Hi folks

Throttle bodies are spitting back during tickover and low revs, when at full chat there isn't a problem and lambda seems about right, haven't had this issue before when the engine was on carbs so not sure where to start; valves? timing? other?

30 sec Video uploaded on youtube:

video

seems to be mostly cylinder 2 with 3 and 4

All suggestions welcome

Alex

What spark control are you using? Are you getting tracking between HT leads?

[Edited on 28/4/11 by coyoteboy]

its VEMS (but practically the same as MS).

I edited after you replied, sorry. Are you running wasted spark, and do you have a decent gap between ign leads to prevent tracking and "mis" firing of the plugs?

wasted spark and leads are touching or close at a couple of places, havent spotted any tracking between leads, will fire it up in the dark to check (leads are new).

Yep, I had that when I initially went to bike TB's. They have some quite unusual flow characteristics at very low throttle openings. Try retarding the ignition at idle and getting the idle speed back up by opening the throttle stops a little bit. This serves to get the butterflies a little more open which will help your throttle response as well as hopefully fixing the spitting back.

Thoughts off the top of my head would suggest wasted spark could, depending on valve overlap and/or valve seating issues, cause back firing into the throttle. Same with the tracking obviously, if the valve is open when tracking occurs, I'd start by just separating all the leads and seeing if it improves.

Thanks for the tips, splitting the leads up is a real quick option, if no joy I'll then change the timing a few degrees and see if that helps.

As already mentioned, try adjusting the retard at idle - it worked for me

I had same trouble with my weber alpha setup, turned out to be an air leak.

Wound the timing back at lower rpm cells then a full fresh tank of Shell V-Power and 20mins of auto-tune and its running the best ever, still quite a few cells to hit yet but the difference is billiant.

Nice one, valves not quite shut by the time it fires then it seems, at least the correction will be aiding your economy at idle now!

[Edited on 3/5/11 by coyoteboy]

Sptting back at idle and cruise is caused by one of only a few things:

Ignition timing a mile out
Balance
Too lean (which could be caused by wrong jetting or an air leak).