Honda CB400 Super Four cutting out

[iansoady]

I've had a couple of very pleasant rides on the Super Four.

https://i43.photobucket.com/albums/e398/iansoady/bike/CB400/_86_zpsdzxtaico.jpg



However, it refuses to idle once warm and cuts out, requiring a rest and then substantial winding over to start. It then idles quite lumpily.

The first time I decided it was the daft vacuum-operated petrol tap so modified that so it didn't need vacuum and blanked off its pipe. But clearly there's more to it than that. It's not blocked tank breather as when I open the petrol cap there's no inrush of air.

I am thinking it's mucky pilot circuits but never having had a bike with more than a couple of carbs, what do those with more experience think?

It runs beautifully at higher revs although I keep looking for a couple more gears at the top end.....
____________________
Ian
1986 Honda XBR500
1952 Norton ES2

[jaffa90]

Pilot jets work mostly up to 2/3,000 rpm.
On a cold start can the choke jets let the engine rev evenly?

[Robby]

It's an old bike, and may have spent some time standing in the past. Could be fuel, could be electrical.

Pilot/mixture circuits are a possibility, and I find they're the one bit of carbs that doesn't clean itself - the other jets tend to sort themselves out with a bit of fresh petrol.

If you're lucky you might be able to access the screws without taking the carbs off the bike. In that case it's a case of winding each one in, counting the turns, and writing down the settings. Then unscrew, recovering the tiny washer and O-ring. Blast some carb cleaner down the hole, and reassemble - leaving the screw the same number of turns out.

It's also old enough for coils or their mounts to be knackered, HT leads are probably toast, or something deeper.

[Tankie]

carbs off, bowls off and clean them out , blow all the jets out with compressed air, check the float heights, remove the air screws, counting the turns in first and blow through, reassemble and refit.
If the machine has any diaphragms fitted on the side of the carb bodies these also need checking for perforations
____________________
Always learning, everyday is a school day

[redeem ouzzer]

Split carb bank, ultrasonic bath, rebuild with new seals. There can be no half measures.
____________________
Be a REAL MAN!

[iansoady]

Thanks all.

It has been standing since I bought it in the autumn and hadn't done much before that (maybe as the previous owner had the same problem......)

I've taken the plugs out - they're clean, properly gapped and a nice colour. I've also lubricated the choke mechanism to ensure it's not sticking and upped the idle speed to 1,300 (it was actually way too low below 1,000 so that wouldn't help). Standing in the drive running up to normal temperature it seems fine but we'll see. I've also chucked some silkolene fuel treatment in - any straw in a storm.

I know I should do it properly but TBH this is supposed to be my reliable modern bike (old bike - what a cheek!) and I have far too many other things to do rather than fiddling with it so I don't really want to take the carbs off......
____________________
Ian
1986 Honda XBR500
1952 Norton ES2

[iansoady]

Finally tracked down to one of the plug caps showing an infinite resistance. Replaced with a new one and just been for a very nice 40 mile ride.

I have to say I was reluctant to believe that it wouldn't just run (even if badly) on the other 3 (or maybe 2 given that coils are shared) but there you are.
____________________
Ian
1986 Honda XBR500
1952 Norton ES2

[jaffa90]

Well done but you never answered my post.

[redeem ouzzer]

[jaffa90]

Quiet simple really like you, if it ran evenly on the choke then it would be a pilot problem

[P.]

Jaffa just fuck off you waste of oxygen.

[Rogerborg]

Ooh, why the kicking? Should I be putting the boot in too, or saying leave it, he ent worth it.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[Evil Hans]

[jaffa90]

Whoah, that`s awful, i think i`ve forgot more than you know!

[P.]

Jaffa is 1k idle fanatic/autistic cunt.

[Rogerborg]

One time, right? Everyone gets one.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[P.]