The Stovebolt Site Forums The Shop Area Engine Shop Vacuum Leak or Pump?

For some reason I can't get my truck (3200, 235) to idle below 1100 rpm without immediately stalling. It starts right up with the choke pulled out. Once it is pushed in at around 1100 rpm it dies. I've done the following:
1. Checked for intake manifold leaks with a propane torch. Did not notice a change. But can't idle below 1100 rpm so it's hard to tell if anything changed.
2. Borrowed a friend's homemade smoke machine and shot some smoke into the intake. No visible leaks.
3. Tried 3 different carburetors. - all stalled out at around 1100 to 1000 rpm.
4. Checked ignition coil with gauge. It's normal.
5. Replaced spark plugs.
6. Points and condenser look good. Tuned truck a few weeks back and dwell was normal.
7. Replaced fuel filter from tank to pump.
8. Replaced fuel filter from pump to carburetor.
9. Checked vacuum advance line for leak. Nothing found.
10. Tee'd in fuel gauge between mechanical fuel pump and carburetor. Got around 3.5 PSI at 1100 rpm. It held at 3.5 for a few minutes after the engine was off.
11. Vacuum gauge shows 19" of mercury at 1100 rpm.

The manual says that the fuel pump should operate at around 3.5 to 4.5 PSI. Is that at a normal idle of say 600 rpm? Isn't the PSI reading of fuel delivery inverse to vacuum? High fuel PSI to low intake vacuum and vice versa? Does that PSI reading sound normal or below normal for that rpm? I'm getting discouraged at this point. Not sure how to proceed.
Thanks.
Mark

Mark,

What procedure did you use to set the engine/ignition timing?

What procedure did you use to adjust the carburetor idle speed and mixture?

Today's condensers are pure crap. Try another new one.

Your description of the problem is classic LATE- - -VERY LATE timing symptoms. Get the pointer and ball aligned in the window in the bellhousing with the engine stopped. Loosen the clamp screw on the distributor and rotate the housing clockwise until the points close. Turn it back CCW until they BARELY open. Tighten the screw down. This procedure sets static timing. The engine should start and run at a normal idle speed of 500-600 RPM as long as you have the basic carburetor adjustments close to correct. Lightly bottom the idle mixture screw by turning it clockwise, and back up about 1 1/2 turns for an initial setting. I would be very surprised if following the above procedure EXACTLY does not result in a good idle.
Jerry

I did set the timing a few weeks back at 10 degrees BTDC. It's possible it slipped. That's the only thing I didn't check. I've never had that problem before. I'll recheck. Hope it's that simple of a fix. Thanks.

What year 235?
What carburetor?
What cam?
What kind of ignition/distributor?

How did you choose 10 degrees BTDC?

If you have a vacuum gauge, you might want to try using it to tune your engine (carburetor mixture & timing).

All this bovine scatology about setting timing with a vacuum gauge trips my trigger. It confuses the rookies who don't know any better, and annoys the experienced people who realize there are better ways to figure our when to fire the spark plugs!

Rant mode off, back to our regularly scheduled fountain of guesswork and misinformation!
Jerry

It usually gets/keeps a stovebolt engine running just fine.

Here's a little more sophisticated vacuum gauge timing technique - I hope it does not trip your trigger.

Point taken- - - - -doing the vacuum gauge thing at cruise RPM with the vac advance disconnected has a much better chance of accomplishing something useful than messing around at idle speed. If the engine happens to be running under partial load on a dyno, it works very well. All you stovebolters with a dyno in your shop and a few decades of using it can probably do very well timing with a vacuum gauge!
Jerry

Everything is stock on my truck. 56 235. Not sure what size cam. But it's stock.

I use a timing light not a vacuum gauge for timing. I use a vacuum gauge for setting the carb air fuel mixture. What puzzles me is that the truck was running great and all of a sudden it died at normal idle at a stop light. It ran with the choke out but with a rough idle. Which had me thinking that it was a vacuum leak. I could never find a leak. It could be that the distributor may have moved and wasn't tightened while driving.?? I don't know. I'll re-time it today and see if that works.

darn, if I knew that I would of not of thrown out my dyno last week.

To the rest of us that do not have a dyno all I know is that my dad used a timing light. I used a timing light. So I have to ask why are people avoiding using a timing light.

The trucks left the factory with the timing set to spec and they ran well. So why is it that not using a timing light now not the best way to go?

I have seen pre computer, computer controlled carburetor, fuel injection computer controlled engines run well set at spec.

Yet people continue to address the symptoms by changing the settings instead of finding out the problem.

Reminds of the time where a customer traded in a late model PFI engine car or took a factory lemon buy back towards a new car. A mechanic ordered a factory rebuilt engine or new block with heads. He did the install and the engine would not idle, fouled out the spark plugs fast and then would not run. He put in a new set of plugs and they fouled out. The car was put on the side.

After a couple of months I was asked to look at it. First problem was back then a target motor was not ordered. Target motors were new and complete. Just drop them in.

Second problem because it was not a target motor the factory rebuilt/new but not complete did not come in with an intake manifold. So the old intake was installed with the original FI system and whatever original sensors and emission control system parts.

I found a EGR valve stuck wide open and about 2 other electronic items, can't remember now what those other things were. I changed the parts and it ran like a new engine. Which it should because it was a new engine. Jeffrey

Has someone already mentioned doing a vacuum leak on the vacuum advance unit?

Have you tried using a propane bottle vacuum leak tester?
Jeffrey

32vld, Original post #1.

" . . my dad used a timing light. I used a timing light. So I have to ask why are people avoiding using a timing light.

The trucks left the factory with the timing set to spec and they ran well. So why is it that not using a timing light now not the best way to go?

I have seen pre computer, computer controlled carburetor, fuel injection computer controlled engines run well set at spec.

Yet people continue to address the symptoms by changing the settings instead of finding out the problem.
.
."

Good questions and good point.

I think the engine in question is a 1956 235.

1. When those factory timing marks were made (on the flywheel), the octane/characteristics of the gasoline was significantly different from modern gasoline.

2. No offense to anyone's truck/engine, but 90% of us have a non-original engine and/or a non-original flywheel. The flywheel was originally marked for a specific engine, in a specific year. There were more than one timing settings/marks used on high-pressure 235/261 engines. Flywheels and clutch assemblies on our trucks might not be original.

Hotrod Lincoln's timing technique and the vacuum-gauge timing technique do not care about the timing marks being original or correct. TDC/UDC is all that is needed to get you close.

Another aspect/benefit of using an inexpensive vacuum gauge is that it works on 6v and 12v systems (yes, I know the engine in question is 12v).

By the way, a vacuum gauge can also be used in finding out problems.

Finally, 90+% of our 235/261 (and 216) engines have an easily accessible vacuum port. This timing technique is easier, cheaper, quicker, and less dirtying than most other techniques. And, it allows us to quickly check/adjust the carburetor mixture setting before we disconnect the vacuum gauge.

Sorry to waste your time, but you asked.

That's what I'm here for. Jeffrey






I mentioned testing the vacuum advance unit because when people check for vacuum leaks they will check for bad gaskets, lines, and hoses but sometimes they forget to check the vacuum operated units. I know my memory is not as good as it used to be. And assume yours and others are bad as mine. Jeffrey

I don't even remember my name or what I had for dinner most of the time.

This is a tough one. The symtems are classic for the plug in the discharge passage in a Model B carb falling out. But trying 3 different carbs & getting the same results rules that out. What about a vacuum leak in the plastic spacer under the carb. It needs a gasket on both sides & sometimes can have a crack in the spacer. Its possible a stuck intake valve is letting compression back up into the intake manifold.
I agree a vacuum gauge is a poor way to set timing. You will generally wind up way too much advanced.