Stumped...only runs on one cylinder....runs on 2 without the exhaust connected....

[ialonso]

I had not used the boat for a 2 years due to having kids. I got ready to get the boat back in shape (99 Sportster 1800), and did a LOT of work.

I pulled the engines and provided them to a local mechanic that rebuilt both top and bottom ends, and rebuilt the carbs.

Also had to replace starters, coils and solenoids.

I put everything back together and took it out for the first time, taking it VERY easy with the fresh engines, and after 40 minutes, aftr having planed once already at 6,000 RPMs, I tried to do the same a second time. Unexpectedly the right engine cut out, and did not come back on.

Once back, I determined that engine would fire one cylinder, but not the other. Tried switching plugs and wires, but issue would stay with the cylinder, while the other cylinder would run with either wire and either plug. In desperation for an answer, I took the exhaust pipe off, hoping to find a restriction of some type, and to my surprise the engine fired up the second cylinder with some delay while running without a pipe.

I am totally stumped. The mechanic just let me know that the compression is indeed 150 still, as he just checked it.

He will be checking it out other possibilities soon.

It's not a very complex motor, I am absolutely stumped as to why this is happening.

You can see fuel squirted out by the accelerator pump when hitting the throttle, I tried with Stater Spray as well, so I am somewhat confident it is not Fuel.
The other cylinder fires OK with both wires and both plugs, so I am confident it is not spark. The fact that it runs without the pipe really has me confused and also discounts the issue being spark...

Compression is 150 according to the mechanic, he just told me minutes ago.

I'm really stumped.

Any takers? Anything that could have come loose to cause this?

 

[parrothd]

Rotary valve, the brass gear probably stripped, maybe a screw fell into the intake? Front port is probably open and the rear is closed so no fuel can enter the cylinder..

 

[ialonso]

Could be...I've done that once before...both screws that affix the flamearrester tray to the carb were there though.

If the rotary valve was stuck or the gear stripped, would it run without the exhaust pipe? and would the pressure be able to reach 150 with the intake open the entire time on that cylinder? I don't really know, but it could be.

 

[parrothd]

rotary valve does nothing with compression..

 

[ialonso]

Really? I thought that's why it closed? To keep the air from escaping.... So how does that valve work?

(Looked it up....)
http://en.wikipedia.org/wiki/Two-stroke_engine
still not 100% sure....

 

[parrothd]

The valve determines when the new fuel/air mixture is allowed into the lower chamber. The valve is "open" during the compression stroke..

On a 4 cycle engine, if the intake valve was stuck open during the compression stroke you'd get 0 compression, but for a 2 cycle engine the reed vales/rotary valves are used to time when the air fuel mixture is allowed into the lower chamber, compression actually only happens when the piston is past the intake and exhaust ports which then closes the intake and exhaust ports..



http://science.howstuffworks.com/transport/engines-equipment/two-stroke2.htm

 

[Phildoo]

I have the exact same issue. Totally weird. I've swapped plug wires and still runs the same. I run it and shut it of repeatedly and it eventually comes out of it and runs fine. Went thru the carbs and no problems. I'm leaning towards exhaust also. Any ideas?.. This is the same cylinder that blew the piston recently. It has 150/150 compression.

 

[ialonso]

I don't hve my boat with me yet, but I figured it could be tested using a flashlight down the air intake/carb, and observing in the spark plug hole for light on each Cylinder as it reaches the bottom while turning the engine? (Not sure this would work, just brainstorming)

Is there any way to test without dissasembly?

 

[parrothd]

Take the spark plugs out, air filter off and look down into the carbs with a flash light and open the throttle(engine off), if you turn the pto you should be able to see the port open and close.

 

[ialonso]

Suggestions for a single carb set up?

 

[Gamerse]

Don't you guys have water box valves? What if the thing has filled up at low speeds because the valve is stuck open. Water is a pretty good restriction.

Did you say you started the engine without a pipe?

I took the exhaust pipe off, hoping to find a restriction of some type, and to my surprise the engine fired up the second cylinder with some delay while running without a pipe.

What did that sound like???

Reach down and give the water box on the fouled engine a shake, if it sloshes, compare it to the other.

Anyway, waterboxes are pretty easy to swap. Since you have two, try swapping in the other and see what happens.

Pipes are big and hollow, would have to have a pretty huge restriction....

Another thought, check the inside of the pipe for a cracked water jacket.

:cheers:
Ernest

 

[Gamerse]

p.s. Check all your water lines to make sure none are backwards

:cheers:
Ernest.

 

[ialonso]

We have WINNER: parrothd hit it, and on the first try too. The rotary gear is GONE. Happened right after the rebuild, and I'm not sure why at all. He says it all looks good otherwise, and suspects a worn spring. I find it suspect that it happened immediately after the rebuild.

Thanks for the help and ideas!!!

 

[Phildoo]

Hmmm. Interesting. Mine does the same thing but comes out of it and runs fine the rest of the day. Could gear be partially stripped?

 

[El Toro 33809]

When they strip typically the entire gear doesn't grenade but it would highly unlikely to run intermittently. It that IS what's happening, go play the lottery. The odds are probably better and luck is on your side.

 

[Chester]

We have WINNER: parrothd hit it, and on the first try too. The rotary gear is GONE. Happened right after the rebuild, and I'm not sure why at all. He says it all looks good otherwise, and suspects a worn spring. I find it suspect that it happened immediately after the rebuild.

Thanks for the help and ideas!!!

Check the rotary shaft for straightness. If the shaft is bent, it will always shear the brass gear.

Chester

 

[Chester]

Hmmm. Interesting. Mine does the same thing but comes out of it and runs fine the rest of the day. Could gear be partially stripped?

No it's not possible. It will not run at all.

Chester

 

[ialonso]

Do you have dual carbs or single?

 

[Phildoo]

Dual carbs. I did a comp test and as usual dead on 150/150. On the lake today. IvalwYs get there early to beat the noobs and test run the boat. Ran perfect. Docked and waited for friends to show up. Finally get there take off and same thing, bogged down on one cylinder. Crazy. Get to sandbar and shut off then restart to go get more peeps. Ran fine. Don't get it. Ran fine the rest of the day. What the he'll would cause this????

 

[ialonso]

Dual carbs. I did a comp test and as usual dead on 150/150. On the lake today. IvalwYs get there early to beat the noobs and test run the boat. Ran perfect. Docked and waited for friends to show up. Finally get there take off and same thing, bogged down on one cylinder. Crazy. Get to sandbar and shut off then restart to go get more peeps. Ran fine. Don't get it. Ran fine the rest of the day. What the he'll would cause this????

Since you have two carbs, it could be that the failing cylinder is being fed by a flaky carb. If it happened again, and it was my boat, I'd spray starting fluid on the failing carb at it was doing that. if the issue is temporarily fixed, it's the carb.

 

[Phildoo]

I thought it was the carb. Went thru them and still same issue. Don't get it. Something with exhaust maybe??

 

[ialonso]

I thought it was the carb. Went thru them and still same issue. Don't get it. Something with exhaust maybe??

I continue to think that carb rebuilding is a dark art. Did you replace all the replaceable parts? needle and seat? any other rubber and diaphragm parts? and did you clean it truly everywhere in every single passage?

I'd confirm with my suggestion before deciding absolutely that it's not the carb. I know some people are against using starting fluid as it can create other issues. You might also just drop a couple ounces of gas in there if it makes you feel better.

The only way I know to fully diagnose the issue with one part being suspect is to either swap the suspect with a known good, or "bypass" the suspect part (such as spray fuel on behalf of the carb, bridge the solenoid, etc) . rebuilding the suspect part and declaring it good while the problem remains is tricky ...it could be that the part rebuild was not successful.