MTFCA Forum

Trying to figure out why my newly restored engine doesn't idle well while driving and to keep it from starving, I have to keep it reved higher than I should. I am thinking it may be an intake leak. Also the motor does not accelerate good even in the garage. It coughs and stumbles trying to rev the motor. I'd like to hear what others say before I remove and replace the manifold gaskets and rings

Richard,

The easiest way to find an intake leak is to...

Start the car and set to low idle...

Take some kind of petro spray can with the red delivery straw ( carb cleaner, etc) and spray with the tip near the ring...

Speeds up, you have an intake leak.

I also recommend to not use copper crush rings twice should you decide to pull the manifold. False economy to me...I buy them by the bagfull and even if I suspect a lopsided pinch...I start again from scratch! Just my way and for the record I find I never need to go back a third time. I also used a dab of grease to hold everything together since I wasn’t born with 4 arms.

If you don’t find an intake leak, open up the mixing screw 1/4 turn...you just might be pleasently surprised

A few things: I don't know exactly what was done to "newly restore" the engine. If you put in new pistons, rings, and tightened the bearings, your engine will be tight for a few hundred miles and needs to be broken in by driving slowly and being careful not to overheat and will loosen up as you drive it and gain more speed and power.
If you replaced the head and manifold gaskets, you will need to re-torque them after the engine has been warmed up. Do this after the engine cools off if you have an iron head, and do it while hot if you use an aluminum head. You should tighten the head to 50 ft lbs. Do this several times after each time you drive the car until it remains at 50 ft lbs. The manifold should be around 30 ft lbs.
Also did you do anything to the carburetor when you worked on the engine? If not, maybe it needs cleaning and re-adjusting. If it sat a long time with old gas in it it can get gummed up and would cause engine misfiring. What, if anything did you do with the ignition system? Was it running smoothly before you worked on it and what if any changes did you make? Sometimes Wiring can be grounded around the timer which will cause a continual spark or no spark at all on one or more cylinders.
So you see it can be any of a number of things causing the problem. If it runs smoothly, but just very slowly and not idling well, I would suspect maybe the timing could be off. So you do need to check the timing.
Anyway, check out the easiest things first, one at a time until you find out for sure what is causing the problem.
Norm

If you installed new al pistons and did NOT hone the piston pins, there is your problem.

George describes a good way to check for an intake leak. You can also use a propane torch, unlit and move it around the gaskets.
Some engine paint is attacked by some carb cleaners....
It would have to be a pretty large leak to effect off idle much.

I also agree with Norm in checking timing, and other minor things before you start taking anything apart.

And yes, we need more info on what was done to rebuild the engine.

Here's what was done engine wise. Reground crank, newly rebabbited rods fitted with Timesaver, new Stipe cam and fitted bearings, new valves, valve job with Neway cutters, lash set at 10 thou, new rings, pistons and wrist pins, new timing gears. Crank and flywheel dynamically balanced, rods balanced at both ends, pistons balanced too. Re ignition, I've installed Patterson coils a new Roller timer, and new X plugs. I'm getting good spark at the plugs. Thinking the problem was with the carb, I switched out the carb with others that ran fine on other cars. Same result. It almost acts as though the float is too high because it seems to load up at low rpm. Did a compression test and numbers were 45 lbs on 2, 3 and 4 and 40 on 1. Is it possible I simply need to run the car some? Never had this issue before but one time I had rough running engine after a rebuild and it turned out to be sticking valves. After I opened up the guides, the problem went away.
Wondering if I goofed up and didn't align the timing gear marks correctly but the car runs strong at driving speed. Got me stumped.

I wonder if .010 is too tight on the exhaust valves? Doesn't Stipe recommend .010 on intake, and .012 on exhaust?

Are the manifolds stock? I received a high volumn intake manifold as a door prize on one of the tours and installed it on an otherwise stock engine. The symptoms I had were that it ran very good at high speeds on relatively level roads, but was very sluggish at low speeds after shifting and going up hills. I tried changing the float level but it didn't help. I am using a holly NH carburetor. When going up hills it actually ran better when I backed off the gas. Finally I installed a stock intake manifold and it runs great. Anyway, what I have found is that some parts work well on modified engines but I think the problem I had was that when I opened the throttle, there was just not enough vacuum to suck in the gas so it was running lean. You can get the idea if you pucker your lips and blow. You will get a strong blast of air which you can feel with your hand about a foot from your mount. But if you open your mouth, and blow you don't feel it. Anyway, it could be just something you modified which caused the problem.
Norm

It could be a valve seal problem, at higher speed the problem would disappear. I would check out the intake manifold, carb gasket first. One of the old timers here would take a hammer and tap on the intake manifold near the clamp and re-tighten it. With it coughing at an idle means it is lean, but if the manifold is leaking it can be both lean and rich. Have you looked at your pugs?

Yu asked about symptoms of a leaking intake manifold. So far I haven't seen what happened to my Model A 45 years ago. I kept hearing a whistle and couldn't figure out what was causing it. At a local Model A club meeting, SOP was for attendees to ask questions after the main meeting, posing their Model A problem du jours to the "old timers". I had no sooner finished asking my question when from around the room, two or three "old timers" answered in unison: "Leaking intake manifold". Wow! With that much instant unanimity, a leaking intake manifold MUST be the problem. The next day I changed the manifold gaskets and the whistling went away. Lesson learned.
You didn't mention that your engine was whistling. But if it is, check the manifold gaskets.
Marshall

There are several methods for checking for a vacuum leak, I am a proponent of just water spray, the engine will stumble immediately. Was the head planed? As mentioned previously, adjust the mixture, an 1/8 - 1/4 turn can make a big difference. Mine starts with one turn and runs on 7/8 turn. It will not start with 7/8 turn.

The compression test results sound more like an engine requiring a valve job, than one that was recently finished. Lash and/or guides too tight need to be confirmed or denied.