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I have a ’65 396-425 horse car with which I’m having timing and running problems. Initial timing advance is supposed to be 10 degrees BTDC without vacuum advance, but the car does not idle well and has significant throttle hesitation. The car does not begin to run better until at least 16-17 degrees initial advance and at that point still has some hesitation although idle has improved greatly. The car really runs great at 20 degrees + initial advance but16 degrees BTDC seems to be the most I can go before I begin to encounter detonation. I’m currently running premium and 104+ octane boost. I’ve checked everything I can think of short of putting a degree wheel on the engine and checking cam timing. I’ve checked valve lash and it is in specification. I’ve rebuilt the carburetor, added new plug wires and plugs, and changed the distributor cap and rotor. I’ve even checked the timing mark on the balancer to see if it has slipped (I did this by inserting a rod into the #1 spark plug hole while rotating the crank with a large ratchet and, although not very precise, it appeared that the timing mark was in fact, correct). At this point, I’m at a loss as to why it takes so much advance to make the car run right. Any help would be greatly appreciated.
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Re: ’65 396 timing/running problem
Have you checked the vacuum advance function against specs? Does it key off full manifold vacuum or is it ported?
What are the numbers on the vacuum can bracket?
Duke -
Re: ’65 396 timing/running problem
describe the method you used with the "rod inserted in # 1 plug hole". unless ya used a dial indicator, estimating TDC is just that, a rough estimate. you might want to pull the left head and set up a dial indicator to get qaccurate TDC then look at the timing tab to see if the outer sleeve of harmonic balancer has in fact slipped on inner portion of harmonic balancer, a common problem. mikeComment
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it is easier to use a spark plug hole piston stop
to find TDC than to take off a head. GM sells one part#12364087Comment
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Re: ’65 396 timing/running problem
Clay,
Is this an engine that ran properly and then suddenly started to run bad or has it ever run properly since a rebuild? If it ran well last month but poorly this month, that would tell us a lot about what to look for. It would eliminate a lot of guess work. There are several things that can cause this problem besides timing. Use the method that Clem described above to make sure the timing mark/dampner is accurate. A 396 should run/idle well with no vacuum advance at all so I wouldn't think that would be a possible cause for the problem that you described.
MichaelComment
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Re: ’65 396 timing/running problem
Agree with Michael - the 396 uses ported vacuum (little to no vacuum signal present at idle - dependent on where your idle screw is set), and these cars and L72's will idle fine without vacuum advance (for tuning purposes). In my experience, on all my cars that only timed well with "lots" of advance, I found the distributor was off a tooth or even two. This requires pulling it and re-indexing it with all the particulars set up properly and in factory positions; distributor body in the initial position the factory uses, distributor shaft indexed with cam properly, and the number one wire on the proper tower on the cap. I have seen more cars with the number one wire on the wrong tower and then compensation made by changing the rotor one or more teeth, and turning the distributor body outrageous distances to make up for it. Note that these cars can be made to run fine by playing "catch-up" with the wire position/rotor index/dist. body position, but if you only have one component in the "wrong" position, it usually requires lots of advance to make it run (i.e. if it idles and revs with 20 + degrees but won't with about 10 degrees, one of these set-up parameters is wrong).Comment
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Re: ’65 396 timing/running problem
If the timing index line on the balancer outer ring is DIRECTLY in line radially with the keyway in the hub, the balancer is OK (assuming it's not a post-'69 balancer); the original spec to shear the bond between the elastomer isolator and either the hub or the ring was over 1,000 ft-lbs, so that's unlikely.Comment
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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.Comment
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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.Comment
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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.
DukeComment
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Re: ’65 396 timing/running problem
Are you sure you have a chevy dist. cap ? not all delcos are created equal. If you bought a new cap make sure the # 1 tower is 2 over from the screw.could be a nos pont. ?Comment
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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.Comment
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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, WayneComment
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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.Comment
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