Exhaust Bridge?

[packfill]

Ok, I can't keep a top end going in this 1998 RM125. Is it because I'm not relieving the exhaust bridge? The cylinder had a fresh bored sleeve. The piston was turned the right way, and ring end gap checked. Maybe the water pump isn't working?

[94cr rider]

sleeves suck the make your stuff run hot

[packfill]

sleeves suck the make your stuff run hot

Gee, thanks for the help...

[94cr rider]

it also sound like its lean

if you cant keep a top end in it post some pics of the last top end

[highmarker]

how is it failing?

[Tailender]

Does it call for drilling oil holes in the piston? Maybe that would help.

[packfill]

It fails by galling and sticking the ring all around. All the pistons I've put in already have holes. Can't remember the main jet size, but it is one larger than stock. Has anyone relieved the exhaust bridge? Let me get a pic...

Edited February 5, 2012 by packfill

[94cr rider]

are you useing 32.1 u need to make it richer

[Tailender]

If the ring is sticking all around, it's not the exhaust port. It would only be a problem by the ports. Like excess wear and grooves in piston sides at bridges. Maybe not enough oil in pre mix? What mix are you running?

[packfill]

We are running 36:1

[highmarker]

Might wanna try a piston that doesn't need holes to survive, if ones available, They make good pistons but also have some applications that are problematic. Since it keeps happening there is another root cause,

[Tailender]

Are you sure you have the correct size piston? May sound stupid, but it's something to check.

[Padgett]

Your problem is heat. Few things to think about are;

1. Stater timing (line it up back to stock with the marks).

2. Air leak or lean jetting.

3. Low coolant flow. Very well could be stopped up radiators, replace or have them cleaned.

4. Spark plug with the wrong heat range.

Forged pistons should be drilled for the bridges, cast pistons don't need to be drilled. However, I never had a cast piston that was drilled for the bridges crack. They will for sure will crack between the drilled holes if you run the piston way to long.

Edited February 5, 2012 by Padgett

[packfill]

Thanks! I'll check those things out...and relieve the exhaust bridge.

[zig06]

Whom ever is boring your cylinder should already be relieving the exhaust bridge and chamfering the ports. Check with them before you pull out your dremmel.

How about posting some pictures of the failed parts?

And for what it's worth, any bike that has a bridge in the exhaust port ABSOLUTELY needs to have the piston drilled.

Tailender and Padgett are on to something, if the ring is sticking all the way around it's not the ports. You either have an air leak, or are running really lean (possibly from a clogged carb). Granted, we are assuming that you are not doing a bunch of high speed runs up and down a gravel road ~ that would destroy any MX 2 stroke top end.

[highmarker]

you can relieve the exhaust bridge a bit, But it was put there to support the ring, cut it back too much and you'll have problems. Without seeing the parts we are all guessing on the cause and repair.

[packfill]

Ok. Here's a pic. We are running 36:1. Stock mainjet is a 175. It has a 178. Bike idles fine. He may have been pinning it across the lake bed, but hey, we all pin it. This is his 3rd top end in as many times out. I don't think he was going that hard. The cylinder isn't bad and I honed it with a nice hatch. New weisco with predrilled holes ready to put in....

Edited February 7, 2012 by packfill

[packfill]

And when I mentioned the exhaust bridge at the shop that did the bore, the sales guy didn't know what I was tailking about. I think he was trying to get rid of me since someone called in a favor for me. They bored it for $40. And the ports were all chamfered nice.

[94cr rider]

thats not from the exast port

[Padgett]

You sure the bike is running at 36:1? 4oz of oil per gal is 32:1, 3.2oz of oil per gal is 40:1. The piston in the photo looks like it had straight fuel ran threw it. Just tossing that out there to think about.