Garelli Gran Sport Low Compression Acting Like It's Running Rich

I bought a Garelli Gran Sport and I was told that it only had one of the two piston rings in it. I tried to run it anyway and it would run, but only if it was on full throttle all the time. While it was running it would act like it's running rich (it would backfire a lot and could only run if there was nothing blocking the intake like an air filter or the air filter housing). I thought it was just running rich, but I did a compression test anyway and it only had around 73 psi. I will obviously be throwing in another piston ring to get the compression back up. Does it make sense for low compression to cause symptoms that seem like it's running rich?

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

if you are running on one ring there is no point trying to diagnose a problem now....its only running on one ring!....get a new ring, gap it and check out the piston and jug when your puttin it in.

then you can look for other problems...

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

It does sound like it's too rich . Have you checked the timing ?

For most motors , 73# is not enough compression to run correctly .

Running a single ring is not your problem , unless that ring is too worn and/or the gap is too great .

I am running a single ring in my FA50 double ring piston . The motor starts easily . Runs smoothly and does it's 'normal' 30 MPH top speed . But , take off is a bit 'less' .

There are plenty of single ring pistons .

Before you buy anything , pull the top end apart and measure what you have .

Piston to cylinder clearance ; ring end gap ( in the cylinder should be as little as .008" and definitely no more than about .014" - .016" ) .

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

I have a spare engine and I'm just taking a ring out of that. I'm going to put that ring in and try it out and see how it does. I would just throw the spare engine that I have on it, but the threads for the flywheel nut are completely messed up and it has no spark.

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

i always suspect the garelli stator side crank seal, it has a tendency to leak, pop out, dry out, etc.

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

I put in the other piston ring so that I now have both in and it runs significantly better. It will actually idle now. However, I did another compression test and it only has around 87 psi.

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

> Daveen Bavari Wrote:

> -------------------------------------------------------

> I put in the other piston ring so that I now have both in and it runs

> significantly better. It will actually idle now. However, I did another

> compression test and it only has around 87 psi.

Did you make any measurements ? If so , what are they ?

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

I have not, I need to get a feeler gauge and measure it.

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

I checked the piston ring end gap and it's around .017". The service manual says it needs to be between .012" and .039" so I'm good there.

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

It'll run better when that ring conforms a little. Check compression after a few hours of riding time. Make sure you're holding the throttle open when you check it.

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

I checked the compression again today and now it's around 115 psi. The new problem I found is that with the air box on, it only goes around 15 mph. The solution I found was to take it off. If I run it without any air restrictions, I can do close to 30, but it's a lot louder now.

Re: Garelli Gran Sport Low Compression Acting Like It's Running Rich

You need at least a screen over it. You can drill the airbox out a lot.

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