Dellorto PHBL 22 AS Jetting?

[Samurai]

Hi Bob, that's a very good point! I must admit that being a bit hot and bothered by the time I got back (having spent all morning playing around with the idle settings!), that I completely forgot to pull the plug before I shut the garage door and walked away for a break.

After your suggestion, I've just gone out and checked and you appear to be spot on, the plug is very black and sooty!

Ok, looks like going down a couple of needle jet sizes seems to be the right way to go anyway, but probably not raising the needle position much afterwards.

Should I drop two or three sizes do you think? The 268k jet appears to be the second richest jet out of the 5 available for this carb. My gut feeling, based on how many sizes of idle jet I've had to drop, is to go straight for the weakest 260k, the next size up being a 264k. Not holding anyone to anything, but do you have an opinion?

Many thanks for the help so far chaps, it really is much appreciated

[Bevel bob]

I have a 260 in my 250cc bike, although its a different carb .

[Samurai]

Hi Bob, thanks for giving me some confidence in my gut feeling, I think I'm going to order up athe 260k and a choke jet maybe half the size of the one fitted too.

Does anyone have any experience of needle jet kits, made for Dellorto carbs, by a company called BGM? They seem to supply many of the Lambretta and scooter dealers with them and a set of 8 common sizes needles is about the same as one genuine Dellorto needle? Normally I'd steer clear of anything that's not genuine, but loads of companies seem to be selling these, although most are currently out of stock in the UK.

When I was a camera operator in the print industry many years ago, I always used to say if you're having a problem with the exposure and need to make a change, at least make a dramatic change so you can see your moving in the right direction. More often than not, doubling or halving the exposure would turn out to be bang on! Wonder if that will transfer to carburation? Guess I'm about to find out

[double diamond]

The starting (enrichening) circuit that is deployed with the “choke” lever is completely independent of the main fuel metering circuit. So installing a leaner needle jet will not lean out the starting circuit. But it sounds like a leaner needle jet and a leaner starting jet are going in the right direction. Ideally, you should end up with the needle clip somewhere between the top and bottom grooves. The clip in the top groove indicates that you need to go leaner, which the 260 needle jet will do.

If you’re able to wrap the intake stub with PTFE tape and fit the carb over it, I would suggest the fit is too loose. The carb should be a snug fit. When you tighten the clamp on the carb, you shouldn’t be able to twist the carb on the intake stub.

Matt

[Samurai]

Just a quick update on the jetting in progress.

Having followed all the advice you guys have given, I can now report the following positive developments. With a .35 idle jet fitted, together with a .40 choke jet and the lighter 6.5 gram floats, the bike will now start easily from cold with full choke and about 1/8th throttle and once warm will tick over nicely without choke. The mixture is set 1 1/2 turns out and the idle speed screw 4 turns out. Currently the best results are with the original 268k needle jet and a D31 needle on the lowest notch e.g. richest position.

Set like this the bike revs cleanly up to about 3/4 throttle but starts to big down after that when stationary. On the road and under load it is a bit juddery under acceleration from about 1/8th to just under 1/2 throttle but settles down ok once cruising. However it still gets stuttery from 3/4 onwards, but the main jet fitted is a pretty weak 98. I've ordered up a 105, 110 and 118 mains, together with D42 and D43 needles, which should be a bit richer too.

I'll keep you posted in case any of this helps someone else in the future and saves them a bunch of cash on jets