VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Odd fuel problem

Context: '28, vacuum canister, rakx-o carb. no leaks
Problem: engine runs smooth and steady, good idle, good acceleration, but when the vacuum canister fills, the engine stumbles. Once it is full and fuel stops flowing, everything returns to normal.

ideas welcome.

Sounds like a leaky shut off valve in the vacuum tank. With a full vacuum tank, disconnect and plug the line to the intake manifold. The motor should run for several minutes without problems.

You could also have a sunken float.

Perhaps I did not explain it correctly. The stumbling only occurs when the float drops and opens the valve to allow the tank to refill. Once refilled, and the fuel refill cycle is complete, the problem goes away. If the valve was leaky, wouldn't gas get aspirated through the vacuum line into the engine and cause other issues? And if the float was the issue, would it not cause similar issues?

The other thought I have is that the engine is running a bit lean and when the valve opens, the extra air is leaning it out enough to make it stumble.

You may be on to something. The sound of the engine running when the tank is pumping is different than when it is full and shuts off. You may be missing the restrictor in the fitting at the intake manifold. This depends on the type vacuum tank you have.

oh no doubt it could be missing. This car has many oddities. The vacuum tank is the one with the tin (not cast) top and i think 4 small screws. What does the restrictor look like?

Here are pictures of the restrictor use on a small type vacum tank ( with 4 screws)


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Damn you guys are good!! Many thanks. I will be out in the garage checking tonight, but I have a feeling this part is not on the car. I might have to fabricate a restrictor of some sorts, unless someone here has one they are willing to sell/donate to me.

shawng
I believe that if I were you I would try to trade the older model vacuum fuel pump for the correct one for the '28 engine. You appear to have an original 28 intake, right?

I do have what appears to be an original intake, and from my research, I thought the vacuum tank on the car was a 28. I thought the stamped steel top was for 28 and the cast was for earlier years, but correct me please if I am wrong.

At this point, the car runs quite well, so if I can make it run better by making a restrictor, great. A new vacuum canister will only happen if I find one at a swap meet. I am not going to purchase one any time soon.

If it needs a restrictor it probably isn't for a 28, the 28 didn't need one. No I wouldn't be ready to buy one either, but I figured that someone with a 28 pump on a 27 or 26 or a 25 might be interested in having the correct one, so they would want to make a trade. I figure that even if a 28 really didn't need the compression coupling valve (restrictor) if it had one it would be ok. Kinda like a guy that wears a belt adding suspenders for "just in case". I have a spare intake from what I supposed is a 28, I'll look at it and see if it has a compression coupling valve. Them old dude engineers that designed the 1928 Chevrolets were pretty smart with vacuum and mechanical devices, weren't they? Distributor and coil on one side, the sparkplugs on the other side....the aircleaner on one side near the exhaust heat and the carb in the cool air on the other side...Ha!

I checked mine and there is something there, but there is no valve in it and it will not come out of the intake. time to play with some fabrication

It is normal for a '28 to have a rougher idle when the vacuum tank is filling. At least all three of my '28s (with original 4 screw vacuum tanks and restricters in the manifold) do it to some degree. When driving down the road I have not sensed a difference in performance when filling. Of course because of engine noise and sitting in the seat, I can't tell when it is filling.

Thanks Chipper (and Mr. Mack too) for all the advice. I should have said last night that the opening in the manifold is indeed restricted (tiny hole) and not line size, so I guess I do have the restictor and no check valve as in Bluzone's diagram, which you say is correct for 28.

as a side note, while all this is going on, it looks like my float has decided to become a submarine and the carb is leaking. UGH!

I checked my vacuum pump and it also is a 4 screw one, so I reckon you are right about that.

Whoohoo, nice to be right. Tomorrow is experiment day. If it works out, I will report back. if not, I will be silent. While i had the carb off to fix the leak (loose seat for the needle valve), I looked up in the manifold and sure enough there is one of those thingies like in Bluezone's picture, just no check valve.

I think that is what you will find in any original 28 intake, and the 27 should have the check valve.

To me it is a good idea to get the vacuum source down in the main vacuum stream just above the carb.

It is a simular situation to using the base of the carb as a source of vacuum, just below the throttle flapper valve, for the vacuum advance in the 216 and 235 engines, rather than the one down on the intake where the wiper vacuum source is. Is that good engineering? I think it is.

Experiment complete. Here is what I did today. For all the purists, stop reading now. For all the geeks, continue. I went and bought an anti-syphon fuel valve (check valve) from the local marine supply shop. it has a very low crack pressure so, it works well in this application. i put it inline right where the fuel line comes up from beneath the car. then after it is the fuel filter. Now when the car runs, the filter remains full and the engined has way less air to pull before the vacuum tank fills with gas. It has greatly reduced, not eliminated the stumble when refilling. For the $20 in parts, it is worth it. The fuel filter i am using is more of a strainer, http://www.jegs.com/i/Spectre/865/2369/10002/-1, but in this application, works much better than a paper filter, or the sock GM suggests you use to filter the gas while refueling.

Good result. Achieved a similar outcome using a marine primer bulb. Have never had to use the primer bulb as such, but the one way valve ensures the line is allways full and you dont get that minor miss when the suction cuts in.

Regards

Ray

so.....you put the check valve in the gas line from the fuel tank in the back of the car? I don't see how that would keep the problem of stumbleing when the vacuum pump is filling the inter chamber with gas if the stumbleing is due to low vacuum in the manifold, which may cause decreased flow thru the main jet in the carb, but it may keep the fuel from siponing back into the fuel tank when you are going up a steep hill and vacuum in the manifold is almost zero, keeping the vacuum from pulling gas into the vacuum pump chamber. Ok, I haven't ever had that problem with my 28. The fuel filter settlement bowl always stays full and gas does not run back into the fuel tank

Well what about the vacuum line from the manifold to the vacuum pump? Isn't that the line that has been the main item in the discussion?

This is the fuel filter I use on the old Chevies, I always turn it so that the glass bowl is down, alowing the water and crap to settle in the bottom of the bowl (settleing bowl)

I installed the primer bulb (one way valve) just below the full filter, so it is approx just in line with the bottom of the vac tank. No problems with line from manifold to vac tank

Ray

1928isgreat, I see great minds thing a like. I thought about a primer bulb, but i like the metal valve better. Personal preference. Mr. Mack, as I said, this fix reduced the stumble. It is still there when vacuum drops, but it is reduced in duration. Once the rain stops, I will take her out for a drive and see how it performs. As to the filter, I do like yours better, but I could not pass up an $8 solution.

Good, I am sure that is an improvement. My inline 6 volt AC electric fuel pump ar the rear of the car that I use to fill the vacuum pump reservoir after long periods of storage(hidden up near the wheel well) has a check valve built in I installed it before we took the roaring fours up in the New Mexico mountains near Taos on a VCCA National pre -1929 tour several years ago, I didn't ever have to use it, the vacuum pump worked fine and the unpowered electric pump didn't seem to restrict the fuel flow to the front of the car either. I had to use second gear a lot though. The old 28 engine never missed a lick except one time when the too short coil high tension wire almost pulled out of the distributor cap.
I do use modified sparkplug wires made for a VW, they stand the heat better than the original type wires. On a hot day in the Texas summer heat on the highway, I had the original wires almost melt the insulation where they cross over above the head.



I have a bucket full of the settlement bowl/filters that I have collected over the years. Some are Morine and some are A/C Delco, all work about the same. I found a WIX filter that fits inside the bowl and works fine.

Some good news. Tonight was the first cruise night I have gone to at the nearby mall. The car drove like a champ. No stumble at all. I am so happy!!

Well done SHAWNG. Isn't it a great feeling when you overcome an annoying problem like that. So simple but the car runs so much smoother

Regards

Ray

Yes sir! It makes you feel smart when you finaly figure out what Louie Chevrolet had in mind when he engineered that car. I just this week figured out a vacuum fule pump problem my self. Have driven the car 30 miles this week with no problems. I think Louie is proud of you.