Brixton help needed please!

[Jw73]

Good evening all!
I own a Brixton bx125 (fuel injected). I used to have a problem of it being hard to start from cold for months. I now have the opposite. It's starts first push of the button but after idling for maybe 30 secs it stalls.
After starting again, I'll ride off and it seems to bog down and lack power at certain revs. When clutch pulled in at my first junction the revs will drop and won't idle as it then stalls.

Starting it again takes me about 15 secs and I have to use lots of throttle to do this. After its started again it will run fine all the way to work when engine is warm but sometimes when warm it will stall at traffic lights if I come off the throttle too quickly after I've stopped. Not all the time though.

It's driving me mad. Could it be a valve clearance issue? Oil changed recently as well as spark plug and air filter.

[TominMO]

So there are basically six areas to consider: fuel, air, spark, valve timing, valve lash (gap), ignition timing.
EDIT: Adding sufficient compression to list.

-- Air is rarely a problem, unless the filter is insanely dirty or someone stuck a potato in your exhaust.
-- Clearly you're getting fuel and air, since it runs for a while.
-- If valve timing or ignition timing were off, it would run very poorly or not at all.
-- So to me, the most likely issue is valve gap. Check it and report back. When was it last done?

But another issue could be a partially-clogged fuel injector; i.e. good enough to get it running, but not run well. Wouldn't hurt to dump in a small amount of fuel system cleaner. I'd do it for a tank of gas. If the bottle says it treats 20 gallons for example, I'd use 1/10 to 1/8 of the bottle for two gallons. In my experience with cars, fuel system cleaner takes about 100 miles to do the job. At that point it will run smoother and stronger.

Always go with the cheap and simple fixes first.

[Jw73]

Thanks so much for the reply. You explained it really well!
Ive done 10000 miles and the valve clearance hasnt been checked for thousands of miles to be honest. Oops... I do my own oil and plug changes but left the valves alone as it was running OK for a long time.

I cannot find any clear instructions for what the clearance should be. May be 0.04mm-0.06mm intake and 0.09mm-0.11mm exhaust from what ive read?!

Do a lot of chinese engines have a similar spec? I think my brixton engine is based on an old suzuki gn or gs 125 but with injection fitted.

many thanks again.

[TominMO]

Personally, I would go with .06 mm intake and .10 mm exhaust. If it's off either way by .01 mm, not a big deal.

In our experience on this forum with Chinese bikes, very often they come from the factory with gaps that are far too tight. Like when they are assembling the motor, they have only one .02 mm feeler gauge and just set everything with that. So what happens is that as the motor heats up, the valve stems lengthen, compression is lost because the valve no longer closes completely, and the bike stops running until it cools off. Very common. The exhaust valve is the bigger problem with overheating, as it only ever sees the hot burnt gases going past it. The intake valve only sees cool air and fuel entering the chamber, so is usually not such an issue. But too tight would mean it does not fully seat either; and too loose would mean it does not ever fully open, for good breathing.

I looked up what that bike was and found a review article. Very nice looking retro bike.
https://www.bennetts.co.uk/bikesocial/reviews/bikes/brixton/brixton-bx125---first-ride-and-review

[Jw73]

Brilliant, thanks so much for the detailed reply. I'll have a go at the valve clearance this weekend. Yeah, they look nice. I have the cafe racer version. Had it over 2 years now but there has always been problems with it.
Guy at work has offered me a Yamaha YBR125 for £700. 12 years old and 25000 miles on the clock but very well maintained and rode lovely when I had a little ride.
The parts for the Brixton are getting hard to find in the UK now so I may take up his offer soon?!

Thanks again for the great advice. Really helped me out! Ive just put some Redex in the fuel tank as you suggested cleaning the injectors. Fingers crossed it does the job! Ill get back with any results from the redex and valve adjustment.

[Jw73]

Problem now solved, fingers crossed!

I couldn't do the intake valve clearance today as the boltheads were tight and the spanner started to round the heads off.

Moved swiftly onto the exhaust valve clearance and couldn't even get a feeler gauge underneath it. Slackened the nut off and set it to 0.1mm as you suggested. Runs perfectly now! Pulls up hills and feels like it has more guts on the straights now. The idle speed seems a bit high now till it warms up, but maybe it was meant to be that way. Who knows! It didn't stall at any junctions or traffic lights so I'm so chuffed. Feels like a different bike. Will see how it runs tomorrow to work and back. Thanks so much for the logical advice. Much appreciated.

[TominMO]

A good way to break tight nuts or bolts free is to tap on the spanner* with a hammer or something, to set up vibration. This very often helps get the stuck part loose.

* That's wrench for you slackjawed Murricans.

[Jw73]

It doesn't help that a lot of the nuts and bolts on my bike are made of the softest metal known to man haha...

[TominMO]

It doesn't help that a lot of the nuts and bolts on my bike are made of the softest metal known to man haha...Kinda like refrigerated butter.....

[Jw73]

Kinda like refrigerated butter.....

Indeed. :)