VCCA Home Forums TECHNICAL - GENERAL Trucks and Vans Leaky Carb on a 216

OK, I'm a little frustrated. Thought I was a half-way competent mechanic on these old trucks, but they keep teaching me new lessons.

I'm talking about a 1953 Series 3100 ½ tom pick-up, affectionately know as “Jed.” The engine is the correct '53 216 cubic inch, everything is bone stock, being restored the very best I can do. Jed has been under restoration for several years, I keep stopping and working on a different car, and coming back to him.

My problem is the carburetor. I have just installed a rebuilt Rochester Model “B” carb, which is the correct one. It leaks gas everywhere! This was rebuilt by a friend of mine, who is pretty darn good. He has restored a prize winning 30 sedan, a '68 Vette, and has rebuilt several carbs for several guys we know, always successfully.

Now the strange thing is, this is the 2nd carb that has had this same problem. Before this one, I had gotten one from Doyle Stokes, long time carb rebuilder that is always in the G&D. It leaked, too!

They leak at the shaft of the throttle butterfly, and around the horizontal “seam” where the two halves of the carb body meet. Also seems to be leaking at the bottom of the accelerator pump linkage. Leaks a lot! It was flooding gas all over the manifolds, everything soaking wet.

After the first leaker, I pulled off the manifold to repaint everything (gas had washed off some of the paint, and stained it).

It's almost as if there is too much gas pressure into the carb, and it can't contain it.

I have pulled a lot of carbs on and off of a lot of cars, from the 20's to the 60's – but I have never experienced this. The fuel pump is rebuilt, the fuel line to the carb is dry. I'm tired of repainting the manifold, and taking carbs on and off.

Has anybody else ever experienced this situation????

yes, there isn't a cheap fix for the throttle shaft leak but that isn't much of a problem, unless it is really bad, manifolds aren't supposed to be painted anyway! Ha! if the rest of the carb is doing it's job, the fact that a new kit is put in right isn't all there is to the Rochester, They do have a problem with warped bowls once in a while, but that also can be deceiveing, the proper float level will do wonders sometimes , also be damn sure the right bowl gasket is in place, sounds to me either you have a leaking needle valve or the wrong float height, or a bad float. The generic kits have a couple different bowl gaskets. Also what kit you use also makes a differance, the best ones I have found are the NAPA 2-5158 for around $12.

Since you have owned the truck was there ever a time when the carb/man. didn't leak?

What internal changes have you made to the motor?

Fuel pump changed?

Wish I could help you better. :( :(

Thanks for your thoughts, gentlemen. The throttle shaft leak is not the worst of it, not even worried about that, if it was dry everywhere else!

The bowl just keeps flooding over, no matter what I do. Went back at it this afternoon. There was no junk under the needle valve, which is new of course. Also the seat is new. Blew through it, and it shuts off when you tip it upside down, like it should. Cleaned everything up again, set the float (which floated OK for 24 hours in gas) so that it would shut off even earlier, put it all back together – and – yep, gas all over everything when I fired it up! Damn! When I hit the accelerator, I even got gas squirting up out of the little tube that comes up at a 45 degree angle (not sure what to call that tube).

See, the thing is, if it was shutting off the flow of gas as it should, the gas level should never even get as high as that main horizontal seam in the body of the carb, where the gasket is. So theoretically, standing still on level ground, it shouldn't leak at that seam even if the bowl was warped a little, or if the gasket wasn't perfect.

MrMack, as a matter of fact we did use the NAPA rebuild kit.

Gator, yes, it didn't used to leak before restoration. Now, everything is new – fuel pump, all new lines, everything. And it is all absolutely stock, exactly like the Shop Manual. I have made ZERO modifications to the inside of the engine, or anything else, for that matter. All I did inside was hone the cylinders, new rings, original pistons, etc. Probably should have gone oversize, but it was right there at about .0030 of wiggle. Did a valve job, with new springs. Pulled one shim on the Mains. The usual stuff. Didn't build it to drive 100,000 miles, just to be a driver and go to shows, cruise-ins, and goof around. But totally stock in every detail.

I guess I'm saying I tried all the reasonably obvious stuff, and I thought maybe somebody had experienced this Twilight Zone type flooding-over problem, and it was something that was not obvious. One of them “you have to learn it the hard way” kind of things. But sounds like you guys are all thinking the same stuff I am thinking, so move over Rod Serling!

Anybody got an NOS Rochester Model B 7004600 still in the box????????

Guru,
What happens if you tap on the carb. with it runnning? If you can get it to run long enough. I have found that even new needles and seats that have been wetted with gasoline and then dried out will stick. I have tried cleaning the invisible 'glue' with every type cleaner I can think of but have never found anything that is fuel (or fool) proof. I have resorted to putting a note in all the carbs that I rebuild warning of the problem and recommending the light tap on the carb to dislodge the needle from the seat. Once it gets dislodged they seem to work for a while without any leakage problems. Before you tell me it is a particular type of needle and seat, I have found the problem with all types (rubber tipped, brass, stainless needles). I sure wish I had a better answer but so far it has eluded me.

Oh yes, I first thought it was only the rubber tipped ones and was because I was storing them upside down and the rubber was glueing itself to the seat. So I started blowing them dry and storing them right side up. Instead of sticking closed they stick open. Not everyone but enough to be frustrating.

Does it start leaking as soon as it warms up? and all the time it is running or just when you shut it off?, hopefully it runs. Exactly what model B is it? just a strait model B with manual choke? Are you sure the float is not rubbing on the bowl, or the float levelis bot correct? it should be checked with a float level gauge (part # 3696192) that is in addition to the float drop (1 3/4" on most carbs.) Did you check the valve and seat by blowing thru it when the float is up (by turning the carb upside down), it should also hold a vacuum, With as much flow from the accelerator pump you may be pressureizing the whole float bowl, this pressure should be vented out thru the pump discharge valve this is a 3/16" ball valve controlled by a spring to release perculated vapor pressure that may build up in the accelerator circuit, the tube that emerges above the choke flapper should vent the carb bowl, if gas comes out there when the accelerator is pumped there is definately something happening in the accelerator pump circuit. Have you checked the fuel pressure on the fuel pump side?

OK, here's an update.

ChevyChip, it runs well enough, but I have been immediately shutting it down, say after maybe 30 seconds of run time – because there is gas all over everything! I tried the tapping, even with a small ball peen, but gas just keeps pouring out. I like your theory about the sticking valve. That's what it acts like. This has, by the way, the rubber-tipped style. I noticed that when I had it apart and hold up the top half of the carb with the float, and turn it up-side-down and back, frequently the valve would not fall open. However, when I blow threw the fuel inlet, it would immediately pop open. So it is sticking at least a little.

MrMack, yes, it leaks IMMEDIATELY, as soon as it starts. Not sure how to describe exactly what Model B it is. It is the correct carb as used 1950-53 on the ½ tom pickup, # 7004600. Float is not rubbing, nor does it hit the gasket. You said-

“…the tube that emerges above the choke flapper should vent the carb bowl, if gas comes out there when the accelerator is pumped there is definitely something happening in the accelerator pump circuit…”

I think what is happening there is simply that the entire chamber is full of gas, and the vent tube is the only way out! other than down the venturi, of course. By the way, we enjoyed that article that Klinkerbelle wrote – Boy Howdy!

What I have done is, I have discussed this whole mess with my friend that I mentioned in the original post, and with Doyle Stokes. I have this morning sent one carb back to my friend, and the other back to Doyle, and they are both going to go through them again, now knowing exactly what a problem I have been having.

Additionally, I removed the fuel pump, which works fine, and Doyle is going to test the output pressure from it. He says he might possibly have gotten a wrong spring in it (the one that regulates the pressure output), and that might be letting it put out too much pressure.

So those 3 items are in the mail, and we'll see what happens when they get back…. I'll let you all know. Thanks again for all the input!

ChevyGuru, I think you will get your problem solved in short order, since both of those gents now have a handle on the problem they will conquer that problem!
Also, thanks for the compliment, I will pass it on to Klinkerbelle, even though it may just make her ego grow, she has become quite a "Motor-mouth" if you know what I mean.

Don't know your friend but I do know Doyle and he should be able to solve any model B carb problem.

I read on another site, that the cure for a warped bowl is to mount the disassembled bowl on a heavy steel plate and place it in an oven for a couple of hours. Have any of you tried this ?

If you go to the 1949-1954 site and look up Right Carburetor post. It was back in Jan time. It might help. Good luck

I have had this problem many times. An old timer recomended I stick my rebuilt carb in a coffe can full of gas overnight. I have never had one of these carb leak again after I tried this trick.

The Rochester carbs are famous for having warped air horns [ from over tightening of the of the air horn screws ]. The only way to remedy this is to " deck " the airhorn & the carb base [ much like decking a head ]. I have seen alot of of novice mechanics try to use a double gasket to remidy this problem. The double gasket approach does nothing but mis-align the venturies & the thing works even worse.

Well, boys, the problem is solved!

Got my rebuilt-rebuilt carb back from my buddy, and reinstalled it of couple a days ago. He said he couldn’t really find anything wrong, and that there was no serious warp-age issues with it. Then yesterday, I got the fuel pump back from Doyle. Doyle had a note in there with it, saying something to the effect of "sorry, the fuel pump was 'over pressure.' “ Seems he had a wrong spring in it, and it was simply putting out too much pressure!! He changed that back to a correct spring, and now we have everything hooked up, and it's bone dry and sits there and runs like a 216 should!

Also wanted to say “thanks” for all these good thoughts, and Autoteching, the string you sent me to was a most interesting bunch of information, too.

Now, all I gotta do is build the rest of this truck around that smooth running 216…..

Wow, this is one to remember! So it is possible to rebuild a fuel pump with the wrong spring and cause the leak. I have never really looked inside a fuel pump but the cam stroke must be charging the spring. I would have thought that it was the other way around, that is the cam stroke would be doing the pumping.

I think maybe that he was meaning a spring on the bypass valve in the fuel pump? Those old Rochester carbs are known for not being able to handle much fuel pump pressure, I think it is the way the needle valve and float linkage is set up. (IMHO) That is something to remember when you hook up an electric fuel pump, I have seen a fuel regulator used to reduce the pressure at the carb to below 3 psi.

The cam stroke causes the diaphram down or suction stroke and the spring applies the pressure. If not all fuel in chamber used by next cam stroke cam only moves diaphram required travel to refill chamber in pump to the predetermined level (cam travel).

OK that sounds logical. I know that there had to be a method of either bypassing ala a valve or some other method to keep from increasing the pressure a bunch when the float closes the needle valve, The rochester needle valve can be overcome by excesive pump pressure, some require l7psi or less to function. I have found in a stock engine with only one one barrel they operate better with 3psi or less which may require a pressure regulator between the pump and the carb like when an electric pump is added. Some positive displacement pumps use a bypass valve controlled by spring pressure.

The spring controls the pressure and the needle and seat in the carb control flow. They will work all day without fuel. Without restriction or fuel the diaphram will travel the full stroke allowed by the cam or diaphram which ever is the lesser.
The current fuel injected system generally use a centrifical pump capable of about 30 psi and most need to be submerged in the fuel for cooling. It is best if these pumps are not left run without fuel.

True, however the electric pump I was referring is not the in the tank pump that is in common use with the F.I engines in today's cars, TBI =15psi and TPI= up to 30psi. I was referring to the add on booster pumps some set at 6 to 7 psi and more, the ones that are aftermarket, as seen on the hot rods with Chevy inline 6s and v/8 engines with multi carbs.

Yep, we were talking about the bypass spring. My buddy that rebuilt the carb says he tested it under pressure, and the float will manage to close the needle valve up to about 6 or 7 psi, and after that it can't keep it closed. Doyle was saying that the spring was letting the pump output as high as 10 to 12 psi, which is way too mcuh for the carb to handle. If I remember right, Doyle said the fuel pump should only put out 3 to 5 psi, but I might be remembering that wrong. But it would make sense if the carb can only handle 6 psi.

ChevyGuru, those test figures agree exactly with what I have found. I believe that it is related to the lever arm of the float and the float's bouyancy. in gasolene.