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re jetting question

Clint

CB750 Enthusiast
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What's a good starting point for new jets.i put pod filters on and shortened my exhaust about 8in.do i need to change the pilot jets or just the main jets?
 
What are the jets in it right now? More than likely you will need at least one size bigger on the mains but maybe more. You may have to raise the needle too. Same with the pilot jet, you might have to but you might not if the motor is otherwise stock. Jetting is something you will just have to try things out and see what it needs.
 
does it bog, or have a flat spot? if it runs good, leave it. otherwise you will have to go up on the pilot jets and the mains like Digger said.
 
need a drastic increase in the pilot jets, the standard air breather has restriction normally, and the carbs are tuned to this because of it. you may not have to bother with the main jets, but certainly need to go a lot bigger on the pilot jets. cheapest solution? put the breather back on.
 
I would but i don't have the original air box.i have one for 76 carbs but not the 77 carbs that I'm running.
 
You are running 77 carbs? Does the accel pump work good from all the carbs? Make sure is gets a good shot of fuel out of all four or you will get a bog. I would check that then bump the main one size and raise the needle one slot and play with the idle mixture screw and see if you can get rid of most of the bog. It is getting hard to find pilot jets for the newer carbs so I would try adjusting first because I think you can get most of the bog out without a pilot change. I run the older round top carbs but I have stock pilot jets and running a 836cc motor with pods and 4 into 1 exhaust. Bottom line you gotta play with the jetting to see what your setup really wants because settings can be different from motor to motor and different exhaust lengths, etc.
 
I doubt they are all getting good fuel. These carbs sat for teen plus years.I basically just sprayed them with carb cleaner, stuck them on, and dumped some sea foam in the tank.I planned on cleaning them better later on.I was just getting anxious to fire it up and see if the motor ran.
 
Maybe I should start with cleaning them up first and see if I still need to change anything after that.
 
I would for sure do the clean first so you know all the jets are cleaned and not restricted. If the accel pump doesn't work right you will have a big bog and chances are the diaphram is hard and the check balls are suck or sticking. I would clean the carbs and go from there.
 
Diaphragm is shot.is falling apart.I'm surprised it even ran.are the check balls the things on the diaphragm housing and should they move?
 
There is a check ball in the diaphragm housing, its i the very bottom. You dont take it apart but you can check by blowing in one direction but not be able to in the other. They do get stuck very easy from old gas. you can try cleaning it out and then blowing out with air and see if it is working. There is also a check valve in the float bowl that needs to be checked like the other one. It should take very little to unseat the check valve, dont use a lot of air pressure. If it is stuck soak it and blow it out and do several times if needed to unstick the check ball.
 
Clint, just for FYI on my '77 F I am using 42 Idle and 120 Main Jets on a stock airbox but the MAC 4-1 pipes.

I have a spare airbox you can have if you want it. It has a crack along the left side that doesn't appear to be functional - just cosmetic,
 
I might take you up on that.I'm going to see if i can get it running good with the pods first though.i thing my main problem was the cracked up diaphragm though.I'm ordering a new one and cleaning all my jets up.i think that's going to help allot.thanks
 
Hey Clint,
This may just be redundant for me to say this to you but, ...Make sure you you clean out every little orfice with some kind of carb cleaner. I mean use the little straw that comes taped to the side of the can and look for the cleaner to come out somewhere else. I mean double and triple check each little orafice to see the cleaner come out somewhere else. Then use compressed air and do the same thing. I don't know how many times I thought I got every little passage then realized I missed one because the bike still didn't run well then had to take the carbs off again. It's easy to miss one.
I would have cleaned the carbs out first especially if they have been sitting around in the dust.
Don't know how many times you've this before and I don't mean to insult you but my suggestion is to be as thorough as possible when doing this. Of course, if you're going to be experimenting with jet sizes and adjusting the height of your needle, you'll be removing and reinstalling the carbs allot.
Ed
 
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