engine missing

[tripower]

After I changed my valve cover gaskets a few weeks back on my 348 3X2 I noticed I had a miss while driving. It is only noticeable when I am not accelerating and not decelerating. The car will lurch a bit. A noticeable miss and under hard acceleration I will get detonation. I checked all to the vacuum lines and everything looks good. I also noticed the diaphragm for the vacuum advance was making a lot of noise so I checked to see if it was good and it checked out fine. The passenger side rear carburettor stud is drilled and is the factory vacuum source for the vacuum advance. When I take a vacuum reading from here it is very radical bouncing back and forth on the vacuum gauge. This would account for the diaphragms noise I believe but, what would cause this? I checked the trimming and it is good. Also set the dwell. I have the 948 Distributor. I am wondering if the cam is going flat...:dunno Any suggestions???

[Ronnie Russell]

Dan, Wished I could help, but a high vacuum surge or miss can be caused by a myriad of things... Needle in the haystack. I would not guess flat cam because there would have to be noticeable increase in rocker arm noise on that particular lobe. Good luck, afraid it has to be " back to the basics".

[tripower]

Dan, Wished I could help, but a high vacuum surge or miss can be caused by a myriad of things... Needle in the haystack. I would not guess flat cam because there would have to be noticeable increase in rocker arm noise on that particular lobe. Good luck, afraid it has to be " back to the basics".

Ronnie Thanks...I will keep digging...:doh

[jester]

Did you do a compression test yet? :dunno

[fatride]

The vacuum needle bouncing around usually indicates trouble in the valve train area. Check for broken springs bad valve guides rockers etc. Detonation may not be associated with the miss. Also check the heat riser valve in the passenger exhaust manifold if you have one it could be rusted shut. I think it would cause a faulty vacuum reading and maybe detonation. Check exhaust for restictons. Make sure that the secondary carbs are closing all the way.

[tripower]

Did you do a compression test yet? :dunno

Fred, Haven't done that yet...

[tripower]

The vacuum needle bouncing around usually indicates trouble in the valve train area. Check for broken springs bad valve guides rockers etc. Detonation may not be associated with the miss. Also check the heat riser valve in the passenger exhaust manifold if you have one it could be rusted shut. I think it would cause a faulty vacuum reading and maybe detonation. Check exhaust for restictons. Make sure that the secondary carbs are closing all the way.

Ray, Secondairy carbs are closing...Will check the other seggestions and get back...Thanks

[CDNpontiac409guy]

Dan, to me... it sounds like an over-advanced condition while at part throttle.
Need to be absolutely positive about total advance ( vacuum disconnected and plugged ). With all in by no higher than 2800 ( 2500 preferred ), you cannot have any more than 34 degrees... ever.
Now, connect the vacuum advance, and take note of how much advance it puts in.

To check if it IS over advancing at part throttle, disconnect the vacuum advance, and go for a normal drive. I'm suspecting that your miss will be gone.

very basic stuff...
just some thoughts:dunno

oh.... when checking vacuum at that one rear port...
ABSOLUTELY... you will get a bouncing needle. It's practically isolated to one cylinder.

[CDNpontiac409guy]

Sorry Dan... completely forgot...
You say that you're getting a miss and pinging under hard acceleration ?
SURE sounds like too much timing ( could be caused by too weak spring in the vacuum advance unit... allowing additional advance at lower than "should be" vacuum conditions ). Also, it's a classic symptom of a lean condition.

[tripower]

Dan, to me... it sounds like an over-advanced condition while at part throttle.
Need to be absolutely positive about total advance ( vacuum disconnected and plugged ). With all in by no higher than 2800 ( 2500 preferred ), you cannot have any more than 34 degrees... ever.
Now, connect the vacuum advance, and take note of how much advance it puts in.

To check if it IS over advancing at part throttle, disconnect the vacuum advance, and go for a normal drive. I'm suspecting that your miss will be gone.

very basic stuff...
just some thoughts:dunno

oh.... when checking vacuum at that one rear port...
ABSOLUTELY... you will get a bouncing needle. It's practically isolated to one cylinder.

Aubrey, One of the first things I did was disconnect and plug the vacuum advance and took it for a drive. It still had the surge or miss. When I checked the timing I set it at 12 degrees with vacuum disconnected and plugged. Then with the vacuum line connected the timing was 36 total.

[skipxt4]

Dan: I know this may be too basic but, you said it happened after you changed your valve cover gaskets. You think maybe, you have a bad plug wire? Skip:)

[CDNpontiac409guy]

Still haven't narrowed it down, Dan.
12 initial ?
That's lots.
If at an idle, you're connecting the vacuum advance, and getting 36...
WHOA !!!
too much !
Need to set TOTAL advance... initial and mechanical only ( ALWAYS vacuum disconnect ).

Could you get back with this ?

I still think it's a timing / lean issue:dunno

[rstreet]

Dan:
Just to add to the thinking I also am running a 948 distributor and had advance and total timing issues. One big problem I had was I had failed to check total timing after adding a new vacuum advance device. I had to restrict the total vacuum advance then recheck total advance then success! An old Sun distributor machine sure helps out!
Robert

[tripower]

Dan: I know this may be too basic but, you said it happened after you changed your valve cover gaskets. You think maybe, you have a bad plug wire? Skip:)

Skip, I did think thats what it was at first but, they check out fine.

[tripower]

Dan:
Just to add to the thinking I also am running a 948 distributor and had advance and total timing issues. One big problem I had was I had failed to check total timing after adding a new vacuum advance device. I had to restrict the total vacuum advance then recheck total advance then success! An old Sun distributor machine sure helps out!
Robert

Robert, One of my buddies just got rid of one of those Sun Machines. Wish he still had it. I think that I am getting to much total advance. Thanks for the input.

[tripower]

Still haven't narrowed it down, Dan.
12 initial ?
That's lots.
If at an idle, you're connecting the vacuum advance, and getting 36...
WHOA !!!
too much !
Need to set TOTAL advance... initial and mechanical only ( ALWAYS vacuum disconnect ).

Could you get back with this ?

I still think it's a timing / lean issue:dunno

Aubrey, 36 total at about 2500- 2800 rpm with vacuum advance connected. Guess I need to check that again.

[models916]

Timing tape is $6.00. Stick it on and check the total.

[fatride]

Aubrey, 36 total at about 2500- 2800 rpm with vacuum advance connected. Guess I need to check that again.

With the vacuum advance connected with the engine at a steady 2500/2800 you should be advanced much more than 36 dgs. I would expect to see 36 dgs with out the vacuum unit hooked up. You do seem to have issues with the timing. In my experience you would not be able to have an initial or static advance of 12 dgs with a stock 948 dist. With that much initial advance you would be over advanced at peak rpm.

[jester]

Did you do a compression test yet? :dunno
I repeat, Did you do a compression test yet?