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hi perf engine runs bad. how to diagnose? #5461809 06/24/20 09:08 PM
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tomcat27 Offline OP
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we built this a few years ago - Dart 351 block with Eagle stroker kit. ran strong and good. until now. I strongly suspect one of the kids trashed on it; and he also replaced the ignition box with a different unit, anticipating to add nitrous (against my wishes). I know its hard to diagnose without seeing; but how do I proceed?
it misses, stumbles, cant hold a steady idle, and has only middling power. compression is good - 180. the timing makes NO sense: when we built the engine, it had 35 degrees total advance. I dont know the initial timing; likely around 20? right now it needs 50 degrees of initial timing to run somewhat smoothly. Yes, that makes NO sense. we checked that TDC is correct and aligns with the timing marks. I've swapped the original MSD box in there and it still runs poorly. replaced. I don't know what to check next - or why the timing is so wacky. vacuum seems to go up when you accelerate. I know. makes no sense. my buddies in our "garage club" are puzzled.

Last edited by tomcat27; 06/24/20 09:10 PM.

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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461819 06/24/20 09:18 PM
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The cranking compression is high....Especially with any kind of performance grind which I assume it has? I would consider checking Cam/Valve Timing.....Maybe it sheared the dowel pin on the camshaft?


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Re: hi perf engine runs bad. how to diagnose? [Re: clinebarger] #5461824 06/24/20 09:22 PM
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tomcat27 Offline OP
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pull the timing cover? it's a mid-performance grind. 12:1 compression; or maybe 11.5:1


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461839 06/24/20 09:40 PM
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Astro14 Online Content
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If it takes 50 degrees of timing to run right, then I suspect that the chain itself has jumped a tooth, or two.

So, the mark for TDC is fine, but the cam is nowhere near where it should be when the crank was at TDC. Cam timing off by 15 degrees explains everything: weird timing to get it to run, high compression, poor idle, lousy power, and vacuum.


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461843 06/24/20 09:42 PM
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Yep, pull the cover.

This happened to me on a 302 Ford many years ago. Not a high performance car, but many of those symptoms, and the chain (with the nylon silencer on the gears) had jumped a tooth on the cam gear.

New timing set. Ran like a dream.


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461860 06/24/20 10:13 PM
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Pull the dist. cap??

Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461915 06/24/20 11:39 PM
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You only have :
1) fuel/air mixture correct (including bad gas)
2) ignition at the right time
3) valves open & close when they should

Let us know what you find!


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461938 06/25/20 12:07 AM
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Is the 180 compression on all cylinders, or did you just check one? I agree with pulling the timing cover. I might pull the valve covers first and make sure all the rocker arms are in place and moving.


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461978 06/25/20 03:32 AM
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Stick a breaker bar on the crankshaft bolt and rock it back and forth while watching the distributor rotor.


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5461988 06/25/20 04:41 AM
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ms21043 Offline
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...also while you're rotating the crankshaft back and forth with the breaker bar, is there any unusual feel or movement of the crank damper & pulley assembly?


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Re: hi perf engine runs bad. how to diagnose? [Re: Astro14] #5461994 06/25/20 04:56 AM
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Originally Posted by Astro14
If it takes 50 degrees of timing to run right, then I suspect that the chain itself has jumped a tooth, or two.

So, the mark for TDC is fine, but the cam is nowhere near where it should be when the crank was at TDC. Cam timing off by 15 degrees explains everything: weird timing to get it to run, high compression, poor idle, lousy power, and vacuum.



Yup, this.

If it's easier to pull the passenger side valve cover, you should be able to check that both valves are closed when it's on TDC on #1 on the compression stroke. If they aren't, then you know valve timing is way off.


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5462002 06/25/20 05:31 AM
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If any valve on the #1 cylinder was not closed at TDC on the compression stroke, that cylinder would have no compression.


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5462004 06/25/20 05:42 AM
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I've never seen cam timing jump towards advance in a running engine. I suppose it's possible. When cam timing does jump, it retards. If the engine is still able to run, symptoms include low compression on all cylinders and low vacuum.

Last edited by ms21043; 06/25/20 05:45 AM.

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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5462009 06/25/20 05:49 AM
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Trav Offline
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What distributor is this engine running, is it running a vacuum advance? For the vacuum to increase as rpm's rise you are taking a reading from ported vacuum.
It certainly could be some sort of of cam timing issue and I would almost bet it is but check the obvious first like the distributor controls eg connecting to manifold vacuum instead of ported, etc.
What cam is in this engine, compression is high for anything much more than stock.


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Re: hi perf engine runs bad. how to diagnose? [Re: tomcat27] #5462047 06/25/20 06:44 AM
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tomcat27 Offline OP
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No vacuum advance in the MSD distributor. Cam is custom 226'duration intake 230 exhaust 110 separation with 565 lift. Compression was 180 in 5 cylinders, 160 in one, 170 on another. The two lower cylinders are not next to each other. I couldn't get the 8th.


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