1982 k750 wont run right

okay, so I have had two kz750's. The first one was bullet proof and ran great this one is a bit more difficult.

I have had the bike about three months. I have compression tested it twice. The first time and second time were consistent. Each cylinder had at least 145 psi. This bike has 2k miles. The spark plugs are new. The petcock was rebuilt but still doesn't work off the vacuum i believe that is floods the engine. I tore into the carbs tonight again because it was running weird before

Running weird to me is that every time I applied any type of throttle to the bike it would stall until 3k rpm then it would let loose and act normal. Under 3k the bike would respond very slowly until it could catch up then at 3k rpm it would accelerate perfectly (while in neutral)

I re-cleaned the carbs tonight again. I installed everything and it immediately would go to 5k rpms. I messed with the idle screw took it off choke and could get it to run at 1100rpm. After that it would not respond to throttle (again) I am at a loss with this build. I don't know what to do because I feel it has to be simple.

Do I need to buy a completely new petcock? Why does the throttle react so slowly? I also dub into the carb idle mixture scews. I removed the caps from them today. I set them at two rotations out. Could the high rpm be a result of running too rich? I did a full electrolysis to the gas tank because of severe rust. Does the tank need to be cleaned of debris?


I feel like it is a simple fix but need advice from the experts:)

Thank you for all your help in advance and hope we get this figured out soon!

Just some suggestions

Each cylinder had at least 145 psi.

That's good.

the petcock was rebuilt but still doesn't work off the vacuum i believe that is floods the engine.

Does that mean that you are running ithe petcock on Prime which would be fine - but the carb needle valves are not doing their job and hence flooding? Still some rust in there?
or
Are you running on Prime and Is the vacuum line still attached and flooding is via the Vacuum line ?

If you removed the vacuum line did you cap off that vacuum port on the carb ?

I installed everything and it immediately would go to 5k rpms

Any time this happened to me it was because I didn't seat the carbs correctly in the carb boots or didn't tighten down the hose clamps or forgot to replace the carb vacuum caps. In short - air leak.

Do I need to buy a completely new petcock?

When you rebuilt the petcock do you stretch out the little spring from WWWWW to WWWWWWWW ? Might be worth a try?

Why does the throttle react so slowly?

Could be to much gas as you say or still some rust in the carbs resulting in blocked jets or sticking floats. I presume you have an inline fuel filter fitted.

I feel like it is a simple fix but need advice from the experts:)

It probably is a simple fix but I'm not an expert. The above suggestions might help until one comes along
Good luck

You just took off the caps to the pilot screws? That may be your problem. Take the screws out and sprat through all the passages. Could be some crap in there. Did you check your spark advance? It needs to be free, lubed and working well.
Steve

You didn't mention how long ago you owned your prior 750. Was it a while back?

Also, you didn't mention whether the symptoms you are experiencing are when cold, when warm, etc.
These old bikes dislike modern gas. My 440 has a similar hesitation when it's cold, but that goes away once it's warmed up. Usually a 1/4 mile (which from my house is 2 stops), or a couple minutes idling on half choke is sufficient to do the job. Most people have a preferred fuel additive to help as well.

Otherwise, what everyone else said. When I was growing up, I'd work on snowmobiles with my dad, he'd make me clean a carb 3 times before he'd entertain the notion that anything was wrong that didn't have something to do with the fuel air mixture. He was usually right.

Given your tank comments, might be prudent to add an in-line filter if you don't have one already,

I have exactly the same problem with my 1980 kz750.

I'm working through it on wednesday coming. I'll contribute if I discover anything...

Hey guys sorry for the late response. But yes I reinstalled all the vacuum lines. I ordered new carb manifolds because i sprayed a little starter fluid and rpm increased slightly. I also adjusted the idle setting I suppose. It is where the throttle cable connects to the carbs. I had it way too tight which caused the high rpm. Once i adjusted it properly it ran great reacted fine.

I took it for a spin and got a mile away. it died. It would start. I cranked and cranked and finally fired back up but I had to hold it about 2k rpm and ride hoem quickly only for it to die right before I got to the drive way. It really seemed like the bike was flooded. I am so confused as where to go on this thing!

I adjusted teh spring to wwwwwwwww lol and it still doesnt seal the petcock. However, I have cleaned the carbs multiple times and do have an inline filter. I responded to a later reply if you would like to read the other diagnostics from yesterday.

If the seals in the petcock are good and the seat is not damaged, then even pretty light spring pressure will seal it off unless it is in the PRI position.

You pull the plugs?
They'll tell you if it's flooded vs. lean-out.
If you feel good about your fuel system, and you've already verified good compression, you could also pop out your plugs and check the color of your spark (or use a multimeter and run the electronics check-out procedures in your manual, but those are not as fast and a lot more boring). Blue spark is good, yellow not as good, no visible spark is bad. Failing ignition can result in weak spark at low RPM, and can cut out once the bike warms up. Seems unlikely, but could make for a nice break in the fuel system diagnostic routine.

I hope I'm not hijacking, but seeing as we're having the same issue, I have an update on my version of the original problem...

First I double checked valve clearances, because I understand that there's no point in doing much else if the valves are out of whack. Everything was in spec.

Then I checked timing with a light. everything seemed to be correct there.

Then I rechecked the carb seat and airbox boot seals. everything seemed to be okay there too.

I put in a new fuel filter, and new clear fuel line. here i noticed an interesting and odd thing. no matter what I did there appeared to be a considerable amount of air in the fuel line. once it ran it appeared that the fuel in the fuel line was full of small bubbles, like it was sucking in air at some point in the fuel delivery between the tank and the carbs. I rechecked all the hose clamps with no change. I'm not sure where the air is getting in, but the amount of air seems to be dependent on engine revs. even when setting it up so that there is no air in the line before starting, more air bubbles appear.

that's as far as I got. Have you checked fuel delivery/air bubbles on your KZ? I'm not even sure if it's the cause of the problem, but there is definitely a correlation. Anyone else out there able to shed light on air in fuel lines?

Did you ever get your problem sorted?

I finally got to work on mine just in time to get to the handbuiilt show in Austin. I sure as hell didn't want to show up to a bike show in my buddy's 2004 ford focus...

anyhow. I took apart the petcock and noticed that the seals and the plastic body were full of rust and pretty ragged, so I replaced the petcock, and all the remaining fuel lines.

complete success! maybe you're suffering the same thing.

Good luck