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SIL GP, doing my head in! What's wrong?

Technical help for Series one, two and three Lambrettas. Models include the Li, Li Special, TV, SX, GP, Serveta and API/SIL models

SIL GP, doing my head in! What's wrong?

Postby JohnnyW » Fri Aug 27, 2021 1:08 pm

After 35yrs of fiddling about with these things, I am perfectly aware that this is a 'how long is a pice of string' type question, but here goes......(and forgive the essay, but I feel it needs a full explanation ;)
Wife's standard GP200, SIL, bought it maybe 5 years ago from a lad in Portsmouth, nice honest scoot, just needed a bit of TLC. New petrol tap, new tank, head off, a bit of a clean up, checked timing etc, sorted. Done a good few rallies since, including Soria, (and that's from Stirling, so we do some miles every ride).
Coming back from Port Rush last year, she just stopped, lost power. Got recovered. Turned out that she had a sunken dome on the piston, which I put down to what appeared to be a blowing head gasket (evidence of gas escape). The barrel seemed fine, but I decided to put a rebored one on that I had, really good leak down, and away we went.
Or at least I went, as I do the running in of course :(
Now I know this barrel was bored pretty tight, at 3 thou, so although I was careful with it, I soft nipped twice over maybe 250 miles of running in, in 30-50 mile stints. Not happy, but thought I would persevere. After the last nip however, I looked at the plug, and it seemed a bit lean. I decided to change the carb, as I had a Jetex on it, and a mate was always haranguing me about putting a dellorto on, so I did. Still standard jetting, as it was a standard pipe.
And then, it started okay, but wouldn't run properly. Wouldn't rev out. Was very 'chuggy' as I tried to accelerate.
Bugger this, back to the Jetex.
The same.
Eh?
Now, I had ridden 30-odd miles home after the nip up, so didn't expect this.
Tried another SH22, the Scootopia one. Same.
Right, that nipped top end must be worse than I thought, change it for one I have bored out to 5 thou. Good leak test again.
Same, if a good deal more rattly from the piston slap :P

So it must be electrical, surely, but no idea how I got home okay for those 30 miles then?
Checked and cleaned earthing straps. Same.
Plug, swapped. Same.
Cap. Same.
HT lead, Same.
CDI swapped. Same.
Stator checked, met all the resistance criteria, but swapped anyway. Same.
Is it the exhaust coked to f**k? Swapped to a BGM V4. Same.
So now I suspected the wiring, and now I get into my weak area...... It has the round junction box, into which the stator wires go, and then reappear into the CDI/Regulator etc.
My first query is that there is only one green wire to the CDI, yet the ignition switch works fine. Am used to two.
I kind of would like to bypass the junction box and loom, but the stator wires, (which are not colours I am used to btw, at white, brown, green and yellow) are too short to reach the CDI, so I would need to extend them temporarily, (thus introducing another worry, but only to erradicate the loom as the issue). What's the best way of doing this if I did? How can I prove/disprove the loom?

And then, the wieredest yesterday. The speedo light was a right bodge, with a bare wire just jammed in the side of the fitting, so I put one of MB's new ones in, and tidied up a bit of the electrican tape etc, the horn connection wasnt a tight fit etc.

She started up again as usual, but this time better. Would tick over, and rev much further, but not 100% at all. Surely this shouldn't have made a difference? Looks a good spark, good starting. Choke is coming off fine etc.

So, as far as I see it, I have changed everything with proven parts, and nothing made any difference, other than a speedo light!!!

But, it still wont run. Chugg chugg. Like it's too rich.

Heeeelp!!!!!

PS - No white smoke or smell, but just drained the gearbox, maybe 300ml came out, so some use, but the last fill was in Belgium when her clutch went, so nothing really amiss to report I'd say.

PPS - Am off for a lie down in a darkened room ;)
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Re: SIL GP, doing my head in! What's wrong?

Postby MickYork » Fri Aug 27, 2021 2:02 pm

What does it run like with bellows off ?
Could the brown cdi wire be a dis-coloured red ?
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Re: SIL GP, doing my head in! What's wrong?

Postby missing lynx » Fri Aug 27, 2021 2:28 pm

Tbh I would ditch the round junction box and go straight from the stator to the cdi and regulator imho they are pointless and a weak link as they corrode and the sockets become loose over time. Have you checked the green wire isn't arking against something in the headset?
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Re: SIL GP, doing my head in! What's wrong?

Postby JohnnyW » Fri Aug 27, 2021 3:36 pm

MickYork wrote:What does it run like with bellows off ?
Could the brown cdi wire be a dis-coloured red ?


No, definitely brown. Odd, innit.
It's the same with the bellows off, just seems to belch fuel, like as if the timing is off, which it definitely isn't
Cheers
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Re: SIL GP, doing my head in! What's wrong?

Postby JohnnyW » Fri Aug 27, 2021 3:39 pm

missing lynx wrote:Tbh I would ditch the round junction box and go straight from the stator to the cdi and regulator imho they are pointless and a weak link as they corrode and the sockets become loose over time. Have you checked the green wire isn't arking against something in the headset?


That's what I want to do, but as I said, the wires on the stator are too short. I can put a new stator on of course, which I would need to do. Problem is, I am not sure of the implications with the rest of the loom....as I say, I am not the best with wiring. I have a new MB simplified loom, but daren't start the job! The other thing is that we plan on a strip down over winter for a paint job, so it would all be best replaced then.....I just want to get the engine running properly first if poss.

I have looked as best I can, and can't see anything obvious with the green, but that's the lines I am currently following.

Cheers

(Am working away for a week, so won't get chance to fiddle again for a while)
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Re: SIL GP, doing my head in! What's wrong?

Postby missing lynx » Fri Aug 27, 2021 4:26 pm

JohnnyW wrote:
missing lynx wrote:Tbh I would ditch the round junction box and go straight from the stator to the cdi and regulator imho they are pointless and a weak link as they corrode and the sockets become loose over time. Have you checked the green wire isn't arking against something in the headset?


That's what I want to do, but as I said, the wires on the stator are too short. I can put a new stator on of course, which I would need to do. Problem is, I am not sure of the implications with the rest of the loom....as I say, I am not the best with wiring. I have a new MB simplified loom, but daren't start the job! The other thing is that we plan on a strip down over winter for a paint job, so it would all be best replaced then.....I just want to get the engine running properly first if poss.

I have looked as best I can, and can't see anything obvious with the green, but that's the lines I am currently following.

Cheers

(Am working away for a week, so won't get chance to fiddle again for a while)

All you need to do is match the wires from the stator Loom with the colours on the cdi and regulator it's as simple as that and if you're not sure sticky's book has the wiring diagram in. You could try another flywheel just in case it's loosing it's magnetism
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Re: SIL GP, doing my head in! What's wrong?

Postby Wack » Fri Aug 27, 2021 10:23 pm

Have you considered a twisted crank,any sign of flywheel rubbing on the pick up etc?
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Re: SIL GP, doing my head in! What's wrong?

Postby Fast n Furious » Sat Aug 28, 2021 1:28 am

The easiest way to detect a twisted crank is to look at how concenric the flywheel is running when at tickover..... or measure the "run out" with feeler gauges between the edge of the flywheel and a point on the mag flange over one flywheel rotation.
If the run out seems excessive, then the crank webs have probably twisted on the pin.
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