94 Waverunner 3 - Mikuni SBN 44 jetting question Flame Arrestor

[robcarync]

My yamaha WRIII has been a nightmare Craigslist rebuild, but I am certain I am at the last thing to fix:

First, the ski is converted to pre mix. The entire fuel system has been replaced (lines, filter, check valves, NEW CARB, etc) New spark plugs, etc.

The ski has ~140 PSI compression on each cylinder.

And now I realized something: The previous owner never had the "Flame Arrestor Element" , or the stock metal screen inside the plastic box. I just now connected the dots and realized that with out the restriction in the intake tract, the manifold pressures will be off, and essentially make it difficult for the carb to draw the fuel in through the small, stock jets and needle/seat etc.

My Questions:

1)

Would this metal screen cause a problem if it's missing? Not from the flame arresting capabilities, but for carbeuration? The ski has basically no power past 1/4 throttle and seems to always run lean. It seems that with a much freer air flow, the vaccum pressures would be lower, and would thus have a harder time drawing fuel through the small stock jets etc.

2)

Should I increase the jetting size, Needle/seat, spring to accomodate the less restrictive air flow? How do I know what sizes are necessary? Currently, the ski has the stock plastic box but NOT the metal screen. I don't want to deal with rejetting if this is going to be a difficult guess and check process for figuring out the springs/jets/needle/seats.

3)

Should I purchase a stock flame arrestor from eBay for ~$65, or purchase an after market flame arrestor for around the same price? Is it necessary to rejet with an after market arrestor?

I would like to either:

A) Purchase a flame arrestor (stock or after market) that will work with the current stock jets in the carb

OR

B) Purchase new jets that will work with the stock air box without the metal screen.

I do not want to purchase an aftermarket arrestor and still have to re-jet!

 

[Faztbullet]

Re: 94 Waverunner 3 - Mikuni SBN 44 jetting question Flame Arrestor

Would this metal screen cause a problem if it's missing? Not from the flame arresting capabilities, but for carbeuration?

It may run just a tad lean at WOT without it but is not a restriction for carb or fuel system. It missing will not cause this problem.

Should I increase the jetting size, Needle/seat, spring to accomodate the less restrictive air flow?

You need to determine if its a lean problem, slowly closing the choke while running will richen mixture and if rpm picks up the carb has a problem.

Is it necessary to rejet with an after market arrestor?

No.......

The ski has basically no power past 1/4 throttle and seems to always run lean.

Verify that its a lean problem, if its not look at the reed blocks as they are prone to cracking...


http://www.atlanticjetsports.com/yamaha_-_flame_arrestor_carb_chart.htm

 

[robcarync]

Re: 94 Waverunner 3 - Mikuni SBN 44 jetting question Flame Arrestor

I see, I initially did not think the flame arrestor would cause a carb mixture problem but my friend mentioned he had the same type of thing happen on his carbed 4x4...the previous owner took the air box off to increase the air flow, but the freer air flow caused the pressures to change, but after installing larger jets, the engine ran fine.

The mikuni owners manual specifically says that when using a less restrictive intake, the pop off pressure must drop and the jets will have to be replaced due to the differing manifold pressures.

Also, atlanticjetsports has the stock mikuni carb, and the high performance mikuni carb. The high performance carb had the choke removed for higher air flow, and it also had larger jets and a larger needle valve than the stock carb with the choke. This lead me to believe that freeing up the intake path as little as removing a choke, would cause a change enough needed to rejet. Thus, completely removing the flame arrestor element, would probably also change the pressures enough to need a rejet. Is this incorrect?

I just ask because these were all my sources that made it seem plausible that the intake pressures are off without an intake restriction, which would cause a calibration issue. As I have said, I have rebuilt the fuel system, rebuilt the original carb 2 times, purchased a brand new stock carb to eliminate re-build errors, replaced all gaskets on the intake side, replaced exhaust gaskets when I was replacing the starter, checked the compression, checked all check valves, lines, filters, etc. The reeds looked fine when I had the manifolds off replacing gaskets. I have ridden the ski before the engine started running rough, which leads me to believe the drive line is functional. I have replaced the spark plugs, and have verified the spark plugs are sparking by using an inductance timing light.

That gives me Spark, Air, and Fuel. Capacitor Discharge Ignition like on this ski rarely goes bad, and I have already verified the plugs are firing. There is plenty of air, and plenty of premixed fuel. I installed clear fuel lines running to the carb to verify the fuel is flowing. All lines are secured with hose clamps to ensure air tightness. Everything just seems to point to the mixture of the air and fuel. A brand new stock carb ran the same as the original carb after I rebuilt it. The machine is completely stock except for a missing flame arrestor. It just seems thats the only variable I haven't accounted for so far.

Any other ideas? Maybe I'm missing something.