What can cause a Mikuni Carb to lose its prime after sitting 2 hours?

[777funk]

If I restart in 10 minutes, it will fire right up. If I wait a few hours, it's like I ran it out of gas. I have to crank for 3 or 4 15 second intervals before I get a sputter then it's back to normal.

EDIT: I see someone before me replaced fuel lines. I don't believe there are any check valves... I see check valves on the return line and breather line on the fuel system diagram. I bet this is it.

Both skis have rebult carbs that hold pressure and pop off at the correct pressure.

 

[Lothsahn]

Are you holding the choke when you try to start it?
Is your choke functioning? (check to make sure the cable is hooked up and adjusting the choke on the carb)
Have you replaced the fuel lines? The fuel lines can dissolve internally and make a green goo which clogs up the carbureator and can make the engine run lean (and destroy itself).

Even with a properly rebuilt carb, you have to pull the choke all the way out when the engine is cold and crank for a little while. My GTS starts after about 5 seconds of cranking when cold, and instantly when warm. 3 or 4 15 second cranks is not normal.

It sounds like you may have a fuel delivery issue. If you haven't replaced the fuel lines, do so first. You'll fight carb issues forever. If you're unsure if the lines have been replaced, see if the fuel lines are grey. If they're grey, they're probably original and need to go.

When is the last time you rebuilt the carbs? The fuel pump in the mikuni is a plastic diaphragm with two one-way valves driven off vacuum from the engine. It's a short hose between the engine block and the mag (forward) carb. If that hose is leaking, you may not be getting good fuel pressure.

The next place to look is the small fuel filter inside the carb. If can get clogged and not allow as much fuel through. This can especially happen if you haven't replaced the fuel lines. However, if you're going to tear the carb apart, buy an official, OEM Mikuni rebuild kit and rebuild the carb. Don't buy one on Amazon--go to a reputable 2cycle seller, such as pwcmuscle or osdparts.com). The non-OEM carb kits don't work well.

I rebuilt my carbs on my 1996 GTS and the difference was night and day. It idles smooth, revs well, launches as you'd expect, and starts easily.

Good luck!

P.S. if you're asking why carbureators require a choke, and why they can be hard to start while cold, it's because gas can condense inside a cold carbureator and intake manifold, causing not enough fuel in the mixture. You have to covercome that condensation until the engine starts and warms up.

 

[777funk]

All good advice. In this case, I have it running out of a clean container of mixed gas. I tried it choked and without choking. I can tell it's not primed because it takes quite a while to get it to fire solid (sputters and coughs until it's primed), but if I give it a splash of gas, it fires right away and runs for a second.

I'm thinking a check valve is bad or gas is simply leaking out. I cleaned the carb but everything "looked" to be ok and was pliable so I just put it all back together with the original parts. I have done this many times with chainsaws with success. I have a carb kit on it's way, I'm sure there are one or more parts leaking.

Once it primes, it runs great and restarts. But let it sit a couple hours and it's slow to start. I have another of the almost identical jet ski (and Mikuni carb) and it fires up instantly (without choking it). So I know it's possible.

The first place to start is to replace the parts in this carb. I can tell whoever had it before me used ethanol 10% gas and let it sit a few years.

 

[etemplet]

It will also not start if it is flooded which happens when the needle/seat is leaking. The longer it sits the worse it gets. You could pressure up the fuel system with air to see if you have any leaks. These carbs are funny. Sometimes these bastards start up as soon as you hit the button after sitting 6 months in the weeds. LOL When you figure that out let me know. Good Luck !!!

 

[777funk]

It's starting great with a little gas down the carb so I'm guessing it's not getting fuel at the jets until the pump gets it primed. Then it runs great.

 

[JeremyD615]

You need to pull the carbs and open up the fuel pump side of the mag carb (if we’re talking about a dual carb setup.) Take a look at the check valves, and look to see if there is an obvious spot where the clear plastic diaphragm is leaking around the edges. I’ve seen multiple instances of the diaphragms not completely sealing and having trouble building enough pressure to get started. Usually in those cases, just taking them apart and resealing the pump side of the carb fixes it.

 

[777funk]

I have two of these and both start and run great. BUT, start the next day and it's crank crank crank until the fuel system is primed. It then runs great again. I checked over the carbs and all looks good (including the pump check valves).

EDIT: I see someone before me replaced fuel lines. I don't believe there are any check valves on the lines themselves... I see check valves on the return line and breather line on the fuel system diagram. I bet this is it.

 

[Lothsahn]

I have two of these and both start and run great. BUT, start the next day and it's crank crank crank until the fuel system is primed. It then runs great again. I checked over the carbs and all looks good (including the pump check valves).

EDIT: I see someone before me replaced fuel lines. I don't believe there are any check valves on the lines themselves... I see check valves on the return line and breather line on the fuel system diagram. I bet this is it.

I agree with Jeremy. It should not take this long to prime the lines. I rebuilt the carb in my 1996 GTS and replaced the fuel lines at the same time. After replacing the fuel lines, I did nothing to prime the lines and the sea doo started after about 3 seconds of cranking. When the fuel pumps work, they pump fast.

I am not aware of any check valves in the fuel lines, but someone else here may know.

I would rebuild the carb(s) with genuine Mikuni parts.