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Big Dirt Bike Jetting Help


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I have a CB750 Hillclimber set up to hill climb that I am having problems with.

1976 Cb750 with a 836 Big Bore kit

Straight Pipes

Pod Filters

I am running a 120 Main

#40 slow

Idle jet is set at 1 1/12 turns

My needles are set 1 notch below center (4th notch)

Altitude 202 Meters

Humid- Hot as @#$% (upper 90s most of the time)

I do run 103 octain gas in it. And I am running a slightly hotter plug. D7EA instead of a D8EA

The bike idles nice and once you are past 1/2 trottle runs nice. But if you roll off the throttle and get back on it-it stumbles. As long as the bike is reving high- It will run good also. I have tried 133 main and it is to rich but 120 seems to be too lean. If I run with the choke set 1/2 way it runs great. One of my questions is- Do I need to change my needle back to the number 3 notch. And Should I go for something between the 120 and 133?

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Can anybody help Me.­čśĆ I am at a loss for a fix. ­čśĆ I have tried every combination I can think of. I have had conflicting information from other supposed experts that run this style of bike. Thanks for looking ALL.

B

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Yes they are stock carbs. It happens on flat ground and on the hill. If I roll off the throtle and try to come back on it it loses power and has to clean itself out. If I keep it on the trottle and don't come off it runs. But the bike has to be reving very high.

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I started at 110 and worked my way up to 133. I had it at 133 and a guy that races these size bikes said I was too rich and to drop the needle one notch. He told me to change my needle setting to make it leaner so I would have better bottom end response. So I lowered the needle down one notch. And he told me to open my idle jets to 1 1/2 turns. They were at 3/4. Before I changed It would blow black smoke. It ran good as long as I kept the revs extreamly high. It ran so good that I spun the back tire and ruined a new terraflex tire and tube. So the other day I changrd the jets to a 120 and made sure the bike was warm and tried some holeshots. It was ok until I got off the throttle and got back on it then it stumblled. I then tried slowly turning up the choke it still did it untl I got to half choke then it ran good. Am I correct in assumeing that I need to richen up the mix? But When I go to 133 (which is the next siz jet- I am not able to find any between) it is to rich and I go back to having a problem with it running. Since I changed the jet size should I go back to the middle notch on the needle?

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I know nothing of that carb but will offer a few thoughts;

1: Very common misconception that bigger engine needs bigger jets. Its actually the other way around, bigger engine draws more air in, the air sipons gas so it automatically adjusts the fual air mix for the bigger engine, in addition as the volume of air increases (with bigger engine) the efficency of the siponing effect increases. Perfect jetting with the bigger engine should require a very slight decrease in jet size from perfect jetting at original engine size. As the original setup was intended for milder use, good economy and smooth application of power in a corner I would suspect original Honda jetting to be a little on the lean side so stock may be just fine.

2: If it where mine I would toss stock and adapt to a set of Kehein FRC carbs from a newer than 2002 sport bike. First because I understand them second because they have an accelerator pump which I would want on a hill climber and third they are designed to be mounted on a slope as the sport bike tech of that era had the cylinders sloping so on the CB which is more upright the carbs would be closer to lever when headed up hill and fourth, bigger venturi area = more power

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