no compression front cylinder-96 GTX

[matthechat]

I've been having a lot of trouble with this ski. It was fouling plugs so I cleaned the carbs and adjusted them to spec and it ran a lot better...but it was really hard to start so i checked the rotary valve clearance (.014") it's within spec but it has some small grooves in it. I put it back together it started backfiring at over 5k rpm on the water so i took it back apart and checked/adjusted the rotary valve, took it back out and it would start but wouldn't plane off because of backfiring. So i pulled the plugs and put my thumb over the holes and cranked it. Plenty of compression in the rear cylinder, none in the front...is it possible that the exhaust valve is stuck open or is that even possible? I thought i'd ask before tearing into it looking for a broken ring or a hole in the piston haha. Compression was 145 in both cylinders a couple weeks ago.... Thanks!

 

[custompartsguy]

use something other then your finger, you could hurt yourself

 

[matthechat]

it was just a quick test because i was on the water. i then used an actual compression tester. 145 psi in rear, 0 in front. Is it possible that the exhaust valve is stuck open or is a broken ring about the only possibility?

 

[rookie101]

it was just a quick test because i was on the water. i then used an actual compression tester. 145 psi in rear, 0 in front. Is it possible that the exhaust valve is stuck open or is a broken ring about the only possibility?

There is no exhaust valve on a two-stroke. You're going to have to pull the head and have a look. If it really is 0 psi I would expect at the very least you will be doing a top end rebuild.

Aaron:cheers:

 

[custompartsguy]

yeah, I would say to take the head off and have a look, 0 psi is not good.

 

[jwick86]

Are you positive the release on the compression gauge was not still releasing air? There is no way you would have 0 psi unless there is no piston. Even if BOTH rings were fried to bits and missing, there would still be some compression from the piston being in the cylinder. Even with a hole in the top of the piston, there is still some compression. Check the gauge and try it again before tearing it all apart.

 

[matthechat]

I just went and rechecked it to make sure. It is definately 0 psi in front (mag?) cylinder. The other checks at 145. I'm pretty mechanically inclined so i do know a little about what i'm doing. I removed the exhaust manifold and the piston is in fact in the cylinder and it does move. So I guess i'm gonna go ahead and pull the head and see whats going on in there. I've got a shop manual so I've got some reference material. Can i remove the head with the engine in the ski?

 

[matthechat]

wow

I think i'll just let the pictures speak for themselves here. This thing was actually still running. somehow. Anyway whats a top end rebuild kit gonna cost me? Can i get new piston sleeves or do i just have to machine out the old ones? also, there's a lot of crap that fell down from the piston into the crank case. Is that a problem/how should i deal with it? Thanks a lot. I appreciate all the help hopefully i can have her on the water here in a few weeks.

O yeah, i went ahead and added a picture of the rotary valve cover. I was wondering about the scuffs in it. Would they potentially cause hard starting? although, that good sized hole in the piston is probably a somewhat bigger contributer.

While i'm here, does anybody have any other suggestions. As in things I should check/replace/clean. Thanks!

 

[timmyboy76]

if the "scuffs" on the RV plate, didnt extend to each hole, than it be ok (slots for oil to puddle at), but since it does, you can send to SBY for re-surfaced one (100 buks), or take gander at ebay.

Wit hcrap down in journal area, wouldnt be bad idea to split the cases, and have look. Find top-end kit on ebay (wsm) for 200 buks, but make sure, you "bore" cylinders firsty, to order right size piston kit.

For suggestions....GO THRU THE CARBS, before firing up new rebuild.:cheers:

 

[custompartsguy]

looks like you did some nice damage there, if crap went down in the case then you will have to split the case. Looks like you melted that piston

 

[kustomkarl]

That engine looks pretty bad. At the least I would do is split the cases and check the crank.If it ran even for a very short time like this it would ruin the crank bearings, with all the tiny pieces dropping in the bottom. It might be cheaper to get a rebuilt or try the "parts" link at the top of the page.

Karl

 

[custompartsguy]

:agree:
I am with Karl on the idea of another motor option!

 

[matthechat]

Well I'll have a look inside the crank before jumping at a new engine, i suppose i'll have to pull it out of the hull. I checked the bore of the cylinders and they're around 3.260" i guess it's been bored before? My shop manual says stock bore is 3.228" Can i go bigger without doing something to the rave valve? What size should I bore to now? and what size pistons will i need?

 

[kustomkarl]

Ok here is what I found: On the 1996 GSX Model # 5620 with a 787 engine, stock configuration RAVE valve guillotine has a minimum clearance of 0.5mm (.020 in.) to the cylinder bore, measured at the center line of the cylinder. On the 787 engine, only a first over size is available that is 0.25mm(0.10in.).The guillotine will have a minimum clearance of 0.375mm(0.15in.) with the cylinder bore. This is the minimum clearance the guillotine should be used with. Less clearance will require reworking of the guillotine to achieve proper clearance and radius.
Ok, we are on the same page... as a direct quote from the manual. you can not cut the stem of the RAVE valve as the guillotine radius and length is what must be modified. The stem and bellows will stay the same.( if you cut the stem you limit the travel of the RAVE valve, that's not the goal here)
When the RAVE valve is reworked you must keep checking measurements to have the clearance. I hope this helps you with your rebuild. Let me know If you need more info, or help.

This info is similar except of the exact model seadoo.

Karl

 

[custompartsguy]

the only person that will be able to tell you exactly for a fact what size over bore you will need is a machine shop.

 

[matthechat]

Well i'm a little confused on the rave valve clearance kustomkarl. Is the 0.5mm (.020") the actual distance from the rave valve to the wall of the cylinder or something else? Also, if that is the case, mine has already been modified...because the cylinder has a bore of 3.275" (.047" bigger than stock (3.228") which would mean that the rave valve would already by non functional if not modified.

I took the jugs to a machinist and he told me I would need 1.5mm greater than stock pistons (82mm)(3.228") which are...I can't find actual piston diameter, only bore size. First of all, is it even possible to get 1.5mm pistons and if so I would appreciate some more info on the rave valve modification if there are any threads you could direct me to.

 

[EvilVtec]

That cylinder looks just like one we worked on the other day, and its problem was oil pump failure to that one side, this one in particular was the rear cylinder. Piston was burned up on the same side, same fashion, same everything. My advice to you would be to rebuild the top end, thoroughly check out the oil pump or replace it, replace the oil lines with new clear ones, and when you get everything back together, make sure you watch the oil travel down the clear lines and into the intake manifold. No oil injection will destroy an engine quick.

Hope the rebuild goes well.

 

[salmonkiller01]

wiseco sells pistons up 2mm over stock so I assume others will too. You will definetly need to go thru your fuel and oiling system to see what caused the destruction.

 

[matthechat]

Ok, sbt http://www.sbtontheweb.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=60-107&Category_Code=60-1&Product_Count=4 is the only rebuild kit I can find that offers 1.5mm over stock pistons. Does anyone have any experience with these? I found a lot of kits on ebay for ~$170 but they don't offer larger than 1mm oversize. If anyone could point me to one that'd be great! although if sbt has a great product i'll go with that.

This sitehttp://fullboreonline.com/tech_tips.htm has some good tips for engine rebuild and break-in. Pics of how to modify rave valve after cylinder bore. I'd buy their rebuild kit but it only comes in 1mm oversize...

Thanks for all the help guys. I've been through the carbs already and I'll be sure to check out the oil pump and put on the clear lines. Would it be better just to go premix?

 

[timmyboy76]

matt..i'd re-sleeve it, with 1.5mm over, aint no more for it to bore to.

 

[matthechat]

solution!

I found a set of jugs and pistons from a parted out ski that was still running with 135psi in both cylinders (OEM bore!) on ebay for $180 http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ru=http%3A%2F%2Fshop.ebay.com%3A80%2F%3F_from%3DR40%26_trksid%3Dm38%26_nkw%3D390056043912%26_sacat%3DSee-All-Categories%26_fvi%3D1&item=390056043912&_rdc=1&viewitem=. I'm pretty happy with this find because now all i need is a gasket kit and probably some rings. Anybody know where i can get a top end gasket/rings kit or will I have to buy rings seperate? The heads should be here in a few days.

Also, the info gauge has been sundamaged i guess and i can't see through it very well. I think i read about the film that covers the lcd going bad. Is it hard to change out? I hear the gauges are a PIA to get out.

 

[timmyboy76]

I got gasket kit for it, just missing the 2-red o-rings that go around the spark plug molds, but you could use ur old ones...pm me if interested $35 w/ship'n.

 

[matthechat]

back on the water!

a pair of jugs, pistons, and rings off ebay and i got her going! Just got back from the river. thanks for all the help! I took the nylon drive gear out of the oil pump and I now premix my fuel. I left oil in the reservoir full and all of the lines hooked up. I have 150 psi in both cylinders. I mixed the fuel 32-1 for the break-in and took it easy for the first hour keeping it under 5500 rpm with lots of variation. After about an hour and a half I went ahead and went WOT a few times and all was well. But when I stayed WOT for probably 15 or 20 seconds it lost power and almost died but kept running. I'm afraid it was a near seize. I took the seat off and noticed the filter had blown out of the carb cover (i hadn't put the plastic carbuerator enclosure back on yet) i thought that was odd since it hadn't been backfiring or anything. I've read somewhere that running with no filter can cause it to run lean and running lean can cause a piston seize. I put the filter back in and opened the low speed needle another 1/8 turn and stayed away from WOT for the most part and all was well but i'm still kinda concerned. should I open the highspeed needle a 1/4 turn or so just to be safe?