Topic: Which jet for my 1978 CB750F?

[Foyil100]

Hey all,

I've looked everywhere for this answer and seem to be getting conflicting opinions.. I have a stock 78' CB750F with the stock 4 into 1 exhaust, however, I am running pods. If it helps for sea level purposes, I live in So.Cal (Newport Beach).  I just rebuilt my carbs and upped the jets from 105 (stock) to 110, based off the suggestion of another post I just ordered some 125 main jets - is this a good choice?  I plan on syncing the carbs next weekend but should I do it with the 110s or the 125s?

Thanks!

P.S. if it helps at all, the bike runs really rough, some carbs spit fuel, and i can't really get it to run smoothly..

[Hon3ybadger]

P.S. if it helps at all, the bike runs really rough, some carbs spit fuel, and i can't really get it to run smoothly..

Spitting fuel from where?
Round top or PD carbs?

[martin99]

Potential popcorn thread...

Here is what I did. This is assuming your carbs are squeaky clean.

Fitted larger main jets (in my case 130, I would think 125 would be fine to start with).
Move needles up by one notch from stock. PD41s have adjustable needle clip positions which makes life a lot easier. If you have PD42s you will have to shim them. Do a search, plenty of 'how to's' on the site.
I stayed with stock pilots, but have them turned out 2 full turns (as opposed to 1 3/4 stock).

How does it run? Pretty good, anything above 4K rpm and it absolutely flies. Lower down pretty acceptable too but will never be as smooth as a standard set-up. I suggest you have a good read of all the pod threads on here, there are loads of variables and you need to be prepared to invest the time in getting it right. There's a lot of folk on here who don't think it's worth the hassle, and I can understand why.

Before you do anything you need to find out why your bike is running rough. 3K tune up is a must, verify your ignition system is operating correctly, timing is good and you have a good spark on all cylinders. Make sure you haven't got any air leaks on the intake rubbers. With your carbs, check your accelerator pump is working properly and that the fast idle cam is set. Did you pull the pilots? You can't clean them properly without doing so and if clogged they will affect the idle. Synching is the very last thing you do, it shouldn't matter what size main jets you have in there as they will not be coming into play at idle speed.

Best of luck and welcome

[Foyil100]

Thank you VERY much for the detailed post!

First to address Badger - it is spitting from the the open side of the carb where the pod goes, almost sounds like a backfire every maybe 15-20 seconds but this is only happening on 2 of the carbs.

Oldskoolnuts - I will go through the checklist that you provided and make sure everything is within spec, I've also been meaning to check the rubber on the intakes for leaks. One thing I did notice which I thought was really odd was when I put my hand up against carb #4 (farthest right) it has really strong pull or vacuum whereas the other 3 really didn't at all.. If I covered up carb #4 with my hand the engine would die but when I did it to the rest nothing happened. This led to to do a compression check and my findings were as follows: #1) 150psi, #2) 145psi, #3) 132psi, #4) 130psi..so everything there seemed okay.

I clearly have some more research to do as I am unfamiliar with the "fast idle cam". I did however just pull the pilot jets last night (did not do this when I originally rebuilt the carbs) and it seemed like 2 of them were really clogged so I cleaned them out with a thin wire and carb cleaner and I'll reinstall the carbs this weekend and see how she idles.

Thanks again for the help! I'll keep this updated!