Topic: hello and 78 CB750K carb questions

[grouchy]

hello, new 1978 750K owner, I'm new to these old sohc bikes and I have some basic carb questions.
My other bikes are a 1977 GL1000, a 1966 CT90, and a 2001 FZ1.

The 78 CB750 looks real nice for a bike that old, the chrome wheels and original four exhaust pipes sparkle.
The original airbox has been removed and pods fitted.

The bike runs when hot with occaisional popping. When its cold it runs like crap, it backfires and pops and stalls when the throttle is cracked. The idle speed has to be turned way up. Choke helps the popping.
The hotter the engine gets, the better the bike runs, after 20 minutes its rideable with occaisional burble.

Reading this site I suspect the slow jet may have to go to a #42. I took the carbs off and pulled a float bowl.  I suspect the slow jet is the one that I can't get out. How do you remove it?

When I took the carb rack off (whew!, pried it off without breaking any cooling fins) I found the idle mixure screws. Right on the bottom must make them a booger to adjust. I found that the p.o. had them set at 3 to 4 turns out instead of one and a half. Would this account for the above problems? Should I set the screws and put the rack back on?

The whole bike looks clean, the insides of the carbs are spotless, the plugs, points and condensors are new, the whole bike looks great to me except for this problem. Should I try to get a original airbox or try to get the pods right?

A 78K airbox is rarer than hens teeth on ebay, will any other years fit?

[brewsky]

Welcome!
Sure sounds like lean at slow speed to me.

You need to pull the slow (pilot) jets out and make sure they and the circuits are clean.


The holes are very small and clog easily.

The screws are mix screws as opposed to air screws on these carbs, so more out = richer.


There are several posts on pulling them out (even though the service manual says not to)

You can download the srevice manual from the FAQ section.

Find an airbox on ebay if you can, I've seen several pop up. I think the 77 and 78 are the only ones that will work with the PD carbs.

[flybox1]

hello, new 1978 750K owner, I'm new to these old sohc bikes and I have some basic carb questions.
My other bikes are a 1977 GL1000, a 1966 CT90, and a 2001 FZ1.  whats the number stamped on the right end of your carbs? something like PD42b  this is important.

The 78 CB750 looks real nice for a bike that old, the chrome wheels and original four exhaust pipes sparkle.
The original airbox has been removed and pods fitted.

The bike runs when hot with occaisional popping. When its cold it runs like crap, it backfires and pops and stalls when the throttle is cracked. The idle speed has to be turned way up. Choke helps the popping.
The hotter the engine gets, the better the bike runs, after 20 minutes its rideable with occaisional burble.

Reading this site I suspect the slow jet may have to go to a #42. I took the carbs off and pulled a float bowl.  I suspect the slow jet is the one that I can't get out. How do you remove it?  pull and twist straight out with pliers.  it will come out

When I took the carb rack off (whew!, pried it off without breaking any cooling fins) I found the idle mixure screws. Right on the bottom must make them a booger to adjust. I found that the p.o. had them set at 3 to 4 turns out instead of one and a half. Would this account for the above problems? Should I set the screws and put the rack back on? the screws add more fuel to the mix. the PO probably twisted them out to compensate for the increased air from the pods.  more than 2 turns out and they are maxed.  dont make any changes until you sort your slow jet size.  40 or 42 might work.  only testing each will do.

The whole bike looks clean, the insides of the carbs are spotless, the plugs, points and condensors are new, the whole bike looks great to me except for this problem. Should I try to get a original airbox or try to get the pods right?  you can get it right with pods, but it will take a few trials, and removals of your carbs.   i would suggest the 40 slow jets, and raising your slide needles by one .5mm shim to start. this will increase the amount of fuel into the circuit to better match the air from the pods.  then, you can fine tune the fuel screw for better idle (1.5 turns out) and mid range by adding another shim to the slide needles if needed. you'll probably want 120 or 130 main jets as well  

A 78K airbox is rarer than hens teeth on ebay, will any other years fit?  another option is the ANTIPOD.  do a search for it here on the forum

[xnoahx]

I would first pull and clean those jets.  Like flybox said, just grab em with a pair of pliers and gently twist and pull.  On mine I let them soak a little in some carb cleaner and then ran a single strand of copper wire through them a few times.  Followed this up with a blast of carb cleaners through them and then a blast of compressed air.  When you hold them up to the light to look through them you should see a very small hole but it should be clear and uniform.  I have stock airbox on mine and a 4 into 1 exhaust and I run 42 pilots, 120 mains, stock needle position (too much work to shim!) and screws out about 7/8.  Runs pretty good and starts easy.  Still started easy with the 35 pilots but I like the 42s better.

[millerza]

hows the accel pump working? does fuel squirt from the little posts?

[grouchy]

I just managed to pull the slow jet with a pair of vice-grips. It's a #36, the main jet is a #130.
I guess that explains the problem.

I only scarred the jet a little with the vicegrips, but it shouldn't matter since the inside hole is still round.  I may need them again if I go back to the stock airbox.

Off to buy bigger jets

thanks for the replies