Hitchcocks Forum

Alls well when riding on half throttle but when I give her full revs I get a backfire and out pops the exhaust from the cylinder head.i,ve tried everything except replace fuel with fresh.
I have checked timing,changed main jet in carb,changed plug and points.tried all types of exhaust paste.
When returning home bike runs fine on low revs until she's opened again to full throttle.
Next stop new carb?
Any ideas?.

had a similar problem on my 350. Tried altering timing, changing jetting - nothing helped. Could be a bad exhaust valve. Do you see any vapors coming back out through the carburator when you shut it down? In my case the exhaust valve was bad.

Backfire,i call it a frontfire in the exhaust system cause by unburnt fuel in there firing,have you checked the valve clearances? A compression test should reveal a poorly seated exhaust valve.Remember these carbs are instant and throttle should be proggresive to avoid wasted fuel.

That cement/paste stuff is useless to seal the exhaust, lasts a few days before its blown out again.

Any old bathroom silicon sealant will do although high temp stuff logic says must be preferable.

Clean all carbon from the port and roughen with wire wool to provide key.. Thin smear around the edge of the port and around pipe, refix and leave overnight to cure..Do Not start engine in the hope of speeding process.... The addition of a finned cooling ring has the advantage of allowing a larger area to be sealed and then clamped... This will help with popping on run down with closed throttle...

Regards the back/misfire on full tilt a few suggestions.. disconnect fuel pipe from carb and open tap,
fill a container and check constant flow.. Float height correct in whatever carb you have and that the float valve is allowing fuel into the chamber at sufficient rate,ie blocked.

Advance/retard stuck.. weak or broken exhaust valve spring. Coil breaking down at high temp.
cheers gwilly

Hi guys----- MIKE82, could enlarge a little as to the sequence of events ?? ie }-- is this happening as you accelerate from mid-throttle up to full throttle OR is it when you close the throttle from fully open down to mid-position ??( in all gears as well ??)-- Perhaps a bit of the bikes background, history, current set-up etc might provide a clue ?? JAFFA90---- Hello mate, you still using that "Frontfire" terminology ?? Didn't we discuss this a few months ago ?? I think you are on your own there with that one...try this}--- http://en.wikipedia.org/wiki/Back-fire ----------------

Alan R,hello pal,nah won`t have it,a backfire also kickBACK is when the engine goes BACKwards after firing and fires through the inlet valve and carb (back of the engine),a front fire is as said but occurs in front of the engine down the exhaust pipe.Your link has two definitions and should be made seperate somehow.L.O.L.

Come on AlanR your not biting (l.o.l.),when i picked my bike up ( 2008,535cc lightning) the guy showed me how to start it by kick cranking and decompressing for a few turns and then a kick back on starting,i`m convinced that`s why the primary chain was too slack with the engine going backwards and stressing the chain using the short run,for now the mikcarb choke (useless)is not used,i just turn the idle screw in half a turn and cover the conicle air filter with a cone shaped hood and start with NO throttle.

Hello JAFFA90---------- I'm not biting as such, more in the way of an interested "nip" (Chuckle)--- I also disagreed with that Wikapedia description as in my book anything happening within the carb. is a SPIT-back as opposed to a BACK-fire which occurs in the exhaust system..... OK, each to their own I suppose, but you need some sort of standardisation in terminology otherwise it's a return to the early days of Engineering chaos...... That starting technique you were shown is definitely a no-no, if only from the extra wear and back-stress that the oil pump drive spindle will be getting....Try using LES H's way---- it's much easier.(where is our good friend these days ??)............ MIKE82, sorry---trying not to hi-jack your thread... What's your answer to my earlier questions, it will def. help in diagnosis ??