Infamous Venturi Dribble

[DylanS]

My engine builder recommended a 70 jet. He did all of the modifications to the carbs I'm running and I've had a super tough time getting rid of a 1/4 throttle dead spot. Also had rear carb dripping like yours. I've tried different popoff pressures, seats, springs, and jetting. I was able to move the dead spot Lower or higher in the throttle position but never eliminated it. Plus terrible fuel economy. Finally got a gauge and measured it. Tried the 80 jet and it cleared right up. Thing you have to consider is your tuner can give you starting points but every ski is different. What might work on one ski might not work on another. Definitely worth checking especially if nothing else is working.

Very odd that it seems to be only on the rear carb if it is a fuel pressure issue.. anyone have any idea on why that is? See I had assumed I would need a bigger return jet too but when Ed told me to run specifically a 60 I guess I didn’t bother questioning him because he’s Ed from full spectrum lol. He had mentioned something along the lines of moving from a 60 to a 65 jet in your return will change the fuel pressure by 2.5 lbs but I can’t exactly remember. I’ll have to give it a check regardless, he seemed busy and didn’t explain his reasoning and I was just happy to hear from him. I assumed my case pressure would be higher on my stroker and I would need to compensate with a bigger return jet..

 

[fastgtfairlane]

I'm running a stock stroke 62t ported cylinder bored to 83mm. 35cc domes with tnt as chamber blew 197/200psi compression on a mac comp gauge. I'm running the "R" series black mikuni 44s. I was shocked at how much more pressure my setup has than predicted. I've tried several jetting and popoff settings from reverse jetting to higher popoff with bigger jets and could never get it dialed completely. Since I've changed the fuel pressure I'm gonna put the original jetting and popoff recommended to me and see how it runs.

 

[DylanS]

I'm running a stock stroke 62t ported cylinder bored to 83mm. 35cc domes with tnt as chamber blew 197/200psi compression on a mac comp gauge. I'm running the "R" series black mikuni 44s. I was shocked at how much more pressure my setup has than predicted. I've tried several jetting and popoff settings from reverse jetting to higher popoff with bigger jets and could never get it dialed completely. Since I've changed the fuel pressure I'm gonna put the original jetting and popoff recommended to me and see how it runs.

Interesting post your results then!!

 

[kevbo]

The rear of the engine moves more than the front because there's less dead weight back there. Ive always been able to use the strongest springs on dual carb applications with good results.

 

[DylanS]

Glad I started this thread before I checked anything!!!

Piece of something in the seat.. lol
Must have been some dirt somewhere after the fuel filter
I’m going to double up on check valves to be safe.
Will post results
Thank you everyone for your info and input

 

[DylanS]

No more dribble, thing is screaming on the stand. Sounds better then I’m used to!!!

 

[Harbor]

Glad I started this thread before I checked anything!!!

Piece of something in the seat.. lol
Must have been some dirt somewhere after the fuel filter
I’m going to double up on check valves to be safe.
Will post results
Thank you everyone for your info and input
View attachment 411344

Glad you got her fixed!

Note: One way to isolate if it’s a carb issue or vibration causing it, is swap the carb front to rear... This is what I did and found neither carb dribbled when on the front cyl and both carbs dribbled when on rear cylinder....

 

[Jcary85]

Glad I started this thread before I checked anything!!!

Piece of something in the seat.. lol
Must have been some dirt somewhere after the fuel filter
I’m going to double up on check valves to be safe.
Will post results
Thank you everyone for your info and input
View attachment 411344

Glad it’s fixed! This might sound stupid but check carefully there’s not already two check valves there. They often stick together and it’s really hard to tell there are two.

 

[Wiseguy92]

I had a 927 with the same
Issue with 48 specs. The return pressure is important. I think I was bigger than Ed recommend like 70 or 80 but don’t be afraid to go bigger and lower it to see how it responds.
What I found to help the most was making my own high speed check valves from thicker material. You can buy plastic “precision” brand shim stock and make what ever you want. Stock valves are .0015” thick. I finished at .005” on one carb and .007” on the other. Don’t want to stiff just enuff to stop the dribble. Made a world of difference


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[DylanS]

I had a 927 with the same
Issue with 48 specs. The return pressure is important. I think I was bigger than Ed recommend like 70 or 80 but don’t be afraid to go bigger and lower it to see how it responds.
What I found to help the most was making my own high speed check valves from thicker material. You can buy plastic “precision” brand shim stock and make what ever you want. Stock valves are .0015” thick. I finished at .005” on one carb and .007” on the other. Don’t want to stiff just enuff to stop the dribble. Made a world of difference


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Yeah another user on here generously sent me a sheet or two of the thicker plastic a year back, it does make a big difference. I had the ski out today and it’s running pretty good without any major tuning. Ended up moving my pop off up to 28psi. There is a decent bit of raw fuel being dumped ontop of the butterflies as the engine decelerates with the throttle closed but from my understanding there’s no way to combat that outside of rasing pop off/check valves. I don’t have any jets up here at the lake besides a 75 but I’ll give her a try at some point in the return line. So far so good. Pretty flat on the bottom but it screams on the mid/top. But I haven’t played with anything quite yet.
I had tried all sorts of return jet sizes a year or so ago but I’d like to spend a day at the lake and properly test each one and catalog my results when the ski is a bit more dialed in.
I need to get out with a fuel pressure gauge and get a good baseline before I go ham tuning it in.
Having issues with the coupler blowing off the exhaust and it isn’t from being too hot. Thinking about trying this prototype centerbleed pipe at some point too.

A buddy of mine had a 850 with the exact same pipe and MSD set up on dual 48’s and it was absolutely insane. We have never been able to get our 927’s to run quite like that motor!! The potential is there but the tuning isn’t ! Lol

 

[Wiseguy92]

Sounds like your on it. Lots of pieces to get dialed for that bottom end to really sharpen up.
I burned out a few pipe couplers before I got my mani inject jet correct. Think I was around a 110. I also used inline flow controls I designed to keep the pipe and box from loading up on water going trick to trick which can really soften the power level.
My WDK pipe had a foam block with two lord mounts for support up in the front. That motor is twisting hard on its mounts, the pipe needs to be able to move with it, that could be why your tearing the couplers out. McMaster carr has nice large OD water hose you can buy by the foot. Much less $$ than silicone and they last If the conditions are right.


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[DylanS]

A buddy of mine had a set of gp800 kidney blocks laying around and it seemed to clear up that dribble off idle and under deceleration while being able to run a lower pop off. Got everything sorted out but it’s very soft on the bottom and there is no hesitation on response after putting a minute on the ski at 1/4 throttle consistently. Pulls hard on the top and mid.
I can’t remember how my Dasa felt anymore and it was the only other big bore I had to compare, but I swear I should be getting a hit sooner than 1/2 throttle lol.
the cyl’s were race ported by Lowell so I’m starting to suspect the raised exhaust ports may be the culprit. Someone PLEASE remind me what the throttle response is like on an actual freestyle motor!!

 

[Wiseguy92]

Here is my 927. You can her and see how responsive it is when all is right.


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[DylanS]

Here is my 927. You can her and see how responsive it is when all is right.


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That’s the ticket, thanks man.
Yeah mine sounds good, but there is a slight hesitation on the bottom, almost a rich condition if I had to say so.
I swore I didn’t need to be half way on the throttle on the last 927’s to have it hit but there’s all sorts of weird stuff going on with this motor.
the pipe stinger diameter was necked down too for when it was being run on a 850 something. Going to try some gp800 style kidney blocks and a lower pop off. Might be chasing my tail on these cylinders with the high exhaust ports.
That video was sick though haha !! What hull??

 

[Wiseguy92]

It’s the RIP hull glass. I loved that ski! Just sold it a few weeks ago as most of my boys don’t ride any longer.

I believe the stinger on my WDK pipe was 40 to 44 mm. I cut the original off and went much bigger for freestyle. Not sure if that would help you or not but it did help me


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[DylanS]

It’s the RIP hull glass. I loved that ski! Just sold it a few weeks ago as most of my boys don’t ride any longer.

I believe the stinger on my WDK pipe was 40 to 44 mm. I cut the original off and went much bigger for freestyle. Not sure if that would help you or not but it did help me


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It’s the RIP hull glass. I loved that ski! Just sold it a few weeks ago as most of my boys don’t ride any longer.

I believe the stinger on my WDK pipe was 40 to 44 mm. I cut the original off and went much bigger for freestyle. Not sure if that would help you or not but it did help me


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I’m going to measure mine tonight (ended up being 38mm) but it’s for sure way smaller then it should be. Would make sense why the hit comes on in the mid/high too, would be at least part of the problem.
thanks for the info man!!!
Just measured the stamped pipe stinger, the ID is 49 mm.