what degree do you set thr timing at tdc with or without the vaccum connected thanks

 

how is your engine set up? Oem cam, heads, etc? I'm now talking about a 76 L48 which is mine. If so and a stock l48 then I think the factory was something like a purposely retarded 7-8 degrees before tdc. It kept pollutants down but took power away. In both cases it's according to what I've read and done set with the vacuum advance unhooked from the factory HEI ignition and plugged. I reset mine to 13 degrees tdc and it runs much better and gets about 8% or so better fuel mileage too. Re attached the unplugged vacuum hose then checked it all and checked advance when pushing the rpms up too to be sure both vac. and centrifugal advance were kicking it well beyond initial timing settings.

If you have new cam and heads, etc. then I don't know what it ought to be. The time honored method is to start it somewhere, run it, look at it, adjust until you like it. The engine will tell you by knock under load if you've advanced it too far.

The famous Lars papers talk about something like a total advance of about 36 degrees when you combine initial setting, vacuum advance which comes with the engine rpms going up and then the final fairly large mechanical advance from thedistributor's mechanical weights and springs moving outward as the rpms expand and that would be at about 3,000 rpms and up fully kicked in or slightly below that if you recurve the distributor mechanical centrifugal advance to get full advance in lower on the rpm scale. If you have non stock engine then it can vary from that depending on the engine.

Lance P.

 

Total advance of 36*, does not include vacuum advance, only mechanical centrifugal advance + initial. With the addition of vacuum advance, you should be about 52* maximum.

 

I thought that the initial advance was added to by the vacuum advance and then when the centrifugal, mechanical weights and springs, advance, kicked in it went beyond the vacuum advance point and the vacuum advance no longer mattered and that it was 36 degrees approx total at that point.

I reread Lars' paper on timing and you are absolutely right...it's 36 degrees without vacuum advance and 52 all up at faster rpm speeds with the vacuum advance hooked back up for his version of optimum. He says initial timing can be 16-18 with springs to adjust and recurve centrif. so all is in at 25-2800 rpms versus factory springs at redline or so. Mine is all in at 3,000 rpms so someone before me put a kit in but didn't go all the way down to 2500 rpms. I have a new kit sitting here but it runs well enough now as is without me looking for the next spring down and screwing something that is working up.

Thanks for correcting. There is lots to learn here.

Lance P.

 

What are you wanting to do? Get the engine started? If that's the intent, set the timing as specified on the tune it sticker. After it's running, you can make changes.

 

Per GM specifications, set timing at 8 degrees BTDC when the engine is idling with the vacuum advance line removed and plugged.

 

Lance:

I don't know or care what my initial timing is. I set mine @ 34* all in @ 2400 rpm. I am using 1 medium and 1 light spring in my Accel distributor. If (when) I was using the stock distributor, I used the stock weights, not the ones that came in the kit. I don't remember whose kit it was, but the 1 medium and 1 light spring gave me 36* all in by 2800 rpm.

If you have engine modifications and your engine vacuum is lower than stock, Lars has a paper on the specifications for vacuum cans. I have 13" at idle, because of my modifications, so I had to use a different can, to get what I wanted.

See my signature for my modifications. I also have a '76.

 

12 degrees for Automatic Trans...

 

Glenn, That's my general understanding but it was 40 years before that I used a timing light so I set it to initial and it ran much better then checked that it was advancing a lot. I now know how to use the total advance setting on the gun so the next time I set it up I'll check the total. I know something substantial happens at 3,000 rpm 'cause I can feel a kick and the secondaries I would think have long been kicked in by then. I read that the factory set all in advance at close to 5,000 rpm so someone before me must have recurved it. I have the kit but will wait till winter to mess with it. I did wondery why you had to change weights and also some kind of little washer for the weights versus just changing springs. What I have is working so well now that I'm not screwing it up for summer but this winter I'll have time and it will be cooler in the garage too. I haven't opened the kit which is for my HEI distributor and read the instructions yet but I understand it the way you laid it out basically.

Thanks.

Lance

 

I did not change the weights, either on the stock HEI or the Accel distributor. Only changed the springs.

Most people say that the stock HEI weights work better than the aftermarket.

According to my GM specs, stock '76 L-48 distributor maximum centrifugal advance is 22* @ 4600 rpm.

 

OK. I give up. How do you set timing to anything past 14 degrees. The metal tab on the timing chain cover only goes that far???

 

Fully degreed balancer or dial back timing light. I have both.

 

This thread is perfect timingfor me. I've just taken over my Dad's '75 stock auto L48 and plan to check the timing in the next couple of days. The sticker under the hood says 6* at idle up to around 22* at 4200 rpm. These numbers don't really match what you guys are talking about. @LarsPearson, I think our engines are very close to being the same so I think I will try your 13*. Can anybody else provide any recommendations or suggestions? The car runs quite well but seems a bit hesitant sometimes when I step on the gas from a stop and doesn't seem to have the power I remember as a kid.

 

Well, if you have a stock engine and only want to pass smog, then go for it.

I set mine per Lars. 36 degrees mechanical all in by 2800/3000 rpm. Another 16 on the vacuum can. I end up with about 16 mechanical at idle but run manifold vacuum to the can so end up with 24 or so at idle usually.

Since I don't drive at idle and don't have to pass smog, I decided to tune for performance vice stock.

 

The 75 was the low water mark in horsepower on the small block from the factory. However, you can make significant changes by doing the timing correctly. At idle with the vacuum unplugged from the distributor and plugged. set it at something like 12-14 degrees before tdc. It runs then more for power and less for lower pollution which is why GM retarded the timing. I would suspect you also might check to be sure your plugs and wires are good. The gap used for my small block is .045" with new 8 or 8.5mm wires. I think the 75 also has the 2-1-2 exhaust system where the manifolds get "y'd" back into the old fashioned Pellet Catalytic converter at least on the 76. If it's the same and original it more than likely by now is if not all crapped up, at least partially crapped up. I replaced my entire exhaust system andwent to true duals through two bullet wire grid inline cat converters into flowmaster 40's I believe and with all those changes I got 750 more rpm at the top and much more uniform power and more speed all around. Better mileage too. The old cat converter can be refilled I think but if it's original it more than likely is time for it to go to the old cat converter grave yard.

For whatever it's worth...

Lance

 

A couple of comments FWIW:

Get a dial-back timing light. The analog ones can be had for under $50 at Summit and on Amazon.

The factory settings will make your engine run noticeably hotter than having the timing advanced according to Lars' instructions. For me, properly advancing the timing was the single biggest factor in winning the battle against high coolant temps in the summer.

Besides Lars' paper, there are good instructions for timing and a paragraph on ported vs. manifold vacuum here:
http://forums.corvetteforum.com/c3-t...long-post.html

Your vacuum source matters. (My 2c, I switched from ported to manifold vacuum.)

Check the condition of components: springs, weights, vac can, bushings--I was having trouble getting good results and found I needed to replace my springs, vac can and the bushing on the vac can. After that it was much easier to dial in the correct settings.

Lastly, I've never experienced symptoms of detonation, but with my total timing + vac advance at approximately 53* I always use 93 octane anyway.



Chris

 

The math doesn't work here. if you have 16* at idle, vac adv disconnected, and a vac adv that pulls 16*, you SHOULD have 32* of advance at idle, with vac adv hooked up. (16+16=32) Something is wrong somewhere (maybe my brain, I may have just totally lost my mind, hope not!)

I have a 1980, L48, stock heads and bottom end, Edelbrock 2101 intake, Holley 600, block-hugger headers and 2 1/2" dual exhaust, about the only real mods, setup my timing as above, (16* initial, 36* @ 2800 rpm, vac can with 16* advance) and I run 87 octane gasoline with no problems at all. The stock CR gives you alot of wiggle room. I plan to upgrade to 64cc heads, I'll probably need to run the good gas then.

Scott

 

Yeah, my math is off. Probably closer to 14 mechanical and 30 total but honestly, I never check the timing at idle. Like I said, I never drive at idle so why check it there.

 

Aww, man, I just totally called you out! Sorry about that, wasn't trying to make ya look foolish or anything, just figured there was a mistake.

If ya set it at it's all in point, and then check it at idle, it makes it easy to do a "quick check". Mine is set at 36* under full mechanical advance, and that comes out to 16* with the vac adv off, 32* with vacuum adv hooked up. I can whip a timing light on it real quick, and if I see 32*, I can be pretty sure nothing has changed from how I set it up.


Scott

 

Your math depends on your vacuum source! Ported vacuum is sourced from above the throttle plates, and produces no vacuum at idle. This, combined with very low initial settings from the factory (book spec on my 1968 327 is 4* initial) was meant to raise EGTs (it does!) for pre-cat emission control. If he has ported vacuum as opposed to manifold vacuum, he will see his initial 16* at idle, even with a fully functioning vacuum advance.