engine knock and other issues - Pontiac GTO Forum
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post #1 of 11 (permalink) Old 03-26-2019, 05:35 PM Thread Starter
 
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engine knock and other issues

Ok so lets start from the beginning.

I think i'm over fueled after sending my holley 750 off to davinci carbs in sealy, TX. that's thing one. the 750 is on a my 350, so a bit big, but probably correctible with some smaller jets. That's not my concern...

I'm somewhat worried as the engine has started "breathing" a lot. Not such an issue with vacuum, but at idle the breather cap is smoking. Again...probably overfuelled.

But what REALLY is troublesome is that now, when at full throttle, I'm getting a nasty knock/ping, and have started blowing black smoke. Is this also a symptom of being overly rich?

last question is if anyone could recommend the holley jet size for this setup (750 cfm on a 350)

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post #2 of 11 (permalink) Old 03-26-2019, 06:20 PM
 
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Ok so lets start from the beginning.

I think i'm over fueled after sending my holley 750 off to davinci carbs in sealy, TX. that's thing one. the 750 is on a my 350, so a bit big, but probably correctible with some smaller jets. That's not my concern...

I'm somewhat worried as the engine has started "breathing" a lot. Not such an issue with vacuum, but at idle the breather cap is smoking. Again...probably overfuelled.

But what REALLY is troublesome is that now, when at full throttle, I'm getting a nasty knock/ping, and have started blowing black smoke. Is this also a symptom of being overly rich?

last question is if anyone could recommend the holley jet size for this setup (750 cfm on a 350)
You give us very little information to go on, so it could be a ton of things gone wrong, adjustable/fixable or not.

Stock 350? Rebuilt 350? Built up 350? If built up, what has been done? Compression ratio? What octane do you use? Timing specs - initial, total, what RPM total comes in at? Fresh tune-up? What type spark plugs? Points or Electronic? PVC system working? Engine Vacuum? Trans type? Rear end gears? Exhaust system? Engine temps?

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post #3 of 11 (permalink) Old 03-27-2019, 07:42 AM
 
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Pontiac Jim is right more information is needed. A smoking breather at idle means way too much pressure in the crankcase,....we were just talking about PCV systems on another thread.....the engine must breath, inhale and exhale. So that must be checked.

Knocking and pinging means the cylinder is firing from compression and not properly timed spark. An improperly timed spark can also cause black smoke out the tailpipe as the mixture is not completely burned and is full of fuel as it enters the exhaust. But Overly rich mixture can do that as well. You are probably really dumping fuel in that 350 at full throttle,...fuel comes then not only from the primary jets, but the power valve and PVCR, and the secondaries and even initially from the accelerator pump.

Make sure your cooling system is correct as well, as the temps may be too hot, at idle and even at full throttle.flowing carbon in the cylinder from over heating can fire the spark. It could also be mechanical and PJ asked about the engine specs. Low octane fuel in a high compression engine will make it knock. The right octane and turning back the timing may help that.

Timing, fuel, cooling system and PCV systems must all be right. Tackle each one, get it to idle right and drive easy first. Before you tackle full throttle issues.

You will get it!!
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post #4 of 11 (permalink) Old 03-27-2019, 12:37 PM Thread Starter
 
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ok so i should clarify that everything is stock apart from the intake (edlebrock performer) and carb (holley 750, built by davinci). It is definitely a bit on the rich side.

so without further ado: Stock 350, mildly low compression on cylinders 7 & 8. pontiac automatic (2 sp), 3.23 diff, running marine fuel (full oil, 93 octane), petronix ignitor 3 with recommended coil and gapped plugs, 172 degree thermostat, PVC relatively recently replaced.

Cant speak to RPM, no tach. Same for temps, only a warning light.
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post #5 of 11 (permalink) Old 03-27-2019, 03:02 PM
 
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Make sure you have vac to the PCV....what is your idle vac also? Make sure you have a functioning fresh air source for the crankcase.

When you idle and up to about 2590 RPM you run completely on the idle circuit, no primaries

That is the mixture screws. 2 or 4 and the throttle screw that sets the rpm by exposing the transfer slot.

Also make sure you choke is set right and first deal with idle settings when warm is you can get it running. Choke is cold starts only.

Cooling system check temp with thermo gun ...
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post #6 of 11 (permalink) Old 03-27-2019, 03:11 PM
 
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If that is ok you need to check timing....base timing with vac to distributor plugged should be 8 to 12,... is so take it for a drive with leaving dist vac plugged and see if knocking is reduced..

It will be more sluggish but won’t hurt it....if the knock goes away....your timing may be too advanced from the dist weights.

Smoking from crankcase could be a lot of things so see if you stop that with your checking temps timing etc
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post #7 of 11 (permalink) Old 03-27-2019, 03:16 PM
 
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Make sure you verify the oil pressure....put a gauge on it if you have not already...
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post #8 of 11 (permalink) Old 03-27-2019, 03:18 PM
 
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Those radar temp guns are cheap....may help you see what is going on.....could be running hot...
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post #9 of 11 (permalink) Old 03-27-2019, 06:21 PM
 
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ok so i should clarify that everything is stock apart from the intake (edlebrock performer) and carb (holley 750, built by davinci). It is definitely a bit on the rich side.

so without further ado: Stock 350, mildly low compression on cylinders 7 & 8. pontiac automatic (2 sp), 3.23 diff, running marine fuel (full oil, 93 octane), petronix ignitor 3 with recommended coil and gapped plugs, 172 degree thermostat, PVC relatively recently replaced.

Cant speak to RPM, no tach. Same for temps, only a warning light.
OK, stock 350 2-Bbl engine. Factory 9.2 compression/265 HP on a good day. First, the Holley 750 carb is too big. So it is going to run rich and you are probably giving up throttle response. Yep, you can jet it down, but still too big on a stock engine. The engine also does not have the cam to take advantage of the bigger carb. A 600 CFM would have been a much better selection.

Mildly low compression on 7 & 8 is definitely not a plus and you are probably pushing some oil into the cylinders. I'll also bet the valve stem seals are shot and your vavle guides could be worn, both can allow oil to enter the cylinder. If after the engine is shut off and sits for a bit and you fire it up and you experience that puff or blue smoke out the back - seals/valve guides are in need of attention.

Hopefully, the timing chain & gears have been changed out. The nylon cam gear is notorious for chipping and breaking off teeth and the chain will get sloppy. Once you start pushing past about 70,000 miles, it should be changed out. If it was a GTO w/400, I would not go 60,000 without changing it as they wear faster due to higher valve spring rates and most people beating on the cars. That said, if the chain & gears are sloppy, it will affect your timing accuracy and trying to adjust your initial and total timing via the distributor will be almost impossible as it will move around on you.

2-speed trans is not ideal. Low gear has a 1.76 ratio as opposed to a 3-speed automatic having a 2.52 1st gear ratio for much better acceleration. Takes more HP/Torque to move the car with a 2-speed on take-off and through the entire Lo range - so this puts a load on the engine and over carburetion is a bad thing.

So, in my opinion, your selection of add-on parts are incorrect for your engine. Aggravating the situation is a worn engine. The 350CI is a fine engine and has build-up potential, but adding components to a stock low-performance engine has to be done carefully and with matching parts. Many will go too big, which in my opinion has been done here, rather than add parts that are better suited and often not what you want.

If you have to use the Edlebrock Performer & Holly combo, go with a smaller carb like a 600CFM/vacuum secondaries and dial it in. Check the chain/timing gears for excessive play and replace if needed. Dual exhaust is needed to improve exhaust flow/breathing. Dial in your timing to maximize engine HP/Torque.

Without a cam change, you are not going to get big HP/TQ results over a stock 350CI by going to a bigger carb. The aluminum intake saves weight, but is no better than the stock intake so no real gains there. You might be adding 15-20 HP at most using a carb matched to the engine.

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post #10 of 11 (permalink) Old 03-28-2019, 08:09 PM Thread Starter
 
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Thanks Jim,

i knew i was a bit oversized with the 750 but in the grand scheme of things i've been planning a stroker kit for this for a while now and maybe it's just time to bite the bullet and make the leap. once piece at a time. engine rebuild just happens to be a big piece.
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