Warm Engine Idle Issue

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73Swinger18

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Camarillo, CA
Please help me from throwing in the towel. I already threw all the parts I could think of at it. My 318 stumbles at idle when it is heat soaked then sits at idle for several minutes (not hot outside here in S. California). Here's the details on the engine...

- Original 318 bored 0.020
- Original heads with larger valves (1.88/1.60)
- Classic Holley 600
- Weiand Stealth intake manifold
- Comp Cam 20-223-3
- Hedman long tube headers
- One 20 micron filter right before the carb
- 5/16" rubber fuel line from chassis to mechanical pump/pump to carb
- 18 deg initial, 36 deg mech, vac connected
- 11 inches of vac while in drive
- 68 jets, 4.5 power valve
- Mixture screws out 2 turns (highest reading on vacuum gauge)
- Carb float levels adjusted just under threads of opening
- Carb heat insulator spacer installed
- Idle set at 800 RPM
- Curb idle set to where I am still on the idle circuit
- Spark plugs read good. Almost a full turn of color on the base with no heavy soot. Porcelain clean.
- Compression check is 135 PSI on all cylinders with no vacuum leaks.

I have swapped out the distributor, rotor, cap, wires, coil, module, ballast resistor, plugs, and fuel pump with no changes. I also rerouted the fuel line away from the hot areas. Nothing seems to help. I ordered some insulation sleeves for the fuel lines today in hopes it may be from vapor lock. When it is stumbling, I look down the carb and the fuel squirts in normally. I figured if it was vapor lock it might spit abnormally. Idle/mixture screw adjustments have no affect while it is stumbling as well. Timing is steady. Any other ideas for me to try while waiting on these insulation sleeves? Thanks for your help. This is the final discrepancy I need to work out with this car and I can finally relax and enjoy it.
 
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Please help me from throwing in the towel. I already threw all the parts I could think of at it. My 318 stumbles at idle when it is heat soaked then sits at idle for several minutes (not hot outside here in S. California). Here's the details on the engine...

- Original 318 bored 0.020
- Original heads with larger valves (1.88/1.60)
- Classic Holley 600
- Weiand Stealth intake manifold
- Comp Cam 20-223-3
- Hedman long tube headers
- One 20 micron filter right before the carb
- 5/16' rubber fuel line from chassis to mechanical pump/pump to carb
- 18 deg initial, 36 deg total timing, vac connected
- 11 inches of vac while in drive
- 68 jets, 4.5 power valve
- Mixture screws out 2 turns (highest reading on vacuum gauge)
- Idle set at 800 RPM
- Curb idle set to where I am still on the idle circuit
- Spark plugs read good. Almost a full turn of color on the base with no heavy soot. Porcelain clean.
- Compression check is 135 PSI on all cylinders with no vacuum leaks.

I have swapped out the distributor, rotor, cap, wires, coil, module, plugs, and fuel pump with no changes. I also rerouted the fuel line away from the hot areas. Nothing seems to help. I ordered some insulation sleeves for the fuel lines today in hopes it may be from vapor lock. When it is stumbling, I look down the carb and the fuel squirts in normally. I figured if it was vapor lock it might spit abnormally. Idle/mixture screw adjustments have no affect while it is stumbling as well. Timing is steady. Any other ideas for me to try while waiting on these insulation sleeves? Thanks for your help. This is the final discrepancy I need to work out with this car and I can finally relax and enjoy it.
Is this stumble happening upon accelerating from a dead stop or just while stopped and idling in gear ?
 
Is this stumble happening upon accelerating from a dead stop or just while stopped and idling in gear ?
At idle and a little hesitation when accelerating. Once about 1500 RPM or so, it will stop. Drive it around for a few minutes and it behaves for a while, then acts up again once standing at a stop for a few minutes.
 
Sounds like possibly the fuel boiling? My 340 dart was doing the same thing on warm day, had hesitation and not idling well, I opened the hood and took the air cleaner off with it running and had fuel coming out the accelerator pumps on my holley carb. I shut It off and fuel continued to squirt out both accelerator pumps. After it cooled down it stopped and was back to normal.
 
Sounds like possibly the fuel boiling? My 340 dart was doing the same thing on warm day, had hesitation and not idling well, I opened the hood and took the air cleaner off with it running and had fuel coming out the accelerator pumps on my holley carb. I shut It off and fuel continued to squirt out both accelerator pumps. After it cooled down it stopped and was back to normal.
I thought this may be the reason too. I have installed a carb spacer which helps and adjusted the fuel floats. I adjusted them to where fuel would trickle out of the fuel bowl holes only when rocking the car. I haven't noticed fuel dribbling onto the plates after the float adjustment. I may consider getting a carb heat shield as well.
 
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What cam are you running and how much timing is the vacuum can pulling?



EDIT I see the cam still how much timing does the can pull?
 
yep, fuel percolating. Move the filter to vertical beside the water pump and install the thicker carb base gasket. That's what the factory engineers did. Cured my 67 273 too.
 
I was going to mention that I didn't see anything about a carb insulator, but others did already.
 
yep, fuel percolating. Move the filter to vertical beside the water pump and install the thicker carb base gasket. That's what the factory engineers did. Cured my 67 273 too.
I'll try moving the filter and I think I'll order a heat shield for the carb too. Any other ideas? Thanks everyone.
 
yep, fuel percolating. Move the filter to vertical beside the water pump and install the thicker carb base gasket. That's what the factory engineers did. Cured my 67 273 too.
Speaking of fuel filter location. Anyone recommend putting it before the pump like the manual states? It would be even further away from any heat source.
filter.jpg
 
Factory set up doesn't always apply. I wouldn't want it or those rubber hoses near a header tube.
 
The can does not pull any advance at idle.


I know the vacuum cam doesn't pull any timing at idle. You posted you had 36 total with the vacuum advanced hooked up. If the can pulls 10, then you have 26 total and that isn't enough.


That's why I asked. So again, how much timing does the can pull?
 
I know the vacuum cam doesn't pull any timing at idle. You posted you had 36 total with the vacuum advanced hooked up. If the can pulls 10, then you have 26 total and that isn't enough.


That's why I asked. So again, how much timing does the can pull?
I installed an FBO limiter plate and springs. 18 initial, 36 mechanical at 2900 RPM, 60 with vac.
 
You may need a stronger Power Valve. measure your vacuum with the engine as lo as it will idle, divide your vacuum reading in half, that is the size power valve you will need.
 
You may need a stronger Power Valve. measure your vacuum with the engine as lo as it will idle, divide your vacuum reading in half, that is the size power valve you will need.
The lowest vacuum reading I got was 10 that's why I put a 4.5 valve in. Are you recommending I go lower?
 
I installed an FBO limiter plate and springs. 18 initial, 36 mechanical at 2900 RPM, 60 with vac.
Unless I misread...you say you haven't adjusted the floats yet?
If you haven't...adjust them right away before doing anything else, it be just pushing fuel in and cleaning up with rpm and more timing..
 
Unless I misread...you say you haven't adjusted the floats yet?
If you haven't...adjust them right away before doing anything else, it be just pushing fuel in and cleaning up with rpm and more timing..
I did adjust the floats. I adjusted them right below the threads on the cap screws. I am thinking I may need to bring them up a little?
 
It was the floats! Worried more about them boiling over after shutdown, I adjusted them right below the threads. I brought them up to where they were trickling out pretty good at idle and it fixed it. Thanks everyone. I am stoked.
 
Seeing all the stuff you have replaced and tried, I'm thinking an inlet needle and seat leaking slightly allowing the fuel level in the bowl to creep up, at idle, and finally start to cause overly rich mixture. Driving will erase the problem as the floats are then calling for more fuel and the small leak is insignificant. Had this very condition on a /6 decades ago.

Find someone with a ignition scope and tailpipe sniffer. Then you can rule out ignition OR a fuel going rich/lean condition. Just my 2c.
 
Damn it! I was premature with the victory. It's still there. I could have sworn the float fixed it because it had corrected it on spot and during my test drive. I'm going to pull the float bowl and inspect....
 
Seeing all the stuff you have replaced and tried, I'm thinking an inlet needle and seat leaking slightly allowing the fuel level in the bowl to creep up, at idle, and finally start to cause overly rich mixture. Driving will erase the problem as the floats are then calling for more fuel and the small leak is insignificant. Had this very condition on a /6 decades ago.

Find someone with a ignition scope and tailpipe sniffer. Then you can rule out ignition OR a fuel going rich/lean condition. Just my 2c.
As you can see, the adjustment didn't fix it. What should I be looking for when I take this float off? How did you find yours?
 
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