Carb rebuild and still issues. Go figure.

First of all let me say the normal thanks. This is by far the best source of info on these old bikes that I’ve found and we’re all better owners and the bikes are all better for it. Everyone sharing their experience is much appreciated.

Bike is an 81 KZ650 H1 stock motor, MAC 4-1 and K&Ns (this is how I bought it). All I’ve done is ditch the wire rims (which were sorely in need of respoking) for a set of mags off a KZ550. Besides, I’ve always been a mag guy. Went to start her up a few weeks ago and no go, nadda, nothing. Not even a maybe. First thing in my head – carbs. I’ve always thought the carbs weren’t quite right – always been a bit tough to start cold and never really idled worth a dang. Did much reading (mostly here and some on Mikuni site) and jumped in. Would have loved to just send them to WG but just can’t swing it right now (youngest is 4 and mommy is still in a stay at home position which doesn’t pay jack ) and besides, I’ve always wanted to learn carbs.


Symptoms include:

Dang near impossible to start cold. Basically have to crank the idle screw up about 3 – 4 turns and have the choke on for it to start. This make no sense to me as I know the choke is an enrichment circuit that is only functional with the throttle closed (negative pressure negated when throtle is opened yadda, yadda, yadda – like I said been to Mikuni’s web site. I get it at least at a basic level). I’d assume that cranking the idle screw up would basically negate the operation of the choke/enrichment. Any feedback on that would be great. Once she’s warm I can basically brush the start button and she fires right up.

A bit lean (plug check after about a 15 minute ride). I’m suspecting fuel service level/float level. I’ve bench checked the float level but just measurement (23mm) but haven’t done the clear tube test. The reason I suspect fuel service level is that rolling the throttle open she’ll pull just fine (and hard) to red-line but snapping to WOT during hard acceleration she breaks up around 7K. Other than the 7k break up, impossible cold starting and the plug evidence of a lean condition she seems to be close. Wait, with all those issues maybe she isn't close.


Checked/changes:

Float level bench checked to 23 mm. Clear tube test/verify this weekend.

New intake boots, fuel lines, fuel filter, and vacuum lines.

Air screw plugs removed and screws checked/set to 1.25 turns out (seems to idle the best there). I’m concerned about this. My thoughts are the plugs were the seals for the air screw circuit – no O-rings on the srews. Parts diagram shows the CAN version had O-rings on the screws and no plug. This is most likely part of the lean condition if my thoughts are correct about the plug basically acting as the O-ring and sealing the circuit from air leakage. If so I’m thinking I could just get the CAN part # air screw and O-ring. Thoughts on that?

Jets are 15 pilot and 112.5 main. I expected to see something larger on the pilot. According to all the reading I’ve done here I was expecting to see something like 20 pilots, 120 mains and 4th clip on the needle but not sure if those numbers are applicable for VM24s or the displacement. Smaller on both jets thann expected and the needle only has one clip. I’m wondering if the previous owner put a kit in - maybe stage 3 or some other aftermarket setup. The VM24s do have a multi clip position needle stock, right? At least the parts diagrams show it has the 5 position needle stock.

Haven’t done:

Valve check. This is simply because I know myself. I’ll end up doing a tear down. I’m a sucker for a hot cam lobe at idle. I’ll end up pulling it apart and putting new cams in and you can't just do cams so a Wesco kit or bottom end refresh at least and I just can’t afford that now so I’ve resisted due to the temptation. That being said I’ll go there if need be – but may need a anti rebuild support group at best, 10 step program as worst if I'm to stay married.

Coil check. I’ll most likely do the WG mod (couldn’t hurt that’s for sure) but haven’t as of yet. She’s always idled rough so this may fix that. New intakes help the idle a great deal so I’m sure there was leakage there.

I tried to provide as much info as possilbe (maybe too much). Let the teaching begin, I'm a good student.

Thanks in advance,
Tony

Tony, welcome to the site. Great to have another KZ650 here. How about a picture?

Common wisdom says to do your troubleshooting in the following order:
1) Engine Compression, dry then wet.
2) Ignition (for god's sake do the WG Coil mod).
3) Carburetion (follow you service manual). IMHO, the clear tube method is the only accurate fuel service level test. Sync your carbs.

You can do this in other orders but it will cost you time and $ (don't ask :blush: ).

The idea is that playing with the carburetors won't solve these underlying issues.

Best of success!

PS
Please add your year, model and location to your signature as this will get the best responses... and it's considered real friendly too.
Bill

With the pipe and K&N pods I think your jetting is too small. Explains the bad idle and breaking up above 7000. With Emgo pods and stock exhaust my 81 kz650 csr was jetted at 17.5 pilots (needles in my CSR didn't have adjustable clips either) and 115 mains.
Do you have a baffle in the MAC? If so it probably doesn't flow any better than the stock system, so wouldn't need any bump in jets. If you have no baffle in it then the mains would probably have to jet up to 117.5.

New intakes help the idle a great deal so I’m sure there was leakage there.

You should perform the carb cleaner test to see if the carb holders are sealing properly. Warm the engine up and idle it down as low as it will run without dying. Then spray carb cleaner around the joints where the carb holders seal against the cylinder head. If idle changes (especially is engine dies) the carb holders must be replaced. This seems like it wouldn't have that much effect on the running of the bike, but it really does. If the carb holders leak I would replace them first before changing other things and getting too far off the baseline. Ed

Thanks for the quick replys. Based on what I've read over the last month or so I was thinking the jetting was a bit small as well. No, the 4-1 does not have baffles (again, how I bought it). It's loud as hell, the wife hates it. I kind of like it. Kind of. I'll try the 17.5 and 115 jets to see what that does. I have the carbs off anyway.

I agree Bill and thanks for your input. With the milage on the bike (30K) I need to check compression (if for no other reason than to just know what it is) and at this point valve check is almost a need more than a check. I'll just do my best to keep from tearing it down. I do want to ride this year.

As far as the WG coil mod goes, that was planned anyway so I'll do that this weekend as well. Would love to just replace the stock coils (seems like I"m triming the wire ends about once every 3-4 weeks) but just not in the budget right now.

I've done the spray test on the boots after putting the new ones on (thanks Jeff at Z1). Found a cap that was leaking and replaced. That was last week.

Here are a couple of pics. I'll get an avitar one soon. First is last year before getting it running again after the new paint, 2nd is about 2 days ago with me wondering what the hell is the problem. :S

BTW - any thoughts on the air screw cap/lack of O-ring? Should this be addressed or are the US VM24s fine with no O-ring on the screw?

Thanks,
Tony

Lol, files were too big had to re-size. Here you go. It's a start.

Hmmm.

I'd up the pilots and the mains. Right now I have the jetting in my signature and am looking at raising the needle another notch, and upping my mains a step or two. I'm running emgo pods and stock exhaust, though looking at modifying the exhaust to open it up a bit.

Definitelly check all the mechanical things and electrical. Carbs are a big enough PITA without spending hours on them due to a misdiagnosis of the problem that turns out to be something else.

I forgot to add, thats a nice looking bike you have. As for the desire to modify it, just give in to the dark side now.