kind with todays gas. If nothing additional in the tank then your mixture is too rich, change carburetor and if the proble still exists then you probably have a loose valve stem or worn-out passage(s).

Carbs are not really that complicated. If you say the float has no leaks, how did you test the float? Shake it? That might be ok if there is enough fuel in it to shake. Otherwise heat some water in a pan to almost boiling, no bubbles in the pan. Dip the float in the hot water and look for bubbles coming out of the float. If all is ok, have you checked float level? Does the float have the metal needle and seat or the flat rubber plunger "needle" and seat? They have two different height specs. Even before you disassemble the carb, look down the carb while the engine is running at idle and look for fuel dripping from the boost venturi. That indicates a fuel level problem, either float, height, or needle and seat. Too high a fuel level will cause dripping. Needle and seat work may be in the offing.

My neighbor is a prime example, as danger working with carburetors. He was attempting to pore some fuel in the carb in order to start it. It backfired, caused him to fall and spill all the fuel on his face and upper body. He spent several days in a burn center just to save his life. He is fairly well healed now with scars all over his head and chest and shoulders. No ears, some nose, badly scarred chest.

Even to look down in a carb to check the action of the accelerator pump, you should have a plastic shield. Better yet, use a mirror and set up a 90 degree line of view.

Didn't mean to disrupt this thread, but that needed to be said.

Loss of Power-- causes

1. Loss of compression -- test with compression gauge. 90 to 70 pounds good. If one cyclender is low and the rest have a close range then the low one is your problem. Could be a blown head gasket.

2. Too rich mixture because of some defect in carburetor. Did you look for "grit" in the bowl under the float valve?

3. Late ignition timing. Use a tming light. Check the "ball" on the fly wheel in the square hole. See where the needle is pointing to the "ball" when the timing light flashes.

Due to work and family obligations, it has been a while, but I wanted to provide an update of my problem. I sent my carb to Doyle Stokes and he sent me a replacement. He said he needed time to rebuild mine which he said was a mess, likely from contaminated fuel. A new gas tank for this 33 is $875 as it apparently has a unique gas filler neck. So rather than buy a new one, I removed, flushed, rust proofed and sealed my tank. I also flushed the fuel line to the pump. I will get the tank back in the car hopefully this weekend. I just did a compression check with a new compression gauge and a cold engine. I got a consistent reading of 70 pounds in every cylinder. Not sure, but I guess that is acceptable for an original engine with 82,000 miles.

I will replace the carbon black spark plugs, add a new fuel filter, cleaned air filter, and new muffler, and pray the problem was the carb. The muffler was broken down inside so I am sure it was not helping matters.

As soon as I get it all together, I will advise the result. Again thanks to all for your assistance. If anyone has an opinion on my 70 pounds of cylinder compression, I would like to hear it. Seems a bit low to me as I was expecting 90 pounds.

Best regards,
Jolo

Just checked the TANKS INC website and they do not make a gas tank for either the 1933 Chevrolet standard series or the 1933 chevrolet master series .

Very surprised !

mike lynch

Rock Valley Antique Auto Parts will build a 33 tank.
http://www.rockvalleyantiqueautoparts.com/catalog.htm

No, 70 lbs. is good enough. Remember that from my Grandfather and my Dad. Below that you will have to do something. The main thing is to have them read all the same within 10 lbs. Remember that this is not a high compression engine! 90 lbs. is top reading as far as I am concerned. While you are at the bottom of this you are still good. If you have one that reads 45 or 50 lbs. or less; then that is the one you want to look at. It is the odd reading that you are looking for. From what you have said, I would look elsewhere for your problem. Rings are good.

An engine running that "RICH" almost undoubtly had a carburetor problem.
It also will have a crankcase full of gas.

Thanks. I can't wait to get the tank back in this weekend. It has been off the road too long

I am sure that you are going to have it fixed; what with the new-rebuilt carb and new gas tank. I wish you good luck. I hope you let us know how it went this weekend!

Well, just started the car after I:
flushed, rust proofed and sealed the tank,
flushed the fuel line and installed new fuel filter,
installed rebuilt carb,
installed new muffler,
drained and changed oil. It was black as coal and the consistency of water,
converted to 12 volts
installed electronic ignition and new wires.

The car is running terribly rough, smoking and won't idle at low rpm. Tried setting the timing but the car is running so poorly it was a fruitless exercise.

Tomorrow I will pull off the valve cover and possibly the side cover. Something must be broken inside. It may be time for an engine rebuild.

Back to the drawing board.

Check the valve springs for breakage.
Did you ever do a compression test?

Consistent 70 lbs in all cylinders. Just pulled the valve cover. Valve springs fine and push rods appear good. Will have to pull side cover tomorrow. However, I am thinking it is an electrical issue. I may put the points and condenser back in as I could have a bad electronic ignition module. I checked to be sure all plug wires are correctly installed and rotor looks good and no obvious cracks in distributor cap. I have a new coil and wires and I cleaned the plugs. It starts fine. It really runs like a problem in the ignition

Stay with the standard points system and you will be better off. The electronic ignition tends to cause issues.

I have the original ignition, too. Sometimes those new fangled so called "improvements" can cause all sorts of problems.

Have you set your carb? Carefully and lightly screw the one adjusting screw in all the way. Just seat it very lightly all the way in. Then back off one and one half turns. (Some 1/4 turn from there one way or the other to get max gas milage.)

Does it make any difference in the smoke?

Thanks Terrill. Had to work on my son's truck this weekend but I did just pull the electronic ignition and will reinstall the points and condenser this week. I really think, and hope the Petronix ignition is the problem. I solved my problem with burning up points (got stuck a few times) when the guys on this forum diagnosed a bad coil. I really should have left it at that and not installed the electronic ignition.

We shall see. I will keep you posted.

I installed Petronix in one of my cars and it lasted one week before it failed and left me stranded in the middle of traffic downtown on a Friday late afternoon.



I totally agree!

I just started the car and backed it out of the garage for the first time in about a year. Problem solved. As the Junkyard Dog suggested, I have a Defective Petronix Electronic Ignition Module. I re-installed the points and condenser and all is good. I did not get the car to running temp, but it does have a miss and when I put the air filter on, the engine stalled. I hate to mess with the carb adjustments but I will at least up the idle speed. Not sure why it is missing, but I set the points using the hand crank. I will get a timing light and tach on it tomorrow, but the missing is likely something else.

Thanks

I am not surprised that the Petronix is defective.

Jolo
Quite often the air filter will cause a slightly enriched mixture. With air cleaner fitted increase the idle speed slightly then adjust the mixture screw in (lean) until the engine stumbles the richen about 3/4 turn. After the mixture adjust then readjust the idle speed.
Tony

Thanks Tony, I will give that a shot. I hope that evens out the idle and missing. Since part of my problem was a bad carb (the other, the Petronix), should I be careful with my selection of fuel? Should I use ethanol free marine 87 octane, or should I use premium or an additive? Also, when not driven for a while, should I continue using Sta-bil in the tank? I don't want to ruin another carb. Thanks

Ethanol is some bad crap. Use Ethanol free gas if you can and no additives are needed in the gas. When your car is parked for a period of time ad some Sta-Bil to preserve the gas while the car is sitting.

Thanks, that is what I thought. I suspect newer cars are better equipped to deal with the corrosive impact of ethanol. I can get marine ethanol-free 87 octane locally, so I will use it whenever I can.

Great! You will be better off using the Ethanol free gas whenever you can.

Anything ethanol free is the better but 91 or 95 will be better again. Down here we can get 98 octane but I have found with my vehicles 98 makes little difference except in my wallet.
Tony

A 1933 was a very low compression ratio engine and designed to run on 50 octane gas. Todays 87 is much more than required and running a higher octane gas can actually be deteramintal. The higher the octane the slower the gas burns.

On that miss...after the engine is warmed up to "normal"; have you tried pulling the choke all the way out until the engine dies and then shove it back in.... in time to let it continue to run; several times? This may clear some crud that is in there out. I have used this to clear my car and truck both. I agree on using non ethanol gas. (But if this is all you can get then I use an additive that removes water.) Our modern gas is way to "hot". You might find that using 89 octaine (non ethanol) will cause you to have to set the spark into an advanced position. I think that the higher the octaine the more advance is needed.

Adjusted the carb but the car is still running rough and smoking and chokes out with the air cleaner on. Guess I need to do a leak down test this weekend?

Spray wd-40 around the bottom of the carb and on that thick black gasket. See if it stops. If so you have loose carb nuts!

With the amount of smoke and fumes coming through the vent tube and the rocker cover vents, I think I have some blow by and possibly bad rings. The car had been running fine after I replaced the coil, then suddenly started acting up with the black exhaust smoke, which prompted the carb rebuild. There are no noises coming from the engine. I guess the Octane Selector changes the Vacuum Advance. Mine is set to zero. I have 33 and 34 repair manuals and I cannot find anything on how that should be adjusted. I have found some threads on this forum for the Octane Selector that will likely shed some light on the proper setting for this.

I am hoping this is not engine trouble. It has 87,000 original miles.

Starting black smoking after a coil replacement highly suggests an ignition problem and not rings or other mechanical malady. That does not mean it can not be a coincidental failure. Just going with odds. Making a change that results in a new observation is more likely due to the change.

The black smoke indicates too much gas being burned AND BEING BLOWN INTO THE OIL. The thinned out oil could cause excessive blow-by.

Black smoke is unburned carbon from incomplete combustion. Either too high fuel to air ratio or lack of enough "spark" energy to have burn rate fast enough to combust everything before the pressure reduction stops the burning. Though most often too much fuel it can be due to timing and/or weak spark at the plugs. Because of the design of the combustion chamber in the head a strong spark is necessary in the early 6 cylinder engines.

I guess it is still running rich. I will go through the ignition again. Seems I should also try another new coil. I believe I have a spare. I put Castrol GTX 10w 40 high mileage oil in and it does seem awfully thin to begin with. Notwithstanding I may have gas in the oil, once I sort out the running rich issue, what grade oil do you suggest? Can I use a 20w 50? I have seen on these posts that 10w 30 non synthetic is the recommended grade. I am in a hot climate-South Carolina.

I will also reset the timing and put the Octane selector to +8 and see how it starts and runs.

10W-30 is what I use in all my old Chevys. I live in as hot a climate as SC. DO NOT use too high a viscosity oil as it will not properly lubricate the cylinder walls and wrist pins. They are lubricated with an oil mist generated when the dipper passes through the trough. Some of that mist also helps lubricate the rod bearings.

If you crank over the engine with the ignition on, spark plug removed (wire still attached) but grounded against the block should be able to determine if you have a strong spark at the plugs. It should be bright and make a "snap" sound. If tested at the coil the spark should jump a 1/2" gap and also make a "snap". While you have the plug out is it black? Should be if you are getting black smoke from tail pipe. Clean them all before putting back. Also check the gap. Should be between 0.025-0.040". Many of us prefer the wider gap as it produces a hotter spark at the plug.

Very helpful. Thanks.

I have good spark at all plugs. I installed brand new plugs gapped at 0.040. Car started easily. I ran it for two minutes then pulled the number 1 plug and it was already black. Even with the rebuilt carb, the engine is running rich. The car ran ok but has a miss. I did not get it to running temp. I am thinking this is a compression issue. I got cold readings of 70 lbs in all cylinders but with 12 volts. So the true reading would likely a bit less than 70 with 6 volts. Unless the rebuilt carb is defective, I think I potentially have bad rings? I adjusted the idle mixture screw on the carb 3/4 from closed then adjusted from there to even out the idle. With the mechanical fuel pump, I doubt I have too much fuel pressure.

I highly suspect the "rebuilt" carburetor. Those engines will run fine with 70 psig cylinder pressure. Are you sure that the metering rod and throttle plate are coordinated? Was it set with the proper gauge? Is the float properly set?

Also, does the "rebuilt" carburetor have all of the correct parts for that particular model of carburetor?

Thanks for the quick responses. I am not sure about the carb. I will contact the rebuilder today. If no luck there, I am willing to try another carb from someone else.

Here is a simple test I learned from an old time mechanic I
apprenticed for when I was about 15 years old! Probably not the most sanitary, but it works! Remove the brass fitting from
between the fuel line and the carburetor; wash and rinse the
fitting with hot soapy water, then rinse with cold water. Now, remove the carburetor and turn it upside down to drain all the gas. Re-install the 'now clean' brass fitting. With the carburetor in it upright (normal) position, blow through
the fitting; air should go freely since now the float has dropped and the needle and seat are open! Next, turn the carb
upside-down, and repeat! Now, the float should have fallen down and closed the needle and seat! You should not be able to
blow through there at all! If you can still blow air under these circumstances, either the float is stuck, or the needle
and seat is leaking! From everything I have read on this post,
I am convinced the problem is carburetion. I hope you have made sure the choke is open, and tried running the engine with
the air-cleaner removed!

Smoke comming from the valve cover after shut down is normal. I would think that doing a compression test would show a low reading on a bad ring. You will have to Re-test each cyclender again to make sure one of the rings did not blow out during your starting. If it was my car, I would have put oil in the cyclenders before starting after setting a long time. Some rust could have been there from an open valve on the cyclender wall. This rust could possably have caused a ring or rings to fail before getting lubrication during start up. TO PROVE IT IS NOT A BROKE RING...Removeing the plug and place compression gauge. Take reading, If low, put oil with a oil can into the cyclender in question; replace compression gauge and see if the reading comes up. If comes up...bad ring.(Note: some people who know the engine will be in storage for a long time place oil into the cyclenders to save the rings on re-start. The engine will smoke a lot upon start up until the oil burns out. Also rings have to "seat" and will make smoke until seating is finished. Set Timing with selector on "0". Then move Selector to improve starting and gas milage.)

Octane Selector: Make sure the slide under the Selector is well oiled and free to move. Set the Selector on 8 or 10 Advance for the regular, non-ethonal gas of today. It has 85 to 89 octane in it. 10 is the max Advance on the Selector. On my car that is towards the fender (advance or +).

If it was my car I would re-test each cyclender.

I think I can finally shut down this very long thread. I spent a few hours on the car this morning. I just took it out for the first time and it ran well albeit only at 25 miles per hour; I did not take it out on the highway yet. When Junkyard Dog and Chipper kept pointing at the carb as the problem, I figured I must have it set up wrong. So I started all over and set the idle mixture as lean as I could and have the car run smoothly, set the idle speed then reset the timing (it was off slightly) and the car suddenly started running well. I cleaned all the new plugs and pulled one out after running for a while-no black on the electrodes. So I think I am there finally. I want to thank all the valuable input from everyone and I hope other backyard hackers like me can benefit from this novel. To sum up, I had multiple issues-a bad carb and a bad Petronix electronic ignition. I am back to way the car should be; with the points and condenser. The only non original things are 12 volts and a new fuel sender and auxiliary fuel gauge.

Now if I can only get where I am going without overheating. Thanks again to everyone that contributed to my success.

JOLO.........its nice to see somebody who has been helped with info from fellow hobbists come back and actually say thank you.....

a rare occurance

mike lynch

Don't know how bad the overheating issue is but just because the water blows out the radiator cap doesn't mean it's overheating. My '34 runs fine with the water about two inches below the top of the radiator.

Hi Ron
I was idling a long time in the driveway and the temp gauge rose above 200. The car is due for a radiator flush (lots of rust under my Eagle radiator cap) and my water pump is leaking slowly, so I will flush the radiator and replace the water pump and thermostat and see if that solves the problem. Idling for long periods is not helpful and it was hot down here in Hilton Head yesterday. If that does not work, I will take the advice from a G&D article and install a Chevy Truck fan. My last resort will be an electric fan, but my luck has not been good introducing modern solutions to my car. The closer to stock, the better.

Thanks Ron

the stock 4 blade low pitched rad fan just cannot move enough air at idle.

IN a large long thread in this section where-in MILLN PHIL did the tests on the truck fan on his 35 std coupe. His results were proven successful . I believe the truck fan has a pitch angle of 28 degrees and draws more air thru the radiator.

Find it and read it .

Its not always a whole lot of backward flushing and boiling out.
Fins need to be tight to the tubes.
Tubes need to be clean.
Water pumps need to tight with no leaks.
thermostats need to be 180 and operating properly.
engine timing must be set to stock specs.
plus no leaking cylinder head gasket


mike lynch.............

Thank you Mike, very helpful

I am Phil, the guy who wrote the'Truck fan blade article. If
you look at my story again, you will see that I discovered it
was an flow problem by placing and electric fan about two feet
in front the radiator and watching the temp gauge while the
engine was still idling. When I did that, and before doing anything else, the additional air flow through the radiator
dropped the temperature about 20 Degrees! The larger fan blade
did the trick for me, and I have heard from several people with similar experience! Good luck, and let me know how it works out!

Pay attention to what Phil has said about the fan. Also, If the engine sat without water (dry) for a long time there will be "sheet rust" that has broken off and is stuck on the "head gasket" holes blocking water flow into the head. I have tried flushing and other products without success. The only way that I have be able to solve this overheating issue is to remove the head. Use a wire in all holes to losen all rust. Blow out with compressed air. Do the block too. That stoped the overheating. She runs nice and cool now.

Funny how we know the right thing to do but walk around it until we're forced to do it right way.

What you are risking by running hot is a broken head or cracked block. Replacing a head gasket compaired to the cost of a new head is nickle/dime vs. big bucks.

I just did this to my head. It soaked in the tank for 3 days and flushed out. I put evapo-rust in it for 2 days and took a power washer to it and the inside looked like a new casting. Another 2 cups of scale came out. It can be bought at Advance Auto and the same thing WD40 brand can be bought at Walmart. It is completely biodegradable and can be reused. I plugged the holes on the top and leveled it. 1 gal is enough.

I just reviewed the Evaporust videos on YouTube. Very interesting. Great to know. Thanks, I am always learning something on this forum.

In these years (33 to 36) they placed water "jets" in the heads. These jets shoot water onto "hot spots" in the head. These jets are easy to plug up as the hole is about pencil size. When they plug the head runs hot. It is real important to keep these free and totaly open.

1934 was first to have water jets in the head.
First with the BLUE FLAME cylinder head.

Yes, Jets, 34....I think his overheating is still due to rust chunks hanging up on the head gasket preventing correct water flow.

Well the overheating is now the least of my worries. I ran the car and tried to go over 35 mpg-no power. On my way back home the car boiled over. When I got home, I allowed the engine to cool a bit and I retested the compression with the engine at normal operating temp (as opposed to cold when I got consistent readings of 70 in all cylinders). At operating temperature (after the engine boiled over and cooled) my readings were:
1. 60
2. 59
3. 59
4. 60
5. 60
6. 60

I then shot oil in each cylinder and retested
1. 80
2. 80
3. 70
4. 70
5. 80
6. 85

Next morning I tested again
1. 68
2. 65
3. 68
4. 65
5. 79
6. 79

Can I now assume the rings are bad and I need an engine rebuild? If I need an engine rebuild, the closest major cities likely to have a good engine shop that might even consider this rebuild are Charleston (2 hours) and Savannah (1 hour). I will check the VCCA resources, but any suggestions are appreciated.

Thanks

Your engine might only require a valve grind and ring job with a good inspection of the head for cracks. I don't think you need a shop in a major town as your engine is similar to many straight sixes local shops have done. I have work done in my small town with excellent results. It is not how big the shop, it is all about what the workers know. You may have to be their parts runner but what of it. There was a big rebuilder in LA Calif. that hired my friend that had only average mechanical aptitude. He told me some strange stuff they did.

Trying to follow along here with your descriptions;

If the car won't go over 35 mph on flat ground, and you have at least 70psi compression, you need to find the real problem, as it is not compression related (if 35 mph is true!)

A person who worked on cars for 50 years would be looking at many things; Like ignition timing not advancing at all, obstructions in intake or exhaust passages, main jetting problem, etc.

I'm not saying that you won't need to pull the head at some point, as one strange thing I found on a 80 chevy C10 inline 6 cyl that had that very strange smog exhaust recirculation system; the intake valves had so much black carbon build up on the stem side, that the valve looked like a pointed Ice Cream cone! The valves still opened, but the air could not flow fast enough, to go over 40mph on the interstate, floored.

I certainly would not condemn the entire motor quite yet, until I found out why it can't go faster than 35. Heck, I suppose the brakes could be tight? it's near impossible to diagnose every single possibility over the internet. :)

I agree with frank32....There is another problem.

Sounds to me like you are starving for gas beyound 35 mph.
I would look at the screen in the top of the glass bowl fuel pump to see if it is clogged up with fine grit. Check to be sure cork gasket is good and glass is tight against it when put back. Look at the screen in the carb on the high speed jet. Low speed jet is good to about 30 mph.... See the underside of the carb. Upside down. There are covers with straight slots for screw drivers. Behind these are screens. Could be some fine grit got in there and blocked your high speed jet. Look at your float level in the "well" on carb to be sure you are not to low on the float and running out of gas while driving. Check the Accelerator Pump Plunger Arm to be sure you are in the middle hole. Make sure the throttle lever adjusting screw is not stoping the pedal from making it all the way to the toe board inside the car. All this providing timing is still dead on. Pull out choke at 35 to see if it goes faster. If so then your air mix is wrong. Carb needs checked then.

I haven't been following too closely but is it possible the valve springs are weak? Could someone have replaced them in the past? This would cause the valves to float at higher speeds.