Another WOT issue...

[jimatsamrayburn]

So was riding around and the ski just bogged down at WOT after an hour or so of riding...with a few stops.

I've pretty much replaced everything that needs to be replaced over the past year (see list below).

One odd symptom that I have not seen when I searched the forum is that top speed WOT comes back when I give it a little choke and then bogs back down when I let go of the choke. This mean it's running rich or lean?

Don't think it's electrical since everything has been replaced. Starts and idles fine. At 1/3rd throttle gets up on plane and runs about 25mph.

Pulled both carbs off to clean the filters yesterday. Front one (rebuilt last summer) had some debris so cleaned that out. Rear carb filter had a hole in it. I have an inline filter so just put it back so it would seal up properly.

I'll order a rebuild kit for the rear carb but want to see if I can get it running for the weekend as is...

Thoughts?

1996 SPX 717
Compression high end of spec
New fuel lines
New gas
Fuel selector works fine
Front Carb rebuilt (did it for the fuel pump)
New Rectifier
New plug wires
All new starting system
New plugs (.020 gap)
Fuses in the grey box are good
Rebuilt jet pump
None of the gauges work...next winter's project.

 

[JeremyD615]

It sure sounds like a fuel/carb issue... If the choke helps, it means you are lean. You don’t want to keep riding it until you find the issue. I’d go ahead and rebuild the cabs again. I know you did them recently, but that hole in the filter has me concerned. The inline filters don’t always grab everything, and there’s a good chance you’ve got gunk in there somewhere.

 

[mikidymac]

Two new genuine Mikuni carb kits with needle and seats.
I would also replace the fuel selector as the old ones are very well known to suck air as are the o-rings on the fuel strainer.

Aftermarket inline fuel filters have been known to starve for fuel at WOT and in my opinion are not necessary.

If pulling the choke at WOT solves the issue the you are starving for fuel and running lean.

 

[Sportster-2001-951C-Stock]

Yep, lean! Piston meltdown in 6,5,4,3..... The low hanging fruit would be a fuel line inward air leak, more than once replacing the small cup back on the fuel/water separator I failed to tighten enuff. It's plastic and feels brittle so was avoiding breaking it.

The only type of in-line fuel filter I trust to reasonably clean grungy fuel are the large spin-on type due to they have a large surface area (I use these for polishing fuel), all others clog easily or pass too much trash. Maybe in-line for fuel injection would be okay but I haven't tested one (FG986B test on my to-do list) for flow in a low pressure system.

It always surprises me how much trash is in a clean tank, too much trash (a big no-no anyway!) can clog the fuel selector anyway, so best to clean the tank if dirty and keep it that way.

 

[mikidymac]

Yep, lean! Piston meltdown in 6,5,4,3..... The low hanging fruit would be a fuel line inward air leak, more than once replacing the small cup back on the fuel/water separator I failed to tighten enuff. It's plastic and feels brittle so was avoiding breaking it.

I agree Sportster, about 20 years ago I gave up on servicing the strainers so for $22 every ski I work on gets a brand new OEM one.

 

[jimatsamrayburn]

Ok cool thanks for the quick responses. Actually misspoke a bit. It is not an aftermarket inline filter....just a new stock one (above the fuel tank). But was curious about aftermarket ones. Stock one is kind of small.

The hole in the filter is/was very suspicious. Maybe I'll pull it apart (again) and double check for blockage. I've got it down to about 15min to completely remove and disassemble them...well that's if I don't drop the Allen wrench and have to fish it out from under the motor.

 

[Sportster-2001-951C-Stock]

I agree Sportster, about 20 years ago I gave up on servicing the strainers so for $22 every ski I work on gets a brand new OEM one.

Yes, these OEM strainers rank highly on the cheesy scale, although they do catch trash. I can buy spin-on style with the housing for about the same price, have considered but haven't tried shoe-horning one into the Sportster.

 

[mikidymac]

I have a suspicion that the vacuum operated fuel pumps on these are really designed to push fuel and are very poor at pulling. The stock strainer is very low resistance and is all that is needed unless you have a bunch of trash in your tank or source really dirty fuel but that’s a different issue.

Pumps in general are really made to push liquids not pull them under suction.

With any aftermarket inline filter you would be restricting flow on the suction side and that could be the reason quite a few have had problems at WOT with them.

 

[Sportster-2001-951C-Stock]

As a solution to the nasty, corrosion-prone below deck aluminumtanks many outboard 2-stroke boats were fitted with, we began fitting them with the spin-on type housings and this quickly became standard practice. Every boat we sell now is still fitted with one and fuel quality has never been more important. They're quite effective, large and flow well.

But you're quite correct about flow restrictions becoming an issue, it doesn't take a lot of debris to jam the USCG required fuel tank anti-siphon valves.

 

[jimatsamrayburn]

Ok so still having the same issue after a full carb rebuild.

Brought it back in, put on another rectifier and bypassed the fuel selector...so RES hose it going straight into the filter which it going into the newly rebuilt carbs. Still same issue so it's not a fuel, rectifier or coil issue...all new.

I'm going to recheck the compression but doubt it just blows...usually slowly goes down unless its a blown head gasket.

Any other suggestions?

 

[Sportster-2001-951C-Stock]

Were you able to confirm the low speed circuit of the carbs was absolutely clean? It's not always an easy task, many others have failed at this particular issue only to finally replace the carbs and shizam, the problem is resolved. The aluminum Mikuni's are particularly stubborn is this respect b/c aluminum isn't the best choice of material for a carburetor body, it corrodes too early and often, you can practically hear them corroding on a quiet night.

It's important to diagnose correctly though, if lean hesitation caused by running out of fuel is resolved short term by pulling the choke (discussed many times in these hallowed halls) to get over the hump (aka flat spot), this is the classic symptom of fuel system issues causing lack of fuel.

FWIW, it's easy to pressure check the fuel line, I do it by disconnecting the fuel line from the carburetor and blowing backward toward the tank while listening for bubbles from the pickup tube inside the tank. If there are leaks in one of the various fittings, even the fuel tank nipple itself, most times you'll hear the air leaking out, and it shouldn't require 12 breaths to get three bubbles to rise.

 

[Sportster-2001-951C-Stock]

This is the 717/720, correct? Some had a rubber tube for the fuel pickup inside the tank so a couple other issues that can result in fuel starvation are the fuel tank vent being clogged and the pickup tube rotting off inside the tank.

Loosening the fuel cap to see if the problem disappears or filling the tank completely are ways of testing for these.

I don't think the '97 used a rubber tube, by then most were the plastic fuel bafle I believe my '95 had the plastic baffle. BTW, rare but I've heard once or twice the plastic baffle can crack internal of the tank.

That reminds me, I need to pull my baffle out b/c the fuel gauge stopped working.

 

[jimatsamrayburn]

Ok thanks guys will try that... Might just pull the whole tank. After the carb rebuild the choke trick wouldn't work...just tried to stall as it should do.

Just bought a '19 Spark this morning so this may end up being a winter project...appreciate all the comments. I'll post the fix when it happens.

 

[Sportster-2001-951C-Stock]

Personally, I think the 720 would be more fun to ride but do as you must!

 

[R-u-n2it]

Remember these fuel systems have an extra element in the fuel system equation, manifold vacuum. If you do anything like aftermarket flame arrestors, they will lower manifold vacuum, and this vacuum is the only thing that operates the fuel pumps which in turn delivers pressure to open pop-off, as well as pull fuel through the metering fuel jets. When applying choke you are richening the mixture AND creating more manifold vacuum. Full throttle bog/stall can be tested for too low vacuum from using less restrictive flame arrestors by blocking some of the arrestors surface area with duct tape, raises manifold vacuum like a choke for those that have eliminated the choke. My modded XP 785 only has 12-13 lb pop-off, any higher and I get hesitation when I nail the throttle to launch. That's with Buckshot 46s and case porting + modded rotary valve cutout. With the jetting "perfect" I would still get the hesitation exiting a corner from the high RPM low throttle setting when entering a corner due to the wetted surface of the bottom end drying on deceleration, then the very 1st application of throttle on exit there was momentary hesitation until the wetted surface had equalized back to the throttle on level. I'm talking fraction of a second but a big deal when you position yourself for the launch acceleration and it doesn't happen immediately. If your problem is due to this phenomenon it can be confusing till you correct it and experience the difference. Sorry for the long post but all points are needed to wrap your head around it if you haven't done so already. Lee

 

[Sportster-2001-951C-Stock]

Good point r-u-n2it, it should always be noted the factory airbox is an essential component of the factory tune, without it or in cases it's been modified, lean air/fuel mixture is likely and thus changes to the tune must be made accordingly, including pop, jet diameters and mixture adjuster settings.