Fuel Starvation?

[Techfan85]

90Sn with 701 61x/61x, won't turn over when cranking. If you add fuel to carb it will run okay (not great, but runs good, idles bad). But then as soon as it stops it will not start again, gotta add fuel to the carb. The primer has been taken out of the circuit. Ran great just 4-5 weeks ago, but I snapped the pole. Replaced the pole, and it sat for a little (thought I had all the fuel ran out of it though) and have not gotten it to run right since. May have crimped the throttle cable a little. When you start it will idle really high and even a little throttle will ramp it way up. Also sounds like maybe only one cylinder firing? 150 psi comp on both

Did have an issue that it wouldn't stop when lanyard pulled, had to turn fuel selector to off. Have not replicated that though.

Replaced inline fuel filter on one side, had some garbage built up in it, but not in the actual filter, it was in the bottom of the filter where the fuel sits. See attached photo. Can't get it to move around in the filter.

So when I go to crank, doesn't seem like a lot of fuel moving through, but it is running through the return line, so that should be good. Spark is good.

Could it be fuel selector switch? Starving enough fuel so it won't crank? I pulled carb apart, was a little grimy, but not bad and the conical filter was almost completely clean. Its all cleaned and back together, no improvement.

I know I just threw a handful of items out that don't seem to have a lot in common. Not really sure what my next issue to check is. Thanks guys!

 

[Tyler Zane]

For starters you can pressurize the system by blowing in the vent line. It will push fuel up threw the feed line and eventually out the carb and threw the return. Fuel pump may be getting to weak to prime the system. I dont recommend running fuel systems out like you said you did.

Check pulse line, one way valve on tank, blow threw all the lines and finally rebuild/clean carbs with mikuni kits. Be sure the little bitty internal carb filter is clear too.

A broken reed will cause a lack of pulse signal reducing fuel pumpage aswell.

 

[Techfan85]

Should I be able to blow through the vent line either way? It seemed to be totally clogged (pulled from the tank and tried to blow through it). Internal filter on carb completely clean. I pulled the vent line and tried to start, but still wouldn't turn over.

For starters you can pressurize the system by blowing in the vent line. It will push fuel up threw the feed line and eventually out the carb and threw the return. Fuel pump may be getting to weak to prime the system. I dont recommend running fuel systems out like you said you did.

Check pulse line, one way valve on tank, blow threw all the lines and finally rebuild/clean carbs with mikuni kits. Be sure the little bitty internal carb filter is clear too.

A broken reed will cause a lack of pulse signal reducing fuel pumpage aswell.

 

[Tyler Zane]

The vent should allow air to go into the tank but not out. They do fail in a manner you described, getting stuck closed. From my experience, when this happens, the engine will run fine for a few minutes and then just suddenly die down.

 

[john zigler]

Go through your fuel system completely. Rebuild carb, fuel pump, and replace needle and seat. Use ONLY genuine Mikuni brand parts. Do not cheap out.

Replace fuel lines, and fuel filter. I like the Yamaha type water separator filter over the little peanut types like you have.

I would also replace the fuel select switch. They too can go bad.

Remember, a lean condition can and will end up in engine failure.

After you are done be sure to run only pure gas, NON-ethanol.

 

[Techfan85]

Thanks. I have a Yamaha water separator as well as the inline filters. The selector valve is 6 months old, but I do want to inspect it. Having issues determining how they come apart though...is it just a threaded connection? Fuel lines are new as well. Was running great up until I broke the pole and replaced it. It also sat for a while over that time (4 weeks or so) but I flushed, greased and lubricated

 

[Techfan85]

Okay guys, so its the off season, time for an update:

Cleaned out carb twice, all lines cleaned, fuel selector good, replaced plugs, still same issue. So going to rebuild the carb, but I have come across this problem, the screws in the valve body assembly are totally corroded and heads are wallowed out, no way I am getting them out without drilling them. Screws #40 in attached. My questions are as follows:

This obviously prevents me from getting to the jets, so the jets could be clogged. Anything else there that could be bad that I can't get to and would cause this?
Should I just get a rebuilt carb? I am not sure that I can drill these out without causing other issues (I am not that talented haha).

Thoughts or suggestions? Everything else seems to be fine with the carb & the rest of the fuel supply, so I am assuming something is bad beneath the valve body assembly that I can't currently get to.

http://www.sudco.com/Diagrams323728/expbn.gif

 

[Tyler Zane]

There are a few options.

-Drill the heads off, pull valve body off and use needle nose vise grips to remove the remaining threads.

-Cut slots in them and use a flat head, preferably and impact of any sort.

-Use and extractor set.

In the future , go strait for the impact when removing any screw on the carb. Some like the screwdriver type you wack with a hammer. I reserve those for big heavy jobs. My 12v bosh impact driver is more than enough for a carb job and makes quick work of it too.


Now back to your running issue.. you have stated several times you have cleaned the carb. If you have not removed the valve body, you never actually cleaned it. The first places to gum up are the transition ports and low speed jet because they are the smallest holes. Fully disassemble the carb and clean/rebuild it right this time. Ultrasonic cleanet, Chem dip, compressed air etc.

 

[Techfan85]

There are a few options.

-Drill the heads off, pull valve body off and use needle nose vise grips to remove the remaining threads.

-Cut slots in them and use a flat head, preferably and impact of any sort.

-Use and extractor set.

In the future , go strait for the impact when removing any screw on the carb. Some like the screwdriver type you wack with a hammer. I reserve those for big heavy jobs. My 12v bosh impact driver is more than enough for a carb job and makes quick work of it too.


Now back to your running issue.. you have stated several times you have cleaned the carb. If you have not removed the valve body, you never actually cleaned it. The first places to gum up are the transition ports and low speed jet because they are the smallest holes. Fully disassemble the carb and clean/rebuild it right this time. Ultrasonic cleanet, Chem dip, compressed air etc.

Whoa, whoa, you mean I actually have to clean every part and not just some of them? haha just kidding. Previously it was just the internal filter that was clogged, so I was considering that "cleaned" but you are right, I never actually cleaned the carb. Thanks for setting me straight on that. Going to get into it later today, will update on what I find but I bet its the low speed jet.

 

[Techfan85]

Okay, so got those screws out, this time for real use air and blew everything out, used carb cleaner (a lot of it), put back together, same issue. Let me sum up the issue: It will run fine if you can get it primed to start. But it will not start at all if you don't dump fuel in the carb (no primer). Debating just adding a primer and calling it done, but I know thats not actually fixing the issue. Any thoughts on next step? I can still rebuild it with new parts (haven't done that yet), but honestly everything seems to be in good shape. Beginning to think maybe its not the carb. Thanks guys.

 

[smoofers]

I would look very closely at your fuel pump and check valve diaphragms (#s 2 & 3 ). The check valves can get creased (very hard to see with the naked eye) and affect the performance of the carb fuel pump.

Honestly though, looking at your original post, if your motor is running away at idle you could possibly have an air leak in the motor some where. The fact that it wouldn't shut off without the lanyard points this way too. That begs the question, how old are the crank seals? An air leak is serious in that it can easily roast your motor.

If you're doing maintenance in the off season I'd do the following at a MINIMUM, given your symptoms:

• Rebuild carb with a Genuine Mikuni rebuild kit.
• Pressure test the carb
• Pressure test your motor
• If pressure test fails, fix the leak (new crank seals, intake gaskets, etc)
• New fuel lines and filter (you might have already done)
• Check compression
• Check your reeds (could explain why your carb isn't pulling fuel)

 

[Techfan85]

I would look very closely at your fuel pump and check valve diaphragms (#s 2 & 3 ). The check valves can get creased (very hard to see with the naked eye) and affect the performance of the carb fuel pump.

Honestly though, looking at your original post, if your motor is running away at idle you could possibly have an air leak in the motor some where. The fact that it wouldn't shut off without the lanyard points this way too. That begs the question, how old are the crank seals? An air leak is serious in that it can easily roast your motor.

If you're doing maintenance in the off season I'd do the following at a MINIMUM, given your symptoms:

• Rebuild carb with a Genuine Mikuni rebuild kit.
• Pressure test the carb
• Pressure test your motor
• If pressure test fails, fix the leak (new crank seals, intake gaskets, etc)
• New fuel lines and filter (you might have already done)
• Check compression
• Check your reeds (could explain why your carb isn't pulling fuel)

Thanks, the single run away I haven't had it repeat that, so might have been a one-off with a minor air leak. Now I know how to deal with those, but will pay attention if it does again. No idea on how old anything is with it other than what I have replaced.

I think you (and lots of others in this thread) are right. Next step is rebuild the carb, see how it performs then. Definitely could be an issue that "looks" fine but isn't as you stated. Learned that with my S/S switch. Only item that gives me pause is the fact that it will run once its cranked, just absolutely won't crank without priming it.

 

[Tyler Zane]

So let me get this straight, you actually cleaned the carb this time but didnt bother to rebuild it? Your killing me smalls...

 

[Techfan85]

So let me get this straight, you actually cleaned the carb this time but didnt bother to rebuild it? Your killing me smalls...

Haha I didn't have the rebuild kit yet! Was checking if maybe there was an obvious issue and cleaning the jets fixed it. Don't worry, I only half tightened everything so taking it apart for the 10th time will be a breeze!

 

[Techfan85]

Update, rebuilt carb with Mikuni rebuild kit, starts like a champ. Still gotta take it out and ride when the weather isn't total trash though. Thanks guys. Clearly my "eyeball test" wasn't any good & replacing everything helped. Anybody looking at this in the future, I would advise to spend the $45 to get the rebuild kit and start there. As was previously pointed out, those little plastic check valves can be crimped & you not notice it. Thanks again all.

 

[smoofers]

Glad to hear you got it fixed, I bet it feels good.