Kz650 compression after rebuild

Hello everyone

Just finished to rebuild a kz650 b1 engine using

K700 wiseco kit
Stock head
Kz750 cams
Dyna s
Vm29 smoothbores

I am in the tuning process but i have a question about something that starts to make worry

When the engine warms up, it becomes very hard to start. Impossible with electric start. Very difficult with kickstart. Makes a lot of resistance. Requires a lot of energy on the kickstarter to turn the engine

I have read here and there that to use a kz650 stock head would be the best choice in order to have smaller ports and because of that a faster flow of mixture and fumes, together with a high compression.

I am starting to think that there is too much compression and i should switch to a kz750 head

Anyone could give some suggest or info about this subject?

Thank you

Vic

Are you are running your bike while it is stationary with no air flow?
That will cause it to run hot and over heat and possibly seize.


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Lots of guys over the years have run that Wiseco kit. The head should be fine. Most likely a carburetoration mixture issue. VM29's flow quite a bit of air for an engine of that size.

I have plugged an oil thermometer. Never been higher than 90° celsius. The problem occurs when the thermometer says more than 60° celsius.

So you think too big carbs will make the engine hard to crank when warm?

I'll try to fit a rack of 26 and see if it helps

Thank you

Lot of us have run 29s and my only problem with them was a slight hesitation off idle like almost all have.......comp should be fine too mine is right at 165 after break on mine so just slightly over stock.....look for a problem else where.... jetting, sync, timing, valves tight, advancer etc.

Which jetting and tuning works best for you?

I decided to try with stock airbox and k&n air filter, plus a zr7 4into1 header and an aftermarket road legal muffler

I am in the process of testing different jets and tuning on the vm29 sb to find the best match.

That problem about the engine hard to crank when warm is concerning me a little

Metal expands when it gets hotter.
Pistons get bigger in diameter and cylinders get smaller.
Piston to cylinder wall clearance is reduced.


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This makes sense to me.
I really can't understand how carbs, jetting, advacer unit or ignition timing could make an engine hard to crank when off and warm. Maybe valve clearence but i set them to spec.
Anyway i am here asking because there is a lot of people here who know much more than me about those subjects.
So if there is anything which doesn't make sense to me it doesn't mean it is impossible but probably it's just something i don't know yet.

So about compression, before to run a compression test, someone could confirm the symptoms i described is something normal for a just rebuilt engine, something most of you have experienced after rebuilt, or something went wrong during the rebuild?

Could i expect this to solve by itself after break in period?

Probably 15 pilots and the clip in the middle...... Maybe look up the clear tube test on here to be sure the fuel level is close...... I ran 17.5s on the stock motor but I had the carbs open (no cleaner) for years. Have new boots all ready to put the 29s back on but havn't yet. Mainly just make sure the advancer is working right (oiled up and free) and the timing is where it should be..... See you check valves so I remove that. Maybe a problem with your starter ??? Not sure like you say a lot more folks are sharper on here than me too so I might be a mile off on your problem.

vict182 wrote: This makes sense to me.
I really can't understand how carbs, jetting, advacer unit or ignition timing could make an engine hard to crank when off and warm. Maybe valve clearence but i set them to spec.
Anyway i am here asking because there is a lot of people here who know much more than me about those subjects.
So if there is anything which doesn't make sense to me it doesn't mean it is impossible but probably it's just something i don't know yet.

So about compression, before to run a compression test, someone could confirm the symptoms i described is something normal for a just rebuilt engine, something most of you have experienced after rebuilt, or something went wrong during the rebuild?

Could i expect this to solve by itself after break in period?

From what you describe your engine is making a lot of compression and now the starter motor is having a hard time turning over the engine. My first thought is that the starter motor is weak or your battery is low. You may want to check your charging system to make sure it's putting out the proper voltage. Regarding the starter motor, I'm not sure how to test that. It's possible that the brushes are worn or the motor is somehow diminished in a different way. Only way I can think of to test that is by trying a different motor. I don't think the root cause of the issue traces back to too much compression.

Nessism, he says it is hard to turn over even with the kickstarter so the electric starter is probably not the issue.
I have an 810 kit with GPZ750 cams, ported 650 head, APE valve springs and running stock VM24 carbs. I do not have any issues with engine resistance hot or cold. The stock starter spins it like it did prior to the big bore kit, my pistons are also HC.
When you crank it over manually is it hard to turn over all the time or only in certain spots? Is it hard to turn over even with the ignition off? Are you 100% certain of the cam timing?
If cam timing checks out and it only occurs when warmed up it probably is going to be the piston to cylinder wall clearance. Do you know what they are and who did the machine work?

Cam timing is correct.
The problem occurs only when the engine is warm

Machine work has been done by a well known shop in this area. They have a very good reputatiion as far as i know. But oh well they can make mistakes too. So i start to think that the problem is in the piston to wall clearance

Any test i could run about it? Compression test will point out that issue?

The only thing i know is that the cylinder block installation was nice and easy following the workshop manual instructions

So if that is the problem it could be also the reason of my very rough idle i guess. No matter which jet or tuning i try. Always nice and smooth idle when cold and then it becomes rough when warm. And it does make sense to me. If i make a great effort to crank the engine with the kickstarter i guess also that small amount of gas at idle has the same problem..