TOPIC: Kz750 starting problem

Hi,
Why am I having trouble starting my bike. Half the time it'll fire right up but it's a pig the other half.
Carbs have been sonically cleaned.
Coils and leads tested.
Relay mod for full power at coils
New starter clutch
Head skimmed
Valves lapped and shims all set.

I think it's slightly worse if not started for a few days.

Any ideas please?

Fuel evaporating out of the floatbowls maybe?  How about trying priming the bowls for 30 seconds or so before trying to fire it up?  Also check for plugged choke pick up tubes and the passage in the float bowls.  They often plug.

Please add your bike's exact year and model to your signature line.  There were many different KZ750 models including 2 and 4 cylinder models, and they did not all use the same carbs, ignition systems, etc.  Thanks.  Ed

Done my mistake 

I think I havnt got the option to prime,it’s just off,reserve or on. Got me thinking now though....
Carbs are definitely clean,I’ve lost count how many times I’ve stripped em. This has been going on for a looooong time. I asked this question before I did the mods and replaced starter clutch.

Most petcocks of that era have three positions: On, Res, Prime
There is no OFF.  

One other thing to check is the choke operation with the carbs installed; make sure all the choke butterflys open and close like they should when the lever is actuated.  The linkage is fiddly and delicate so you might want to check after the carbs are installed even.

The petcock has been changed hence the lack of prime,oh well that explains that.
Anyway,the starter stopped working so I stripped it and couldn't believe the state of the commutator,very worn/grooved. Also the motor was full of oil and oil that had turned to sludge. The brushes were worn to nothing.
What happened there,is there a seal on a bearing that had let the oil through?
I sourced a good used starter and the bike has been firing right up,well,so far so good.
The old starter was spinning the engine over ok but it didn't want to start. With this starter all seems well, what could have been happening?

The old starter may have been drawing so much power that there was not enough left to fire the ignition.

Problem seems solved,still starting on the button. Wish I'd looked inside the starter ages ago.
Oil leak has all but stopped too.

Well,I’m back with the same problem AGAIN!!,it sounds like the starter clutch is slipping now. I press the button and it cranks over a couple of times(but not enough to actually start it) then it stops cranking the engine and the starter motor speeds up a lot. It had a new starter clutch last year. I’m using Motul 3000 4T 20w50 standard mineral oil. Any ideas folks?

Pipes66 wrote: .................... It had a new starter clutch last year. .............

What components of the starter clutch were replaced? Ed

Complete starter clutch,a previous mechanic tried a repair kit twice so I bought the complete unit as I could see the “ramps” were very bad.

Did you also replace the gear? The surface that the rollers grab can wear and cause slippage (see image below). Ed

No,I didn’t replace that,guess I’d better strip the s clutch out again!!

So I’ve replaced the starter gear,as you said it was shiny and worn. It’s back together and I’m still having problems. I hit the starter button and it’ll turn over but then stops but I can hear the starter spinning up,I un press the starter button and when the s motor slows down it engages again and stops with a small thud.
I suppose I’ve buggered the s clutch again coz I didn’t replace the s gear.
I reset the valve clearances,they were all over the place.
I also replaced the coils as I didn’t think the spark was very big and blue. I have already done the 2 relay mod. I still havnt got a big blue spark though.

Well that sucks lotta work ...... hate to give advice like this for the fear of not being correct on the solution but maybe new rollers again (not the whole unit) to go with the new gear?? and when mine went out it started in the spring when it was in the 50s and 60s and I was using 20/50 Mobil one...... had read that a few people were using synthetic oil and blamed that on their st clutch problem so I switched to 10/40 Valvoline (not saying syn oil was the problem Ed and many others have used it for years) just saying the slightly slippery oil and thicker oil when cold "might" have been a factor

. As soon as I did the problem went away and it was fine till fall when the first 60 degree day hit then it slipped again. What I think was happening with mine was it right on the edge of going bad and the extra drag for the 20/50 in cold temps was enough to get it to slip??? Just something to think about and somewhere around here there's a thread on using a newer model 750 starter clutch with a improved design ...... I wish I would have seen this before but would be a lot of extra cost and trouble.

I replaced the whole unit, roller, springs and gear around 5 years ago and its still going strong but threads like this worry's me.

I agree,I think the oil is a factor. I swapped to 20/50 because I couldn’t get 10/40 in mineral,it was only available in semi (I know it’s mineral) and I found that semi was pissing out the valve co er and base gasket whereas mineral didn’t.
Perhaps I’d be better off using 10/40 semi,what do you use?

I switched to 10/40 Valvoline bike oil but it might have just been a band aid and might not be the solution...... search "starter gear solution-zr7s part" for the thread on the improved set up.

I did search the other day when it was first mentioned,but I thought it looks exactly the same parts. I also had a look on eboy,there was only 550 ones on the Uk site,they look th3 same too.

The only 10/40 mineral I can find is used in quad bikes,I might give it a go.

I’m using Yamalube 10/40 mineral.

Example:
www.motorspeed.no/yamaha/ymd65031-00/yamalube-m-4-10w40