some carb and ignition problems.....
Thread Starter
Racer
Joined: Sep 2002
Posts: 402
Likes: 0
From: Kingsport TN
some carb and ignition problems.....
guys, i have a 750 speed demon carb and stock hei with recurve kit.
when i accelerate, it does good until about 3000 rpms, where it starts to lose power and jerk, (sorta like cruising with too much advance), it doesnt do this if i dont go wot, and if i am cruising and go wot it does it also, if i accelerate slow, i doesnt do it either.
brand new fuel pump, with mr gasket fuel line, its running about 9 lbs pressure at idle.
i changed the sec spring the other day, it was too light, and it help somewhat, but the chug is still there, and it starts to backfire of some sort at higher rpms.
any help would be greatly appreciated, im going to the track thursday night.
could the popping be valve float?
[Modified by SharkBit, 4:48 AM 5/5/2003]
when i accelerate, it does good until about 3000 rpms, where it starts to lose power and jerk, (sorta like cruising with too much advance), it doesnt do this if i dont go wot, and if i am cruising and go wot it does it also, if i accelerate slow, i doesnt do it either.
brand new fuel pump, with mr gasket fuel line, its running about 9 lbs pressure at idle.
i changed the sec spring the other day, it was too light, and it help somewhat, but the chug is still there, and it starts to backfire of some sort at higher rpms.
any help would be greatly appreciated, im going to the track thursday night.
could the popping be valve float?
[Modified by SharkBit, 4:48 AM 5/5/2003]
Le Mans Master
Joined: Feb 1999
Posts: 6,807
Likes: 0
From: Hoosier
Cruise-In III Veteran
Re: some carb and ignition problems..... (SharkBit)
Not an expert, but........I'd be thinking it's too lean. I'm not familiar with Demon carbs but 9 lbs pressure sounds high. Most carbs run at 5-7 lbs. At excessive fuel pressure you could be overcoming the needles which is hiding the lean calibration at lower rpm's.
Burning Brakes
Joined: Feb 2002
Posts: 752
Likes: 31
From: Melbourne Australia
Re: some carb and ignition problems..... (SharkBit)
Mine had the same behaviour.....my ignition timing was way too retarded. This was because I stupidly set the idle timing with the vacuum advance still connected!!!. Brain fade....
Problem went away instantly after I fixed that, but not before I'd unsuccessfully changed the fuel pump, filters and rebuilt the carb.
The reason it behaves OK when not at WOT is that you have some vacuum advance to compensate for the retarded mechanical advance.
It might not be your problem at all, but I thought I'd mention it just in case.
Good Luck,
Joe
Problem went away instantly after I fixed that, but not before I'd unsuccessfully changed the fuel pump, filters and rebuilt the carb.
The reason it behaves OK when not at WOT is that you have some vacuum advance to compensate for the retarded mechanical advance.
It might not be your problem at all, but I thought I'd mention it just in case.
Good Luck,
Joe
Drifting
Joined: Jul 2002
Posts: 1,254
Likes: 1
From: Edinburgh
Re: some carb and ignition problems..... (jyounane)
Been having a similar problem with mine since fitting the 650 Demon but talking it through with Lars, we decided it was ignition since the distributor came out to fit the manifold (Lars quote; "most carb & fuel problems are ignition problems.") so I have been playing around with the timing without success so I am now moving to the electronic ignition conversion. At Lars suggestion I will put the old points operating distributor in and see if the problem goes then. I am also looking at the HT leads breaking down. Not a lot of help really except with the advise to look to your electrics rather than the carb. :cheers:
Thread Starter
Racer
Joined: Sep 2002
Posts: 402
Likes: 0
From: Kingsport TN
Re: some carb and ignition problems..... (Graemeinvette)
ive checked everything in distributor, except the pickup coil in the bottom of the distributor.
it did this with the old holley i had, but i sorta thought it was vaccum related (it leaked like a siph (sp?))
but now, i dont know, ive asked everyone and all they ask is dumb questions, to check, like the obvious. i run ported man. vaccum to the advance, so it doesnt matter if its hooked or not at idle.
any other suggestions? i dont think its too lean, the plugs are sorta sooty
it did this with the old holley i had, but i sorta thought it was vaccum related (it leaked like a siph (sp?))
but now, i dont know, ive asked everyone and all they ask is dumb questions, to check, like the obvious. i run ported man. vaccum to the advance, so it doesnt matter if its hooked or not at idle.
any other suggestions? i dont think its too lean, the plugs are sorta sooty
