running on 3 cylinders unless spark plug changed

Hi !

So i bought a Z500b1 from 1981. after getting it started it worked fine for a few weeks but then started to work on 3 cylinders only (cylinder 4 not firing).
I thought it was the spark plug so i changed it and it was fixed for another few weeks but then it happened again.
This time i removed the plug and checked and it's oily. i'm thinking that's the problem, oil in there is not normal.
What can i do? Is there a better fix than getting a new spark plug every time it gives out? Can that be dangerous?

Thanks
Manu

The most common cause for that would be a bad valve stem seal. A new set of Viton seals will fix that problem.

Is there any smoke from that side of the exhaust system?

Are you sure it is oil? An overly rich air/fuel mixture on that cylinder could foul the plug, leaving it wet with fuel. It is also possible that the plug wire or the cap are failing, and not providing reliable power to the plug.

well indeed it smoked when i changed the plug...
Normally it doesnt smoke but i've lost some level in oil...
I might replace the whole ignition coil to see if that helps but the seals would be a good idea.

How about like, the piston rings?
M

I would not replace the coil unless the secondary wires are molded into it. Even if that is the case, the caps can be replaced individually. You might want to put a multimeter to work to see for sure if one of them is the problem .

It is possible, though much less likely, that you have a broken ring. A leak down test will usually confirm the health of the rings.

It is almost always cheaper and less frustrating to test each of the systems to identify the problem or problems, than to randomly replace parts hoping to accidentally stumble upon the solution.

Thanks ,

I wonder how to prform the Leak down Test. Also it seems weird that i would have a damaged ring since the bike runs super well when running on all cylinders. no problems.

Also, the bike starts fine if i insert a new spark plug. Seems like something is maybe it go wrong. Could that be the cap?

M

Here is the plug i removed. I installed that one like 1 month ago. I changed with nex plug and the bike runs fine. No blue smoke nothing but it back fires a bit only when it's standing. As soon as i rev the bike it's smooth...

Any thoughts?

As already suggested Valve Guide Seals.
Not just mileage , but age kills these rubber bits .
I had the same condition once and that fixed it
Dave

My bet is on the seals.
Steve

Thanks guys ! So these www.suzukiperformancespares.co.uk/produc...lve-stem-seals/3553/ ?

I also saw that this type of wear on the spark plug could be unproper ignition voltage to the plug. Since I changed the ignition coil for 1&3 but didn't do 2&4 that could be a fix too.

Will first change the coils and then see how it goes. I guess for the valve seal I need to take the engine apart...

M

swingmanu wrote: Thanks ,

I wonder how to prform the Leak down Test. . . .

A leak-down test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor-man's leak-down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air. Or just use a portable compressed air tank.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

Thanks a lot ! seems like some serious work ahead of me !
I will first change that ignition coil and see if anything improves !

M

swingmanu wrote: . . . Since I changed the ignition coil for 1&3 but didn't do 2&4 that could be a fix too. . . .

One coil fires plugs 1 and 4 (outside plugs). The other coil fires plugs 2 and 3 (inside plugs).

Good Fortune!