LCGB Forums

Now then chaps, I have finished rebuilding after my RTA, but I'm having some running problems.

GP150....
I fitted an AF175 cylinder and piston with new rings. But can't get it to idle. Starts only with near full throttle and no air filter. Once going, it basically needs plenty of revs, but will die if left.

I've tried 3 carbs, and still have the same issues. What has concerned me, is that I've been unable to get any squish reading, despite leaving both gaskets off and just using gasket sealant.

I'm certain its something to do with the head, but what? Compression seems good and sparks well.

Any advice welcomed

Did the piston come to the top of the cylinder with the head off?

Only starts on full throttle
Won't idle
Dies on closed throttle

Suggests pilot too small maybe?

What carb and jetting do you have?

Big squish won't help.

Take the head off and check the deck height as Coaster has suggested.

The piston is probably a couple of mm of the top of the cylinder.
I decided to try my old 150 head and it runs much better and will tick over with high revs. But still doesn't like an air filter and won't even consider firing up without choke.
At high revs it sounds sweet, but I'ts under load.
Question is: start again and get a new piston and cylinder kit or just a new piston??

what carbs have you tried and how did you jet them ?

Is your casing Indian? Some of them are known to have variable heights on the gasket face for the cylinder base leading to problems setting squish.

Do you have a base gasket or packer fitted? If so how thick is that?

You could always have a little machined off the top or bottom of the cylinder to improve the deck height of the piston at TDC. If you took it off the top the port timings would be the same, taking it off the bottom would alter them slightly.

You need to be careful with a 150 57mm head on a 175 - it really should be opened out to match the bore.

Yeah it's Indian, but had ano Italian 150 cylinder and head and ran great.
I can only assume that it's the cylinder that's out, as even with no gasket, the piston does not sit flush.
It's probably a case of - if it ain't broke, don't fix it and put the old 150 back on.
Shame because the AF cylinder has been opened up and cleaned really well.

I think Ive counted three posts so far asking about your carb / jetting and every reply you give has completely ignored this.

I would suggest actually answering the questions people are asking if you actually want some meaningful help?

Sorry Donnie, no offence intended.
Too wrapped up and stressed doing a full rebuild. But thanks for your input. It's not a carb problem, which I originally thought.


I've finally found the problem.
Wrong size rings. I bought the cylinder and piston together ( 61.96) but without rings which I bought separately.
When I measured the rings in the cylinder ( which I should have done in the first place!) The gap is huge. So I need a different size ring/size to suit the piston.

Answers of ring size welcomed. Present piston has the slim type.