340 IDLE ISSUE

chargere 2 · Sep 2, 2016

OK GUYS THIS ONE HAS ME SCRATCHING MY HEAD, I HAVE A 340 IN A 72 DEMON , REBUILT (CAM ,PISTONS , REBUILT IRON HEADS , NEW SUMMIT BRAND (WEIAND ACTION PLUS COPY ) THE ISSUE I HAVE IS THAT WHEN THE ENGINE IS IDLING AT 700 ,IF I SLOWLY RUN THE THROTTLE UP TO SAY 2000 AND SLOWLY COME BACK DOWN , THE IDLE GOES UP TO 800-850, UNLESS THE THROTTLE IS SNAPPED BACK, THEN IT MAY RETURN TO THE 700 ,IF THE IDLE IS ADJUSTED WITH OUT THE SNAP BACK OF THE THROTTLE (WHEN IDLING AT THE 800-850 )THEN WHEN THE THROTTLE IS SNAPPED BACK IT WILL GO DOWN TO 600 ISH, THE THINGS I HAVE CHANGED TO TRY AND FIND THIS ISSUE (AFTER 4 DIFF CARBS , AND UNHOOKING ALL LINKAGE)-IGN BOX,COIL ,BALLAST, RESISTOR, COMPLETE DIST (AFTER CHANGING SPRINGS AND ADDING A FBO LIMITING PLATE TO THE ORIGINAL) THERE ARE NO VACUUM LEAKS AND I HAVE DISCONNECTED THE PCV VALVE. NOW THIS INTAKE WHEN I REMOVED IT TO LOOK FOR A VACUUM LEAK I FOUND A SMALL CASTING HOLE BETWEEN #6 AND#8 CYL. I HAD THAT FIXED (WELDED) AND CHECKED FOR ANY OTHER ISSUES (NONE FOUND) NOTHING I HAVE DONE CHANGES THIS ISSUE AND WHEN DRIVEN ,COMING TO A STOP THE IDLE IS NOTICEABLY HIGHER UNTIL THE THROTTLE IS PUSHED DOWN AND LET SNAP BACK - THE CAR IDLES WITH A SMOOTH 17.5 INCH VACUUM AND AT 20DEG INT TIMING WITH THE ADVANCE NOT STARTING UNTIL 1400 RPM

idrivemopar · Sep 2, 2016

What carb are you using? Thermoquad by chance?

chargere 2 · Sep 2, 2016

idrivemopar said:
What carb are you using? Thermoquad by chance?

I HAVE HAD A THERMO , A HOLLEY AND 2 DIFFERANT STREET DEMONS ON IT

PurpleBeeper · Sep 2, 2016

From what you're describing, this sounds like a throttle cable issue. It could also be a weak carb return spring. This assumes that all 3 carbs were 100% free-moving.

I have seen "smutz" build up on Holleys when they've had the idle set the same for 20 yrs. or more. The oily, dirty "smutz" builds up along the "throttle blade spring" & the idle hangs up at that location, even if you're turned the idle screw down (out) to a lower idle setting.

I've also seen this issue on my own six pack. When I added the nitrous plates that raised the carburetors, changing the angle the throttle cable comes in at vs. the center carb. It confused the heck out of me. I cleaned & cleaned the "smutz" off the outside of the center carb....it was better, but still hung up. My final solution was to add a small inner spring in addition to the stock 6-pack carb return spring. Now it's a little harder to push down on the gas pedal, but the idle drops back down exactly where I set it & doesn't hang up any more. Maybe an extra spring would work for you? Or shorten one loop of the return spring to make it stiffer?

You could also have a throttle blade slightly hanging up on a carburetor gasket, but that doesn't seem too likely if you've changed carbs 3 times.

If you have a bunch of carburetor spacer on the engine, maybe put a stack of washers under the throttle bracket to "raise it up even with the carb" and use two longer intake bolts?

chargere 2 · Sep 2, 2016

PurpleBeeper said:
From what you're describing, this sounds like a throttle cable issue. It could also be a weak carb return spring. This assumes that all 3 carbs were 100% free-moving.

I have seen "smutz" build up on Holleys when they've had the idle set the same for 20 yrs. or more. The oily, dirty "smutz" builds up along the "throttle blade spring" & the idle hangs up at that location, even if you're turned the idle screw down (out) to a lower idle setting.

I've also seen this issue on my own six pack. When I added the nitrous plates that raised the carburetors, changing the angle the throttle cable comes in at vs. the center carb. It confused the heck out of me. I cleaned & cleaned the "smutz" off the outside of the center carb....it was better, but still hung up. My final solution was to add a small inner spring in addition to the stock 6-pack carb return spring. Now it's a little harder to push down on the gas pedal, but the idle drops back down exactly where I set it & doesn't hang up any more. Maybe an extra spring would work for you? Or shorten one loop of the return spring to make it stiffer?

You could also have a throttle blade slightly hanging up on a carburetor gasket, but that doesn't seem too likely if you've changed carbs 3 times.

If you have a bunch of carburetor spacer on the engine, maybe put a stack of washers under the throttle bracket to "raise it up even with the carb" and use two longer intake bolts?

WELL, I AM SURE ITS NOT THE THROTTLE CABLE , LIKE I SAID ,THATS THE FIRST THING I UNHOOKED THINKING IT WAS HANGING UP, IT DOES THIS WITH NO CABLE OR KICKDOWN LINKAGE HOOKED TO ANY OF THE CARBS - THE CARBS (EXCEPT FOR THE THERMO) ARE ALL LESS THAT 2 YEARS OLD

mopar 3 B · Sep 3, 2016

Turn off your caps. Have you checked carb to intake alignment? Are the return springs pulling straight or at a angle? Carb gasket clearence ok?Almost anything can put carb linkage in a bind.

chargere 2 · Sep 3, 2016

mopar 3 B said:
Turn off your caps. Have you checked carb to intake alignment? Are the return springs pulling straight or at a angle? Carb gasket clearence ok?Almost anything can put carb linkage in a bind.

Yes i have checked alignment on all 4 carbs, i have moved the springs around at differant angles and when this happens the throttle is on the screw, almost like a vacuum leak but goes away when the throttle is snapped

65-440 · Sep 5, 2016

same issue, 4 different carbs ? ... have you sprayed around the intake/head mounting surface ? make sure theres not a small leak there ..

chargere 2 · Sep 5, 2016

65-440 said:
same issue, 4 different carbs ? ... have you sprayed around the intake/head mounting surface ? make sure theres not a small leak there ..

yep ,i even checked the intake and head surface to be sure no leaks ,you can see were the gaskets have left a mark when compressed (nice even marks all the way around the ports), not sure what i am missing with this issue ,i think maybe i am going to get rid of the Chinese intake and but an edlebrock to see if it fixes it ,if not it will rule it out

philcollins · Sep 6, 2016

What is your initial advance? I had the same problem, was running stock distributor. (was bought with) now running 20 initial, 14 mecanical, and idle nice at 850 rpm.

chargere 2 · Sep 6, 2016

philcollins said:
What is your initial advance? I had the same problem, was running stock distributor. (was bought with) now running 20 initial, 14 mecanical, and idle nice at 850 rpm.

well like i stated, the int timing is 20 deg , 34 mech - with the advance not starting until 1400 rpm, i have changed springs and added a fbo limit plate and also installed a complete different dist, with no change, when the idle does go up to the 850 the timing stays dead on 20 deg, i really thought it was in the timing but when i checked it at the high idle it is always at 20 deg

Bigal75 · Sep 6, 2016

Have you ever checked the timing when it's doing this? Perhaps the advance mechanism is binding.

Darthomas · Sep 6, 2016

Original factory thermos had a "hot idle" bleeder thingy on the bottom near the middle, rectangular.....supposed to open when things got hot and keep idle clean.
I dont think mine ever worked.
Ran rich at idle in high heat.

chargere 2 · Sep 7, 2016

Bigal75 said:
Have you ever checked the timing when it's doing this? Perhaps the advance mechanism is binding.

yep, stays at the set 20 deg int when at the high idle

340 IDLE ISSUE

chargere 2

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idrivemopar

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chargere 2

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PurpleBeeper

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chargere 2

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mopar 3 B

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chargere 2

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65-440

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chargere 2

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philcollins

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chargere 2

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Bigal75

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Darthomas

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chargere 2

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