Blaster 38mm SBN pop off modification

[96superjet]

So, I took a set of 38mm Mikuni SBNs and pulled the choke shafts, pressed in new brass primer nipples and plugs, swapped high and low speed adjusters with T handles and installed new rebuild kits on each. They're now sitting on an OEM intake which is connected to a stock 701 motor with a Factory B pipe limited chamber. Originally these carbs had a pop off value in the mid 50s, but I've now pulled two of the three screens and removed the choke plates and shafts. I'm certain this reduces air velocity, which would require me to drop the pop off value, but how much of a drop?

I switched springs and was able to get it down to 26 PSI using the stock 1.5 needle and seat. Is that too low for simply removing two screens and choke plates and shafts? Should I be somewhere in the 30s or even in the 40s? Anybody who's done a similar mod and and is running a good setup,would you please weigh in.


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[air blair]

You lost me on pulling 2 of the three screens. Going to am air filters your right on the money or 2.0 n&s to get it a we bit lower


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[96superjet]

You lost me on pulling 2 of the three screens. Going to am air filters your right on the money or 2.0 n&s to get it a we bit lower


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I'm not using aftermarket flame arrestors, I'm sticking with the OEM air box. Inside the air box, directly above the carbs is a layer of three stainless steel screens which are designed to keep any sparks/flames from shooting back out of the carbs. The three screens are also a bit restrictive as far as air flow is concerned. You can disassemble the rubber grommet which holds the three screens in place and remove two of them, thereby increasing the quantity of air which can get to your motor, but as a side effect reduces the velocity of the air as it speeds down past your carbs. Imagine using a larger hose on your shop-vac, more air movement, but slower velocity.

This is why I'm wondering about just HOW much lower on pop off to go. I went from stock mid 50s to 26 PSI. It's half the pop off value. Is that way too far?


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[air blair]

I seen an old oem wetjet similar to what you have done wonder what their pop off is set oem at


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[McDog]

You don't have to change anything after a screen reduction and choke removal. At the most the screws could be opened up a tiny bit more.

 

[Big Kahuna]

Only reason to change popoff would be if you changed needle and seats.

Main Jet: 135 (130 stock)
Pilot Jet: 70 (stock)
High speed screw: 1
2
turns out from closed
Low speed screw: 1 1/4 turns out form closed
Needle & Seat: stock (55 psi)
Spring: Stock (95 gram

High Speed is showing a typo on their site. cannot remember if it should be 1/2 turn out or 1 1/2 turn out.

 

[smithrookie]

I just put together a 701 blaster with a riva red pipe over the winter. Went with 140 mains and 75 pilots with all but one screen removed on stock air box and originally set it at 40psi pop off... ski would barely even run. I lowered it to about 30 psi with the 65 gram spring on the 1.5 needle seat and it ran much better but still felt a bit week compared to other motors I've had so I swapped a 2.0 needle seat and lowered pop off to 24psi and it really took off and feels great. Everything I've read says the dual 38s like a lot of pop off but some motors act differently

 

[Quinc]

I just put together a 701 blaster with a riva red pipe over the winter. Went with 149 mains and 75 pilots with all but one screen removed on stock air box and originally set it at 40psi pop off... ski would barely even run. I lowered it to about 30 psi with the 65 gram spring on the 1.5 needle seat and it ran much better but still felt a bit week compared to other motors I've had so I swapped a 2.0 needle seat and lowered pop off to 24psi and it really took off and feels great. Everything I've read says the dual 38s like a lot of pop off but some motors act differently

I am going to give that a try on my blaster. I think I am right around 38psi right now.

 

[G-Body]

I seen an old oem wetjet similar to what you have done wonder what their pop off is set oem at

Both of the wet jet 46's that I have disassembled were 1.5n/s gold spring 55psi pop off. I swapped coffman sizzler pipes on both motors and ended up putting the pop off much lower on both. Both have 2.0 n/s one is an 80 gram spring, the other motor uses a 65 gram cause I didn't have a second 80 gram. The 65 gram spring is slightly rich on high speed deceleration.

 

[2HIGH4U]

38's love around 38-40psi

 

[96superjet]

I just put together a 701 blaster with a riva red pipe over the winter. Went with 140 mains and 75 pilots with all but one screen removed on stock air box and originally set it at 40psi pop off... ski would barely even run. I lowered it to about 30 psi with the 65 gram spring on the 1.5 needle seat and it ran much better but still felt a bit week compared to other motors I've had so I swapped a 2.0 needle seat and lowered pop off to 24psi and it really took off and feels great. Everything I've read says the dual 38s like a lot of pop off but some motors act differently

@smithrookie, was the motor you used in your build a stock 62t case, 61x cylinder motor or had the cases or cylinder been ported?

Remember a two stroke motor is nothing more then a heavy air pump and if you port either the case or the cylinder, you are changing the amount as well as the velocity of the air flowing into the motor through your carbs. I suspect switching to an aftermarket exhaust also changes these factors as air can escape your heavy air pump much faster and with less resistance.

Point is, if you built your Blaster with a stock 701 motor, then we have very similar setups and the motor (air pump) should function similarly.

Post script: do you have an aftermarket higher compression head on your Blaster?


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[smithrookie]

My motor is stock besides a riva red and a ada girdled head running 160 psi compression