LCGB Forums

Hi all, long time since I've posted for various reasons, but I'm back with a query regarding slow running. Just returned to Pompey from 'Operation-2-stroke', a five day tour of D-Day beaches with eight other riders of vintage Italian shopping bikes.

My GP125 (Allstyles 186 kitted) ran beautifully on the road but has never really been able to maintain a tickover despite a two hour dyno session with JG Scooters at the IOW last August. To be fair to them it was literally dark when they finished and they simply ran out of time. The slow running screw is turned up to the max but it isn't quite enough.

So, is it a case of fitting a larger pilot jet? It's a PHBL25 carb. There are no leaks and the ignition timing is set at a very conservative 16 degrees. Exhaust is via BGM big bore V2.

The pilot jet may be too big and is flooding the carb' ....... on the other hand the pilot jet may be too small and the carb' is being starved.

Are you running an air filter ? is it better or worse without the filter ?

Timing could be too far retarded is another option.

Hope that helps

As you state no details of slide or jetting, hazarding a guess it sounds as though the pilot jet may be too large. Piston ported could be anything from high 30's to 50 plus as far as the pilot.

If you can get the pilot in the correct ball park area, that's when the mixture screw comes into play by allowing optimum setting. For instance, a 40 pilot may be too small, but even so, the mixture screw should still have an effect via adjustment & it's then intuitive as to whether a bigger or smaller jet will be required. I'd bet that whatever your pilot may be stamped that it is too big....

Anyway, this is a good read & can explain far better than I can, precisely & methodically how to remedy what you have:

https://www.vespaonline.de/wsc/attachme ... -guide-pdf

(With acknowledgements to the O P of this pdf )

Don’t want to play with mixture really as it runs so well from tickover to redline. Is 16deg really too retarded in these days of crappy modern fuel?

Are you sure it’s not the cable wants adjusting ? Has it stretched

Toddy wrote:Are you sure it’s not the cable wants adjusting ? Has it stretched

Was just going to type the same thing!

Pompeydave wrote:Don’t want to play with mixture really as it runs so well from tickover to redline.

That response suggests to me all the more reason to gain a better understanding of your chosen carburettor.....

Pompeydave wrote: Is 16deg really too retarded in these days of crappy modern fuel?

Hmmm. Not certain what it is that you are asking, but with static ignitions on even fast road tuned engines, I have yet to set the ignition any further retarded (from OEM) than to 19 DBTDC. However, I always stipulate super unleaded which is not quite so 'crappy' as the plain unleaded.

It's not a cable problem, I've checked that. I only mentioned crappy unleaded for those times when you're away at a really and you can't get Super unleaded. So are we saying then that the 'herd' figure of 17 BDTC is too conservative as I'm always (when available) using Esso Supreme- 98 RON and zero ethanol if sourced from a garage supplied by Esso Fawley?

Would 19 BDTC using Supreme be still probably ok? And how would that affect the jetting?

How we measure our timing (in most cases) is not that accurate. There's plenty of chances of being out by a coupling degree . If your 16 is slightly out and under 15 that could be your problem.
I'd still plump for the carb' but wouldn't rule out other options. A lightened flywheel can also struggle on tickover.

What is your carb' spec' ? Are you using an air filter ?

My TDC mark is definitly 100% accurate and I'm at 16 BTDC. I'm using a standard air filter. Flywheel is standard weight (BGM). Not sure on jets, I'll take it apart and have a look.

MickYork wrote:The pilot jet may be too big and is flooding the carb' ....... on the other hand the pilot jet may be too small and the carb' is being starved.

Are you running an air filter ? is it better or worse without the filter ?

Timing could be too far retarded is another option.

Hope that helps

+1

you're still goin thru the airbox, so have you removed the airscoop baffle?
drilled the airbox & removed the swan kneck?
using a breathe sweet filter?

By the slow running screw do you mean the tickover adjuster screw? If so have you check it's moving the slide when you screw it in and out I've known them to have damaged tips so they don't actually work also I wouldn't think it would be a breathing problem as you say it runs well when the revs are up which requires more air. When it's cold does it tick over with the choke out?

I'm using a standard air filter but via the later modified Innocenti airbox. Taking the air filter off makes no difference, it still dies. The slow running/tickover screw does move the slide up and down, but I do wonder whether it's too short? It isn't damaged, but clearly if it raised the slide by just a touch more I'd have no problems. When the bike is cold it will tick over on choke.

There is a procedure that should be followed for all two stroke carburettors. Fundamental, as all engines have to go up from the lower range & back down again is controlled by pilot in conjunction with the mixture screw.

Control of the tick-over (which might appear to now be the nub of the problem ) is in general set by an idle adjust screw. If the screw acts upon the slide, then if either wear, it may mean that a fast enough tick-over may not be achieved. In any case, presuming a spring & washer to be fitted, often a substitute shorter spring can be fitted, thus allowing a nat's more of the screw to go in as the spring can often get coil bound.

An easy-peasy check is to temporarily remove the spring completely from the idle adjust screw & check if that makes all the difference.

As it's been suggested, my opinion is that there is a herd mentality concerning what is a safe retardation of ignition, but the same could be said about the choice of carburettor bigger than OEM. That's not meant to be a criticism, honest(!) but my belief is that those owner/rider/tuners that are involved in all aspects of engine tuning/optimising are likely to have a far greater understanding of causes & effects whilst being open minded to things that get done to other two stroke engines besides Lambretta. Personally, my current rule of thumb for a static ignition is 19 DBTDC within the top half of the rev range & a tick-over will be achieved with a J/Lui/Vespa PX/AFR lightweight flywheel. That said, were I to build an engine that revved substantially more, I would expect the setting to be different, but by then I'm almost certain to opt for one of the retarding set-ups to hand.

Unfortunately, anybody can call themselves a tuner, even though, principally, all they do is assemble the parts. IMHO a 'proper' tuner will be capable of preparing an engine to a specification that will deliver good power & reliability, even if some elements of the work must be subcontracted out, such as welding, boring, plating etc.

Ok, if I advance the timing to 19BTDC presumably I’ll need to have if dynoed all over again?

Are you totally certain that your chok plunger is sealing off the choke jet? that will have a big affect on tickover once warmed up.

I have used 17 degrees on my kitted scoots (TS1, Avanti, GT240 and 200 and the only time that it wont tickover is when I forget to push the thumb choke fully back and that and I find zero diffence between super or E10

Pompeydave wrote:Ok, if I advance the timing to 19BTDC presumably I’ll need to have if dynoed all over again?

I've been honest as to what I do. A mate of mine ran 21 DBTDC with a mildly ported 200, PWK 30 & expansion chamber. BUT...he used Super Unleaded. It never had issues & was a joy to ride.

So, TBH I agree with you that there is a 'herd' following concerning retardation, or to a great extent. Without proper research, it's difficult to be assertive as to what is 'right' & the likes of me can only say what works on my tuning.

All that said, I suggest you initially re-set to 17-1/2 DBTDC & do plug chops @ 1/4, 1/2, 3/4 & WOT. Tape/marker pen/embroider(?)/Tippex the marks for throttle positions on the RH bar & fit an auxiliary toggle switch near the throttle to kill the engine otherwise readings are meaningless. I doubt you'll need another Dyno check. See how that goes. It should be more responsive & if encouraged by the results, try a bit more advance.

Ignition timing is such an influence on an engine, two or four stroke. I once owned a split screen camper that was set retarded. I advanced that until it just started to pink, then backed it off a bit. The difference was amazing! It made me change the front cross plies because with the increased performance, you only had to fart & it would change direction

BTW. Despite my very poor view of aftermarket electronic ignitions, I KNOW that a good electronic ignition with a good advance/retard system is a massive boon to a two stroke engine. I used a Motoplat on track in the 80's that initially advanced then retarded up to about 10° so setting was an initial faff, but worth it for the increased punch out of hairpins. I just feel the need to correct the view that poorly made electronic ignitions from India (mostly) are going to improve reliability over OEM contact breakers. Good job Esther Rantzen is not on the case!