Pop off psi, what should I be aiming for?

[superjetincairns]

i have a freshly rebuilt 701 superjet motor, 82mm bore, riva red pipe,msd enhancer, k&n filters, ports mildly cleaned up, stock 38 mikunis, 75/135 jets. 1.5 needle &seats with 95gm springs. 1st ride out, won't rev past about 3500-4000rpm. At anything over 1/3 to 1/2 throttle it just dies, i confirmed it on the trailer.starts well & idles nicely. I checked the pop off pressures and both measured about 42-45psi. I've seen most people run 2 mm n/s with 95g springs which should give me 25-30psi (from memory) according to the mikuni chart. I swapped through all of my spare springs and got my pop off pressure down to 27-28 psi but havent been back on the water yet. Any thoughts on what pop off pressures I should be aiming for with 1.5mm n/s. Am I going the right way here or do I need to run 2mm n/s with the 95g springs.

 

[Harbor]

Got pretty close to the same set with type 4 dry pipe with 2.0 needle and seat

Can't remember jets and spring though.

 

[tomski]

If it dies you are lean.

 

[superjetincairns]

I figured it was too lean, like sooo lean it wouldn't even run up above half throttle. It felt like the pop offs weren't even opening at all. The 95 Gm springs came with my jets but when combined with the smaller 1.5 mm n/s result in a pretty high 45 psi POp off pressure. My 65 Gm spring/1.5 mm n/s combo will give me approx the same pop off pressure as the 95gram spring/2mm n/s combo according to the mikuni chart. Once I get my footholds & turf finished I guess I'll find out.

 

[JetManiac]

42psi popoff is good for your dual 38 setup with a/m flame arrestors. 38s like a higher popoff than larger and a/m carbs.

You may have other issues?
What are your high speed screws set at?

 

[Blue]

I agree with Jm. Sounds like other issues to me. Even if it was lean it who still rev up and eventually melt down the pistons.

 

[mybad]

My engine is setup like yours. I run 24 psi. Dont go by the spring....only popoff gauge.

 

[superjetincairns]

I set the screws at everything from 1/2 a turn to 2 turns out, made no difference at all. I even swapped back to original jets from before I rebuilt it, felt a little crisper but still wouldn't rev at all at anything over 1/2 throttle. I didn't have the pop off springs with me at the time so I didn't get a chance to swap & try them. I'm hoping it isn't the enhancer or coil causing me issues.

 

[JetManiac]

I set the screws at everything from 1/2 a turn to 2 turns out, made no difference at all. I even swapped back to original jets from before I rebuilt it, felt a little crisper but still wouldn't rev at all at anything over 1/2 throttle. I didn't have the pop off springs with me at the time so I didn't get a chance to swap & try them. I'm hoping it isn't the enhancer or coil causing me issues.

Check carbs again for issues, unless you feel it is electrical related somehow.

 

[Quinc]

My engine is setup like yours. I run 24 psi. Dont go by the spring....only popoff gauge.

This^^ I bought 3 of the same spring and all where different with the pop off gauge.. You can also take a ball point pin spring and cut it down to what you need.

 

[superjetincairns]

I did rebuild the carbs with a pretty average looking wsm rebuild kit and noticed that the little aluminium block on the diaphragm that actuates the pop off valves was a lot smaller than the original mikuni part. Is it possible that when the diaphragms are being pulled in they aren't touching the actuating arms and opening the popoff valves due to the reduced thickness of these little pieces of aluminium. They are at least a millimeter thinner than the mikuni parts.
Has anyone experienced this kind of thing happening? I didn't adjust the pop off actuating arms at all but think maybe I should or put the original mikuni diaphragms back in and see if they make a difference. They do look a bit crinkly and sad though.

 

[JetManiac]

Do you have the original oem popoff arms? Recommend putting those back in and to read the mikuni sbn manual concerning arm height.

 

[superjetincairns]

I didn't change the arms when I rebuilt the carbs but it has been raced and modded before I got it so anything could be in there really. Would you recommend I put the 95gm springs back in there when I put the original diaphragms back in and run with the higher 42 psi pop off pressure as a starting point. I haven't even started it up yet since I changed to the lighter springs and I'd rather go with changeing one thing at a time.

 

[JetManiac]

Do you know if the carbs were modded at all because any mods can change how they respond to jetting and popoff. It sounds like lower popoff has worked for many posting here, but I know that 35-40 psi has worked for us.

Another thing to check is the restrictor in the slave carb cover to make sure it is not blocked. Also check all jets and passages for blockages.

 

[superjetincairns]

I'm not sure what was done to the carbs apart from jetting and removing the chokes. I stripped them completely and put them through an ultrasonic cleaning bath before I reasembled them so im pretty sure nothing's blocked. I'll try the original diaphragms to begin with and go from there I think.

 

[Vumad]

Didn't read all the posts. I my stock motor stock porting is bored 1mm, stock carbs. Jetting is 75/140, popoff of 18. Ski runs great. Tuned by bad boys racing. Could go a tad leaner in the surf but I'm a rec rider and need that extra fuel for bouys.

 

[superjetincairns]

ok so I finally got back to the boat ramp for some more testing today. running it on the trailer. same jets as before, 75 pilots, 135 mains, low speed screws 1 turn, high speed 1.5 turns. jm supplied 95 gm popoff springs with 1.5 mm needle and seats.(measured 42-45psi popoff pressure)
I switched back to the original mikuni carb diaphragms in case the replacements were limiting the n/s opening as the aftermarket ones have a smaller actuating nipple than the originals (whatever you want to call it, the bit of alloy in the middle that presses on the arms)
It starts fine, idles reasonably well and feels crisp down low, but get to half throttle area and it bogs and dies still. pretty much the exact same symptoms as before. cant get it to rev past about 4000rpm with any load on it, i.e. in the water. It will rev to 7000 easily with a quick blip when its running on the hose in my driveway.(neighbours must love it)
Im going to try the lower pop off springs which will get me down to around 30 psi from the current 42 psi popoff pressure. I suppose i'll check the carbs for anything untoward while they are off. it has fresh fuel, mixed at 40:1, new ngk iridium plugs, and a fresh rebuilt engine from the crank up, 195-200 psi compression(not too sure of guage accuracy)
starting to do my head in!
Any bright ideas welcomed!

 

[Harbor]

Take out the iridium plugs too

 

[Fro Diesel]

My 38s like 22-28psi aftermarket arrestors

 

[superjetincairns]

What's wrong with iridium plugs? They've always been the best plugs for every engine I've had but this is my first superjet engine.