Topic: K8 running lean / jetting questions.

[Captain Ron]

Hey guys so I have a 1978 K8 engine in a f3 frame and it’s currently running lean ( especially 3/4 ) pipes are blueing quickly and spark plugs white. I also can’t tell if there’s an exhaust leak or if that’s just from the pipes burning (I attached photos). I’m running a k&n filter in stock airbox. 35/110 jets in 42b carbs. I’m not sure what kinda header my 4-1 is but I’ll also attach a photo. Floats checked with clear tube method and look to be at 3mm from gasket mating surface. Any help would be greatly appreciated, this forum ( and Hondamans book ) are the only reason I’ve made it this far.



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[newday777]

Welcome aboard Ron
With 2 cylinders lean I would venture to say you have plugged fuel ports in the carbs. The PD42A carbs are more involved than earlier carbs.
How did you clean your carbs, as in what process? Spray clean only?

[newday777]

BTW the header looks (hard to see underneath with your pictures to know for sure) like it might be a 78 F2 header collector with possibly a Kerker slip on muffler.

[Captain Ron]

Welcome aboard Ron
With 2 cylinders lean I would venture to say you have plugged fuel ports in the carbs. The PD42A carbs are more involved than earlier carbs.
How did you clean your carbs, as in what process? Spray clean only?

I entirely disassembled the carb rack and cleaned all parts before reassembling. I replaced the brass (stock mains had been replaced with 130s). With that said I used jets from 4-1 and after some more reading I’m realizing it might make a difference if I use genuine Keihin brass. I’m going to order some new jets from jetrus and was trying to decide if I should try and go up to a 40 slow and maybe a 115 main. With conversion and shipping to Canada it’s going to be about 100 bucks to replace them and I’d rather not replace them tooo many times lol


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[robvangulik]

You cleaned all parts, but did you also clean the passages the jets and airholes lead to? Compressed air and a few soft copper strands can do wonders....

[Captain Ron]

You cleaned all parts, but did you also clean the passages the jets and airholes lead to? Compressed air and a few soft copper strands can do wonders....

Yahh I cleaned air holes with a high e guitar string and carb cleaner. When I tear them down again (potentially to rejet) I will double check everything is clear with some compressed air as well


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[newday777]

You cleaned all parts, but did you also clean the passages the jets and airholes lead to? Compressed air and a few soft copper strands can do wonders....

Yahh I cleaned air holes with a high e guitar string and carb cleaner. When I tear them down again (potentially to rejet) I will double check everything is clear with some compressed air as well


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Spray carb cleaner is most likely your problem. 
Either a deep soak or ultrasonic cleaning if old gas was left in the carbs for a long time to clear out the hardened gas.

[newday777]

You cleaned all parts, but did you also clean the passages the jets and airholes lead to? Compressed air and a few soft copper strands can do wonders....

Yahh I cleaned air holes with a high e guitar string and carb cleaner. When I tear them down again (potentially to rejet) I will double check everything is clear with some compressed air as well


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What was the outcome of your carb problems? You should reply back as an update so others can learn that read through looking for answers to their problems.

[Captain Ron]

I rejetted them using genuine keihin brass and that helped a bit. #2 was still running a bit lean but I rode it for the rest of the season. Going to dig back into them in the next few weeks. Will update with a solution


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[Ellz10]

Subscribed - curious to know the outcome here.

[denward17]

Subscribed ...........