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Hi everybody, long time reader, but this is my first post. I'm having some serious issues with my bike. It is a '81 Honda CM400c, it sat for 4 years. I cleaned the carbs, changed the oil, put in a new battery and fresh gas, and fired her up.

She runs, but she runs very poorly. Sometimes it will idle at 1200 for a while, but it will start to go nuts and crank it self up to 4000, and it won't come back down. The only way to get it back is to kill it and restart, or adjust the idle speed just slightly- and it drops like a rock and stalls.

The throttle acts weird too. When it's not surging to 4000 rpm, the throttle is rediculously sensitive right off idle. The slightest touch and it shoots up to, you guessed it, 4000 rpm. It's like there is no 1200-3999, it's like a switch. It's impossible to hold the engine at 2000, but easy to do at 6000.

I have not balanced the carbs (but I didn't mess with that setting either, so it shouldn't be too far off) and I did not change the plugs as I didn't have a socket that fit.

Where should I start?

Sounds like a typical lean-mix problem, usually caused by blockage in the carb passageways, petcock, or fuel lines. However, vacuum leaks are often the culprit as well, so don't rule that out just yet..
How did you go about cleaning the carbs? Did you totally disassemble them and follow a particular procedure, or just "wing" it?
I think first I'd check the vacuum lines and carb boots for leaks/cracks. Then, I think I'd go back and double-check the pilots, since they supply the gas for low RPM/part-throttle situations. You may be able to improve it by adjusting the pilot screws out a little, but that won't help unless the pilot jets themselves are clean.

Two things. It sounds like you need to clean the carbs again, perhaps more thoroughly. That may mean removing the jets and cleaning the passageways. The other thing is to sync the carbs. that could be a contributing factor.

I cleaned the carbs on my Yamaha 3 times before I was satisfied with performance. Obviously I wasn't thorough enough, the first two times.

I'm definately suspicious of the carbs still being dirty. I'll have to rip them apart again and see what I missed. It's just rather hard to see if those microscopic holes are open or not. I followed my shop manual, but it wasn't very thorough (though I thought I was). Also, I didn't use a carb dunk product, just the cleaner in a can thing. I did soak them in it for a few days however. I'll have to check for vacuum leaks (any suggestions how?)

Thanks!

A carb dip is naturally the most thorough way to go but some folks have had success with the spray. You just have to be sure to follow up with compressed air while the stuff is still wet and be prepared to repeat the process several times. BTW, you don't want to "pressurize" any part of the carb with the air - always hold the nozzle back away from the orifice. I've heard of guys ruining parts - like diaphragms, for example - by jamming the tip too close.
A good visual inspection is probably the best way to start investigating vacuum leaks. Squeeze and bend the tubing all along its length and pay particular attention to connections. Assuming you can get a constant idle speed, you can also test by spraying the tubing and boots with carb cleaner or WD40 while it's running, meanwhile listening for RPM changes.

One quick question, since I can't get to the bike till next weekend - I'm just thinking.

If the needle circut was lean in both carbs (real lean) would that explain this type of behavior?

Thanks!

Update: Next weekend I'll finally be able to bring the bike home, so I can really work on it. I'm thinking I must have reassembled the carbs wrong. There's just no other explanation for it.

There is no vacuum or intake leak.

New plugs, decent spark.

Choke is operating correctly, when all the way off there is some slack in the cable. Same with the throttle cables.

I managed to get it to idle just fine after it warmed up, however, once I reved to 3000 rpm, it would start to climb and climb all the way up to 6000. Only by loading the engine with the weight of the bike could I get it back down again. However, once down it would idle ok. I'm thinking possibly the jet needle(s) are bound up hanging open and dumping enough fuel in to keep the engine reving. I rode it up and down the driveway for a bit, and if I kept it under 3 grand it was ok, but any higher and it would take off! I could cruze at 20mph in first gear without using the throttle!

What do you guys think?

Sounds like a lean condition. Either from an airleak or slightly clogged jets. When you re-clean the carbs, make sure the intake boots are in good condition and seated properly.

Turns out that the tiny washer I missed really didn't have anything to do with the problem. But I fixed that anyway.

The culpret? Clogged needle jets! Apparently, some gunk got back down there after the rebuild. But after cleaning them out again, she's working just fine.

Now I need to balance the carbs and she should idle like factory.


Thank you all for your help! It is much appreciated!


Oilstain

hey guys i have same problem, except for my #3 cylinder is cold and fuel is expelling out exhaust manifold...shouldn't I atleast get "fire" even in lean condition.... spark seems good, intake boots are new and sealed well, carb is "professionally" rebult ( a freebie from boss' race shop), though was stated that they were heavily corroded. Any suggestions...I have pre-synched throttle valves (fly's) with a bread tie wire, was told should get me by... also i've set floats, did have fuel overflow on #3 carb out of venturi area it seemed just once while off bike (hmmm). never backfired with all that fuel.suppose was lean! But why no BANG? and REVING?