LCGB Forums

Hi guys first post on here so go easy on me haha, some of you may know me from "the other forum" but here goes. Engines a Mugello 186 bored to 64.5mm and ceramic lined and fitted with a 61/115 crank, all built by Harry Barlow, was using a Scootopia clubman that was extremely good with the air filter and air box but recently I fitted a Darrell Taylor modified TSR evolution (although I'm not sure how as me and Harry spent several minutes on our hands and knees with a DT one and a TSR one and neither of us could see any difference!) but never mind that's not the problem.

The problem is this. Whilst trying to jet it in, the PHBH that is using an x7 needle AS267 atomiser I just can't get it to run right in the mid range, seems woolly leading up to mid and through it, needle on bottom clip with a washer underneath, do plug chop, plug white.... try again this time with an AV 266, pulls much harder from low down but as before plug white. This time I try an AV 268, can't get it to run right through the mid range splutters as if far to rich.... plug chop, plug white, I never got as far as trying the top end power but fitted the biggest main jet I have 129.

Done a leak down test and there are no leaks really blubbery low down with lots of smoke tried both 50 and 55 pilot but makes no difference, also tried the air screw, can screw it all the way in or all the way out but again makes no difference.
Timing at 18 dbtdc, fuel tank and tap all clear and 300 float needle valve.

It's like the faster I go the leaner it becomes, I'm totally flummoxed any ideas guys??

And just for a bit of fun it's booked in for a dyno run, let's have a guess how much power it will kick out. I'll start with 16 Bhp and 13 foot pounds. What ya reckon?

I would suspect that you've identified the problem:

"the biggest main jet I have 129"

I would try running it (not far!)with one of your smaller atomisers but with no main jet fitted & prove that the main jet is the problem.

That should prove, one way or the other, whether that is the restriction that it should be, having gone from a Clubman to an expansion chamber.

I hear we're your coming from WT1, but I Dident even get into main jet territory I simply taped up the throttle and marked 1/4-1/2-3/4-open positions and ran it at a steady half throttle, and as I say the 268 was spluttering like mad (this was for about a mile and a half btw) plug chop and white.

The dyno is Friday so I think I'll leave it to them to sort out cheers for your reply mate.

One other thing though the carb is second hand and I noticed a fair amount of scuffing on the bore of and on the slide so I wonder if it's simply worn out?

Taking the main jet out is an old racer's trick to see if the atomiser is big enough, which a 266 ought to be.

I think a white plug is dangerously lean, hence my suggestion, despite the fact you consider the set-up rich elsewhere, which it may well be.

You haven't said what slide you have in it, but I'm guessing around the 40 cutaway will be about right.

Plug chops are not the easiest things to perform (de-clutch & cut engine @ a certain area in the range whilst maintaining control!) Krypton Factor stuff!

BTW. I'm presuming you have checked all passages & that the float(s) are not upside down as they can be in certain of the later Dell'Orto's...

I'll have a good look tomorrow. Interesting what you said about the float, when I was looking at it it Dident seem right somehow, thanks again.
Yes slide is a 40.

Took it to Chalkie,in the end we settled on an AV 266 atomiser, x7 needle and a 125 main jet to get the air/fuel ratio right, still a bit lean half throttle, worn slide on worn carb, got a new one ordered.
The perils of second hand shit eh?

Scooterdude wrote:Took it to Chalkie,in the end we settled on an AV 266 atomiser, x7 needle and a 125 main jet to get the air/fuel ratio right, still a bit lean half throttle, worn slide on worn carb, got a new one ordered.
The perils of second hand shit eh?

You might find that you need the next richest needle as well with a fresh slide. Off the top of my head, I think it is an X13

Just checked the spec of the new carb, it comes with an x2 needle but will try an x13 as well. I got the carb from wasp performance, can't believe how good the prices are never seen them before PLUS, (and this is the biggie).... they answer the phone yay!

Well it's back to square 1 again. New carb arrived fitted it as per the dyno set up.......scooter says no

Splutters like a twat everywhere, experimented with 264, 266, 267, 268 atomisers all at different combinations with x7 and x2 needles, upped the main jet to 129, managed to get it past the spluttering and onto the pipe and really spanked it up the A6 for a couple of miles when I pulled over for a plug chop the packing in the end can was smouldering plug was ford diamond white!
Now I've seen similar set ups using smaller main jets and bigger atomiser/needle combos so now I'm beginning to wonder if I'm having problems with my needle valve/float (900gms) and (300) ? When I shake the carb I can hear the float flopping up and down alright.

Warkton what am I trying to find by taking out the main jet? After all there's a crossover point we're main jet and atomiser work together.

You had sounded as if you’d settled on the AV266 atomiser.

As I’d said:

I would try running it (not far!)with one of your smaller atomisers but with no main jet fitted & prove that the main jet is the problem.

Is something you can still do with the AV266 you'd seemed settled on, but I don’t understand why you are not trying an X13 needle which is richer ‘sooner’ than an X7.

I've got an x13 on order so will give it a go when it comes as I will your suggestion about trying without main jet.

The more I think about it though I'm sure it's float related: I replaced the 200 needle jet on the new carb with the 300 that I already have and put the old float back in as the metal tang on the float had been pinched together to accept the much smaller round nipple of the 300 needle, the new 200 one is bigger and square. I'm wondering if there is a special clip I now need to fit the 300 needle in the tang of the float?

When I turn right and then straighten back up the scooter immediately splutters before clearing itself..only does it when going right, which is beginning to make me suspect the float height is wrong, the valve is for whatever reason malfunctioning or the float is sticking on the float bowl.

Not sure what the float height should be so will find out and check.

Cheers for the help and advice.

The float needle valve needs to be bigger than the largest main jet that you are ever likely to need.

Therefore, the safety factor for a 200 is about 3:2 with your 129 main jet (200 ÷ 129 = 1.55) which is fine IMHO.

Personally, I’ve only replaced them in VHB carbs (Ø36 mm & bigger) for flat out racing which should be using more fuel than most!

I have a 250 here if you want to borrow it, but you really could swap back the 200 to prove your theory, @ least for now.

Thanks for the offer WT1, but i took it all to bits again earlier and on comparing the two floats in each carb it was clear that the old one was at the wrong height so fitted the new one and managed to fit my existing 300 float valve, x7 needle second clip from bottom, AV 266 atomiser and 129 main jet.

On taking it for a blast everything was as it should be, clean off the mark and up to just after half throttle then spluttered as it went on to the main jet (just as id hoped) so now in an ideal world i can work down on the main and everything should be fine.
Did some right turns everything good and as you would expect.

Did a half throttle plug chop and it was running rich, which again is what id hoped for, engine much cooler almost able to take plug out with bare fingers, again a good sign.

Will have a play with the x13 needle when it comes but from what ive seen this evening im guessing i may well stick with the x7 but move the clip down one.

One thing i dont understand is why have several different size float/needle valves? why not just have the large one and have done with it?

Scooterdude wrote:
One thing i dont understand is why have several different size float/needle valves? why not just have the large one and have done with it?

Sensitivity! One size doesn't fit all

By that, most people know that smaller float valves exist to, say, restrict supply from a pumped fuel line, perhaps in a kart. So, if you can understand that concept, you can probably relate to the fact that float levels may be more consistent with a float valve that's not too big.

The smallest sizes that you can safely use, allow lots of little sips rather than a lot fewer gulps, so the float level is more accurately maintained, as I understand it.

A consistent fuel level in turn makes the carburation more consistent.

You can prove this in turn, if you try a heavier (four stroke) float in place of what you have, as I found out! The carburation enriched as if the choke was still on!

I ran x7 .. top clip .. av 266 on mine with 228 main ...300 float valve .. 58 pilot worked fine

Got it all sorted now guess I should have posted an update but forgot. AV 268 atomiser, x2 needle top clip with a washer, 50 pilot, 134 main jet. Seems extremely over the top compared to many of the set ups on the carb section on here and on the cam lam site but anything less and the plug is white.
Been out for the first decent run today 109 miles with this pipe on, absolutely hated it one minute it's on the power next minute it's dead, used two tank fulls of fuel, putting it back to the original settings with air box and clubman later just as soon as it cools down a bit..