Carb Theory & Practice

[DylanS]

Weird. The tech at yamaha said they did it specifically to prevent fuel dribbling in the carb because the engine had crazy vibration at idle. 1200 non pv. It's pretty well documented. And yanah came out with a specific fuel plate for the mikuni carb for the problem. Didn't mention emissions at all.

Tim couldn't tell me why my movi dribbled fuel at idle. Round. And Round. And Round. And Round. Was told 19-20psi popoff. Went to 37. That was like 80-100 hours ago.

Strokers have a ton of signal on the engine. When you hit 7k and let off, the signal is still trying to open that needle. 19 psi is less than half of what mikuni engineered the carb for.

I'll continue to listen to them. But appreciate the conjecture.

I do see what you’re getting at, I’d imagine you had to go a fair bit richer on your pilot jet?
Basically you’re compensating for a high pop off pressure (that’s preventing the shake from constantly moving the needle from the seat) with a richer low speed jet?

Also as far as the emissions end goes..

 

[Sparkplug]

Have you tried too double up on the siphon valve ? that took care of my dribbled on a pair of FS 49s, Novi has a great right up on checking return fuel pressure that might help with dribbled too.

Yup. And I've drill my own fuel plate the offset passage. I tried everything man. I chased the issue on a 1200 for 2 months. It was an absolute nightmare. I spent more time calling random yamaha shops, than I did working on the ski. Because theory didn't seem to matter. It turns out it was a known issue all along. And basically the carbs have to be tuned incredibly perfect for idle. Otherwise nothing will seat the needle

 

[Sparkplug]

You run 37 on what setup?

Dasa 1100. Novi 48. Powerfactor dry. Torrent 155. Xfr hull. Rec and free style riding.

Piston wash is absolutely perfect. It will fire up with no choke after 4 or 5 days. If it sits longer I gotta choke the carbs.

It will absolutely rip your arms off from idle to wot. It passes the 1/4 throttle test perfectly.

Is it perfect? Idk. It runs way better than when I got the carbs, and they were set EXACTLY to what the builder recommended. Maybe being 100ft altitude matters. Idk.

Buddy has a 1100. He was running 20 popoff. Had issues starting and a low lean bog. He bumped it to 25 psi with my recommendation, and it made the starting issue go away. The lean big turned out to be a adjuster issue. Or maybe it was popoff combined. Who knows.

All I know is, low popoff skis always crank for days. Always need a choke or a prime when they sit. And always seem to flood the engine with the 1/4 throttle test. This is straight up tray time talking. At the river. Pulling carbs. And seeing seat of the pants.

 

[kevbo]

We use popoff pressure as an easy way to check and adjust the main needle and seat. This pressure as a tuning tool is a little misleading though. The pressure on the pump side ranges from slightly negative up to 6 psi. The tank pressure variance will add to or subtract from that number. This is why I prefer to completely vent the tank.. The pressure on the carb side is whatever the little spring is pushing on the needle. Popoff pressure is meaningless until the pump side pressure begins to rise. Then there is only a 6 psi difference. In practice, the popoff pressure will affect idle and pilot jet tuning noticeably so it needs to be the the same when multiple carbs are used.. It will also affect how well the needle seals. Fortunately we use pulse pumps that never build much pressure and a have a limited number of springs so we cant get this setting too far out of bounds.

 

[WFO Speedracer]

All of those 1200 NPV Yamahas came with ridiculously lean pilot settings. you could crank on them for days before they would start, Yamaha knew about this and didn't care , it passed the emissions check so it's good to go right ?

Nope wrong, bad wrong, I have fixed numerous 1200's by readjusting the low speed screws, when I say readjust I mean a turn to a turn and a half out, what does that do, it lets the ski take off without hesitation and crank without the choke 99% of the time, so a ski that came in and would not crank without pouring gas down the carb left cranking without the choke, did Yamaha know , sure they did , did they fix the issue , nope, did they care nope.

The Yamaha 1200PV engines don't start well either, high popoff, ridiculous low speed settings and too small pilot jet and a cat converter inside the exhaust pipe, did they know, you bet, the fix , yank out the cat , put a chip in the exhaust heat sensor plug, and change all the jets to Europeon spec, you know those skis that didn't come with cat converters.. other than that the only way one will crank is hold the choke wide open, hold the thrttle wide open till it tries to light off and then it will start, it starts this way only because it is now pulling fuel through the main jets.

 

[Jdbarnes89]

We use popoff pressure as an easy way to check and adjust the main needle and seat. This pressure as a tuning tool is a little misleading though. The pressure on the pump side ranges from slightly negative up to 6 psi. The tank pressure variance will add to or subtract from that number. This is why I prefer to completely vent the tank.. The pressure on the carb side is whatever the little spring is pushing on the needle. Popoff pressure is meaningless until the pump side pressure begins to rise. Then there is only a 6 psi difference. In practice, the popoff pressure will affect idle and pilot jet tuning noticeably so it needs to be the the same when multiple carbs are used.. It will also affect how well the needle seals. Fortunately we use pulse pumps that never build much pressure and a have a limited number of springs so we cant get this setting too far out of bounds.

Hi I was just reading up on this article, and I'm in the process of raising pop off pressure I think, waiting on parts to arrive tomorrow... Anyways I have a et967, dual novi 48, pfp ran dry, and have tested fuel pressure and ended up with a 110 jet to get to the 1.5psi at idle, I'm guessing the big bore stock stroke must have a ton of signal? anyways carbs are at 20psi with 125 low and 115 high jets, 3/4 turn both high and low, but I cannot for the life of me get it to idle worth a damn, it's smokey and so rough it vibrates the teeth when kneeling down lol, anyways ordered new rebuild kits and a 2.0 NS was thinking of 115gr spring which should be close to 35psi, also ordered 130 and 135 low jets for the increased pop off, just wondering if I'm on the right track here?? Also am I going to need to play with the high jet after changing pop off??

 

[Flash-FX]

When I test/pop-off (dual carbs) my goal is to get them as close to identical as possible. On the bench, metering side cover off, and drained lines (you know this already). The actual pop-off is really just a hiss. Get them to start hissing at the same time. Gently press down on the front needle to get a reading on the rear carb (while watching the gauge), then do the rear carb. A paper towel covering the open side of the carbs is a little insurance for not to get a fuel spray in the face when listening closely for the hiss leak. The closer the pop is with the 2 carbs, the closer the LS adjuster needles (turns out) will be for tuning. Of course this is all relating to the idle circuit here. You probably will have to stretch or squeeze the end of a spring to fine tune the "leak" to get them the same. Start with the highest # psi spring out of the package and fiddle with the low one to match. Another thing to remember is to get the needle control levers to be level with the carb body, a good starting point. I use a floor bike pump for air.

Besides the carb tuning issue, another thing (among many others) that makes for a rough running/smoking motor is leaky/chipped/bad sealing reeds. They will give you a false rich condition that can make you chase you tail thinking it's definitely the carbs/jetting/pop-off. Nice tight sealing reeds are what every one needs. Reduces issues.

For a rough comparison, I have a set of Full Spectrum 48's in combination with a power factor pipe semi wet/dry. When running this on a Xscream SS900 it wanted 26psi pop-off. This same setup on a SS Dasa 850 needs a 18 psi pop-off and a smaller pilot jet. With both engine combo's the final LS needle setting is between 1/2 and 5/8 turns out. Idles perfect.

 

[kevbo]

Start it out of the water and close the bottom screws until the rpm goes way up. Then open until it slows back down. Two strokes can run with a very rich mixture. You want it just rich enough for good response from idle.

 

[sjetrider]

I have been messing with carbs for 25 years. I still do not understand EXACTLY where the pop off comes into play. If one or some of you gurus has a suggestion, I am all freaking ears. I am surrounded buy guys smarter than me that have rode the ski and lead to some of the changes made below.
T1 freestyle setup hull, case and cylinder ported Maniac motor, PFP dry, 760 ZEEL electrics , 192 compression and a 155 OEM pump currently a 5/12 pitch Skat mag swirrel , cut back prop. I know WFO is gonna tap out cuz it aint stock LOL!
I know I have had them too high and the ski would struggle to get to pipe but not really ever have a LEAN bog.
I seem to always end up between 18-20psi on aftermarket 48mm and up carbs.
I am dealing with a set of retail 44's now and chasing what feels like a low end, JUST OFF IDLE seemingly rich bog.
Started pop off at 18, low speed jet 125 / high speed 115. It was crazy rich,'
went to 117.5 pilot / 120 main still rich
Down to 110 pilot up to 122.5 main and bumped pop off to 25 still rich, very bottom but better
im at 105 pilot 122.5 main and 25 pop off and I drilled the returns in the carbs and running a 75 return jet (after trying a 60 then a 100) the 75 seemed best and idles smothest.
has 1.5 psi fuel pressure at idle and 5 when revved up on trailer to 3/4 throttle
I still need to run the bottom screw 1/2 turn out to get best results, still a slight rich feel right off bottom. (at 1/2 turn out it idles crazy and high out of water not in water, idles good at 3/4 turn out when out of water. )
I thought, hell, I am chasing a prop, so pitched it down and really NOT much change.
The ski comes on super nice just past 1/4 throttle, but dropping into a setup wake I let all the way off throttle and that slight bog is driving me NUTS!

 

[sjetrider]

Don't misunderstand, this ski runs pretty damn good! We are talking a very early off idle issue that gets better the more I go in on bottom screw but never gets PERFECT.

 

[kevbo]

Make sure the top screw is tuned slightly rich of peak rpm. Then step up the pilots until the hesitation goes away. They always have to be four cycling at 1/4 throttle for best response. I have had dual 38s and a single 44 that could run cleaner at part throttle than any dual 44 or 46 arrangements.

 

[Jdbarnes89]

When I test/pop-off (dual carbs) my goal is to get them as close to identical as possible. On the bench, metering side cover off, and drained lines (you know this already). The actual pop-off is really just a hiss. Get them to start hissing at the same time. Gently press down on the front needle to get a reading on the rear carb (while watching the gauge), then do the rear carb. A paper towel covering the open side of the carbs is a little insurance for not to get a fuel spray in the face when listening closely for the hiss leak. The closer the pop is with the 2 carbs, the closer the LS adjuster needles (turns out) will be for tuning. Of course this is all relating to the idle circuit here. You probably will have to stretch or squeeze the end of a spring to fine tune the "leak" to get them the same. Start with the highest # psi spring out of the package and fiddle with the low one to match. Another thing to remember is to get the needle control levers to be level with the carb body, a good starting point. I use a floor bike pump for air.

Besides the carb tuning issue, another thing (among many others) that makes for a rough running/smoking motor is leaky/chipped/bad sealing reeds. They will give you a false rich condition that can make you chase you tail thinking it's definitely the carbs/jetting/pop-off. Nice tight sealing reeds are what every one needs. Reduces issues.

For a rough comparison, I have a set of Full Spectrum 48's in combination with a power factor pipe semi wet/dry. When running this on a Xscream SS900 it wanted 26psi pop-off. This same setup on a SS Dasa 850 needs a 18 psi pop-off and a smaller pilot jet. With both engine combo's the final LS needle setting is between 1/2 and 5/8 turns out. Idles perfect.

Well last summer I replaced the vf3 reed cages with new ones for this same issue and it ran exactly the same as before, just send like no matter what I do I can't get the thing to idle worth a damn, but I haven't ever changed the pop off throughout all of this so I'm wondering if the pop off would contribute to a rich low fuel screw, thinking maybe I'd raise it from 20psi to either 25, or 35psi then rejet if need be... Forgot to mention I ran dual 46mm on this engine with same 20 psi and had same issue, but on the last 780cc engine they ran great, besides the pfp all the parts on my current et967 came from my 780cc that blew a crank, but up until that the 780 would idle great, so I'm thinking it has to be something with the strong carb signal or something???

 

[Pro-pulsion]

Yep I body carbs are some of the worst to modify, it is also one of the main causes of failure on 720 Seadoo engines running a Factory pipe, everyone wants to blame top end seizures on the pipe, it's the carbs actually causing the problems.

In the case of the 650SX you have so many people with aftermarket parts syndrome, they just swap intakes and carbs without thinking about the exhaust manifold and exhaust pipe restriction, then they wonder why their skis isn't as fast as their buddies ski that has the right combo of parts on it.

There is also a known issue with the stinger exit diameter being to small and restrictive which had a hand in melting pistons. I know of one that had a rather large weld bead just before the exit and a rather small diameter that I setup on the CNC mill where I bored it out by about 2mm. It made the exit tube walls quite thin but they're still good enough to be strong and really opened up the exit diameter. It would take me a while to find the info now but I think Factory was the one to figure it out because they later changed the exit diameter from I believe it was 34mm to 38. The HX I did some mods to was going to use this pipe but at the end he didn't want to chance it so I did some intake modding and stock pipe drying out along with a few other little things. He absolutely loved it, said it was the best it had run since he bought it. I wish we would have ran the pipe but the plans for the higher performance build that would have used it fizzled out. He loved the stock mod HX so much he stayed with it and then eventually fizzled out freeride entirely. The pipe is still sitting on the floor at the old shop just hanging out with the boys whenever we show up lol.

 

[Flash-FX]

I have been messing with carbs for 25 years. I still do not understand EXACTLY where the pop off comes into play. If one or some of you gurus has a suggestion, I am all freaking ears. I am surrounded buy guys smarter than me that have rode the ski and lead to some of the changes made below.
T1 freestyle setup hull, case and cylinder ported Maniac motor, PFP dry, 760 ZEEL electrics , 192 compression and a 155 OEM pump currently a 5/12 pitch Skat mag swirrel , cut back prop. I know WFO is gonna tap out cuz it aint stock LOL!
I know I have had them too high and the ski would struggle to get to pipe but not really ever have a LEAN bog.
I seem to always end up between 18-20psi on aftermarket 48mm and up carbs.
I am dealing with a set of retail 44's now and chasing what feels like a low end, JUST OFF IDLE seemingly rich bog.
Started pop off at 18, low speed jet 125 / high speed 115. It was crazy rich,'
went to 117.5 pilot / 120 main still rich
Down to 110 pilot up to 122.5 main and bumped pop off to 25 still rich, very bottom but better
im at 105 pilot 122.5 main and 25 pop off and I drilled the returns in the carbs and running a 75 return jet (after trying a 60 then a 100) the 75 seemed best and idles smothest.
has 1.5 psi fuel pressure at idle and 5 when revved up on trailer to 3/4 throttle
I still need to run the bottom screw 1/2 turn out to get best results, still a slight rich feel right off bottom. (at 1/2 turn out it idles crazy and high out of water not in water, idles good at 3/4 turn out when out of water. )
I thought, hell, I am chasing a prop, so pitched it down and really NOT much change.
The ski comes on super nice just past 1/4 throttle, but dropping into a setup wake I let all the way off throttle and that slight bog is driving me NUTS!

When does the pop-off start?* It generally happens when the butterflies start to open and expose airflow past those little drilled metering holes, located above the closed butterfly. The lower the pop-off (richer), the easier it is to let more fuel flow. The higher the pop-off (leaner) the harder it is to suck fuel through those little holes.
You change this variable with larger needles and seats and or softer springs (making it richer). The opposite is true when you go with smaller needles and seats or stiffer springs, (making it leaner). Sorry if this is "old news".

*Providing that the butterflies are not worn away at the shaft/bore ends. Worn carbs are a waste of time to attempt to get tuned right, Air leaks past the wrong spots.

Good aftermarket carbs are easier to tune than say OEM dual 44's and or 46's. Gotta remember what they were designed for. My 94 FX came with a 61X motor and a single 44. It had a 1.5n/s and about 43psi pop-off, and a stock flame arrestor (pretty restrictive). It ran well with a smooth power band from idle to WOT. Now take a pair of them and try to make them work just like that a 62t case. Nope. You can get the idle and the WOT to work, but every thing in between will be either a little rich or lean... or both. Aftermarket carburetors (not talking annular carbs here) have a relocated booster (more efficent) that helps even out the metering transition. Refining the mid throttle operation.

OEM ported cylinders can be real screamers that make great mid & top end power. But they can't compare to a large CC power valve cylinder which is designed to provide a wider power band, especially in the lower RPM range. The 155mm set back mag takes a lot to get going, especially from a idle. Big motors have trouble spinning them up from a idle too. And the practice of "a big motor, a big prop" doesn't always work out.

In a last attempt to get closer to your goal, you might need to bore the nozzle? Try even a lower pitch impeller? Maybe add a spray bar to that dry pipe and tune it for a lower hit?

Does this help clear things up? Or is it still clear as mud...

Once again...the "manual" https://mikunipower.com/Manuals/SBN_Manual.pdf

 

[Jhucke]

I apologize this comment has nothing to do with the popoff discussion.

I would be really interested if someone has ever tested for mass flow rate, intake velocity, etc to spec a carb size based on real data and calculation for differing engine sizes, port shapes, other variables. I know this industries primary testing is trial and error, seat of the pants tuning, though, so I would not hold my breath.

 

[Jdbarnes89]

Well I found this in the manual and decided to raise pop off from 20psi to 26, not sure why but the levers are dead flush with carb body, and with a 2.0 seat and a 115gr spring I ended up at 26 dead even between the carbs, even checked the pop off tester for accuracy

 

[sjetrider]

When does the pop-off start?* It generally happens when the butterflies start to open and expose airflow past those little drilled metering holes, located above the closed butterfly. The lower the pop-off (richer), the easier it is to let more fuel flow. The higher the pop-off (leaner) the harder it is to suck fuel through those little holes.
You change this variable with larger needles and seats and or softer springs (making it richer). The opposite is true when you go with smaller needles and seats or stiffer springs, (making it leaner). Sorry if this is "old news".

*Providing that the butterflies are not worn away at the shaft/bore ends. Worn carbs are a waste of time to attempt to get tuned right, Air leaks past the wrong spots.

Good aftermarket carbs are easier to tune than say OEM dual 44's and or 46's. Gotta remember what they were designed for. My 94 FX came with a 61X motor and a single 44. It had a 1.5n/s and about 43psi pop-off, and a stock flame arrestor (pretty restrictive). It ran well with a smooth power band from idle to WOT. Now take a pair of them and try to make them work just like that a 62t case. Nope. You can get the idle and the WOT to work, but every thing in between will be either a little rich or lean... or both. Aftermarket carburetors (not talking annular carbs here) have a relocated booster (more efficent) that helps even out the metering transition. Refining the mid throttle operation.

OEM ported cylinders can be real screamers that make great mid & top end power. But they can't compare to a large CC power valve cylinder which is designed to provide a wider power band, especially in the lower RPM range. The 155mm set back mag takes a lot to get going, especially from a idle. Big motors have trouble spinning them up from a idle too. And the practice of "a big motor, a big prop" doesn't always work out.

In a last attempt to get closer to your goal, you might need to bore the nozzle? Try even a lower pitch impeller? Maybe add a spray bar to that dry pipe and tune it for a lower hit?

Does this help clear things up? Or is it still clear as mud...

Once again...the "manual" https://mikunipower.com/Manuals/SBN_Manual.pdf

All good information for sure.
I am putting a Bun chamber on this week and prior to picking that up I was going to put a spray bar on the pfp
I agree prop may be still too tall. Wet pipe may give it that low end bump that gets me through it. If not…… back to the pump

 

[sjetrider]

Thank you for your input.
By your description and something I have contemplated. Maybe even higher pop off. Which will be a first for me , but so is a damn 105 pilot lol

 

[DylanS]

^ After blowing up many a big bore engine I’ve recently gone back down to a 781 that I case and cylinder ported.
Carbon freak, 781, total loss, pfp w/ rrp carbon chamber, 50mm full specs, 155 setback skat mag w/ 6/12 prop. Carbs specs @ 24 psi, 132.5 low, 122.5 main 3/4 low 1 high adj.
Ski pulls hard from top to bottom.
A lean combination between low speed and pop off will sometimes seem to be a rich hesitation when putting around with these freestyle setups. I personally prefer to be in the mid 20’s on pop off and play with the low speed jet until I find that crisp combination. Helps with the fuel dribble too.

But take it for what it is comparing 44’s to 50 full specs.
Have you talked to Chris ? Those guys have their engines pretty figured out. To me a 105 pilot sounds insanely lean.

 

[waxhead]

I alway say the low speed jet determines how much fuel and the pop off determines when. if you have a massive low speed jet you can up your pop off to make it run cleaner. you can also get a low speed jet to work sooner with a lower pop off. But in reality just set your pop off on a mikuni to about 20 psi and jet around it