backfiring, rich or lean...

[A Yerbury]

started her up after 3 weeks standing but had a lot of stalling backfiring...ok so I tried the usual, hotter plugs, juice up the batt a bit, a dollop of pro fst and a quick tightening on all pipe flanges and carb hoses (the jubilees) better now...but the backfiring got me thinking about what causes it and -ahem- what actually is it thats popping? cheers chaps.
Alex..

[DesmoDog]

what is pro fst? Mine does the same

[Duncan]

Back firing is usually associated with loose exhaust joints and usually worsens the closer the leaking joint is to the engine. Your engine uses back pressure in the exhaust (to a much lesser degree than a 2 stroke) to function but changes in back pressure affect gas flow through the engine and also how and where / how quickly the mixture burns and backfiring is just an explosion in the wrong place / time.
If you were to make radical changes to the exhausts by fitting straight through systems etc the mixture, valve and ignition timing needs adjustment .
It could also be caused by a sticking valve or tappets set wrongly.
Normally a bike will tolerate cans being changed but too weak a mixture will cause spitting back thro the carbs as well as preignition and overheating and even holed pistons.
Could your bike have been spitting back as Monsters will do this when cold, they often fire up on one cylinder and as fuel draws through to the other carb it chimes in and spits and spats for a little while. You often hear a noise that sounds like the carb diaphragms being popped open too when it spits back. If you were running open carbs you might even see flames at this point and my old Triumph once caught fire from this where it caught back to the leaking carb tickler.

[A Yerbury]

cheers duncan, most informative. Ive just re impregnated a k and n with oil, so my initial suspicions were that it would be running a bit richer than normal (its open and when dynoed I suspect the filter was a bit dry and it was set up thus) so i took the filter off to have a peek and "lean it out" a bit mixture wise but it was the same and there was this spitting of mist from the top of the carbs, although that should be normal when looking down at it as the lid and filter are off giving a weak mixture? ah well I ve got a standard lid in the post so I can make some comparisons when I get that and do a bit of drilling.
thanks again.
Alex.
ps checked the flanges on the pipes as that was a bit iffy when I got it, they still seem fine although because of the angle of the pipe and flange getting a good purchase down there is a bit of a bother. so to sumarise then backfiring would not be a result of too rich a mixture?

[A Yerbury]

d dog pro fst is made by silkolene and helps prevent carb icing on monsters and many carbed bikes in cold/damp conditions. (ps pay attention as the stuff gets a look in here at least once a week!!)

[spacemonkey]

Oi mush, have you checked the down pipe of the rear pot where it joins the main exhaust manifold? That's where mine was leaking and it was difficult to trace. I had to have a bit cut out from the 'cuts' so that the jubilee type clip could be tightened sufficiently.

[A Yerbury]

oi mish mash, the jubilee s arent on the exhaust side? just flange and collets (the nations fav drag artistes) next time get some bmw jubilees 50p cheaper and they are the narrow size.
al

[spacemonkey]

Can't describe it too well, but if you follow the downpipe from the collets/flange past the shock, and roughly just under the swing arm pivot it is there that the pipe clamp on to the x shaped collector pipe bit.

[A Yerbury]

aha! I assumed it was all one piece? cheers. fingers crossed for no bellypan removal...also does anyone know what the factory standard fuel mixture adjusters are set at? DS says 3 turns out from fully in is "average"

[Fosse Foxfight]

No guys.....the most common form of "back" firing is incorrect ignition timing. This is usually when ignition occurs when either an inlet or exhaust valve is open. So.....QED, either the ignition timing is out or the valve timing has slipped...your belts ok Yerbs? The "fire" that occurs when an inlet duct is taking in air is more of an "out of placefire" :p ....which is it Yerbs?

Sean

Bouncebackability....ahem....

[A Yerbury]

but in that case it would be occuring all the time, possibly a slightly lean mix will suffer when cold or starting after standing? Ive upped the idle tick a pinch and this has helped with initial stalls as it was set a bit low. after Ive done a few things with the new lid I may richen the mix slightly, obviously just a quarter turn or some such on both which is easily reversed.

[rjc]

If this is happening after the bike's been standing for some time it might be nothing more than humidity/corrosion in the ignition circuit (HT leads very likely). Bear in mind a weak ignition at the plugs is going to look like rich running cos it won't burn the fuel properly.

Richard

[Fosse Foxfight]

If this is happening after the bike's been standing for some time it might be nothing more than humidity/corrosion in the ignition circuit (HT leads very likely). Bear in mind a weak ignition at the plugs is going to look like rich running cos it won't burn the fuel properly.

Richard

To be fair, this is probably the likeliest, moisture in the ignition circuits.

[DesmoDog]

Mine had just warmed up when I parked in very chilly shade. Jumped on a couple of hours later and didn't let it fully warm up. It misfired for the first mile of so and then was fine. Does this sound the same?

[A Yerbury]

yeah it does, I was used to it last winter and seem to have forgotten about the damp as have not been on the darn thing much..........Incidentally has anyone bothered with iridium etc etc plugs on a monster? I doubt they would make any diff on an old carb but theres no harm in asking? Also last year I had remus gp s and the sil backfire pop is a bit more pronounced methinks.

[A Yerbury]

although speaking of damp in the leccy bits is there any silkolene etc stuff to prevent this or is it a case of wait for the warmth to kick in?

[spacemonkey]

If i had mine running it would be Pro FST time in the morning..

Mine stopped backfiring once I had fixed the down pipe clamp.

[Scotty]

whats 'overrun' then ?

[A Yerbury]

I know what it sounds like. Here space cheers for the tip! located the bugger but it wouldnt budge either way so sprayed a bit of wd on in the vain hope (we know it rarely works but hey...) that the bolt might free but nothing. "oh well that must be tight enough then" and left it at that, fired up engine for idle fiddle and a smidgen anti clock on each carb for richness but whats this??? ah the wd fumes smoking of that joint I sprayed...although as the engine fired the fumes undulated in time with the motor! so there is a wee leak there. something for the little mechanic chappie to sort out before I leave.
Alex.
pro fst in the morning.............the smell of victory!

[spacemonkey]

If you get it free (mine was a bugger evidently, so I did the lazy option and got the Shop to do it!) you could reassemble it with firegum or similar exhaust sealant paste.

[A Yerbury]

before I go Im getting the seals done at the front, and a bargain replacement sachs put on as mine is a mess (I may be a ponce and get the spring blasted then powdercoated red too) I dont have a voltmeter so Ill get the fella to check that...oh and bleeding the rear as I got bored of that job after about 2 futile hours..so while its being tweaked on the bench Ill get that seal looked at. cheers again, firegum you say. also the wd trick and watching the smoke move was serendipidous! as you say its in a dirty spot, is a tricky angle and like most stock allen bolts its made of mozzarella...