Rough running?

[olly]

I'm running a 1275 A+ which has an LCB freeflow exhaust and electronic ignition but otherwise std Ital lump. Its done about 85k I think.
Intermittently it will run rough and lack power (i guess timing). It is almost like its running on 3. However it only happens as you apply power, ie first third throttle fine but push it past this 'line' and it runs rough. back off slightly and its fine again. The wierd thing is that one day its there and the next it will run great. It will even 'fix' itself in one journey. It isn't related to wet weather, engine heat, air temp or driving style.
vacuum advance retard all works fine (moves when u suck it). Carb was rebuilt. When the engine is in its ok phase it runs very nicely. tried replacing dizzi cap, leads coil with others but not new ones.
checked plugs, 1 and 2 are good, 3 is a bit sooty and four is right colour but more brown deposits than 1+2. gaps all ok. tappets all good.
any ideas guys?
At the moment my thoughts are that no3 doesn't always fire, but why?

[Cam]

I had similar effects when driving my 1380 Ital engine up through the gears. My problem was that I was running the standard SU fuel pump and it was starving it of fuel when I was accelerating too much. I replaced the pump with a FACET fast road pump and the problem disappeared.

[Kevin]

The symptons are similar to when the dashpot oil is low, but as you have a sooty plug it could be a valve starting to play up, have you done a compression test ?

[olly]

Cam,
Could well be the prob... have had issues with the pump in the past, first ran rough after it stopped and I had to bash it to get it going again...
Funny that it should be intermittent though?
where did you get the uprated pump from?

Kevin,
Dashpot oil is OK - just below where the piston cowling bulges if you see what i mean. Don't know about comp. as no tester but I can't hand start engine as CR too high - the handle just gets bent!
However this is an old engine which I run on unleaded so the valves are probably not too happy.

Cheers all
olly

[Cam]

Funny that it should be intermittent though?

Maybe the pump is on it's way out.............

I got the pump from Minisport: http://www.minisportshop.com/

I got the up to 150 BHP pump kit. BUT I had to rear mount the pump next to the tank (as it is a pusher type). I then ran braided hose up into the engine bay where I used a FACET filter king (also from Minisport) to filter and drop the pressure from about 5.5 PSI to less then 2 PSI. Otherwise the needle valve on the carb float chamber won't close and you'll get fuel everywhere!