importance of the correct return jet sizing

ryan0

ryan0

Location
kent island maryland
sneds said:
I wonder if others with totally different setups would see the same type of results? Can anyone comment?

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Click to expand...
2lick said:
Great write up and information on something I have never seen discussed. Thank you!

My question is does the Novi recommendation of
  • 1-2 psi at idle
  • 4-5 psi at 3/4 to wide open throttle
Apply to all motor/carb combinations?

I'm curious to check/set mine this week. Wonder if I'll notice any difference?
Click to expand...

Let me be the first to say i am no carb expert but my 2 cents like i said before a drilled return is a drilled return and novi tech says for best performance you should verify your setup is in that psi range. no where do they say what engine type this works for aka there are wayyyy to many variables in a engine build to list the magic return jet size for any one setup hence why you always should test it....

Again this test is only useful on carbs with the return drilled out. if you do not know what that is click on the link on reply #12

Also this is very important if you find out you do need to go up in return jet size like i did your most likely going to have to re jet your carbs cause it will run leaner due to the fact the fuel pumps are not pushing the fuel thew as hard as it was before..
 
OCD Solutions

OCD Solutions

Original, Clean and Dependable Solutions
Location
Rentz, GA
I bet if everyone tuned their fuel pressure within the same range, jetting specs would also become far more consistent from ski to ski.

I had about a dozen issues I was fighting with my first Seadoo BB build but there were 3 or 4 very little details that made tuning absolutely impossible. It took speaking to guys like Tim at Novi and a few other very well known engine builders, (Huge shout out to Bill O'Neil of Watercraft Magic), before I dug down far enough and got the key details that mattered the most. Getting the correct pitched prop for my setup and properly tuning the return circuit were two of the major turning points in my project.

They say the devil is in the details and this topic is a prime example of that.
 
ryan0

ryan0

Location
kent island maryland
@OCD Solutions Exactly my point to a tee and why i made this post i hate fooling with carbs with a passion and im sure there are a bunch of people on here that feel the same as i do and if that fuel pressure isn't in spec your going to be forever chasing your tail with tuning...
 
OCD Solutions

OCD Solutions

Original, Clean and Dependable Solutions
Location
Rentz, GA
Yes, and certain engines have their own little quirks that one needs to be aware of and tune for. In my case with the 951 bored out to 1040 and then mated with a Coffman pipe, it had very little to almost no pulse signal at an idle. It was recommended to run dual external fuel pumps for this reason as fuel starvation down low was very common. The problem then was that once the pipe came on, the pulse became very strong and therefore the fuel increase was staggering. I did as much as I could with tuning the fuel pressure and still ended up reverse jetted at something like 155 pilots and 130 mains and a serious lean condition around 5500RPM that I never could tune out.

You really have to start looking at your power plant as an ecosystem. Everything needs to work in concert and each system must compliment the other. Once you see it from that perspective, building and tuning becomes so much simpler.

Also, tuning and setup is far more math than intuition or instinct like most would like to think. The experts spout numbers off the top of their head but they have all done the math at one point so they can do that. I promise you, do the math at least once and suddenly all that 2-stroke smoke magic turns into good usable science.

But, if everything worked the first time, what would we ever learn?
 
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sneds

sneds

Location
Wasaga Beach
Has anyone gone through this process with Novi's and a Dasa 10mm 1100 or 5mm 900 by chance? I plan on ordering a gauge to test mine but, I thought if someone has done it already...

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ryan0

ryan0

Location
kent island maryland
At the very least get the idle to 2500 rpm 1-2psi fuel pressure dialed in on a trailer. Heres the deal tim said a stock mikuni SBN carb has a built in restrictor the size of a 30 jet hence the 60 return for duel carbs. He's drilled out this restrictor since he started building carbs back in '95 his reasoning was so you can fine tune you fuel pressure and to be able to get your fuel pressure back in spec. he said it becomes especially important to check if you have more than 800cc's or a stroker crank.
 
kevbo

kevbo

Location
dfw
Any fuel line pressure has the effect of lowering popoff. This will change your idle and pilot mixture a lot. He was able to set the idle mixture with line pressure alone. This is why using published carb specs is often foolish. The only advantage of low line pressure is so an air bubble won't lean the fuel mixture as much.
 
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sambo

sambo

I built a 4 mile stroker on 760 sleeves. I get lean hesitation when snap open wot..And can not get it to bog on wot. Even with super high speed jets. I'm.on daul 46s 135loe jet and 190 high on 3 turns and it still will not get enough fuel. I even removed the high jet and still no bog. I'm running stock return holes so surely this fuel pressure would be too high. But why am I still getting lean symptoms. There are no air leaks it's all been pressure tested and hold pressure for 30 minutes with average of 1 PSI drop over the 30 minutes.
Anyone had these issues. Cheers
 
V

Vortex

Location
Southeast
I was messing with this today with a ss ported 735, dry pipe, 48 novis. Started with 60 jet. Had about 3-4psi at idle, and it remained right around that range with 70 and 80 jet (closer to 3 with 80). Ended up trying a 140 jet and this got me within the 2psi range at idle-2500. And it did seem to idle a little smoother and the engine even had a kind of different "note" to it, hard to describe but in a basic sense, it sounded more like a stock ski. However I think this was too large of a restrictor as I had the carbs lose their fuel prime after the ski sat for maybe 10 minutes. Went back to 80 jet for now.

I don't run a dual primer kit at the moment but I think this in conjunction with the larger return jet may be on the right track.
 
stefan

stefan

Location
denmark
Vortex said:
I was messing with this today with a ss ported 735, dry pipe, 48 novis. Started with 60 jet. Had about 3-4psi at idle, and it remained right around that range with 70 and 80 jet (closer to 3 with 80). Ended up trying a 140 jet and this got me within the 2psi range at idle-2500. And it did seem to idle a little smoother and the engine even had a kind of different "note" to it, hard to describe but in a basic sense, it sounded more like a stock ski. However I think this was too large of a restrictor as I had the carbs lose their fuel prime after the ski sat for maybe 10 minutes. Went back to 80 jet for now.

I don't run a dual primer kit at the moment but I think this in conjunction with the larger return jet may be on the right track.
Click to expand...
maybe try a 100 next 80 too 140 is a big gap
 
stefan

stefan

Location
denmark
Just bumping this thread i measured my return fuel pressure att ideel, and it came in att 3.5 psi this was in the water i also have a 60 jet in the return i changed it too 120 (smallest i hade) and pressure droppede too 1.5 psi. It made night and day differens every thing worked better more power better hit more stady ideel. Only bad was it lost fuel pressure when sitting 30 min and needed too crank more for first start up, soo i bought a 110 jet and it helpede. My setup is
ET 1107
Msd tl
Fs49s
PFP
155 mag
 
ryan0

ryan0

Location
kent island maryland
Funny I hopped on here for the first time in a while looking up some other stuff and saw the notifications from this old post. Glad to hear it helped a few out.

one other thing I’d like to add to help you guys out it’s not as fine tuning as carb jets i.e. it takes 20 points to change a 1/2 psi so for instance if you have 3 psi with a 80 jet if you go to a 100 jet it should be 2.5psi.
 
H

Hillclimber

Location
Norwich United - Kingdom
ryan0 - I think you have saved me going mad! I live in the UK and been struggling to get my V8 Evinrude fitted with 8 Mikuni Super BN 44-40 carbs running correctly. Its fitted into a Sports race car. I am at the end of the 3 year build project and the engine suffers exactly the symptoms that your post describes. I spent all last weekend at a race meeting trying to get it running "Cleanly" but always appeared rich even with 80 pilot and 100 mains. At wits end, I joined and posted : http://www.pwctoday.com/showthread.php?t=494601 : and one of the posters lead me to this post of yours.
The SBN Mikuni manual mentions the fuel restrictors but not their importance - hence my tackling the jets rather than the potential cause. The race season has now finished here so will attack the problem with a new resolution after Christmas ready for a real run next year. THANKS for your post. attached is engine with CVT transmission.2012-07-11 03.27.25.jpgShelsley.jpg
 
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