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i called demon on this one and they dont know what to tell me. i have a 350 bored .060 over. speed demon 650 carb. edelbrock airgap rpm intake, dart platinum 200cc heads. comp cams XE268 cam, 1.5 roller rockers. Petronix Flamethrower II ignition, MSD wires. Timing is correct and vacuum gauge is rock steady. the problem is--i have to adjust the idle mixture screws every 4-5 days or so. when the problem begins the car doesn't want to idle on its own... dies when i come to a stop unless i powerbrake it to about 1,000 rpm. the car seems to do OK at idle if i take it out of gear., it will idle but even then it's a little skitish and the idle will fluctuate like its starving. every time it has done this i play around with the idle mixture screws giving it more gas every time. this always fixes the problem and the car runs perfect, i've done this about 4 times now. each time lasting 4-5 days like i said. i only have to adjust the idle mixture screw anywhere from 1/16'' - 1/8'' each time. it hasnt always been the same barrel each time. i'll adjust one..wait and if it doesnt work i turn it back until i get to the right one. and when i do it fixes the problem instantly. any ideas? with this kind of maintenance im thinking about going back to a holley!
 

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Save a horse, Ride a Cowboy.
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If you keep turning them out, sooner or later they will fall out. :D

Check when the spark advance is BEGINNING to advance. You probably need to adjust it so that there is no centrifugal advance starting before 1000 rpm.

Your springs are probably not snug to the posts, or too soft of springs giving you a fluctuating advance at idle rpm.

Slow idle to your usual. Read timing mark. Read again at 1000 rpm.
Start by putting slightly tighter springs on for a test.
Read idle timing again. If it changed, reset by turning distributor.

Bet it helps. :thumbup:

added=
One other thing to consider is if your throttle plate is too close to the end of the transition slot.
 

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I had similiar issues like this with my demon. Remove the bowls and check the idle passages, you`ll likely find they have machining debris in the passages. Other than that, check the fuel pressure, make sure it`s not too far on the high side, when my fuel pressure was giving me fits, I`d have to do the same thing your doing, it was always due to the pressure overcoming the idle circuit and it would force fuel through the circuit resulting in a flood.
 

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Discussion Starter #5
i'll get a fuel pressure gauge for it then...what range should i be within? it has the stock mechanical fuel pump, come to think of it its the oldest thing on that car besides the body.
 

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Discussion Starter #6
xntrik.... the distributor springs/spark advance are all good things to check and i'll have to check it out. but my thing is...why does the car run great and respond well to adjusting the idle mixture screw. to me.. if i have 'too much' advance then i would never be able to get the car to run correctly until i addressed that problem. i remember having that problem on my stock motor that needed lighter springs... nothing i did made it run right until i replaced them. i just dont get why the idle mixture fixes the problem but only lasts for a number of days. as far as too much fuel pressure... (i'll check it) but it's a stock mechanical pump. this is a higher performance motor that if anything should require more pressure. i know its not getting too little because once again i can fix the problem and have it working great for about 4-5 days. whats the likely hood of these idle mixture scews vibrating out of adjustment? the car does vibrate quite a bit with the cam and large exhaust. which the exhaust by the way is hardmounted (welded) straight to the car. i get a lot of noise and vibration from inside the car because of it. i didnt mention it because the "techs" at demon seemed to think that was impossible.
 

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Sagacious said:
xntrik.... the distributor springs/spark advance are all good things to check and i'll have to check it out. but my thing is...why does the car run great and respond well to adjusting the idle mixture screw. to me.. if i have 'too much' advance then i would never be able to get the car to run correctly until i addressed that problem. i remember having that problem on my stock motor that needed lighter springs... nothing i did made it run right until i replaced them. i just dont get why the idle mixture fixes the problem but only lasts for a number of days. as far as too much fuel pressure... (i'll check it) but it's a stock mechanical pump. this is a higher performance motor that if anything should require more pressure. i know its not getting too little because once again i can fix the problem and have it working great for about 4-5 days. whats the likely hood of these idle mixture scews vibrating out of adjustment? the car does vibrate quite a bit with the cam and large exhaust. which the exhaust by the way is hardmounted (welded) straight to the car. i get a lot of noise and vibration from inside the car because of it. i didnt mention it because the "techs" at demon seemed to think that was impossible.
Adjusting the idle mixture is NOT "fixing the problem". It is temporarily changing it.

You said =
"OK at idle if i take it out of gear., it will idle but even then it's a little skitish and the idle will fluctuate like its starving. every time it has done this i play around with the idle mixture screws giving it more gas every time. this always fixes the problem and the car runs perfect.....it hasnt always been the same barrel each time."

So this is a 4 corner idle?

answer =
You really can't tell if the dying or fluctuating is because of varying fuel or varying timing. All you said was that in gear you had to pedal it to 1000 to keep her motating.
If opening them up cures the problem, open them up another 1/4 turn and just drive it. seriously. see what happens. You said 1/16 to 1/8",,,,, Idle screws are not that sensitive.

I was thinking inconsistent timing, causing a rpm/ vacuum fluctuation, or sticking advance. When you are on the edge, 2* makes a difference.
The distributor might also be worn, or the advance sticking. Put in tighter advance springs as a test, see if it idles better.

Look at the transition slots.

Check for crud in the internal passages, but it changes hole to hole. So that means pulling it apart, flush and blow with carb cleaner through all passages.

If you're worrying about the idle screws turning, gently turn them in and count the turns. Clean off the outside with electrical contact cleaner and put a dab of silicone on the screw...or get some "torque seal" (like whiteout)marker and put a dab on them. Typewriter whiteout works too. Put enough on there so that if the screw moves you can see the crack in the whiteout. Trouble is it heat shrinks a lot and might fool you.
 

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Demons are famous for not being clean when brand new. machining debris can give you fits.
 

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Technical Support Barry Grant
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Sagacious,


A few quick questions about the carb:

1) Have you checked to make sure it was clean?

2) Have you tightened the bowl screws?

3) Were the screws in the same location as they were when the car ran best previously?
 
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