hi perf engine runs bad. how to diagnose?

[OVERKILL]

Originally Posted by tomcat27
I checked the piston position on cylinder one against the timing mark.


Pull #1 spark plug, rig up a piston stop and verify 0 on the balancer is actually zero. Start there.

 

[tiger862]

Originally Posted by tomcat27
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.

I would now have to remove whatever is needed for cylinder 8 to be tested. If over revved the valves on that cylinder could have floated and bent a pushrod or put a hole in piston.

 

[DuckRyder]

My thoughts:

Verify the distributor is advancing ( that is - that it isn't locked or a spring broken etc. ) MSD distributors come with heavy springs and have adjustable advance via replaceable springs and stops, they have 19 degrees advance and aren't all in until over 4000 from the factory IIRC. Ive seen the lighter springs rust and break in long term storage.

The cam card - get it out. Get out your cam degree kit verify the events. (you will verify the timing marks in the process)

What is the IVC at advertised on that cam, because the 050 events and 12:1 (or 11.5) static sounds like a recipe for disaster to me, but a few degrees could make a huge difference in dynamic.

If you don't find any issues there, you're gonna need to check compression on that last cylinder, look for broken valve springs and stuff like that... Is it a Hydralic Roller?

 

[Jimmy_Russells]

If you have gone over the timing/advance with a fine tooth comb, I would pull the valve covers, look for broken springs, bent pushrods, broken rockers etc. If that all checks out I would pull the timing cover and make sure that's all lined up. Third step I would check compression in the last cylinder.

 

[ms21043]

Longshot here but assuming you have a 351W, is the cam ground for the 351W? If it's a 302 cam (pre-302 H.O.) the firing order is not the same as 351W. If you're sure of the cam and firing order, disregard, if not you could try the 302 firing order. Easy too do and costs nothing..

 

[PontiacHO]

Floated the valves and bent the pushrods. Still has good compression but runs poorly.

 

[Silverado12]

Originally Posted by OVERKILL
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.



This was my guess too. I've had a balancer go bad in a Plymouth and a Mustang.

 

[deanm11]

I'm with the camp on valve cover removal and inspection - top end/push rods are a common over-rev casualty; fairly easy remove and inspect. Also, possible to have good compression and low power due to low lift.

 

[Trav]

If I had it here I would probably throw the original distributor and related pieces in it (if they were still available) just to eliminate or condemn the MSD system. You say he switched the box out but did anything else get changed or damaged? 30 min job so why not, if it is no different you have a better direction.

Edit: I remember doing a car years ago that someone had rebuilt the engine on, it would only run on 4cyl when cold then all 8 would kick in and run super until it was floored then 4 cylinders dropped off again. After hundreds at the dealer, the guy called it a day.
I Mickey Moused an old point dist in it just for the sake of diagnosing it, it ran like a champ. Turns out they had pinched 2 wires reinstalling the engine.

 

[CT8]

Reading all the above posts some cover the basics of diagnosing others wild guesses,, start with the easy stuff [ the distributor as Trav suggested] until you find a problem..

 

[stroked93]

The distributor in in a SBF has a roll pin through the gear. If that shears and spins it will retard the ignition timing forcing you to advance it further. Pull the distributor and look at that pin, its your culprit. The oil pump is driven off the same gear and can cause drag if it gets junk in it and it will shear the oil pump drive, or shear pin in the distributor.

 

[Trav]

AFAIK if the pin breaks its out she goes on the spot, no ignition at all and no oil pump being driven. I guess if one end of the roll pin broke and the rest of it remained jammed in there I suppose its possible but I never saw anything like that.
With some performance cams I have seen the dist gear teeth get pretty messy though.

 

[tomcat27]

I will start tearing into it again late Sunday and will certainly provide updates. it may be slow - mom has surgery this week. I will start with distributor, timing cover.

 

[OVERKILL]

Originally Posted by tomcat27
I will start tearing into it again late Sunday and will certainly provide updates. it may be slow - mom has surgery this week. I will start with distributor, timing cover.


Before tearing it down, verify zero on the balancer as I instructed. It doesn't require disassembly and is an easy check. Rubber ring SBF balancers are known to spin.

 

[OVERKILL]

Originally Posted by Trav
AFAIK if the pin breaks its out she goes on the spot, no ignition at all and no oil pump being driven. I guess if one end of the roll pin broke and the rest of it remained jammed in there I suppose its possible but I never saw anything like that.
With some performance cams I have seen the dist gear teeth get pretty messy though.


That's correct, if the pin breaks, the dizzy doesn't spin and the engine stops.

 

[zrxkawboy]

Any updates on this?

 

[another Todd]

I am in Astro14's camp on this one. He Stated it needs about 50 degrees (30 more than before) to run somewhat smooth. Flogging engines can be hard on timing chains and gears, I have seen several on the road thrashings result in a jumped tooth.