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Old 07-19-2004, 08:34 PM
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Timing Advance concerns

Hello all,

In my last posting, many of you were very helpful in diagnosing the engine sputtering and knocking problems with my 1968 Buick Skylark convertible. This time, I have a few timing questions. So we don't have to go back to my long-winded last post, let me quickly reacquaint you guys with what has been done so far.

The previous owner went to a local engine shop and had a rebuilt, 1968 (the numbers stamped on the block confirm this) Buick 350 long block installed. Everything on the car is pretty much stock (as far as my novice eyes can tell) including heads, intake and exhaust manifolds, etc. I purchased a correct model Rochester Quadra-jet 4MV off eBay and had it installed on the car. I also pulled the points in favor of a Pertronix Ignitor 2 and an MSD Blaster coil (No ballast resistor needed. The car comes with a 1.8 ohm resistor wire) Unfortunately, despite the car's stock appearance, I have no knowledge of the rebuild specs. None of this was specified on the receipt for the engine work, so I am trying to track down info from the shop itself. However, it was approximately three years ago. In addition, I had a new dual exhaust system (with 2-1/4" pipes and Magnaflow Turbo mufflers) installed. This took care of the stuck heat riser and the rusted glass packs.

That should be everything. Now, here is the general problem. She seems to be lacking in power. While she gets a decent jump off the line, she loses a little steam in the higher rpm ranges. I don't feel comfortable even passing someone at freeway speeds. Now, she bogged out in the past when I attempted to floor it. It turned out that my mechanic didn't properly tune the Quadra-jet. I purchased a Chassis Manual for my car which had detailed tuning instructions for the carb. These tweaks helped a little with acceleration (and eliminated the warm-up stumble from my last post). However, it still felt a little weak.

Then, I recalled the timing issue. It seems the car requires an initial timing setting of 25* BTDC. I know this is much higher than the average 12-14* many people advise. My mechanic claimed it wouldn't run well at any lower number. Testing it with my light (with the vacuum advance disengaged and blocked off) the total advance comes to 47* at 2500-3000 rpm. Add on the 14* (I believe) of vacuum advance and we're up to 61-62* of advance. That has to be too much. I tried to lower it to 19-20* recently (adjusting the idle to maintain 700-750 rpm). It ran better. However, when it cooled off, it was hard to start. It wanted to stall, and when it did stall, it puffed gas back up through the carb. Resetting the distributor to 25* advance, I did a compression test. All cylinders register around 145 psi. Someone in another post did say that an engine might need that much advance if the compression is below 150 psi.

Now, I have been trying to read up as much as I can on the boards and the Knowledge base about timing in general. I know that 12-14* initial is preferred. Total (initial + mechanical) should be around 38-42* at 2500-3000 rpm. I have checked the weights and springs. While a little corroded, they appear to move freely. The timing light shows that it does give 20* advance as the rpm's increase. There is no slop in the timing chain. As far as I can tell, the tab on the balancer matches TDC.

My questions are these:

- Is there anything I can do about this 25* initial advance?

- If I am stuck with 25* initial, how can I compensate at higher rpms?

- Should I compensate at all? Since the engine needs a high initial, does it also require a higher total timing? (meaning the lack of power might be in my imagination?)

- Should I install an adjustable vacuum can and hook it up to the manifold vacuum port? (This way I start at idle with, say, 18* on the distributor + 7* on the vacuum can which equals 25* initial. Then, as I accelerate, the vacuum drops out as the mechanical kicks in, giving me the total 38-40* advance by 3000 rpm. I don't know. Just Mark's crazy theory of the moment)

- What the hell causes the spitback through the carb at the lower timing settings?

Please bear in mind that I know this topic has been discussed a billion times over. It's just that I have just finally reached a saturation point in my research. To quote Albert Brooks, "My 3% is swimming!". If anyone of you fine folks could just direct me to any threads I might have overlooked or a Knowledge Base article that might help, well, I'd be extremely grateful! No need to re-type old information.

Of course, any personal theories you might have, I would love to hear 'em..

Many thanks in advance for your time!

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Old 07-20-2004, 01:57 AM
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Will it idle smoothly with the timing where it's supposed to be? If not, look for a vacuum leak near the base of the carb in the plenum area. I've seen a few that would only run with excess timing and a fast idle when they had a big hose off or a gap in the carb gasket, etc. Failing that, you might check to make sure the #1 piston is really at TDC when the timing marks say it is.
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Old 07-20-2004, 07:33 AM
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I am pretty sure that #1 is at TDC and the mark on the balancer matches. I turned the crank by hand (socket wrench and a breaker bar). Unfortunately, I don't have a guage, so I was kind of spit balling. Perhaps I should try it again to be sure.

I will take a look for the vacuum leak as well. I am pretty sure all ports and hoses are accounted for. My mechanic (who really is good at his job) indicated that the first carb, a larger Q-Jet, was bolted on fairly loose. He checked the gasket, and finding it to be good, bolted on the new carb. The timing before was somewhere around 25*. He found he had to keep it there afterward as well.

The book actually recommends the timing be set to 0* on this model. Anywhere close to that setting and the car bucks and wheezes or doesn't start at all (with the aforementioned spitting back through the carb). When I recently reset the timing, the car was warm and the idle was around 750 rpm. I was only brave enough to set it down to 19* initial. It did run much better for a while (good idle, good response) Unfortunately, it seemed to develop an attitude when it cooled down and I tried starting it again.

Thanks for the advice. I will check it and get back to you all. Any more thoughts?
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Old 07-20-2004, 08:13 AM
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145 pounds of compression is pretty low with a stock type cam. DO you repeat the test with oil?

With compression figures as low as 145 pounds running large amount's of timing is not unusual.

Another useful tip would be to ignore the factory service manuals. the specs given in those manuals are way to generic. You figure the manufacturer has to compensate for a driver who lives in the desert and a owner in the mountains. They need to set up the car before delivery to the dealer but the assembly line doesn't know which location. With so many locations available it would be impossible to customize every car. So a performance advantage comes in to play. The manufacturer doesn't know who ar where the car is going so the specs given are VERY generic.

12-14 is normal for a Chevy 350, but Buick's have less efficient chamber design and need more timing advance.
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Old 07-20-2004, 12:47 PM
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You could try installing the points again to rule out any problems with the pertronix module. If runs better with points then you might look into replacing the pertronix.

John
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Old 07-20-2004, 05:02 PM
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Thanks for all the tips so far folks. I really do appreciate them.

Propaniac, as far as the points go, the initial timing was 25* back when I had them in the distributor as well. When my mechanic did the first tune-up, he had me go over to a performance parts store and pick up a new set of points. It was after this he told me that the timing needed to be at 25* or better. When, I put in the Pertronix unit, nothing changed. I don't believe that is the problem. Thanks for the suggestion though.

Johnsongrass, I am going to do the compression test over again. First, in the normal fashion. Then, as you suggest, with a shot of oil in the cylinders. In reading up on compression tests, I saw that people only did this if one cylinder was reading way off from the others. Since all my cylinders were within 5* of each other, I didn't even think of that. Now, if compression does increase, does that mean I am in for some valve work?

Thanks again. If you think of anything else, please post it. I am thirsty for the knowledge.
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Old 07-20-2004, 10:16 PM
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Okay, it's update time!

I went to the parts store and purchased some oil (I am using Mobil One 10w30 grade) and one of those hand squirters. Since it was so damn hot in the valley today, I didn't get a chance to work on the car until just before sundown. So, the work is somewhat incomplete.

First, a visual inspection showed all vacuum ports were occupied. I did find a small diameter hose coming out of the intake manifold that seemed to be going nowhere. Then, I found a similar diameter hose coming out of the firewall, also disconnected. I assumed they were part of the heating system and attached them. I would check for any vacuum leaks at the gasket after I did the compression test.

With the compression test, as it was getting late, I only got to do a couple of the cylinders. I found my original notes, and was surprised to discover that the number I quoted you earlier was wrong. The highest compression I could get was 140 psi. Three cylinders were at 140. Four others were at 135 and the last made it to only 125. I checked the odd numbered bank this time. I got exactly the same results. A squirt of oil in the cylinder made no discernable difference (except that the needle seemed to jump to the end number a little faster.) I believe I was performing the test correctly. I set the throttle wide open. I made sure the butterflies on the carb were also open. I grounded out the coil wire. Finally, I pulled out each plug and tested it one at a time. I made sure to turn it over several times untiul the needle stopped. I know some say pull all the pugs out while others tell you to do them one at a time. I went with book instructions.

I fired her up just to see if I could detect any kind of a leak from the carb gasket. I couldn't tell a thing. However, the car seemed to run just a little bit nicer. (I can't imagine that tiny little hose had anything to do with that) After it warmed, I took it for a quick test spin. It seemed to run okay, except that now there is a 1/4-1/2 second lag when I hit the pedal. Then , the power comes on. Not spectacularly, but it's there. Of course, I don't know if this new development is timing or carb related. So confused...
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