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Author Topic: (SOLVED)Vacuum Advance Induced Detonation.  (Read 2202 times)

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Offline Glitch666

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(SOLVED)Vacuum Advance Induced Detonation.
« on: September 06, 2015, 02:32:47 AM »
Well finally completed going through the motor on my 76. Everything is up to snuff and painted, looking good. Put in a ranger roller cam with new lifters and attached the new(only 800 mile transmission). Got it in the car today and fired up and timed. Runs smooth at 10 degrees. Except giving it any throttle results in spark knock (not driving, idle in the drive way). Disconnecting the vacuum advance and it goes away and runs perfect. Backing the timing down until it runs like crap still results in detonation with the vacuum advanced hooked up. I know the head was previously done shortly before I acquired the car. Don't know if it was maybe shaved. It had it before the engine was pulled, thought it was the lifters/cam... .. Turned out to be detonation....

UPDATE: Turns out number 1 had a broken compression ring and a cracked skirt that fell of when I pulled the piston. Honed and replaced with the engine still in the car and she runs fantastic.  Was lucky and caught it in time. Was able to hone out the damage.

Offline C. M. Wolf

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Re: Vacuum Advance Induced Detonation.....
« Reply #1 on: September 06, 2015, 06:05:42 PM »
I had to deal with something very similar to this on my 70 Chevy-built 350cui engine. I changed the intake over to a 0-50/Performer-400 Edelbrock intake.. this(& along with the different carb, Crane-valve-train, special-ground cam, pistons, ported & polished heads, HD-50 oil-pump, etc), changed the vacuum ratios on the engine which caused mistiming on the distributor's vacuum advance. it took a sharp fellow I knew some 'shiny' math to finally get it all right. I finally had him build me a correct vacuum advance diaphragm unit that worked perfect.

..and after having to re-jet the new carb.. lol

Michael


Offline ixplod

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Re: Vacuum Advance Induced Detonation.....
« Reply #2 on: September 06, 2015, 06:35:30 PM »
Where is the vacuum advance hooked up? The best place on a Pinto is the vac tree on the intake (rear driver's side). If the vac advance is hooked to venturi vac the advance will increase with throttle opening. With manifold vacuum, it SHOULD lose advance when the throttle is cracked open.

What does the timing look like with the vac advance hooked up? Screw the "rules" and see what happens to the advance if you pull the line while it's running. Pull the line and put your thumb over it. The idle SHOULD get rougher. The high manifold vacuum adds more timing while at idle and the engine has little or no load.

Personally, I've never seen this problem - even with 26 degrees initial advance on a 302 Ford. Started hard, but pulled like a beast....until it started pinging. Backing the timing back down to 16 allowed it to work all around.....jus t without the oomph on the bottom end.

There has to be a solution to this!

1980 Sedan - 2.3, 4-speed

Offline 74 PintoWagon

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Re: Vacuum Advance Induced Detonation.....
« Reply #3 on: September 06, 2015, 07:39:30 PM »
Vacuum advance should be connected to manifold vacuum, not ported vacuum that's a smog thing..
Art
65 Falcon 2DR 200 IL6 with C4.

Offline dennisofaz

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Re: Vacuum Advance Induced Detonation.....
« Reply #4 on: September 06, 2015, 08:39:05 PM »
Hi, I would try whay Art said and hook the dist. to manifold vacuum.  In my 74 with a 2.3 the harmonic damper outer ring shifted and i cannot time it with a light at all.


Dennis

Offline Glitch666

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Re: Vacuum Advance Induced Detonation.....
« Reply #5 on: September 06, 2015, 09:27:22 PM »
There's no smog stuff left on the car. The only vac line that's hooked up is the advance can. It's hooked to the ported spark vacuum by the mixture screw. I did try hooking it to manifold vacuum and backing the timing back and it ran worse, causing rough idle and backfires. I just finished test driving without the advance hooked up but it pings under acceleration. Runs like a raped ape though... Manifold and ported vacuum act exactly the same except at idle. It would really only be hooked up to manifold vacuum if I had a big cam that needes alot of base timing to idle correctly. I'm gonna try some high octane fuel and see it helps.

Offline 74 PintoWagon

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Re: Vacuum Advance Induced Detonation.....
« Reply #6 on: September 06, 2015, 09:38:11 PM »
Ok, your mistake was to back your timing down, set your timing where you had it and hook the vacuum to manifold, it will idle high because of advance at idle so just turn your idle down, you'll see how much smoother it runs..
Art
65 Falcon 2DR 200 IL6 with C4.

Offline 74 PintoWagon

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Re: Vacuum Advance Induced Detonation.....
« Reply #7 on: September 06, 2015, 09:42:22 PM »
In my 74 with a 2.3 the harmonic damper outer ring shifted and i cannot time it with a light at all.
Dennis
You can bring it up to TDC and remark the damper, but you really need to replace it if it moved before the ring flies off..
Art
65 Falcon 2DR 200 IL6 with C4.

Offline Glitch666

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Re: Vacuum Advance Induced Detonation.....
« Reply #8 on: September 06, 2015, 10:43:34 PM »
Ok, your mistake was to back your timing down, set your timing where you had it and hook the vacuum to manifold, it will idle high because of advance at idle so just turn your idle down, you'll see how much smoother it runs..
I think you missed my point, Its not that I cant get it to run smooth. Its that it pings no matter what even if the vacuum advance is disconnected and plugged. Also I know how vacuum advance works. Ported vs manifold has been a debate for many years. It advances completely the same except at idle. I don't have an issue at idle so increasing my timing there does no good. They both will advance the same at above idle and full throttle. Again so no matter what, if I give it gas and it pings above idle it will do it regardless if I use ported or manifold.
Also my goal was to retard my timing back to eliminate my detonation issue but keep the base timing up for a smooth idle. But I couldn't get it right, ran like crap no matter what.

Offline 76hotrodpinto

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Re: Vacuum Advance Induced Detonation.....
« Reply #9 on: September 06, 2015, 10:55:59 PM »
I'd start with a compression test and a vacuum test. Maybe even stuff the fuel supply line in to a can of fresh gas, and see if your fuel is wonky.
1976 half hatch 2.3 turbo w/t5.

Offline Glitch666

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Re: Vacuum Advance Induced Detonation.....
« Reply #10 on: September 06, 2015, 10:59:20 PM »
The gas was fresh regular That I put in last night after the tank was drained before first start up. This engine had this same issue before I tore it down. I also installed a different distributor during the go through, also the different cam and new lifters. Im definitely gonna do a compression test next weekend.

Offline 76hotrodpinto

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Re: Vacuum Advance Induced Detonation.....
« Reply #11 on: September 06, 2015, 11:15:53 PM »
Another trick to try. Hook the vac line to a brake bleeder pump, and mess around with different levels of vacuum, see if you can see a trend or discover something. Be sure to cap manifold fitting. Not very scientific, but it gives you control over the vacuums full range.
1976 half hatch 2.3 turbo w/t5.

Offline ixplod

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Re: Vacuum Advance Induced Detonation.....
« Reply #12 on: September 07, 2015, 05:59:42 AM »
The gas was fresh regular That I put in last night after the tank was drained before first start up.

Run a fuel line into a can of fresh 92 octane and see if it makes a difference? Maybe the head was shaved a lot. That would have been something to check while the engine was apart.

My parents' '80 Mustang (bought new in late '79) developed a similar issue at about 75k, making a pinging sound whenever accelerating (even lightly). We tore the engine down in and found absolutely nothing unusual. Dingle balled the cylinders and put it back together with new rings and bearings. They traded it off at 98k, so I never found out the cause nor a solution.
1980 Sedan - 2.3, 4-speed