Topic: CB750K0 Carburettor Problems

[Steph550]

Hi all,

My other half has a 1970 CB750K0 which has been causing problems for a while now, it looks stunning but has had consistent carburation problems which we can't solve.

The bike is reluctant to pick up at RPMs less than 3500 - when the throttle is opened it initially dies and then suddenly picks up. There is a considerable amount of hesitation when riding at around 30-40mph. It is also a nightmare when trying to pull away, for example at a junction, he has to rev it and ride the clutch to stop it from dying half way through pulling out. However, the tickover is very sweet, and at higher RPMs (for example, cruising at 50mph plus) it picks up fine.

The carbs have been balanced several times now, just after they have been balanced it runs beautifully, but once it's been left for ten minutes it's all over the place again. It has been checked for air leaks (by spraying fluid onto the engine to see if it temporarily improves) and there are none. Plugs all look perfect.

The bike has been restored to a very questionable standard by a 'professional' company, and it has taken 9 months to even get it running and on the road. However we are now really stuck with the carbs. They have had new carb kits fitted, but we are beginning to think that the carb bodies themselves are worn, and the bike will never run properly with these carbs on it.

Has anyone else experienced anything like this? My other half is now looking to source another set of carbs for it, if anyone has any they would be prepared to part with..?

Cheers,

Steph.

[74750k4]

If you've got the stock airbox, and exhaust on it, it sounds like fuel starvation, otherwise you need to look at those first two items, and your  jetting, maybe fuel flow into the carbs. bowl screen clogged???

[Sam Green Racing]

Stephi, I'll see what my mate has, I can see an early set of carbs on his shelf. 

http://forums.sohc4.net/index.php?topic=135444.0;all

Sam.

[flybox1]

what is the status of the idle and pilot jet? main jet and emulsion tube?
have you yourself been in to clean and inspect their condition, or did you soley rely on the 'questionable' restorer? 

[Steph550]

Thanks for the responses.

The bike is completely stock. We have had the carbs apart and the jets are new, however we don't know what quality overhaul kit has been used, they could be cheap aftermarket ones.

Cheers,

Steph.

[Johnie]

Did you save the old jets, etc? Basic rule of thumb is to keep the OEM parts in the carbs as they usually just need a good cleaning. My KO and the other bikes still use the OEM parts. I hope you still have them or can get them from the shop. The people doing your work may not know a lot about doing vintage carbs, adjusting floats, etc. These are a lot different than the newer carbs of today. I would question those carb kits. Do you know if this questionable mechanic even put in the right size jets? Does he know the air screw settings? How many miles on the KO? Complete worn carb bodies is doubtful. Do you know any history about this KO? How is the compression? Did he properly adjust new points, install new condensors, properly adjust timing? I think I would first be looking for a different mechanic. Have you checked the area for other vintage bikers for help? Let us know how you come out.