Carb Issues

[etemplet]

I have an issue with the Mikuni Carbs. Seems like the lever has a long way to move before it begins opening the needle. I don't see any measurement on that. I am looking at carbs for a 657 SPX and I sure do see what you guys mean about aftermarket kits. The Bellows are WAY stiff and the button on the bottom is about half the length of the Mikuni Part. Do you guys bend the tab so the lever will lift the needle sooner? Any rule of the thumb on that??

 

[mikidymac]

Take a look at my carb threat. The lever should be level with the carb body, in post #7 I show checking it with a razor blade.

 

[etemplet]

Thank Miki... I did read your carb thread thoroughly. My question about the lever that actually contacts the needle pushing downward DOES NOT physically LIFT the needle. It only relieves the pressure ON the needle so fuel pressure can push past the seat and into the diaphram chamber. I know I am getting a bit picky but that is all I can figure. This is been a point of interest to me since my first carb repair last year. At present I am pretty good and building them....I am very slow and meticulous but I can't tune them for crap. LOL

 

[mikidymac]

You are correct. The engine vacuum pulls the diaphragm down against the spring pressure, lifting the arm and allowing the fuel pressure to push the needle off the seat. Keep in mind the pop-off is just a test and not how the carb actually functions because it is the diaphragm that moves the arm and not fuel pressure like when testing pop-off.

 

[etemplet]

I wouldn't make any money building "these" carbs. I think I have as good as I can get them. Without the insight of members I'd still be poking around a bit. Changing parts don't make these babies work. After talking with Matt this morning I measured the carbs on the SPX. Scratching my head... one is a 38mm the other is a 40mm.

 

[R-u-n2it]

Miki, been awhile since I've had the carbs apart and trying to get my head around something, you say vacuum opens the fuel needle, where is the vacuum port into the chamber, and if there is vacuum present what prevents fuel from being pulled into the port? I always thought the diaphragm was to mitigate the barometric pressures from altitude and atmospheric (weather) changes. I thought Pops F said this so I never gave it any thought. Also closed throttle produces high vacuum when least fuel is required and full throttle has low vacuum when fuel requirement is high. I ported the cases on my xp and with 46 Buckshots have to run 12-13 pounds on pop-off so it doesn't bog when launching from idle to full throttle. Like I said, been awhile since I jetted this, 20+ years.

 

[mikidymac]

The vacuum is pulled through the pilot and main jets.

 

[R-u-n2it]

I agree sort of, vacuum pulls the fuel through the jets. If vacuum through the jets controlled the opening of the fuel needle, to overcome spring pressure to get fuel at wide open throttle (low vacuum) the engine would flood at low throttle settings (high vacuum), would it not? Checking pop-off uses pressure to operate the needle and seat to simulate engine operation, yes? As long as one knows what jets to use the technical principles are for nerds anyways {;o)

 

[etemplet]

Well back on the GTXs for more carb tuning. My wife's GTX is pretty strange. I believe my carb set up is still not spot on but I am learning. The ski sits at 35mph and sits and sits and sits at Wide open throttle... she hits a wave or for some reason unknown to us... it finally kick in and hauls azz up to a tad over 50mph. When she shuts down... same thing all over again. I opened the high speed screw to 1/2 turn out on PTO carb, previously it was zero. Now the ski stalls at 28mph and sits and sits and sits then.... takes off when it feels like it. I may have proved the ski is running rich. Last week I ordered new needles and seats. The Other GTX runs better but neither are making top RPM. The other GTX is barely reaching 6500. Tomorrow's plan is check the spark plugs and spend some on the carbs on my wife's ski. Then test and tune on Sunday.

Could leaking needles cause this?? Input, help is appreciated.

 

[R-u-n2it]

Is the carb setup stock, specifically the flame arrestors?

 

[R-u-n2it]

A pop-off setting that's too high for the available fuel pressure will react this way, an easy test is to restrict the airflow somehow, 1 way is to cover half of the intake with duct tape, this will increase manifold vacuum and since the fuel pump uses alternating vacuum and pressure (pulse) from the bottom end increasing vacuum will increase fuel pressure and allow the pop-off to open sooner. When pop-off is almost right but a little high, when you hit the spot where acceleration stops you can feather the throttle by barely opening it (increasing vacuum) and it will take off where keeping it full throttle (low vacuum, no air restriction) and it will never take off. It will make sense eventually, just like a choke works, restrict air = more vacuum and you pull more fuel through.

 

[etemplet]

Is the carb setup stock, specifically the flame arrestors?

Sorry for the delay in responding but I've been away from the computer. Not sure what you mean on the flame arrestors but I have stock jetting and Mikuni parts. On Sunday I installed Genuine Mikuni needles and seats just to be sure and reset the pop off pressure from 23 psi to 32 psi. The ski responded by getting up on top much better but not 100% it was still Sluggish down low and in the transition. Basically the ski is running a good bit better with a 10psi difference in pop off pressure but it isn't pulling hard at all on the bottom.

Note also that in April we took these ski out and they both ran well enough for us to have a blast with the power they were delivering. They were a little rich down low but they got up really quick. and pulled really hard. I did nothing to these skis and both suffered a drop in performance. I've been chasing this for a month. Pulled the carbs apart again twice, clean, check, set pop off, etc....

Both skis are clearly getting too much fuel. Since I have the same problem with both and had very little issues with them in March when the weather was cooler, that leads me to believe that the higher density altitude is richening the mixture but I am surprised at the degree of the effect. Just doesn't seem right to me. That said... my problem could be sticking rave valves. Perhaps the Mystik oil I am using (API-TC) isn't the best suited for the valves. I have not pulled any of them yet but plan to this week. We cleaned the flame arrestor as well.

I read the skis were set up stock for a 750' density altitude. It's all strange but I am looking at every possible issue. I must eliminate the raves because if they are not working properly.... they engine would run rich because it can't burn the fuel fast enough.

 

[etemplet]

A pop-off setting that's too high for the available fuel pressure will react this way, an easy test is to restrict the airflow somehow, 1 way is to cover half of the intake with duct tape, this will increase manifold vacuum and since the fuel pump uses alternating vacuum and pressure (pulse) from the bottom end increasing vacuum will increase fuel pressure and allow the pop-off to open sooner. When pop-off is almost right but a little high, when you hit the spot where acceleration stops you can feather the throttle by barely opening it (increasing vacuum) and it will take off where keeping it full throttle (low vacuum, no air restriction) and it will never take off. It will make sense eventually, just like a choke works, restrict air = more vacuum and you pull more fuel through.

We had the issues you mention and still do to a degree. It takes a second or three now for the ski to take the fuel but it pulls like hell when it does. This issue was much more pronounced with the pop off at 23psi. The ski would run at 35mph, throttle position didn't help no matter what we did. Then all of a sudden, the ski would blast off. At times it took off when we hit a wake. The jolt seemed to wake the ski up. I played with the throttle, snuck up on the 35mph, hammered, eased off eased up, hammered then sometimes the ski would blast off. This was at 23psi pop off. Next time out a week later, I opened the PTO Carb high speed 1/2 turn out. The skis response was to run at 28mph and was even more sluggish to jump up to high speed.

So.... with the mikuni needles and seats and 32psi pop off. The ski "Always reaches maximum speed when you give it throttle it is just sluggish through the transition. My cousin played with it and says it come on strong at 6100 RPM. He adjusted the raves and got it to come on a bit earlier in the RPM range. This is really takes off. This whole mess is kinda fun playing with but I'd like to get it right so I can understand what is causing the problems.

 

[etemplet]

So I have another kicker.... I just rebuilt the carbs on a 96 XP... same engine as the GTXs. The XP is a work in progress but I wanted to put it in the water. That thing... ran like a raped ape. Pulled hard from a standstill all the way up. I'd love to get the GTX throttles to respond that way. I realize the XP is much lighter so I don't expect that type of performance but the GTXs were pretty impressive in April. I am considering taking the carbs off the XP and putting them on one of the GTXs. Anyway I sure do appreciate your detailed technical analysis of the situation. I hope I gave you enough information to help you to offer further advice and assistance.

 

[R-u-n2it]

1) Is the oil 100% synthetic? If not it will burn dry and leave carbon that sticks the valves, synthetic leaves a black goo but doesn't stick the valves. 2) Did you put the correct sized needle and seats in, the size and the pop-off setting work together. 3) How did you "set" the pop-off, different springs? There are 2 different weight springs that look almost identical used and not new but one weight is wound the opposite direction from the other three, even Andrew Buck (Buckshot) didn't realize this until I told him and he supplied carbs to over 75% of the Pro racers for years back in the early Seadoo days. Were you turning left when you got the bog? Stock intake manifold oriented the carbs with the diaphragm such that it affected the needle and seat, many had the left turn bog and/or rpm drop, I didn't since I had a Watercraft Magic intake that use the rubber spigots that are used with the 46mm Buckshots on my XP and the 47 billet Buckshots on the 787 mod HX. I run 13 lbs of pop-off on my 787 XP. Your throttle shafts are end to end if stock, mine are vertical with a bellcrank and rod between them. As for the flame arrestor, if you can figure out a way to restrict the airflow through them some it will either run better or worse, if better your pop-off is too high or something else (like needle and seat size) is limiting fuel delivery, if it runs worse the pop-off isn't too high or at least not the only problem. You used WD-40 every time you tested pop-off right? Address each of these and note the difference. And YES, switch carbs like you said, test and if good THEN open the good carbs up and note the Mains and low speed jet size, the needle and seat size (might be a 3.5, been 20 years...) test pop-off and check what springs, if I remember there's gold, silver, black and an dark green that looks almost like the black but 1 of them is wound opposite of the other 3. Sorry for the long post but you need to get those turkeys running, it's summer.

 

[R-u-n2it]

Oh yeah, set the RAVEs red top flush with the black holder, and using synthetic oil you will eventually get the black goo running out, that means you need to change the o-ring on the stem as it gets brittle and leaks. If it still leaks might be time for new valves or holders or both, likely just the o-ring, I've put many many hours on my XP at 7600-7800 rpm, it's a zinger.

 

[R-u-n2it]

YOO HOO, etemplet, you got to let us know what you figured out, invaluable to anyone looking for answers, you know how that feels.

 

[etemplet]

I am on vacation and catching rays, swimming and a little jet skiing so I am not checking my computer stuff that often. Here goes....

Previous Pop-Off setting was 23psi on both skis. The ski with the bigger bog problem is the fastest top end. I set the Pop-Off to 31-32 pounds. I get a better seal and ski has less of a bog. I recently purchased mikuni needles and seats so I have the right stuff in there now. Carbs... I've cleaned and checked 5 times.

I pulled the rave valve covers off because I didn't like the goo. I wanted to see what was going on. I do have lots of clearance around the valve stems and guide. Nothing melting but I will fix that when I get back home. I thought the raves might be sticking because the ski is sluggish to get the power on line... "warp drive" LOL

I do use the full synthetic Mystik oil.

Water control valve - I suspect of leaking as shown by the pictures I posted. >See below<. After reading about the WCV I am wondering if it can be causing other performance problems.

After I installed the mikuni needles and seats and reset the Pop-Off higher the ski does a much better job of reaching top speed. Before this.... it would sit at 35mph no matter what you did to the throttle then all of a sudden take off. Then you'd spend time trying to figure out what you did to make it go. LOL

I may have mentioned... when I opened the high speed screw on the PTO carb 1/2 turn.... the ski would not get over 28mph. So.... logically it makes me think I am getting too much fuel.

That said... I am not sure I have genuine mikuni springs on the carbs for the pop-off. I was thinking a spring is a spring as long as it works for the pop off. That might not be the case. I ordered the genuine mikuni springs but they have not arrived yet.

Hope this helps. I hate this crap but love the chase.... When I get home I am gonna check compression. I have about 10 or so hours on the rebuild.

 

[R-u-n2it]

The water control diaphram looks OK, I have a jet inside mine, when exhaust pressure exceeds spring pressure it shuts off most of the water at high RPM and the engine reacts as if it had a longer skinny pipe, at least it does that withthe Factory pipe with water injection right after the head pipe. I need to find some pics, I've been all about finding a Yamaha FX 1.8L HO to mod for the rough stuff. The pop-off springs come in gram weight, I think I used 80 gram springs. Didn't find pics of my XP but found some of a guy named Lee Stone I took, 1st is 1st loop of a double backflip.

 

[etemplet]

I didn't witness any leakage on testing but I found rusty water stains on top of the muffler. I wiped them off and will check after my next ride. What about some high performance parts for the GTX? I'm ready to make mine faster than the one my wife rides. Needs some attitude but.... we have a few XPs I need to get running.... they have attitude.

 

[R-u-n2it]

How about a Mel Miller 951 race engine, lol. Seriously a GTX is heavy. Get it running good and sell/trade for a 951 XP 99 or newer, no DI fuel injection ones. This style XP cut's through the chop and carves. I have a factory service manual and with the strange problems you're having you should go back to square 1, get it running good then olay around with tuning. This is the web site for the FICHE FOR ANY PWC, QUAD, STREET OR DIRT BIKE, SNOWMOBILE from Yamaha, Suzuki, Honda, Kawasaki, KTM, and pretty sure Husky www.hlsm.com exact same as the dealers use.

 

[etemplet]

OK, got some new data on the jet ski with the bogging issue. Here's how it works. If the prop/engine can reve past a certain point it rocks and rolls. Let it drop below and it will run smooth but not "take off" I think that point is a bout 6 grand on the tach. Soon as the prop comes out of the water and you get some RPMs the ski RIPS for as long as you keep it on the step (so to speak) Let it drop below and your doin about 35 and it take a minute to get past the flat spot.

I've done everything I can to the carbs. Perhaps the water valve may be an issue. I cleaned and checked the rave valves and they are smooth and clear. This ski ROCKs hard when you get it in the kick azz mode. Maybe I need to look at the prop. Ya never know who put What on there.

 

[etemplet]

How about a Mel Miller 951 race engine, lol. Seriously a GTX is heavy. Get it running good and sell/trade for a 951 XP 99 or newer, no DI fuel injection ones. This style XP cut's through the chop and carves. I have a factory service manual and with the strange problems you're having you should go back to square 1, get it running good then olay around with tuning. This is the web site for the FICHE FOR ANY PWC, QUAD, STREET OR DIRT BIKE, SNOWMOBILE from Yamaha, Suzuki, Honda, Kawasaki, KTM, and pretty sure Husky www.hlsm.com exact same as the dealers use.

I have a 1998 951 XP and one for parts. Matt says it has rossier heads. I'm gonna be workin on this one soon. I also have a 96XP that kicks butt. I have work to do on it as well but it is way fun. I'm not really interested in going all that fast, I just want to get the GTX to perform as designed. I have two GTXs that I am keeping for my wife and I. I still prefer the smaller skis for me though.

 

[R-u-n2it]

If the original color of the 98 engine was white that's the problematic engine, the 99 and up are silver-grey. Like I said in the beginning, I think your pop-off is a little high. The pulse (pulse is alternating pressure/vacuum) runs the fuel pumps, fuel pumps supply fuel to the carbs to go faster, going faster makes higher pulse (higher vacuum/ higher pressure) which allows more fuel to start the whole process at the next level of rpm, and if your pop-off is a little too high it stumbles along very slowly getting a few more rpm and in turn allowing a little more fuel till finally it takes off. I went through the same thing learning with a 94 650XP. Put on aftermarket flame arrestors and if I nailed it nothing would happen because the lower restriction arresters didn't allow the engine to get fuel, as the piston goes down on intake it pulls air, with less intake restriction it's easier for air to go by the arrestors than it is for that air to pull on the fuel pump diaphragm, but if I'd feather it, give a little throttle and back off a tiny bit and now I'm making more intake restriction with the throttle butterfly plates and the XP would take off quicker. I bet you put it on the hose and give it the smallest amount of throttle to raise rpm and it will rev since there's no load on the engine, therefore for the same throttle setting as in the water it will make a higher rpm out of the water with no load. I learned that when I gave it throttle and you feel it start to fall on it's face to quickly back off and then repeat with some throttle but backing off just when you think it's going to stumble and it would take off. And I had set the pop-off to what I was told is stock. My 785 XP with the big 46mm Buckshots will literally will launch when you stab the throttle and my pop-off is in the 12-13lb range. You tried everything else, just drop yor pop-off a couple lbs from where it was when you started. You never said what size N&S you used and what size was in it originally, Needle and seat size works together with spring size on pop-off, the N&S is what the pop-off spring tries to keep closed and fuel pressure (from the pulse operating fuel pump) is what tries to open the N&S. Sorry for the length, just trying to explain it from every angle. Remember, low pulse = low fuel pressure, low fuel pressure and less N&S opening, less opening = less rpm building, like a dog chasing it's tail, pop-off is set to gives just a little more fuel to go but low enough when throttle released the fuel shuts off preventing engine flooding. Jut try lower pop-off. Don't forget, what was and is the N&S size.... Lee

 

[R-u-n2it]

Found a pic of the XP so took a pic of a pic.....