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..

[Bitsa (Ralph Wright - RIP)]

Steph,
I can not help you with your problem at the mo but I am sure the other k0 owners will come to your rescue.Though they had four cables the rest apart from jetting etc can not be much different between the rest of them.If at a last resort you want another set I do know where I can lay my hand on some albiet there condition is unknown.
Cheers
Bitsa

[Trigger]

It sounds like they are sucking air or a load problem. Will sleep on it and talk to some carb racing engineers tomorrow and get back to you Steph.

[JamesH]

Have you tried running the bike on a later set of std carbs with the twin linkage? At least a known good set of carbs would tell you if yours are suspect, Did the bike come from the dealer in Ireland that have ceased trading?

[Steph550]

Thanks for the replies.

We want to try and keep the bike original if possible, as the rest of it is completely stock. It may be worth running it with a later set of carbs to verify that it does solve the problem, but that just means buying an extra set of carbs which adds to the expense!

Cheers,

Steph.

[Bryanj]

Does this have the 4 into 1 throttle cable? If so they were always a pain to set up and the cables frequently bind up giving wierd balancing problems, i don't know if new cables have been fitted or are available but that is one, expensive, thing to try

[tarmac]

Quote Steph,..(Has anyone experienced anything like this)?

  "yes I have, on my cb750 f2" but it was related  to the spark plug cabs which were shorting out badly and was only spotted
by my son after numerous wild goose chases,....definitely worth a look for sparking in that area.

     hope this helps,.terry

should add that 4 new plug  caps fixed the problem and bike now runs superbly

[UK Pete]

Quote Steph,..(Has anyone experienced anything like this)?

  "yes I have, on my cb750 f2" but it was related  to the spark plug cabs which were shorting out badly and was only spotted
by my son after numerous wild goose chases,....definitely worth a look for sparking in that area.

     hope this helps,.terry

should add that 4 new plug  caps fixed the problem and bike now runs superbly

+ 1 ON THAT I have also found what i thought to be a carb problem to be a coil related problem, i could have sworn it was carbs by the symptoms
pete

[JamesH]

+2 on that. Worth pulling the plug caps and putting a multi-meter across the plug caps individually and across the pair of ht leads from each coil to check the impedence. Should be 5kohm caps and 15 on the coils

[philward]

Are your carb to cylinder head boots the original items? As we all know, 40+ year old rubber goes really hard and doesn't seal very well. At 30 squid a set, it might be worth a try if the advice on previous posts doesn't work. As these symptoms are normally an air leak issue, might be worth a try.
Phil

[hairygit]

If the bike is sporting those appalling metal clad plug caps that could be the cause. As Bitsa will tell you, Honda dealers were advised to remove them to prevent similar symptoms to what you are describing. I know people who still use them like to keep the bike original, fine if it lives in a museum and doesn't move, but more trouble than you can imagine on a regularly ridden bike!>

Sent from my GT-S5839i using Tapatalk 2

[Bitsa (Ralph Wright - RIP)]

Hairygit
OR GET WET,
funny the more I think about it they fitted them shock frackin things to the whole range of bikes.Also if memory serves well soon after removing the metal shields the caps started falling to bits.
Bryanj might recall this better than me.
Cheers
Bitsa

[K2-K6]

If the ignition HT supply is playing up you do need to verify that first as you'll get round and round in circles trying to balance the carbs if it's not firing properly.

Start it in the pitch dark without the tank on it and you'll see straight away if the caps/leads are shorting out as they light up like a christmas tree (apologies for mentioning christmas this early in the year, only 270 days to go  ).

Carb balance in my experience will never give that severe a symptom as you describe if they are just close to sorted which also points to influence from something else.

If it does have the metal type plugs caps, as the others say, they never really worked when new....nearly everyone with any intention of have the bike run reliably used NGK, two straight ones in the middle and two cranked ones on the outside cylinders.

[Bryanj]

Part of the PDI on every Jap bike I ever did was to rip the steel shield(s) off and usually the caps lasted 2-3 years

[tarmac]

+2 on that. Worth pulling the plug caps and putting a multi-meter across the plug caps individually and across the pair of ht leads from each coil to check the impedence. Should be 5kohm caps and 15 on the coils

thank you for that usefull information james, I can make good use of that.

                                        cheers,.. terry