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Old 07-04-2004, 04:08   #1 (permalink)
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Vacuum advance

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Old 07-04-2004, 05:33   #2 (permalink)
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Re: Vacuum advance

Did it ping with the previous dissy, (same advance)?
What fuel and compression ratio are you running?
Open chamber or closed chamber heads?
You retarded it 2 degrees from what?
Are you sure the timing marks on your harmonic balancer are correct?

I was encouraged by the sparky who rebuilt and recurved my dissy to remove the vacuum advance altogether. A modified engine doesn't deliver the same vacuum signal as a stocky, so I'd rather rely on a mechanical advance instead, which is governed by RPM.
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Old 07-04-2004, 13:36   #3 (permalink)
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Re: Vacuum advance

Make sure its connected to the ported connection on the carbie not full manifold vaccum. The ported connection as piped to above the throttle plate where vaccum is effected by throttle openings
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Old 07-04-2004, 15:32   #4 (permalink)
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Re: Vacuum advance

Quote:
Originally Posted by Rhett
Did it ping with the previous dissy, (same advance)?
What fuel and compression ratio are you running?
Open chamber or closed chamber heads?
You retarded it 2 degrees from what?
Are you sure the timing marks on your harmonic balancer are correct?

I was encouraged by the sparky who rebuilt and recurved my dissy to remove the vacuum advance altogether. A modified engine doesn't deliver the same vacuum signal as a stocky, so I'd rather rely on a mechanical advance instead, which is governed by RPM.
fuel = Mobile 8000 98 octane
closed chambers opened out
went from 12 degrees back to 10 degrees
stock balancer: can you put it on wrong?

What makes me think it is the vacuum advance is the way it ping on and off the throttle at cruise. 2000-3000 rpm, 80-100kmh. This would be at highest manifold vacuum, so the vac advance would be fully on (i think). Stops pinging when given a bootfull.
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Old 07-04-2004, 15:32   #5 (permalink)
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Re: Vacuum advance

Quote:
Originally Posted by northiam
Make sure its connected to the ported connection on the carbie not full manifold vaccum. The ported connection as piped to above the throttle plate where vaccum is effected by throttle openings
yes the one that sticks out the right hand side of the front metering block (750 Holley)
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Old 07-04-2004, 21:30   #6 (permalink)
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Re: Vacuum advance

No you can't put the balancer on wrong, but outer rings (where the timing marks live) have been known to spin off centre. This means that the TDC mark isn't actually at TDC for number 1 piston.

Something else to try is to block the vacuum advance hose and see if it stops pinging.

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Old 07-04-2004, 22:11   #7 (permalink)
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Re: Vacuum advance

To check for TDC I got an old sparkplug and knocked the middle out of it. Drilled the middle out completly and used a tap to put a thread where the middle was to accept a long bolt. Ground the bolt end round. Wind bolt out so the round end sticks out about 1 to 2 cm past sparkplug thread. Put this in Number 1 cylinder and tightened up makeing sure first that number 1 piston is as close as possible to BDC. CAREFULLY turn engine over by hand clockwise and as number 1 piston approaches TDC it will stop against the end of the bolt in the middle of the old sparkplug. Mark the balancer where it lines up with timing pointer. Now CAREFULLY turn engine over by hand anticlockwise until it stops against the bolt again. Mark the balancer again where timing pointer is. The middle of these two lines is true TDC. With a bit of luck it should be where the balancer is marked with TDC. BUT, I have done this on a 302w that the TDC mark was about 10 deg retarted useing this method.
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Old 07-05-2004, 01:38   #8 (permalink)
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Re: Vacuum advance

OK, what I really want to know is whether the vacuum advance would cause pinging as described? At cruise rpm when on and off the gas ,stopping when given a bootfull??? Have I correctly diagnosed the problem? I have read the supplied instructions for removing the vacuum advance. The metal thread screw in the drawing has been substitued for a self tapper with a hideous socket head (torx or something) And there are no SHIMs in the kit, what are they are talking about?? My next step will be to email MSD.
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Old 07-05-2004, 03:52   #9 (permalink)
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Re: Vacuum advance

You can stop the vacuum advance without pulling the dizzy to bits.(for the sake of testing it)

Just block the pipe that runs between the carbie & dizzie by pulling it off at the dizzy end and jam a centre punch or similiar in the pipe. No need to worry about blocking the dizzy off as the baseplate in the dizzy will just sit in the position its at with no advance.

Test drive the car again. If it still does it talk to MSD or a mechanic that knows how to re-graph a dizzy.

The vacuum progressively increases with revs to progressively increase the advance of the timing by pulling the baseplate in your dizzy around. With a good timing light you should be able to watch the timing mark on your front pulley progressively advance further as you increase revs. (don't try to this while driving along and be careful not to stick the timing light into the fan)
So at idle your timing is say 10 degrees advanced BTC but at say 3000 rpm it might read 30 degrees BTC.
Depending on your compression ratio, cam etc you might want all your timing (the total advance) in by say 3000 rpm so it does not advance the timing more than 30 degrees total. So as you increase revs beyond 3000 the timing stays at 30 degrees.
Hence the saying "Initial timing 10 degrees and total timing 30 degrees but all in by 3000rpm"
A mild Clevo don't like more than about 36 degrees total and would need to be all in by say 3500rpm.
Its possible you may have a broken spring in the dizzy that returns the baseplate to its neutral position or the spring/s may be too weak
If you discover it has too much total advance too early and decide to go without any vacuum advance you will have to do what the spec sheet says or employ the services of a mechanic or auto electrician who know what they're doing
Good luck
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Last edited by shaynet; 07-05-2004 at 03:56. Reason: Forgot some detail
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Old 07-05-2004, 04:12   #10 (permalink)
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Re: Vacuum advance

I dont think it needs regraphing. It only happens at CRUISE rpm when ON and OFF the gas. If it needed re graphing wouldnt it ping at full throttle? It has been factory preset to the slowest (least likely to ping) available advance curve. The cam manufacturer (Crow Cams) said to leave it like this. Inital advance = 10 degrees and blue stop bushing = 20 degrees. Total advance = 25 degress, fairly conservative stuff.
If I was going to test it without the vacuum advance I would take the pipe off the carburettor end and put a little plastic block off cap on it. One of the FOUR I had to buy in a pack to get one to block off the tube at the base of the carb. Will test it tomorrow.
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