Larry is this right??? you buick guys actually have cars w this kind off timing??? ive never owned anything i didnt put 12-14 initial in ever ever?
That is the factory spec for a 69 Riviera. Larry's advice will essentially add advance in at idle which will make the vacuum reading go up. Basically, he's bumping the initial timing up through the vacuum advance
Yes, try it. Anytime you run that kind of retarded timing, the engine will have a tendency to run hot in a stop and go scenario. The 1972 cars were timed at 4* BTDC in conjunction with the Transmission Controlled Spark emissions system. The thermo vacuum switch would apply manifold vacuum to the distributor vacuum advance if the engine coolant temperature went over 220*. I would think the 69 cars were probably the same. That thermo vacuum switch probably has a manifold vacuum input. Have a look at a page from the 72 Chassis manual. Note the highlighted passages. Most of the early cars used manifold vacuum for the vacuum advance. Ported vacuum was for emissions reasons.
Thanks for including the page. I read a similar piece on the thermo vacuum switch in the 1969 Chassis Service manual. We will give it a try on the 1968 Riviera...
Larry, two questions: 1. When going to manifold vacuum, rpm goes a little bit higher, do we need or is it ok to adjust the rpm back to 550 rpm in D range? I think we still need to do so for checking ignition timing right? 2. We still use the T-piece between vacuum canister and manifold vacuum of carb with a vacuum hose that goes to the stock air cleaner element. Is it ok to have it connected like that? Thanks.
Yes to both questions. You want to check your ignition timing at a low enough RPM so that there is nothing being added by the mechanical advance. If you like the way it performs, you can leave it like that. The engine will run cooler in the summer heat during stop and go driving.
The engine smoothes out immediately at idle when using manifold vacuum and throttle response is better too. We will check what we will have at vacuum with a gauge later this week. Thanks so much Larry...
Thank you too Jason... Yes, it does. We will recheck initial, vacuum canister disconnected at 550 rpm in D, then reconnect the vacuum advance and leave the rpm idle the way it is. I bet when the engine is warm it will only be a little bit higher then connected via thermo vacuum switch, ported vacuum carb.
Larry, an update here: We checked for vacuum with a gauge and now had 20" of vacuum. We noticed the idle rpm at park was at 950, we lowered it to 800 rpm and now have 18" to 19" of vacuum...initial timing at around 12 vacuum advance connected to manifold vacuum 800 rpm in Park. Vacuum advance disconnected at 0 TDC 550 rpm. It idles perfect. We do notice a little hesitation sometimes (not always) when we slighty push the throttle...hesitation from the carb. We want to determine this...any advise would be appreciated.
I edited my last post... At around 12...out of scale. Vacuum advance connected via manifold vacuum at 800 rpm in park. Happy Birthday!
The spec on the vacuum canister is 14-18* @ 16" of vacuum. So it should be that if it is timed at TDC without vacuum.
Yes, it is good. Timing is just fine. At first we had via manifold vacuum 20 inch of vacuum at 950 rpm and around 14 initial. We lowered the rpm idle a little bit to 800 rpm...so we now are at the specs i mentioned. Thanks.
It isn't really initial timing, it's vacuum advance so it will drop out under heavy acceleration. That is where the weights should swing out and provide mechanical timing. If I had one of these distributors, I'd reduce the mechanical timing so I could run more true initial timing. I think the engine would like that even better.
Well, like I said, the timing increase is from the vacuum advance. As you step on it, the vacuum advance will go away under moderate to heavy acceleration, so it may be retarding a bit at first until the mechanical advance starts to build with increasing RPM. If you want the engine to really run it's best, I'd rework the mechanical advance so that instead of having 30-34* of mechanical advance, you had only 20*. That would allow you to run 12-14* initial and have the 32-34* at WOT. Get it all in by 2500-3000 RPM, and limit the vacuum advance to 8-10*. That would give you a very noticeable increase in performance. I'd also look at your accelerator pump to see if it was providing a fuel shot at the very instant the throttle is opened.
