Visit The Dirt Forum for More Information

Author Topic:   need help pulling vacuum
Dirt Roller

Total posts: 21
posted July 25, 2002 10:21 PM
i have to pull 17in of vacuum at 900rpm. rules are cast iron heads and intake, no porting, rodchester 2gc unmodified. i run 24deg initial timing 35 total, my modified carb will nly pull 15in, my stock 16.5, are there any tricks i don't know about, i don't like to lock the mechanical advance out because it gets hard to start, i have tried leaning out the idle circuits, i run variable duration lifters with zero lash. i have tried two cams, both should pull and the motor was just freshened. any help will be greatly appreciated.

Dirt Roller

Total posts: 20
posted July 26, 2002 09:19 PM
we had to go to a 450/461 lift to get ours up

Dirt Newbie

Total posts: 2
posted July 27, 2002 01:14 AM
you could back the rocker arms off some more, thats what weve had to do

Dirt Full Roller

Total posts: 55
posted July 31, 2002 09:51 AM
Have you tried adjusting the idle screws?
You should be able to increase vacauum enough
with idle mixture ***** to pull up over 17.

Dirt Full Roller

Total posts: 70
posted August 01, 2002 01:34 PM
Our track doesn't have a vacuum rule, so I haven't tried these. But here are some ideas that may work:
- higher compression ratio theoretically should pull more vacuum at idle
- find a cam will lowerer overlap
- advance the cam a few degrees
- find a cam with less duration and higher lift....I believe higher duration causes lower vacuum, lift shouldn't effect it as much
- or if they use your tach for checking rpm, change the tach settings to a six cylinder engine. Then 900 on your tach would really be 1200 rpm. (You would have to calculate your max rpms as well). 1200rpm should give more vacuum.
Other people may have tried these, so feel free to comment as to whether they will work or not.

Dirt Roller

Total posts: 21
posted August 01, 2002 09:50 PM
thanks for the suggestions, i can't change the tack, they use a handheld, i have tried leaning the idle air ***** down, as well as at their highest vacuum point. i am at 0 lash, have advanced cam, swaped to a smaller cam. i can barely pull it when i put on a track legal(stock 500cfm) rodchester. thanks for the help, tim

Back to the Archives