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Thread: Demon Carb Help

  1. #1
    Taylorman
    I just ran my new carb for the first time yesterday. Its a Mighty Demon 850 on a single plain Victor intake on my 455. I read through the manual to make sure the initial setup was correct before i installed it. Bolted up and went to the river. It ran great, i was really pleased. Picked up 100 rpms over my old setup with tunnel ram and dual edelbrock 600's. However, i can't seem to get it to idle down. I like my boat to idle at 1000 rpm's. Its idling at 1200. The Demon carb has a butterfly adjustment screw on the primary and secondary butterflies. I adjusted the one on the primary and it did not have an effect on idle speed. I could not slow the idle down. In the manual, they stessed several times that the butterfly position is really important or else and off idel stumble could occur. So how do i fix this?
    Also, at wot, i saw 4800 rpms, then if i let off the throttle just a hair, the rpms would go up to 4900. Whats up with that?
    Kevin

  2. #2
    oldbuck40
    I just ran my new carb for the first time yesterday. Its a Mighty Demon 850 on a single plain Victor intake on my 455. I read through the manual to make sure the initial setup was correct before i installed it. Bolted up and went to the river. It ran great, i was really pleased. Picked up 100 rpms over my old setup with tunnel ram and dual edelbrock 600's. However, i can't seem to get it to idle down. I like my boat to idle at 1000 rpm's. Its idling at 1200. The Demon carb has a butterfly adjustment screw on the primary and secondary butterflies. I adjusted the one on the primary and it did not have an effect on idle speed. I could not slow the idle down. In the manual, they stessed several times that the butterfly position is really important or else and off idel stumble could occur. So how do i fix this?
    Also, at wot, i saw 4800 rpms, then if i let off the throttle just a hair, the rpms would go up to 4900. Whats up with that?
    KevinKevin it sounds like on the idle deal i know on mine that the butterflies were real close to the manifold and gasket. i used a 2 inch spacer on top of the intake and noticed the gaskets were a little small and causing the butterflies to drag.??? had to trim a little,, so far as the rpm deal it may be a little fat on the top end,,call demon they have allways been very helpful...

  3. #3
    HP350SC
    Is the throttle linkage allowing the carb. to return to base idle? Make sure the tang is resting on the idle speed screw. Then make sure you have full travel at WOT, but not so much that it pulls butterflies past vertical. You want to make sure your pedal throw matches the carb throw.

  4. #4
    GofastRacer
    I just ran my new carb for the first time yesterday. Its a Mighty Demon 850 on a single plain Victor intake on my 455. I read through the manual to make sure the initial setup was correct before i installed it. Bolted up and went to the river. It ran great, i was really pleased. Picked up 100 rpms over my old setup with tunnel ram and dual edelbrock 600's. However, i can't seem to get it to idle down. I like my boat to idle at 1000 rpm's. Its idling at 1200. The Demon carb has a butterfly adjustment screw on the primary and secondary butterflies. I adjusted the one on the primary and it did not have an effect on idle speed. I could not slow the idle down. In the manual, they stessed several times that the butterfly position is really important or else and off idel stumble could occur. So how do i fix this?
    Also, at wot, i saw 4800 rpms, then if i let off the throttle just a hair, the rpms would go up to 4900. Whats up with that?
    Kevin
    You adjusted the front butterflies and the idle didn't drop, did you back off the secondaries?? Push the throttle pedal all the way down(I hope you have a stop)and look at the butterflies, they should be straight up, if they went over center that means your not at WOT, if you are straight up and you gain RPM when you back off the carb may be too big???, been down that road before!.. :notam:

  5. #5
    REAL HP
    I just ran my new carb for the first time yesterday. Its a Mighty Demon 850 on a single plain Victor intake on my 455. I read through the manual to make sure the initial setup was correct before i installed it. Bolted up and went to the river. It ran great, i was really pleased. Picked up 100 rpms over my old setup with tunnel ram and dual edelbrock 600's. However, i can't seem to get it to idle down. I like my boat to idle at 1000 rpm's. Its idling at 1200. The Demon carb has a butterfly adjustment screw on the primary and secondary butterflies. I adjusted the one on the primary and it did not have an effect on idle speed. I could not slow the idle down. In the manual, they stessed several times that the butterfly position is really important or else and off idel stumble could occur. So how do i fix this?
    Also, at wot, i saw 4800 rpms, then if i let off the throttle just a hair, the rpms would go up to 4900. Whats up with that?
    Kevin
    You made a smart choice dropping the 600's, thats for sure. What size cam do you have in your 455? Who recomended that carb for you?

  6. #6
    Taylorman
    Ok, on the idle deal, i backed the secondary idle screw off 1/2 turn and that fixed it. On the wide open throttle deal, the butterflies are opening all the way and are vertical at wot. The linkage does not allow them to open past the point where they are vertical so the possibility of them opening to much does not exist. Not sure what the deal is there. I'll have to figure it out in time i guess.

  7. #7
    GofastRacer
    Cool, that's one problem solved! Now on to the next!..

  8. #8
    Taylorman
    To remove the float bowls, is there a way to drain the fuel out before you remove them so fuel doesn't spill all over the place?

  9. #9
    oldbuck40
    To remove the float bowls, is there a way to drain the fuel out before you remove them so fuel doesn't spill all over the place?get a small bowl/cup or somthing to catch the gas and pull one of the lower bowl bolts!

  10. #10
    Taylorman
    Alright guys, i need some help here. I still have this problem. Where should i start. Could it be it going lean or rich at that rpm? Should i re-jet. Im no expert on reading plugs. Runs great throughout the rpm range. It just slows down a couple hundred rpms when the throttle is to the floor. When i let up just a hair, the rpms go back up.

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