Engine running rough

Rigz

Member
Mar 1, 2018
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orlando
Hi guys iv been having some trouble with an engine that I got recently. It's a pk80 type with 40mm intake 40mm stroke. Balanced crankshaft and just generally cleaned up ports with an rt carb and Dax banana expansion chamber pipe. It's got about 150 miles on it. The trouble I'm having is when I transition from high rpm like say 5000 down 3000 or so it runs rough and misfires. Not 4 stroking actually missing and hesitating pretty bad. Now if I run the motor at low rpm at just a low cruise it's fine but when I give it some gas and rev it up, going back down to cruising speed runs rough. When I give it a lot of gas it takes off and runs really smooth over 5k rpm. I haven't had it past 6500rpm.
I'm thinking the plug might be going (champion rj7y )....the cdi might be going. Maybe it's a timing issue or that 40mm intake is too big and dumping too much air/fuel out the carb at low rpm. I'm also thinking that dax pipe might be too big and designed for really high rpm. I also removed one of thw bottom gaskets and added a thinner one to the top which did not do anything accept increase compression a bit.Iv also skirted the piston at the intake side. Any thoughts would be greatly appreciated
 

crassius

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Sep 30, 2012
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if it has always been that way, I'd look at carb - if it just started, I'd look at last thing I changed
 

Rigz

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Mar 1, 2018
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Its akways been that way. I was thinking that but I had that carb on another engine that ran great. Iv tried several different jets and the one in it now works the best. A little on the rich side but not to much. The other motor had about 450 miles on it with that carb, maybe its time to open it up and clean it out really good although it looks pretty clean taking the bottom off and looking in at the needle and flot. I know that it doesn't take much to clog them up.
 

crassius

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Sep 30, 2012
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in latest rainy weather, mine are growing green fuzz on mains pretty quickly - alcohol added to gas here pulls water out of the air

was last motor same stroke & ports as this one?
 
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Rigz

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Mar 1, 2018
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The last motor was a 32mm intake and 38mm stroke. I fried the piston rings due to to much rpmThat motor didn't start really running its best till over 200 miles so I was thinking maybe the problem will go away with more breaking in although it has not gotten any better since i first rode it.and the old motor never had that problem at all.The plug also came from the other motor.
I really like that plug but it does have a good 600 miles on it. The motor starts up super easy and idles really nice and steady at about 1600-1800rpms so I'm kinda stumped. I do have an nt carb that I might try just to compare and I also have a new DAX rt carb that I could compare the old one too. It's the Runtong carb that most people call the "racing" carburetor. I'm also not using ethenol free gas. Iv been using 93 octane due to the increased compression which is the same gas I used in the other motor. I have the upgraded cdi with the red wire.
 

crassius

Well-Known Member
Sep 30, 2012
4,032
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USA
the 40mm motor can be felt at seat of pants to have more power than the 38mm

you may need to try different carb or diff jetting
 

Rigz

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Mar 1, 2018
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orlando
I wanted to try the makuni 16 or 18 but I can't find an intake that will fit. Do you know where I can find one for the 40mm intake? I like the adjustable air/fuel ratio. Seems like fine tuning will be easier. I have a bunch of jets but they are not marked or marked wrong. So using my wire Guage bits I measured them and the Jets that I ordered that are supposed to be 61-65 are all the same size. I can fit a .072 bit in all of them. The .073 is to small and the .071 is equally too big in all lol. I used the jet that came in my carb from dax that was .069 and that was waaaaay to rich the bike was 4 stroking like crazy and smoking so I tried the .071 and that was better but still rich so I tried the .072 better still but still a bit rich. So I soldered and drilled it out to .073 but my bit broke rrrrrrr so I tried the .074 and broke that too gggrrrr. So I tried the .075 and got that to work but that wound up being way to lean and the motor was really hard to start and ran very bad. So I'm sure the .073 is the right one but I broke it so I need to order a new set
 

Rigz

Member
Mar 1, 2018
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orlando
That's a good idea thanks. I was using a pin vise becuase I don't have a chuck that small but next time I will do that. It was my first time working with bits that small and fragile
 

Rigz

Member
Mar 1, 2018
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orlando
The problem wound up being the spark plug. I got an E3.10 and the problem went away. I also tried an NGK BPMR6A which ran even better. I think the E3.10 is a colder plug becuase it took longer for the motor to get fully warned up. I couldn't find an actual heat range chart for the E3 but the the E3.10 compares with the champion CJ7Y which is a colder plug than the NGK BPMR6A.