Topic: '75 CB400F non-mechanic needs help, stalling on low rpm

[broken]

I've had my CB400F for about 5 years now, I had to do some highway driving ( I usually just drive around the city ) and had her mostly wide open for 20 minutes or so, after which she has not been the same, after exiting the highway she stalled at the next light and would not start.

I figured she'd overheated and gave her some time to rest, after which point she did start, but ever since I have to keep the rpm up at stops or she stalls out, sometimes that doesn't even work. Also a lot of sputtering at low/mid rpms.

I'm pretty green with engines, if anyone has suggestions clear steps would be great. Cheers!

Phil

[750goes]

How long since you gave it a tune up?

I would possibly replace the plugs for a start point.

Clean or replace the air filter as well.

wind up the idle a bit to compensate..

[broken]

Had a tune up last year. I'll look into the air filter and new plugs. This really did happen "all of a sudden" though, she was fine two days ago, now after this one ride I can't go from A to B without stalling and refusing to start somewhere along the way...

[kghost]

Whats the inside of the tank look like?

Any possibility that rust and what not has finally clogged an Idle jet?

[Gordon]

Check that the battery is fully charged and is holding a charge when a load is put on it.  Make sure nothing slid around under the seat and is now blocking off the air intake.  Check valve clearances.  Check ignition timing and points.  Basically, do a full tune-up so you either find the problem or eliminate those things from the list of possibilities and have a good base to start from.

[broken]

Thanks for the suggestions folks. Battery is fully charged, new this year. Nothing's clogging the air intake.

I'll have a look inside the tank, in layman's terms how would I check the idle jets and valve clearances?

[Gordon]

I'll have a look inside the tank, in layman's terms how would I check the idle jets and valve clearances?

The first step is getting and reading through a good maintenance manual. 

Have you checked the battery since this started to happen or are you assuming it's in good condition because it's relatively new?  New batteries can go bad, too, especially if there's a problem with the bike's charging system.   

[broken]

I've got the clymer manual that I'm looking through as well. The battery is good, I checked it and charged it just a before this started happening. Old battery was trouble and it was obvious, once it died there was no starting nuthin'.

Phil

[kghost]

The best way to check your tank is to drain it.

Then pull the petcock. On the petcock is a finger screen. If you find rust and debris on the screen...well you can rest assured it is in the carbs.

Carbs have very tiny orifices in them. Both in the idle jet and the Emulsifier tubes.

Before I drained the tank I would in order:

1. Be certain the battery is fully charged. A weak battery gives a weak spark at Idle and low RPM. Causes misfiring and bogging. Once the RPM picks up theres enough power from the charging system for a good spark. Also the recifier can lose a diode and then your charging at a reduced rate...leading to a discharged battery.

2. Valves are worth checking but given the circumstances...kinda unlikely. You'd certainly hear the change in valve noise.

3 Check that your points have not closed up or otherwise gone out of time. Sometimes when new points are installed they wear rather rapidly and close the gap...plus if your mechanic was less than diligent they may have wandered.

IIf the battery is good and the timing is on...baring Carb clogs...its should idle.

I'm betting you've picked a bit of trash up in One or more carbs.

[Gordon]

Cool.  

Just follow the procedure in the manual for checking/adjusting the valve clearances.  It's pretty straight-forward.  Someone could type out the procedure for you, but there's really no need when it's right there in the manual.  

To check the idle jets, it's possible to get the float bowls off the carbs with the carbs still on the bike, but IMO it's less hassle overall to just pull the carbs to get at them.  This will also allow you to get at all the fuel passages easily to make sure they're all thoroughly clean.  

If you find there is a jet clog due to debris in the gas, then clean out the tank and filter, and replace the fuel lines before reconnecting anything or you'll just clog them right back up.  

[broken]

Thanks, I'll go through the valve clearance check in the manual. I may have found a tell tale clue though, when I switch the tank to reserve the problem is all but gone, perhaps a large chip of rust has settled in the main tank line...

[kghost]

Its possible.

It should be a rather large clue that you have trash in the tank.

Also its not uncommon for the plastic tubes coming out of the petcock to crack or otherwise come apart.

However generally its the screen covering them thats plugged.