Bonneville America Forums Forums Technical Forums General Tech Missfiring at low speed

Can anyone help?
I have a 2002 TBA I got a few weeks ago and am having a problem with the engine missfiring at low speed. OK doing 60/70 on open roads but around 20/30 it starts to splutter. Plugs replaced, new HD leads, carbs cleaned out - anything else that could be looked at. The bike appears to be standard, the only after sales mod appears to be the fitting of the Triumph short pipes. Love the bike but this is driving me crazy!

I had a similar problem that turned out to be a coil failing.
http://www.bonnevilleamerica.com/forums/...true#Post374640

Hello Steve and welcome to the site. Could you give us some more info? What exactly were you doing when the engine begins to miss? Accelerating? Cruising? Idling?

Have you adjusted the idle screws or tried to balance the carbs? Dont forget to inspect for vacuum leaks. The vacuum caps and intake clamps/rubbers should be checked for tightness and cracks.

I find usually if the bike runs fine at higher rpms, it is a fuel or air related problem. I wouldn't change any coils until everything else has been verified.

Who cleaned out the carbs....Why?

Hi,
the problem seems to occur when cruising at low speed.I am not technically very competent, so when I said cleaned out the carbs I mean check there was no crap in the bowls stopping fuel getting through. I got someone to do this for me.
I will inspect for vacuum leaks and that he vacuum caps and intake clamps/rubbers are tight and crack free, once I have looked them up and find out what they are lol.
As for balancing the carbs I think that is a bit beyond my skill set and will get some help.
Strangely been out today and not really had a problem.

imho coil failures are counter intuitive. That is, they show signs of failure more at idle and off idle then at half throttle or above where they appear fine.

Draining the bowls is fine, but nothing like dropping the jets and inspecting them. Drop too the air mixture screws as a little bit of foreign matter even there will cause symptoms such as those your motorsickle currently exhibits.

You know what Jim, you might have something there. I just reread Rob's post and the problem he had. It really does appear counter intuitive. I guess the old pickup coil problem and running on one cylinder is the same....why?????

Steve,check everything out first. Look closely at where the carbs attach to the engine...those rubbers and clamp should be tight and free from cracks.

If you can get your hands on a known good coil, then you can swap them out one at a time to see if the problem follows a coil?

yeah Greg. Even though I wouldn't look at the coils first.

Steve, to help narrow it down see if a simple fuel test will point the way. Albeit, even that test may be intermittent. But if it does even once indicate fuel, then a floater is in the fuel system, probably in the idle circuit. Heck could even be a bit of refuse sucked up into and obstructing the idle jet (read varnish). The down and dirty fuel test? Does the engine when stumbling, get a kick in the rear when the choke is pulled out?

Being a 2002, the carbs could stand to be properly cleaned even if they are not at fault. But do not clean them until you know it has to be fuel related. (money concerns, chasing your tail etc.) For sure look at all the fittings as rubber juices fades away.

To get her running look at the vacuum port rubber caps 4 of them, hum... you don't still have the air injection on it do you? If so one of the reed valves could be messing you up... The connection where the pipes attach to the motor too. (4 nuts). The crossover pipe (underneath the beast). The header pipes to slip-on connections. (clamps on each pipe near the forward controls)

Good luck. You'll get it sorted soon enough!

Hi,
took it out for a quick run, 4 hours, over country roads, through towns and on the motorways to vary conditions/speed and try out the down and dirty fuel test. The bike never missed a beat? The only thing I had done was give the bike a good clean and polish so I may have accidentally knocked something back in place. Anyway I am going to leave well alone for now and see what happens. Thanks to you all for the help.
Steve