RPM Issues

Ok, the good news! I just got my first bike, a '76 KZ 400, 10k miles, decent shape, and CHEAP. The bad news, it wasn't running on cyl 1, but I took the carb apart, cleaned it up, and it fired right up.

More bad news, I took it for a ride around the block, and it does great as long as I don't ask it to get above 3500 RPM, then cyl 2 doesn't keep up and if I baby it I can get 5k, but its touch and go if it will keep up on cyl 2. Cyl 1 does great. I don't really know squat about this stuff, but I'm really good at winging it, so I don't know if this is a problem with the cyl 2 carb, and it needs adjustment, or if its the condenser that just can't keep up.

Additional info...It starts first kick every time with the choke on, but will not start easy without, even if its been ridden for 20+ min., but as long as I'm riding and not trying to change gears, it does great with the choke off. I have the clymer manual, but if the info on this is in there, I haven't found it, and haven't have much time to look for it. Anything you all can tell me I would appreciate it. Thanks!!!

KZDaffid

Did you just clean #1? How are you determining that the cylinders aren't working?

I'd imagine that if #1 was clogged, it probably wasn't the only one. Clymer suggests checking things in a specific order -- compression, valve clearance, then carbs, I believe.

You might be able to get it running by cleaning the carbs, although eventually you'll want to shim the valves, verify compression, adjust your points (or just replace them), etc.

Check your spark plug coloration to see if they're getting fuel. The information on reading plugs is in the Clymer manual...

Clymer is good as a reference, but you have to know how things are organized. Read Chapter 3 as a minimum for general maintenance.

Something I've learned: fixing the wrong problem often takes longer than diagnosing the right one.

"Did you just clean #1? How are you determining that the cylinders aren't working?"
Well, When first working on the bike, #2 was the only one running, as could be told by the exhaust note (seperate pipe for each cyl), and the dry cold plug in #1. I was pressed for time which is why I didn't take the other one apart.

"You might be able to get it running by cleaning the carbs, although eventually you'll want to shim the valves, verify compression, adjust your points (or just replace them), etc."
I did put a compression tester to it, and it has great compression on both sides. I don't have a clue as to what shiming the valves is about, and as to replacing points, I'm going to do that and the condenser, once I can find replacements. I have locked into ebay, but alas, as with the carb, I haven't had time yet to look much, so if anyone has any suggestions as to where to get them, that would be great.

But just to be over clear, I know that both cyl's are getting gas at idle and low RPM's, its only after higher RPM's that it cuts out. I can hear that #2 is the one that goes out from the change in exhaust noise, but I don't know if it is because the points need replaced along with the condenser, or if its the #2 carb that just needs to be toyed with. Between that and the (possibly related problem) idle/choke issue, I think I'll be all set. Thanks for the return post, and please, anyone with any know how on this I welcome your input!

KZDaffid

... but it's still a good idea to check the plugs.

If the plugs look very rich, then I'd start by checking the ignition system. If the plugs look very lean, I'd assume that whatever fuel makes it into the cylinder is being ignited, and I'd start with the carb.

Carburetors run fuel through different paths depending on how far the throttle is opened. Partially blocked jets may cause the bike to perform poorly at higher engine speeds because it leans out as you can't get enough gas through to keep the mixture right.

If you'd like to replace the points and condensers, they're $12 for your bike ( here ). Replacing them isn't a bad idea *if* you're willing to adjust them properly afterwards, but I still think that checking the spark plugs is cheaper and faster than replacing parts until something works.

Since one carb was plugged, if I didn't want to pull out a spark plug and look at it, I'd probably tear them both down completely and make sure that all passages are *completely* clear, then adjust it back to Clymer's specs (especially the fuel or air screws, whichever is appropriate for your bike), reinstall, making sure that all fittings are tight.

EDIT: Also check out www.kz400.com for more specific information on your bike.

Post edited by: GargantuChet, at: 2005/11/07 16:33

Vacuum leak? Choking it makes it run rich, and with it off it could be too lean to run with it off. This would also limit your RPM's. Check the fittings on the carbs.

All right, I pulled the plugs today and #1 was clean (for a plug) while #2 was black and fluffy, which I learned from my manual was bad. So, I pulled off the carbs, and cleaned everything out/off/up. Put is all back together, put some gas in it, and it fired right up with the choke on, but started spewing white smoke heavily out the exhaust. It was also sitting around 6k rpm. So I immidiately turned the choke off and it died. It will stay on without the choke on if I play with the throttle, and I have my full range of rev's...I would assume that I just need to check the adjustment of the points for the idle?