Newly rebuilt 351W problems
 

I'm still having trouble getting my rebuilt 351W running smoothly.

Problem: The car starts instantly. Runs a little rough until it warms up. Then it runs well for about 15 minutes or so, then starts to get rough. For those 15 minutes, accelleration is good and smooth (haven't gotten on it much since I only have 50 miles on it total). After the 15 minutes, the acceleration is not as smooth. No backfires, just some hesitations through the rpm range. Also, the lifters *seem* louder and more clackety. If I let the car sit for an hour and cool off a bit, it seems to run well again for a few minutes, then starts to run rough again. Temperature guage reads a steady 190 degrees throughout all of this once warmed up.

What I have:
1982 351W (non roller) block 40 over.
Comp cams XE retro cam with hydraulic roller lifters.
Roller rockers set with 1/2 turn after zero lash on cam base curve - warm engine.
AFR 185 heads
Edelbrock AirGap Performer intake
Holley 670 carb with vacuum secondaries
100amp alternator
Proform distributor (HEI with built-in coil)
Mallory 6AL ignition box
Normal volume oil pump - warm idle pressure = 20, cruising pressure = 50
Castrol dino oil 10W-30.

The initial timing is set at 12 degrees with the vacuum advance disconnected and plugged. Intake manifold vacuum is around 14 at idle. Relatively stable with a mild fluctuation. It won't idle at less than 1k. Have sprayed everywhere with carb cleaner looking for air/vacuum leaks - none found. Starts instantly. I'm not sure what total timing is - not sure if I measure this with the vacuum advance attached or not. I'm also not sure if the 1200 rpm idle is causing some of the mechanical advance to come in at idle - not sure how to test this since I cannot get the idle below 1000 rpm. At 1000 it is rough, at 1200 it is smooth. I have adjusted the air/fuel to be 1/8 turn out from maximum rpm - same on both sides. This is about 7/8 turn out from seated. Secondaries are closed. I tried to go with the square slot, but it would then idle at 1200 with the idle screw backed all the way out - and the air-fuel screws were inoperative. At this setting, I would place my hand over the secondaries and the car would bog down.

So, I'm not sure what is going on. I'm not sure if this is a carb tuning issue, distributor problem, electrical connection fault, or mechanical engine problem. Why would it run well when it is warm, but rough when it is hot? Any thoughts? I am almost ready to change to an MSD distributor with external coil, buy a different carb (or fuel injection system). However, I would like to know what's wrong instead of just throwing parts at it.

Any help will be appreciated greatly!

What is fuel pressure running at?
Floats set properly?
Have you tried pulling the carb and cleaning out the passages?

Sorry - forgot those.
Fuel pressure at 6, floats set correctly at "a trickle of fuel" coming out the windows.
I had the carb off a 2 months ago when I had to replace the cam. I sprayed it with carb cleaner then. I bought it new from Summit and have had ethanol fuel stabilizer in it since I reinstalled it.

There are centrifugal weights under the cap that swing out as rpms increase. Remove them or use heavier springs to keep them in and this will help you block out advance at lower rpms for testing purposes.

Not sure this is the root problem though.

What timing light are you using?

14 inches of idle manifold vacuum, yet won't idle under 1000 rpm says the mixture is lean, spray some carb cleaner into the carb to see if rpm picks up.

Then get a QFT carb, you won't look back.

Thanks, I will try to lock out the mechanical advance to set initial timing. What is the recommended total timing?

Not sure that the high idle is due to a lean condition. I still cannot lower the idle even with the mix screws out 2 turns.

I think if it were
mine I would start with the carb. I have multiple carbs on my bench to work with and generally will change out to try and eliminate one aspect. if that is not possible for you then I would go back to the basics and pull the carb and recheck everything. Generally speaking if the car starts fine I would think the ignition is fine

Start with the basics. Check/replace your fuel filter- it may be clogged with debris from the tank or from alcohol in your fuel degrading lines and other items.
Then check your carb for debris in the jets and other orifices- some small pieces may have gotten past the filter or come from the pump and/or fuel lines ahead of the filter. Blow out your carb well with air to get everything out of the jets, install, readjust.

IMHO, this is a carb/fuel problem and something is getting into your fuel system clogging up the works. Get that cleaned out and you'll be golden again.

Bob

You say you changed cam 2 months ago. Did it have this problem before the cam change, and was that the extent of the changes? Trying to relate root cause to what parts changed. Sounds carb related to me. Are you sure all vacuum ports on the carb and manifold are hooked up? Full advance of 34 to 36 degrees is typical with vacuum plugged. Good luck.

I flattened 3 cam lobes on a tappet cam on initial startup. Yes, the idle was high before the cam change but it wasn't running long enough to tune. That was when I went to the roller conversion kit. Looked and sprayed everywhere looking for a vacuum leak.

Which vacuum port are you using for your vacuum advance? With your set up, you need to use the "ported" port. If connected to straight manifold vacuum you will have a higher idle tha can not be reduced. Simple test. Remove and plug vacuum advance and try to adjust your idle.

Yes, I am using the appropriate ported port. Vacuum advance is disconnected for timing adjustments and idle adjustments.

With all you described the carb is the issue. If you can not kill engine by seating the a/f adjust screw she is getting too much air. On idle placing hand over secondaries would bog engine also a sign of air flow. Float height on a vac 650 should be just below opening and have to bounce a little to see gas dribble out -float a liitle too high for a Holley. bet ya secondary throttle blade is open a little for whatever reason. Jeff C

PS.... ignition quality check = spark at plug should be a neon blue color, yellow or orange hints to a prob

I don't know how to check the color of the spark. As noted previously, fuel just barely trickles out the windows. Placing my hand over the secondaries bogged the engine when I had the square slots, that was before I closed the secondaries. Now it doesn't bog when I cover the secondaries.

If I turn the mix screws in on one side only, it gets very rough and almost dies. If I turn both screws in, it will die. My buddy has an Edelbrock 500cfm carb that I can try to see if it works.

Any thoughts on why it runs well when warm but then gets rough when it is hot (after 15 minutes of driving)?

Engine block and intake temperatures can greatly exacerbate carb and fuel issues because of fuel vaporization and expansion. I think bolting on a good old Holley or Barry Grant 650 would tell a lot.

I am not knowledgeable on aluminum heads or cams, but is it possible that your valve lash is set too tight, and the valves are not fully closing when the engine gets hot ? Another thought is to check the choke fast idle screw to make sure it has not slipped off its cam, causing binding that won't let the throttle come to idle.

Doc,
Checking color of spark can be done by pulling any spark plug wire off its plug. Insert an old plug in the wire you removed and place the plug threads against any metal (header tube or any engine part) where you can see it easily. If you do not have an old plug it will jump out off the boot end anyway. Now start the engine to see what color jumps out the wire or across the old spark plug. Let it run just long enough to see color of the spark. PS... avoid a polished valve cover as spark may discolor a small amount and do not hold the wire or move it while running or you may get zapped. LOL.

A cold engine will run with a very rich mixture (think about a choke) but a warm engine wants a better air/fuel ratio. Closing just 1 side should kill the engine and should not have to seat both mixture screws to kill engine. Try the loaner carb to save a bunch of time and grief for yourself. Jeff C

Pete made me think of throttle cable as well as his hints. Try his and check for slack on cable too.

What you describe could be a few things actually.

Could be vapor lock, plugged gas tank vent or fuel line or a failing coil, among other things. I like the idea of bolting on a buddies carb and then testing. That would eliminate the carb from the equation.

Then I would start with the coil. Not sure how you would do that without replacing the entire distributor. They usually show signs of failure when good and hot. Replace with a friends and see what happens. Next would be vapor lock, or fuel lines partially plugged. Fuel lines running too close to headers or other heat source. Carb could have fuel boiling in the float bowls. When the car is hot, pull off the air cleaner and listen and watch for fuel boiling out of the bowls.

Got to be something simple. Heat is the catalyst.

Well we seem to have hit the big three!!! Air-Fuel-Spark!!!....I had a similiar problem that I chased for a while. It wound up being a bad coil....had a very small crack in top that would arc...