hi perf engine runs bad. how to diagnose?

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Jun 25, 2009
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Chicago, IL
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.
 
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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?
 
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.
 
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.
 
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!
 
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.
 
Stick a breaker bar on the crankshaft bolt and rock it back and forth while watching the distributor rotor.
 
...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?
 
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.
 
If any valve on the #1 cylinder was not closed at TDC on the compression stroke, that cylinder would have no compression.
 
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.
 
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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.
 
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.
 
When you say you verified TDC--did you pull a plug and verify that the piston was actually TDC? It sounds like it but figured it couldn't hurt to ask, not sure if it's possible for the balancer (or timing marks) to have wandered with time.

The rotor, does it have play? Just to rule out wear in the distributor gear.

Compression sounds good but any chance of wiped lobes on the camshaft? I'd think nitrous taking out the bottom end first but who knows.

In some ways I'd be wanting to pull the engine and go through it--if I suspected someone beat on it I'd probably want to pull the bearings and check things out. No sense in wasting a bunch of time on diagnosis when there is a good chance it needs to be gone through thoroughly. Kinda extreme I know but it' sure beat scattering parts.
 
Originally Posted by ms21043
If any valve on the #1 cylinder was not closed at TDC on the compression stroke, that cylinder would have no compression.


Serves me right for posting with 2 hours of sleep and no coffee
lol.gif


I was trying to suggest checking the valve timing approaching, at, and then just after TDC on #1. If it has jumped time, it should be noticeably off relative to zero on the balancer (assuming the balancer hasn't slipped).
 
Originally Posted by supton
When you say you verified TDC--did you pull a plug and verify that the piston was actually TDC? It sounds like it but figured it couldn't hurt to ask, not sure if it's possible for the balancer (or timing marks) to have wandered with time.


Yes, another good point, if this has a stock Ford balancer with the rubber ring, they can slip. Using a piston stop to confirm 0 on the balancer should be one of the first steps.
 
I checked the piston position on cylinder one against the timing mark. as far as "beating on it": I was not in the car but heard that he did 7k power shifts and also, at 35 mph, dropping it into 1st and stomped on it - breaking the tires loose. SMH. I previously had the rev limiter set to 6500 - and that was turned back to open. I also found a power steering line blown off at a factory crimped connection, and the front seal blown out of the PS pump - which leads me to believe that it was over-revved. Nitrous was never filled, or fully functional. and, after this episode, will not be on the car. I was against it the first 20 times it was brought up, but my wife said I should allow it as a "learning experience". !?!?!?!? I dont see her out in the garage diagnosing, fixing, or paying for this.

as far as "next steps" - I kinda agree - and don't have a problem disassembling the engine. I would just hate to tear it all down and not find the real problem. strong possibilities include: cam timing issue, carburetor issue, distributor, vacuum leak. I also had a friend suggest a twisted crank. really? The engine has less than 2000 miles on it. top quality parts were used throughout. until I find the problem I wont know for sure, but have the theory that this was caused by abuse. A certain person will not be driving the car once its fixed.
 
Try setting initial timing by highest manifold vacuum instead of timing marks. If that improves things I would suspect damper.
 
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