Hi Guys, After much reading through this site (awesome info here!), I've decided I couldn't find a thread describing the issue I'm facing. I'll apologise for the long post, too... I've got a '96 600 Monster with some minor mods (K&N filter, Remus twins, 140 main jets, 42.5 pilots) that was fouling plugs like mad. This was traced back to a failed vacuum petcock and was sorted. First time I fire the bike back up, it runs OK at idle/using fast idle lever, but when revved up to around 3500-4000rpm it starts misfiring like crazy! I've tried everything I can think of and I've gotten nowhere. I'm hoping that someone here has faced a similar problem or is cluey enough to help out. So far I've done the following: Fresh plugs Fresh coils New leads Checked static & dynamic timing Reset air gap in ignition pickups Checked resistance of pickups, coils, ignition boxes (?), leads, caps - all good Carbs were balanced when the bike was running properly Compression check - within spec, even across both cyls. When I pull the plugs, they have fouled which leads me into thinking ignition. But I'm running out of components to check/replace! Any leads you guys can provide would be incomprehensibly appreciated. Cheers Alex
@Arquebus is good on these motors, as is @Derek. @chrisw is great on all things ignition. There was a similar problem on here fairly recently and it was traced, iirc, to an earth from one or both ignition boxes. Something along those lines anyway. Fingers crossed it's the same problem or something equally simple.
Fuel delivery may be a problem.... It may be reduced just enough to weaken the mixture causing spitting back through the airbox, or backfiring in the exhaust. Once the revs are dropped down it may tick over perfectly alright as enough is getting through. .............is the tank breather clear? .....is the filter and pump allowing enough through? Also, have you checked the tiny filter in the fuel line Tee-piece between the two carbs? It might even be some crud is floating around in the float bowls and blocking the jets when the demand for more fuel is there. As the OP says he has checked timing etc, I'm assuming the belts are correctly installed and not one tooth out. Also, if the plugs are fouling, check the operation of the fast idle lever and the relevant mechanism....the 'choke' needles in the carbs can wear as well.............so the problem may be over-fuelling...............a bit like leaving the choke on.
Hey Arquebus, I'd almost written off fuel delivery because the plugs were so black, but it couldn't hurt to check it all again. Been going around in circles anway . To add to my first post, there's black smoke coming out of the exhaust throughout the rev range. This was not there before the changes. Carbs have been apart & cleaned thoroughly with carb cleaner so there should be no crud in the float bowls, fuel petcock is new, new fuel filter, breathers are clear - I did have an issue whereby pressure in the fuel tank from expansion on a hot day was causing fuel to run through the main hose to the carbs (this happened when the carbs were off, of course). I found the lower breather hose was pinched. Moved it and it seemed to stop happening. Frustratingly, the Haynes manual I have suggests that there is no method for testing the fuel pump? Assuming there are no figures around flow rates or pressure? Fast idle lever is operating OK, though it was a little stiff/slow to retract. It had gotten caught but re-greasing the slider mechanism seemed to solve this.
If this is a 96 model, it will surely have a choke, not just fast idle as on injection versions. Sounds as though the choke may not be coming off properly. How fast does it idle??
1200 rpm or so when it was running well. I can manually make the choke come off. Right now, it won't idle by itself cold without holding the throttle or opening the choke - I figured this was symptomatic of whatever was causing it to run like a pig?
Also, faulty fuel tank breathing can cause fuel starvation due to the vacuum build-up. This doesn't square with sooty plugs though, which suggest over-fuelling.
My thoughts exactly. My mind keeps going toward ignition as there's clearly plenty of fuel getting through there (enough to blow some out the exhaust and ignite it, at least). As far as tank breathers go, there are only two, yes? One in the cap and the main breather hose next to the main fuel outlet...?
In the cap? On my 96 SS it was straight out of the front of the tank, via a welded in pipe first, then a rubber pipe with a one-way valve in it. You could try running the engine with the cap loose
The RH exhaust is blowing more smoke, but there's definitely some coming from each. Mind you, the crossover in the exhaust would make it difficult to judge, no? Which is something I could possibly have done in my haste to enjoy the weather, but I'm 98% certain that it's all back where it should be. I'm going to triple check in the daylight tomorrow, though. Carb cleaning & gasket kit, new petcock, un-pinched breather hose. It wasn't running perfectly to start with, but was more than acceptable prior to the petcock issue. Will do - could be partially blocked I guess. The monsters definitely don't have the pipe in front.
