Horrible rebuild results!?

Today I finally took my rebuilt KZ750H3 LTD out for the first time to start breaking it in. It smoked bad so the rings were changed and valve seals replaced, valves cleaned up, etc. (top end job). Also, pods were added and rejetted for (KeihinCV34's), and the mufflers were drilled out in the back a little bit for different sound.

I start it up and it idles good! Almost immediately I hit a red light. Turns green. Smooth take off, plenty of power and smooth cruse. Hit another red light. Turns green. Horrible take off. Engine sturggling to keep running and no power! After fighting it to about 1500 rpm, it smooth's out with nice power. Hit yet another red light. Smooth powerful take off-no problems. Next red light. Terrible take off....and so on and so on. Seems to happen at lower rpm's, when it happens. I force it home, where it dies as I pull into the driveway. Only consulation is that there's no smoke now!

First I pull the plugs. A little black dry carbon. A little rich!- but not real bad. I had checked the float bowl levels and they were good. The carbs were quite clean when I did the rebuild (and ran fine) so I left them alone. I had changed the stock #62 and #125 main jets to #70 and #142.5 to accomodate the pods, but did nothing with the pilot screws (2 turns out), and besides it seems to be running a tad rich now.

It's like it has a major vacuum leak, but only every other few minutes! I had fixed a problem with the idle reving up and staying there, but that's back also (but only sometimes). I can say that the tank was low, but still on the run setting, not reserve(Hummm...could the tank be so low I should switch it to reserve? No warning light!).

Well, I'm not sure where to start looking! Any suggestions? Thanks!

Start by eliminating the easy stuff.

Get a full tank of high octane gas (in case timing is advanced some).

Fire it up and limp it around the block at moderate rpms to get it hot. Then turn fuel off and drain the float bowls (look for crud).
Remove all the plugs and connect them to thier wires. tape them to the head and do a compression test.
While testing compression, watch the plugs and make sure they are all firing a nice blue spark continuously.

If thats all ok, button up things and turn on the fuel.
Fire up the bike and adjust your mixture screws properly.
Screw each screw in till the rpms audibly drop. Then turn the screw out (counting the # of turns), till it drops again. Then screw it back in to half your count (between the two drop points).

Blip the throttle hard (half or more). The bike should rev fairly easily, and return exactly to idle quickly.
I the idle hangs slighly before returning to idle, your too lean on the screws. If rpms falls below idle then comes up to previous idle slowly, your too rich.

Then perform a vacuum sync, if possible.

Then perform a dynamic timing with timing light, again if possible.

GL!

polkat wrote:

It's like it has a major vacuum leak, but only every other few minutes! I had fixed a problem with the idle reving up and staying there, but that's back also (but only sometimes). I can say that the tank was low, but still on the run setting, not reserve(Hummm...could the tank be so low I should switch it to reserve? No warning light!).

Well, I'm not sure where to start looking! Any suggestions? Thanks!

Other than a throttle cable being pinched, the only thing I've ever seen cause a idling rev that comes and goes is a vacuum diaphragm (not sure on the spelling there) tear/rip.

Just some quick thoughts
142.5 may just be a smidgen to large.
I had to set my pilot screws at 2 1/2 turns out.
If you feel there may be a vacuum leak go back and check that all your hose clamps on carb boots and pods are tight.

Low rpm problems would seem to indicate pilots. The first time I tackled carbs I had to clean 3 times before I got it right.

Did you synch the carbs ?

I have no experience with running in a motor so hopefully the experts will be along soon to advise. Perhaps it will come right with running in ?

Could your pods be covering your carb vents? Are your vacuum ports on the carbs covered/capped?:dry:

No the venbts arn't covered and all the ports are capped except the one to the petcock. I don't think the main jet size is the problem as it runs fine in the higher rpm's, once i can get it there! Lots of good advice here, so I'll get on it and report back. I'll also try the pilot adjustment suggested.

polkat wrote:

Smooth take off, plenty of power and smooth cruse. Hit another red light. Turns green. Horrible take off. Engine sturggling to keep running and no power! After fighting it to about 1500 rpm, it smooth's out with nice power.

Mine did that after I got garbage in the fuel tank and it was getting into the main jets. Tank rinse and inline filters are the fix, also flush the carbs.

:whistle: :huh: :dry: :whistle:

kzrider.com/index.php?option=com_docman&...d&gid=461&Itemid=108

Old Man Rock wrote:

:whistle: :huh: :dry: :whistle:

kzrider.com/index.php?option=com_docman&...d&gid=461&Itemid=108

haha, you love posting that! :woohoo:

yup... Too bad you guys don't read it though... :blush:

reading is for chumps...what can reading teach you? lol

His H3 has Keihin carbs.

As I read, the thought occurred to me that the tank being low, not reserve, _MAY_ be an issue if it close to the reserve point.

My H4 doesn't yet run, but my KZ400 acts just about like you stated when the main tank runs low. Just enough slosh in the tank during starts/stops to suck some air.

I'd try with a full/new tank of fuel, and be sure to include an in-line fuel filter.

The pods can also be tricky. Your rejetting, I presume, was to complement. I don't know a thing about rejetting, so assume what you did was proper.