Air filter missing...

[frogdon]

Hi,

For a while, I've been having problems with my 1993 CB 400 project big one running rich and fouling up the plugs (or rather, at least plug one - clean this one, and it starts to work again).

I've now stripped it down a bit to get rid of the rust on my frame, and I've had a bit more of a look around to try and work out what's going on.

Underneath the rear of the petrol tank is the air box, and at the rear entrance to the air box there should be a filter I beieve, but it ain't there. I think this could cause it to run rich? We removed this when we were previously investigating a lack of turnover - it fell apart when we tried to check it out to see if it was in good order and, to be honest, I completely forgot about it

Looking around, all the pictures I can find of CB400 air filters are cylindrical, but this was a flat rectangle. Are the pictures wrong, or was this some sort of secondary filter?

Any good places to get replacement filters? Oh, and coolant for the rad, which I removed in another fit of stupidity.

Any help would be appreciated, and thanks in advance!

[Superjack44]

Filter is there to stop debris getting drawn into the combustion chamber, coolant is a 50/50 mix of silicate free antifreeze and distilled water .

[frogdon]

Cheers Superjack; much appreciated.

Am I right that it is the air filter I am looking at?
And would you recommend any particular replacement?

[RICKNC31]

If you have no filter in the air box the bike should run leaner as the air filter which is round as standard partially restricts the air flow making the mixture richer and without one weaker i suppose a previous owner put in a wrong filter. standard filters are fine but a washable filter although more exspensive can be used several times so making it cheaper.There is no rear opening to the air filter box the filter is changed through the round opening on the gearchange side by undoing one bolt Best of luck Rick

[Superjack44]

If you have no filter in the air box the bike should run leaner as the air filter which is round as standard partially restricts the air flow making the mixture richer and without one weaker i suppose a previous owner put in a wrong filter. standard filters are fine but a washable filter although more exspensive can be used several times so making it cheaper.There is no rear opening to the air filter box the filter is changed through the round opening on the gearchange side by undoing one bolt Best of luck Rick

Agreed

[frogdon]

Cheers both, that makes a lot of sense.

I now just need to find out if someone's stuck a different airbox on for some reason, and work out the actual cause of it running rich.

One thing I have found, is that carbon-fouled plugs can also be caused by "an inoperative preheating system (carburetor intake air)", so maybe removing the filter has allowed junk into the carb, blocking some of the air, making it run rich.

I think the airbox is going to have to come off....

I shall go and investigate. I'm learning more and more about engine internals as I use this bike

[amorti]

Do you mean the main air filter is missing all together? Or do you mean the little mini one that's in a small black box on a hose? The first is absolutely essential and if missing the bike will run lean, not rich. The second stops your carbs drawing dirty air in when the slides rise and fall. It'll help keep the slides nice but in the short term can be removed.

Show us a picture?

[frogdon]

I thought I was going mad!

It's "the little mini one that's in a small black box on a hose". I can get a pic tonight if you need, but I think you've got it bob on.

I'll check the air box in case anything's managed to get in there, and I'll lube and clean up everything I can see on top of the carb assembly.

[amorti]

I thought I was going mad!

It's "the little mini one that's in a small black box on a hose". I can get a pic tonight if you need, but I think you've got it bob on.

I'll check the air box in case anything's managed to get in there, and I'll lube and clean up everything I can see on top of the carb assembly.

Gotcha. I would say to replace the foam in that box with anything similar, I used uni-foam for air filters but honestly I reckon a bit of washing up sponge would do it.

[frogdon]

You, sir, are an absolute star!