Effect of humidity and air temperature on carb performance

[storkfoot]

I didn't clean the filter last night but I did take it off and stuff some kitchen paper in it for a few minutes to absorb a bit of the petrol/oil mix that had been spat out____I wouldn't say it was soaked but, yes, I can see that this may have been a factor.

Today, it has been much better behaved but I'm not sure whether this was the spitback in the filter, or me blocking what I've always thought was an overflow tube leading 90 deg. down from the float chamber, but now I'm not so sure (Keihin's seem to have loads of overflow type valves some of which need blanking off, like the one for a diaphragm on the top of the bore, some of which I clearly, as you can see , don't understand what they're for___if I get a chance at the weekend, I'll post a pic and show you what I'm on about).

Anyway, I suppose the filter is a limiting factor and does make the carb harder to jet, but the amount of spitback when I ran it open mouthed was pretty bad .

Colin, I stick a plastic bag over the filter when I'm in work for that very reason_____a wet filter is totally useless as I too have found out

Thanks for the replies

[storkfoot]

What needle are you using Storkfoot? I have found the GJH needle works better for me its a bit weaker and has vastly reduced the 4 stroking that used to occur on mid throttle.

If you are weak in the mid throttle position it will hesitate and jerk, a bit like fuel starvation but it will clear as yo go through the week spot. If its bogging to the point of having to stop then its definately too rich. Have you cleaned you filter lately?

Colin, it's the SCK one that came with the carb and it's just marked 28, for 28PWK. I've other needles from SCK which, from memory, are marked 21, 24, and 30. I think, at the weekend I'll take your pointer and try the 24, which I would think would be the next leaner.

It is bogging to the point of stopping, or it was yesterday. Today it has been boggy again. It manifests itself particularly when under load, ie going up a hill. It'll more often than not clear if you come off the throttle, drop down a gear, and rev through it. Sometimes it won't though______this is was happened yesterday____it'll just die. There will be lots of vapour in the filter so the most likely way to start it is petrol off and full throttle to burn off all the excess fuel____I suppose this then saturates the filter too, a vicious bloody circle

Colin, did you get the GJH from Allens?

Paul

[coaster]

No Paul, it was from the German outfit http://www.lambretta-teile.de/.

If you filter is getting soaked with petrol then it might be that the petrol is evaporating and leaving the 2 stroke ol in the element and blocking the air flow. This would be a gradual process I would guess. Have you tried cleaning it?

[storkfoot]

I do clean the filter every couple of months or so but I've never noticed a difference performance wise______you do get a fair bit of black cack out of them but as I clean it in petrol, with a small dash of 2 stroke, there's not a lot to gain from overdoing it, I would have thought.

Having said all that, I will give it a proper clean tomorrow night

[elgarra]

It's like the folks saying their Vespa only runs well when it rains..................and you know they've been keeping their waterproofs under the seat, covering the air intake.

[storkfoot]

What needle are you using Storkfoot? I have found the GJH needle works better for me its a bit weaker and has vastly reduced the 4 stroking that used to occur on mid throttle.

Colin, you're probably aware of this but having spent the morning looking at Keihin needle charts (only to find that I don't have a leaner diameter one than the one fitted ), I believe your taper is the reason why you run a higher main jet that most other folks_____your GJH is the same as the "standard" JJH in terms of diameter and length, but you have a leaner taper. As the taper kicks in just before you go onto main, the bigger main is compensating for the leaner taper____anyway that's my understanding

Anyway, I had some success this morning____raising the clip 1, and leaning it out in 1/4 to 1/2 throttle range has improved the bogging (this was a cold crisp morning though), although it's pretty zingy and the plug colour may be a bit on the light side.

I'm going to run it like this to work and back for the next week and see how we go. Also, going to try and get hold of a JJK needle which is leaner towards 1/4 throttle and may mix well with the 42 pilot.

Thanks for your help Paul

[coaster]

I believe your taper is the reason why you run a higher main jet that most other folks_____your GJH is the same as the "standard" JJH in terms of diameter and length, but you have a leaner taper. As the taper kicks in just before you go onto main, the bigger main is compensating for the leaner taper____anyway that's my understanding

Paul

Paul, I wish that were the case but I arrived at the 138 main while still using the JJH needle. I had bought the GJH several months earlier on a recomendation on the LCGB forum but found it too weak but after establishing the main jet at 138 it was 4 stroking badly at 45 - 50 mph so I tried the GJH and bingo..ish. In the current cold weather the 4 stroking has almost vanished. The size of mu main jet is maybe down to the jet holder, I noticed a number on the side of it recently which would indicate that there are different ones. The only thing is that it doesn't seem to be offered as a variable by anyone selling parts for them, the mystery continues

Hope you've found your problem

[Adam_Winstone]

I believe your taper is the reason why you run a higher main jet that most other folks_____your GJH is the same as the "standard" JJH in terms of diameter and length, but you have a leaner taper. As the taper kicks in just before you go onto main, the bigger main is compensating for the leaner taper____anyway that's my understanding

Paul

Paul, I wish that were the case but I arrived at the 138 main while still using the JJH needle. I had bought the GJH several months earlier on a recomendation on the LCGB forum but found it too weak but after establishing the main jet at 138 it was 4 stroking badly at 45 - 50 mph so I tried the GJH and bingo..ish. In the current cold weather the 4 stroking has almost vanished. The size of mu main jet is maybe down to the jet holder, I noticed a number on the side of it recently which would indicate that there are different ones. The only thing is that it doesn't seem to be offered as a variable by anyone selling parts for them, the mystery continues

Hope you've found your problem

Looks like I might end up needing to order a GJH also as changing to H** needles has been a big step in the right direction, starting to clean up the unrideably rich mid-range of the J** needles. I've noticed that many people seem to be trying to clean up mid-throttle positions by restricting pilot and/or main jets, which goes against the rules of jetting!

I also note that the needles suggested for some Vespa kits are far weaker still on the needle taper so I'm reassured that you and I are not crazy to head in this direction. I like to think that I can get all progression stages to function as intended, rather than making one with another.

Cheers

[coaster]

Blimey 2009 how time flies Adam, trip down memory lane Glad you are making some progress

[Adam_Winstone]

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