I recently rebuild my 355 with edelbrock heads, edelbrock air gap performer, 268h cam, demon 625, moroso HEI. My guesstimate is 10:1 CR I was using it without the advance plugged in, base timing at 14 BTDC. The specs of the distributor, the mechanical adv is 20, so should put it at 34 total (no I didn't check it with a light). Not too bad. I decided to connect the vacuum advance to the manifold vacuum port on the carb last night. I did that and adjusted idle where I wanted it at about 700 in park (500 in gear). Drove it, it drove great - running 91 octane. Its a little warmer today and this afternoon and I'm driving and I am cruising and when I get on the gas I hear a ping for a second and then its fine. I'm thinking the timing is too far advanced with the manifold vacuum plugged in. The vacuum advance should give it about 10-15 degrees when connected, causing a leaner mixture at idle and cruise untill you move the throttle plates and mechanical comes in to play. The engine isn't running any hotter than on a cool day). It has an adjustable vacuum canister on the distributor. I'm almost thinking of disconnecting it again and not using any vacuum advance as I believe 14 is not too much initial and the performance is great and no ping (except this). Any insight or ideas?
You are getting too much lead too quick. Try the ported vacuum or make adjustments to the vac canister on the distributer.
That's why they have an adjustable vacuum advance canister. Work with it until the ping disappears. I would also stick a light on it and see where you really are at idle and again around 2500. SPark
Yeah Ill do that. It appears that turning it out would lower the amount of vacuum advance right? And 16-18 base seems to be the sweet spot for this cam from my research.
this is just my opinion, 14 degrees base timing may be a little much, try 8 to 10, you most likely will not need to do anything else. with the vacuum advance on manifold vacuum your engine will run cooler at idle and cruising speeds
When you step on the gas, vacuum drops and vacuum advance does not advance spark. Vacuum advance improves fuel mileage when throttle is steady (vacuum is high). Phil
Thank you,put a timing light on it please your engine is not the same or gearing or trans as what you read,
Right, that's when it pulls in the maximum amount of advance. But if you watch a vacuum gauge while driving around, you can see how fast manifold vacuum comes and goes, it swings widely back and forth in a split second even when accelerating hard. So the vacuum advance or breaker plate is constantly moving, and it's adding or removing quite a bit of timing on a constant basis when driving, it isn't simply for steady highway cruising.
Its a 1970 Chevy C10 with a turbo 350, 3:07 rear end. I pulled the vacuum advance off last night and set timing to a total of 34, that set base to 16. I adjusted idle and let it go for now. It was hot yesterday and so was the engine. Ill get out on another night and mess around with the vacuum advance with a light on it. I remember on other GM cars, you would set base at 6, then put vacuum on and it would jump to 20.
I had a similar problem with my SBC pinging at tip in when on steady cruise. Total advance was about 34 degrees plus whatever is in the can. Although I am a firm believer in full manifold vacuum- I ended up switching it over to ported vacuum to cure the problem on the highway.
Seems unusual that your distributor has only 18 degrees of mechanical (centrifugal) advance. I would like to suggest you recheck your total advance, (with vac advance disconnected and plugged), if your initial advance is 16 degrees. Phil
I'm sorta wondering aout a couple of things: #1 You say dist.has 18* of mech. advance, but is this dist degrees or crankshaft degrees? #2 You don't mention he rpm at which you reach full mech. advance.
pardon my butting in, the total timing is not important on a 350 in a pickup truck. that is for an engine that sees wide open throttle a majority of the time. base timing set that high will be hard on your starter and ring gear, along with being harder to crank when the engine has a hot soak condition. 6 to 10 degrees is plenty of base timing
How else does the curve get set correctly, every OEM in the last 70 years seemed to think it was important.
If you'll bother to read the first post by the OP, you'll see that even though this engine is in a pickup truck ( what difference does that make anyhow?), it is a pretty stout engine with a good many modifications, and total advance makes a helluva lot of difference, along with initial advance, rate of advance, accuracy of timing marks, etc.
My goodness , you've revolutionized the automotive world ! Nevermind , you're wrong ! Please read what Dirty Old Man says and then do some research , hopefully you'll see the errors in you're thinking .!
As others said, adjust the can. I wild (perhaps dumb) idea I had was a restriction in the hose to slow it down at tip in
I'm measuring timing at the crank with my adjustable snap on timing light. Setting to 34 (with a 16 base) it runs really good. No hard to start after running, no pinging on WOT accell on hills. As for RPM when measuring total advance, I'm disconnecting and plugging the vacuum source and running it up until the timing stops advancing. Seems to be about 3500 RPM but don't have a tach when I can see it while doing it. Everything Ive read on the 268h cam says they like 16-18 base then adjust from there. I have a couple MSD billet distributors as well, I thought about throwing one in, but the moroso HEI, if it fails and I'm out and about on a trip or whatnot, I can get parts at the store. Ill mess with the can, but its running great now. No bogs, no hesitations. I set the secondary air valve spring at 1/2 turn past factory. But I gotta add that when I pull and put my thumb over the PCV valve, it idles much more solid. I have moroso valve covers with no baffles (only the rubber ones with the slits cut in them) and a breather on the opposite valve cover. I don't know if this factors in at all...
10:1 CR..............224* intake 230*exhaust @050 demon carb etc. and your worried about gas mileage?! Come-on! Plug the vacuum advance........put a timing tape on the balancer ....borrow a tach from somebody(with that kind of $$$ in the engine I would recommend a TACH!).....see what your total advance is and what RPM its all in. I thought (here-we-go)...the distrib. had 24 degrees built in it.(Fords do) Add 12 initial you got'cha self 36*. With those heads 34-36* should give max power. Do you have the support hanger on the back of your starter? You may need one with 34-36*. I would get everything sorted out and THEN hook up the vac. advance and play-with-it. I have found cammed-up engines love a lot of initial timing. Really cleans up the idle and more responsive accel. jus say'in 6sally6
While I know nothing about Moroso HEI’s I can tell you that there are a variety of different Delco advance plates and it is the length of the slot that controls the maximum available. In the photos you can see the slot and also how the plates are marked. The 534 means 34* and the CW is for clockwise, this plate is for a CCW rotation Pontiac distributor. Over the years I have seen plates marked 514 up to 538. Some aftermarket advance kits include different bushings to control this, or you could braze and shorten the slot if necessary. A distributor with a limit of 18* will allow more initial to be added without getting into detonation.
I'm not worried about gas mileage at all. I do however want it to be as efficient as possible because that makes the most power. Looking at the specs for the distributor, depending on the advance springs in there, it could be 22-26 mechanical advance. I don't know what springs are in it, but if it was stock installed it would have the lightest springs and be 26 at 3100 rpm. So if I'm running it up until the timing stops moving and only getting 18... hmm, should be more like 42... the specs do say "Variables of your engine will determine different actual numbers"... I have a tach in the dash, just cant see it when I'm in the engine compartment.
A vacuum advance will improve part throttle drive ability,how much mech advance is usually controlled by slot .
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