Rebuild first time fireup....a problem!?

Fired up the engine I just rebuilt (cylinders on up) in my KZ750-4 LTD for the first time today. Started fairly easily and no leaks or any smoke, but cylinder 3 took a lot longer then the others to warm up, and the sound from that sides muffler is different.

I think #3 is firing intermittenly. Then I remembered that when I installed the coils, that cylinder's plug wire seemed a little loose in the coil. Could that be the problem? (Maybe I should put this in electrical?). I'm waiting for the engine to cool down and will look at that plug, but any ideas until then? Thanks!

Generally, even a loose plug wire will not stop the cylinder form firing, unless its falling off (huge gap).

I would take the bike for a short cruise, like 10 mins.

Then turn the idle up to where all cylinders are firing good (with a big fan pointed at the engine), and set the mixture screws, then do a vacuum sync.

You need vacuum guages and a timing light to dynamically set everything, or else the bike will just not run great.

Make sure fuel height in the bowls is perfect, via the clear tube method.

The vacuum hoses will need restrictors that slow down the pulsations of the intake. Any material can be made into one. Just cust a small piece of say plastic into a shape that will plug the hose, then drill a 1 mm hole thru it.
Stick it into the hose end closest to engine.

GL!

polkat wrote:

Fired up the engine I just rebuilt (cylinders on up) in my KZ750-4 LTD for the first time today. Started fairly easily and no leaks or any smoke, but cylinder 3 took a lot longer then the others to warm up, and the sound from that sides muffler is different.

I think #3 is firing intermittenly. Then I remembered that when I installed the coils, that cylinder's plug wire seemed a little loose in the coil. Could that be the problem? (Maybe I should put this in electrical?). I'm waiting for the engine to cool down and will look at that plug, but any ideas until then? Thanks!

To aid diagnosis, could swap spark plug wires at cap ends between cyl #2/3 to determine whether the intermittent firing problem moves from #3 to #2.

If problem stays with #3, it's more likely a fuel/carb issue (or maybe a valve clearance issue).

If problem moves to #2, it's more likely an ignition issue.

If both #2 and #3 crap out :pinch: - - - let's talk about that later. :laugh: :laugh:

Good Fortune!

And still the best way to determine whether a cylinder is combusting, is spraying water mist onto the exhaust headers. Just use a common household hand-held water spray bottle.

If combusting, the water instantly vaporizes.

If not combusting, the water runs down the header pipe.

Using fingers is strongly discouraged, to avoid burning the daylights out of yourself.

Good Fortune!

Congrats on your startup!

Thanks Link14!

I should have mentioned that I did the water on the pipes test after it had run for maybe 1 minute. Got sizzle on all but #3. After about 4 minutes #3 sizzled as well.

So I just went out and pulled the plugs. #1, #2, and #4 had fluffy carbon, which I atribute to the oil I smeared around the cylinders when I installed the pistons. But #3 was as clean and dry as when I installed it. To me, that meant no fuel or no spark. So I decided to swap plug wires 2 and 3 as Patton suggested. But as I was moving the wires, the plug boots for both wires simply fell off into my hand!! The wires are quite stiff by the way...probably very old.

Anyway, the wire for #2 would not reach over to #3, so I put everything back as it was, fumbling to get the caps back tight on the wires. It ran a bit better, but I can still hear missing. Are you guys sure it's not the old ignition wires??

One other new thing; Even with the choke off and the engine warm, if I rev it up past about 3000rpm, it stays there! I can actually see the throttle cable coming back to idle position, but it stays there!? The cable has been lubed and works smoothly, with no kinks evident. I know that the throttle plates must still be open for it to do that, but....why?

polkat wrote:

Thanks Link14!

I should have mentioned that I did the water on the pipes test after it had run for maybe 1 minute. Got sizzle on all but #3. After about 4 minutes #3 sizzled as well.

So I just went out and pulled the plugs. #1, #2, and #4 had fluffy carbon, which I atribute to the oil I smeared around the cylinders when I installed the pistons. But #3 was as clean and dry as when I installed it. To me, that meant no fuel or no spark. So I decided to swap plug wires 2 and 3 as Patton suggested. But as I was moving the wires, the plug boots for both wires simply fell off into my hand!! The wires are quite stiff by the way...probably very old.

Anyway, the wire for #2 would not reach over to #3, so I put everything back as it was, fumbling to get the caps back tight on the wires. It ran a bit better, but I can still hear missing. Are you guys sure it's not the old ignition wires??

One other new thing; Even with the choke off and the engine warm, if I rev it up past about 3000rpm, it stays there! I can actually see the throttle cable coming back to idle position, but it stays there!? The cable has been lubed and works smoothly, with no kinks evident. I know that the throttle plates must still be open for it to do that, but....why?

Could try some starting fluid (ether) sprayed into #3 carb intake while attempting to start up the engine to determine whether #3 plug is firing. If then fires, would indicate fuel issue. If still no firing, perhaps ignition issue.

Might improve connection between spark plug cap and plug wire by trimming back end of plug wire before replacing the cap onto the wire.

Would replace old hard worn out leaking spark plug wires and caps with new Dyna plug wires from Z1E which come complete with caps.

If existing plug wires aren't removable, would replace the coils also with Dyna coils from Z1E. An alternative is plug wire splicers from Z1E to use new wires with old coils.

Racing idle could be from air leak in carb holder, vacuum cap, vac hose, etc. Could test while running while spraying WD40 all around suspected leak areas. If no noticeable performance change with WD40, could try propane (unlit--Duh) sprayed about the suspected areas.

Racing idle could also result from advancer hanging in open position. Check advancer function by watching with strobe type timing light as rpm is varied by blipping throttle. If advancer is sticking, it may often be remedied with just a little cleaning and lubing.

Good Fortune!

Thanks Patton, I'll look further into the ignition wire thing.

The biggest problem right now is the racing idle...but it's not just a racing idle.

I can understand an air leak causing a rev problem with the idle, but this thing idles just fine on it's own. It is only when I open the throttle and rev it up to anything above around 2000. Then it stays there. Wouldn't an air leak cause the problem all the time? I can rev it up to around 1500 or even 2000 and it comes back down just fine. But go above 2000 and it stays there, even though the throttle closes. Very weird!

Hummm...perhaps you are right about the advancer.

If fitted with a vacuum petcock, would check the vac hose from carb to petcock, assuring no cracks or loose hose attachment at either end.

Possible that crack opens enough with increased vacuum at the higher rpm to start gulping and continue gulping. This is of course some wild speculation on my part (far worse than MUFON stuff). :S

Good Fortune!

That tube turns out okay, as does the advancer. Guess I'll be pulling the carbs off to inspect the carb to head rubbers. Anything else I should look at when they are off?

It's kind of obvious, but I'll mention it....when I put my 750 back together, my engine revved similar to what you are experiencing and I discovered that the vacuum line to carb #4 was loose.

Good luck!

I only have a vacuum line to carb #3, which goes to the petcock. The rest are stopped with rubber caps. That line can go to any carb, and I guess that's what you are reffering to. It's good, as are the caps. I'm seriously suspecting the carb to head rubbers now.

By the way, the engine had no gaskets between these rubbers and the head. I guessed that was because the rubbers are fully coated with rubber, and I assembled it that way. Right....?

When I get it all pulled, I'll let you guys know what I find (if...hopefully...anything).