96 gsx 800 4 stroke-ing up to 4000 rpm

[Mekanix]

Hi,

First time having the ski out on the lake today after fresh rebuild.

It starts up and run's rough as if it was loading up. almost 4 stroking and the engine shakes.
I thought it would be synchronization and tried to set it again but even with it perfect there was no change in the way it ran.

I tried tuning the low speed and it had no effect.
Its as if the low speed makes no difference until its 3 turns out, at that point I can notice the difference.
But there is literally no tuning ability with the screw at all which leads me to believe there is something else going on.

When applying throttle its rough and the engine shakes up until it hits 3500-4000, (1/4 throttle) then it smooths out but still misses a beat every now and then.

Below that it seems like its missing on one or both cylinders evenly.
Same thing if its on the trailer or if its running on plane.

If I let the throttle fall and pick it back up it feels like 4 stroking until it clears out at 4000. I'm only applying enough throttle to get it to rev up to 4000. I'm not mashing it and backing off. Just applying it smoothly until its around 4000 and then backing off a bit to keep it at that RPM.

I haven't taken it past 4500, It seems to want to "take off" after that point and the engine isn't broken in yet so I'd rather not. But it has surprised me a few times so I had to back off.

The following are set according to the shop manual:

The carbs are stock except choke plate removal and primer install.
Carb's dipped and cleaned thoroughly.
pop off was 32psi.
stock springs.
1.5 needle and seat which are brand new.
Jets are stock.
high speed at 0
low speed at "it doesn't mater " but set it at 1.0

Synchronized the carbs by eye and then fine tuned while running.
Oil pump synchronized with the engine running at 1500 then set to normal idle.



I'm also mixing 50-1 at the moment for break-in. Could it just be the extra oil ?
How sensitive is this thing to oil?

I'm not sure where to start.

Thanks for looking.



PS. One thing I did notice the very first time it fired was that the response was quick and smooth but that was with clean gas just from me priming the lines.

After the mixed gas went through and the oil injection system started to kick in, it didn't have that Zip.

 

[Mekanix]

I tried draining the tank and switching to regular gas with 100:1 mix.

I have two quarts of regular XPS oil in the oil tank for break in.

Also installed the whole airbox. From 4000 on its fine and rev's smooth.

Dropping below that is horrible. As if its really rich or smothered with oil.


How can I troubleshoot this ?



The only thing I changed from what the po had it set at was the needle and seat. Pop-of went from 16 to 32 psi and added the air filter.

 

[Sportster-2001-951C-Stock]

One way to eliminate excess oil would be to disable the oil pump and just use premix. I have doubts it's oil related, sounds like too much fuel if turning the low mix screws completely closed then you could still be getting fuel from the transition ports but I'd have to think pop-off would be the cause for that b/c transition ports don't see vacuum until the throttle plate opens and in fact the diaphragm shouldn't see vacuum if the idle mix screws are closed.

So I have to vote for a pop-off problem or maybe even a missing welch plug but these carbs don't have welch plugs, do they?

Okay, maybe I misunderstood and idle can be dialed in and the problem you're describing is off idle too rich... In that case I'm wondering if someone drilled your pilot jet.

 

[racerxxx]

Did you have the bottom end apart?

Sent from my SGH-T989 using Tapatalk 2

 

[Mekanix]

You might be on to something there!

I checked the numbers on the jets but not the actual size.


It idles but its not the regular smooth tingy two stroke sound. Its like everything second or third hit is a misfire and the engine shakes.

I do have it as close to smooth as I can get it by synchronising the carbs by the way its running.

Off Idle the engine shakes side to side and the only time it smooths out is after 4000. Even backing off very slightly will result in a miss every second or so.

Its like 4 stroking below 1/4 or full on main jet.

Plugs are always wet when I pull them out.

Giving it throttle up to 1/4 throttle feels like its missing on one or the other cylinder equally.

Makes sense that the pilots would be drilled but when I looked at them the ID looked quite small.

What exactly does pop-off do with higher and lower pressure's ?



One more thing. I set the plug gap to spec in the manual. That seems like really small of a gap.

 

[Mekanix]

Did you have the bottom end apart?

Sent from my SGH-T989 using Tapatalk 2

Yes. complete tear down, New crank, new balancer bearings, new top end bored to 82.5mm. New head in perfect shape.
New gaskets and seals.

New 1.5 needle and seat.

 

[racerxxx]

Did you align the counter balance?

Sent from my SGH-T989 using Tapatalk 2

 

[Sportster-2001-951C-Stock]

The other possibility I can imagine is the fuel return may be obstructed causing a fuel pressure increase, assuming your idle mixture is pig rich even with the mixture screw fully closed. The metering needle shouldn't open until the metering diaphragm sees vacuum from through the idle circuit, throttle plates closed, the vacuum normally pulls fuel through the idle mixture screw but if your metering needle could be forced open by too much fuel pressure in that case you might have fuel coming from the transition ports and none through the idle port even with the idle mixture screw closed.

 

[Sportster-2001-951C-Stock]

"That seems like really small of a gap. "

Yeah I know it does seem small, 0.020 or something like that, about the thickness of a credit card, LOL.

 

[Mekanix]

Did you align the counter balance?

Sent from my SGH-T989 using Tapatalk 2

Yes. Checked that 10 times My marks didn't line up properly as you would expect with new cranks and old balancers. So I know for 100% certainty its aligned. Otherwise I wouldn't be able to handle the shaking up at 4000.

 

[Sportster-2001-951C-Stock]

Right, and in addition to the counterbalance is the rotary valve timing. Still, I wouldn't expect the carb to fuel at idle if the idle mix screw was completely closed the diaphragm should see no vacuum and thus there should not be any fuel entering the fuel chamber. If fuel were entering the fuel chamber then it has to be going past the metering needle to get there and from there it cannot go through an idle mixture screw that's fully closed so it has to be fueling through the circiut that has no vacuum, the transition ports or even the main venturi are the only other paths fuel can take to get into the bore and neither of these should have vacuum on them which is necessary to pull the metering needle open.

So, too much fuel pressure is overcoming pop-off, or the seat is leaking. Too much fuel pressure could be the possibility if the fuel return line was obstructed, then fuel pressure would be abnormally high?

I think you should be able to completely remove the pilot jets and still adjust idle mixture enough to lean the idle out, the pilot jet IMO should have no effect on idle mixture, just off idle to about 65-70% throttle it seemed like when I was working my lean bog out, I went up two sizes on the pilot and the lean bog was completely gone. In my case I think the factory jetting was just too lean or maybe the transition ports have some minor oxide that's grown in them over 12 years, I may have just needed to go up one size but that's a different story, the point is those pilots had an effect all the way from off idle to 65%+ throttle. Pilots even contribute at 100% throttle.

 

[Mekanix]

The other possibility I can imagine is the fuel return may be obstructed causing a fuel pressure increase, assuming your idle mixture is pig rich even with the mixture screw fully closed. The metering needle shouldn't open until the metering diaphragm sees vacuum from through the idle circuit, throttle plates closed, the vacuum normally pulls fuel through the idle mixture screw but if your metering needle could be forced open by too much fuel pressure in that case you might have fuel coming from the transition ports and none through the idle port even with the idle mixture screw closed.

Ok now I'm learning.

I didn't check the return into the tank but I did replace all the lines. I'll check that as well.


What effect would the rotary valve being off have ?

 

[Mekanix]

Right, and in addition to the counterbalance is the rotary valve timing. Still, I wouldn't expect the carb to fuel at idle if the idle mix screw was completely closed the diaphragm should see no vacuum and thus there should not be any fuel entering the fuel chamber. If fuel were entering the fuel chamber then it has to be going past the metering needle to get there and from there it cannot go through an idle mixture screw that's fully closed so it has to be fueling through the circiut that has no vacuum, the transition ports or even the main venturi are the only other paths fuel can take to get into the bore and neither of these should have vacuum on them which is necessary to pull the metering needle open.

So, too much fuel pressure is overcoming pop-off, or the seat is leaking. Too much fuel pressure could be the possibility if the fuel return line was obstructed, then fuel pressure would be abnormally high?

That reminds me of one thing.

I left if off for a minute and started it up but this time got on the gas right away, It seemed better and smooth for a few seconds and then loaded up again and didn't rev smoothly again after that.

I Seriously hope i didn't connect one of the returns to the supply's. But I don't think so because when i was finished assembling and testing I connected the pop off tester to both of them together to see if one would pop off before that other.

Would that do it ?

It would be one way to bypass the any type of metering or tuning right?

I have to go look again.



* nope the input and output lines are going to the right places and the feeds between the two carbs are in the right place. *

so back to "whats wrong inside the carbs "

 

[Mekanix]

Also the idle screw was less than 1.5 turns in from light contact. I had it set at two for the first start and then reduced for a 1500 rpm to set the oil pump. After that I set it down slightly to reduce the shaking.
Just to clarify I'm not screwing the idle too far in to make up for some other issue and passing right through the idle circuit. I could see having a low speed circuit problem and fixing it with too much idle and having no tuning ability. ( this was my first theory)

What about having a busted pump diaphram? what effect would that have ?

 

[Sportster-2001-951C-Stock]

Are your diaphragm cover vents taped over? Someone had done that recently, he was painting his carbs and didn't want paint to get into the vent hole in the diaphragm cover so he put some tape over the hole. It took a lot of head scratching to understand why his carbs were constantly flooding because the dry side of his diaphragm wasn't open to atmosphere.

Well, in order to build pressure to test the pop-off you have to block the return line, and fuel is always returning to the tank through the return orifice unless the orifice or return line have an obstruction in them somewhere. If the return was obstructed, then your fuel pressure would be abnormally high and fuel could potentially be forced past the metering needles.

Otherwise, fuel cannot flow past the metering needle seat unless the fuel chamber pressure is less than atmospheric pressure and the diaphragm is pushed into the metering lever by the pressure differential.

So, I dont know about the rotary valve motors well but there are some differences in volumetric efficiency and the timing of the valve as compared to the reed motor. I view the RV motors as a different ball of wax in comparison when it comes to jetting and pop-off selection due to the different dynamics of the rotary valve intake tract.

Do you still experience flooding if you remove the flame arrestor and airbox? Removing those will raise the throttle bore pressure and might be enough to reduce any negative pressure in the bore to the point that fuel stops flowing into the fuel metering chamber.

I could see something like this creating a vacuum spike if the rotary valve timing were late, a strong vacuum spike might occur which pulls the diaphragm into the metering chamber and opens the metering valve.

 

[Sportster-2001-951C-Stock]

"What about having a busted pump diaphram? what effect would that have ? "

Hmm, you would probably have lots of air in your fuel line and all kinds of fuel metering problems but that one is easy to test just put a mighty-vac on the pulse line.

 

[Mekanix]

Are your diaphragm cover vents taped over? Someone had done that recently, he was painting his carbs and didn't want paint to get into the vent hole in the diaphragm cover so he put some tape over the hole. It took a lot of head scratching to understand why his carbs were constantly flooding because the dry side of his diaphragm wasn't open to atmosphere.

Well, in order to build pressure to test the pop-off you have to block the return line, and fuel is always returning to the tank through the return orifice unless the orifice or return line have an obstruction in them somewhere. If the return was obstructed, then your fuel pressure would be abnormally high and fuel could potentially be forced past the metering needles.

Otherwise, fuel cannot flow past the metering needle seat unless the fuel chamber pressure is less than atmospheric pressure and the diaphragm is pushed into the metering lever by the pressure differential.

So, I dont know about the rotary valve motors well but there are some differences in volumetric efficiency and the timing of the valve as compared to the reed motor. I view the RV motors as a different ball of wax in comparison when it comes to jetting and pop-off selection due to the different dynamics of the rotary valve intake tract.

Do you still experience flooding if you remove the flame arrestor and airbox? Removing those will raise the throttle bore pressure and might be enough to reduce any negative pressure in the bore to the point that fuel stops flowing into the fuel metering chamber.

I could see something like this creating a vacuum spike if the rotary valve timing were late, a strong vacuum spike might occur which pulls the diaphragm into the metering chamber and opens the metering valve.

I painted the carbs by hand with a brush so no tape.But thats a good one!

I'll check to see if there is anything that fell behind the diaphrams. I'm doubtful but hopeful though.


First time starting it was with the just the flame arestor. Today was the first time with the complete airbox. No change except maybe slightly worse off idle.

 

[Mekanix]

"What about having a busted pump diaphram? what effect would that have ? "

Hmm, you would probably have lots of air in your fuel line and all kinds of fuel metering problems but that one is easy to test just put a mighty-vac on the pulse line.

Its one more thing I'll check. I wouldn't have any throttle response in that case though I would think.

 

[Mekanix]

I'm still new to how these are supposed to act. It seems to come one the pipe at 4500 and fall off just below that. Is that just coming on the main jet or is it the raves opening up. It feels like a powerband on a dirt bike. Not much power and then everything...

Or is that just my issue causing it to react that way?

I haven't been able to find any GSX videos to see how they normally act.

 

[Sportster-2001-951C-Stock]

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?

 

[Mekanix]

Not burning its not that rich. But I do get a bit of smoky taste at low speeds.
And I only smell gas from the carbs when I had the airbox off.

One thing I just found out and fixed is that my vent and return lines were backwards on the fuel sending unit.
I blew in both and both are free flowing. SO I'm at a loss as to what the actual difference is.

Does the vent side of it have a valve that closes that I can't see inside the sending unit? That would do it wouldn't it ?

I mean it would shut off the return line.

One thing I noticed is that I didn't spill any gas from the return line going into the sending unit when I pulled the line off. 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.

I blew through the return line and i herd some fuel go into the tank but I thought I would have gotten more fuel than that.

 

[racerxxx]

I keep reading this over and over, the engine shouldn't shake, very little that is. Did you have the magneto off the flywheel?

 

[Mekanix]

I keep reading this over and over, the engine shouldn't shake, very little that is. Did you have the magneto off the flywheel?

Yes I did. It isn't a ballance issue I don't believe. The engine is smooth when it revs up and winds down. No shaking or vibes. Its just cruising at below 4000 is like I stuck a 4 barel on this thing.

It doesnt feel like a mechanical comstant shake its a combustion or miss kind of shake.

Like pull a plug wire off any engine snd watch it shake kind of thing... best I can describe it.

 

[racerxxx]

The Mag has to go back on the flywheel a specific way, not just "back on". Did you line it up properly?

 

[Mekanix]

Yup. With the hole on the flywheel lining up with the trailing edge of the trigger I believe it was.
I mean if I was a screw off advanced the engine would only backfire through the carbs. And retarded would make it very hard to start and spongy I would think.