Eratic timing..... possible causes ?
Thread Starter
Racer
Joined: Apr 2002
Posts: 415
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From: loughborough Leics ENGLAND
Eratic timing..... possible causes ?
Before i get to deep into the engine bay has anyone got any experince of similar problems with their Vette.
I ran my 90 at the track last weekend and it was suffering from intermitent pinging once it gets into top gear on W.O.T. ( auto )
The car did not feel as quick as the last time at the track in the last 1/8 mile with a new cam and ported heads ( although i picked up .8 sec on my previous best )
When i got home i put a timing light on her and it was rock steady at 8 deg's and then it hesitates and shows 90degs before going back to 8 deg.
It is random so i am not sure were to start, the plugs, leads, rota and cap have less than 3k miles on them ( gen GM parts )
Were do you guys think i should start and what should i look for ?
:cheers:
I ran my 90 at the track last weekend and it was suffering from intermitent pinging once it gets into top gear on W.O.T. ( auto )
The car did not feel as quick as the last time at the track in the last 1/8 mile with a new cam and ported heads ( although i picked up .8 sec on my previous best )
When i got home i put a timing light on her and it was rock steady at 8 deg's and then it hesitates and shows 90degs before going back to 8 deg.
It is random so i am not sure were to start, the plugs, leads, rota and cap have less than 3k miles on them ( gen GM parts )
Were do you guys think i should start and what should i look for ?
:cheers:
Burning Brakes
Joined: Jan 2001
Posts: 1,169
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From: Manhattan
Re: Eratic timing..... possible causes ? (BLUE ONE)
a few things
did you disconnect the brown? wire by the fire wall to disable the electronic spark control?
have you had a custom program burned for your new heads/cam?
you should hook up a diagnostic scanner to the ALDL port and record some runs, most likely you just have too much timing for your new setup and WOT in top gear.
did you disconnect the brown? wire by the fire wall to disable the electronic spark control?
have you had a custom program burned for your new heads/cam?
you should hook up a diagnostic scanner to the ALDL port and record some runs, most likely you just have too much timing for your new setup and WOT in top gear.
Instructor
Joined: Oct 2000
Posts: 152
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From: des plaines, il
Re: Eratic timing..... possible causes ? (BLUE ONE)
When i got home i put a timing light on her and it was rock steady at 8 deg's and then it hesitates and shows 90degs before going back to 8 deg.
W(h)ere do you guys think i should start and what should i look for ?
W(h)ere do you guys think i should start and what should i look for ?
I'd also record a scan session, looking for strange rpm/timing readings that could indicate an ECM or pick-up coil fault.
Race Director
Joined: Aug 2002
Posts: 13,356
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From: tucson AZ
St. Jude Donor '04 & '05
Re: Eratic timing..... possible causes ? (BLUE ONE)
Have you checked the timeing
(spell) chain....or the harmonic balancer being off???
:confused:
[Modified by oldace84, 5:44 AM 9/7/2002]
(spell) chain....or the harmonic balancer being off???
:confused:
[Modified by oldace84, 5:44 AM 9/7/2002]
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Thread Starter
Racer
Joined: Apr 2002
Posts: 415
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From: loughborough Leics ENGLAND
Re: Eratic timing..... possible causes ? (oldace84)
Well thanks for all your comments, i think i have got half the problem sorted now :confused:
I found one H/T lead with a rip in the insulation.
AND ............a distributor gear that has worn to a knife edge on the teeth which allowed the rota to slop around quite a bit.
I have now fitted a new gear and the ping has gone under WOT all the way up to 120 mph ( did not want to go any faster, on the road anyway !!!!!)
But timing still moves around a bit at 800 rpm, ( up to 90 deg every now and again and yes the brown wire is disconnected and it has got a new chain and the car has only 27k on it )
As rpm increases though it is rock solid with no movement if you hold it at a given rpm. So i will be back at the track next week end and see if i can get in the 12's and or see if the problem comes back
:cheers:
Burning Brakes
Joined: Feb 2000
Posts: 1,206
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From: USA USA
Cruise-In II Veteran
Re: Eratic timing..... possible causes ? (BLUE ONE)
"AND ............a distributor gear that has worn to a knife edge on the teeth which allowed the rota to slop around quite a bit."
Scorp was right on the money$$$!!!!!!!
Glad you found it!!!
Matt
Scorp was right on the money$$$!!!!!!!
Glad you found it!!!
Matt
Melting Slicks
Joined: Aug 2001
Posts: 2,516
Likes: 0
From: Lockport NY
Re: Eratic timing..... possible causes ? (BLUE ONE)
I have now fitted a new gear...
Too much end play will allow the dist timing to jump around; most notable on accel/deccel, but at idle it will be erratic...
You can (roughly) check this by pulling up on the rotor with the cap off...If you can see any rotation of the rotor, you have too much end play...
...And if you can hear it, you have way too much... :eek:
[Modified by Rich B., 10:21 AM 9/8/2002]
Thread Starter
Racer
Joined: Apr 2002
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From: loughborough Leics ENGLAND
Re: Eratic timing..... possible causes ? (Rich B.)
You might be onto something with setting the end play.....I didn't set it :eek: :eek: :eek: i just put the new gear on, but i was suprised how much there was with the old gear and the fact it was no better with the new. This ecessive end play could be why the gears wear in the first place :crazy: . What should it be set to ???? i guess mine was about 20 to 30 thou and although the shaft would ride up and down there was no slop in the gear mesh with the cam :bb
At least it was not to big a job to do if i have to do it again
:mad :mad :mad
At least it was not to big a job to do if i have to do it again
:mad :mad :mad
Team Owner
Joined: Mar 2000
Posts: 83,383
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From: Boston, MA
I found this one out the hard way. My tach had been completely screwy over 3500 for about a week and the car seemed 'off'. Then one day I gave it come gas and then she came to a stop. 3 of those razor sharp teeth had sheered off completely so that the distributor wasn't spinning anymore.
Thread Starter
Racer
Joined: Apr 2002
Posts: 415
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From: loughborough Leics ENGLAND
Re: Eratic timing..... possible causes ? (BLUE ONE)
Just one other thing you mentioned about it being noticeable more on deceleration, guess what, mine used to pop and bang in the exhaust when decelarating down hills etc and now you have mentioned it, i didn't notice anything today when i took her out so this is really making a lot of sence.
Thanks for making it all make sence at last :D :D :D
:cheers:
Thanks for making it all make sence at last :D :D :D
:cheers:
Melting Slicks
Joined: Aug 2001
Posts: 2,516
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From: Lockport NY
Re: Eratic timing..... possible causes ? (BLUE ONE)
What should it be set to ???
You need some clearance for oiling, same as any shaft/bore in a motor.
I've found that a lot of the older shops have a Distributor Dyno that they used to set up old point-type dists; they usually have the shims & heavy-duty roll pins in stock and can set you up in less time that it takes to explain it...Literally...
Thread Starter
Racer
Joined: Apr 2002
Posts: 415
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From: loughborough Leics ENGLAND
Re: Eratic timing..... possible causes ? (Rich B.)
Thanks again, i will get it out and do it right this time. This really could be the reason there are quite a few cars with the gear problem, rather than it being the gears them selves I hope other people can learn from this too.
Makes it worth doing it again almost. :smash: :smash: :smash:
:cheers:
Makes it worth doing it again almost. :smash: :smash: :smash:
:cheers:
Melting Slicks
Joined: Aug 2001
Posts: 2,516
Likes: 0
From: Lockport NY
Re: Eratic timing..... possible causes ? (BLUE ONE)
Here's a guy that does C2 dist restorations...he recommends .010" end play for those...
http://www.tispecialty.com/articles/article6.htm
This is a site that recommends .015" for HEI dists...
http://rmcavoy.freeshell.org/HEI.html
Mine was along the order of .060", so anything near .010" - .015" was fine with me... :D
[Modified by Rich B., 10:59 AM 9/8/2002]
http://www.tispecialty.com/articles/article6.htm
This is a site that recommends .015" for HEI dists...
http://rmcavoy.freeshell.org/HEI.html
Mine was along the order of .060", so anything near .010" - .015" was fine with me... :D
[Modified by Rich B., 10:59 AM 9/8/2002]
Drifting
Joined: Aug 2000
Posts: 1,251
Likes: 0
Re: Eratic timing..... possible causes ? (BLUE ONE)
Cam walk?
You say you have a new cam & heads,If you have high rate valve springs the pressure drives the cam forward .If the cam moves it causes distributer gear turn, & wear.
If you have installed a "cam button" then my thought is mute.
Also theres a brass bushing inside the distributor shaft housing.If its worn the side to side motion can make timing move alitttle.But not 90Degrees.
Anyway its something to think about when fine tuning it. :chevy
You say you have a new cam & heads,If you have high rate valve springs the pressure drives the cam forward .If the cam moves it causes distributer gear turn, & wear.
If you have installed a "cam button" then my thought is mute.
Also theres a brass bushing inside the distributor shaft housing.If its worn the side to side motion can make timing move alitttle.But not 90Degrees.
Anyway its something to think about when fine tuning it. :chevy
Safety Car
Joined: Jan 2001
Posts: 4,386
Likes: 689
From: Boise ID
Re: (scorp508)
I found this one out the hard way. My tach had been completely screwy over 3500 for about a week and the car seemed 'off'. Then one day I gave it come gas and then she came to a stop. 3 of those razor sharp teeth had sheered off completely so that the distributor wasn't spinning anymore.