Compression test

michwil4 · Post by **michwil4** » Wed Nov 14, 2007 1:18 pm

Have a problem with getting my trailfire to idle, one plug looked oil fouled, the other looked white.

Compression test resulted in 70 on one side (the white plug), 145 on the other. I didn't know I was supposed to hold the throttle open while I was cranking, so the readings are a little high. I put a little oil down the low hole, and it came right up to the other reading.

I just got this sled from a swap meet in Brule WI. The guy said he just put rings in it and couldn't get to run right.

Is there common problems I should be looking for given the limited history I know about the sled when I tear it down?

Thanks
Mike

07_Z570 · Post by **07_Z570** » Wed Nov 14, 2007 1:59 pm

Where did you learn the oil trick? That should tell you what you need.. I mean. everybody that knows that trick knows whats wrong.

Well, anyway. Get a engine Rebuild, Rings, and Hone the Cyl's while your at it.

michwil4 · Post by **michwil4** » Wed Nov 14, 2007 2:03 pm

Thanks for not being condesending.....

michwil4 · Post by **michwil4** » Wed Nov 14, 2007 2:11 pm

Are a trailfires cylinders chrome or can they be honed?

07_Z570 · Post by **07_Z570** » Wed Nov 14, 2007 2:28 pm

Sorry, didnt mean to sound like an @$$
I'm Not Up on all the JD Engines.. However I Know how to test for certain things.

You Dont want a Chrome Smooth Cyl. wall.. You want a guide for the Rings to rotate around the pistion, (Hone). I'm Not going to go all out Explaining Why. I am not All that Great typing on computers. But Anyway. Somebody may Chime in, and Explain it for you.

Just get looking for an Engine Rebuild kit. and Somebody that Knows how to Hone Cyl's.

Danzig · Post by **Danzig** » Wed Nov 14, 2007 3:24 pm

Put on the brakes!!

Rings should not spin. If they spun the end gaps would sooner or later line up thus causing a problem with blow by or loss of compression at the end gap area.

I am not sure about this particular engine, but the pistons I am familiar with currently have a pin in the piston rings groove area on the piston to keep the ring from turning.

If this cylinder is chrome lined, you do not want to hone this surface.

I have never seen "an engine rebuild kit". Everything will need bought piece by piece. Gasket "Kit". Pistons and rings. Crank will need sent out to be built unless you can do it your self which I have yet to see one person on this forum do it due to balancing it and getting end to end runout within specification. Needle bearings for your rods...etc.

Maj_hazard · Post by **Maj_hazard** » Wed Nov 14, 2007 3:39 pm

Hi Mitch

If the cylindars are chromed don't hone as it will remove the chrome...

I don't know if your model had chromed cylindars or not . I'm sure some of the more knowledgeable members can advise on that..

If the engine was rebuilt and shows such awful compression on one side it would appear they just threw in a set of new rings and never checked the tolerances of the bore. If you can get the heads off try micing the bores and comparing the values to the specs for your machine.. If I remember correctly a sticking ring will also show an increase in compression with an oil test.

Just my 2 cents worth..

Thats 2.2 cents in US dollars

Cheers

Mike

Liquifire 4/6 · Post by **Liquifire 4/6** » Wed Nov 14, 2007 3:53 pm

The cylinders in all new Trailfires were chrome bore. That being said, when you're dealing with a sled that's nearly 30 years old and has gone thru 6 or 7 owners, anything is possible. People do strange things...like installing Arctic Cat engines with steel liners in Trailfires (I bought one of those) or simply getting a liner installed when a cylinder went bad. If it has a steel bore, it should be quite easy to spot by looking at it...or use a magnet. You can give chrome bores a very very light honing....so I am told, but I really wouldn't recommend doing so. Be sure to use the correct rings for a chrome bore cylinder. Did you say your rings spin? That's scary....they should not rotate much more than a milimeter or so. Make sure the piston still has the little ring gap locating pin within the ring groove.

michwil4 · Post by **michwil4** » Wed Nov 14, 2007 4:57 pm

No, I didn't say my rings spin...07 z570 guy said they are supposed to.

I was just wondering if the guy who recently put rings in this thing may have done something wrong that was common. Would honing a chrome bore wipe the rings out right away?

07_Z570 · Post by **07_Z570** » Wed Nov 14, 2007 10:55 pm

Yea, like i said. I'm not up on the JD engines. I am in 4-stroke Racing Engines.

"Honing a bore, when properly done, leaves a fine crosshatch of valleys in which oil collects. A perfectly smooth cylinder wall wears rings very quickly." From-
http://www.circletrack.com/techarticles/ct...and_honing.html

Danzig · Post by **Danzig** » Wed Nov 14, 2007 10:59 pm

Nick-
Are you then saying the rings spin on a 4 stroke engine? Whats the purpose of honing the cylinder wall then, whats the gain? I am lost on this can you explain?

07_Z570 · Post by **07_Z570** » Wed Nov 14, 2007 11:06 pm

Yes. if the rings didnt rotate the Cross hatch would wear quicker.

harleysportster · Post by **harleysportster** » Wed Nov 14, 2007 11:29 pm

I would like to add something here:
I was taught that honing the cylinders creates cross-hatch grooves which allow oil to cling to cylinder walls for lubrication and a better seal. It could be possible that the rings on a non-pined piston rotate. This seems strange since most mechanics stagger the ring end gaps during the installation of pistons.

07_Z570 · Post by **07_Z570** » Wed Nov 14, 2007 11:37 pm

Yea, You dont want the gaps to line up... As Long as the Rings are Free, and your Cylinder walls are not smooth, each ring should rotate with the hatch. A Stuck ring with the gap lining up, with a non stuck ring would result in loss of compression (blow by)

Danzig · Post by **Danzig** » Wed Nov 14, 2007 11:37 pm

Nick-
Where did you learn this information?