Rough idle and dieseling?
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Melting Slicks
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Rough idle and dieseling?
I just installed an ATK 383 crate motor with a Trick Flow 650 carb in my '77. The car wont idle well below about 1100 RPM and diesels?. I used the stock Distributer. There is an exhaust leak just before the right side muffler which I think doesn't matter Any ideas?
Race Director
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did you set the total mechanical advance timing to 34-36 degrees as specified by ATK?
did you check the float levels of the primary and secondary float bowls?
did you adjust the idle mixture screws?
did you check the float levels of the primary and secondary float bowls?
did you adjust the idle mixture screws?
Last edited by MelWff; Mar 12, 2020 at 04:14 PM.
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You stated you used the stock distributor but didnt say which one, L48 or L82.
The l48 distributor has 22 degrees of mechanical advance so to achieve the 34 to 36 degrees ATK calls for you would have to set initial timing to 12 to 14 degrees and use non-stock advance springs.
The l82 distributor has 16 degrees of mechanical advance so to achieve the 34 to 36 degrees ATK calls for you would have to set initial timing to 18 to 20 degrees and use non-stock advance springs.
The l48 distributor has 22 degrees of mechanical advance so to achieve the 34 to 36 degrees ATK calls for you would have to set initial timing to 12 to 14 degrees and use non-stock advance springs.
The l82 distributor has 16 degrees of mechanical advance so to achieve the 34 to 36 degrees ATK calls for you would have to set initial timing to 18 to 20 degrees and use non-stock advance springs.
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
I would be willing to bet the secondary throttle plate is open too far. Trick Flow is like the Holley design. There is a transfer slot in the secondary system that needs to be adjusted for a 0.020 gap exposure. Rather than a bunch confusing paragraphs here, try YouTube for the procedure to correct this. The carb will have to come off and be flipped up-side-down.
Basically the carb is dumping extra fuel even though the primaries are almost closed at idle. Might have a bit too much vacuum advance too.
Basically the carb is dumping extra fuel even though the primaries are almost closed at idle. Might have a bit too much vacuum advance too.
Last edited by HeadsU.P.; Mar 12, 2020 at 06:36 PM.
Thread Starter
Melting Slicks
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Float bowls were rechecked. L48 distributor with 12 degrees initial and springs were changed per Lars several years ago. I did not check the secondary clearance issue though. Will get with you tube tonite!
Racer
Joined: Oct 2001
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From: Portage La Prairie Manitoba
ATK makes several versions of the 383...… which one have you got?
Thread Starter
Melting Slicks
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
I just have the basic long block where I reused the water pump, distributor and starter and had to buy a carb and air cleaner and manifold.
Dr. Detroit
Joined: Mar 2012
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From: New Braunfels Texas
Check all of the things that MelWff said to check and I will add put a vacuum gauge on it and see if the needle is steady. A bouncy needle is an intake or carb leak.....you can sniff out the exact location of the leak with an unlit Propane torch cracked open slightly......a vacuum leak will suck the Propane in and change the idle. This also works awesome for multiple carb motorcycles.....
Jebby
Jebby
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From: Virginia
Are you 100% sure that your vacuum advance is hooked up to manifold vacuum, and tested it with a vacuum gauge?
The idle mixture helped a bit, but what really got my engine cool and stopped the dieseling was setting the timing correctly, and then going back and setting it even more correctly.
The idle mixture helped a bit, but what really got my engine cool and stopped the dieseling was setting the timing correctly, and then going back and setting it even more correctly.
Thread Starter
Melting Slicks
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Thank you all who responded to this thread
I feel like an idiot but it seems like it may just be float level all along. With the air cleaner off I can see the carb spitting up. I lowered the levels slightly and the erratic idle and running on are all but gone. The cars pulls much stronger also. Should the levels be set with the car hot. Do I need a carb spacer, it's already very tight in there even with a drop down AC ?
I feel like an idiot but it seems like it may just be float level all along. With the air cleaner off I can see the carb spitting up. I lowered the levels slightly and the erratic idle and running on are all but gone. The cars pulls much stronger also. Should the levels be set with the car hot. Do I need a carb spacer, it's already very tight in there even with a drop down AC ?
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Carb is spitting? New ball game now.
Your accelerator pump arm is set too tight. Every little vibration is causing the pump to squirt / spit raw fuel.
Using a feeler gauge between the pump shot arm and the linkage, set that gap around 0.012 - 0.15.
More importantly, check it again at WOT, engine OFF. If its too tight, you can rupture the pump shot diaphragm.
You don't know how to make any adjustments to the pump, look on YouTube. Takes two wrenches, very easy to do.
Your accelerator pump arm is set too tight. Every little vibration is causing the pump to squirt / spit raw fuel.
Using a feeler gauge between the pump shot arm and the linkage, set that gap around 0.012 - 0.15.
More importantly, check it again at WOT, engine OFF. If its too tight, you can rupture the pump shot diaphragm.
You don't know how to make any adjustments to the pump, look on YouTube. Takes two wrenches, very easy to do.
Last edited by HeadsU.P.; Mar 13, 2020 at 12:59 PM.
Le Mans Master
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From: Cool Northern Michigan
Not true. Both idle & WOT should be checked.
At WOT, damage to the diaphragm if too tight.
At idle, the pump will dribble and cause lots of idling issues.
At idle, if too loose there will be a hesitation or bog off idle.
That's why there is some lee-way; 0.012 --- 0.015.
At WOT, damage to the diaphragm if too tight.
At idle, the pump will dribble and cause lots of idling issues.
At idle, if too loose there will be a hesitation or bog off idle.
That's why there is some lee-way; 0.012 --- 0.015.
Last edited by HeadsU.P.; Mar 13, 2020 at 09:05 PM.
Le Mans Master
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From: Cool Northern Michigan
Holley manuals, Summit Racing video and the like claim 0.015 is perfect.. What the feeler gauge reads at idle (or closed throttle) maybe different than the reading at WOT.
Using a feeler gauge between arm and adjustment screw. There should be some drag with the feeler gauge at 0.015.
However, I have seen videos claiming 0.012 clearance. Less than that and I get a dribble spit, on my Holley 4150 at idle. I assume Trick Flow is very similar.
You will just have to play with it to see what works best for your carb. If its a double pumper, you have to set both pump arm clearances.
But regardless, your carb should not be spitting fuel at idle.
Using a feeler gauge between arm and adjustment screw. There should be some drag with the feeler gauge at 0.015.
However, I have seen videos claiming 0.012 clearance. Less than that and I get a dribble spit, on my Holley 4150 at idle. I assume Trick Flow is very similar.
You will just have to play with it to see what works best for your carb. If its a double pumper, you have to set both pump arm clearances.
But regardless, your carb should not be spitting fuel at idle.
Last edited by HeadsU.P.; Mar 14, 2020 at 08:48 AM.
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Thread Starter
Melting Slicks
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Ok .....Idle is set for now. I have to tackle the electric choke which I think is over choked?
Race Director
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Not true. Both idle & WOT should be checked.
At WOT, damage to the diaphragm if too tight.
At idle, the pump will dribble and cause lots of idling issues.
At idle, if too loose there will be a hesitation or bog off idle.
That's why there is some lee-way; 0.012 --- 0.015.
At WOT, damage to the diaphragm if too tight.
At idle, the pump will dribble and cause lots of idling issues.
At idle, if too loose there will be a hesitation or bog off idle.
That's why there is some lee-way; 0.012 --- 0.015.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
You can insist on whatever you believe. but I can vouch that the pump adjustment at idle is just as important as WOT. Some, say its more important and not to worry about WOT clearance.
One adjustment is for preventing diaphragm damage (WOT)
One adjustment is for bog or hesitation issues (closed throttle).
The WOT clearance is seldom an issue. I have never ruined a pump diaphragm. The closed throttle clearance setting assures that a shot of fuel is instantaneous when the primary or secondary throttle begins to move. However, you do not want the setting too tight though or the vibration from the engine, transfers to the pump arm and dribbles raw fuel into the carb causing horrible idling issues.
It takes a steady hand with a feeler gauge to set it just right, making adjustments with two small wrenches. As the feeler gauge is inserted at idle, you will note that a shot of gas will make the RPMs stumble briefly. Persistence and fine tuning will pay off.
One adjustment is for preventing diaphragm damage (WOT)
One adjustment is for bog or hesitation issues (closed throttle).
The WOT clearance is seldom an issue. I have never ruined a pump diaphragm. The closed throttle clearance setting assures that a shot of fuel is instantaneous when the primary or secondary throttle begins to move. However, you do not want the setting too tight though or the vibration from the engine, transfers to the pump arm and dribbles raw fuel into the carb causing horrible idling issues.
It takes a steady hand with a feeler gauge to set it just right, making adjustments with two small wrenches. As the feeler gauge is inserted at idle, you will note that a shot of gas will make the RPMs stumble briefly. Persistence and fine tuning will pay off.