Infamous Venturi Dribble

[DylanS]

Fired ski up tonight after finishing the build, front carb is completely fine, rear is spewing fuel at idle and throughout rpm range. The motor is a 927, and they are notorious for shaking the fuel out, but the front carb is completely fine.
Full spec 48’s, brand new 1.8 n/s, pop off set at 26psi and checked a million times. External fuel pump, pulse from front cylinder and back cylinder pulse capped. Same kidney blocks, check valves are new. Butterfly’s set in sync. What could possibly be making this big of a difference between the two carbs. Both cylinders are within 2 psi of eachother.
I am going to try doubling up on check valves on that carb and checking the Ns and pop off again tomorrow but I am baffled as to how this could be a carb (or cylinder) specific issue.
Thoughts?

 

[user_error]

You might have a piece of debris in your needle seat preventing the seat from sitting properly in the needle, allowing fuel to flow by. Happened to be once, I took the carbs back apart and found a tiny piece of funk in the bottom of the needle seat.

 

[Harbor]

Fired ski up tonight after finishing the build, front carb is completely fine, rear is spewing fuel at idle and throughout rpm range. The motor is a 927, and they are notorious for shaking the fuel out, but the front carb is completely fine.
Full spec 48’s, brand new 1.8 n/s, pop off set at 26psi and checked a million times. External fuel pump, pulse from front cylinder and back cylinder pulse capped. Same kidney blocks, check valves are new. Butterfly’s set in sync. What could possibly be making this big of a difference between the two carbs. Both cylinders are within 2 psi of eachother.
I am going to try doubling up on check valves on that carb and checking the Ns and pop off again tomorrow but I am baffled as to how this could be a carb (or cylinder) specific issue.
Thoughts?

My 16 mil does that too, front carb is fine, only rear carb is effected... doubling up the check valves on both carbs helps some people..

A different part number kidney block that has the passage make a right angle and dump higher up fixed mine... this passage is what the check valve covers... there is a thread about it and the part number for the kidneys are in it... think they were $86 new each.... I might not have time to look for the thread though...

Here is what I bought

 

[Jcary85]

Agreed with others - could be debris. I’ve also had the n/s outer oring fail causing the same thing. As others said, make sure check valves on kidney block are doubled. I think I also had this happen once and it was broken reeds causing a lot of outward pressure up the carbs which was enough to pull fuel out the Venturi surprisingly.

 

[Harbor]

I bumped the other thread Dylans, can’t copy as my phone is broke.

 

[DylanS]

My 16 mil does that too, front carb is fine, only rear carb is effected... doubling up the check valves on both carbs helps some people..

A different part number kidney block that has the passage make a right angle and dump higher up fixed mine... this passage is what the check valve covers... there is a thread about it and the part number for the kidneys are in it... think they were $86 new each.... I might not have time to look for the thread though...

Here is what I bought

I’m going to try a doubling up on the check valves tonight, and I’ll check the NS again. Just seemed odd to me, reeds are new, brand new everything and a fresh filter. I remember that kidney block trick, I’m going to order a set!! Thanks man

 

[smoofers]

This may sound stupid, but verify that the diaphragm button is not pushing the needle arm down when the square cover is screwed on.

 

[Wetbuffalo]

I’m going to try a doubling up on the check valves tonight, and I’ll check the NS again. Just seemed odd to me, reeds are new, brand new everything and a fresh filter. I remember that kidney block trick, I’m going to order a set!! Thanks man

What reeds are you running?

 

[fastgtfairlane]

Are you running a restrictor in the return fuel line? Have you checked your fuel pressure with a gauge?

 

[DylanS]

Are you running a restrictor in the return fuel line? Have you checked your fuel pressure with a gauge?

Yeah, 60 main as per Ed’s suggestion. I didn’t check pressure but if that was the issue both would be leaking instead of one or so I assume

 

[DylanS]

What reeds are you running?

Three stage Riva cut reeds for Full spectrums manifolds

 

[DylanS]

This may sound stupid, but verify that the diaphragm button is not pushing the needle arm down when the square cover is screwed on.

I had checked this when I was doing the pop off as well as arm clearance to carb surface but I will double check tonight

 

[fastgtfairlane]

You might want to get a fuel pressure gauge and check it. Could definitely cause issues you're having. According to novi, 1-2psi at idle is ideal. 4-5psi at wot. I have a ported 735 with dual 44s. 70 jet was recommended, actually got a gauge and checked pressure. Was at 3.5psi at idle. Put a 80 jet in there and it immediately ran smoother. 80 jet put me at 2.5psi at idle. Still high but much better. Going to try a 85 next. Youd be surprised how much difference it will make. Theres a really good thread on here worth reading. I'll find it and post for you

importance of the correct return jet sizing

I recently discovered after spending countless hours on here searching for advice and trying every logical jetting combo known to man I started to see a lot of people post issues similar to the problems I was having and struggling to get their ski tuned properly and it always seemed to ended up...

www.x-h2o.com

 

[DylanS]

You might want to get a fuel pressure gauge and check it. Could definitely cause issues you're having. According to novi, 1-2psi at idle is ideal. 4-5psi at wot. I have a ported 735 with dual 44s. 70 jet was recommended, actually got a gauge and checked pressure. Was at 3.5psi at idle. Put a 80 jet in there and it immediately ran smoother. 80 jet put me at 2.5psi at idle. Still high but much better. Going to try a 85 next. Youd be surprised how much difference it will make. Theres a really good thread on here worth reading. I'll find it and post for you

importance of the correct return jet sizing

I recently discovered after spending countless hours on here searching for advice and trying every logical jetting combo known to man I started to see a lot of people post issues similar to the problems I was having and struggling to get their ski tuned properly and it always seemed to ended up...

www.x-h2o.com

Yeah I gave it a read already, like I said i didn’t bother checking because the front carb wasn’t having the issue at all out of the same fuel pump. Ed from full spec was extremely adamant that anyone running anything other than a 60 return jet (on his carbs) was making a mistake when I spoke with him on the phone Because I had also been playing with my return jetting. I will tear it down tonight and if it’s not a obvious issue I’ll start at the fuel pressure

 

[fastgtfairlane]

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.

 

[CD155MX]

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.

So on your ski, less fuel pressure cleared up the dribble?

 

[fastgtfairlane]

Since you have an external pump, setup is a little different. The gauge goes on the supply side instead of the return side when using dual pumps.

 

[fastgtfairlane]

So on your ski, less fuel pressure cleared up the dribble?

It allowed me to run a lower popoff with no dribble which helped clear up the dead spot. The fuel pressure directly affects the popoff

 

[CD155MX]

It allowed me to run a lower popoff with no dribble which helped clear up the dead spot. The fuel pressure directly affects the popoff

Thanks for the insight. (watching this thread and trying to pickup some know-how)

 

[fastgtfairlane]

It was recommended to test while moving however I checked mine under load on the trailer. I will extend the hose to the gauge through the pole to see if it makes any difference in pressure moving vs on the trailer.