won't stay running during break in - Pontiac GTO Forum
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post #1 of 10 (permalink) Old 06-26-2016, 11:30 PM Thread Starter
 
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won't stay running during break in

This is for the engine builder/tuning experts only please. First some history and crucial data. The block is a 74 400, just rebuilt by a good local speed shop. Stroked it to 461 and bored .035. Heads are 1968 #1 6 with bowl work to take them from 72cc to 79cc. Heads also ported. Pistons were selected to give me 10:1 CR. Carb is an Edelbrock 750 that was on the old motor and ran fine. Dist, coil, plug wires, etc are all new "ready to run" (and not on a resistor wire). I am familiar with finding TDC, and setting up the dist on #1 . This was all double checked. FP is new and high volume. Cam is a Comp XE274. Rockers are Harland Sharp 1.6:1. I filled the crank with Brad Penn break in oil and primed the oil pump, and had plenty of pressure on the mech guage. So here's the issue... I went to break in the cam this weekend, and it wouldn't fire up on the first try. So, I gave it a little more timing and it fired right up. I took her up to 2500 rpm and held it there. Eight minutes into the break in it stumbled to a halt. The oil press and temp were good. I restarted it and it only ran 3 or 4 minutes this time before it stumbled out. I played with the timing a bit (30 to 36 degrees @ 3000 rpm), but no matter what, I can only get it to run 2 to 4 minutes at 2500 rpm or above. I only took it up to 3000 rpm, but when I try to drop her down to 2200 rpm it becomes erratic and dies. It feels like it would run fine at over 3000 rpm, but I won't try that until it finishes the break in. I thought that maybe my new FP was to blame, or maybe the fuel line I ran was too close to the exh manifold, so I rigged up a simple gravity fed tank directly to the carb. This had the exact same results, which tells me it's not the FP or the fuel vaporizing. So, if the carb ran great on the old motor, and I bypassed the FP, where do I look next? What would cause it to run at high rpm but stumble at anything under 2500? I already disconnected all the vacuum hoses and capped them off, so it's not a vacuum leak there. I also checked for vacuum leaks at the carb base and intake manifold by spraying, again, no leaks. I'm not a professional, but a pretty good shadetree mechanic, but this has me stumped. Could the module in the new dist be breaking down? Could the new coil deliver enough spark for higher rpm but not under 2500? Could the cam have failed in the first 8 minutes, and if it did would it run strong above 2500 rpm? BTW, even after it cooled off completely, I could not run it below 2500 rpm. BTW, because the old motor ran on 89 octane, I added octane boost to the tank to handle the 10:1 on the new motor. What the heck am I missing here? It seems like timing or a massive vac leak, but if I advance it or retard it anymore than I have, it won't start at all, and I've ruled out the vac leak. I'm looking for advice from people that have seen this before or know just what to look for, or how to troubleshoot. Please, no comments about a Holley carb, fuel injection, or a roller cam.

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post #2 of 10 (permalink) Old 06-27-2016, 07:12 PM
 
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Nino, you say you played with the timing, at 2500 rpm what is timing? And how are you getting there. Distributor Centrifigal is totally RPM dependent, so how that is set up matters. Are you getting to 36 @ 3000 by weights and Base? What about a Vacumn canister on distributor or other electronic means to advance the spark....

I would start at timing and then move on as it seems everything else is in play....
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post #3 of 10 (permalink) Old 06-27-2016, 09:42 PM Thread Starter
 
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Nino, you say you played with the timing, at 2500 rpm what is timing? And how are you getting there. Distributor Centrifigal is totally RPM dependent, so how that is set up matters. Are you getting to 36 @ 3000 by weights and Base? What about a Vacumn canister on distributor or other electronic means to advance the spark....

I would start at timing and then move on as it seems everything else is in play....

I have the timing set to 34 at 2800 rpm. No vac adv. No computer stuff, just base and mech adv. Then I tried a few degrees advanced and retarded. No change.
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post #4 of 10 (permalink) Old 06-28-2016, 07:05 AM
 
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Ok, some other possibilities include coil dropping out, swap with another and retry,..cross firing plugs, recheck firing order.....

also carb was fine on other engine, but this engine may demand more or less fuel, and you could be getting a lean or rich stall at the lower RPM's....

You may have to check there, .....also is it RPM's that causes drop out or just "time"... is battery, alt, electrical system dropping out?

Are engine grounds tight?....
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post #5 of 10 (permalink) Old 06-28-2016, 07:15 AM
 
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Also Nino do you have the correct PCV valve hooked up? Is the engine breathing?

Crankcase vapors can build up fast and choke out the engine, the PCV cleans those out through the intake and burns them off.....

3000 RPMs requires a more open throttle plate, while 2500 2200 is tighter....so more air at 3000 and it can burn....

Just a place to check...it is why EGR valves cause stalls as well,...exhaust gas dilution effects the mixture....
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post #6 of 10 (permalink) Old 06-28-2016, 08:01 PM Thread Starter
 
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Ok, some other possibilities include coil dropping out, swap with another and retry,..cross firing plugs, recheck firing order.....

also carb was fine on other engine, but this engine may demand more or less fuel, and you could be getting a lean or rich stall at the lower RPM's....

You may have to check there, .....also is it RPM's that causes drop out or just "time"... is battery, alt, electrical system dropping out?

Are engine grounds tight?....

You think coil could work at 2500 rpm but drop out below that? Hmm. I guess I can try the old one. I did double and triple check the firing order, it's all good.
RPM is definately causing the drop. I can run it over 2500, but not below, no matter how long it runs. This is definately related to the rpm.
Grounds are good. Batt is good

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post #7 of 10 (permalink) Old 06-28-2016, 08:03 PM Thread Starter
 
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Also Nino do you have the correct PCV valve hooked up? Is the engine breathing?

Crankcase vapors can build up fast and choke out the engine, the PCV cleans those out through the intake and burns them off.....

3000 RPMs requires a more open throttle plate, while 2500 2200 is tighter....so more air at 3000 and it can burn....

Just a place to check...it is why EGR valves cause stalls as well,...exhaust gas dilution effects the mixture....
PCV is new and hooked up.

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post #8 of 10 (permalink) Old 06-28-2016, 08:19 PM
 
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Good, think about what changes At 2500 RPM....Spark from Centrifigal advance; fuel air mixture from throttle plate position; any electrical demand can change.....Vacumn changes based on throttle plate position....

after that mechanical, Valve & cam ......the internal engine guys, much better than me, can advise you better on that....

But simpler things first.....rich or lean mixtures can stall an engine, and the fact that the carb ran ok on another engine is different. This stroker has different demands, Vacumn, throttle air speed etc,

so maybe you should start at the carb,...basic mixture screws and throttle plate stuff, a poorly set fast idle screw on choke side for example, ...can really screw up the mixture
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post #9 of 10 (permalink) Old 06-29-2016, 11:07 PM Thread Starter
 
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Solved! Turn out to be a defective brand new distibutor. Best I can figure is the advance wasn't wide enough, so when it was timed at 2800 rpm, it was way off at 1500 rpm. I put my old distibutor in and it's fixed.
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post #10 of 10 (permalink) Old 06-30-2016, 02:26 PM
 
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Super, now you can get her going! Good job staying with it!
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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