Any one ever had this problem?

* Idles well
* Runs great at full throttle
* At mid-throttle it will run OK for a second, slow down, then backfire out the air filter. Then idles fine.

I've spent all day trying to figure out what is wrong. Did a compression test, retorque & re-adjust valves, swapped plug wires for a known good set. Re-tighten inlet manifold bolts. Removed soft-mount insulators and bolted carb straight to the manifold. Nothing has made a difference.

Plug #3 looked like it was running different from the others (cleaner burning). Compression is good. Spraying WD-40 over the manifold gasket and carb base gaskets doesn't make any difference. I don't think there is a leak. I did have some slight leaks but got those fixed.

I know bad plug wires can cause a mid-throttle backfire. But I've swapped wires. Also the slow-down then backfire is unusual. It won't stay running at part throttle.

what type of sidedraughts are they? weber?
did they run properly before and it has just started recently?

popping out of the carbies usually indicates a lean mixture.
Is it happening on one carby barrel or all?

what rpm is it happening and is it only under load
Sounds like an emusion tube problem or an idle air corrector than is too small.

I had this problem twice...time for a carb rebuild...

It ran perfect the first day I got the new 1200. The next day it started doing this and has been doing it since.

It sounds like one carb, I cannot tell if it is one barrel or both. They are Mikuni Solex, Nissan Competition style.

I'm thinking a jet is clogged. Do you think I need to clean the passageways?

Quote:Any RPM, load or no load. Once I move the throttle past 10% or so, I guess that's about 2000 rpm or higher, then it runs OK for a sec, then slows down and backfires through the carb. With a load it backfires almost immediately. To drive it I was stabbing the throttle which was OK for the highway but almost impossible to drive around town like that.

So the little air leaks I had were affecting the idle, now I got everything tightened up (and insulators removed) it idles perfectly once again.

maybe the carbies need to be syncronised

Yep its one of two problems(mabey both)

1: ur carbs have fallen out of sync with each other. its most likely that as u said the were running well once before,adjust ur linkages and mixture screws, its a very fine line between in sync and outa sync i suggest u get a carb syncroniser(vacuum gauge)

2:its running to lean yea could be blocked jet or idle air bleed jets are to big

Got it fixed. Yes, she was running lean.

The problem was the air filters. I neglected to notice the Mikunis have the air jet/float chamber vents in a different place than the webers (the air filters are for webers).

A couple of holes drilled in the right places and she's running like new!



air vents

The 'cough' now only happens with engine cold, no choke -- it's exactly the same symptoms. But now with the chamber vented, no problems after she's warmed up.