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Carb Rebuid = Weak Fuel Pump

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Barky_Dog

Barky_Dog

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Carb Rebuild = Weak Fuel Pump

So, I did a carburetor rebuild on 97 GSI, which has a single Mikuni carb. I really took my time and tried to carefully replace everything that came in the carb kit. Then I reinstalled the carb on the engine.

The problem is- I went to start it up, and the fuel pump is not providing a good suction now. The engine will run off of starter fluid just fine, but the carb never pulls any fuel from the tank.

I took the carb off again last night and opened up the section for the fuel pump. The clear diaphragm and all of the gaskets are there as they should be. The pulse line seems to be providing a strong pulse. I am stumped. :banghead:

I get the feeling that if the carb could actually get fuel, it would run great. Any ideas from the carb Jedi gurus in the community?

Thanks!

Barky
 
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Did you check the fuel filter? Is that full of fuel? If so, the problem is somewhere between there and the carb because it sounds like the carb should be functioning properly. I know it sounds dumb, but did you recheck all the fuel lines? Maybe one got mixed up? Just trying to help since I'm also rebuilding a carb right now and hope to god it works once I'm done!! Good luck man!
 
Are you putting the hoses back on the correct carb ports? The lowest port is fuel in (from the filter). The middle port is engine pulse. The upper port is fuel return to the tank. :cheers:
 
Thanks leepoffaith,

The fuel filter has a little bit of fuel in it, maybe 1/4th full. It doesn't seem to be filling any more than that.

I pulled the line off the filter (coming from the engine) and turned over the engine to see if I could feel any suction. I felt a little bit, but not much. The suction certainly didn't suck my finger in very much. However, I don't know how much suction is supposed to be there.

There are two hoses coming off the carb towards the fuel tank. I did have the baffle out, so I guess it's possible I could have switched them. I did label the hoses when I took them off, and thought I put them back where they each belonged.

Are both the lines coming off the carb supposed to be sucking fuel?

-Barky.
 
Just saw your post, Dennis. You bring up a good point. I need to double-check that. I know the hoses on the carb are correct, but it's possible they could have gotten switched at the fuel baffle.

Let me check that and I'll report back.


Hey Scott- I bought the carb kit from SBT. I assumed it was OEM, but maybe it's not. Hopefully, i don't have to buy a new OEM kit.
 
When working correctly the fuel pump does NOT pull much vacuum on the hose. I put my tongue over the end of the hose in order to determine if the pump is working.

The SBT carb kit is not genuine Mikuni and will give you trouble. Ethynol in the gas will cause the cheap diaphram to ballon up and hold the needle and seat open among other things. Genuine Mikuni parts hold up much better.
 
Hey Dennis. That not the best news regarding the SBT carb kit. That's $47.12 I won't get back. I hope that's not the issue.

I finally ran the battery down trying to start it (10 seconds at a time, of course). It's on the charger now. I will give it another shot when the battery is back up to strength.

Thanks,

- Barky
 
Try blowing into the fuel vent line to pressurize the tank. I did that after changing all my fuel lines and my 787 and 717 fired right up with choke.
 
I'm leaning toward that diaphragm being bad but just to be thorough: pour a little bit of premix fuel into the Carb and a little down the plug holes and fill up the separator. Do that in the reverse order of what I said tho lol. That should give you enough fuel to idle for about 30 to 45 seconds. If it doesn't pull fuel on it's own either the fuel separator seal(or some other leak in the system) is bad or your diaphragm is crap and not doing it's job.
 
Just had the same problem with the same sbt kit. On mine it was the check valves for the fuel pump. They wernt seating properly. I had to reinstall them and kinda rotate them a bit tile they were seating good. Just make sure the fuel inlet flows with pressure and stops on vaccum. Then the fuel return flows with vaccum a stops with pressure. After that mine was working good. Might go with a oem kit before the season i can see this problem happening again. Good luck hope that helps
 
OK. Here is an update. I wanted to narrow down the issue further, so I took some extra hose I have to gravity feed fuel to the carb. The engine ran great. So, I'm 95% sure something is screwed up with the pump section.

Bhk- That's a very good point. Those two plastic check valves may be the issue. I took the carb off and tried blowing into the fuel inlet and then in the fuel return. When I blew into one, some air would come out of the other one. That is not supposed to happen, right?

Guys- I made a quick YouTube video, so you can see the fuel pump section of this carb (instead of me descibing it). Can you please take a look and see if anything looks wrong to you?

Thanks!

-Barky
 
The port with the arrow pointing to the carb u should be able to blow air in but not suck air out.when u blow in itll come out that other side. Then the port with arrow pointing away from carb you should be able to suck air out but not blow air in. Like i said i had to rotate those clear plastic disk check valve and try doing the pressure tests again til they both worked.
 
Thanks, Bhk. I can't believe that two little plastic disks are keeping me off the lake with this ski. Very frustrating! I will try moving them around to see if they will seat better. You're right, though- it doesn't seem like a good long-term solution.
 
Make sure the round plastic disks (check valves) lay perfectly flat after they've been installed. Any little kinks around the edges will cause poor pump performance. :cheers:
 
Well, I am definitely screwed. Those plastic disks (check valves) will not seat properly at all. When I took them off to look at them again, they have creases and bends that are not allowing them to seat properly. Either they were bent in shipping, or I bent them when installing them. And, the rubber parts that are supposed to hold them down are fairly loose- I don't think they are pulling down the check valves enough. :(

I wish I had not removed the old ones (and thrown them away). I figured, new ones are in the carb kit, so replace them. Big mistake.

I don't know if I can buy just those parts. I hate to buy another carb kit just to get those plastic disks. I'll guess I'll call SBT on Monday and see if they'll do anything for me.

Any other words of advice would be appreciated.

Thanks!

-Barky
 
I can send you a couple...do you need anything else? I typically have a $15 minimum....


You should almost send me your piece so I can put them in properly. Did you follow the instructions and put the blue line up?
 
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