Bad running/back fire

[CPH Lambretta]

Last sommer I had problems with bad running and misfiring on my GP, I didn't get the problem sorted before the end of the summer and decided to give the motor a total rebuild over the winter.
I did the rebuild but still have the same problems with bad running and misfiring at high revs .
The original set up was :
Home tuned 175 kit with original standard exhaust ,new 22mm jetex running a remote filter,new bgm electronic kit, new mec crank and Li 150 gearbox.
After the rebuild its running a Harry Barlow tuned 175 kit with a rebuilt 22mm dellorto the rest is the same.
Its still running s**t so tried changing a few parts. So far I've changed the plug,plug cap,HT lead,coil,and stator plate and this hasn't helped.
It starts holding back when it gets to about 5000 revs and backfires if you try to push it anymore.
I have changed almost everything now but still the same problem, today I took the remote filter off to see if that was holding it back and that just made it much worse,it started holding back lower in the rev range.
So I'm thinking of trying a different exhaust (maybe the old one is partly blocked) and checking the fuel flow to see if the fast flow tap is flowing what it should.The motor block was ported for an imola kit previously but the ports on my 175 kits are pretty standard, could this give problems?
Anyone else had the same kind of problems or got any good ideas?
Its really p1551ing me off now
Cheers Nick

[thebassmasta]

TIMING

[coaster]

Have you tried a choke test?

[CPH Lambretta]

Hi Coaster,
Thats not something i've tried.
When trying to run on full throttle and its running badly if you close the throttle to about 3/4 it picks up and runs much better.
Like I say with the air filter off and just the bellows on the carb it runs far worse and holds back/runs s**t at anything over about 1/4 throttle.

[coaster]

I was just thinking that if the choke improved the situation it would point at main jet being too small? Also, you checked the fuel flow through the tap but is it flowing though the float valve fast enough? Just some thoughts, I'm not claiming I know what I'm talking about

[thebassmasta]

where did you get your fast flow fuel tap?

[CPH Lambretta]

the fast flow tap came from Scooter Center,it is the usual omg rear exit type.
I was sure it was something electrical until I changed just about everything and now the air filter thing makes me think its petrol/carb related

[nelson pk]

If you take the filter off and its worse, chances are its running weak as running with a filter on will richen...........filter off.........weaken.
Have you drilled the atomiser on the jetex? Jetex atomisers are notoriously lean for tuned set ups and should be drilled otherwise they do not let enough fuel through.
I think its a 2mm drill bit you run through. There's more info on the LCGB site about this.

From LCGB site:
One tip on tuned machines, it is sometime prefferable to drill the bottom of the atomniser (more so on Jetex atomisers) if you need a richer mid range. Start by drilling to 2.00mm, if you still need more fuel, 2.5mm

[thebassmasta]

has harry set the carb up? personally i dont like the jetex carb but H swears by them, just check the flow of the fuel before you do anything else, you should getting between 300 to 500ml anything less it is suspect, check it on reserve aswell

[CPH Lambretta]

Harry supplied the jets and atomizer. The atomizer is a 1.5.
I never discussed the air filter with harry, maybe its flowing too much air for this carb set up?