The Stovebolt Site Forums The Shop Area Engine Shop 235 with No Vacuum at Carb

OK guys, I'm having trouble running down a problem and am tired of pounding my head against the fenders trying to figure it out. I have a '52 3100 with a rebuilt 235 with less than 500 miles on it. I added the Fenton dual intake and two Rochester B's. Everything was running great. I went out of town for a couple of weeks, got in and drove it down the road to find it with little power and backfiring (mostly through the exhaust but every once in a while through the front carb). Everything seems fine at idle. I checked the timing- it's fine. I sprayed starting fluid along the vacuum line from the distributor to the carbs. I sprayed all around the rear carb gaskets and even into the throat and there was no change to idle. I sprayed into the front carb throat and idle reduced slightly. I sprayed all around the front carb gaskets and idle dwindled to nothing and it shut down and wouldn't immediately restart. Here's what has me stumped. If there was a vacuum leak at the base of the front carb, wouldn't the engine still run off the rear carb even when the starting fluid was sprayed at the front? Any wisdom you guys could share would be greatly appreciated.

Your problem is one too many carburetors, but I'm sure you really don't want to hear that. Starting fluid is a messy, not very effective way to chase a vacuum leak. A much better method is to use a Propane torch. Turn the gas on, don't light the flame, and pass the torch nozzle around the gaskets and other possible air leak spots. Be sure to pay close attention to the intake-to-head joints. Aftermarket manifolds have a nasty habit of being hard to seal against the cylinder head.

Getting the engine to run as well on two of the same carbs that are designed to support the engine with one of them is going to be a frustrating experience. Dual carb systems are supposed to have much smaller carbs so they can share the fuel metering equally. Two full-sized carbs are definitely the wrong way to go.
Jerry

Thanks for the advice. I've heard of the torch method but never used it. I'll give it a try. The frustrating thing is the two 1 bbl carbs had been working great for months and were working great one day, the next time I started her up- crap. I'll probably take the carbs apart and clean them while I'm at it. It looked like the fuel stream in the front carb was a little too much like a dispersed spray instead of a solid stream. There might be something clogging that fuel input. I'll report back what, if anything, I find. Thanks again for the tip.

I don't have any experience w/a 2 carb Stovebolt 6 but I do have experience w/multi-carb setups on various other engines so I'll throw some basic checks out in hopes it'll help.

1st easy check would be to remove both aircleaners and let the engine idle.
Slowly cup your hand over the throat of the front carb and listen/watch to see if you hit a point where the engine rpm picks up. If it does your front carb is running lean, troubleshoot accordingly. IF it stalls you probably don't have a vacuum leak on it. Key is to do it slowly. If at any point it picks up you probably have a vacuum leak. Keep in mid that in any case you will stall the engine if you totally cover the opening.

If it doesn't alter the idle do the same for the rear carb, if it does troubleshoot accordingly.

Troubleshooting accordingly: use Jerrys suggestion on using propane or Mapp gas to check for the vacuum leak. I've successfully used carb cleaner but using gas may be quicker.

To add to Jerry's suggestions also spray around the throttle shaft where it pivots in the carb base as worn shafts and/or bosses are known vacuum leak sources.

You could also have a stuck float in one of the carbs causing a lean condition (stuck closed)or stuck open which causes an over rich (flooding) condition. Are you seeing black smoke when it starts missing?

I've used a "snail" carb flow meter to setup multi carbs on other engines but I haven't tried the Stovebolt 6 as of yet. They are very helpful in getting both carbs setup...

Stumbling under load can also be bad points, the low volt coil wire grounding out on something (in dist, engine block) cracked dist cap, wet dist cap etc.

Good luck & Semper Fi
Dave

Great tip abut cupping the hand over the throat Dave. I'll definitely try that before the propane check. I don't see any black smoke when it's missing. The backfiring out of the tailpipes is a rapid popping- nothing really big. Just a pretty constant rapid pop. I've done the 12 volt conversion and added a Pertonix coil. I have all new plugs and wires so I'm pretty sure the ignition parts are all good. I used a vacuum gauge that is placed over the throat of the carbs to set them up (float in the gauge rises and falls in a tube depending on vac amount) and had them tuned so they were running extremely well before one day just crapping out.Thanks again for the tip and the insight.

Semper Fi,
Rick

Ok. I tried cupping my hand over the throat. The front carb has a lot of vacuum and sucks my hand down if I get anywhere near the throat opening. The rear carb is another story. I have to put my hand completely over the throat before it affects the idle speed at all. Having said that, I've tried the propane thing around all of the carb gaskets, connections, etc and cannot find a source of a leak. Could it be somewhere else? Intake? Head?

After not being able to find the leak and speaking with Carbking (thx Jon), I bought a compression tester and checked all cylinders today. Here's what I found. #1 and #2 were 129 pounds, #3-#5 were 120 pounds. #6 was 110 pounds. Also, I looked (again) at the plugs after just a few minutes of run time. #1 and #2 looked fine. #3 and #4 had a fair amount of carbon. #5 and #6 had a lot of carbon. I have a pic of #1 and #6 in Photobucket (link below) showing the difference between the two. Can anyone tell me what these varying compression measurements mean and if that's enough to cause one of my two carbs to have almost no vacuum? Any words of wisdom would be greatly appreciated. I'm about to pull out what little hair I have left. Thanks to all.

http://s42.photobucket.com/user/abbynormal44/library/