Misfire - Backfire

[lcjohnny]

Last week I had the good luck:thumbsdown: to ride from Bristol to London & back in the Fog. I also had to use the M4 in the morning to get there on time :fou:

So 35 miles of queued traffic doing 35mph ish in the fog and the bike started misfiring losing power and backfiring at 1/4 to 1/3 throttle. As soon as i opened hte throttle it cleared again. Did it again in the fog & dark coming back.

Oh experienced ones:hail: what should I look at first? float levels, crud in the floatchambers, plugs, leads? what do you suggest? Jon

[Yorkie]

Check Pedro's reply on this thread!

http://www.ukmonster.co.uk/monster/showthread.php?t=19968&highlight=carb+icing

Carb icing i think!

Yorkie.

[scrumpster]

Funnily enough mine was doing exactly the same thing last week, but it's been fine in the wamer, dryer weather this week. I have now started to fill up with super unleaded, got my pro FST and ordered some iridium sparkplugs :biggrin:

[nik_the_brief]

Yep, definitely carb icing - it's that time of year again.

Silkolene Pro FST all the way - their website says this about the problem:-

"Pro-FST: British Technology Engineered in Britain.

Pro-FST (Fuel System Treatment) was developed in 1992 following an unofficial request from Kawasaki Motors (UK) Ltd., for something that would deal with the particularly British problem of carburettor icing in damp, cold weather. (It was also specifically a motorcycle malady. Cars have room for heated air intakes.) KMUK were harder hit at that time than the other 3 big Japanese MIC manufacturers, because they sold a lot of 'GT' models to dispatch riders, who obviously covered a lot of miles in all weathers; there were also a lot of complaints from the 'off road' fraternity.

The problems were:
Poor starting, irregular tick-over, sticking throttle, stalling at Iow speed, re starting problems, high fuel consumption and general rough running. What caused this was the formation of ice crystals in carburettors, blocking slow-running jets and jamming throttle slides.

The reason why the ice formed was because carburettors act as refrigerators!
The drop in pressure across the jet bridge plus the heat absorbed by evaporating fuel drops the temperature by several degrees; this will freeze out any moisture in the incoming air, especially when it's only a few degrees above zero to start with. In really freezing weather there is no problem, because once the air temperature drops below zero, the water drops out as frost, leaving dry, cold air.

The cure:
is to add something to the fuel which will dissolve the ice as it forms, without upsetting any applecarts: no fuel system corrosion, no ill effects on octane rating and so on. KMUK actually hired an environmental
test chamber for the trials, and sent videoed results to their masters in Japan, who had always refused to believe there was a problem!
After a few tries we came up with the answer, later sold as Pro-FST. 1% of this would
deal with normal icing conditions, and 2% would handle the worst they could throw at it. (Fog at 1 degree C) Apart from something that dissolves the ice, we included a synthetic low-temperature lubricant to lubricate carburettor bits and to act as an upper-cylinder lube to cut down cold-start ring and cylinder wear. Also, because some motorcycles are 'weekend only' transport, a touch of anti-corrosion additive was included to look after the top ends of engines standing idle in damp garages.

John Rowland, (R & D Chemist)
(Your writer has run into this one! The throttle slide in the AMAL carburettor of my 990cc Matchless engine, on a Morgan 3-wheeler, would sometimes freeze up so badly that I had to drive on the ignition cut-out like a Sopwith Camel pilot! I am now a Pro-FST user.)

PS: Pro-Boost only has a minor effect on carburettor icing; it is primarily an octane booster. Likewise, Pro-FST
has a minor positive effect on octane rating, but it is not a good idea to use it as an octane booster, because it's a carburettor de-icer!"

[lcjohnny]

Thanks guys

Really good info... I will get some FST .
and do the iridium plugs help at all?

I once had an LC350 that used to freeze the carb slides at half mast when v cold. That was quite interesting as well....:flamed:

I spose :hissy: this means the carburettor warmers that I switched on (and had no effect) are flaming useless??

Jon G

[grillphil]

This could be a plug cap breakdown. Check the resistance. They should be 5 ohms. Cheap fix too

[rimmy2000]

I had a misfire/backfire problem and it turns out the Plug cap is playing-up. Should be a swift fix. Make sure you check this.