�65 396 timing/running problem

[Clay Thomas]

[Duke W.]

[Mike M.]

[Clem Z.]

it is easier to use a spark plug hole piston stop

to find TDC than to take off a head. GM sells one part#12364087

[Michael H.]

[Everett Ogilvie]

[John H.]

[Wayne M.]

If Clay's got a List 3124, the vac advance is ...

...not ported. (Don't think GM went this way until emissions '68-69 ?). To verify, squirt some carb cleaner in the vac. advance port on the metering block (#4456) and the juice exits on the base plate slot that joins the front Phillips screw, and not in the primart throttle bores, either above or below the butterflies.

[Everett Ogilvie]

Re: If Clay's got a List 3124, the vac advance is

I believe the 3124 is ported. A friend and I rebuilt two of them last year and my recollection is that we specifically checked for this and they were ported (I will double check this with my friend because the memory is going...). Additionally, so are 3247's on 66 L72s, so Holley was doing this before '68-'69.

[Duke W.]

Re: If Clay's got a List 3124, the vac advance is

It's a lot easier to just pull the signal line off the vacuum can. If the idle speed/characteristics don't change it's probably a ported vacuum advance.

You can also check vacuum at the signal line and compare to manifold vacuum from a known full vacuum source. That will tell you for sure!

All L-72/71 have ported vacuum advance. There are so few L-78s around I've never gotten a definitive answer.

Duke

[Bill W.]

[Everett Ogilvie]

Definitive Answer - I Stand Corrected...

Wayne, you are correct and I have to apologize for my error. I had my friend check one of the 3124's that is still sitting on his bench, and it is full, not ported vacuum (through the base plate, as opposed to a ported hole above the throttle plates in the main body). I could have sworn... I guess my memory is now fully, officially gone.

[Wayne M.]

Ported Spark Advance started in 1968, per ...

... the Sept/Oct issue of Chevrolet Service News. It was part of exhaust emissions regs, and consisted of 1. Controlled Combustion System and 2. Air Injection Reactor System. Seems the only 1968 engine that didn't use the ported system was the 307 cu.in. version.

Dug this up last night, and was going to post.

Regards, Wayne

[Everett Ogilvie]

Re: Ported Spark Advance started in 1968, per ...

That may be the date of an integrated Chevrolet system (with AIR components etc.), however Holley calls their part of the system "timed spark" and the 3247 in '66 definitely has the ported vacuum source. I own a very early 3247 dated 585 and it has ported vacuum as have all the numerous 3247's I have owned, dated over the entire production year. Sounds like Holley utlized this on some of their carbs before Chevrolet integrated it into a complete emissions system.