Monobloc fault?

My D7 BSA Bantam has a really annoying fault that I can't see how to fix at all. It will start and run only if I keep the tickler button on the carb pressed in. It has a 375 monobloc carburettor. Without the button pressed it starts and just dies after a minute or so. When I first looked at it I thought the float needle was stuck so I cleaned it up but that didn't fix it. I fiddled about for ages, changed the float needle, tried putting a shim under the float needle housing, checking for leaks, but nothing makes any difference. The only thing that could be wrong is that the float height must be too high, mustn't it? There is no adjustment for float height on the monocloc though. I haven't had the bike runing on this carb, although it seemed ok on a concentric I tried it with. Any ideas?

Is the main jet blocked?

Is there water in the jet holder?

Has the needle in the throttle slide dropped into the needle jet?

Does the carb have correct jets and parts for two-stroke use?

Either the float is too high, or you have a fuel flow issue. I tend to suspect the latter. If the float was too high, it wouldn't starve. More likely is would rich-foul the plug.

Triton Thrasher said:

Is the main jet blocked?

Is there water in the jet holder?

Has the needle in the throttle slide dropped into the needle jet?

Does the carb have correct jets and parts for two-stroke use?

Click to expand...

I have completely stripped and cleaned it. It has new jets and needle and the petrol flows freely all the way down to the banjo at the top of the float chamber. There is only one type of main jet isn't there? The float needle seals and all I can imagine is wrong is that the float is riding too high and keeping the float needle shut for too long. If I keep the float depressed it works! There is no adjustment for the float and I can't see any stress marks or damage to show that anything might have been bent.

I would replace it altogether with a spare Mk1 concentric if I knew what jets to put in. Is there a table somewhere listing jet sizes for different applications?

Berticus said:

Triton Thrasher said:

Is the main jet blocked?

Is there water in the jet holder?

Has the needle in the throttle slide dropped into the needle jet?

Does the carb have correct jets and parts for two-stroke use?

Click to expand...

I have completely stripped and cleaned it. It has new jets and needle and the petrol flows freely all the way down to the banjo at the top of the float chamber. There is only one type of main jet isn't there? The float needle seals and all I can imagine is wrong is that the float is riding too high and keeping the float needle shut for too long. If I keep the float depressed it works! There is no adjustment for the float and I can't see any stress marks or damage to show that anything might have been bent.

I would replace it altogether with a spare Mk1 concentric if I knew what jets to put in. Is there a table somewhere listing jet sizes for different applications?

Click to expand...

If the float was "riding too high" that would mean the float chamber was full and thusly it couldn't starve out. Also floats set too high make for a rich mixture as the fuel begins being drawn up into the spray tube earlier than it would were the float level set lower. Perhaps the float isn't dropping on it's own and doesn't admit enough fuel until it's forced to by your holding down the tickler.

Sounds to me like your low speed fuel circuit is clogged. You need to soak the bare carb body in a one-gallon can of Berryman's Chem-Dip for about 30 minutes, then flush well with water, and blast with conpressed air in every orifice. Some people like ultrasonic cleaning; but I've found Berryman's to be a better solution unless you happen to have an ultrasonic cleaning kit (I still like berryman's better).

Thanks,

I'll try that. Can I get it in UK (or Belgium)?

Berticus,

Is the float brass? Because if it is, it should have an adjustable tang between the pivot that butts up under the float needle. On all of my beloved Monos they all had a brass float and the tiny little tang was adjusted by pinching it with pointy nose pliers and gently levering it up or down to raise or lower the float level.

The other thing is that there was a different spray tube on the needle jet outlet for the 2 strokes. Better check with Burlen Fuel Systems UK as they stock brand new Monos and spare parts. Google Burlen and you should be OK.

Mick

Spot on Grandpaul! It must have overlooked the pilot jet when I cleaned it the first time.

Mick, I didn't realise that you could bend the brass tag on the float, I'll remember that for next time.

Thanks for all the help guys, the little bantam is buzzing again!

There's a good lad!

Well done.

Berticus said:

Spot on Grandpaul! It must have overlooked the pilot jet when I cleaned it the first time.

Mick, I didn't realise that you could bend the brass tag on the float, I'll remember that for next time.

Thanks for all the help guys, the little bantam is buzzing again!

Click to expand...

I have a bit of problems with pilots jets on two strokes. I think the oil in the fuel tends to clog them if they sit too long.

swooshdave said:

I have a bit of problems with pilots jets on two strokes. I think the oil in the fuel tends to clog them if they sit too long.

Click to expand...

Quite right.

Another major mistake some folks make is removing the injector pumps and pre-mixing fuel, without re-jetting to compensate for heavier visosity of fuel mixed with oil!

(Don't ask how I learned this as a youngster)