Carb Pop Off Pressure

[the MfM]

2000 Yamaha ls2000 65u non PV 1200 Mikuni SBN 44

Apparently it’s the greatest kept secret on the Internet. Anyone know the factory spec?

 

[the MfM]

My carbs had the gold springs and I measured 70 psi with the old springs. I bought Genuine Mikuni kits and the new gold springs are a bit shorter. With the new springs I measured 55 psi.


I tested with the old levers and new levers with no changes.

According to the mikuni chart the gold springs with the 1.5 needle should be @55. So that’s what I’m going with unless someone has a reason they Should be higher... the other spring in the kit is longer and held till 85psi.





@4x15mph

 

[4x15mph]

Sorry, i didnt test pop off since when i rebuilt i reused the springs to keep it simple. I do habe a mikuni manual that i can send you if you PM your email

 

[the MfM]

Sorry, i didnt test pop off since when i rebuilt i reused the springs to keep it simple. I do habe a mikuni manual that i can send you if you PM your email

Thanks, the mikuni manual is where I got that chart from.

I could not find any spec in the Yamaha service manual. Since I’m getting the 55 the chart says I’m supposed to with the gold springs and 1.5 needle I’m going to try that and see how it runs. The old springs might be from an aftermarket kit.

 

[4x15mph]

Thats a great manual. I sent one rack out over the winter and they are brand new but i’m not sure the adjustments will be perfect. Fingers crossed since it is hard to adjust now that they are all installled. Started it up today and engines fired right up. One side is a new rebuild that i also just did. I added some new rims - doesnt it look fast?

 

[the MfM]

What did you use for the initial H L settings?

 

[4x15mph]

For the rack i did i used the service manual
Low speed: 1 plus/minus 1/4
High : 3/4 plus/minus 1/4 for cyl #1, 3.
1 plus/minus 1/4 for cyl 2

 

[the MfM]

That’s what I was planning to go by... probably start on the rich side of those settings though.


Here’s a thread talking about the right recipe...but with an aftermarket flame arrestor so they are 1.5 out on the H side. 50-55 pop off though.

GP1200 carb recipe

Hi all, I'm about to fit GP1200 carbs on the Waveraider and wondering what jetting will work best. I dont have them yet but theres a full set incuding intake manifold and reedblocks ready to buy. Until I see them though I've no idea what's actually in them, are they 1.2 N/S with 100 lows and 130...

greenhulk.net

 

[the MfM]

Saw this on the group k site

http://www.groupk.com/yjetboat.htm

Sounds like the gold springs aren’t what were original... need to set the low out a little bit more with them.

 

[Mainah]

Quick backwoods tip if ever needed. You can test pop off pressure by taking the chuck off a pedestal bicycle pump and forcing the hose over the nipple on the carb. Of course a dedicated pop off tool works too.

Mikuini carbs are sensitive on the low side once everything else is right. Make sure low and high are adjusted right prior adjusting main. My experience has been that the main can get you close but it is usually the low that needs tweaked. All assuming right spring, good float and needle, right pop off, right jets, good needles, etc..

 

[4x15mph]

@the MfM How do you plan to adjust the settings once you have the rack installed and are on the water. I don't think I have access to the low/speed screws since they are buried below the flame arrestor and even with that off, I am still not sure how I can fine tune.... how have you done this

 

[the MfM]

@the MfM How do you plan to adjust the settings once you have the rack installed and are on the water. I don't think I have access to the low/speed screws since they are buried below the flame arrestor and even with that off, I am still not sure how I can fine tune.... how have you done this

I haven’t tried adjusting while on the boat. I was happy with how the spark plugs looked after a high speed run. Was a little rich at idle but never bogged down on acceleration so I left it alone.

I think the L side is easy to get to but the H is buried behind the fuel lines and under the air Box?

 

[the MfM]

Quick backwoods tip if ever needed. You can test pop off pressure by taking the chuck off a pedestal bicycle pump and forcing the hose over the nipple on the carb. Of course a dedicated pop off tool works too.

Mikuini carbs are sensitive on the low side once everything else is right. Make sure low and high are adjusted right prior adjusting main. My experience has been that the main can get you close but it is usually the low that needs tweaked. All assuming right spring, good float and needle, right pop off, right jets, good needles, etc..

Bike pumps not a bad idea. I only had a 0-30 psi tester. So I looked in the garage and the first thing that came to mind was an old gauge off one of those disposable Freon refill cans.

 

[the MfM]

Alright here’s my latest question/confusion....

I have the LATE sbn 44 carbs. Apparently the early 44’s used a slightly different fuel pump. The late 44’s went back to use the same fuel pump as the 38’s

Different gaskets and o rings are included in the kit for both versions.

When I took apart mine they all had the large o ring (part 1a) and a gasket (part 1) on either side of the clear plastic pump diaphragm. (Part 2)

From the diagram it’s pretty clear that you leave out the gasket below the clear diaphragm in favor of the O-ring for the sbn38’s.

It’s not clear about the late 44’s... it seems the lower gasket wouldn’t be needed though?

 

[the MfM]

The way they were

 

[the MfM]

They way I think it should be

 

[Mainah]

Sorry no idea which way for your application.

 

[4x15mph]

Not sure and i remember the mikuni kit instructions were clear as mud

 

[the MfM]

Yep not exactly obvious. And the insert that comes in the kit makes no mention of late vs early versions of the carb....


But I found my answer. Whoever rebuilt the carbs last was NOT supposed to use that extra gasket.

 

[4x15mph]

@the MfM - I just went through my emails back when I sent my carbs out. I asked for the settings and I was told they used:
1.2mm seat with 95 gram spring

Pop off wasn't tested since I was told they were all new OEM parts. I didn't push them on this since they are the experts

"They did recommend adding some castor base lube to the oil package...injection or premix
Castor oil protects the engine and carb internals like nothing else. it is expensive,, but you only need to use 1/4 of it in place of your regular oil. Maxima 927 or Klotz Benol"