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Discussion Starter · #1 ·
Hey, got a problem with my MSD distributer. First of all they dont make an adjustable vacuum advance. I have set the initial timing at 8 degrees and have drove the car with no problems at all. Then a mechanic put a vaccuum modulator in the tranny to fix a locking up of the converter problem. Tranny works great, but problem is when the car is in fourth gear and you are gradually climbing or generate a light load on it, the engine detonates like crazy. If you drive normally and manually down shift the car, it takes off like a bullet with no detonation at all. He had told me that the advance in the distributor was way too much. Under a downshift load the vacuum drops which does not present a problem at all, but under a light highway load the vacuum pulls the timing up near 50 degrees hence the detonation. My question is, do I get an adjustable vacuum for my MSD, or is there a way to fix the advance in the distributor to only go to a certain point and no further. Hope this makes sense to someone, cause I am at a total loss.
 

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i think MSD distributors all have adjustable mechanical advances in them. they give you different bushings to limit the amount of advance it allows. if they don't make an adjustable vacuum advance can, get an aftermarket one. i think their small housing units are the same as GM point distributors, and the HEI unit would use an HEI can.
 

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Why did it start when the modulator was changed? Unless you have a lot of gear in the car or a huge engine even part throttle in high gear will cause the vacuum advance to go back to zero. Will it down shift without putting your foot all the way to the floor? If you have the adjustable kind of modulator try adjusting it to downshift sooner.
 

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I can't think of an overdrive transmission that has a vacuum modulator, but I get the idea. Basically, OD has loaded the motor to the point that you're hearing the ping. Fine tuning timing is a cut-and-try process. My uncle in law
, and engine builder, often used to pin the advance mechanisms on GM HEI distributors to limit the advance. Surely that MSD dist has a limit adjustment, or can be modified relatively easy. What about the next heavier advance springs.

-dnult
 

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Discussion Starter · #6 ·
Went through the car yesterday and here is what I did. I plugged the port on the car where the vaccuum advance was hooked to and then plug the vacuum canister as well. When I looked at the total timing it was at around 32 degrees. OK. Took it for a drive and it still detonated like crazy in fourth gear with the converter locked up when you hit the gas. Tried it with a small load up hill with the converter locked and a small amount of gas, same thing. When I tap the brake first(unlock the converter) and hit it, no detonation, when I go up hill and tap the brake and give it a little gas, no detonation. I have a 700r4 and the guy who worked on it hooked up a vacuum module in the tranny to lock the converter. Since the vacuum advance is plugged it shouldnt go above the 32 degrees total timing and it doesnt. So I unhooked the vaccuum going to the tranny which does not allow the converter to lock up and ran it the same way. Down shifted beautiful and no detonation, car was very happy. Hooked up the vacuum advance and still keep the vacuum to the tranny for the converter plugged and drove even better!! For some reason I think when the converter locks up it puts a load on the engine that it doesnt like. Almost like it cant unlock fast enough and make the adjustment causing the engine to detonate. So what now? Do I drive the car as is with the converter not locking up? Or do I hook an electricall connection to a button on the B&M handle and push it whenever I am on the highway to lock the converter and press it when I want to unlock it to punch it on the highway? I thought the 700r4 was suppose to have an electrical connection to do this not a vacuum connection? Any thoughts?
 

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Be careful about running your 700R4 unlocked. Depending on the model year, some 700R4 would self destruct due to a lubrication problem.

You're right about the transmission putting loading the engine, but it's not the transmissions fault. By the way, the vacuum modulator you spoke of is probably a vacuum switch to interrupt the 12V supply to the transmission that enables lockup. The switch is external to the transmission. If the switch is adjustable, you might soften it up a bit so the converter drops out at a higher manifold vacuum.

Make sure you know where TDC is. Has the harmonic balancer been changed? If so, the marks could be off.

What RPMs is this pining occuring? If below 3000, try the next stiffer set of mechanical advance springs (or change just one to a stiffer spring).

What is your compression ratio? If it's high, you may be at the limits of your fuel's octane rating.

-dnult
 
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