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Sounds like you are not truly at TDC, when at true top a leakdown tester shouldn't move the piston if you have a hold of the crank with a breaker bar and socket on the crank bolt (if you roll off top then it'll be tough to hold).
I don't have experience with Motors assembled by someone else, but i'd assume they could screw lots of things up so I'd be degreeing the cam as part of my sanity check if this was my motor (not that it would cause your symptom) and resetting the lifter preload. (someone else gets credit for suspecting the lifters, again i do it all myself so it would be a major f-up if i messed that up so i didn't even consider that for your issue).
OP
You're probably doing LDT right ... but if not ...
... you shouldn't use TDC (Zero degrees) for every cylinder ... instead, each piston should be at top of it's respective stroke.
have you done a compression check on all eight?
Jackson, we are using the top of the compression stroke for the cylinder under test. Everything we've viewed on-line has recommended against trying to hold the engine from turning by hand, but we even tried that. Maybe we need a bigger breaker bar...<grin>
We haven't done a compression test on all cylinders yet, but we will as soon as we get a chance.
Use a breaker bar with a 5/8 socket on it and hold it into place while testing leakdown at 100 psi. The lower your test pressure....the more inaccurate the leakdown test will be. 100psi loads the rings and helps push on the valves which would simulate combustion pressure to a point......much more so than 50 psi. I would just run a compression test from here......note the color of the plugs on 5 and 8 and rule out the long block at that point.....that way you can carry on and narrow it down.
Bouncy vacuum gauge indicates something amiss.....was the intake removed at all for anything? Intake leaks will cause a bouncy needle and low compression.
Anyway...compression test it and post your findings.....
Compression Test done; all cylinders within 130-140 range
Leak down - no leakage seen on any cylinder
Pictures of spark plugs:
I had my tech and an experienced GM mechanic whose shop is behind ours each drive the car to make sure they agreed we were chasing the right problem. They now think it's a fuel delivery problem because at acceleration the stumble goes away and performance is fine. At steady state cruising, the stumble is obvious. So now we're starting to look closer at the Holley 670 double-pumper that came with the engine. I've also got a former employee who is a good engine builder helping out by phone. Based on the pictures of the plugs, he has suggested changing the primary side accelerator pump cam to position 2 to see if that improves things.
You are running VERY lean. You say it is a 670 Holley. I imagine it is a Street Avenger...not a double pumper...E-mail Lars for his paper on how to set up Street Avengers due to lean condition. Pretty common thing.....
Double pumpers do not come in 670 cfm and it would be very doubtful that you would have a lean issue with one even out of the box.
Get some fuel in this thing and it will respond.
Jebby
Last edited by Jebbysan; Aug 24, 2019 at 11:18 AM.
Here is a picture of the carb. My understanding is that this configuration, with two accelerator pumps, is what makes it a "double-pumper". We have the primary pump off to check which power valve is installed. It's a 6.5.
From the GM Performance engine installation guide:
We already know there isn't a 10.5 power valve installed (it's a 6.5) and the guide doesn't provide a part number that the Holley website recognizes as valid.
Here is a picture of the carb. My understanding is that this configuration, with two accelerator pumps, is what makes it a "double-pumper". We have the primary pump off to check which power valve is installed. It's a 6.5.
What size jets and nozzle. If it has 64 jets in the front and 76 jets in the rear, you need to change them. 72 front and 80 in the rear
Last edited by Gunfighter13; Aug 24, 2019 at 12:03 PM.
Nope.....it is a 670cfm Street Avenger....the carb in your photo does not have dual accelerator pumps....also double pumpers are not vacuum secondary. This is a common confusion.
The part number is on the front right of the choke air horn: https://www.holley.com/products/fuel.../parts/0-80670
You need to throw some jet at this......sorry GM....but those jet and power valve number are nothing I have ever seen. They leaned it 3 sizes on the primary and fattened it 3 on the secondary......so the front and rear are 12 sizes apart, Holley sets them up 12 sizes apart too and it is dead wrong. Should be 8. Put a 68 jet in it and run it. Also...according to GM this engine makes 21 inches of vacuum? I don't think so.....the 6.5 is the right choice for base line. A 10.5 would never shut! The only thing I have ever seen a 10.5 in is a dump truck with a tiny cam.
Put a 68 jet in it first.
I worked on a ZZ4 69' Camaro years ago and it had a Street Avenger on it......I believe I put 6 jet sizes in it before it responded.....I do not know where GM gets these specs but they do not jibe with anything or anybody in the engine world....way off.
Jebby
Last edited by Jebbysan; Aug 24, 2019 at 12:10 PM.
The 72 and 80 may be where he ends up, but I don't like to jump more than 4 at a time....but you are 8 jets apart, and that is correct.
The power valve open is 8 jet sizes....Holley knows this.....not sure why they do what they do.
Jebby
Last edited by Jebbysan; Aug 24, 2019 at 12:12 PM.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Sounds like a too lean at cruise condition as already noted by others. Therefore, do not adjust/replace the accelerator pump or power valve yet. They don't do anything at steady cruising speed. Try moving up two jet sizes at a time on the primary side until the stumble at cruise is gone. This will help in better fuel economy finding the minimum jet size to eliminate the stumble.
GM's part number for the 670 Street Avenger. It must a superseded number because you punch in the number you have and this comes up. But it is the right carb.......Holley only makes two 670's...a single and dual feed.....and yours is a GM rebox Street Avenger.... https://www.gmperformancemotor.com/parts/19170092.html
Resdoggie is correct but I would jump 4 on this as the plugs are clean as hell....too clean.....I bet you end up at 70 like previously stated above.......
Jebby
Last edited by Jebbysan; Aug 24, 2019 at 12:35 PM.
