Help me diagnose my poorly 7R.

Poor running below 4krpm, wind the throttle on and it gets worse, to the point of cutting out. It then won't start for a few minutes (wet plugs at a guess). Once it fires up, you have to hold the throttle at a certain position till it revs cleanly through.

Once this has happened, you have to clutch it up past 4krpm and it runs fine.

I pulled my carbs apart hoping it was something simple, but nothing obvious apart from Dynojet needles and ever so slightly ovaled needle jets. I put the needles on their leanest settings but no joy, so I took the carbs off and put a second hand standard set on. It's still doing the same thing.

Assuming I've ruled out fuelling by fitting a different set of carbs, next up is the ignition system. I've got a pulse pick up coil, loom and CDI waiting to be trialled.

Am I missing anything glaringly obvious? Perhaps check the reg/rec- see if it's giving the CDI enough juice at low revs, split inlet rubbers (they did look alright, but it was only by glancing at them - gets a bit tedious taking the carbs off every 5 minutes)


Please help!

Nobody likes a split rubber. They looked good to me when I pulled them off, but will check again.

Shirley a split rubber would cause the fueling to get progressively worse as the vacuum increases with raised engine revolutions??

Surging can make a motor run all sorts of rough, do the revs hang before decreasing when you let off the throttle?

Have you rejetted after fitting your new exhaust system?

Have you done anything to the breathers?

No they don't hang, thinking about it I have noticed a delay when blipping the throttle on downchanges (8-10k), there's sort of a split second delay which isn't apparent at low revs (5-8k).

The bike runs huge mains to compensate for the air recirc system on the exhaust. The bike was originally running mains suitable for a snazzy system but it made no difference having Keihin 180 mains or Dynojet 132 mains. Having the jets slightly out would never make it run this bad anyway, flatspot yes, but not this bad.

Have you tried new ht leads and fresh sparkplugs ? Had an issue like this but car related in an old vw. Low revs had a horrible vibration with no real pickup that seemed to get worse as you put your foot. High revs were much better but that could simply have been masking the issue. Checked all sorts and, after speaking to my local mechanic he recommended the above.
What was really odd was that there was no gradual deterioration. One minute it was fine and the next it was st.

Fouling plugs screams bad mixture to me. But the bad mixture could either be the carbs themselves or an air leak.

If all plugs are fouling but firing, it has to be something effective all four like you say, the leaves you with searching for air leaks or trying a spare CDI. I'd bet on it being a carb jetting/air leak issue though.

Dynojet sizes are different to Keihin, and it has the emissions ste back on to lean the mixture out (somehow, black magic I assume).

You can (apparently) run an aftermarket system on standard jetting without any 'major' problems, I certainly had no issue swapping between standard and open can on the last one. If it was a jetting issue, it would have been cured by fitting the new exhaust.

Oh good haha. Still, which ones?

It's running standard carbs with the emissions stuff back on. I have tried to put it back as standard as possible to fault find.

And it's no better?