I'm usually over in the C1/C2 forum, but you guys seem to have more handle on the performance end so here goes.

The engine is a 383 with 10.7 to 1 comp and CC XR282HR cam. Heads are RHS aluminum with 200 cc runners. Long tube headers. Carb is 750 cfm. 4.11 to 1 rear end, M22 Muncie. Altitude here is 5000 ft. The Holley 770 SA carb I pulled off had the opposite problem, I could never get the idle/transition right but the top end was magnificent with the lightest spring and #82 secondary jets.

I have the Speed Demon idle and transition working perfectly. #74 primary jets. No bog on the take off, and decent idle at 1000 rpm. Vacuum is 11" @ idle, and I have a 6.5 power valve. I can run down the road at 3000 rpm/55mph and all is well.

When I bury the throttle and the secondaries start to open, I get a huge bog so I started playing with the springs. The lightest spring was the worst and I went progressively all the way to the heaviest spring and since the secondaries are no longer opening at this point, it doesn't bog anymore. I had #81 jets in the secondary and went to 83's with no change. I ran out of time for today so no more testing. Tomorrow I think I'll go to an 8.5 PV and bigger jets to see what happens.

It sure seems lean in the secondary to me. I've read and followed Lars' papers, the BG website, and everything I can find on the net with no success. I am stumped. Can anyone point me in another direction?

I really do appreciate any help you can give.

 

Bumping for some help.

 

I've run several Speed Demons and Mighty Demons on the dyno here in Denver (5000 ft), and every one of them has run lean. I've had to increase jet sizes 4 to 6 jet sizes on both primary and secondary side to get the WOT A/F mixture optimized, along with bumping the squirter size up 2 to 4 sizes. Don't be afraid to go rich - rich won't cause problems, but lean will. Bump those jet sizes up.

Lars

 

Thanks Lars. I'll keep pressing ahead with it until it beats me or I run it through a crusher.

 

I had a similar problem with my demon. I went to a bigger shooter (2 sizes) and the problem is better now. I havn't had a chance to revisit to finish things up though. Good luck!

Jim

 

This morning's first test was 88's in the secondary with no other changes and the issue improved a lot, but was still slightly starving at 5000 rpm +. I came back and went to a 8.5 PV, problem came back and. Now back to the original PV of 6.5 and 89's secondary.

Rolling out now to try it.

Haven't messed with squirters yet, but the problem is up high in the rpm range, so I will press ahead with what has improved the problem.

 

You need to richen the primary side, too - the primary side has a greater effect than the secondary side. Keep the size split between primary and secondary side at 8 jet sizes - only richening the secondary side won't get you there. If you have 88s in the secondary, you need 80s on the primary to keep the air/fuel ratio through both ends of the carb the same. Rather than doing that, I'd run a 78/86 combo.

 

Thats great advice on the 8 jet size thing. I have a 750 speed demon too (that lars has been a godsend on getting the bog straightened out) but I dont think that it has 8 jet sizes between front and back.

 

I went up on the primaries 6 sizes (74 to 80, 2 at a time) and all I got was a stumble coming off the line on anything bigger than the 74's I started with. Maybe squirters? The acc. pump discharges if you just touch the arm.

 

Lars, did you find the same results at a lower elevation say in New Jersey...

 

I would go bigger squirters before changing the linkage. I had a small block with an 850 on it and it did the exact same thing. The bigger jets were needed but till I changed My squirters The stumble would not go away. Once I got the squirters right and the jets it was unbelievable. Your headed in the right direction exactly as Lars first posted here.

 

I'm getting closer. I went back and re-set the timing to make sure it was correct, and found it a couple of degrees too light. The slight miss is when I the secondaries first open up and goes away if I keep my foot in it. I'm now at 80/86 on the jets and will go up to 80/88 and try again in the morning. It is far better than it was, so I'll keep after it.

 

BG does recommend to increase the timing and squirters size to correct a stumble on their troubleshooting page..... what do you have your timing set at...

 

Lars does however, the entire forum trusts his judgement over Barry Grant any day.Most of the time the stumble is caused by not enough fuel getting shot into the carb when the secondaries open. The only fix for that is bigger shooters.

 

Lars is the man here when it comes to carbs, I've been playing around with jets and squirter sizes for a while now....

 

"90% of all carb problems are ignition problems." (Quote from my "Tuning for Beer" seminar series).

Make sure you have adequate initial timing - you should have initial timing in the 18-degree range with total around 36. It also helps to be running a 12-degree vacuum advance, matched to the engine's vacuum level, and hooked up to manifold vacuum. This will go a long way in curing low rpm stumbles.

Lars