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Blowback through both carbs

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etemplet said:
My thoughts as well. My spidey senses are tingling. LOL
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Running on a hose, water comes right out of the back where it should, runs through the head, through the cylinder, through the exhaust, and out. Runs no differently with or without water. Had the cylinders off, and there was no water in the case. It creates positive pressure on the downstroke, negative on the compression stroke.
 
Testing likely would not show anything as it is an open system. You're the pilot. :) Git R Done. :)
 
As far as fuel pump is concerned, about the only thing there is the valve disks can become warped or cracked, and the round disk seat can become corroded. FWIW, I noticed a few drops of water in mine behind the diaphragm on the crankcase pulse side, both times I had it apart several years back.

I like to visually inspect the valve disks but they should seal fairly well against the round seat so as to not leak backwards much. If the seat is corroded (pitted) enough, the volume will drop off.

Otherwise, they work really well.
 
BoJaXz said:
Running on a hose, water comes right out of the back where it should, runs through the head, through the cylinder, through the exhaust, and out. Runs no differently with or without water. Had the cylinders off, and there was no water in the case. It creates positive pressure on the downstroke, negative on the compression stroke.
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Sounds good. Just mentioned that b/c Jet pumps really can create big water pressure especially at WOT, so not to be overlooked when gaskets leak/pop for unexplained reasons.
 
Sportster-2001-951C-Stock said:
As far as fuel pump is concerned, about the only thing there is the valve disks can become warped or cracked, and the round disk seat can become corroded. FWIW, I noticed a few drops of water in mine behind the diaphragm on the crankcase pulse side, both times I had it apart several years back.

I like to visually inspect the valve disks but they should seal fairly well against the round seat so as to not leak backwards much. If the seat is corroded (pitted) enough, the volume will drop off.

Otherwise, they work really well.
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How do you check those disks? I can hardly tell which side goes down on the new ones. :) I did find some white stuff on my SPI's single carb. The pump wasn't doing much pumping. I cleaned them up and that solved the problem. I don't suppose there is a lapping plate to get those seats back smooth. I may need to make one. If the seats are bad... it is new carb time correct ??

Any idea of the fuel pressure the pumps put out??
 
Oh, and as far as testing the F pump goes, you can attach some tubing to the pulse line of the pump and "resperate" the diaphragm through the tube with your mouth pressure. It should pump fuel up from the tank and out the return line.
 
The disks have some curl in them b/c they're cut from a rolled-up sheet of mylar. The disk goes on with the curled side against the circular seat. The important thing is, the disk should be pressed against the seat the full 360 degrees so it works as a check valve.

And of course, make sure when inserting the rubber grommets not to push your tool through the grommet and puncture the grommet. Some people wrap waxed dental floss around the grommet to pull it into/through the hole, I just use an ball allen wrench + some 2-stroke oil to shove it through.
 
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You might be able to lap the seats depending on how bad they are but if they're that bad then I wonder baout the small passages in the low speed circuit of the carb, surely those would be corroded.

I'll just say, a lot of guys go round and round only to wind up fixing their problems by tossing a shiny new set of carbs on. Make sure to get the right ones and personally I'd check the jet #'s before tossing anything on new or used.

There's no published spec for fuel pressure, I'd imagine a few pounds.
 
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etemplet said:
How do you check those disks? I can hardly tell which side goes down on the new ones. :) I did find some white stuff on my SPI's single carb. The pump wasn't doing much pumping. I cleaned them up and that solved the problem. I don't suppose there is a lapping plate to get those seats back smooth. I may need to make one. If the seats are bad... it is new carb time correct ??

Any idea of the fuel pressure the pumps put out??
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Genuine mikuni disc's have a blue line on the side that goes up when installing.

I believe pulse pumps go anywhere from 3 psi at idle, up to 7 psi wot.
 
BoJaXz said:
Genuine mikuni disc's have a blue line on the side that goes up when installing.

I believe pulse pumps go anywhere from 3 psi at idle, up to 7 psi wot.
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Thanks and yes I can see the blue line but what side is it on? LOL
 
etemplet said:
Thanks and yes I can see the blue line but what side is it on? LOL
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It faces you as you install it. The side without the line is the side that sits against the seat. So the blue line faces out on both discs with the other side facing the valve seat.
 
etemplet said:
Hard for me to tell which side the blue line is ON. :)
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If it has a mean potato-chip curl to it, as if it was cut from inside the roll of mylar sheet, it's best to place the inside radius against the seat and then install the grommet. Lay the disk on a flat surface to determine which side is curled if at all. Some are barely curled or not at all in which case it doesn't matter which side is placed against the seat. The important thing is an 360 degree seal that doesn't leak appreciably, so if you obtain that you're there.

The biggest thing in most cases is the small passages (pilot holes Mikuni calls them in their manual) are imperceptibly corroded, causing a part throttle lean hesitation (and probably piston-eating detonation that you can't hear).

As far as manipulating the metering arm lever goes, make sure you're using the correct spring/metering needle seat then your pop-off measurement is obtained by bending the metering arm slightly. Get the two carbs pop as close as possible matched within 1 psi if you can. The flush with metering plate/chamber thing gets you in the ballpark but confirming pop is the best practice.

Also, the flame arrestor aitbox design determines where your pop needs to be, no airbox or high flow then pop needs to be lower. Moded or missing airbox will cause lean hesitation thus has to be corrected for and IMO isn't worth the trouble unless maybe you're a competitive racer.

In that case, tweeking carbs based on atmospheric conditions, tearing into them daily, polishing the needle top, metering seat and arm mirror smooth to obtain the perfect repeatable fuel delivery. This is all a bit much IMO, like swapping impellers on a quest to find the perfect one for a particular race. Custom anodized billet carbs with minimal low speed circuit, chrome-plated twice pipes, custom shaved heads, developing a throttle technique, programmable advance curve, and on and on....

My 2-stroke philosophy:

I'm a big believer in being on the rich side of the mixture kneepoint mostly b/c I don't like rebuilding engines. I'd rather ride knowing it's gonna restart with barely more than a bump of the starter. I don't shoot for that extra 25 RPM WOT for my purposes it makes no difference so I listen for brief hints of 4-stroking in my carb tune and that's my happy spot, some indication of being adequately rich. If earlier fouling is the price yet performance remains snappy I'm willing to go one range hotter.
 
That's a great attitude. I'm struggling getting these skis where I want them. Not fast, just close to spec as I can. Trying to find what is limiting performance. Of course, I have nothing to compare them to. I believe they should pull fairly strong and consistent from the first application of throttle to top end. Both are fun to ride as they are but being a perfectionist.... I have to understand why it's not doing what it it should be doing. I am off to work on these babies again. :) I too prefer mine a little rich without fouling the plugs.
 
Good Call !! Funny thing in all this and it makes me want to rejet. In May of this year, this ski was running at 6800 RPM indicated. Now it is at 6500 indicated. The only thing that happened was the weather got HOT.
 
Other things can happen too of course, like a small exhaust leak mat be filling your bilge with carbon monoxide you will never see or smell, ride with seat off as a test.
 
As far as rejetting goes, the RV motors don't seem to respond when low speed jet is increased. The 951 carbs do respond, that's how I got rid of my part-throttle lean hesitation although I probably could've just been more aggressive while cleaning the pilot ports and accomplished the same thing. All aluminum mikunis eventually have issues with pilot port corrosion.
 
Theses carbs are not lean, they are rich. Getting the Pop Off away from the bottom number helped. I think part of the problem or maybe the whole problem with one ski was it had aftermarket needle and seats. When I put the Mikuni parts in the carbs.... with the same spring the pop off jumped by 8psi.
 
etemplet said:
Theses carbs are not lean, they are rich. Getting the Pop Off away from the bottom number helped. I think part of the problem or maybe the whole problem with one ski was it had aftermarket needle and seats. When I put the Mikuni parts in the carbs.... with the same spring the pop off jumped by 8psi.
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Where is it rich... idle, part throttle or top end WOT?
 
Update, took it to the lake today, has a mid range hesitation. From idle to 1/4 was good, then from 1/4 and up, it hesitates. If I feather it or slowly increase throttle, its fine. From 1/2 to full it's fine. But where the pilot and main jets overlap, it hesitates. Adjusting low speed screws did nothing. Hitting 6800rpm wot. Pop off was around 19 or 20psi, I wonder if I need to lower the pop off. Thoughts? Sinking it in the lake is still a viable option...

Another weird thing. The pto side oil injection line, it's pulsing, like with each stroke of the piston, the oil moves around like crazy inside the line. I've tried 3 banjo fittings, bled the lines, same thing. If I crank and hold the pump wide open, it flows as it should, but while running, it just moves around.
 
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BoJaXz said:
Update, took it to the lake today, has a mid range hesitation. From idle to 1/4 was good, then from 1/4 and up, it hesitates. If I feather it or slowly increase throttle, its fine. From 1/2 to full it's fine. But where the pilot and main jets overlap, it hesitates. Adjusting low speed screws did nothing. Hitting 6800rpm wot. Pop off was around 19 or 20psi, I wonder if I need to lower the pop off. Thoughts? Sinking it in the lake is still a viable option...

Another weird thing. The pto side oil injection line, it's pulsing, like with each stroke of the piston, the oil moves around like crazy inside the line. I've tried 3 banjo fittings, bled the lines, same thing. If I crank and hold the pump wide open, it flows as it should, but while running, it just moves around.
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In the case of the oil injection, make sure the spring-loaded ball check valve in both oil injectors isn't stuck open or gummed up. I don't recall how much pressure is necessary to dislodge the check ball off it's seat (mouth pressure?) but if you're down there messing around with that feel free to refresh my memory. A stuck open injector check valve will allow oil to drip into the crankcase and flood the crankcase with oil while sitting over a period of time. Be careful if you try to remove the injector it will probably snap/break off so it's better to blow them out with solvent (carb cleaner) and compressed air to get them cleaned out.

Part throttle hesitation makes me think the pilot holes (mikuni term) aren't fully clear but also the mylar reed type check valve covering the hole on the metering plate may be responsible if it's an aftermarket mylar reed may be too stiff (too thick) and doesn't allow the high speed circuit to flow early enough.

Hopefully that makes sense.
 
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