Pop off pressure dual 38’s
- Thread starter TURBO JET
- Start date
- Location
- Ontario, Canada
Start with 32 psi - 65 gram shiny silver spring on 1.5 N&S, if you find it won't power up quite right, try the black 80 gram spring at 38 psi. Keep in mind this is just the typical numbers that turn up most often and by no means the standard. Every engine will respond differently and will need it's own fine tuning.
curious on why you recommend going up in popoff if it hesistates off start. If it is a clean hesitation I usually recommend going down in popoff assuming it is idling fine. IMO, stock 38’s can run decent popoff pressure, but once you go to far you get a clean hesitation with sometimes a hard start.
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john zigler
Vendor Account
- Location
- wisconsin
We run 1.5's with a 95 gram spring. Pilot jet ill make a difference on what you need for pop off specs.
- Location
- Ontario, Canada
It's not so much about hesitation as it is about timing. If the pressure is too low the fuel deleviery will occur too soon and make it either dribble out of the bombsite and be lazy on powering up, or if it's too high fuel won't arrive until it's too late making lean hestation. Starting off low is easier to identify lazy response for most people than a lean hesitation. I just went through this with another group of guys, they swore up and down their ski was rich but to a trained ear it was clearly lean. They ended up proving my point when they found a fuel leak, corrected the leak and it was back to it's normal. But the point is when we opened the screws way up and pounded the fuel to it, they could hear and feel the difference. Flooded and lazy. I like to start off low and lean out from there, higher popoff in a manner of speaking is doing that, restricting fuel delivery until a certain time/popoff pressure. If it's too high not enough fuel will be delivered at the right time, hence a lean condition. That's when you hear that rev limiter like sound at about 3/4 throttle, not enough fuel at the right time.
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- Location
- Ontario, Canada
That looks like it says 75, which is also the typical starting point. 75 pilots, 135 mains and tune from there if needed. Mine is running mostly the same setup as yours with 32 psi popoff, 1.5 N&S, 75 pilots, 135 mains, I believe the low screws are at about 1 to 1-1/8 turns out and highs are at about 1-1/4 turns out. And change your springs, the numbers you have are way too far off for shiny silver springs. Those sound like the stock springs which should be popping at about 55 psi. You may need to clip a spring too to get the numbers you need. Just clip about a 1/4 of a spiral at a time, it doesn't take much to change their pressures by a lot. My dual 46's needed both springs clipped to get an exact 21 psi. One needed a little more than half a spiral removed. They're very inconsistent so you really want to pay attention to that. And do your popoff testing according to each carb. Don't use one carb for all the testing, what one will give for pressure, the other might not due to slight changes in the passage ways of the carb. As tooling wears out during manufacturing, the passage ways will change slightly. Just a little bit smaller of a passage way will increase internal pressure.
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Vumad
Super Hero, with a cape!
- Location
- St. Pete, FL
I have been told in the past that cutting the springs changes the way they work, and that it is better to bend the arm very slightly. However, I would think that would change how they work with the diaphragms. Not sure which way is better. However I did find (in my limited experience) that when sticking with genuine mikuni parts I had a lot more consistency in my spring performance. The WSM and SBT parts were a lot more inconsistent where 2 brand new springs of the same tension gave different numbers even in the same carb with the same arms and needles. I also had a lot of issues with leaking needle and gasket kits with the SBT. That definitely makes me concerned about the accuracy of their jets. I think it's very important to start with making sure to have clean quality parts.
I also have very similar numbers as pro-pulsion. Shooting from the hip but I think mine are identical except I have 140 main jets. I posted them in another thread.
- Location
- Ontario, Canada
I actually have found WSM kits to be really quite good and noticed the Mikuni kits were very inconsistent lol. I can agree though that the SBT kits are just terrible. I noticed one day after only a few hours on SBT kits that fuel was squirting out of the metering diaphragm vent hole. Took the carbs apart and the rubber diaphragms would spin on the little metal discs. I believe they're supposed to be vulcanized in place or really well pressed at least. The fuel was getting past the rubber portion at the center post of the discs where they push on the needle valve arm, filling the atmospheric side and squirting out the vent hole. I went back to the WSM diaphragms and haven't had a problem since. Similarly, my ski is running on WSM kits now for 5 years and counting. I find with the Mikuni kits, the metering diaphragm will roll up like tacos when you expose them to air after they've been in contact with fuel. That and they swell and distort making them less than fun to get back in place. I usually have to let them dry out before putting them back in so they can come back to their original shape and size.
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- Location
- Ontario, Canada
That's still a little on the high side but good that they're equal at least.
Ok it’s back together because I did it last night at midnight lol so going to ride it today but now I know and I’ll take them apart again.
Do they sell different springs or should I change the needle to get a lower pop off
- Location
- Ontario, Canada
I'm pretty sure you can buy just springs. Get in touch with JetManiac for that, he's an awesome guy and really good prices...quite backed up on processing right now but he will get back to you. And yes you can change to a 2.0 needle, that should put your popoff down to about 34 going by your current numbers and needle.
- Location
- Annapolis, Maryland
They sell a bunch of different springs and needles/valves. Here is the table from the Mikuni SBN handbook that shows you the different pop off pressures you can set. If you haven't read through that yet you really need to. Repeatedly... lol I have read it at least a dozen times, not kidding.
Yup I’ve seen it that’s why I thought mine was high but then I didn’t know if that chart accounts for the B-Pipe and head thanks for responding I yes I’ll read it thoroughly
I have noticed that it’s hard to start some times which means I think correct me if I am wrong but that the pop off is to high
- Location
- Annapolis, Maryland
So the B-Pipe and Head have no affect on what the pop off pressure will measure. However, I believe those mods will sometimes require a change in the pop off pressure, if that makes sense.
As for the hard starting and pop off pressure. This is where things get a little complicated, and I myself have been dealing with very similar tunning issues on my home ported 718 that I just put in my new XFR. Someone correct me if I am wrong but unless you were seriously off on pop off pressure I don't believe that is what is causing your hard starting. Starting is controlled by the low speed adjuster and that is what you need to adjust to fix your hard starting.
I think about it like this, and again if I am wrong someone please correct me. But with the throttle bodies almost closed in the idle position, you should be generating so much vacuum that you can easily overcome the pop off pressure required to deliver fuel. But once you start to open up the throttle bodies past the idle position and up to 1/4 throttle this is when the amount of vacuum your motor generates drops and will vary depending on multiple factors such as how restrictive your intake filters/flame arresters are and how free flowing your transfer ports and exhaust system are. This is where adjusting the pop off pressure comes into play, not at idle. You have to adjust pop off pressure to suit the amount of vacuum generated by your motor during this transition phase from idle to over 1/4 throttle .Then once you are past about 1/4 throttle your motor should be once again generating so much vacuum, that the pop off pressure has little to no affect on fuel delivery. Take a look at this graph from the manual that shows where each circuit comes into play.
One more really important thing you need to check, is that you need to make sure that your carbs are in synch. Unscrew the idle adjustment screw so that it isn't touching and make sure that both butterfly valves are in the exact same position when closed and that they open up in synch. Use the screw mechanism that connects the shafts in between the two carbs to adjust this. When I put my motor back together I never reset mine and it made starting and tuning almost impossible.
- Location
- California
I would agree, for over 25 years of working on these things have always been told to never cut the springs.
Ok so just got back from the lake which is my test bed before hitting the ocean tomorrow and at first the ski ran like poop then it sorta cleaned itself out where it would run but not even 80% so I came back in and pulled the fuel pick up out noticed the reserve line was dry I disconnected the lines coming out of the fuel selector and blew on them and I must of cleared something out because we put it together and it ran better
So Now Full throttle and three quarter throttle are the same also still hard to start
My friend pulled away from me on his SXR and that shouldn’t happen
I still want to get the pop off pressure correct but also feel that the high and low will need adjusting
So Now Full throttle and three quarter throttle are the same also still hard to start
My friend pulled away from me on his SXR and that shouldn’t happen
I still want to get the pop off pressure correct but also feel that the high and low will need adjusting