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From: Into the Mystic And yet, despite the look on my face, you're still talking TN
St. Jude Donor '09 thru '25
need help with timing
I think I'm chasing my tail here. L82 4 speed that was just gone through, rings, bearings, cylinders honed, block decked etc. Cleaned up the crank, connecting rods and pistons and reused them. Comp Cams XE268 and Dart 165/72 iron heads. Stock Q-Jet rebuilt by Lars several years ago that has performed flawlessly.
I have set the timing per Lars excellent papers. I have 36* mechanical advance and with a VC1853 vacuum can, I have about 31* at idle and 51-52* at higher rpm. Here's the problem, I have a nasty part throttle jerk/chug up to about 2500 rpm, then it's fine. No knock, just the jerk.
Reading Lars paper, he says this could be the result of too much vacuum advance at a lower rpm.
I am using the gold MRG springs but I still get all the mechanical advance to 36* coming in around 1500 rpm instead of all in by 3000 rpm. I'm thinking this coupled with the vacuum advance is too much advance at a lower rpm and is causing my issue?
Do I need to somehow slow down the mechanical advance? Heavier springs? Lighter weights?
I can disconnect and plug the vacuum advance, drive it and the problem all but goes away.
Chased this for two days now and I am stumped.....and somewhat frustrated
Hello,
I just sorted my timing out too so maybe a suggestion or two could help.
First try and back off the initial timing a couple degrees and take a power run. If this solved the problem leave well enough alone.
Second make sure the weights are moving freely after the spring swap and not binding during advance.
Also miti vac the new advance can. Make sure you have the unit fully advancing 2* below the initial. Your base timing seems too high.
Marshal
i think i'm chasing my tail here. L82 4 speed that was just gone through, rings, bearings, cylinders honed, block decked etc. Cleaned up the crank, connecting rods and pistons and reused them. Comp cams xe268 and dart 165/72 iron heads. Stock q-jet rebuilt by lars several years ago that has performed flawlessly.
I have set the timing per lars excellent papers. I have 36* mechanical advance and with a vc1853 vacuum can, i have about 31* at idle and 51-52* at higher rpm. Here's the problem, i have a nasty part throttle jerk/chug up to about 2500 rpm, then it's fine. No knock, just the jerk.
Reading lars paper, he says this could be the result of too much vacuum advance at a lower rpm.
I am using the gold mrg springs but i still get all the mechanical advance to 36* coming in around 1500 rpm instead of all in by 3000 rpm. I'm thinking this coupled with the vacuum advance is too much advance at a lower rpm and is causing my issue?
Do i need to somehow slow down the mechanical advance? Heavier springs? Lighter weights?
I can disconnect and plug the vacuum advance, drive it and the problem all but goes away.
Chased this for two days now and i am stumped.....and somewhat frustrated
Those are the correct springs. With stock GM weights, they usually are all in about 2600 or so.
I could find it on my machine, but over the net all I can suggest is see if the weights are stock,
Measure your idle vacuum, and cross it to Lars HEI vacuum can chart.
I think your timing settings are fine, at least as a starting point. All in by 1500 is way too early though.
At that level all of your mechanical advance would be present plus all the vacuum unless you're full throttle.
Slow down the mechanical with stronger springs would be my first attempt. If that cant get it then yes you can reduce the weight of the weights. If they are stock weight I can't see why they would be too heavy though.
Maybe they have been altered at some point in the past.
It's possible someone may have even altered the shape of the weight which would change the timing advance curve.
Throw up a pic and maybe we can see if they look stock.
From: Into the Mystic And yet, despite the look on my face, you're still talking TN
St. Jude Donor '09 thru '25
Originally Posted by REELAV8R
I think your timing settings are fine, at least as a starting point. All in by 1500 is way too early though.
At that level all of your mechanical advance would be present plus all the vacuum unless you're full throttle.
Slow down the mechanical with stronger springs would be my first attempt. If that cant get it then yes you can reduce the weight of the weights. If they are stock weight I can't see why they would be too heavy though.
Maybe they have been altered at some point in the past.
It's possible someone may have even altered the shape of the weight which would change the timing advance curve.
Throw up a pic and maybe we can see if they look stock.
That's exactly what I'm thinking, thanks. It's a new distributor, don't know what's going on with it. The springs that it came with do the same thing. They are a slightly larger diameter, think I will try to put one MRG spring inside the larger spring and see what that does.
That's exactly what I'm thinking, thanks. It's a new distributor, don't know what's going on with it. The springs that it came with do the same thing. They are a slightly larger diameter, think I will try to put one MRG spring inside the larger spring and see what that does.
From: Into the Mystic And yet, despite the look on my face, you're still talking TN
St. Jude Donor '09 thru '25
Got more going on that just a simple timing issue, I think now. Double springs so I get all the advance in about 2500 rpm, chugs and jerks and gradually smooths out by about 3000 rpm with the vacuum advance. Disconnect and plug the vacuum advance, absolutely no difference. I did notice the #5 plug and sometimes the #6 plug are both arcing to the headers. Is that enough to steal the spark? We used thermal insulation on the plug boots and it's very close to the headers. Got to be some kind of ignition problem.
From: Into the Mystic And yet, despite the look on my face, you're still talking TN
St. Jude Donor '09 thru '25
It's amazing how much better these engines run when they hit on all 8 cylinders It wasn't timing.
Replaced #5 and #6 plug wires with my old wires. Problem solved. One boot had a hole burned almost all the way through from arcing to the header. Supposed to be good wires, too. Guess I'll buy a set of ceramic boot wires now.
need help with timing
It wasn't timing.