Topic: carb jets for 78 cb750f

[seat cover man]

what size jets are original in my 78 cb750f....im having a hard time getting this thing to idle. i have to just about choke it to death or start it with starting fluid and keep it reved wide open to run? i "cleaned" (to the best of my ability) the carbs and noticed the two jets were 125's and 35's....i dont have a factory air box for it either....please help

[mick750F]

  The original main jets are 105 while the slow jets are 35. In order for these carbs to work properly all jets must be clear, all passages must be clear and the accelerator pump must be working properly. The slow jets which are a "push in" type must be carefully pulled in order to clean them. It sounds as though you have at the very least clogged circuits somewhere, either jets or passages. If you're trying to run it without any type of filter you aren't going to be getting satisfactory results either.

   You can down load a factory manual for your bike in the FAQs section of the forum.

Mike

Changed jet size to correct 105. Obviously I had a brain cramp posting 110 as I've had a few different 77/78 K/F carbs apart.

[seat cover man]

i have some pods will that work for the jet sizes in it? it also has a header with no muffler

[bollingball]

Until you get a proper exhaust and air filter you are going to have a lot of fun trying to tune this bike. Well fun if you enjoy taking carbs on & off +1 on what mick750F
said carbs probably need better cleaning. Start at the gas cap and clean down to end of overflow drain tubes. Not saying it can not be tuned for this setup just not the way I would do it. Way to aggravating

[Ryan6838]

Stock mains on the f are 105 and k had 110. My bike had pods when i got it, bike needed jetting but im going to a stock airbox with k&n in it. Theres alot of argument over pods, from what ive seen there a pain to tune and inconsistent. What filter and exhaust do you want to run? Tune for what youll run.

[seat cover man]

well i can buy a set of the simpler flat top carbs for the earliear ones will they give me issues with the newer motor?

[Ryan6838]

Why change carbs? Any carb you run that hasnt bee cleaned completely and rebuilt takes tuning.  By round tops and your in the same boat. If you dont want to tune it yourself take it a shop.

[bollingball]

Ryan6838 Are you sure about those # the 750 K8 came PD42b with 105 main 35 slows The F model had PD42a with the 110 mains. Think about it like this f model a little faster means a little more air & fuel more fuel=larger jet I believe the 77-78 supplement corrects a misprint on the sizes in the original manual.

[Ryan6838]

Look at rebuild kits 105 for f and 110 for k, look at honda parts fiche 105 f amd 110 k, stock carbs off my 78k 110 jet. I read it somewhere it was something about the   to 4 exhaust actually had more flow or something. I cant recall exactly. I read a bunch of stuff about it when i was buying my rebuild kit. K kit was like 28 and f was 15 and only diff was the jet.

[Flying J]

I had my F running open headers and pods. So loud it made kids cry as i drove by. Really it happened. But i could never get it to run great. I now have stock intake box and a mac exhaust. Runs great. I liked the look of pods but settled for an ugly airbox and a good running bike.