GSX 97 Engine Back Firing above 4,200RPM

[peter Nosbor]

I have a GSX 97 with a 787 engine.
The motor starts easily and idles okay.
The RAVE values are reasonably clean and can move.
I have run the engine without the RAVE covers and can see them move to a move open position as revs increase.
The problem.
The engine revs to approx 4,200 and then begins to backfire badly.
Note: Holding the RAVE valves closed (with the covers off) seems to make it a bit better, but only marginal.

I have a recently replaced the MPEM and found this did not change the situation.

The carby was overhauled by a jetski mechanic and the machine has backfired ever since. Is it possible the rotary valve is out?

Can someone help me with this problem.

By the way I now have a good spare MPEM for a GSX97 278 001 201 for sale.

Also, the baffles in the water box are on their way out but there is no restriction in the exhaust air flow.

 

[seadoosnipe]

Rotary...

Under normal conditions, backfiring is caused by the rotary valve. Either out of time or the woodruff key on the shaft is sheared.

But, since your saying that this has occured since the carbies were rebuilt, I'd be pointing to them. The rpm you mention is when the carbs are in transition between the pilot (low speed) jet and the main (high speed) jet. If the pop off pressure and needle seat lever are not properly set, then this may be your source of backfiring. It's probably only one carb that is off.

Also, missing the sync between the two might cause this. Normally, not having them properly in sync will cause one cylinder to be lean, the other rich. But in your case, if the carbs are not built properly and also out of sync, I can see this happening.

I'd have the carbs looked over again; by another mechanic at that.:cheers:

 

[jwick86]

Another method I use to use when racing snowmobiles was to start the motor, and at an idle pull one spark plug cap off. If the motor comes down in idle the spark is good, but in your case if the backfiring stops then you have isolated the problem to that cylinder. I also have had bad spark plugs cause this since they can't fire, even brand new ones. If the motor dies when you pull the spark plug boot just put it back on and restart it and try the next one. Don't worry, you won't get shocked unless you can touch bare wire under the insulation on the coil wire. The rotary valve shouldn't be out of time if he only touched the carbs. As Snipe said, check the pop off spring and make sure its transitioning over to your mains.

 

[seadoosnipe]

Shocked?....

You can get shocked without touching a bare wire. If you have a crack in the insulation anywhere, that 40,000 volts will find a ground. Most of the time, you are the ground.

The best way would be to read your plugs to see if you have a weak cylinder.

You can hear the poppin of a plug wire that's foul. You can see it if it's dusk or dark. It is a small electrical arc from the wire to the point of ground. I've done what jwick86 has suggested and it works...dont' get me wrong. I've had the hell knocked out of me too from a wire with bad insulation.:cheers:

 

[jwick86]

I understand what your saying Snipe about the wire being cracked. I should have specified that I ONLY grab the boot as they are least likely to have a crack and the rubber there is thick and somewhat pliable.

 

[rookie101]

Just a thought......

I may be way off here, but given the number of threads that we have had in dealing with cabs being "stuck" to the intake manifold, is it possible when the mechanic did the overhaul he pulled the intake and messed up the rotary valve timing?

I don't know enough about the Rotax to be sure on this, so it is really more of a question than an suggestion.


Aaron

 

[seadoosnipe]

Yes....

I'll agree with you in that context. But I don't think you can ever be 100% sure your not going to be bit....

Like I said, I do it too and I've also been bit. It smarts...:ack:

The best thing to do is use a pair of electrical gloves or electrical pliers, that way....your sure!

Yeah,......call me a sissy, I can take it.:lols:

And yes, your right Aaron, he may have taken a short cut if the carb bolts were frozen; it is a 97 model.

 

[Chester]

A bad voltage regulator does some strange things. Disconnect the red wire to the voltage regulator & try again. If the backfiring disappears, replace the voltage regulator.
Also, it's a bad idea to disconnect a plug wire while the engine is running (especially at 4200 RPM). You could fry the ignition coil. That is why there are grounding posts for the plug wires.

Chester

 

[jwick86]

The grounding post for the plug wires are for when the plug wires are not being grounded to anything. It is bad to crank a motor over not giving the spark a place to go and just build up inside the magneto. That can fry the mag or the trigger. And yes pulling the plugs off at 4200 rmp is not a good idea, that's why it was stated to do so at an idle.

 

[Chester]

The grounding post for the plug wires are for when the plug wires are not being grounded to anything. It is bad to crank a motor over not giving the spark a place to go and just build up inside the magneto. That can fry the mag or the trigger. And yes pulling the plugs off at 4200 rmp is not a good idea, that's why it was stated to do so at an idle.

In his case, it idles fine.
The spark will build up in the ignition coil, not the magneto. When you pull the plug wire, you are creating an open circuit in the secondary windings in the ignition coil. The magneto coils will still have a closed circuit and will still send a current through the primary windings in the ignition coil. The current going through the primary windings induces a high voltage current (over 20,000 volts) through the secondary windings to fire the plugs. This is why I don't recommend pulling a plug wire ever when the engine is running on a PWC.

Chester

 

[peter Nosbor]

Course of action

I plan to check the voltage regulator first.

Then plan to remove the spark plug lead of each cyclinder and see whether I can isolate which cylinder (carby) the problem stems from.
I will then ground the spark plug on the posts and then try to rev it through the 4,200 RPM - see what happens.

Does the MPEM change the MPEM ever change the spark timing or is spark timing pretty well fixed for a two stroke?

How difficut is it to re-tune the carby?
This includes checking it and checking the timing of the rotary valve.
I know how to use a spanner, not how to build a nuclear power plant.

Can you advise where I can get detailed instructions.
I don't mind paying a bit if I can do it myself.
Can this be done without removing the engine from the jetski?
Otherwise I will be paying a bit to get a mechanic to look at it anyway.

Seadoosnipe - What did you mean with your post
"And yes, your right Aaron, he may have taken a short cut if the carb bolts were frozen; it is a 97 model."

Thanks for your coments and help - It is very much appreciated.
The funny thing is, I am trying to get this jetski running so that I can sell it.
Young children are not all that conduscive to playing.
I do not want to sell someone a dud.

 

[jwick86]

Everything depends on your mechanical ability on how difficult it is. Checking the carbs are not hard if you understand them, and checking rotary timing is not hard if you understand how to do it. In those older model machine's they have problems with the carb bolts oxidizing to the carbs and not coming undone, then causing the mechanic to pull the rotary valve cover off and possibly changing the timing in the motor. Pulling the boots off one at a time and only for a few seconds will not hurt anything, I have done this on $12,000 race motors and they still run fine. Not just by luck, many racers do this to isolate problems. But first before you worry about doing anything to fix it, find it, or atleast isolate it first.

 

[dharvey]

Mpem

I have a GSX 97 with a 787 engine.
The motor starts easily and idles okay.
The RAVE values are reasonably clean and can move.
I have run the engine without the RAVE covers and can see them move to a move open position as revs increase.
The problem.
The engine revs to approx 4,200 and then begins to backfire badly.
Note: Holding the RAVE valves closed (with the covers off) seems to make it a bit better, but only marginal.

I have a recently replaced the MPEM and found this did not change the situation.

The carby was overhauled by a jetski mechanic and the machine has backfired ever since. Is it possible the rotary valve is out?

Can someone help me with this problem.

By the way I now have a good spare MPEM for a GSX97 278 001 201 for sale.

Also, the baffles in the water box are on their way out but there is no restriction in the exhaust air flow.

How much are you asking for the mpem?

 

[peter Nosbor]

Problem Solved

There were 2 issues.
The rotary valve timing had been adjusted and was out by 15 degrees.
But the real issue was the "new" stator that had been installed. It was an aftermarket unit and was not meant for the 787 engine.
The correct stator has been fitted and the machine now works like a charm.
Thankyou all for your advise and comments.

The original marine engineer used for the service has been taken off my xmass list.