Vomiting Carbs

[Dorny119]

Hi all, here is my setup:

Dual 46
Low Jet: 130
High Jet: 132.5
Low Screw: 1 out
High Screw: 1.5 out
Pop Off: 18psi

-760 (84.75mm) with Mildly ported cases and Mild/Heavy Ported Cylinder
-B-pipe Mod chamber
-ADA head with 37cc domes, 170psi (raised ex port, less trapped volume)
-Hooker 10/16

(I am still tuning my ski, as i have been constantly changing parts and modifying things as an experiment).

Last time I rode my ski i noticed smoke emitting from the carbs after i shut it down. Also, if i pushed it into the water to force water up the exhaust tube, smoke would billow out AND i could hear the reed pedals humming a tune. So, i pulled the reeds (some kind of old dual stage reeds) and sure enough a couple pedals were opened up. So, i just replaced them with new VF3 reeds and put a Wax intake manifold while i was at it.

NEW PROBLEM: Ski will not idle on the stand unless i open it way up to probably past the low speed jet orifice. When running, it is spitting fuel up into the air in the form of droplets. I will say it sounds extremely snappy compared to the last time i revved it on the stand (old worn reeds?). But i am puzzled by the spitting fuel.

THEORY: Could this be that my low jets are incorrect? Could the worn reeds have been tricking me into improperly jetting my ski? I meticulously tune my skis, changing one jet at a time etc etc... I never ran this ski with new reeds before, as the first time i ever ran this motor was this season with those old reeds.

Any words? Posted is an image of the droplets resting on the blade. Thanks!

 

[Quinc]

that is really rich, what size engine and pipe?

https://www.pwctoday.com/forum/pwctoday-riding-groups/midwest/midwest-pwc-riders/badgerland-jet-pilots/306829-addendum-to-the-mikuni-sbn-tuning-manual

 

[Dorny119]

that is really rich, what size engine and pipe?

https://www.pwctoday.com/forum/pwctoday-riding-groups/midwest/midwest-pwc-riders/badgerland-jet-pilots/306829-addendum-to-the-mikuni-sbn-tuning-manual

oops. see my post again. I added details. and yes i found my tuning method very similar to the addendum.

 

[Quinc]

I am running a very similar setup and my jetting is:
135 Pilot 1turn
105 Main 3/4 turn
19psi

*And I think I am rich on the main still.

 

[fastgtfairlane]

When you say 18 psi, is that measured or is that using the mikuni chart? Also important to list the n/s and spring you are using. Obviously important to verify popoff with a gauge. It sounds like you have fuel dribbling out of the venturis causing an overly rich condition.. if you have a 65 gram spring that can cause a fuel dribble due to vibrations. To compensate and keep the pressure relatively close, you can increase the n/s size to run a stiffer spring. Also important to note that when you change your popoff you might have to re jet as well.

The other possibility could be a fuel pressure related issue. More common on larger and higher compression engines that produce a stronger pulse to the fuel pumps. To fix this, you can remove the fuel pump and drill out the restrictor inside the return hole that goes back to the tank.. next you'll run a mikuni main jet in the tee outlet from the carbs. Now your pressure is adjustable with a mikuni main jet. This also equalizes the pressure between carbs. To find out what jet you need, you'll connect a fuel pump between the carb outlet and tee. Increase jet till 1-2psi at idle and 4-5 psi at wot.. this modification will make tuning the carb easier.

Your jetting also sounds pretty rich as well..

 

[waxhead]

I would try 135 low and a 120 high. you have our high velocity intake manifold on it. due to this you will not have as much fuel drop out due to the increase in runner efficiency. Your jetting will be rich. I know when we made this manifold and tested it that we had to lean it out. Basically under 280 feet per second fuel no longer stays in suspension. When you have a manifold that opens out you lose velocity and fuel pools on the bottom of the of the intake and runs into the engine. Because of this you need to run more fuel to accommodate it. There is less drop out on the high speed port runner and so you dont waste as much fuel

 

[Dorny119]

My pop off is 18psi measured. My N/S is 2.3 and i do not know what my spring is. I typically use what i have as long as the colors are the same, as the colors tend to be deceiving so i just go off of measured psi.

Yes i agree that my mains are rich. Like i said i am still tuning as i am changing things frequently. I recently had a different issue (long story, piston/wall clearance related) that was making me think i had a lean condition at high rpm, so i richened it way up to rule that out. I have since rectified that problem (poor piston/wall clearance, piston seizing during WOT). The mains shouldn't be causing me this issue here since my problem is with low throttle openings without a load.

There is no fuel dribbling from the venturi. I double checked that while it was running. Keep in mind that this is a new issue and the only difference is two days, and new intake/reeds.

Wax, thanks for the info. I was actually concerned about the rapid increase in cross sectional area for that reason. Glad to see your explanation.

Well, i will put 135 pilot and 127.5 main to start then. I won't go lower than 127.5 main however because last time i had 127.5 in, i didn't like my piston wash. It was ok but a little lean looking. Again, this is an idle issue i am having.

 

[kevbo]

You can power tune the main circuit. A little rich of peak power with good response is all there is to it. Nothing else matters when its at full throttle.

 

[Dorny119]

You can power tune the main circuit. A little rich of peak power with good response is all there is to it. Nothing else matters when its at full throttle.

Thanks Kevbo. Yes and that is how ingot to 127.5 before. I was using a tach. Of course that will change now with my new parts.

 

[waxhead]

I always tune of the low speed jet, Why they never called it the everything jet confuses me.
I will always run the biggest low speed jet and the smallest high speed i can. This always creates the most low to midrange torque, the top end stays the same.
The main jet is just to trim in my opinion. If its to small then it will not pull up on to the top.
A lot of people over prop and this makes it way harder to tune a ski as you pulling around in to a high gear. Tuning becomes way more finecky

 

[kevbo]

One problem I had with an overly rich pilot/popoff was those times I had to cross a lot of rough water at slow speeds. It sucked the tank dry nearly twice as fast. I agree most riders overload their engines and are then very hesitant to change it. Changing an impeller is a lot harder than turning a screw.

 

[Dorny119]

alrighty so i did confirm tonight that there is fuel dripping from the venturi. I raised my popoff to 21psi which is the highest i can see being practical in my application. No point in raising it to see if it fixes the problem if i can't go much higher than that when i start to re-tune the ski.

So.. this means drilling the restrictor and putting a restrictor in the return lines. Gosh i really hate the fact that that is necessary tho. I am trying to build a high RPM ski and i worry about fuel pressure at higher revvs. Also, it's just one more thing to tune and consider for diagnosing. There must be another way to stop the dribbling fuel from the mains?

I checked for a clogged return line. It's clear. I really should replace my needle to see if that's the issue. Sometimes when i do a popof test i get a hiss instead of a good pop. So that may be an inticator. I will also try a stronger spring with a larger N/S and aim for a similar popoff. Maybe the anti-siphon valve too?

 

[MTRHEAD]

21 psi is a what most would consider a “low” pop-off with oem 46’s. Based on the very little info posted I sure wouldn’t go lower Also airfilter styles have an affect on the pop-off circuit

As other have said over propping can get you in a place you can never get it correct. 10/16 hookers can load a motor harder than most think

 

[fastgtfairlane]

you could also have a leaking oring around the seat or even a bad needle. when you check popoff, you can also go to a few psi below the actual popoff psi and let it sit with air in it and see if the needle drops to diagnose a leak. i can understand not wanting to drill out the restrictors but it can be a necessary thing to do. most all of the full specs, novis etc come with drilled restrictors and require a main jet in the return tee. ironically, its not a modification you hear mentioned very often.

i would try re jetting first since you changed reeds and your intake manifold. im not sure about vforce but i know with boysen reeds, they mention having to rejet when using their reeds over stock. if you increase your low jet, you can go smaller on the main jet. i like to have my low speed screw at 3/4 to 1 turn out and high speed 1/2-3/4 max and will adjust jetting until i get there.

 

[waxhead]

One problem I had with an overly rich pilot/popoff was those times I had to cross a lot of rough water at slow speeds. It sucked the tank dry nearly twice as fast. I agree most riders overload their engines and are then very hesitant to change it. Changing an impeller is a lot harder than turning a screw.

Yes your right reverse jetting uses a lot of fuel. Some one said to me years ago. You have to feed the ponies

 

[Dorny119]

This may sound silly but does anybody think that the fuel dribble can be made worse by having the carbs oriented with the diaphragm on the outside? I see almost everyone's setup has the diaphragm on the inside facing the motor... That is how i had mine before i made this reed change and it never dribbled. Any thoughts?

 

[MTRHEAD]

This may sound silly but does anybody think that the fuel dribble can be made worse by having the carbs oriented with the diaphragm on the outside? I see almost everyone's setup has the diaphragm on the inside facing the motor... That is how i had mine before i made this reed change and it never dribbled. Any thoughts?

Doubt it. You have described other fundamental issues. Did you check those orings around seats? I had some fail this year that caused fueling issues. They dry out and let fuel by. What needle/seat size are you using? If it’s small and you’re using the real small spring to get the pop off sometimes those don’t hold under vibrations.

You can always try a higher pop off and see what happens? Assuming orings are good

 

[Dorny119]

I have not checked those yet, as i am going to purchase a 2.5 and stronger springs ( i don't know what color is in mine right now, as the colors tend to be confusing, and i did not have them labeled, but i will collect a labeled set from now on). AKA i will putting in a fresh o-ring any way. Just need time to go grab those from the local shop (showtime).

I did change my jetting to 135 pilot and 125 main, placed two anti-siphon valves on the kidney plate, and oriented my carbs how i had them before (they were digging into my exhaust tube anyway). The collection of those things did help! The ability to idle improved. However, it is still awful. The last thing to do before i go drilling out my restrictor is to get new needle/seet/oring and up the spring weight!

 

[MTRHEAD]

I would keep your 2.3's, make sure they are not shot and maybe even 2.0's if your into tuning. (that's me) I own multiple 60+ MPH standups and none of them require 2.5's even with 46 Mik's or Novi 50mm Carbs. Only time I ever "needed" 2.5 was racing with MS109 fuel or in my race only GP1200R. Get yourself a set of 115, 95, and 80 springs (cheap)

Your jetting numbers still sound heavy in a general sense... Also sometimes motors that are ported or cleaned up like smaller or similar size jets as stock. Hard for people to wrap their heads around that but it's true many times.

Recommend reading post #2 and then spending time reading that Mikuni tuning guide/example

 

[Quinc]

You got to go with 1.8's! The springs are different if you really look at them. But you need to check with a gauge. Even buying two of the same springs may give different results.

@MTRHEAD When you test pop off on dual carbs, do they both pop at the same time, or is within 3-5psi good enough?

115 gram = gold spring
95 gram = dull silver spring
80 gram = black spring
65 gram = shiny silver spring