TAC Tech => Mechanical => Topic started by: Journeyman17 on October 25, 2022, 06:43:51 PM
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I am at TDC # 1 cyl compression stroke the distributor is pointing just past the #7 cylinder. I am gm guy so i want to turn distributor to number 1 and go gm firing order but everythin i find says #1 wire goes over by #7 cylinder??. Sorry im newbie to trans am. I should mention it is a 1975 400 motor
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In reality, the position of the rotor when cylinder 1 is TDC of the compression stroke is only important relative to the position of the plug wires meaning you can “put” the rotor pointing any direction as long as plug wire #1 ends up on the same location and the plug arrangement follows the correct firing order.
From the factory though, yes, the rotor typically points somewhere between the firewall and #7 and the firing order progresses from there counterclockwise; 18436572. I’m not a Chevy guy but I want to say Chevy motors typically had the rotor pointed toward #1 and the distributor rotation is clockwise.
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Ok i agree with all that. Now im trying to figure out if im a tooth off or not on the distributor
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Is there a reason these distributors are clocked this way? If it makes no difference i will prolly turn it toward # 1 like im more used to
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Shouldn’t make a difference other than wire length/routing
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it is to allow for rotating the dissy without running into problems with the vacuum canister
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You can still position the vac can wherever you want independent of where the rotor is positioned.
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I've seen conflicting examples of where the dist. should be located, from the factory manuals, to chiltons, or Haines, and others.
And you can't be 1 tooth off. because the bottom of the distributor is slotted, and can only go in the right way, or 180° out.
You can put your #1 anywhere you want like mentioned above, by taking a long flathead screwdriver, and turning the oil pump driveshaft in the dist. hole, to where you want it to be relative to your distributor gear slot, and rotor. And when you drop it in, remember to take into account it will twist when it engages with the cam gear, B4 it settles in. It might take a couple try's to get it to point where you want. And when you finally install it for good, like Aussi mentioned, the vac can needs to go where you can adjust it W/O hitting anything.
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Ive figured out the timing thanks for the replies. Car runs good now
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Glad you got it worked out
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Thanks for concluding J. Congrats. :-)
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Ok i guess i still have timing questions. I had it set (without vac adv) at idle 18 advanced and 33 advanced at 3000 rpm. I hookup the vacuum at idle speed timing jumped to 33 and 48 @ 3000 rpm. I guess i have alot to learn. Wth?? So i turned the distributor all the way to ~ to barely get back to 18 at idle and 33 full adv but it ran better without the vac hooked up. Fresh motor still has not been out of the barn yet
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With the exception of the 403, I've never seen Pontiac run their initial adv., with more than 10-12°. (W/O manifold vacuum hooked up). But most engs. do like more advance...except to the point of starter kick back, or detonation. Do you have either of these problems? When you set your initial, (vac. adv. blocked) at around 700-800 RPMs, when you hook up your canister, it should shoot up in RPMs, and smooth right out. Then you can turn your mech. idle screw down to 700-800.
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It was running good until i hooked up vac. And retimed it. Update tho i just found out im hooked to the manifold vac. Port. Im gonna put it on the other side of the carb to the port vac and try again
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With the exception of the 403, I've never seen Pontiac run their initial adv., with more than 10-12°. (W/O manifold vacuum hooked up). But most engs. do like more advance...except to the point of starter kick back, or detonation. Do you have either of these problems? When you set your initial, (vac. adv. blocked) at around 700-800 RPMs, when you hook up your canister, it should shoot up in RPMs, and smooth right out. Then you can turn your mech. idle screw down to 700-800.
Actually stock for the 400s (in 77 anyway) were 16-18° BTDC depending on what trans.
Journeyman, your method of setting your initial timing was correct. Set the timing with the vacuum can disconnected. Timing will increase with the can connected to manifold vacuum at idle. That’s normal and not necessarily undesirable. If you didn’t though, ensure when you disconnect the vacuum from the can, make sure the carb port where the hose is blocked so there are no vacuum leaks. Sounds obvious but sometimes I’ve seen people get confused. Myself included when I first got to messing with my car years ago.
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Actually stock for the 400s (in 77 anyway) were 16-18° BTDC depending on what trans.
I've heard this before, but never saw it listed, until now Brian. (As you posted the year.) The only year they did that. Sounds like a ton of advance, but maybe cause it was ported vacuum, (and I'll bet the vac. canister was calibrated for less), they were able to get away with it. Altho the fuel quality was disappearing quickly, you could still get leaded high test, (Sunoco 260), which would allow this much advance. Of course with the catalytic converter, you weren't supposed to do that.
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Interesting. Didn’t realize that was a one year thing.
EDIT: shows the same in my 78/79 manual also. I know the early 2nd gems were much less initial timing. Maybe 77 was when they upped it
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I disconnected vac advance and plugged carb. Reset initial timing to 18. Idled good thwn i plugged into the ported vac port which had 5 psi of vac at idle.. the rpm went up and seamed to run better but again it added loads of advance. At full advance it was almost 50. Funny thing is it seams to like it. But im afraid to drive it like that. Remember too when guys are talkin about stock timing and emissions. I have no emmisions. 5c heads milled .008 off. 231/240. .470/.470 cam. Edlebrock rpm intake, headers and 750 Edlebrock carb. Overdrive trans with 2200 rpm converter
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I would use manifold vacuum for the dist. and back down on the throttle blades. So much better for throttle response, eng. runs cooler, and gets better mileage to boot. It's a win, win. You can always tailor the advance curve , if needed.
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The full manifold vac was too extreme. I think im just gonna run mechanical only till i get exhaust on here.