GS 1998 backfire

[jst4sho]

The engine is still backfiring through carb when opening throttle. I've overhauled carburetor and tried several adjustments to smooth things out but nothing helps. Engine ran strong the last time on water until it died and stranded me on lake. Carb goop really bad but did a rebuild kit and cleaned everything. Fuel lines replaced too. I've not changed any settings except low speed screw but always return to spec forum has told me to use which is out one and one quarter turns. High speed screw is at spec 0 turns.
No other electronics changed from when engine died. Engine will fire right up and idles normal.
I have some mechanical background and am confident with the carb overhaul since I had such great help on the forum. Thanks to all who guided me.
What are the reasons this thing would be backfiring through carb after the overhaul? It was not doing that before the engine died on the lake....ron

 

[Nate Surveyor]

I hope this helps:

I used to have YZ 80 Yamaha. I ran it hard, and finally it needed a new piston, rings, and rebore the cylinder.

I ran it for a long time, and it finally started backfiring.

The mechanic told me that the problem was that the crank pin was worn enough to allow the timing to be off enough, to make it fire down the intake, effectively burning the fuel air mix in the crankcase. It would run fine, until I put it to over 7/8 throttle, and then it would explode in the crankcase. (I never have confirmed the mechanics story...)

Now, if your timing on the motor was off, it could do precisely that. Just firing too soon. So, set the spark timing a little later. If it stops, then you have found the problem. If it was caused by a worn crank pin, then you may need bigger help. (unless of course, the mechanic on my YZ was not truthful.) In either case, check timing.

Maybe this will help.

I don't know how to time one of these engines.

Maybe a premium membership ....

Nate

 

[jst4sho]

SeaDoo GS has not been on the water more than once or twice in a season and I got it new in 1998. It may be ten years old but it is just does not have very many hours on it. My children were grown and in college or living out of my town and I worked six days a week and ten hours a day. I just never got on the water a lot. When I look at some of the forum photos in the carb rebuild I think to myself that those really look corroded. Mine looks new inside and out except for a few bumps from the dock or beaching.
That doesn't mean something could not be 'worn' out but it is highly unlikely unless the running lean with the filter clogged could have caused something to go bad fast.
I know it points to timing Nate but if it was running fine up until it shut down because of the fuel lines why would the timing be off after a carb rebuild. If it is the timing how do I try to set it back a little. Do I have to get into the engine somewhere? From what I see in the manual it looks like I would have to get behind the intake but it is just not real clear. If you or someone could make that more clear as to where I go and then what I adjust once I get in there then off I go for more fun.

 

[Nate Surveyor]

I don't know how to check the timing. Or if it could have slipped somehow.

All I can say is that the symptoms are like a timing issue.

Maybe somebody else will have the answer.

Nate

 

[seadoosnipe]

Timing?........

The timing on the 85 hp, 717cc engine is pre-set. There is no "adjustable" timing on this or a few more models of the Rotax engine.

The timing is normally considered the rotary valve. If it's not set correctly, it will cause backfire.

I know you say it's not new, but really has no hours on it.....that doesn't matter. Parts can wear out, from just being stored and not lubricated.

There are a couple guesses I'm going to give you.

1. If the rotary valve has never been removed, then you can move to step 2. If it has, then you need to remove the carbs and the rotary plate and reset the plate. You'll also want to check the backlash clearance on it. This rotary shaft is geared to the crankshaft through a worm gear. It's possible that the brass gear is damaged.
2. The pickup or trigger coil. You can remove the magneto and make sure that the small pick up coil is in it's correct locationand is not bent or damaged.
3. CHeck that the magneto flywheel is still keyed. We had one member last year, that when he removed his magneto cover, the armature was laying inside the cover...........

I hope this gives you somewhere to start. The problem will not be in your carbs.......Problems with the carbs will make the engine run rich, lean, cut out, stammer, hesitate and flood out.............but will never be the likely cause of backfiring.

 

[jst4sho]

Thank you Nate and Snipe...I'll see what I can do....ron

 

[jst4sho]

Sorry to take so long to get back to the forum. I changed gears at the beginning of October and took back a part time job.
Anyway, I finally checked the compression on the 97 GS and both cylinders read 135. I'll be getting it all covered for the winter today I hope before a cold front arrives tonight.
It still backfires so that problem not resolved. Nate and Snipe had just asked for a compression test and I wanted to let all who have helped me with the problem know the readings

 

[seadoosnipe]

Compression....

The compression is o.k.....You worked on the carbs. Have you checked to see that the accelerator pump works correctly?

If you are saying that the ski ran fine till it stranded you on the lake, what happened? Did it just quit? Did it stumble, spit and sputter, then quit? How do your spark plugs look?

Look to see if your sucking air around your manifold and carb. Backfiring can also be caused if your rotary backlash is worn out of spec.:cheers:

 

[jst4sho]

Ski was only on the water one time this summer and twicelast summer. Last summer was a check by the mechanic after he said he cleaned the carb and one ride by me. This summer after taking out of storage and doing preseason procedure it did not run really good at idle or on applying throttle. It would sputter as throttle applied but then catch on a zoomed pretty strong. I road for less than a half hour anywhere from just planing out to full throttle and just before it shut down I had run at or near full throttle from the middle of our small lake to a cove area. I let it idle down and it just died and would not restart until I joined this forum and you all walked me through the fuel line replacement and carb overhaul. It will now start and throttle up but backfires at RPMs above the low speed jet. Accelerator pump works fine when I checked. That is where I am now and the ski is ready for winter after procedure for storage. All I need to do is get it covered and towed to the back yard for the winter hibernation. Next spring I will try again but at least I am much more experienced than before you and Nate nurtured me along. It could be the timing but my suspicion is leaning toward fuel still and I will check and recheck all those lines and the carb and the tank