Thumpr & timing oddity...
 

So i'm running a thumpr cam in my 357 sbc dome pistons with an hei streetfire dizzy and after fighting to get it to run properly i just gave up on using a light. now i managed to get this thing to run beautifully by ear and test drives....i threw the light on it and i'm getting a reading of 2 degrees AFTER tdc and 8 btdc with advance hooked up and it runs great! now i've got people telling me i'm one tooth out on my dizzy but as far as i kno you cannot be out a tooth on HEI distributors, it would be one plug tower out but then it would run like absolute garbage which isn't the case. i'm wondering if my time light is hooped? (its my father's from the late 80's lol) i rather enjoy the truck running great and the power the cam gives me but i'm uneasy about the atdc timing...anyone run into this? :chevy:

Re: Thumpr & timing oddity...
 

It is very possible that your timing mark on the balancer is "off". You need to establish an absolute "zero" with a piston stop, especially before you make any other changes/modifications.

The whole thing about being "a tooth off" is not the case here. That only comes into play when you cannot turn the distributor enough to get the timing where it needs to be. The vacuum canister would bump into the back of the intake manifold, carburetor, firewall, etc...

Re: Thumpr & timing oddity...
 

as Longhair said, it sure sounds like the mark is off. possibilities are a slipped outer ring or the wrong damper.
is it a new damper?

Re: Thumpr & timing oddity...
 

Or you did not get the timing chain gear teeth lined up correctly. Common mistake to be a tooth or two off and never get the balancer to line up correctly.
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Re: Thumpr & timing oddity...
 

Yah its an older damper which I had set for true tdc at installation of cam so there is possibility of slippage I will have to do a follow up on that I'm kinda thinkin an 8" would be a good addition anyway. I will just drive it for the time being seeing as it runs and drives great finally...the thumpr cam is deffinitely a pain when it comes to ignition timing but a great cam once figured out. If its not the balancer I'm thinking my distributor is over advancing cuz at one point I had it at 13 btdc and no vac advance to get it to run well enough to drive and I lived with it till I realized I never tried below 8 degrees..
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Re: Thumpr & timing oddity...
 

Are you running on timed vacume or full vacume port? I had an issue with my truck before I swapped heads of it wanting to run full manifold vacume to dizzy to run right. Just some food for thought. Balancer is a good option too been there before
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Re: Thumpr & timing oddity...
 

That is completely not true. The timing chain controls the cam (valve timing).....even though the distributor is driven by the cam, it is installed seperately and timed independantly.
The distributor is installed based on the timing mark on the balancer (crank), so "theoretically", you could have the cam timing off and the ignition timing could be spot-on.
I'm not saying that this would be good thing, it wouldn't run well that that either...but, the two are completely independant.

Re: Thumpr & timing oddity...
 

:haha: is the timing light on #1 plug wire? :haha:

Couldn't help myself...;)

Regarding what shortbed70 mentioned about ported timing - never use it, ever.

Re: Thumpr & timing oddity...
 

You laugh, but on an 84 S10 2.8 #1 is NOT the same size as a SBC. :uhmk: And the truck will run (sort of, like on 3 1/2 holes)) timed to the wrong cylinder.

Remember #1 is the most forward cylinder not the left front!

Re: Thumpr & timing oddity...
 

So your saying full vacume all the time? That's what I usually run anyway.
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Re: Thumpr & timing oddity...
 

Dizzy timing is something of an art. Especially with cams with alot of overlap.
It all depends on the engine and how good it runs at idle and across the rpms.

Some engines run much better at idle with a boat load of advance. This helps it burn cleaner AND cooler.
Lets say something like 26 initial degrees makes it idle just like you want (some can go as far as 50!).
Now, you can't set the dizzy to 26 degrees initial timing because when it reaches its full advance (20 degrees more) at around say 4000rpm, you will be at 46 degrees of advance! Way too much.

So now you set your initial timing to 16 degrees with the added 20 degrees of mechanical and you have the magic number... 36 degrees total advance! But now it won't idle worth a crap.

Enter Mr. Vacuum advance. He can add 10 degrees of advance to the party. Hooked up to manifold vacuum it advances the timing another 10 degrees, but only at idle, cause when you hit the go pedal the vacuum advance disappears and the mecahnical takes over! And now it idles again because you have 26 degrees of advance at idle!

Moral of the story is each dizzy needs to be set up for the engine combination its going in. Manifold vacuum or port vacuum? # of degrees of mechanical advance? Initial timing to idle right? ..............

All the numbers above are variable except the 36 total degrees of advance WOT. That number doesn't change much.

So to just drop in a distributor out the box 9 times out of 10 doesn't have the right mix unless its bone stock and meant for that setup. That's usually why so many folks have problems with their carb and can't get it to run right. Its usually not the carb!

Anyway my $.02

Wow, did that make any sense?

Re: Thumpr & timing oddity...
 

I need some more info, how old is that dist ? Is it points, stock type HEI or aftermarket? Did you have it serviced or just throw it back in again? Your cam timing really has nothing to do with actual dist timing. If the engine was balanced with that damper you need to replace it with that same size damper. Could you give me the cam part # so I can see the recomended rpm range and such. Are you using stock type heads or Vortec, Vortec heads take way less timing than stock style 23 degree ones.

I have a rather large cam in my ride and it idles at 1k rpm. My initial timing is 14 degrees with a total of 35 at 2500 rpm. Timing lights need to be recalibrated every so often but if the one you have is not a dial back style go buy one.