Topic: cb400f running issues and possibly a karma issue

[cbcoker]

My cb400f must be some sort of punishment for some injustice I've done in the past, retrobution, karma, call it what you will. here goes.

Ridding along the bike has hesitation unless its getting a little choke, even when warm. Cylinder #2 isn't getting as hot as the rest and I plan to lower the float bowl a mm to see if that fixes that. To my knowledge the main jet is standard, but it must be getting some highspeed lean out because it won't go past 9k, no matter what. Thinking of putting a larger main in, any suggestions on sizes? Who sells main jets? Also the plugs suggest incomplete combustion as they are all sooty. I've bought hotter plugs to see if that works. Carbs have been cleaned and synchronized, valves adjusted, and filter cleaned. Bike has aftermarket pipes, four into four, but stock air filter. All plugs have spark.

What does the idle mixture screw do? Does it have any affect on the bike when its not idling? I have that turned out 2 turns on each carb. If I accelerate the bike in neutral, it is slow to return back to idle, what could be the possible culprit?

Anything you have to say is helpful, unless its criticism, which is only hurtful

[750goes]

slow return to idle - one of the symptoms would be the engine sucking extra air from badly sealed carburrettor boots (engine to carb side) .....check for leaks when idling  - sparingly spray some starting fluid around each of the boots one at a time - and if the engine revs higher then you know it is leaking (keep an extinguisher handy if you are sloppy with the spray)...as the bike gets warmer also tighten up the boots if you can - they become a bit softer and can leak a bit more than when they are cold... possibly invest in some newer clamps may also assist..

Is the throttle cable lubricated and running freely........
Is the return spring strong enough to snap the throttle closed .....

keep us advised of progress

 

[Jay B]

If you running stock except for exhaust I would think it should be jetted close enough to go beyond 9k. But maybe not. There are usually jet kits available on eBay, they have several different sizes in them.

[flatblack]

A few contradictory things going on here...

One, you say the plugs are sooty, indicating a rich condition. Two, you say it needs choke to eliminate hesitation; that would normally indicate a lean condition. Three, it won't rev past 9K; could be anything.

What happens when you dial in the idle mixture screws to, say, 1/2 turn from bottom? Three turns out? What about plug chops -- done any?

On the slow return to neutral, is the linkage hanging up? Get off the bike and rev it while watching the linkage. If the linkage returns to the idle position but the revs stay above idle, it's a carb issue.

Also, how does the engine behave when decelerating? If it chugs or surges, it's lean. But I don't think it's lean.

If you want to screw around with jetting, try http://www.siriusconinc.com for the parts.

Finally, if you still have the stock pipes, put them back on for a baseline, get everything sorted out, then put the 4-4s back on.

HTH...

fb

[cbcoker]

750goes, you could be right about the carb boots, no matter how hard I try they only get so good especially #3. I'll look into that.

Flatback, I wasn't sure what the idle mixture screw does so I'm trying not to f-with it too much. I don't mind changing it, I write down most things, its just I'd like to know more about what it does. What are "plug chops?" Don't believe the throttle linkage is hanging up, I've cleaned and greased most things, but its possible. All my cables have been lubed and the like. Thanks for the jetting link, I'll check that out.

I'm getting mixed signals too. It's hard to start, like its a lean thing, yet the plugs are sooty, like its incomplete combustion. I checked spark yesterday and am going to install NGK D7AE plus instead of the D8AE, thinking that could cure it. Better clamps around the carb boots might be an option, might try regular old hose clamps instead of the little metal jobbers that come stock.

[flatblack]

Flatback, I wasn't sure what the idle mixture screw does so I'm trying not to f-with it too much. I don't mind changing it, I write down most things, its just I'd like to know more about what it does. What are "plug chops?" Don't believe the throttle linkage is hanging up, I've cleaned and greased most things, but its possible. All my cables have been lubed and the like. Thanks for the jetting link, I'll check that out.

I'm getting mixed signals too. It's hard to start, like its a lean thing, yet the plugs are sooty, like its incomplete combustion. I checked spark yesterday and am going to install NGK D7AE plus instead of the D8AE, thinking that could cure it. Better clamps around the carb boots might be an option, might try regular old hose clamps instead of the little metal jobbers that come stock.

Idle mixture screw: Turn it in for richer mixture; out for leaner. Start with 1-1/2 turns from bottom. If you haven't done so already, pop off the little plastic caps that limit the screws' travel.

Spark plugs: My manual says the NGK D8ESL is stock.

Plug chops:

1. Warm up the bike, ride it.
2. On an open stretch with little/no traffic, use a constant throttle setting for a minute or two, then hit the kill switch.
3. Pull to the side and pull some plugs. (It's preferable to do some planning and have the tools with you... <grin>.)
4. Read them for color/condition at the throttle setting you used in #2.
5. Repeat as necessary for different throttle settings.

On the carb boots and leaks, warm the engine and then get off. Grab a spray can of something combustible; WD-40 should work. Starter fluid/ether is better. Sparingly spray it around individual carb boots while the engine is idling. Any change in the idling will pinpoint a bad carb boot.

Hose clamps: I got some recently from McMaster-Carr in stainless. Haven't put them on a Honda yet, but did install them yesterday on a two-stroke Yamaha I picked up this week. Work great. Try www.mcmaster.com and page 228. The Type 316 stainless in 5/16-inch width is what I bought.

One other thought: Is this a newly rebuilt or resurrected motor? 'Cause I wonder if the head needs to be retorqued. Maybe try the idle/spray trick around the junction of the cylinders/cylinder head?

But, WTFDIK?

HTH...

fb

[cbcoker]

Flatblack, thanks I've been doing this job solo so far and its been a lot of trial and error. I certainly will try the plus chops and check the booties. Also, I'll turn the mixture screws back a half turn and go from there. Again, I appreciate your wisdom, you are a gentelmen. tell you how it goes tomorrow.