Still backfiring Mighty Demon 750
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Still backfiring Mighty Demon 750
Talked with Demon tech. I was advised to advance the timing and back off the idle mixture screws and the idle speed screw. I did just that, the timing is set at about 15 BTDC and the idle speed screws were turned out one full turn. The car still backfires at low rpms and stumbles if not stalls when getting off the clutch in first gear.
I turned the mixture screws out to two full turns to see if it was a lean problem and it backfired even easier. I pulled off the aircleaner to find that the carb was on fire. So the last one wasn't a lean backfire, kinda makes me wonder if they were all rich backfires...?!
I pulled the carb off and pulled the bowls off, the correct jets were installed. I also pulled out a spark plug, black, the engine was obviously running rich. But I ran these spark plugs with the Q-Jet for 1,000 miles so I'm not sure which carb turned them black. Could this be the cause of a backfire?
I'm so lost guys.
I turned the mixture screws out to two full turns to see if it was a lean problem and it backfired even easier. I pulled off the aircleaner to find that the carb was on fire. So the last one wasn't a lean backfire, kinda makes me wonder if they were all rich backfires...?!
I pulled the carb off and pulled the bowls off, the correct jets were installed. I also pulled out a spark plug, black, the engine was obviously running rich. But I ran these spark plugs with the Q-Jet for 1,000 miles so I'm not sure which carb turned them black. Could this be the cause of a backfire?
I'm so lost guys.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
What you have is lean popping. Does the MD have a power valve? I'm running the Speed Demon. It's stock power valve is a 6.5. I run a 5.5
You have to have the Acc arm linkage adjusted to dribble at the most minute movement of the throttle.
My Demon had trouble from day one and I tried drilling out the squirters to get the biggest fastest shot. But the problem was the pink plastic cam had been installed wrong from the factory. So it wasn't in the position to give the squirt at the first movement of the throttle cable.
Start your car and have it idling. looking in the carb and just move your linkage just a hair so your not causing a full squirt. You should see a wet dribble. Without the extra enriching your going to get the backfire.
Good luck it can be fixed. It's a learning curve
You have to have the Acc arm linkage adjusted to dribble at the most minute movement of the throttle.
My Demon had trouble from day one and I tried drilling out the squirters to get the biggest fastest shot. But the problem was the pink plastic cam had been installed wrong from the factory. So it wasn't in the position to give the squirt at the first movement of the throttle cable.
Start your car and have it idling. looking in the carb and just move your linkage just a hair so your not causing a full squirt. You should see a wet dribble. Without the extra enriching your going to get the backfire.
Good luck it can be fixed. It's a learning curve
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
what do you mean idle speed screw? that should be screws....you are making the same adjustment to both correct?
have you checked to make sure your accelerator pump is adjusted correctly ?
and where are you on the float settings?
have you checked to make sure your accelerator pump is adjusted correctly ?
and where are you on the float settings?
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
you know i was thinking about this .....double check all of your spark plug wires and make sure they didn't get switched somehow.....
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Originally Posted by bobs77vet
what do you mean idle speed screw? that should be screws....you are making the same adjustment to both correct?
have you checked to make sure your accelerator pump is adjusted correctly ?
and where are you on the float settings?
have you checked to make sure your accelerator pump is adjusted correctly ?
and where are you on the float settings?
I have not checked the accelerator pumps. I pulled the carb off of the car. I'm going to watch the video and adjust them tomorrow, any advice on those?
Floats are set to the middle line by the sight glass.
Originally Posted by bobs77vet
you know i was thinking about this .....double check all of your spark plug wires and make sure they didn't get switched somehow.....
Originally Posted by gkull
What you have is lean popping. Does the MD have a power valve? I'm running the Speed Demon. It's stock power valve is a 6.5. I run a 5.5
You have to have the Acc arm linkage adjusted to dribble at the most minute movement of the throttle.
My Demon had trouble from day one and I tried drilling out the squirters to get the biggest fastest shot. But the problem was the pink plastic cam had been installed wrong from the factory. So it wasn't in the position to give the squirt at the first movement of the throttle cable.
Start your car and have it idling. looking in the carb and just move your linkage just a hair so your not causing a full squirt. You should see a wet dribble. Without the extra enriching your going to get the backfire.
Good luck it can be fixed. It's a learning curve
You have to have the Acc arm linkage adjusted to dribble at the most minute movement of the throttle.
My Demon had trouble from day one and I tried drilling out the squirters to get the biggest fastest shot. But the problem was the pink plastic cam had been installed wrong from the factory. So it wasn't in the position to give the squirt at the first movement of the throttle cable.
Start your car and have it idling. looking in the carb and just move your linkage just a hair so your not causing a full squirt. You should see a wet dribble. Without the extra enriching your going to get the backfire.
Good luck it can be fixed. It's a learning curve
Thanks guys, really appreciate the help.
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
There is .025" between the primary accelerator pump arm and the throttle linkage that pushes it. So yes, the throttle does move a little bit before it pushes on the accelerator pump. Could this be the problem?
Also I noticed that at WOT the secondary pump in almost completely depressed whereas the primary pump still has a decent amount of travel. (Much more than .025") So is there a preload recommendation for these accelerator pumps?
Also I noticed that at WOT the secondary pump in almost completely depressed whereas the primary pump still has a decent amount of travel. (Much more than .025") So is there a preload recommendation for these accelerator pumps?
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by torqvette
So is there a preload recommendation for these accelerator pumps?
yes there is but i don't know what is off hand.....they talk about that on the video....your floats are in a good range ....and when the car is running they stay in that range ? if so we can rule out a fuel delivery problem.
check to make sure all those spark plug wires are on nice and snug
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Burning Brakes
Joined: Oct 2004
Posts: 801
Likes: 17
From: Middletown Delaware
I just sent my new speed demon carb back to demon for repair with the same issues that you have. The tech told me that if it backfires out carb with a large flame it is to rich. Any way I just talked to tech yesterday and they found a spring issue with the secondary float bowl float. Float was not shutting off causing a rich condition. I had brand new plugs in this engine before I started and they were carbon fouled in about an hour of troubleshooting. The tech that helped me at Demon was Eric at extention 121. Good luck
Instructor
Joined: Sep 2005
Posts: 180
Likes: 1
I also have a mighty demon on my 71 ls6. You need to use a vacum gauge to check your idle mixture screws. With 4 corner idle screws, mine are adjusted out only 3/4 of a turn!!! 2 turns is way rich!! If you dont have a vacum gauge, start at 1 turn and go in (Leaner)This may not be your only problem, but will result in the plugs being fouled out
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Well I got the carb working ok, I think. It's not tuned perfectly yet, but it doesn't backfire or stall at low rpm. I guess it was a problem with the primary accelerator pump. But now I think I'm having distributor issues.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
if you don't have a properly working distributor fine tuning of the carb will be imposible.........when a distributor is shimmed and lubed it will perform great
ok give us the details....final float setting....final turns out????
what was the magical fix the accelerator pump?
ok give us the details....final float setting....final turns out????
what was the magical fix the accelerator pump?
