Hi all,

I'm New enough to road riding and purchased my first bike last year.
A beautiful black 1993 bandit 400 with just 12k miles.

When I got the bike the previous owner had installed a Remus Shorty exhaust and said that it hadn't been running perfect since. Unfortunately he had lost the original so i'm stuck with it. When I test road the bike it pulled perfect up until 8k rpm at which point it had a constant missfire until throttle was reduced allowing me to get as high as 8.5k but no higher (Approximately 20% throttle).

Long story short I bought it and when i got it home i started by cleaning the carbs in an ultrasonic cleaner. I notted down the jet sizes 102 mains with a 32.5 pilot, I set the float height to 14.5mm as the manual and forum suggested and the idle jet screw 2.5 turns out.
All o-rings and seals looked to be in good condition.

I reassembled the bike and took it for a test ride and lone behold it was fixed or so I thought. After a few days the missfire came back. I repeated this cleaning multiple times and it fixed it each time but for less and less time.
The last time I was able to get past 8k it was only 4 miles till it started missfiring again.

The past few times I cleaned them out I also cleaned out the tank. The previous owner had lined the tank with what looks to be epoxy, bits of which were flaking of and being pulled up against the screen of the tap.
But these last few times I have been unable to fix the missfire.

Things I've done
Spark Plugs Changed (NGK CR8EK)
Ignition Coils Changed
HT leads changed
Spark plug caps changed for NGK 5Kohm
CDI opened up, found some water and light corrosion, cleaned up and closed up (Possible cause?)
Ignition pickup changed
fuel line changed
Carbs Balanced.

Bike Starts cold with no choke and no throttle needed.
charging circuit puts out correct voltage as in manual.
carbs look absolutely spotless before and after cleaning.
Bike free revs to redline no trouble.

I will try to make a video of the issue to better describe it and include photos at a later date.
Any help is greatly appreciated as I am at a loss at this stage as to a fix.
I'm getting way to fast at cleaning these carbs for my own good.

Update #1

I had installed a carb rebuild kit a few months ago but since then was not able to get the bike running right.
Last night I re-installed the original parts into the carbs, test rode no difference.

Today I took out the plugs and they looked like this cyl1 on the left and 4 on the right. These plugs have about 300 miles on them, Plug 3 looks to have a slightly weaker spark than the others and didn't improve when cleaned.(Tried in all other plug caps)
I set the needle 1 slot from the top (leaner) and will try to ride it with no airbox as this is how it was when I was last able to get it to run properly.

I would have test rode it by now but the throttle cable snapped at the grip.

If the root of your problem is the carbs... then what you're getting at around 8,000 rpm is either an over-rich condition or a too-lean condition. Based on another hint you dropped in your description (the fact that the bike will start from cold without needing any choke) I would bet that your problem is likely an over-rich condition that gets so bad at around 8,000 it's reaching a borderline almost-too-rich-to-burn situation in one or more cylinders.

Did you remember to put the rubber plugs back on the pilot jets? They usually get lost on the floor when you pull off the float assy.
This will give you very rich running. Item 8 on this page : https://www.fowlersparts.co.uk/parts/59 ... carburetor
Do you have bst32 or 33mm carbs?

moforockband wrote: ↑

Mon Jun 03, 2019 11:09 am

Did you remember to put the rubber plugs back on the pilot jets? They usually get lost on the floor when you pull off the float assy.
This will give you very rich running. Item 8 on this page : https://www.fowlersparts.co.uk/parts/59 ... carburetor
Do you have bst32 or 33mm carbs?

I Have The bst33 mm carbs.
As far as I remember they never had the plugs in since I've owned it. The previous owner did say he had the carbs done when he put on the exhaust so it might explain why it runs good for a bit and then dies.
Thanks for the help, looks like another thing in the shopping basket.

Yep, that's yer problem, the carbs will run approx 25 MJ sizes richer without them :D

Update 2
I Rode the bike with the needles set one notch leaner and no airbox on after cleaning the plugs.
It has been running perfect since apart from a small flat spot around 10-11k rpm.
The plugs arrived today and have been installed, I will let you know if it starts to foul the pugs again and misfire.
Thanks for the help
Ryan.

Did you put the rubber bungs on? Running w/out the airbox will mask a rich running condition.

Currently I have everything as standard, airbox on needles at factory heights and bungs in.

Dont know if this issue has been solved yet, but i recently took out my rubber plugs (bungs) from the pilot jets just to see if there would be a difference and there was a huge difference!
My bike would hit the rev limiter at 15k rpm and with rubber plugs removed it hit a massive wall at 11-12k rpms.

Also make sure your airfilter is not dirty when i first got my bike i chased a rich condition for months thinking the carbs needs new jets and needles. I removed the filter, cut it open, took the old material out, bought a sheet of filter foam, cut it to size and fitted it. Sealed the filter up and all my problems were gone. I also had a bit more airflow to up my jetting and get a bit more accelaration

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