150 VMAX carb gets on plane and then loses power

[bassinman215]

Pulled the top two carbs today. Very clean inside. Main jets looked great - no clogs. Rather than disassemble further, I'm reinstalling.

Fuel pumps and anti siphon valve are on order. Will keep everyone updated.

[StevenMCC]

Mine is doing same thing. I just ordered both pumps, but what is the anti siphon valve?

[bassinman215]

Anti siphon valve is on the gas tank. Its in the nipple that screws into the fuel fitting. I tried to take my entire fitting and pickup line out of the boat before realizing they are two separate pieces.

In my Stratos 285 I had to remove the livewell to get access to the anti siphon valve.

I have a new anti siphon valve in hand that is the wrong size. But, the amount of pressure it takes to work is lower than the valve I took out. Will update when I get the correct valve in as well as the two fuel pumps.

[bassinman215]

Took it out today. Fires right up. Accelerates fine. Cruised around for about 15-20 minutes at 45 mph without any issues.

However, my top speed is down to 52. Should be 57-58. None of the plugs were fouled but one looked a little dry. Will continue to troubleshoot and keep everyone posted. Pressing in the choke just caused it to lose speed.

[bassinman215]

Update: Haven't water tested the boat but went through everything I could think of.

There seemed like an excess amount of fuel under the cowling. Tightened the drain plugs on the carbs as well as the carb nuts that hold it to the intake manifold. Also fixed a gasket between the lower carb and the air box that wasn't sealing.

It also appears that the carb butterflies are not opening all the way. There is still 1/8-3/16" of travel left in the throttle roller when the throttle is wide open. I suspect this is why I'm not getting top speed. Should the carbs open to the stop at WOT?

All I can figure is that I messed up the adjustment on my carbs when I pulled two of them off. My idle is 625 rpm in neutral in my horse trough. That seems lower than I remember - IIRC it was 750-775 before.

Thoughts? Suggestions on what to do next, other than try a link and sync?

[cwilt]

if you pump the bulb and fuel comes out of the carbs...you need to re-adjust the floats/needle valves. there should be no fuel under the cowling, could cause an explosion. and yes the butterfly's should be open all the way at full throttle.

[bassinman215]

Needle valves seem to be sealing fine. No fuel comes through when I pump the bulb.

Fuel seemed to be a few drips around the bowl drain bolts. Not a lot of fuel, but enough to make me go looking for a source. No visible leaks when I start it up in the horse trough.

[bassinman215]

FIXED!!!!!!

Not exactly sure what the final problem was. Ended up cleaning the main jets, syncing the carbs, and replacing a gasket between the air box and the bottom carb that was partially blocking a venturi. My initial problem was the anti siphon valve at the tank.

After doing all of this, the carbs aren't adjusted to hit the throttle stop. They still have an 1/8" of clearance.

I'm convinced two strokes run on black magic and voodoo...

[Topwater All Day]

I'm a little late to this party, but I'm having the exact same problem with my '99 VMAX 150 Carb - starts, idles and runs great until I hit 5k rpm, then it drops down to just over 4k rpm and won't go higher. I checked all the fuel lines and filter, replaced the fuel pumps, put new plugs in and finally just had a shop rebuild the carbs. It's still doing the same thing. When I push the mechanical choke at the key, it gets worse and the boat slows down - too much fuel???

The shop that did the work re-inspected the carbs last week and he found a very small piece of debris in one of the carb bowls. It ran a little better, but still did the same thing yesterday on the water. I'm wondering if they all need to be looked at again?

Getting pretty frustrated dealing with this, and I'll never own a carb motor again! Any other ideas would be greatly appreciated! Hoping it's not the fuel siphon valve as that will be a pain to replace...

Thanks!

[Texas Rig]

Just went through the reduced rpms at WOT with my 2 stroke 150ProV. The carb check is easy to identify a fuel flow problem. Get a buddy and remove the cowling and intake cover over the carbs. Move throttle lever or hot foot to WOT position and see if butterfly (not choke plates) are laying flat. If so, you just determined it is not a throttle issue. Get a small container that you can spray gas from... I used a squeeze lime juice bottle. You and your buddy head to the lake, he sits on the back deck and you ease up on plane to WOT. He sprays gas into each barrel of the 3 carbs. If you have a fuel restriction then as soon as the gas hits the problem barrel, the motor rpms will jump up. If this test reveals no increase in rpms then I would check the coils in the stator.

[bassinman215]

Guys - Do the anti-siphon valve if you have these symptoms. If you can access the tank, its a simple job.