Technical Strange Rochester B stalling issue

I’m working on an original ‘51 Chevy 216 with the stock Rochester B that I’ve rebuilt. The engine starts, idles and revs great, and the car accelerates smoothly with good power. My problem is that when the car is jolted hard enough, the engine will sputter for a second or two, sometimes stall, but usually levels right out.

It will happen if the iron powerglide is in neutral, park, or in gear. The carb seems to operating perfectly otherwise. It responds properly to the mixture screw, and is sitting at about 1-3/4 turns out. I’m pulling a steady 20psi of vacuum. I’ve verified that I don’t have any other vacuum leaks. I believe I’ve isolated it to the carb. It happens when stopping sharply, but I can easily make it happen in the shop when firmly rocking the car back and forth in park or between some wheel chocks. It seems that fuel slosh is a big part of the issue. I can ALMOST cure the issue by adjusting the float height from the spec’d 9/32” to a crazy 17/32”. The spec of 9/32” is very universal with these carbs, even in marine applications, so this doesn’t sit right with me at all and I’d like to solve it. The float tests perfectly and looks like every other B float I’ve seen. I’ve wondered if it just sits lower/heavier than others? I know warping is a common issue with these carbs. Although the bowl gasket does get a little wet near the fuel inlet, I’d say it’s one of the better sealing carbs I’ve seen. The gasket shows good contact on all the sealing surfaces so I doubt there’s an internal leak, at least a significant one. When I rock the car and make it sputter, I don’t see a drop of fuel in the Venturi dribbling or leaking from any source so it’s hard to blame flooding. Same goes for when I rev it, I’m not seeing sloppy performance (dribbling) in the Venturi. I’m wondering if the fuel sloshing is temporarily causing some sort of a venting issue if it sloshes the right way, causing temporary starvation? I can kill the engine immediately if I put my finger over the bowl vent.

I’m going to continue to pick the carb apart and make sure I haven’t missed a clogged passage. This is a weird one in my experience.

 

Is the fuel pressure stable and within specs?

Is there a float drop spec to be considered?

Everything tight and well sealed between the throttle body and the bottom of the float bowl?

 

“Jolted hard”, are you sure it isn’t an electrical issue as a wire that’s ready to completely loose connection? Like a distributor to coil wire or else where in the power supply for ignition to happen?

 

It’s not electrical. The ignition, starting and fuel systems are completely new using stock style parts (plus a Pertronix pickup and coil) and retaining the 6v. I’ve also replaced most of the critical wiring and have inspected what I left behind. At this point I wish it was an electrical problem.

The parts-store fuel pump impressed me with a near perfect 4 psi. They are usually higher than 5 for me.

 

All the gaskets seem to be sealing pretty good. I’ve tested with light without a gasket (typical method on these) and I have good imprinting on the sealing surfaces.

 

Float drop is spec. I’ve considered replacing the float for the hell of it since they’re still readily available. But I hate guessing and throwing parts at stuff without testing/verification.

 

Ooooooffff ! Be careful what you wish for!

 

The problem is worse when the float is at spec, when I lower it (less fuel) the problem almost goes away, but if course the lean issues from low float level comes in to play now.

 

You might have been to this site already, but if not there is some info here on troubleshooting the idle circuit on the Model B.

https://www.carburetor-blog.com/knowledge-base/rochester-b-bc-bv-idle-system/

There's also a schematic showing where it picks up fuel for the idle circuit. For such a simple carb it sure seems to take the long way around the barn.

 

Yes I have, they’ve put together a complete manual for these that’s amazing!

There may be an internal fuel leak to vacuum that I can’t see. Im stumped

 

I spent most of the day with the car, and have verified all the details mentioned earlier. It’s definitely related to fuel slosh, specifically with fore and aft, not side-to-side movement. With the float at 1-9/32” (spec), I can rock the car as hard as I want side-to-side and have a really hard time getting it to stumble. But if I give the car a firm nudge back and forth, it stumbles very easily. I worked the float level down 2/32” at a time and saw improvement every time, all the way up to about 19/32” where it became impossible to get it to stumble, but I lost consistent accelerator pump function since it was starving for fuel due to the low fuel level. I brought the float back to right about 16/32” and the car works almost flawlessly for the setup. Really harsh stops or launches will make a small stumble, but Sunday cruising and everything works great. It doesn’t starve for fuel around curves or during hard acceleration.

I think I’m going to have to let this one roll like this with a recommendation for a new carb if it won’t reasonably hold the tune.

 

Richen your idle mix up to 18" of Vacuum & see if that helps.

 

This comment is still troubling to me. But I can't think of any logical explanation why.

 

After days of running well with the modified float level, I decided to put a new set of gaskets in it after being disassembled so many times. After unscrewing the needle valve seat out, I found a weird crescent shaped hole under the needle valve’s washer. I’m not sure if I just completely missed it during the rebuild, or if it’s a repair that failed sometime after I put it back together, but most importantly I’m not sure if it’s supposed to be there at all??? I don’t remember seeing a hole like this in other carburetors, and I’m don’t see any mention or pictures of a hole like this online yet. Does anyone have any input, or a picture that would confirm whether or not it’s supposed to be there?

I don’t see a reason why it would be there, and it would make total sense of the sloshing issue.

 

This sounds like a job for @carbking ! ! !

 

It appears to show up on this how to http://devestechnet.com/Home/RochesterCarb

 

I thought I saw it on one particular picture from that website, but it almost looks like a shadow to me as well

 

Here are some others

 

There’s no way it’s supposed to be there, in my research. According to the schematics, it should be causing poor fuel metering due to a vacuum leak to the fuel bowl, and causing the fuel sloshing stumble issue. I took apart another Rochester B and it’s not there. A few pictures online that seem like they have it must be shadows. I had a hard time taking this picture without a hard shadow.

I ordered some Marine-Tex, here tomorrow. I’ll give it 48 hours to cure.

 

I got the small hole patched up and it cured my issue. I was able to set the float height to spec and the car will not stumble at all when sloshing the fuel. The throttle is a touch more crisp off idle, too.

 

I had a similar issue with the new motor in the 51 Ford. Everything told me it was fuel level. That part was Correct however it was the End result of a different problem. Along with the New motor I installed a New fuel pump NAPA brand #BO173P. In fact, everything related to Fuel was new including the Tank. Short story, it was the new fuel pump. In fact it was 3 new NAPA pumps. They ranged between 10 and 14 lb's of pressure. Who has ever had 3 new defective pumps in a row? I've learned it's not a new problem and NAPA has known about it for Years. Their advice is to add an inline regulator. No thanks! It's defective parts, they don't care. I bought a new $200.oo plus fuel pump made in the USA Edelbrock brand and end of issue. First start and solid as a rock as a pump should be I had 6.5 lbs. at the Carb. It's called Quality Control because the built in USA pump is built by people the Give a S--t not made in China to save $$$. Put a gauge on the line at the Carb. I can't tell if you have a new pump on the motor or not but can see you've got sealer on the 2 fittings. Makes me wonder if you have recently replaced it and due to blind faith installed your problem yourself just like I did.

 

@Pist-n-Broke you read my 3 fuel pump story.

 

Boy these old cars are very simple but it doesn't take much to upset them. And there's times that they can make you pull your hair out.

 

Kudos to @BOOB on the finding of that hole!

Speaking of NAPA, my local store is owned/operated by a friend. I shared a shop with a local racer/mechanic where we did general repairs. My '63-1/2 Galaxie had broken the left engine mount, so I replaced both.
2 days later, (mild 390, not 'fierce') broken driver side again. Replaced it, NAPA. Then again, a week later.
My NAPA counter man is older, and sharp. He got ahold of their jobber, and I was awarded a pair of USA made 'hi-zoot' mounts. Took 3 times, but...

 

Your story is parallel to way to many I found doing my research. This is not just a NAPA story. It seems to be under many different names on the Box but all are in the $30.oo to $55.oo range and All say made in China and look to be the same housings. In all my years I never needed to install a pressure gauge on a new pump until lately to find a Fuel problem. I have an Ace carburetor guy and have had for over 40 years. I never once questioned his work until this event. He is actually the one that put the gauge on the running motor and showed me the issue. Man, did I feel like Crap.

 

I have actually filed a Labor claim through NAPA. I'll let you know how that goes too.

 

I see the machined spot he pointed out and thought, it's been there since it was manufactured how can it just now be an issue? I sure hope it's the end of things for him, but I'll wait and see.

 

Hate to bring it up on this thread but back to the tread I did. One member mentioning testing every pump before going out the door. I hope he made up some sort of equipment to simulate pump being in the vehicle for testing. R&R is money bottom line.