Rough running at steady throttle under light load

PostPost by: Larry C » Thu Jun 09, 2022 6:51 pm

I have a newly rebuilt twin cam. bored +.040, Sprint cams, QED head with Sprint valves and steel crank. Rebuilt Webers with Sprint spec jets, air correction jets, etc. The car runs fabulously when accelerating or when under load, such as a steady speed but up hill. When under light load, at rpms between 1,000 and 3, 000 especially, there is a popping from the exhaust and periodic lurching. Get back on the throttle and it is fine. Checked cylinder compression-all fine. Timing on the mark. New distributor with Petronix insert, new plugs. Plugs look good when pulled. Currently bewildered. I also balanced the carbs and used a ColorTune to set idle mixture.
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PostPost by: mbell » Thu Jun 09, 2022 8:51 pm

My guess is that your issue jet is too lean.

You likely need a richer jet, the question is which one... You can just buy one of to and try them out or take it to a rolling road and get it tuned properly.
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PostPost by: Mazzini » Thu Jun 09, 2022 8:57 pm

How accurately are your cams timed?
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PostPost by: 2cams70 » Fri Jun 10, 2022 9:16 am

Don’t try and change any calibrated parts in the carburettor until you are 100% sure what you have is as good as it can be and importantly that it’s been assembled correctly. As always in these situations ask yourself how was the engine running before and what changes have happened in the intervening time. Target your process of elimination as to the cause accordingly. Does sound like a mixture issue but that can be caused by many things and an air leak somewhere or incorrect assembly or damage is more likely than wrong jet sizes.
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PostPost by: Larry C » Fri Jun 10, 2022 10:31 am

Thanks for the suggestions. Cam timing looks excellent based on timing marks. The Webers had benefited with different jets-the mains and the air correction jets were not "Sprint spec." What is curious, and makes me think I may need to go through the process again carefully, is that the car ran like a charm for the first 50 miles or so after fitting the Sprint spec parts. I like the suggestion of a vacuum leak. That will be my next investigation.
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PostPost by: Craven » Fri Jun 10, 2022 11:40 am

Check your emulsion tubes are F11.
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PostPost by: TBG » Fri Jun 10, 2022 11:54 am

I have popped this on before but it may help. The QED suggested jetting was hopeless! D


Spot jetting.jpg and
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PostPost by: Larry C » Fri Jun 10, 2022 12:22 pm

TBG--Very interesting chart. I do have one question. In your chart, in the final line labeled "perfect", it seems the idle jet was changed from F50 to F59. Is that correct, or was that a one-digit-off typo?
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PostPost by: TBG » Fri Jun 10, 2022 4:42 pm

A typo I think but I will pull a jet and check for you! NB change to emulsion tube also. D
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PostPost by: mbell » Fri Jun 10, 2022 9:04 pm

The change from good running to not good running didn't happen to occur when e10 fuel was brought in did it?
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PostPost by: TBG » Fri Jun 10, 2022 9:14 pm

No - I use ESSO fuel that does not I think has this dreaded E10 nonsense in it. D
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PostPost by: Davidb » Sat Jun 11, 2022 5:59 pm

I went through something like this with mine recently. I found that setting up the idle/slow run screws was the answer-in combination with balancing the carbs properly. Remember, the idle/slow run jets effect the engine up to over 3000 rpm. I used an Empi carb synchroniser plus, and this was important I found, a timing light tachometer. (turn off the timing light to avoid distracttion-you just want a quickly readable tach). Having balanced the carbs, make slight adjustments to the slow run screws and Wait a five seconds or so for the idle to settle-don't rush it.
I finally spent quite a bit a time doing this and the car runs better than it ever has.
'65 S2 4844
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PostPost by: TBG » Sun Jun 12, 2022 8:07 am

Larry - it is 50F9, so it was a typo. Sorry. Corrected one below, D

Spot jetting.o2.jpg and
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