96 gsx 800 4 stroke-ing up to 4000 rpm

[racerxxx]

If you look in the manual the "bump" is just at the hole on what I'll call the leading edge (looking at it as a clockwise rotation), but with the bolt circle you can probably get enough out of it to put it at the trailing edge. So, it's pull the carbs and re-verify them, or pull the mag cover and check that.

 

[racerxxx]

I might not start with it off like that, but you can't rule anything out unfortunately until you check everything--UGH

 

[Mekanix]

Yeah, wel there are a number of things all going on at once, like the boat getting up on plane and breaking free, the raves opening up too early could make the low end mushy, the amount of water in the tuned pipe down low on the throttle could be insufficient, that pipe fills up to keep the chamber size matched and then clears out as the power band comes in too. Oh, and the timing advances as all this is happening....

Maybe if you just try putting around at no-wake speed, say 2500-3000 RPM and hold the throttle in one position, does it load up on fuel and choke out? If too rich, it will, RPM's will just keep dropping till there's so much fuel the plugs can't fire. If you can set the idle mixture too lean by turning the screws in that it runs out of fuel and dies at least then you can balance the amount of fuel it needs to maintain a constant idle that doesn't drop off due to loading up. I mean if it can sit there and idle without loading up and finally quitting, then it's time to open the throttle and hold it at a higher RPM and see if it loads up enough to the point it loads up with fuel. If it keeps loading up with fuel that means you have to keep adjusting the throttle or restarting with WOT to clear it out. A little four stroking is just a sign there's almost too much fuel at a certain spot in the throttle and you might be able to clean it up with just a pilot size change but if it loads up and chokes on fuel then it's not close enough. Four stroking on the trailer doesn't count for much unless it's in the water, I'd expect it to four stroke on the trailer, like holding a dirt bike at part throttle in neutral, those will also four stroke no-load, Ding, ding, ding, ringing the expansion pipe. It's when it's pushing water the four stroking should clean up. Pull it out of the water and you can't really tell anything except it should sit there and gurgle like four stroking at idle. That should clean up in the water with the impeller loaded, then crack the throttle a little and preferably the response should be crisp, hold it part cracked in the same steady position and the R's shouldn't start dropping off like it's loading up.

Could be something besides just carburation, could be running like it's supposed to. Seriously, I think you have to trust your instincts some when it comes to knowing when there's too much fuel, do your eyes burn and do you smell raw gasoline?

Now to answer the rest of this:

If I put around below 4000 holding the throttle in one position it feels rough as if I gave it lots of fuel but no air. It will not load up though, It will continue to run like that and missing a beat every now and then.

Setting the screws all the way in makes it slightly smoother but still runs the same way. It idles with the screws all the way in and even if i let it idle really low and turn the screws out, I don't see a difference until 3 turns out. Then it starts to flood out and die.



From a stand still I can get on the throttle right away and it will take off without hesitation. I can back off and reapply with response. But when dropping below 4000 and slowly coming back onto it I feel the missing. But after 1/4 throttle its back up to 4000 rpm and working just fine.


On the trailer it seems fine. I can start it and it zings with just a little throttle but then again its already at near 3000 rpm so the transition from 3 to 4000 doesn't take long.

 

[Sportster-2001-951C-Stock]

"vent and return lines were backwards"

There are no differences in those on my boat, I don't think either one of them protrude into the fuel. Definitely not the vent. Mine has a pressure relief valve up higher in the vent hose to bleed off excessive vapor pressure, yours may be open to atmosphere like most boats.

"At back at the carbs when I pulled the return line off There was only a little bit of fuel in the T from the carbs."

You could try pulling fuel through the pump and carbs from there, that's how I prime mine just after installing the dry carbs. You should be able to pull fuel from there and I'm sure you can.

A miss on one cylinder? Could even be a coil but that should show up at WOT where it's harder to fire a plug.

You never did say you confirmed the rotary valve timing is dead-nuts.

 

[Mekanix]

I might not start with it off like that, but you can't rule anything out unfortunately until you check everything--UGH

Yeah I hear ya. But your picture did remind me of something.
The two larger holes in the rotor left an image on the flywheel. When I reassembled I lined it up so that the holes matched the image.

I'll hook up a timing light and make some TDC marks to see where it is just to be sure.

 

[Mekanix]

"vent and return lines were backwards"

There are no differences in those on my boat, I don't think either one of them protrude into the fuel. Definitely not the vent. Mine has a pressure relief valve up higher in the vent hose to bleed off excessive vapor pressure, yours may be open to atmosphere like most boats.

"At back at the carbs when I pulled the return line off There was only a little bit of fuel in the T from the carbs."

You could try pulling fuel through the pump and carbs from there, that's how I prime mine just after installing the dry carbs. You should be able to pull fuel from there and I'm sure you can.

A miss on one cylinder? Could even be a coil but that should show up at WOT where it's harder to fire a plug.

You never did say you confirmed the rotary valve timing is dead-nuts.

I wouldn't say a miss in one cylinder, Its a miss in one or the other or both but my plugs a probably going to foul soon I can feel that now more than before.


At WOT its nice and smooth, Everything above 4000 is trouble free and smooth.


I set the Mag cylinder at what I measured to be TDC and on my block there was a factory impression of where the rotary valve should be so I used that but besides that I haven't made sure with anything else.

The edge of the valve was just starting to close the mag side port I believe. But I can't remember. I knew that when I installed the plat I could be 1 tooth off and it would be a big difference in the location of the valve. I even had to flip it over once to get it to line up properly with the alignment mark on the face of the casing.

 

[Sportster-2001-951C-Stock]

"Setting the screws all the way in makes it slightly smoother but still runs the same way."

Well, I don't think it's pulling fuel through the idle port if the screw is closed, the fuel has to be coming from the transition ports which aren't in a vacuum region unless the vacuum is a brief spike that pulses the entire intake tract into a negative pressure instead of a longer duration low pressure event. I vote for a rotary valve timing issue.

 

[Mekanix]

"Setting the screws all the way in makes it slightly smoother but still runs the same way."

Well, I don't think it's pulling fuel through the idle port if the screw is closed, the fuel has to be coming from the transition ports which aren't in a vacuum region unless the vacuum is a brief spike that pulses the entire intake tract into a negative pressure instead of a longer duration low pressure event. I vote for a rotary valve timing issue.

Ok, would the top end still be as smooth as it is with the timing off ? Wouldn't that make one cylinder work better or worse than the other ot would it just not run at all?


And at low idle where the butterfly valves are almost closed I should be able to starve the engine if I turn the screws in right ?

 

[Sportster-2001-951C-Stock]

"Ok, would the top end still be as smooth as it is with the timing off ? Wouldn't that make one cylinder work better or worse than the other ot would it just not run at all?"

I've never set one of these up myself, just ridden them and they're smooth as glass. But, as engine speed increases I'd imagine the pressure in the intake tract stabilizes considerably. One clue might be the engine cannot reach maximum power due to a mistimed(late?) rotary valve.


"And at low idle where the butterfly valves are almost closed I should be able to starve the engine if I turn the screws in right ?"

That's what I would expect, kinda doubt it should be fulling fuel from anywhere else but through the idle port but a vacuum spike might be enough to generate a brief negative pressure great enough to pull fuel from the transition ports.

Surely someone must have some experience with the symptoms of mistiming the rotary valve and can speak with some authority, I'm only capable of expressing an opinion which isn't based on experience. That and $7 gets you a cup of coffee at Starbucks.

How about this, does it ever spit fuel out of the carbs? Reed valve motors often will at idle, especially if one of the reeds is lazy. Not that this is any way to compare but I wouldn't think spitting fuel at idle would be something to embrace.

 

[Mekanix]

How about this, does it ever spit fuel out of the carbs? Reed valve motors often will at idle, especially if one of the reeds is lazy. Not that this is any way to compare but I wouldn't think spitting fuel at idle would be something to embrace.

It doesn't spit fuel back out. I know what your talking about with reeds. I've had some on dirt bikes that did not seal and you could feel pressure coming back through the carb.

From 4000 to 6000 its completely smooth and the one time I pined it for a second it was completely smooth like I would expect it to be.

I appreciate your input. It always seems to be easier to come up with ideas when its someone else's problem but when its your own, Its hard to think of what you missed

But now I have things to check like:

restricted return line
timing
rotary valve
jet size...



I just started reading this : http://www.mikuni.com/pdf/sbn_manual.pdf
Its not the exact same carb but the illustration is close enough to get an idea.

I was thinking of trying it with the airbox an flame arestor off and looking down the carbs with a mirror to see if there is any visible leaking.

 

[Sportster-2001-951C-Stock]

Here's a document that might answer some questions, further down he talks about rotary valve timing in this passage:

"Curiously, the best port-closing timing for a very wide variety of disc-valve engines is about 65-degrees after top center, but before deciding to apply that timing you should consider that any appreciable delay in closing the port, after the piston has started down from top center, will cause some low - speed blowback. At higher engine speeds inertia effects in the intake tract will overcome the slight blowback caused by the delayed intake closing, but there will be a loss of low-speed power. "

http://www.2strokeengine.net/gordonjennings/twostroketunershandbook.php

 

[Mekanix]

Here's a document that might answer some questions, further down he talks about rotary valve timing in this passage:

"Curiously, the best port-closing timing for a very wide variety of disc-valve engines is about 65-degrees after top center, but before deciding to apply that timing you should consider that any appreciable delay in closing the port, after the piston has started down from top center, will cause some low - speed blowback. At higher engine speeds inertia effects in the intake tract will overcome the slight blowback caused by the delayed intake closing, but there will be a loss of low-speed power. "

http://www.2strokeengine.net/gordonjennings/twostroketunershandbook.php

I'm going to pull the cover off. I think I know what I did and it could be the rotary valve...

The shop manual says "NEVER USE THE RIDGE MOLDED IN CRANKCASE AS A TIMING MARK" That's what I did with the piston at TDC. I assumed that the mark would have been where it should be set at. I have to look at that again. I thought the mark was pretty much at the right spot but I guess not.

 

[racerxxx]

You have to use a degree wheel, somewhere you can download a paper one.

 

[Mekanix]

I have one. I remember using it and finding that the alignment on the case was pretty close to what it was supposed to be but I'm doubting myself now

 

[Sportster-2001-951C-Stock]

If you're thinking the RV timing might be off a bit then perhaps removing the air box and flame arrestor would increase the throttle bore pressure enough to enable the idle circuit to become more responsive(slightly reducing transition circuit vacuum signal), to the point where the idle mixture screw position becomes the more critical variable as anticipated? If so, then it might support the theory for late timing of the rotary plate?

Not sure if I laid that out correctly, venturing into a gray area here in terms of direct experience. Dr Honda might have some valuable insight at this point?

 

[Mekanix]

If you're thinking the RV timing might be off a bit then perhaps removing the air box and flame arrestor would increase the throttle bore pressure enough to enable the idle circuit to become more responsive(slightly reducing transition circuit vacuum signal), to the point where the idle mixture screw position becomes the more critical variable as anticipated? If so, then it might support the theory for late timing of the rotary plate?

Not sure if I laid that out correctly, venturing into a gray area here in terms of direct experience. Dr Honda might have some valuable insight at this point?

I pulled the RV out and set it exactly to the spec. Turns out thats where the line on the casing actually is. This wouldn't work for the 717 though so thats probably why the warning was there. SO I'm thinking my problem isn't that.


I pulled the carbs off and partially apart. No evidence of a problem on the Needle side of each carb. When I look at the throttle valve its set so that at idle the edge of the disk is between the two pilot holes.

 

[racerxxx]

I pulled the RV out and set it exactly to the spec. Turns out thats where the line on the casing actually is. This wouldn't work for the 717 though so thats probably why the warning was there. SO I'm thinking my problem isn't that.


I pulled the carbs off and partially apart. No evidence of a problem on the Needle side of each carb. When I look at the throttle valve its set so that at idle the edge of the disk is between the two pilot holes.

Do me a favor--fill in the XXX and XX

Procedure:
1.Found TDC on the mag piston
2.Then align 360 on bottom side of mag inlet port
3.Then find XXX degree and mark the case
4. For closing align 360 with TOP of mag inlet port
5.The find XX degree and mark case


What is the XXX and XX you used?

 

[Mekanix]

Xxx is 146.5+-5
XX is 64.
Valve has 159 degrees of open gap.

 

[racerxxx]

OK, that is good XX should actually be 65, but who's spliting hairs right now. In the pic where is the line on the casing your talking about? Is it a line that someone made or a parting line?? If you look on my RV surface you can see a very faint scribe line at about the 11 o'clock position. That is the 147 degree mark and you can barely make it out but there is a small tick mark for the 65 degree line.

 

[Sportster-2001-951C-Stock]

"SO I'm thinking my problem isn't that."

Yeah, somehow I knew it wouldn't be after tearing everything apart to find out.


"I pulled the carbs off and partially apart. No evidence of a problem on the Needle side of each carb. When I look at the throttle valve its set so that at idle the edge of the disk is between the two pilot holes."

Uncovering the transition ports (bypass holes = Mikuni tongue), eh? Well, it's not the same carb I'm used to looking at, obviously.

Well, I guess at least you can inspect the pilot jets closer for signs of someone drilling them.

 

[Mekanix]

The line is in the exact same place as your but it looks like a factory made mark. Way too clean looking.

 

[Mekanix]

Put it all back together and took it to the lake.

Reset the synchronization between the two carbs and started it up. Set the oil pump with it running at 1500.

Still rich on low end.

Its like the pilot circuit is just wayyy to rich.

The low speed screw had an effect this time. Anything more than 1/4 turn out would choke it out.


I fouled some br9 spark plugs so I put the br8's back in and fouled those.
So I went with 7's and it allowed me to run it longer to mess around. Those haven't fouled at all.

But with new plugs Black fouling that quick it also supports the super rich condition.


Also, When it first starts up, the rpm's climb high and drop back down to idle. It only lasts a second. But its another indication of it running lean for 1 second and then back to super rich.



I looked down each carb and reved it up to 4500. The transition to main jet happens at 4000, and then it really does smooths out.

They ARE NOT leaking or bypassing at idle. In fact I could reach in and plug the port with no change in the way it ran.


I also tried turning the gas off and running it until it starved. I noticed the engine getting smoother as the fuel supply dropped until it was just about to bog out. Then I turned the gas back on and within a second it was back to the way it was.







Now the theory that I understand is that the pilot jet allows fuel thats in the chamber to pass through the two small ports just under the butterfly valve.

And the low speed screw bypasses that circuit and allows raw fuel to enter and at the same time is allowing negative pressure from the manifold to pull on the diaphram and allow more fuel to enter the compartment.



1 what exactly does popoff do in this case? Will a higher pop off make it leaner or richer ?

2 with the screw closed I should only be getting fuel from the pilot right ? Forget about the main circiut. Just the pilot jet itself right ?

So if its too rich to start off with. I will have no turning ability at all with the screw except to make it worse I would think.

 

[Mekanix]

"SO I'm thinking my problem isn't that."

Yeah, somehow I knew it wouldn't be after tearing everything apart to find out.


"I pulled the carbs off and partially apart. No evidence of a problem on the Needle side of each carb. When I look at the throttle valve its set so that at idle the edge of the disk is between the two pilot holes."

Uncovering the transition ports (bypass holes = Mikuni tongue), eh? Well, it's not the same carb I'm used to looking at, obviously.

Well, I guess at least you can inspect the pilot jets closer for signs of someone drilling them.

Both jets had no marks on them. Numbers match what the book says.

I also took a look at the fuel pump diafram and it holds pressure. So its not sucking in any extra fuel from there far as I can see.

 

[Mekanix]

Oh and here is a VIDEO [video=youtube;yDcXswHTET4]http://www.youtube.com/watch?v=yDcXswHTET4[/video]

0-1/8th you can see it hesitating and shaking. Then smoothly applying throttle up to just when the main jet kicks in around 4000.

from 0 to Wot responds without hesitation. it wants to go when your using the main jet.
It does not want to go when that jet isn't active...

 

[Sportster-2001-951C-Stock]

"Now the theory that I understand is that the pilot jet allows fuel thats in the chamber to pass through the two small ports just under the butterfly valve."

If your carb is similar to mine like I think it is, the fuel pulled through the pilot also feeds the idle port, after passing though the idle mixture needle. A shot of carb cleaner through the pilot should prove this.

"And the low speed screw bypasses that circuit and allows raw fuel to enter and at the same time is allowing negative pressure from the manifold to pull on the diaphram and allow more fuel to enter the compartment."

Yes, the vacuum pulls the diaphragm into the fuel chamber to open the fuel inlet metering valve. At idle, this vacuum pressure is highest down stream of the throttle plate so it's pulling fuel through any port that's exposed to the negative pressure region. This is why a restrictive airbox requires a higher pop-off setting, to limit fuel flow.

"1 what exactly does popoff do in this case? Will a higher pop off make it leaner or richer ?"

A higher pop-off will require more negative pressure (vacuum) to pull the metering needle off it's seat, a restrictive air box requires a higher pop-off due to the lower pressure in the intake tract, especially in the section upstream of the throttle plate. The idle/low speed circuit will deliver more fuel if the pop-off is set lower. Adjusting Pop-off to a lower value increases the signal response of the low end fuel delivery.

"2 with the screw closed I should only be getting fuel from the pilot right ? Forget about the main circiut. Just the pilot jet itself right ?"

Correct, my carb has three holes there in the bore which I call the transition ports and Mikuni calls bypass holes.

"So if its too rich to start off with. I will have no turning ability at all with the screw except to make it worse I would think."

Yes, at some point in the throttle plate rotation the low mix screw loses it's effectiveness to control mixture. Depending on the transition port location in relationship to the throttle plate, if the throttle plate is uncovering one or more of the transition ports, then you may only gain control of the fuel delivered through those holes by adjusting pop-off, fuel pressure, or the airbox configuration.