Fine Tuning a 750 Demon
Team Owner
Joined: Sep 2001
Posts: 30,958
Likes: 0
From: Middletown Connecticut
Cruise-In IV Veteran
Originally Posted by gtr1999
So questions:
1-With these carbs screwing in richens or leans the mix?
2-Is the bog caused by the mix screws,float level?
3-Same for the surging @30-40 mph?
This is the first time setting up a Demon so would appreciate your input.
2- might be relative to the power valve, try tightening the spring in front of the throttle linkage.
3- same
if you want, I can swing down tomorrow and have a look
Burning Brakes
Joined: Mar 2001
Posts: 1,211
Likes: 5
From: Nashville TN
what size motor are you running ? I found that the Demon 750 on ly 355 had top be richened up substantially to get it to run right. Out of the box these carbs are lean on smaller motors. I ended up re-jetting and going to a different power valve.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
gtr -
Try dropping your primary and secondary jet sizes by 2 sizes from the stock size. I think you'll find this will make the carb respond a little crisper, and it will clean up the performance issue you're having.
Try dropping your primary and secondary jet sizes by 2 sizes from the stock size. I think you'll find this will make the carb respond a little crisper, and it will clean up the performance issue you're having.
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Sounds like your idle circuit is excessivly rich. I am in the process of fixing this on mine. You can go one of three routes, or all of them if you feel like it.
Increase the idle air bleed diameter in .003" increments.
If you have the "idle-eze" baseplate open it up some
Drill a couple small holes in the primary butterflies
I am still looking for idle air bleeds in 2 packs from BG. You have to buy 10 at a time from Holley. :rollseyes:
Also, check idle vacuum, you may be low enough your pulling a slight amount of fuel from the power valve circuit.
Also, these carbs need a good vacuum signal to operate effectively with larger cams, this is the reason they suggest the 18-24 degrees initial advance. I will tell you that they do like the extra advance, a lot. I run my ignition locked out at 36 degrees.
I have found that 1/2 turn out is what works best for my setup, but it is still very rich.
Good Luck.
Increase the idle air bleed diameter in .003" increments.
If you have the "idle-eze" baseplate open it up some
Drill a couple small holes in the primary butterflies
I am still looking for idle air bleeds in 2 packs from BG. You have to buy 10 at a time from Holley. :rollseyes:
Also, check idle vacuum, you may be low enough your pulling a slight amount of fuel from the power valve circuit.
Also, these carbs need a good vacuum signal to operate effectively with larger cams, this is the reason they suggest the 18-24 degrees initial advance. I will tell you that they do like the extra advance, a lot. I run my ignition locked out at 36 degrees.
I have found that 1/2 turn out is what works best for my setup, but it is still very rich.
Good Luck.
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Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Gary -
Although the BG literature will tell you to use BG jets due to their "superior quality," the Holley jets are fully interchangeable and recommended. Drop the jet size on both sides (pri & sec) by 2 sizes from your stock setting and verify that your PV is a 6.5. Idle mixture screws tend to run best at a setting between 1/2 turn and 7/8 turn out from lightly seated, keeping all 4 balanced along with the balancing of the primary and secondary throttle blade angles as you've done. This should give you a nice crisp setup. Try it out and see how the car behaves. Tag me back if you need additional info or assistance.
Although the BG literature will tell you to use BG jets due to their "superior quality," the Holley jets are fully interchangeable and recommended. Drop the jet size on both sides (pri & sec) by 2 sizes from your stock setting and verify that your PV is a 6.5. Idle mixture screws tend to run best at a setting between 1/2 turn and 7/8 turn out from lightly seated, keeping all 4 balanced along with the balancing of the primary and secondary throttle blade angles as you've done. This should give you a nice crisp setup. Try it out and see how the car behaves. Tag me back if you need additional info or assistance.
