popoff spring rate accuracy

[amp3d]

Hey everyone,
Just in the process of rebuilding the carbs on a "new to me" superjet with dual 38s.
I pressure tested the popoff and I'm getting about 38psi on my gauge with the stock 115gram springs (at least they look like 115g according to other posts on here)

I purchased some 80gram springs and put them in and tested the popoff and my gauge reads consistently around 22-24 psi.
I know that calibrations of different gauges can be off, but these readings seem a little low to be at least based on the charts I've seen on here.

I'm curious if anyone else has noticed this?

When I put the carbs back together later today, should I stick with the 115's popping off at "38" psi or should I give the 80 gram "22psi" a go.
This is a new to me ski with a brand new stock sbt motor dropped in, only ran it for a few minutes with the previous owners carb settings which were a disaster lol

 

[mikesx]

Hey everyone,
Just in the process of rebuilding the carbs on a "new to me" superjet with dual 38s.
I pressure tested the popoff and I'm getting about 38psi on my gauge with the stock 115gram springs (at least they look like 115g according to other posts on here)

I purchased some 80gram springs and put them in and tested the popoff and my gauge reads consistently around 22-24 psi.
I know that calibrations of different gauges can be off, but these readings seem a little low to be at least based on the charts I've seen on here.

I'm curious if anyone else has noticed this?

When I put the carbs back together later today, should I stick with the 115's popping off at "38" psi or should I give the 80 gram "22psi" a go.
This is a new to me ski with a brand new stock sbt motor dropped in, only ran it for a few minutes with the previous owners carb settings which were a disaster lol

Are they genuine Mikuni springs? Aftermarket springs are a waste of time.

 

[amp3d]

Are they genuine Mikuni springs? Aftermarket springs are a waste of time.

The 80gram ones I got are genuine mikuni because I bought them.

I cant speak for what appears to be oem 115.

I guess I'll just run the silver ones that read 38psi unless folks think my 22 sounds like a better reading?

 

[Storbeck]

I've had them measure spot on and I've had them measure pretty far off. Always genuine mikuni as far as I know. I have never really looked into it because I think as long as your pop off is reasonable and consistent it can be accommodate with jetting ( my opinion and not a particularly expert opinion so take that for what it's worth). I've seen bending if the little arm affect pop off, and I've seen a bent arm causing the diaphragm to contact the cover plate and cause low pop off when the cover plate is on, for what that's worth.

 

[Storbeck]

Are you sure you have 1.5 seats?

 

[kevbo]

Using a lighter spring will make the bottom screw and pilot jets richer, it wont have much effect on the main jet and top screw. If its a new engine always open the top screws until it loses power at full throttle and run a tank through it tuned that way. You want to limit combustion temps while the piston is wearing in.

 

[smoofers]

Mikuni springs are all over the map, regardless of what anybody says. I've set up many a carb or carbs where with brand new genuine Mikuni needles and seats, the same arm (new or old) and genuine Mikuni springs I've never come closer than 3psi in either direction. I recommend using a popoff gauge to get a determined repeatable PSI value. I've had carbs with new needles an seats and new Mikuni springs *of the same color* (even swapped arms between carbs to rule out that variable - which arm angle shape, doesn't really affect popoff value) and had a 17 psi difference between carbs. Measure it, don't falsely predict it.

 

[amp3d]

Mikuni springs are all over the map, regardless of what anybody says. I've set up many a carb or carbs where with brand new genuine Mikuni needles and seats, the same arm (new or old) and genuine Mikuni springs I've never come closer than 3psi in either direction. I recommend using a popoff gauge to get a determined repeatable PSI value. I've had carbs with new needles an seats and new Mikuni springs *of the same color* (even swapped arms between carbs to rule out that variable - which arm angle shape, doesn't really affect popoff value) and had a 17 psi difference between carbs. Measure it, don't falsely predict it.

I was able to get 38 over and over on both, so I'm just going to be happy with that for now. At least both are consistent, but a far cry from what the chart says they should be at.

 

[mikesx]

Mikuni springs are all over the map, regardless of what anybody says. I've set up many a carb or carbs where with brand new genuine Mikuni needles and seats, the same arm (new or old) and genuine Mikuni springs I've never come closer than 3psi in either direction. I recommend using a popoff gauge to get a determined repeatable PSI value. I've had carbs with new needles an seats and new Mikuni springs *of the same color* (even swapped arms between carbs to rule out that variable - which arm angle shape, doesn't really affect popoff value) and had a 17 psi difference between carbs. Measure it, don't falsely predict it.

I’ve dialed in carbs with Mikuni springs and aftermarket, and in my experience Mikuni springs stay where you leave them (pop off psi) , after markets don’t. I’ve had carbs that were dialed perfect with aftermarket go lean and after inspection found to change as much as 10 psi. Just my 2 cents.

 

[amp3d]

I’ve dialed in carbs with Mikuni springs and aftermarket, and in my experience Mikuni springs stay where you leave them (pop off psi) , after markets don’t. I’ve had carbs that were dialed perfect with aftermarket go lean and after inspection found to change as much as 10 psi. Just my 2 cents.

I've heard this before as well. Good to know thanks

 

[smoofers]

I’ve dialed in carbs with Mikuni springs and aftermarket, and in my experience Mikuni springs stay where you leave them (pop off psi) , after markets don’t. I’ve had carbs that were dialed perfect with aftermarket go lean and after inspection found to change as much as 10 psi. Just my 2 cents.

I don't disagree with you at all on that.

 

[JetManiac]

Hey everyone,
Just in the process of rebuilding the carbs on a "new to me" superjet with dual 38s.
I pressure tested the popoff and I'm getting about 38psi on my gauge with the stock 115gram springs (at least they look like 115g according to other posts on here)

Did you pull the seats and check the size? If they are the stock 1.5, then popoff expected with 155g is 55psi. Your tester may be off then.

 

[amp3d]

Did you pull the seats and check the size? If they are the stock 1.5, then popoff expected with 155g is 55psi. Your tester may be off then.

I actually replaced one of the needle and seats and can confirm 1.5. The springs I'm assuming are 115 but I can't be sure as they came with the ski.

I did buy some 80g springs and they popped off at 22psi, so perhaps my gauge is off as you suggest

 

[waxhead]

I’ve dialed in carbs with Mikuni springs and aftermarket, and in my experience Mikuni springs stay where you leave them (pop off psi) , after markets don’t. I’ve had carbs that were dialed perfect with aftermarket go lean and after inspection found to change as much as 10 psi. Just my 2 cents.

The spring losing tension should make it go rich to be fair. Im not sure how it would go lean

 

[kevbo]

As an example of how uncritical the pop off numbers are. On keihins or SBN44-46 I could adjust the whole part throttle range by stretching or clipping the spring until it responded perfectly. No jet changes required! Now I use 115g spring and change pilots since I have so many.

 

[mikesx]

The spring losing tension should make it go rich to be fair. Im not sure how it would go lean

Gained tension!! Pop went up 10 psi ! That was my experience with aftermarket springs. To be fair to the aftermarket though, I had purchased bulk from R&R Jetski , anyone remember them?, and they had been on the shelf sorted in a plastic Plano container for a loooong time. Maybe they age hardened? IDK……