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kellymac530 10-10-2012 07:41 PM

stroke twin issue
I know this NOT a jet ski site but many of you are engine gurus no matter what it is. I am having NO success on any PWC sites and I can not figure this out.

First off, I am a decent mechanic, not a pro trained tech, but the guy who everyone of my buddies call to figure something engine out. I have a full shop in my garage, bike lift, 6 hp 60 gal Air Comp, air tools, presses, pullers, TIG 250 Lincoln get the point. I am a do it myselfer and have built many cars and bikes and boats from the chassis up. No way near bragging here but just letting youknow that I am not a carpet salesman playing with tools, I work with my hands daily and am somewhat capable.

That said I am stumped big time. I have raced Jet Skiis and built quite a few of my own and A buddy had me rebuild his Sea Doo a couple years ago and it ran better than ever, new crank, bore, piston & ring kit...yada yada. So another friend asked to trade me a beat up tired BMW R80GS for my labor and parts to rebuild his 550 Jet Ski. No problem. I find out it does run but not well and it dies after running for a few minutes and wont start for an hour or so.

I take the deal. After stripping it down I find a worn thru spot in the water box, I assume that is the stalling problem since that is a common issue. I order a rebuilt crank, gasket kit, oversize piston N ring kit {after having the bores checked to see what size I need} and have the barrels bored, all from a reputable Jet Ski racing supplier. I clean everything carefully, and rebuild it all. Jet Ski engines could not be easier, crankcase splits down the center, crank drops in with a seal on each end and drop the barrels over bthe pistons.

A few hours and it is together it pops right off but wont rev up. It has good reasonably even compression {105 and 108} for an unbroken in 2 stroke. It has GREAT spark, brite blue even in a well lit shop from both plugs. I know the carb works because it was on a running ski before, and I have since rebuilt it just to rule it out and put a carb off of a third running ski on it. I also have lightly sprayed fuel from a spray bottle while cranking and it does not help.

It fires after 30-40 seconds of cranking, it will run with the throttle wide open but will not really rev up. I have since swapped out the entire CDI box from his other good running ski as well as the front magnetto and pick ups and flywheel all to NO avail.

What is wrong?

I have ALWAYS been successful with the concept of, if it has compression, fuel and spark in the right time all motors can run. It has never failed me. This is the first time I have not been successfull with a motor once I deduce down those simple points. I have tried timing it full advance and full retarded and at every notch in between which only about 15* or so total so I am sure it is not just timing.

Could there be something wrong with the crank that was a rebuild? The seal in the center not right?

ANY ideas will be tested and retest if you have an idea PLEASE throw it up. I am out of things to try here.

willis 2000 10-10-2012 08:00 PM

Do these use a vacuum-type fuel pump? Low fuel pressure could cause these symptoms.

caryder 10-10-2012 08:10 PM

Exhaust restriction?
Is a power valve involved? Stuck maybe?


ragtoplvr 10-10-2012 08:14 PM

How is it ported, piston or reed. Are pistons same height from wrist pin.
an there be some blockage in the exhaust.

I would get out a timing light and verify timing, it does sound like massively retarded spark. It could be a wrong crank with key in wrong pace.


kellymac530 10-10-2012 08:28 PM

Thank you folks for throwing out some ideas.

It is a piston port model. The skirts and pins are exactly the same as the old ones, and the ports look to be the same, but I will verify that. Good ideas.
The exhaust is wide open and not even connected to the waterbox right now. It has a good aftermarket exhaust that I have checked thoroughly, no rats nest, squirrels, walnuts...nothing and it is disconnected right now from everything.

It does not even always start sometimes it will crank and pop like it wants to start, but just not quite do it.

I have been messing with it for weeks now.

Timing is something I will mess with again, but I have tried it an many incriments with no real changes.

kellymac530 10-10-2012 08:37 PM


Originally Posted by willis 2000 (Post 19792356)
Do these use a vacuum-type fuel pump? Low fuel pressure could cause these symptoms.

Sorry I forgot to address this reply, Yes it is a vacum fuel pump, not really a pump at all. There is a solenoid that shuts fuel off after the engine turns off so it does not siphon and flood, but while running it is just a simple siphon feed. The solenoid has been tested and works for sure.

Either way, the fuel would be irrelvant just for starting purposes since I have tried to even use a spray bottle replicate fuel spray and tried ether which both will pop but not rev out. There is fuel in there for sure. Is there a way to test or better address a possible fuel problem?

RodT 10-11-2012 10:17 AM

Have you tried a leak down test? It kinda sounds like a leaky main seal, even though they are new, i have had new ones leak. Rod

Twin-shocker 10-11-2012 10:58 AM

You need to pressure test the cases, and check for leaks. Sounds like a primary compression problem.

kellymac530 10-11-2012 02:12 PM

GREAT idea. Didn't really think about a crankcase leak down test. How do I do that on a 2 stroke?

Do I need to make a plate on the intake and exhaust manifold openings and then put a schrader valve or something and then pressurize one side cylinder at a time and see if it is between them or out one end or the other end seals?

Is there a tester to test a 2 stroke TWIN bottom end?

RodT 10-11-2012 06:08 PM

Yes, you make plates to block off both the exaust and intake and on one put something like a tire valve to preserize it, but don't get carried away with the pressure. I'm not sure but I'd do some reserch on how much pressure to use. Rod

ragtoplvr 10-11-2012 06:21 PM

To test crankcase you normally do not want to use more than 10 or 20 PSI. Use soap suds on all the seams and seals and look for bubbles, lots of bubbles.

The exhaust should be hooked yup. some porting schemes need an exhaust to run.

It could also be retarded timing easy to check with light.

Could also be a severe vacuum leak.


Twin-shocker 10-12-2012 09:52 AM

You can make a crankcase pressure tester, but they are easily available and are not that costly. Not a good idea to go over about 6psi when testing.

kellymac530 10-12-2012 10:21 AM

You guys are great. Thanx alot. I will make up some pieces and test it. I thing I even have a 0-30 psi gauge that is good from an old pool filter that I can use to watch the pressure leak down.

As to the exhaust, I don't think I was clear on my explanation of it. The exhaust maifold is on, then the header cone is on where it necks down. What I have disconnected is the last portion of it, the water box and the rubber neck out of the hull. All of the required back pressure is there to run properly, many of us run no waterbox at all but my buddy wants it a bit quieter so I installed a nice all aluminum waterbox but because of all of the adjustments I am having to try with the timing and pulling the flywheel on and off I have the front of the the engine compartment empty. The manifold, the 180* header, and the long cone is all on the ski currently.

For an initial leak down test I could make a small plate over the carb inlet and use a plug in the exhaust cone and pump up both sides at once to see if the leak is in the cases or seal...correct?

Then if that does not prove helpful I would need to make a plate that would isolate the cylinders individually to tell if it is the center seal passing between the sides...correct?

Thanks for all of the help even though this is not a motorcycle the same principles apply to snowmobiles and yami RZ350, and Banshee style motors so the help is MUCH helpful and useful, thanks for your time folks...I owe you a beer. Ever near Elsinore CA shout out, I am up for a ride and a beer.

Twin-shocker 10-12-2012 10:30 AM

You need to do a crankcase pressure test, not a leak down. I would be very careful using a gauge that reads up to 30psi, as you only want 6psi it isnt going to be very accurate. Go much higher than 6psi, and you will probably find the centre crank seal will leak anyway.

kellymac530 10-14-2012 08:16 PM

I am not worried about the gauge. It is a low pressure FULL sweep gauge with VERY clear 1 lb incriments.

So I realize that I am first testing if there is a leak at all, but if that does not find a leak, wouldn't I need to test if it is leaking between #1 &#2?
Wouldn't that require a plate that separates the 2 cyclinders?

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