Poss' carb problem.

[St George]

My SIL 200 Stage 4 is boggy as I build up top end speed. Running a new Jetex 22mm, 50 Pilot, 118 main and choke is fine. It has a new Ramair filter and new standard GP 200 Exhaust. Recently fitted a new AF 200 standard head. New CDI fitted. Starts well, pulls away through gears 1,2 & 3 very quick. Once in 4th it gets boggy and hates headwinds or hills. This is a newly run in engine and the timing is fine. I feel that it is a carb problem ? Advice welcome, thanks.

Colin

[Scooter Paul]

Just some random thoughts; is it a carb with the propper 200 slide? I'm always forgetting but I think the 200 set up has no cutaway on the back of the slide, or is it the other way round? What atomiser are you using? This is quite important too.

Paul

[DaveTomo]

What gearbox, front rear & sprockets? Just thinking if it was the carb it would be boggy in all the gears not just 4th

[St George]

I fitted a second hand 200 Delly Atomiser as the one supplied was no good. So why does it fly through the gears until I get wind resistance or load. Main jet keeps nagging me albeit normally wrong. Lammies are a learning curve. Thanks for reply. Hi Dave Tomo. You sent this before I replied to the last. I have a standard SIL GP 200 Box and sprockets I was told.

[Knowledge]

. So why does it fly through the gears until I get wind resistance or load?

Most of the reason for lack of power in top is to do with insufficient torque. Even though you have a relatively standard engine, you might just have a mis-match of parts that are robbing you of torque. I'm thinking of the exhaust, or porting or carb combination.

Having said that, there are only two types of Jetex carbs, those that work and those that don't.

[Steve J]

Are you sure it's bogging (running over-rich) in top gear? Normal recommendation for a 22mm Jetex on a SIL barrel would be a 122 main jet, and if you've got a Ramair type filter, it's probably flowing more freely than a normal filter setup. To me, it's sounds like it's probably running weak, not rich.
Another 'feature' of the Jetex carbs is that sometimes the fuel passageway (drilling) into the float chamber is too small a diameter, which can cause a low fuel level & weak running at higher speeds. I'm not sure what the diameters are, but I do know that there was a difference between the DellOrto 2/22 and the Jetex. The remedy is to carefully drill out this passageway to the same diameter as the DellOrto - I'm sure someone on here will know what the diameter should be.

[St George]

Steve J. Thanks for a comprehensive answer and it all makes sense. This engine had a JL4 and 30 mm Delly when I bought it (The bike pulled like stink ). I think the crank is AF and con rod is Jap' as I have a 3 mm spacer under the barrels. This is the reason I played with the jetting. The plug is not sooty at all. I did source a standard airbox, would it be worth fitting it or fitting a bigger main. I take on board what you said about the drilling.

[Steve J]

The 122 main jet is the normal one for the SIL Stage 4 setup with a standard airbox and filter, so I'd regard 122 as the minimum size. If you do anything to reduce the obstruction to inlet airflow, such as fitting less restrictive filters, or altering standard airboxes (e.g. drilling extra holes, removing the 'kidney' silencing baffle, or increasing the diameter of the airbox inlet tube), then it would make sense to try a larger main jet. I've got a similar SIL setup with MecEur black crank (standard length rod), 42mm Clubman, and a DellOrto 2/22 running a 123 main jet. I've completely removed the internal baffle from the airbox, and replaced the airbox inlet pipe with the largest diameter one I could weld in (around 40mm or so, if I remember correctly). It's got a standard air filter, but I suspect it's running a bit weak, so I'm going to try a 125 main jet as the next step. There's also a fair amount of induction noise.....

[St George]

Thanks Steve. I cannot get to my scoot at the moment as the builders have invaded my garage with cement mixers etc. Hopefully get to the bike this weekend and swap the jets over. I appreciate your sound advice and will let you know the outcome.

Colin

[Steve J]

Funnily enough, I can't get to my scoot at the moment either because of all the stuff that's piled up in the garage...... I'd like to blame the builders (or anyone else for that matter), but in reality, it's all mine!