Wont rev out and misfiring - carb issue?

[Adam_Winstone]

You can often only go so big on the main with the standard 175/200 atomiser before the rich atomiser causes it to flood between 1/2 and 3/4 throttle, never allowing it to get onto the correctly sized bigger main. This situation often results in riders limiting the main size, in order to keep progression clean, only to run weak at full throttle.

You should probably be getting this along to a decent dyno centre.

Adam

[victor]

Thanks Adam,

Still goes fine through the revs, little bit of bludder when you hold it at 50/60% throttle with no load, but no difference from 110.

If I read you correctly, I'll get problems at 1/2-3/4 when I go too big? If so I'll just continue until that happens and go down a step or two...

I'm going to go for a race tour in about 2 months from now and after that I'll get it dynoed, just need to understand who can do that in London...

[Adam_Winstone]

Experience with 22mm and 22-24.5mm carbs (so your 20mm will differ slightly) has shown that mild road tuning (porting or the fitment of clubman or expansions) can require that the motor actually needs a much bigger main jet when you get to 3/4 and above, however, the standard 175/200 atomiser then allows too much fuel to pass (too rich) at 1/2-3/4 throttle. This richness (normally when trying to run larger than a 122 main... NB: a JETEX 123 main is commonly weaker than a Dellorto 122!) commonly results in the motor becoming boggy at 3/4 and never clearing enough to progress to the revs that the motor wants to run at from 3/4 onwards... so it is boggy and tends to max out at about 55-60mph on 200cc motors [very common on SIL 200 motors]. So, riders then find that they limit the main to about 120-123 so that the bike does perform reasonably well and doesn't bog down too dramatically, however, what this does is mean that it is jetted ok enough to progress through 1/2-3/4 but then actually runs weak at full throttle as the motor really wanted a bigger main, which the rich atomiser would not allow! You then run weak at full and can have issues.

The solution for 22mm carbs is to change to a weaker atomiser and larger main (sometimes much larger!) so that you can jet it correctly for all stages of progression, rather than compromising one stage for the benefit of another. Unfortunately, such a dramtic change of jetting (result of atomiser change) requires very careful testing to ensure that you are not putting your motor (and yourself) at risk, which is why a dyno session is to be considered. If you want to do this testing yourself (read up on 'choke tests') then make sure that you have a range of Dellorto jets and atomisers to try as the Jetex parts cannot be trusted (as per the Jetex 123 being weaker than a Dellorto 122 example).

Adam

[victor]

Think you might be right - tried the 122 today and that didn't go very well at all, very rich, felt like driving with the choke engaged although it pulled from 3/4...

It didn't resolve the problem though - could it be that it doesnt get enough air? I've the original, restrictive air box...

Went back down to 116 where it goes incredibly clean, but feel less responsive and powerful than with 118.

118 though is on the limit to be too rich from 1/2 / 3/4 so a bit unsure where to go next

[Adam_Winstone]

Improving air flow would certainly help. Innocenti actually knew what they were doing when they developed the air box design, which was revised a number of times through the Series III series so that the motor could breathe correctly with the free flowing later pipe, 22mm carb and higher revving motors, with revised points, shorter gearing, etc, etc. If your new pipe is trying to get gasses out quicker then it makes sense that it'll be trying to breathe in better too.

In your shoes I'd be improving air flow into the motor and trying a different atomiser with bigger mains.... but at your own risk

Adam

[victor]

Thanks Adam,

Theoretically what would the consequence of not getting enough air on high revs be? Characteristics...?

Are there foam filters available for the standard carb..?

Many thanks V

[Avantone]

As Coaster says, it sounds more like ignition. Be careful about fuking about with jetting on what is essentially a standard motor - if it's misfiring and you think it's too weak, put the choke on when it does it - if it still misfires, it's ignition. Usual suspects, points/condenser, wet/high resistance connection, coil, ht, plug etc etc

[victor]

Hi Tony,

I'm rather suspecting it not getting enough air (or fuel but limited by the tap in that case).

118 had a nice coloured plug whereas it was quite greyish on 112 earlier, although I've been doing about 1000 miles like that, also some longer stretches.

122 was too fat, and it didn't solve the issue.

I don't think it's the ignition - points look good and are adjusted correctly, coil (piaggio), lead and plug cap all new and engined strobed (by me though so you never know ) so there should be no issues there.

What would lack of air result in theoretically..?

[mick1]

Lack of air ((like running with choke on) Too rich a mix and boggy....quite restrictive.....wouldn't have thought it would cause backfiring....dyno costs about £35 plus whatever the soloution may cost....money well spent in cases like this

[bristolmod]

Had similar on my TV175- tracked it down to a semi clogged fuel tap. Worth changing/ checking.