low compression leak down good

[NIL8er]

Just bought this from a guy in Charlotte NC named Anthony Tran( dont buy from this guy hes a crook ), he claimed was just rebuilt. found no spring in rave valve diaphragm for rear cylinder,
1999, 787 rfi, has 90psi compression front cylinder and 145 psi rear. took apart nothing obvious. Leak down held for an hour. Allegedly new rings. What can cause this condition other than oval sleeve/ cylinder or worn rings.. If leak down holds doesnt that rule out rings and sleeve/cylinder?

are holes in cylinder supposed to face rotary valve side?

 

[mikidymac]

Leakdown will tell you nothing about the condition of the pistons, cylinders or crankshaft. It only tells you that the gaskets and seals are good.

For low compression, it has to be worn out cylinders and rings. You can't just put new rings on worn cylinders and pistons or you will have low compression. you have to bore the cylinders to the next oversize and fit oversized pistons and rings.

 

[NIL8er]

Thanks for your reply. I assume that's what the previous owner did. One cylinder is obviously larger than the other bec I put a piston in it and the piston can wiggle whereas the other cylinder is much tighter fit. Weird thing or not is that the one with low compression is the tighter fitting one. I'll get them checked and honed more likely bored and I'll bet the one with low compression is ovaled. When I said leak down it was in reference to cylinders.

 

[NIL8er]

Leakdown will tell you nothing about the condition of the pistons, cylinders or crankshaft. It only tells you that the gaskets and seals are good.

For low compression, it has to be worn out cylinders and rings. You can't just put new rings on worn cylinders and pistons or you will have low compression. you have to bore the cylinders to the next oversize and fit oversized pistons and rings.

So a broken ring or piston wouldn't affect it holding air pressure in that cylinder? Its my understanding even worn rings can allow air to leak in out of a cylinder when doing a leak down test.

 

[mikidymac]

You are confusing a cylinder leak down test which is only a 4-stroke thing since on a 2-stroke there’s no valves or oil rings to check.

On a 2-stroke a leak down test is to make sure there are no gaskets or seals leaking air because on a 2-stroke an air leak will cause it to go lean and blow up.

So same name but completely different test depending on the engine.

If you have low compression and different size pistons some hack tried fixing it so two new pistons and fresh bore on both. Both pistons have to be the same size.

 

[NIL8er]

You are confusing a cylinder leak down test which is only a 4-stroke thing since on a 2-stroke there’s no valves or oil rings to check.

On a 2-stroke a leak down test is to make sure there are no gaskets or seals leaking air because on a 2-stroke an air leak will cause it to go lean and blow up.

So same name but completely different test depending on the engine.

If you have low compression and different size pistons some hack tried fixing it so two new pistons and fresh bore on both. Both pistons have to be the same size.

thanks for clearing that up . Makes perfect sense.
The pistons are not diff sizes the cylinders are.
Strange or maybe not thing is the larger cylinder was the one with 145 compression and the tighter on was the 90psi. I would imagine thats because although the cylinder is large its still round and the rings can seal it for now, the tighter cylinder has ovalized making it impossible to seal????

 

[NIL8er]

Ill be taking it to the machine shop and honing or boring cylinders.
New rings and pistons if needed.

 

[mikidymac]

How do you know one cylinder is larger than the other?

 

[NIL8er]

One cylinder is larger than the other and one piston is larger. I know this bec I checked both with caliper. They're both tight fitting to there perspective cylinder. But def diff sizes. The pistons have the same numbers inside so I suspect one was milled down it has no number on top but the slightly larger one has a 1. I see no reason why the compression is low , its low in the smaller cylinder/ piston. Im swapping out the rubber gaskets on the cylinders to see if that makes a difference. The current ones are brittle and obviously very old.

 

[NIL8er]

Ok from what I'm learning it's not unusual for one piston cylinder to be a bit different size on some engines. Not optimal but it's done.

 

[mikidymac]

No, you should never have two different cylinder sizes or pistons on these.
It is not done unless someone is a hack trying to swindle someone.

 

[WhiskeyD]

Did you check the compression before buying it?

 

[NIL8er]

Did you check the compression before buying it

Lol. I see you.

 

[NIL8er]

No, you should never have two different cylinder sizes or pistons on these.
It is not done unless someone is a hack trying to swindle someone.

I agree. Especially with the rfi I gather. Not optimal but done. I believe this guy was either an idiot or trying to swindle. Probably both. There are theories that it can twist the crank. Small difference but enough where one piston can't even fit in the other cylinder. Hell idk might not even be enough to matter. How close should they be?

 

[NIL8er]

Prob not the best angle but both same angle. You see the difference. Actually hard to see in these pics but there's a difference.

 

[mikidymac]

First, you can't accurately measure cylinders with a caliper, it hast to be a bore gauge.
2nd, it looks like at least one cylinder has rust pitting/staining on it and you will have to have it bored to clean it up anyways.

 

[WhiskeyD]

First, you can't accurately measure cylinders with a caliper, it hast to be a bore gauge.
2nd, it looks like at least one cylinder has rust pitting/staining on it and you will have to have it bored to clean it up anyways.

Yea and if you're going that far I would inspect crankshaft while you have motor apart. Would suck to put the time and money into a new top end just have have a crank bearing let go.

 

[NIL8er]

Yea and if you're going that far I would inspect crankshaft while you have motor apart. Would suck to put the time and money into a new top end just have have a crank bearing let go.

All great advice. And already on it.

 

[NIL8er]

First, you can't accurately measure cylinders with a caliper, it hast to be a bore gauge.
2nd, it looks like at least one cylinder has rust pitting/staining on it and you will have to have it bored to clean it up anyway

Yup boring it is, machine shop guy here confirmed it. Doesn't do outboards or PWCs. I guess ill have to send it out. Group K website doesnt have an area with any info on cylinder boring.

 

[Poormanracing]

One you can't measure cyl like that.guy prob replaced the cyl with a used one I'd guess and dumped it.any numbers or lettering on piston domes ?sea Doo piston would of had aus and an arrow pointing to exhaust.pro x just an arrow.sbt and wsm would have part number.i would consider doing a complete rebuild.are all the bottom end bolts look like they where touched.paint removed on the bolts.

 

[NIL8er]

One you can't measure cyl like that.guy prob replaced the cyl with a used one I'd guess and dumped it.any numbers or lettering on piston domes ?sea Doo piston would of had aus and an arrow pointing to exhaust.pro x just an arrow.sbt and wsm would have part number.i would consider doing a complete rebuild.are all the bottom end bolts look like they where touched.paint removed on the bolts.

Yes I know you can't correctly measure cylinders like that! Like I already said it was all I needed to confirm they were different sizes. One piston had a 1 on top the other nothing but both had made in Japan and prox 01-5510 in side the piston. We've established the last owner just put together with intention to find an idiot to buy it. Anyway it's getting top end and crank.

 

[mikidymac]

Best bet is to call group k on Monday, they are great.

 

[NIL8er]

Best bet is to call group k on Monday, they are great.

I contacted them but I'm going with someone closer. Champion power sports in Denver NC. He's well known on east coast. Has been doing it for 40 years and has built race engines.

 

[NIL8er]

Best bet is to call group k on Monday, they are great.

Are there any markings that would identify an oem crank vs aftermarket?