787 Max RPMs
- Thread starter 6KJR
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bigJake
Well-Known Member
Tony, i coincidentally did several runs last week watching my tach after reading all the posts about cavitation, i kinda remember that I was peaking at 7000 but then it would drop back to about 6900 or a little below. is that normal or an early sign that I'm starting to get a little cavitation?
6KJR
New Member
Wow. I really need to lean mine out! I think I'm only turning high 5's. I need MOAR power and speed. Time to drop jet size.
Dr Honda
Well-Known Member
You may have very light cavitation... but since you are sill below the rev-limiter... I wouldn't worry about it too much. Honestly... that's pretty normal if you have a little clearance in the pump... and you have a couple nicks in the impeller blades.
Dr Honda
Well-Known Member
Didn't we talk about jetting last fall?
6KJR
New Member
Well, it was last spring. And after my car accident everything was left unresolved. Problem STILL is, that everyone's 787 recipe doesn't work for my machine. I am ordering 95 and 115G springs to bring the pop off pressure up. I never did find out what a 787 should be turning for rpms. Mine will never run over 5900. EVER. Now I know that 6500 would be fine. Im thinking that the area I live (sea level wise) causes problems.
6KJR
New Member
Anywhere from 236 ft, 537 ft, and 1082 feet are the 3 lakes I frequent.
Yes my carbs are rebuilt.
Yes I have changed my fuel lines.
Yes I have a new stator.
Yes my coil has been changed as well as plug boots.
Yes I have a new pickup coil.
Yes my compression is great (~155 each side)
And Yes I run premium (91 octane, and for the most part ethanol free fuel.)
My most frequent lakes are the first two mentioned.
Aftermarket flame arresters.
Oil injection delete.
Engines been rebuilt, not sure if its been bored over a size or not. Im not taking the head off to find out.
Top loading grate.
Skat Trak 17-22.
1998 pump nozzle.
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Dr Honda
Well-Known Member
Anything under 3000 MSL doesn't require a change to the jetting. SO... Other than the aftermarket flame arrestor.... it should run with factory jetting. And honestly... if you don't have any internal engine mods... I would bolt on a factory air box to see if you can get it to run properly.
Have you ever gone back in, and checked the rotary valve timing?
Have you ever gone back in, and checked the rotary valve timing?
6KJR
New Member
Haven't checked my timing. Only runs rough at certain throttle. Turns the plugs right wet with fuel after a chop at that speed. Seems like it's dropping cylinders.
Don't have access to stock air boxes.
If it WAS bored over, can you still run stock jetting specs? Wouldn't you want to jet one over?
Don't have access to stock air boxes.
If it WAS bored over, can you still run stock jetting specs? Wouldn't you want to jet one over?
6KJR
New Member
SO after stretching my springs out to get pop off pressure up to 31 I took it out for a test yesterday. Give it a couple primes and fires up.. Runs for a bit then stalls. Odd I thought to myself but I get it goin and take off. Machines warmed up, all the fogging oil is now burned out. Let off the throttle, dies. wont start. Roll over, fire a few times and dead.. Prime it a shot of gas, runs then stalls shortly after (idling).
Out of curiosity I cranked the low speed screws out a turn or two and it will idle now.
Pin the throttle from a stop and it will likely bog or stall. Still not as crisp as I would like it but better then last year.
Why do you think I would be running out of fuel on the bottom end and what would cause the bog off the line?
Brighter note, it was GREAT to get out on the lake. First time this year. Ya I know, Im late.. Had things to get done.
Appreciate the input. Thanks.
Out of curiosity I cranked the low speed screws out a turn or two and it will idle now.
Pin the throttle from a stop and it will likely bog or stall. Still not as crisp as I would like it but better then last year.
Why do you think I would be running out of fuel on the bottom end and what would cause the bog off the line?
Brighter note, it was GREAT to get out on the lake. First time this year. Ya I know, Im late.. Had things to get done.
Appreciate the input. Thanks.
Dr Honda
Well-Known Member
Nope. Most of the time... if you do a "Big Bore"... you lean out the jetting. The reason for that is the vacuum comes up, and it draws more fuel. BUT... with a "rebuild" you are only talking 0.5 to 1 mm. It really doesn't make a difference in the engine size.
AAAAAAAAAAAAAAGGGGGGGGGGGGGGGGGGGGGGGGGG !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
You can't do that. Yes... it will give you a different reading on the pop-off gauge... but you didn't change the spring rate, and you destroyed the real tension of the spring.
Pop-off is only a way we measure the spring tension on the needle and seat... but the actual number has nothing to do with the dynamics of the regulator. Get some new springs, and start over. DO NOT try to make the stretched spring work.
Take the carbs off... make sure they are clean, and set to factory spec... and put in one spring softer than factory. (shoot for the bottom of factory spec) It should run with the aftermarket air cleaners.
Just an FYI... the "Shiny Silver" spring with a 1.5 needle ans seat should give you 32 psi.
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6KJR
New Member
Dr Honda. With the BRAND new 95G springs, my pop off was at 19 PSI. With the 115G springs pop off was at 22. This is too low! The needle and seats are NEW! 2.0. EVERYTHING NEW.... I dont understand why its so low. But the machine has never ran this good before, since Ive owned it. Why do I get to deal with all the stuff that doesnt work right? I donno.
I know stretching the springs is a fail but it was THE only way to get pop off pressure up. With this new pop off pressure I can actually use my mid throttle without it cutting out cylinders from fouling over with fuel. Just not sure why it stalls out on the idle with the needle adjusters in. Im about ready to throw the carbs in the lake. Its the same thing every year.
I know stretching the springs is a fail but it was THE only way to get pop off pressure up. With this new pop off pressure I can actually use my mid throttle without it cutting out cylinders from fouling over with fuel. Just not sure why it stalls out on the idle with the needle adjusters in. Im about ready to throw the carbs in the lake. Its the same thing every year.
Dr Honda
Well-Known Member
The reason it's so low, is because you have a 2.0 needle and seat. You carbs are supposed to use a 1.5.
Now... this is where the "Dynamics" comes in. Since it's really the vacuum signal that is opening the carb regulator... trying to get 32 psi as a "Pop-off" with the larger surface area of a 2.0 N/S... you may never draw enough vac with aftermarket air filters to actually draw the N/S open.
I personally would go back to a 1.5 N/s... but if you want to try to leave the 2.0 in there... I would try a dull silver spring. (95g) That should give you a pop-off of 25 psi. Since you are saying it's down at 19 psi... I have to ask... is the N/S OEM mikuni? This hasn't been an issue in the past... but this summer, I've worked with a few carbs that had aftermarket N/S units, and they would not give the proper pop-off. (they were machined wrong)
With all that said... I went back and looked at your post and you said:
"I dont understand why its so low. But the machine has never ran this good before, since Ive owned it. "
So... is it running or not? If it's running good, leave it alone. If there is an issue... let's try to resolve it. (I thought you were having issues)
Now... this is where the "Dynamics" comes in. Since it's really the vacuum signal that is opening the carb regulator... trying to get 32 psi as a "Pop-off" with the larger surface area of a 2.0 N/S... you may never draw enough vac with aftermarket air filters to actually draw the N/S open.
I personally would go back to a 1.5 N/s... but if you want to try to leave the 2.0 in there... I would try a dull silver spring. (95g) That should give you a pop-off of 25 psi. Since you are saying it's down at 19 psi... I have to ask... is the N/S OEM mikuni? This hasn't been an issue in the past... but this summer, I've worked with a few carbs that had aftermarket N/S units, and they would not give the proper pop-off. (they were machined wrong)
With all that said... I went back and looked at your post and you said:
"I dont understand why its so low. But the machine has never ran this good before, since Ive owned it. "
So... is it running or not? If it's running good, leave it alone. If there is an issue... let's try to resolve it. (I thought you were having issues)
6KJR
New Member
Other then starving on the bottom end and stalling when the jets arent turned way out, and its not very responsive. If your under a load playing, OFF plane, it just doesnt seem to have the snap it should if your on the throttle. If you go to jump a wave and go WOT it hit the rpms it should until you let off and then WOT for the next wave. Make sense? And why if your idling and pin the throttle it will bog and or stall.. I know it seems like this topic is dragging on forever trust me. Its been 3 years. Trying what everyone says works best for the engine and WONT work for me. All the 787 guys say 2.0 N/S 75 pilots stock mains. FAIL. lol...
All the parts sites I order from all CLAIM OEM parts. The last place I got parts from was atlantic jet sports. The place before that was the seadoo source guy, who is now no longer dealing with the machines. Ive thought about going back to the 1.5 needle and seat to try that out but am sick of dumping little bits of money with no results. I wrecked most of my springs trying to get the pop up off.
All two strokes just rip the RPM when you hit the throttle they rev up and take off. My sled is a trippple and theres no waiting for that thing to rev up. Sure its not a twin but it runs awesome.
WHO supplies GENUINE mikuni parts? I was looking at a new site thats closer to home http://onlyseadoo.com/.
If I could get some snap into the bottom end now and keep my mid throttle working smooth as it is now Id be set.
Thank you for your input.
All the parts sites I order from all CLAIM OEM parts. The last place I got parts from was atlantic jet sports. The place before that was the seadoo source guy, who is now no longer dealing with the machines. Ive thought about going back to the 1.5 needle and seat to try that out but am sick of dumping little bits of money with no results. I wrecked most of my springs trying to get the pop up off.
All two strokes just rip the RPM when you hit the throttle they rev up and take off. My sled is a trippple and theres no waiting for that thing to rev up. Sure its not a twin but it runs awesome.
WHO supplies GENUINE mikuni parts? I was looking at a new site thats closer to home http://onlyseadoo.com/.
If I could get some snap into the bottom end now and keep my mid throttle working smooth as it is now Id be set.
Thank you for your input.
Dr Honda
Well-Known Member
It's not 100% clear, but I think I know what you are saying. From low RPM's... if you mash the throttle... it will try to die. BUT... if you are in the mid RPM range... it's crisp to high RPM's. If that is right... keep reading.
First... Parker Yamaha sells OEM Mikuni parts. Also I buy a lot of bike Mikuni parts from "Carburetor parts warehouse". I don't know if they have SBN parts... but you can give them a call. (They are fast, and have good prices)
OK... with the big needle and seat... and the low pop-off... yes, you have to put in a larger pilot jet. BUT... link that to the low vacuum of the aftermarket air filters... and you have to go even bigger. BUT, opening the low speed needle won't help. (totally) That needle is AFTER the pilot jet. SO... you can only get what the jet will give you. In reality, the low speed needle is only to adjust the idle fuel. It's basically useless as soon as you crack the throttle... and it's true use is to limit the fuel when the vacuum is high. (at idle) So... with what you are telling me... you are trying to get the needle to do something it's not meant to do. (add fuel to the transition)
OK.... for a second... let's leave your stretched spring in, and go with your 20'ish psi pop-off.
Knowing all of the issues better... I would put in a monster pilot. Maybe around and 80. (I would also buy 77.5's) With the big jets... you will flow enough fuel to achieve the transition. NOW... you can use the low speed needle to keep the carb from flooding it self at idle. (get it)
Oh... this is also assuming that all of your bypass ports are perfecly clean and clear.
Last thing... even though the pilot has very little effect on the high speed circuit... it will change the overall (full throttle) fulling. SO... you may have to drop your main jets by one size. BUT... don't to that at first. You don't want to run lean. You will have to get the low speed ironed out... and then adjust the high speed.
I know this sounds like a pain... but since you are changing things... jetting has to be done... and one persons settings aren't always good for others. When you are dealing with the loss of signal from the air filters... any other hull mod's can change the air in the bilge.
Personally... if this was my ski... I would get a stock air cleaner from eaby (cheap) and jet to factory spec. The aftermarket air filters are always going to be a pain since they don't give enough signal to handle changes in barometric and altitude changes. Also, they offer little protection on water ingestion. (but that's just me) I know you are probably getting tired of hearing it... but since you said you are getting tired of spending money on the jetting... that will solve your issues.
First... Parker Yamaha sells OEM Mikuni parts. Also I buy a lot of bike Mikuni parts from "Carburetor parts warehouse". I don't know if they have SBN parts... but you can give them a call. (They are fast, and have good prices)
OK... with the big needle and seat... and the low pop-off... yes, you have to put in a larger pilot jet. BUT... link that to the low vacuum of the aftermarket air filters... and you have to go even bigger. BUT, opening the low speed needle won't help. (totally) That needle is AFTER the pilot jet. SO... you can only get what the jet will give you. In reality, the low speed needle is only to adjust the idle fuel. It's basically useless as soon as you crack the throttle... and it's true use is to limit the fuel when the vacuum is high. (at idle) So... with what you are telling me... you are trying to get the needle to do something it's not meant to do. (add fuel to the transition)
OK.... for a second... let's leave your stretched spring in, and go with your 20'ish psi pop-off.
Knowing all of the issues better... I would put in a monster pilot. Maybe around and 80. (I would also buy 77.5's) With the big jets... you will flow enough fuel to achieve the transition. NOW... you can use the low speed needle to keep the carb from flooding it self at idle. (get it)
Oh... this is also assuming that all of your bypass ports are perfecly clean and clear.
Last thing... even though the pilot has very little effect on the high speed circuit... it will change the overall (full throttle) fulling. SO... you may have to drop your main jets by one size. BUT... don't to that at first. You don't want to run lean. You will have to get the low speed ironed out... and then adjust the high speed.
I know this sounds like a pain... but since you are changing things... jetting has to be done... and one persons settings aren't always good for others. When you are dealing with the loss of signal from the air filters... any other hull mod's can change the air in the bilge.
Personally... if this was my ski... I would get a stock air cleaner from eaby (cheap) and jet to factory spec. The aftermarket air filters are always going to be a pain since they don't give enough signal to handle changes in barometric and altitude changes. Also, they offer little protection on water ingestion. (but that's just me) I know you are probably getting tired of hearing it... but since you said you are getting tired of spending money on the jetting... that will solve your issues.
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6KJR
New Member
OK. So, to clarify. Your stating that with a LOWER pop off you will need bigger jets? If the pop off is lower, will it not pop the needle and seat sooner resulting in more fuel in the carbs? Resulting in more fuel to the engine causing flooding.
When I went from 20 to 31 pop off I left the 75 lows in to test it. (try not to change to much at once to mess the results) It wouldn't idle anymore. Just shut off once you let off the throttle and would NOT start unless primed. To me that sounds like the high pop off has stopped fuel flow into the carbs. Then, after opening the adjusters it WOULD idle. It kinda doesnt add up to what your saying, if Im understanding it correct.
EVEN with the aftermarket FAs, when the throttle is closed (idle) the vac on the inside of the throttle blade would be the same. It would still pull the same amount of fuel through with or without the stock breather.
I could see that effecting more of the low-mid-high throttle positions where the throttle blade is opening and the intake is the restriction as opposed to the blade itself.
How much more fuel will a 2.0 over a 1.5 NS flow? If I was to go back to a 1.5 and put pop off about 25 would it be about the same as my 2.0 with 31 pop?
When I went from 20 to 31 pop off I left the 75 lows in to test it. (try not to change to much at once to mess the results) It wouldn't idle anymore. Just shut off once you let off the throttle and would NOT start unless primed. To me that sounds like the high pop off has stopped fuel flow into the carbs. Then, after opening the adjusters it WOULD idle. It kinda doesnt add up to what your saying, if Im understanding it correct.
EVEN with the aftermarket FAs, when the throttle is closed (idle) the vac on the inside of the throttle blade would be the same. It would still pull the same amount of fuel through with or without the stock breather.
I could see that effecting more of the low-mid-high throttle positions where the throttle blade is opening and the intake is the restriction as opposed to the blade itself.
How much more fuel will a 2.0 over a 1.5 NS flow? If I was to go back to a 1.5 and put pop off about 25 would it be about the same as my 2.0 with 31 pop?
Dr Honda
Well-Known Member
No... you don't automatically need bigger jets for low pop-off... but, you said it would only idle with the low mix screw turned out a lot. SO... put in a bigger pilot jet, and use the low mix screw to keep it from flooding. BUT... this will give the added fuel in the trasition to get yo into the higher RPM's. The reason I'm saying to leave the spring alone, is because we know what it's doing. (wrong or right)
As far as the needle ad seat... right from Mikuni's manual... a 1.5 N/S will flow as much fuel a the pump can put out. SO... the only reason to go any bigger is to get your pop-off lower. So, there was no reason to change it in your carbs. All it does, is make tuning harder.
Let's talk Pop-off.
We check the tension on the needle and seat with compressed air on the face of the needle. So... if you keep the spring the same, but increase the surface area of the needle (by putting in a bigger seat) then the pop-off drops. The problem lies in that we aren't depending on fuel pressure to open the N/S. We relay on vacuum on the diaphragm. SO... a 32 psi pop-off on a 2.0 N/S doesn't actually open at the same time as a 32 pop-off on a 1.5 N/S. Also... since I think you have aftermarket N/S in your carbs... who knows where they are actually opening. Since your pop-off is very low... I don't know is the opening is right... or if the seat depth is correct. This is partly why I'm wanting to just leave it alone, and try to jet around it.
Last thing. The low speed screw is kind of useless after 2.5~3 turns out. So... we need a pilot jet large enough to flow enough fuel so the engine will idle well with it set to about 1 to 1.7 turns out.
As far as the needle ad seat... right from Mikuni's manual... a 1.5 N/S will flow as much fuel a the pump can put out. SO... the only reason to go any bigger is to get your pop-off lower. So, there was no reason to change it in your carbs. All it does, is make tuning harder.
Let's talk Pop-off.
We check the tension on the needle and seat with compressed air on the face of the needle. So... if you keep the spring the same, but increase the surface area of the needle (by putting in a bigger seat) then the pop-off drops. The problem lies in that we aren't depending on fuel pressure to open the N/S. We relay on vacuum on the diaphragm. SO... a 32 psi pop-off on a 2.0 N/S doesn't actually open at the same time as a 32 pop-off on a 1.5 N/S. Also... since I think you have aftermarket N/S in your carbs... who knows where they are actually opening. Since your pop-off is very low... I don't know is the opening is right... or if the seat depth is correct. This is partly why I'm wanting to just leave it alone, and try to jet around it.
Last thing. The low speed screw is kind of useless after 2.5~3 turns out. So... we need a pilot jet large enough to flow enough fuel so the engine will idle well with it set to about 1 to 1.7 turns out.
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6KJR
New Member
Well according to your research, if a 1.5 needle and seat will give me all the flow I need, perhaps going back to a 1.5 will be better and I could get my pop off down to a 25 area where I think it would work better with the jets that are there.
Im thinking that once the transition occurs, or starts to occur, its too rich. Which is why the engine wont respond to the throttle as required. (the reason I dropped back the main jets) That way, the pop off could work properly with the air flow/vac as discussed with aftermarket FAs. If I leaned out the bottom end (somehow) I could get better snap from the engine.
I know your opinion if it was your machine, but in my case. What would you try first... going to a 1.5 NS and test pop off with OEM springs? From what you say, going to 1.5 NS and 70 pilots would clear up my issue. 2 jet sizes would make that much difference?
Whats the point in going bigger N/S if the stock one can flow as much anyways?
IF you think that a 1.5 will clear up my problem then Ill go buy them. To me, anything less ten 24 pop off is TOO low on these carbs with a 2.0 NS. If I should order stock low jets I will too. Remember, at the same time, today, other people are telling me that 75 lows and 2.0NS are the best thing going for 800 seadoo engine.
Soon Ill make the thing fuel injected with a aftermarket FI controller so I can tune it with a laptop. Wheres the headbang smiley?
Im thinking that once the transition occurs, or starts to occur, its too rich. Which is why the engine wont respond to the throttle as required. (the reason I dropped back the main jets) That way, the pop off could work properly with the air flow/vac as discussed with aftermarket FAs. If I leaned out the bottom end (somehow) I could get better snap from the engine.
I know your opinion if it was your machine, but in my case. What would you try first... going to a 1.5 NS and test pop off with OEM springs? From what you say, going to 1.5 NS and 70 pilots would clear up my issue. 2 jet sizes would make that much difference?
Whats the point in going bigger N/S if the stock one can flow as much anyways?
IF you think that a 1.5 will clear up my problem then Ill go buy them. To me, anything less ten 24 pop off is TOO low on these carbs with a 2.0 NS. If I should order stock low jets I will too. Remember, at the same time, today, other people are telling me that 75 lows and 2.0NS are the best thing going for 800 seadoo engine.
Soon Ill make the thing fuel injected with a aftermarket FI controller so I can tune it with a laptop. Wheres the headbang smiley?
Dr Honda
Well-Known Member
Any time I have a jetting issues... I almost always go back to factory specs. So, in this case... that is probably what I would do. BUT... since we want it to run with the aftermarket air filters... I would put in the soft "Shiny silver" spring... and give it a try.
One thing... since you are thinking that you are going rich in the transition... I would make sure the return hose (orifice) is clean and clear. If it's plugged up a little... then you will have a hard time with the pilot circuit.
Regardless... knowing that you are way down at 20 psi with the 115g spring... something else is wrong. SO... even if you want to stay with a 2.0 N/S... I would order new OEM units. (you shoudl be at 32 psi with a 2.0 and 115g spring)
One thing... since you are thinking that you are going rich in the transition... I would make sure the return hose (orifice) is clean and clear. If it's plugged up a little... then you will have a hard time with the pilot circuit.
Regardless... knowing that you are way down at 20 psi with the 115g spring... something else is wrong. SO... even if you want to stay with a 2.0 N/S... I would order new OEM units. (you shoudl be at 32 psi with a 2.0 and 115g spring)
6KJR
New Member
OK. Im gonna order 1.5 needle and seats, 2.0 needle and seats. New springs in every size that I havent destroyed already and go from there. When I redid the fuel system a couple years ago nothing was plugged. And nothing has changed engine running wise since then either. Perhaps Ill order stock lows.
Looks like the dull silver is the 95G spring. I think if my pop off was at 25 it would be better, even 26/7. I cant believe how much going from 20 to 30 made a difference in the mid range throttle problem I was having.
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