Backfiring G3LS

[bombardier]

I just had to add my ten peneth worth !
is the needle set at the correct height and (this will sound daft)are the battery leads connected tightly as this was a problem on 250csr with backfiring and poor performance.Hope it helps,you never know.

[AlanRSmith]

Have checked needle height (third from top)and checked the battery leads (cleaned and reconnected them twice).

Not sure what else I can do ??

[lolski]

just a thought have you checked to see if carb flange is flat not distorted also the carb spacer is also flat and no hair line cracks do you have gasket on cylinder head and carb ends you said you checked slide do you have a lot of play does it rattle if so this may give you trouble with air mixture on tick over what type of carb do you have on bike one last thing the cap on top of carb is this a good fit best of luck keep trying and do not give up sometimes its best to walk away a while and come back with new thoughts and refreshed to tackle problems
regards Laurence

[AlanRSmith]

I have checked the carb on glass and it's flat.
Not sure the question re 'gasket on cylinder head and carb end' - can you explain please?
Re slide - sorry what slide re play?
Re carb top - how do I check if it is a good fit?
Sorry but I cannot understand what checks you are suggesting.

[bjork]

quote:
Finally sorted the backfiring - it was air leak in exhaust to manifold. Took me three attempts to fix it.

However the carb I cannot fix.

This is what I have tried (time and time again):-

1) Set mixture screw in fully- then 2 turn out
2 Warm up engine
3) Set tick over up a little using the throttle stop.
3) I Attempt to screw mixture in and out a little to see if it improves tick over .

At this point I can get no further as the more I turn the screw in the faster the engine goes. Screw it out too much and it stops. So I cannot get past the basic 'two turns out'. This sounds like the pilot hole is blocked but I have cleaned it time after time. If I take the screw out I can see a very small amount of fuel.
Also I cannot get it to run on slow tick over - it tends to run fast or it stalls.

Any ideas on what to do?

Hi Alan, reading above it seems you may be not quite getting the balance between the two adjuster screws on the carb? (I'm trying to read between the lines here so please bear with me!)
The horizontal screw is an AIR screw-not a mixture screw-i.e. it only controls the amount of air allowed to go through the idle circuit drillings. So when you screw it out, it allows more air and the mix is weaker. The amount of fuel for idling is controled by the idle jet, which is either part of the brass jet block, (if you have the 'standard' Amal carb-eg 276 stamped on the flange) or it's a seperate screwed in jet on the later 'Monobloc' Amals (eg 376 stamped on flange)
If it were mine, I'd start with the AIR screw set at 1 to 1-1/4 turns out, I think 2 is quite weak and suggests an excess of fuel. If it really needs that much-float level high for instance.
Then I'd start it and try to set the idle speed with the other screw, that is the one pointing up at an angle. You should aim for a slightly higher idle speed to start with. Then back to the air screw again, but only adjust it in or out a little bit at a time, till you get the fastest idle. Then go back to the idle speed screw, the one pointing upwards, and adjust the speed back down to normal. If you do both of these moves a couple of times it should come approximately right. If it does, go and ride it to dry the cylinder out of the oil from the wet sumping, and I bet it will need another bit of fine adjustment as things start to settle back in. Don't try to set the idle speed with the air screw, use the throttle stop screw-the one pointing upwards-then fine tune it with the air screw.
Don't spend too long going over this idle setting with the bike stationary, as the engine will get a bit too warm and the settings will not be right when you come to a stop on the road. Also, err on the side of richness with the air screw, just a tad as it helps to avoid the popping and normally gives a more reliable idle.
If the above doesn't help it may be your carb is badly worn, but as you say it only started doing this after a run, it's unlikely the wear state would worsen that quickly.
Sorry if the above sounds a bit condescending, I don't mean it to be! Hope you get it sorted.

[paul knapp]

Alan, are you doing these adjustments with the ignition timing fully advanced?
If so, retarding the spark should settle the idle speed to be more smoother, where the engine should be more responsive to the carburettor adjustments.

Paul

[AlanRSmith]

Have done all that has been suggested with little or no improvement (poor tickover and backfiring continues) and am planning the following:-

Repack exhaust with firegum (think the gun gum is failing again)
New slider in the carb
New Gasket set for the carb

Fingers crossed this will at least improve the old bike.
Expecting parts by Friday and will then report.
Any other suggestion or ideas welcome.

But thanks to those that have been kind and suggested a number of causes and possibles fixes even if none have yet hit the mark

[lolski]

Hi alan sorry for not making my self clear
on gaskets you should have in order a gasket between carb flange and spacer and gasket between spacer and cylinder head the slide in carb should not have to much play side ways the cap i mean where throttle and choke cables go in when the ring is tight is the cap able to move up and down or twist sorry hope this i more understandable. regards
Laurence

[paul knapp]

An inlet valve guide with excess clearance to valve stem will allow air to be drawn in to the cylinder from the rocker box when the throttle is closed, giving a leaner mixture. This accounts for a hard to tune carburettor and can allow excess oil being drawn in to the cylinder, giving a little smoke when the throttle is opened. The faster the engine is spinning, as when slowing down from road speed, the more air that is drawn in, leaning off the mixture.

Paul