Blowback through both carbs

[BoJaXz]

Hi, I have a 94 seadoo xp with the 657x engine.
I had a couple cylinder bolt heads break in the lake took in some water while limping to the dock. Pulled the engine, the crank had bad bearings, got a reman crank.
Put it all together, lined up the timing mark on the stator, timed the rotary valve. I have 150psi in both cylinders.

I haven't taken it to the lake yet, but on the trailer, If I give it throttle, it hesitates, blows back through the carbs, then revs up. If I go wot, it will bog and die.
It was doing the same thing with the broken head bolts while testing before I pulled the engine. Pistons are still in good shape, as they only have maybe 10hrs on a top end rebuild. Cylinders are perfect, one spot on a piston has water marks, but the Rings look perfect and, as I said, 150psi. Changing the valve timing by one spot either way just makes it run worse or not at all.

Seems like the valve isn't closing soon enough when the piston is coming down, and some of the positive crankcase pressure gets pushed through the carbs causing a lean hesitation for a second.
Any help would be GREATLY appreciated! Valve clearance is also within spec and in decent shape.

 

[GGuillot]

I'd go through the carbs and sort of reset everything in that area. You could do a rebuild and put everything back to factory settings and see what happens.

 

[BoJaXz]

I've done that as well, twice, that was my first assumption when it was hesitating. Is it supposed to blowback through the carbs like that? It just seems that the blowback blows the fuel out instead of sucking it into the crankcase.

 

[Jeffspx]

Did you replace the rotary valve? Or is it the original? Are you certain of timing? Did you use the case marks for timing or a degree wheel?

 

[mikidymac]

Sounds like your rotary timing is off and you did not verify it correctly if you are randomly moving it a tooth forward and back.

 

[BoJaXz]

I used a degree wheel, it hits dead on 65 degrees and about 147 degrees. Manual states 65 and 149 +-5. At TDC on the mag side. Rotary valve has no marks on it, it's smooth, cover has light scratches, no deep grooves. Rotary gear is fine, checked it when I replaced the crank.

 

[mwat65]

Probably a stupid question but did you verify the woodruff key was on the crank before you installed the flywheel ?

 

[BoJaXz]

Yes, I actually had to replace the woodruff key because I forgot to take it off of my bad crank before sending it in as a core. The ski starts and idles fine. But as soon as I give it throttle, I get blowback. Spits air and gas out. If I shut off the gas and crank the engine, it sucks and blows out of each carb, just like a pulse line would do.

 

[Sportster-2001-951C-Stock]

Yep, sure sounds like the RV isn't working correctly so I agree with those above that should be confirmed. I'm curious at idle, is it blowing fuel mist out of the carbs? I'm not sure if that's normal for an RV motor but many reed motors do so this is something I always pay attention to for some reason.

 

[mwat65]

As Miki said it sounds like timing. Did you check your rotary gear for wear when you replaced the crank? I just replaced one with one tooth partly sheared that I didn’t even notice at first.

 

[BoJaXz]

I checked the rotary gear, it was in great shape, I made sure of it before I put the case together.
At an idle, it doesn't spray out that I can see or feel, only when I hit the throttle.

No shavings in the case either, so if there was any wear on the rotary gear, there would have been something, but it was really clean.

Consensus says rotary timing off, so I guess I'll keep playing with the timing. Using a degree wheel doesn't seem to be working, is it more important for btdc or atdc to be dead on? Rotating the crank by hand, I can also feel air come out of the hole right before it closes shouldn't that be closed by the time the piston starts to come down?

 

[Sportster-2001-951C-Stock]

Let me just throw out some thoughts...

Long shot but do you think it's the original RV and/or it was working correctly before? The snowmobile guys have been using different shape valves to tune power band so I'm wondering if maybe this valve has come from somewhere else like a snowmobile or airplane?

How about the exhaust, you didn't leave a rag in there and forget to remove it?

 

[Sportster-2001-951C-Stock]

I've done that as well, twice, that was my first assumption when it was hesitating. Is it supposed to blowback through the carbs like that? It just seems that the blowback blows the fuel out instead of sucking it into the crankcase.

Carbs do have some really tiny passage holes for part throttle and over time these close up ever so slightly due to native corrosion of the aluminum. Lots of guys have gone round and round only to have finally solved the problem by installing shiny new carbs. The message is, the entire low speed circuit (inclusive of those tiny holes and the passage feeding them has to be absolutely clean or else a piston eating detonation and lean bog/hesitation occurs.

 

[Sportster-2001-951C-Stock]

I checked the rotary gear, it was in great shape, I made sure of it before I put the case together.
At an idle, it doesn't spray out that I can see or feel, only when I hit the throttle.

I think it's normal to have some fuel mist at higher speed (high throttle angle) under no load but I don't have a rotary valve motor to verify my suspicion.

I dunno, based on where I think you are and where you came from (roasted pistons?) I'm leaning toward carb issues.

 

[BoJaXz]

The original rv had some small grooves in it, got a used one, exactly the same size. Cover has tiny grooves, nothing you can catch a fingernail on, but clearance is within spec. None of this happened until the cylinder bolts broke and took in some water. It was running great, then the rpms started dropping, then I couldn't get above an idle. That's when I limped to the dock and pulled the motor, replaced the crank... Etc.

The 3 pilot holes are clear, I've blown air through them and used my carb cleaning wires, they go right through the holes, insides of the carbs are clean, including the filters, brand new gaskets, pop-off is about 19-20psi on both. Drops 3-4 psi and holds steady around 16psi. My other ski, same exact ski, I rebuilt the entire engine and carbs, runs perfect. I did nothing different between the two.

 

[BoJaXz]

I think it's normal to have some fuel mist at higher speed (high throttle angle) under no load but I don't have a rotary valve motor to verify my suspicion.

I dunno, based on where I think you are and where you came from (roasted pistons?) I'm leaning toward carb issues.

I bought the ski 3 years ago with a blown top end. The carb was shot, rusted, bent throttle and choke plates.. Etc. I got a replacement carb set from a different xp and rebuilt it along with top end. Ran great until this happened.

 

[Jeffspx]

Let me just throw out some thoughts...

Long shot but do you think it's the original RV and/or it was working correctly before?

That was gonna be my next question as well.
Also, did you try flipping the valve around? In my experience, the valve is not perfectly symmetrical and will change angle if flipped over.
Also a dumb question, you mention that you see pulsing, the pulse line is in the right place correct?
You said you have the same ski, not exactly easy, but have you thought about swapping carbs to see if it changes?

 

[Sportster-2001-951C-Stock]

AFAIK, When an RV is worn, the RV motors become hard to start. Lots of no-start reports b/c of this, starts on trailer but not in water.

I'm not sure how you got to where you are now but of course backtrack and put the pieces together. If the carbs were fine before then they're probably still okay.

Other gotchas are fuel line air leaks like at the water separator cup or shutoff valve, empty fuel tank (this has happened to me, where I coulda sworn there was plenty of fuel but low and behold it was sucking air, lol! Pressurize your fuel system to 5psi and make sure there are no air leaks.

 

[BoJaXz]

That was gonna be my next question as well.
Also, did you try flipping the valve around? In my experience, the valve is not perfectly symmetrical and will change angle if flipped over.
Also a dumb question, you mention that you see pulsing, the pulse line is in the right place correct?
You said you have the same ski, not exactly easy, but have you thought about swapping carbs to see if it changes?

I have thought about swapping carbs, but the "pulse" I'm talking about is just the pressure and vacuum I feel when I put my hand over the intake ports without the carb installed while cranking the engine. The pulse line from the mag side is fine.

I haven't flipped the valve, but I put the valve back on the way it came off, there are stains on the rv from fumes or whatever caused the stains from the crankcase side before the engine was rebuilt. The used rv that is in better shape has the same "stains". Would replacing the crank cause the need to flip the valve backwards? Shouldn't the valve line up with 65 degrees and 149 degrees - ish?

 

[Sportster-2001-951C-Stock]

Water in the crankcase can damage bearings I guess but a real biggie is aluminum dust and grit from exploded pistons gets in them they don't last long (FWIW)

Take a hard look at that RV though, esp see if the shape seems different (might be the wrong one?).

I really can't think of much else unless maybe the ignition pickup or flywheel were changed, those are matched to the MPEM program for CDI ignition timing control and there were changes over the years where magneto position vs TDC isn't the same. Could locate TDC and throw a timing light on it...

 

[BoJaXz]

AFAIK, When an RV is worn, the RV motors become hard to start. Lots of no-start reports b/c of this, starts on trailer but not in water.

I'm not sure how you got to where you are now but of course backtrack and put the pieces together. If the carbs were fine before then they're probably still okay.

Other gotchas are fuel line air leaks like at the water separator cup or shutoff valve, empty fuel tank (this has happened to me, where I coulda sworn there was plenty of fuel but low and behold it was sucking air, lol! Pressurize your fuel system to 5psi and make sure there are no air leaks.

Even after the cylinder bolts broke, the ski would start right up and idle, but wouldn't go above an idle or it would die. Before that, it would start fine in and out of the water.
I haven't tested this on the water after the crank replacement, starting isn't the issue, getting it to Rev up isn't the issue, the hesitation, blowback, and death from an idle to wot is.

It very well could have blowback at an idle as well, but at an idle, the throttle plates are mostly closed, so I wouldn't be able to tell until I opened the throttle, which is when I notice problems.

 

[Sportster-2001-951C-Stock]

I have thought about swapping carbs, but the "pulse" I'm talking about is just the pressure and vacuum I feel when I put my hand over the intake ports without the carb installed while cranking the engine. The pulse line from the mag side is fine.

I haven't flipped the valve, but I put the valve back on the way it came off, there are stains on the rv from fumes or whatever caused the stains from the crankcase side before the engine was rebuilt. The used rv that is in better shape has the same "stains". Would replacing the crank cause the need to flip the valve backwards? Shouldn't the valve line up with 65 degrees and 149 degrees - ish?

The reason for flipping the valve over (going by my memory) is to get the timing dead nuts. The valve is symmetrical but flipping it over changes the position by 1/2 spline?

 

[BoJaXz]

Water in the crankcase can damage bearings I guess but a real biggie is aluminum dust and grit from exploded pistons gets in them they don't last long (FWIW)

Take a hard look at that RV though, esp see if the shape seems different (might be the wrong one?).

I really can't think of much else unless maybe the ignition pickup or flywheel were changed, those are matched to the MPEM program for CDI ignition timing control and there were changes over the years where magneto position vs TDC isn't the same. Could locate TDC and throw a timing light on it...

It's the same shape rv as the original that had small grooves in it. I had the same issues with the original as I am having with the replacement. The flywheel and stator are the originals as well. 5 wire. The stator has a mark to line it up with, I checked that it was lined up before disassembly, and double checked that I had it lined up on installation.

I went through the fuel system last year, had a leak in the fuel cup, bad seal, fixed it, ran great afterwards. No air in the fuel filter, checked the fuel return while cranking, plenty of fuel flow.

 

[BoJaXz]

The reason for flipping the valve over (going by my memory) is to get the timing dead nuts. The valve is symmetrical but flipping it over changes the position by 1/2 spline?

Somewhere around there. I will have to mess with the timing some more. It's very tedious to remove the carbs, intake, and cover over and over again.

 

[Sportster-2001-951C-Stock]

And I'd have to agree when you "give it the throttle" if a plume of fuel spits out of the carbs this makes me wonder if the RV timing isn't right. If fire spits out, this makes me wonder about ignition timing, can't recall ever seeing backfire coming out of the carbs on a 2-stroke but then again if the ignition occurred really late then maybe but then it probably wouldn't idle well or at least the idle screws would need to be turned in way far.

How did you locate TDC for setting the RV, did you use a piston stop or a dial gauge?