Rejetting my Frankenbike...

[Beasty]

Several years ago, I purchased a box o'parts of a CB750 chopper (true chopper. '77 CB750F frame cut up to rake neck, forks for the 750F with 6" longer tubes). The engine it came with is a CB750K with a set of '77 CB750K Keihin Carbs. Had to do a complete rebuild of the Carbs as they were so gummed up even the throttle wouldn't move. So it has stock jets, but Pods and drag pipes (essentially headers).

I saw the post with the cheat sheet of how to calculate the jetting required for the carbs, but am unsure what the stock jets are, and haven't seen what that would be for that year carb.

 

[Beasty]

The gent in the picture is a friend of mine who is a master mechanic (can fix anything from large dump trucks to small 1000W generators), and did a great deal to assist me in this build

 

[Misfit]

Nice looking bike!

 

[Beasty]

Nice looking bike!

Thanks!

 

[Beasty]

Well, the jets that came with the rebuild kit were 115's. The ones that were already in it were 115's. Still trying to determine what is the best jet to use for this beast.
Should I bump up[ to 120 or 125?

 

[dirtdigger]

What does it run like? Take it for a ride. I normally take for a ridec go out on highway wideopen throttle and start closing the choke slowly. If it starts to pick up you are lean if it bogs down it is close to right or on the rich side. Then you can do part throttle runs and use the choke to get close on the needle jet position. Drag pipes are very inefficient and you will have to richen it up to make it run good at full throttle. I have had to take 836 engines out to 140's with drag pipes to get decent power. Actual 4into1 headers are going to be the most effiecient and normally only need a 1 or 2 jet size over stock and make more power then drag pipes. Drag pipes look nice for the looks but you will actually lose power over a good 4 into 1. Only way to find correct jetting is to ride it and see what it needs, there is no one size fits all for particular exhaust etc., to many things affect jetting that you start with stock then adjust for what it wants.

 

[Old_Crow]

For the drag pipes, you can add a bit of restriction to the pipes which adds back pressure to help the bike bike run better. I started out with a flat washer welded to a small bolt. Looked sort of like a lollipop. I kept playing with it until I was down to just a 1" long 5/16" bolt stuck through a hole in the end of the pipe. Took away all of the hesitation without affecting the sound or top end performance.

 

[dirtdigger]

I guarantee the drag pipes in general and "restricted" drag pipes are down on power from a 4 into 1 or an original set of HM300 pipes. Yes there are things to make them run better but the drag pipes lack all of the scavanging of the 4 into 1 and the 750 likes longer pipes if you have 4 into 4 pipes. Drag pipes are for looks and sound not performance for sure. but hey run what you like

 

[Beasty]

There was a bogging down when you would throttle up before it would take off again. We just got the 125's in today.
It is running very well. Still getting a bit of a drag but not enough to care about. This is very different from my old V-Twins. Wow.
Added a couple new bolt-ons. New seat, and a Fork Tool Bag I got from Honda originally for the VTX.
Oh, and heat shield stick-on to protect the leg from hot oil tank.

 

[dirtdigger]

main jets are only part of the solution, needle jet position plays more of a part than anything at less than full throttle. Sometime the slide needs to be milled a little off the bottom to change the shape of the cutout in the slide., affecting lower mid range.