Pro-Form HEI Misfire

[T-Boy's GW]

I have a new Pro-Form HEI Distributor on my 360, 1988 GW. Just got the rest of the ride back together and I’ve notice a misfire on cyl #4. I’ve swapped plugs, wires, compression test, leak-down test, vacuum test…everything passes. I’m either getting weak spark or intermittent spark, so the only thing I haven’t tested is the distributor.
Has anyone had this problem or knows how to trouble shoot the HEI distributor?

[Ryan]

while not the exact same hei, this should get you in the right ballpark:
http://www.ehow.com/how-does_4809570...stributor.html

[Stuka]

One way to rule out the HEI is to move every plug over one, and then readjust the timing to make up for the plugs being off. If the same cylinder is missing, then its not the ignition. If it moves down to the next cylinder, then it is the distributer.

[T-Boy's GW]

Ok...thanks! So, clock the plug wires ccw and rotate the dis cw

[T-Boy's GW]

Ok…I tested the distributor and the misfire stayed with #4. So, what next? I have noticed a vacuum pulse from 13-15 at idle in time with the #4 firing. Could this indicate a problem?

[Stuka]

Ok…I tested the distributor and the misfire stayed with #4. So, what next? I have noticed a vacuum pulse from 13-15 at idle in time with the #4 firing. Could this indicate a problem?

A vacuum pulse can be a bad thing. When you did the compression check, did all cylinders show the same? Was #4 any lower?

You could have a burned valve. It would explain the vacuum fluctuation, and the miss.

13-15 inches is pretty low unless you have an after market cam though. I take it the carb and timing is set dead on? When you put a timing light on, does it bounce around at all?

Also, how are you certain it is #4 missing?

[T-Boy's GW]

A vacuum pulse can be a bad thing. When you did the compression check, did all cylinders show the same? Was #4 any lower?

You could have a burned valve. It would explain the vacuum fluctuation, and the miss.

13-15 inches is pretty low unless you have an after market cam though. I take it the carb and timing is set dead on? When you put a timing light on, does it bounce around at all?

Also, how are you certain it is #4 missing?

The compression is good/same on all cyl. The valves train is new and stock...less then 2 hrs of run time. The timing is good but i'm not sure what dead on is with this HEI system.

I thinks it's a mechanical problem with the valve...
what should i look for?

[Stuka]

Did you replace the came and timing chain then too? If so, is it possible you got the time camming off?

[T-Boy's GW]

Did you replace the came and timing chain then too? If so, is it possible you got the time camming off?

New timing chain assy and cam. I’m sure the cam is timed good. Wouldn’t it cause more than one cyl to misfire?

Have you heard this one?
I did the "paper test" and it failed. paper test is: put a piece of paper by the exhaust pipe and it should flap away. if it gets sucked in, then there is an exhaust valve problem.

[Stuka]

Yup, I just forgot to mention it

So, it looks like it is a valve issue. The question is, what is causing it.

Its just odd that the compression test is fine.

How are you verifying that it is #4 that is missing? Pulling a plug and seeing if the idle changes? or?

[T-Boy's GW]

Yup, I just forgot to mention it

So, it looks like it is a valve issue. The question is, what is causing it.

Its just odd that the compression test is fine.

How are you verifying that it is #4 that is missing? Pulling a plug and seeing if the idle changes? or?

Yes, first thing i did was locate the misfire by pulling the plug wire. I was thinking that it's slow to close but I loosened the rocker and it smoothed out.
I'm not sure what that means?

[Stuka]

It makes me think the valve was not closing all the way. but then the compression check should have failed. Which is odd :/

[T-Boy's GW]

OK…back to work on my project! I have found that the compression is low in every cylinder and that the valves are staying open just a hair. #4 is noticeable because the valve stem is about .010 tall. The rocker arms, rods and lifters are all new and in factory spec. So, what is causing the prob and how should I fix it? A) shorter rods or B) shim under the rocker arms.

[Stuka]

Are the rockers factory also? If they are a different ratio, the rods need to be a different length. But as it is, it sounds like the rods are too long.

[T-Boy's GW]

Are the rockers factory also? If they are a different ratio, the rods need to be a different length. But as it is, it sounds like the rods are too long.

I've got rods ordered...60ths shorter (the next stock size)
Hopefully that works.