Technical Head scratcher. GM crate engine BOG/cutout problem

Hi,
I have 31 Model A with 2002 year Crate Engine 350HO horsepower . It has very little mileage on it 5000km. I purchased a couple months and unable to get engine to run proper. It has a big bog when accelerator is pushed to WOT. It will actual cut its self out all together as long as pedal is held down to floor.

Stuff that I've done so far is:

1.Replaced Holley 770cfm with new Edelbrock 650cfm AVS. I was told to big of carb for motor initially. Was really tough starting before and now better. Also less soot build up on side pipes now.
2. Set idle mixture screws on new Edelbrock with vacuum gage. Getting 17 inch Mercury at idle.
3. Got kick down pedal adjusted so now it will down shift when pedal is to WOT.
4. Checked timing yesterday. Its showing 18 degrees BTDC before making any changes at around 800RPM. I disconnected vacuum advance and plugged off both ends. One on carb and distributor. Is it necessary to do the distributor canister also? Specs for motor is 10 degrees initial and 32 Total at 4000RPM. I'm unsure how to check total mechanical timing as the chrome indicator only goes to 16 degrees BTDC? How do I do the total timing. Do I have to make another mark with whiteout on crank at 32 degrees? I have made three 2 degree changes so far by retarding the timing closer to the 10 degrees spec. Unsure whether its helped. Its currently at 12 degrees BTDC. I run 92 octane fuel and have never experienced any pinging at the 18 BTDC? On occasion on a hot restart it would be tough turning engine over. I read this could be a sign of running to advanced of timing.

Questions:
1. I hear people say they like to see full timing at 2500-3000RPM vs the stock 4000RPM. Would it help my problem putting lighter springs in the distributor?
2. I was told to check my vacuum advance diaphragm to make sure the engine it pulling it and not stuck all together. How can I test this?
3. Does the idle mixture screws need to be readjusted after making a timing change?

I appreciate any help you have to offer. I could be missing something altogether, am I on the right path?

Thanks. Kevin

 

Get a timing light with a dial or buttons or whatever, that lets you use the TDC timing mark to see advance. example:

https://www.summitracing.com/parts/ino-3555

You might be able to change the curve by putting lighter springs in the distributor. See what the curve is now, though, using a dial type timing light.

To check the advance mechanism, find a vacuum port on the carb that has vacuum at idle. Put the timing light on the engine, see what the timing is, then connect the vacuum hose to the port that has vacuum, see if the timing advances.

Good idea to re adjust the idle mixture (and rpm) after changing timing.

There are probably a few other things that might be causing the bog...but it sounds like you have a plan, to start with the obvious stuff, and make sure it's right. Carry on.

 

Is the edelbrock really an AVS or is it possibly an AFB? If it's an AVS (Air Valve Secondary) there is an adjustment that can be made to the secondary air valve opening rate that might make an improvement at WOT once you get your ignition tune-up stabilized.

The secondary air valve is adjustably spring loaded, the same as it is on a QuadraJet. If it's too loose it may be opening too soon and leaning things out. The adjustment is kind of a trial and error affair.

Is it stick or an automatic? If A/T, does it have a stock or high stall convertor? What kind rear axle ratio? A lighter vehicle with a stick or a loose convertor and a lower axle ratio will like a looser adjustment on the air valve. A heavier vehicle with a stock convertor and higher rear gear will like a tighter adjustment which slows down the opening rate of the air valve. How tight or loose is the trial and error part of the equation.

 

Besides a spring change shorten up the curve maybe all in at 2800.

 

I'll just throw this out there.

I had a motor that didn't run right no matter what I did. Finally, a mechanic buddy of mine said "sounds like a vacuum leak". I pulled the intake manifold off and sure enough some of the sealant material had slid off the back of the block when we dropped the intake on.

We cleaned everything up and sealed the manifold again and no more problems.

 

Its an AVS so the secondary is adjustable. Its a TH350 3 speed auto with 1900-2400 stall converter. Rear end gears are 3.45 ish. Vehicle I'm guessing is a little over 2000lbs.What would you recommend. I'm a little baffled as the Holley 770 did the same thing as the new carb. The old Holley secondarys weren't even opening. Did the paper clip test.

 

Its entirely possible. I may have to resort to this.

 

Are you referring to a lighter spring in the distributor. After doing a timing change is it best to readjust the idle mixture screws?

 

Put a vacuum gauge on it, manifold vacuum. this will tell the tale for vacuum leaks, late timing, valve problems, carburetor mixture, etc. They really are the key for good diagnostics and tuning.

 

As far as testing the vacuum advance, it's as easy as removing the dist. cap and sucking on the vacuum hose while watching the breaker plate in the distributor. The vacuum advance should pull it around a few degrees. Stick your tongue over the end of the hose to see if the vacuum canister holds vacuum. Pop your tongue off the hose and the breaker plate should pop back to it's original position as it's spring loaded. Super simple.

 

I set the Idle mixture with a vacuum gage. I had 17 inches of vacuum. How do I do this for timing and how to tell if there is a vacuum leak? Do I just turn the distributor cap with vacuum gage on manifold and try to get max vacuum. Thanks in advance.

 

Google is your friend: http://www.mysandman.com.au/forums/showthread.php/5340-Vacuum-Gauge-Engine-Diagnosis-Chart

https://www.google.com/search?q=win...Q#tbm=isch&q=engine+vacuum+gage+chart&imgrc=_

 

Did you replace the sparkplugs when you got rid of the holley? Its a sure bet they got fouled.

 

This video should get you a close, baseline setting. The fine tuning part will be deciding if the engine likes the spring a bit looser or tighter from the baseline.

Though now I'm a bit concerned that you had the same issue with the Holley.

 

Sounds like a vacuum leak, or gas tank vent problem to me.

 

Is it an actual GM rebuilt engine or a GM engine built by someone else? Does it run well after it gets up to rpm?

 

A WOT bog or stall won't really involve the idle mixture screws.
How do you know it's down shifting at WOT if it bogs and stalls at WOT?

 

You can do this with the engine running, as a quick test, no need to pull the cap. I usually try to have a longer piece of hose and put the timing light on and watch the mark to get an idea of how much it advances and if it leaks down. Usually if the diaphragm is bad its like having a straw in an empty cup. You can also do this with one of those vacuum brake bleeder tools. Dont forget to plug the original vacuum hose.
Have you looked at the fuel delivery?
Does the bog eventually go away? Does it have a flutter sound, and then smooth out? That sounds more like the rear air door, as mentioned. That actaully has to be pretty stiff for a stockish motor.
Good Luck.

 

Fuel pressure
Plug wires
Spark plugs
Coil and coil wire
Points or HEI

 

There is a gas tank vent in the trunk. Fuel pressure is also a little suspect. Runs up to 9psi when starting and after running it drops down and holds 3.5psi. Its fed by Red Holley electric pump. Its suppose to be internally regulated at 7 psig?

 

Might be worth while. They are 14 years old most likely and have some sign of soot build up.

 

Its an actual GM crate engine. Purchased at dealership in 2002. Bought car at estate sale so unsure of how it run in the past. Only 5000km on engine and parts. It runs quite well. Thought it should have more power as the car is 2000lbs. it will spins tires very briefly then peters out. Then when I put it to floor it does the bog cut out. Would it it be wise to put more or less spring pressure on secondary air valve. Do I want it to open sooner or later. The old Holley secondaries wouldn't even open so good chance that on WOT the engine wasn't getting adequate fuel/air? Thoughts??

 

You should be able to floor it with a 2 barrel

 

I pulled distributor and sucked on diaghram. Had to pull hard but was causing it to advance then snap back. Appears okay. HEI cap terminals a little white. Scratched that off. Rotor appears okay. Bog appears when mashing throttle when car is moving. If I ease into the throttle it tends to be okay and can then feel the 3rd to 2nd downshift. The bog will hold the engine out if I hold it to the floor. Will probably eventually stall the car out. This is a stock 330hsp engine do you think I should make the door a little stiffer? Does it sound like at full WOT the engine is going lean? Does vacuum open the top air plate on secondsaries and mechanical open secondary bottom plates?

 

Any chance a rubber fuel line is collapsing internally or kinked ?

 

You think the plugs wires are 14yrs old as well?

 

Because its a simple thing to do, you can stiffen the spring for the secondary air door to see if it helps, but usually that sort of bog is a temporary thing, it eventually should speed up if you stay in it.
It sounds like you are running out of fuel, or the mechanical advance in the distributor is not working. Did you do as squirrel suggested and verify the total timing, and at what RPM its in by? Good Luck

 

An off idle bog is usually caused by not enough fuel from the squirter to cover the transition from idle circuit to main circuit. I am a Holley man. Somebody can tell you how to increase squirter volume on an Eddy. 14 year old plugs and wires and corroded cap not helping things.

 

Maybe it's me but I never had a small block that "wanted" 12 degrees btdc. Nevermind 18 degrees. I have owned a lot of small chevys and they were all pretty mild. They ran best around 8 degrees

Sent from my SM-G900V using The H.A.M.B. mobile app

 

I agree with this, but its also usually accompanied by spits and backfiring, and since you didnt mention that, I ruled that out....maybe I shouldnt have. Easy to check though.