28ml PHBH four stroking

[steve_lawrence]

I recently had an RT195 kit fitted to my Li Special. It has a brand new Dell’Orto PHBH 28ml carb which is proving virtually impossible to set up for a non-spluttery ride. My friend, a motorcyclist and scooter rider for many years has tried many jetting set ups but it has got him beat. The scooter goes well, but once you ease back on the throttle the 4 stroking kicks in. Under load it’s fine, but ease off to steady the speed and that’s when the issue starts. Would a dyno session possibly cure it?
The current set up is 50 pilot, 126 main, X13 needle second clip from the top, 264 atomiser. I’m not using the air box but there’s a foam air filter fitted directly on the carb.
Any advice would be gratefully appreciated.

[hullygully]

Ive used a 55/57 pilot in 2 x mates 28mm dellies on rt195's but with open carb
what's your air screw & slide bud, plus exhaust?
have you tried without filter?

[steve_lawrence]

It’s a 40 slide. I’m not near the scooter at the moment so I can’t check the air screw.

[Adam_Winstone]

Leaking float with fuel in it?

[Warkton Tornado No.1]

The slide is too rich. Dell’Orto carburettors - perhaps unsurprisingly - tend to need the same slides across the range & 45 is the normal choice.

If possible, borrow a 45 slide to try.

The good news - apart from it having a round slide - is that the 40 slide is easily convertible to a 45 via material removal from the edge…..

[Rich Oswald]

A 45 or even a 50 slide would probably help your issue.

[hullygully]

there is also another range of atomisers, the AS range as opposed to the AV's, which have the same no. range but are slightly weaker

[Adam_Winstone]

there is also another range of atomisers, the AS range as opposed to the AV's, which have the same no. range but are slightly weaker

^... I've not noted a response to your previous question / suggestion either: "have you tried without filter?"

There are SO MANY inappropriate filters sold by scooter shops. OK, I appreciate that many are selling them as they're meeting a customer demand, which is often too small as customers want a filter that fits under bodywork, without cutting a hole in the panel, however, these small and restrictive filters are not appropriate. A filter should filter out particulates, not provide a barrier restriction that does not allow an engine to take in the volume of air that it is often gasping for! Any efficient filter will filter out the rubbish but not restrict airflow.

My way of assessing whether a filter is appropriate for use is to jet / road test the carb open mouthed (no restriction to intake), then to add any associated filter elements to the intake and to road test again to see how each element impacts on the motor. Any addition that restricts the airflow and gives symptoms of running rich is identifying that your added element is not appropriate. NB: I say any 'filter element', rather than filter, as any air hose or airbox can also be a restriction (regularly are!). PLEASE NOTE that simply reducing the jetting so that a dyno gives the correct AFR ratio actually means that any restriction in air is matched by a reduction in jetting, which gives the right AFR BUT actually provides too little of both (air and fuel), leaving your motor gasping for more of both!

Any decent filter setup will not cause a well jetted open mouthed carb to suddenly start running rich. Before you can assess a filter, you need to have the jetting set appropriately for your specific motor's need... then you can look to introduce and evaluate a filter.

Good luck in getting it sorted.

Adam

PS - These are lots and lots of reasons why your jetting could be rich / out but you (Steve) but you could well find that this is a filter issue that will not allow you to jet your carb correctly. Remove the filter, sort the jetting, then reintroduce and evaluate the filter.

PPS - I am a huge fan of running filters on carbs but am horrified by the small / restrictive arrangements that many owners strangle their motors with.

[Covboy]

Out of interest Adam which filters do you use or favour?