Trouble setting timing
Thread Starter
Racer
Joined: May 2007
Posts: 296
Likes: 4
From: London Ontario
Trouble setting timing
I am trying to set the timing in a modified 427/390hp. In order to make it run my initial timing needs to be set really high, like 30 deg. The carb is running rich and I am not reading very much vacuum.
Engine has comp thumpr cam, 10.2CR, ported factory heads. 650 holley (Qjet replacement)
Engine has comp thumpr cam, 10.2CR, ported factory heads. 650 holley (Qjet replacement)
Pro
Joined: Oct 2006
Posts: 734
Likes: 0
From: Bryan Texas
Vac may be fine, although I have heard of thumper cams making enough vacuum to run power brakes, not sure 5 would do it. I'd check the intake and carb for open ports just to make sure. It should run with 10-12 initial timing. Have you tried idling the carb up and then run with lower advance?
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Thread Starter
Racer
Joined: May 2007
Posts: 296
Likes: 4
From: London Ontario
The cam I have is just the small Thumpr Cam (~.530/.545 lift). I tried to order the baffle before but they were out of stock, then i forgot about it so I'll ry that when i get home. Hopefully thats all it is, if not I'll try all the other stuff people suggested. Thanks
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
That vacuum reading sounds low- I have the factory ZL-1 cam- and it's like .600 and 270* and I pull 14-15" at idle. I'm also running about 16* initial with 41* total advance on 110 octane fuel. 12.5 compression too.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
With idle vacuum that low, even if vacuum advance was connected, it probably wouldn't pull any timing anyway. I wouldn't worry too much about the carb running rich right now, it probably has just opened the power valve because of the low vacuum. (it thinks it is running under high load because of the low vacuum)
Give us some information to work with. My first question is always: Did it ever run well with the present combination of parts and tune?
Scott
Give us some information to work with. My first question is always: Did it ever run well with the present combination of parts and tune?
Scott
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
Forget what I said before.
That little one shouldn't need that kind of timing and your vacuum should be a lot better.
Has this car ever run properly since the cam was installed, or are you just now getting it running?
If its just gone together, you might want to double check your valve adjustment.
That little one shouldn't need that kind of timing and your vacuum should be a lot better.
Has this car ever run properly since the cam was installed, or are you just now getting it running?
If its just gone together, you might want to double check your valve adjustment.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
At this point you are dealing with an unknown problem. You need to systematically test for and rule out possible causes until you discover what is wrong. Don't take shortcuts. Don't skip things because they are difficult to perform. Baby steps, baby steps, we'll get this figured out sooner or later. Probably won't be me, I'm already starting to feel like I'm getting in a little over my head here, but there are some real smart dudes on this forum who will get you squared away.
Scott
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
Couple things. Since running rich, maybe a carb float bowl overflow problem, as a vacuum leak usually causes a lean condition. Also, you mentioned 5 psi vacuum(assuming negative), which is normally measured in inches Hg mercury. If this is the case, about 10" Hg. Maybe not the greatest, but if a big overlap cam, probably not far off.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Couple things. Since running rich, maybe a carb float bowl overflow problem, as a vacuum leak usually causes a lean condition. Also, you mentioned 5 psi vacuum(assuming negative), which is normally measured in inches Hg mercury. If this is the case, about 10" Hg. Maybe not the greatest, but if a big overlap cam, probably not far off.
Scott
