Hi Guys,

I have purchased and installed a new timing system on my Vette (77 L82, Manual).

I have tried all sorts of settings and just want your advise. It runs perfectly on the centrifugal advance, I currently have it set up to be at 16 initial, 36 all in at 2,750rpm. Throttle reponse is good and it revs cleanly through the range with no hesitation or pinging/knock/pre ignition. It will also rev up to the red line without any problems, which suggested the ignition system is working as it should?

The problem occurs when I connect the vacuum (connected to full manifold pressure). Revs rise as they should at idle once connected. Idle is good and it runs cleanly below 2,500 rpm on a constant throttle.

But if taken over 2,500 rpm (still in the garage with no load) and throttle held constant it misfires fairly heavily. I have tried a couple of vacuum cannisters which vary the amount of overall advance between 50 and 55 (I know we are looking not to exceed 52) but the misfire is still present. If I retard the ovrall timing with vacuum connected back to about 47 the misfire with no load seems to clear.

So 2 questions really:

1. Is the misfire present because I am testing it with no load? Should I run it up the road before going any further?

2. What loss will I make by restricting the amount of advance the vacuum produces (say back to 45)?

Heres to hoping that one of you tming guru's in the know will help me out on this one.

Thanks

Lester

 

Lester, you don't make any mention of the brand or type of ignition system you installed for folks to comment. Also did you check the axial shaft play before you installed the new distributor?

 

I have run itno this problem with a few cars in my shop from time to time. GM changed from ported vacuum to constant intake vacuum for the distributor vacuum pod. The compression of the engines were lowered due to EPA or who ever caused the major power drop in the engines from the earlier years. I always test my cars while driving, because I do not care what it does while stiing still in the shop. I want "real world" results. I have in some cases not attached the vacuum advance pod to vacuum because when the vette was on a chassis dyno the power numbers showed that it was the best for the circumstances that no vacuum went to the distributor. It was set up for centrifugal advance. About 36 degrees total advance.
Also make sure that all of your plug wires are routed away from heat and NOT "zip-tied" together. make sure that thay are seperated. Especially #5 and #7 cylinders. I know you said that you have a good idle but it is something I check...visually.
How many miles on your engine? Timing chain play? and your distributor shaft play like what was earlier mentioned by Scott. And what kind of timing system? Fuel quality, you can't have a rocket ship if you don't give it rocket fuel. Keep us informed.

 

Thanks for the replies guys.
I was going to see what it did up the road, but the misfire is quite heavy (the whole engine rocks when its present). I guess the vacuum prduced at say a constant 70mph will be less than the vacuum created at idle? In which case it will advance it less.
I thought that the fact it will pull the revs cleanly without the vacuum suggested that the leads/plugs would be ok. However I have replaced the leads and plugs as a matter of course.
The ignition system is a Summit racing system, it has a seperate control unit, new alloy distributor and seperate coil. I never checked the distributor shaft play as being new I figured it would be ok (probably a big mistake with most aftermarket stuff). I guess I could pull it out and check it, but I dont appear to have much spark scatter, probably less than 1 degree.
With regards to fuel, I am in the UK and we have a choice of 2 octane ratings, either 95 or 98. I always run it on 98 as with the old distributor it was less likely to give pre ignition.

Unless anyone else has any ideas then I guess I will run it up the road with it at 16 initial, 36 centrifugal all in, and 52 all in with vacuum and see if it still misfires with load present. I will also modify one of the vacuum cans to give me 45 total and see how it compares.

Again thanks for the replies, if any one can think of anything else let me know, any help/advise appreciated.

Lester