97 Gsx 787 Misfire at half throttle

[busta]

Hi guys, in desperate need of some help. I have a 97 Gsx, at half throttle it misfires but also like a dry gravel kind of sound. its still very responsive and great at wot, still get the top speed out of it. It's hard to start, I usually need to give it some gas 1st. I have noticed sometimes there are small air bubbles coming out of the return fuel line at the carb outlet then they stop after about 20 secs. It seems to me that there is a transition issue between the low and high jets but I could be wrong.
Do you think I should try a pop off pressure test on the carbs or could it be a timing issue? As I say though, it still runs great at wot and responsive which has me puzzled.
Any info and advice is much appreciated.

 

[mikidymac]

Have the carbs been rebuilt with genuine mikuni parts. Air leaks at fuel selector or strainer?

 

[busta]

They were rebuilt but they weren't genuine parts I'm afraid. I have already replaced the selector valve and seal at the strainer. Is there an easy way to test the fuel lines to pin point a leak, if not, it will be like trying to find a needle in a hay stack. It's driving me mad.

 

[busta]

There doesn't seem to be any fuel in the return line, it's as if its not holding a vacuum. I take it the lines should be full of fuel if it's well sealed even when the ski is not running?

 

[ah64driver2002]

The aftermarket rebuild kits cause all sorts of problems. I would start by getting a new OEM rebuild kit and see what that does for you.

 

[busta]

Yeah, I agree, that's what I will do, I am feeling it's carb related reading up on them and seems like they after market kits are shit. Cheers for the help guys, much appreciated.

 

[jjsinaz]

I've been chasing the same problem with mine. I'm leaning toward something with the water regulator on the water box. I've read a lot and it seems like that can be the culprit. Let us know if you figure it out and I'll do the same

 

[busta]

I will do Jjsinaz, I will need to look into the water reg thing aswell now. Defo keep each other posted.

 

[busta]

Right, reading about the wr, I'm now thinking this could also be the problem, I have noticed when my ski is idling in the water it sounds alot deaper than usual (like a Harley Davidson) but would it cause the misfiring at mid rpm?

 

[jjsinaz]

Supposedly it can contribute to it. When reading you might also have read that getting rid of this symptom all together is unlikely. They all sputter some when they make the transition from the low to the high speed circuit. I'm just hoping to get mine a little better than it is right now. I checked the WR and found an O-Ring on it where there should not be one (In the narrow groove on the valve block. I'm going to see if that made any difference the next time out and als play around with the adjustment and the RAVE adjustments. I'll post my findings

 

[busta]

Mines never use to run like that, it's only been this year, the thing is, it is still quick and responsive. mines also has air bubbles in the fuel lines jist after the carbs which isn't normal so I'm back to thinking it's a carb issue. I will check the wr aswell and eliminate as much things as I can. It is frustrating. I will keep you posted on my findings aswell.

 

[esahlin]

I have a 96 GTX.....it had a 1/4 - 3/4 throttle hesitation when I first got it and then would run awesome at anything 3/4 to full throttle. I went through all the carb rebuilding strategies (rebuilt twice with Mikuni) and even tried different pilot jets and popoff springs. Went leaner and richer on the pilot jets too with no improvement in the part throttle hesitation. Replaced the fuel selector valve and the fuel filter and all installed new lines.

Finally someone mentioned the water control valve/regulator on the exhaust as a possible culprit and sure enough the water valve had a rusted and broken snap ring in it and a tear in the rubber bellows. Bought some new parts rebuilt the water control valve and fixed the part throttle hesitation problem. Was an easy check and easy fix.

Water control valve/regulator might not be it but its an easy check and a lot less time consuming and less $$ then working the carbs and fuel system again.

I get air bubbles in my fuel lines every now and then and the bubbles don't seem to be affecting how it performs.

I did have one time where I flipped the ski and the gas tank vent lines filled with water and were plugged. THe vent lines that run out through the hull in the front. It ran and hesitated alot and was obviously hard to start. Not saying its your problem. Another easy thing to check.

 

[jjsinaz]

Mine never used to be this bad either. I had the engine rebuilt during the off season and since I've put it all back together, I've had this issue. Been through the carbs twice now. Genuine mikuni stuff in them. Pop off is 38 psi give or take on each carb. I replaced the crappy rusted spring clamp with a zip tie as soon as I got the used water regulator The one that was on the ski when I got it was shot and leaking. I didn't see any holes in the bellows whenever I inspected it the other day and it isn't leaking either. So I am hoping it was the O ring that was not supposed to be there or an adjustment (even though I know you're not supposed to mess with that)

 

[busta]

Thanks esahlin for your info, it will be the 1st thing I'm checking tomorrow, yeah, better checking that before looking at carbs, easier to reach and cheaper, it's crazy to think that would effect your rpm's and misfiring, I would never of thought. I have done all the work on this myself since I got it 3 years ago, engine rebuild, carbs rebuild, jet pump rebuild, new fuel lines, filters, fuel selector valve, etc, so have learned alot but one thing I never looked into or check is the wr, think it's one of those things you stay clear of as it does say do not mess about with it. Every day is a school day with these things, just another thing for me to learn about I suppose.

 

[busta]

Looked at the wr today, never had time to totally strip it out but looking at the red screw cap, it was screwed half way into the black housing, I have now screwed it in tight and unscrewed it out 3 times as per the factory setting, now it's sitting just above the flush of the black housing, would this be right? also I noticed when I to the black cap off that the metal band clamp that holds on the rubber bellow was rusty an a small amount of water was in there to but not much just a few drops. The band didn't seem that tight so I have tightened that up, I'm hoping that's been the culprit but won't know until I get it on the water now.

 

[esahlin]

There is the top stainless steel band clamp on the top of the bellows that can be tightened by the small screw.
Then there is a steel clamp ring on the bottom of the rubber bellows that rusts cause it is steel and actually pokes into the bottom of the bellows and sometimes creates a hole. Its this one that usually fails and causes problems. Most uses a zip tie to replace it....and apply rtv to the hole in the bellows (if there is one).

Not sure if a little water in there is ok or not as the stainless clamp on mine had failed and also the lower steel clamp had failed and created a hole. I didn't adjust the red screw cap and left it to factory settings.

Check that lower clamp and look for a hole if you haven't yet. Here is a post with a picture of how it was fixed by a member.

Water Pressure Regulator

 

[jjsinaz]

Interesting....that pic in the other thread that you posted from Lou Doo you can see that there is definitely and O-Ring in the middle groove on the valve block. I believe those that have said there should be no O-ring there are correct, but this is exactly what mine looked like. Maybe they were putting an o-ring on them very early on but decided it wasn't needed??

 

[mikidymac]

You would think the o-ring should be there to stop them from leaking since it's just plastic to plastic without them but every one I have taken apart didn't have an o-ring and it isn't shown on the parts diagram.

 

[busta]

Thanks for this guy's, mines has those rust marks inside the plastic aswell, I will need to strip it out to check if it's punctured. Now I'm worried that I have screwed out the red knob, there are so many people online saying don't touch the wr knob and then there's lots saying you can adjust it and it won't do it any harm. I'm confused.

 

[jjsinaz]

You would think the o-ring should be there to stop them from leaking since it's just plastic to plastic without them but every one I have taken apart didn't have an o-ring and it isn't shown on the parts diagram.

Yeah, I agree. This is the first pic I've come across that has the o-ring. The only other thing I can figure would keep it from leaking is the bottom of the bellows where it meets the housing.

 

[busta]

So looking at the manual for the 97 787cc engine, my wr has been at the wrong setting the whole time, I have put it back to factory setting, I'm hoping this has solved my issues.

 

[mikidymac]

Also keep in mind the 1997 GSX used a flat washer under the spring to give it more pressure too.

 

[busta]

Thanks Mikidymak, I will need to check this out as there was no washer their either, where should the washer be positioned?

 

[mikidymac]

It should be under the spring on the plastic piston, then the spring, then the plastic cap, then the clip to hold the cap on.

 

[kmarts]

It should be under the spring on the plastic piston, then the spring, then the plastic cap, then the clip to hold the cap on.

I'm glad i saw this... My new WR spring is coming in the mail today. I also didn't have a washer.

Looking at the part breakdown the washer isn't there.


Do you happen to know what size/thickness? can i just grab something that fits from the hardware store?