Problems with my '78 KZ650/Mikuni VM24

This may or may not have to do with my carbs. All I know is since I cleaned / re-jetted, things got worse. On to the details . . .

Bought the bike recently. It wasn't running at first but I changed the spark plugs, charged the battery and put some gas in it and she eventually fired up. But shortly thereafter it started to get harder to start. Then it stopped starting off the electric start altogether and would only start after kicking it for 10 minutes.

I thought I might as well check out the carbs, see if they were dirty and if they were jetted properly for the pod filters and 4-1 exhaust (Dunstal muffler with baffle) the previous owner put on. The carbs were pretty darn clean. I found 17.5 in the pilot circuit and 102.5 in the main circuit. (I should say this is the air screw type carb and not the pilot screw.) Also, the needle was set to slot #4.

I put the thing back together, having cleaned it and re-jetted my mains to 110. Based on all that I've read on this fantastic forum, 17.5 pilot / ~110 main / #4 needle clip is the way to go with my setup. Or at least a good start. I also did a mech sync.

Slight tangent here, I noticed my petcock would flow gas on the RES and ON setting just as if it was on the PRIME setting. I opened it up and found it was missing a spring that closes off the flow if no vacuum is present. Got the part, installed it. But I can't help but think maybe the last guy took it out for a reason.

Anyway, everything is back together now and I can hardly start the bike at all. It's essentially worse than before. I got it running once. With the choke half way on, the bike runs (poorly) only between 1500 and 2000 RPM. It dies at idle and it dies the moment I try to give it anymore than a 1/4 throttle.

FML.

Things I dont know or have not done:
Float level
Compression (EDIT: tested. 135/120/115/120 respectively)
Valve Clearance
Fine Sync
Air screw (EDIT: now at 1-1/2 turn from lightly seated)

Here are some photos of my plugs.



I've been thinking of calling up House MD for a diagnosis cause I'm lost at this point. Oh and thanks to all the contributors on these forums. I've learned so much in so little time. Such an amazing resource. You guys rock.

Pods! :laugh:

You don't need House, but you might need his vicodin by the time you get things figured out if you don't make sure of compression & valve clearances before trying to dial in carbs!

anarki650 wrote:

You don't need House, but you might need his vicodin by the time you get things figured out if you don't make sure of compression & valve clearances before trying to dial in carbs!

Have any suggestions on how to check the compression if I cant get the bike to wide open throttle?

Get a compression tester, the thread in type are better than the rubber boot push in type. Remove all 4 plugs and insert the tester into a spark plug socket. Hold the throttle wide open and hit the electric starter. The gauge on the tester will bump a little with each rotation until it hits the maximum level. That's your cold compression. Now repeat for each cylinder. You ideally want to have 140 to 150 psi per cylinder, with no more than 10% or so variance between any of them. Best to check compression when the motor is at full operating temp, but cold #s will give you a general idea.

anarki650 wrote:

Get a compression tester, the thread in type are better than the rubber boot push in type. Remove all 4 plugs and insert the tester into a spark plug socket. Hold the throttle wide open and hit the electric starter. The gauge on the tester will bump a little with each rotation until it hits the maximum level. That's your cold compression. Now repeat for each cylinder. You ideally want to have 140 to 150 psi per cylinder, with no more than 10% or so variance between any of them. Best to check compression when the motor is at full operating temp, but cold #s will give you a general idea.

I had no idea you could do that. Thanks man. I'll be back ASAP with some numbers.

OK so my cold numbers (in PSI) are as follows:

#1 - 135
#2 - 120
#3 - 115
#4 - 120

Unfortunately I have no idea what to do with this information. Thank goodness for kzrider.com I guess.

Those numbers are ok, a bit low on one, but she'll run with those.

You mention that it got progressively harder to start, and that could be a sign of the battery getting weaker because maybe the the battery isn't getting charged. Start checking the connections from the alternator up to the regulator/rectifier, then up to the battery terminals. You want all connections clean, no corrosion, no dull & dirty.

Then I would set the air screws about 1-1/2 turns out, after lightly seating them.

OKC_Kent wrote:

Those numbers are ok, a bit low on one, but she'll run with those.

You mention that it got progressively harder to start, and that could be a sign of the battery getting weaker because maybe the electrical isn't charging it back up.

Thanks for chiming in OKC_Kent. I've recharged the battery a few times. Thinking that maybe the battery was indeed a little weak, I hooked up jumper cables to a friends truck last night (engine off) and tried to start off that. No luck at all.

I'll start checking those connections. The starter motor turns the engine with vigour when the battery is charged. Not sure if that's worth mentioning.

Thanks for the note about the air screws. I'm pretty in the dark with that one.

EDIT: before changing them they were 3 turns out.

anarki650 wrote:

You don't need House, but you might need his vicodin by the time you get things figured out if you don't make sure of compression & valve clearances before trying to dial in carbs!

I hear you. The one thing is, before I took the carbs off, though hard to start, she ran fairly well.

What kind of shape are the carb holders in? If you've got air leaks from cracked up d holders she'll be tough to run & start....

anarki650 wrote:

What kind of shape are the carb holders in? If you've got air leaks from cracked up d holders she'll be tough to run & start....

The manifold boots look to be in pretty good shape. I think I've figured out the problem.

www.wgcarbs.com//index.php?option=com_co...view&id=12&Itemid=26

Describes my last month of tinkering perfectly. Time to head over to the Electrical thread. Thanks everyone.