I have a serious hesitation during acceleration, and in 2nd & 3rd gears, it will back-fire on me; so I know I have a lean condition and not too much fuel. It happens when I accelerate at any speed; so I can eliminate it being tied to the off-idle circuits. The engine is built like the L79, 350HP/327ci that would have come in the car, except with a 350ci short block. Carburetor is a simple 600cfm Holley with vacuum secondaries, and they are opening at about 4000+ RPM. Float levels are set correctly Timing with vacuum advance is sitting on 25 degrees at idle, and I'm running off of manifold vacuum, not ported vacuum. All-in timing is about 38 degrees and comes on pretty quickly; so if anything, I think I'm getting too much advance too fast, which would tend to help a lean condition. I just changed the accelerator pump nozzle from a .025" all the way up to a .035", and it had absolutely no effect. The next step would seem to be to bump up to the 50cc accelerator pump, but I just can't imagine why this engine would require that much fuel from the accelerator pump; there's just nothing exceptional about it.

Just looking for other ideas or something that I'm overlooking here???

Thanks.

 

You'll need to duct tape a fuel pressure gauge to your windshield and observe the fuel pressure under actual running conditions. If you see the fuel pressure falling under a heavy load you'll know something is wrong somewhere between the tank and carburetor.

 

Considered that, but once it gets past the stumble and into steady state running off of the main venturi circuits, it goes fine; so I don't think fuel starvation is the problem.

 

Maybe its a rich backfire. That's what my 600 double pumper had. I reduced the squirters to 21s and used the reduced pump cam setting, and that helped.

I confirmed that I had a rich backfire, associated with the squirters, on dyno runs that had A/F readout.

The dyno also shows the loss of power associated with the over-rich condition and the engine's recovery as the fuel burns out.

 

You have not made any mention of what the plugs look like. Without a AFR gauge this is the only other way you can confirm your lean theory. Show us your plugs.





This is a picture of mine. During WOT this reads as a AFR ~11.5 - 12.5. Idle is ~ 14. Highway running ~ 14.

I am running @ 2200 ft.

How does it run accelerating from about 1500-4000 in 4th ?
I would be looking at slow acceleration with-in this range.


Also any chance of plug wire cross fire or any possiblity of a wrong plug wire order ?

 

A quick check for timing - pull the vacuum line and plug it. drive the car if it is better, reduce the timing.

 

These aren't great pictures, but there isn't much to see anyway. As I was getting this car ready to bring back to life, I changed the plugs, and the new ones only have less than 35 miles on them. I've double and triple checked the firing order, because, as you know, it's easy to get wires mixed up going through all of the factory shielding, but they are right. They only thing I can't attest to is whether one or more wires are grounding out before jumping the plug gap. I guess that's my next thing to check.




I even started thinking that maybe the outer ring of the harmonic balancer had slipped; so I tried giving it 10 degrees more or less advance, but neither improved the performance.

 

Okay, so I just checked all eight plugs and confirmed that they are getting spark, but I did notice quite a bit of variation between different plugs. A couple put off a decent-looking white spark while the others all had a relatively weak-looking yellow spark. I'm starting to think that maybe the coil is the problem.

 

Dang...I went and bought a new coil before it dawned on me that my distributor has an electronic module instead of the original points. I still have my ignition wired as stock with the ballast resistor and the red wire to the controller attached to the + side of the coil; so I'm not getting full power to the ignition controller. I'm pretty sure I either need to extend the red controller wire back to the switch side of the ballast resistor or just bypass the ballast resistor altogether. Does anyone here have experience with this and know if I'm on the right track?

Update...I wired the red lead into the switch side of the resistor, but that made no difference on the sputtering. When I measure the total resistance across the BR and the primary side of the coil, I get 3.6 ohms, of which 1.5 ohms is internal to the coil. My next step will be to pull the ballast resistor out of the circuit altogether, which I believe should get me closer to the full 42K volt potential of the new 12V coil. If that doesn't give me a better spark and at least help with the sputtering, I don't know what else to do.

 

What kind of vacuum do you have @ idle, and what happens if you unplug the line from the brake booster. Stalls or RPM increase ?

Does the condition change if you remove 1 plug wire or does it seem to run the same ?

When was the last time it worked correctly?
What mods have been done since ?
How smooth is the centrifugal advance curve ?

Also if you get nowhere with electrical / ignition changes I would next eliminate valve / head issues by doing a compression test of each cylinder. This would make sure each cylinder seals properly during compression stroke. The only way then for a back fire is ignition timing and carb problems.

 

S#!^...I should have checked the compression first. The highest was 100 and lowest (2) were 88; so this thing is flat worn out. Shooting some oil in the cylinders, they all came up between 10 and 20%, but still none higher than 110psi. I'd love to think that I could get away with just re-doing the heads, but since I got 20% improvement out of some with the oil, I believe that's telling me that the short block is worn out too.

Anyone disagree that it needs to come out?

 

I would then next check the timing chain to make sure it didn't jump. It could explain the low compression of every cylinder. If TDC of the piston isn't timed to the valve's, you will have a open valve during compression stroke. If that looks good then I would pull. But you might be surprised to find the timing chain is not aligned properly or jumped. The other thing you can do is set the engine for TDC # 1. Pump air into the spark plug hole and listen where the air is excaping. If its via the carb then the intake valve is not sealed. If via exhaust then exhaust valve not sealed. If in the oil pan then the rings are not sealed ( if loud hissing) this is where I expect the air to go.

Get yourself one of these to plug into the sparkplug hole and connect a standard air compressor to. Set for about 5 PSI.

 

You may be chasing multiple issues, and I would fix the ignition and fuel FIRST before pulling the engine.

plugs indicate lean, and you'll need to ensure you are wired properly for your ignition setup first. the carb stumble can be worked through using Lars' paper on tuning. make sure there are NO VACUUM LEAKS!! then go back and make sure there are NO VACUUM LEAKS again!!

If your compression is really that low, then Yeah, time for rings and check the cam/timing chain. BUT, are you SURE you did the compression test correctly? throttle wide open, plugs out of all cylinders, crank over long enough? I would also do a leakdown test and verify no leaking valves, headgaskets, cracks, etc. BEFORE pulling the motor and committing to a rebuild...

 

You can attach the feed to the yellow wire on the windshield wiper motor. There are numerous pictures on the site. The lower voltage can cause multiple problems.

I
If you have a HELI Distributor there is a coil and it does go bad - I had chased a problem on my 1980 truck - HELI's coil was weak... Heli needs 12+ volts...

 

Confirmed weeks ago that there are no vacuum leaks. I've been chasing carb & ignition for weeks now, which is why I was getting so frustrated that nothing I did was helping. I guess I was just hoping for a cheaper, easier answer. Gonna try the leak down test and probably pull the timing cover and check for a timing set slip on Saturday. If that's not the problem, I may go ahead and advance the cam by one tooth while I'm in there just to see if it will buy me enough extra compression to get it running okay in the short term. If that doesn't work, then I guess I know what I'll be doing Sunday...

BTW...I started looking at crate motors just to run some numbers, and I found that GM Performance discontinued the ZZ5. When I called the local dealer, he said they put out a stop-sell and recalled them because of over-rated horsepower. Apparently, they don't really put out the 400HP they claimed. No safety or mechanical issues, he said; just a liability issue. Interesting...I had read good reviews on these crate engines, but now I wonder what they were really putting out???

 

[QUOTE=ranger9812;1590001943]Confirmed weeks ago that there are no vacuum leaks. I've been chasing carb & ignition for weeks now, which is why I was getting so frustrated that nothing I di was helping. I guess I was just hoping for a cheaper, easier answer. Gonna try the leak down test and probably pull the timing cover and check for a timing set slip on Saturday. If that's not the problem, I may go ahead and advance the cam by one tooth while I'm in there just to see if it will buy me enough extra compression to get it running okay in the short term. If that doesn't work, then I guess I know what I'll be doing Sunday...

BTW...I started looking at crate motors just to run some numbers, and I found that GM Performance discontinued the ZZ5. When I called the local dealer, he said they put out a stop-sell and recalled them because of over-rated horsepower. Apparently, they don't really put out the 400HP they claimed. No safety or mechanical issues, he said; just a liability issue. Interesting...I had read good reviews on these crate engines, but now I wonder what they were really putting out???[/QUO

every GM crate I've seen tested put out more than advertised.

I would go with a GM 383 crate and have a great time.

Blue Print is also another great choice.

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