Cylinder 3 and 4 on/off

[OilDrip]

Hello everybody,

Please assist a newbie from Copenhagen, Denmark.

I'm the happy new owner of a partly unrestored CB750 Four K2 which has been off the road for at least 10 years. 27.000 km according to the gauge. Most soft/rubber parts, including carburetor manifolds, O-rings, fuel hoses, tires etc. were replaced with new ones. Oil and air filters and spark plugs were also replaced. After cleaning the carbs the bike sounds great with loads of power. The happy time lasted a couple of days. Thereafter, cylinder 3 and 4 only worked now and then. Today I replaced the two spark plugs (3 and 4) with new ones. The old ones looked oily and black. Hereafter, all four cylinders ran like a dream for 5 kms or so. Then back to running on 2 cylinders. Please advice me what to do.

Please notice that the carbs have not been synchronised yet. This will be done one of the coming weeks. First I need to solve the issue with the unstable ignition or whatever the problem is.

 

[OilDrip]

Initially, I placed my question in a wrong section (under DOHC). Here is what Chris wrote:

Check compressions and valve clearances.Compression pressure should be not less than 140-150 psi. Valve clearances not less than 5 thou. (cold) First, check coil wiring for loose/broken connections.

 

[OilDrip]

And this is what amc49 wrote:

On getting something that old back running, sometimes carb clogging issues can occur if things were knocked loose like with a good cleaning, it can take a couple more times to get the system running 100% troublefree. Check your filters and for rust in the tank, the fuel bowls will have rust in them as an indicator.

 

[Cbdrag]

I swam to be havin the same issue on cylinder 1/2. Plugs are black and sooty. I am also shooting gas out of my left cylinder with a grayish white smoke. I found one of my pilot jets is clogged but I don't know why else it could be happening. I need to find a brass fitting to test compression. But I'm going to think that it's my clogged jet. Take a look at all of your jets. If you have never done it before. Make sure you take pictures and watch YouTube videos. If you don't feel comfortable take it to a mechanic.

 

[amc49]

A clogged jet runs lean not rich, not your issue. Could be indicative though of trash in fuel also clogging needle seat partly open to flood.

 

[OilDrip]

Update: After synchronising the carbs the engine runs on all four cylinders without major hickups. Though, I still experience some random RPM peaks in idle position. I assume this could be due to a float or float needle valve issue. I will replace all float needle valves and seats with a new, original set later this autumn.

 

[amc49]

'...some random RPM peaks in idle position.'

Often a sign of the compression being off a bit, it makes for irregular hits at idle.

Once you are much closer in with the sync often a resync can make that a bit better as well.

Slightly fouled plugs from flooding will do it too, if running right now then driving it may make the idle a bit more even too as the plugs finally burn back to correct. That can take an hour or so.

 

[OilDrip]

'...some random RPM peaks in idle position.'

Often a sign of the compression being off a bit, it makes for irregular hits at idle.

Once you are much closer in with the sync often a resync can make that a bit better as well.

Slightly fouled plugs from flooding will do it too, if running right now then driving it may make the idle a bit more even too as the plugs finally burn back to correct. That can take an hour or so.

Thanks! I really appreciate the information.