Hi Guys, I am looking for some advice on the correct jetting for my '76 CB750 after putting K&N filters on. It has been a nightmare since I started messing with it. I have got it to idle ok and take the throttle fairly ok but once I come close to 4000rpm it just tops out. I have raised and lowered the needle. Gone from #110 to #125 on the main jets. #40 on the idle jets and I still cannot get it to go above the 4000rpm. I have had the carbs out several times.
CB750 1976 Carb Jetting problem
- Thread starter Irish Dan
- Start date
How is your tune up? Have you checked points? timing? Does the ignition advancer work? How are the spark plugs? How is the engine condition?, checked compression lately? You have to start with a solid engine basics and good ignition system. When everything is verified, go for a ride and when riding in the the spot where it doesnt run right start closing the choke.....if it starts to pick up then its still lean, if it gets worse its already rich or weak spark or fouled plugs. you can use the choke at different throttle openings to start seeing if its rich or lean and adjust from there. Somethings not right because pod filters are easy to tune for on these carbs.
madmtnmotors
CB750 Addict
125 seems high for main jet. I'm running open velocity stacks with 836 pistons with a 4 into 1 barely baffled exhaust. I started with 125 jets after stepping up to 836cc but ended up stepping down until I got to 112.5 mains. As dirtdigger mentioned, check everything else first and make sure something else isn't making it worse.
Hi Dirtdigger,
I was running good on #110 main #40 idle jets with its own filter box. The only issue I had was a slight lag when hot leaving from the traffic lights. I will go back to all the basics first with the jetting and see what I have from there first. Thank you.
Surf Dog
CB750 Enthusiast
Start by reading the plugs, all of them at first, to make sure they're close to being the same. If they're constantly the same, you can read only one.
Only make one change at a time.
Also, these bikes don't like to go slow, especially when it's hot out. Mostly keep it above 40mph. City driving on a hot day will over heat it and it'll bog right down, like what it's doing now.
Go bigger on the main jets. I've got K&Ns and a very low resistance exhaust system, I've got the needles all the way up and the biggest main jets they make. Then I went 1 number hotter with the plugs. I had to go to racing clutch springs because with all the torque the clutch couldn't prevent from slipping when I hit second gear.
Depending on the backpreasure in your exhaust, you might not have to go all the way to the largest jets. I'd go up one size at a time. You don't want to blow your pistons or valves.
Always read your plugs after making any changes. And if there is even a little bit of dry black powder or scale on them, or if they're wet, clean them. My bike misfires if it's running even a little bit rich.
If you use the choke to start it, open it all the way as soon as it's running.
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Surf Dog
CB750 Enthusiast
Still, reading the plugs is the most accurate way of determining how an engine is running.
