Fuel Delivery Problems

[DarthRacer]

I've been wrestling with an OS .15 that I can't seem to keep running. Once it gets hot it shuts off. I notice air in the lines almost every time. When I prime to restart, the fuel gets pushed back into the line from the carb by a big air bubble. Any fuel that gets into the carb sunds like it burns off right away. My temps with a temp gun aren't over 280F. Am I doing something wrong here? The engine has compression when cold.

 

[FastEddy]

After you replace the fuel lines and pressure test the fuel tank, You may also want to look for an air leak from the carb.

Take the carb off and check to see that everything is tight. I have had loose throttle nuts drive me crazy.

When you put the carb back on be sure to use a gasket around the base. If you don't have one you can use a little rubber O-ring from the plumbing department of your hardware store. I have also used instagasket sealer even though I here its not the best thing to use.

This is just one of many possibilities.

 

[DarthRacer]

Man, I cranked down on that carb, but an air leak you say??
I notice that RCCA always uses silicone sealer. I may look for an o-ring as you suggest.

How about a leak in the backplate? Same symptoms??

 

[HugeGuy]

Sounds almost like vapor lock. Do you have your fuel lines running near your exhaust at all? If so, try to re-route them. Also, make sure your fuel lines aren't going up too high above the carb fuel inlet.

Last but not least, make sure that you do not have any holes in your fuel line or pressure line. They can be very difficult to spot but will cause odd things like this. Oh, one more thing, make sure the nipple on your exhaust pipe is not plugged(just stick a little needle in the hole).

 

[DarthRacer]

Originally posted by HugeGuy
Sounds almost like vapor lock. Do you have your fuel lines running near your exhaust at all? If so, try to re-route them. Also, make sure your fuel lines aren't going up too high above the carb fuel inlet.

Last but not least, make sure that you do not have any holes in your fuel line or pressure line. They can be very difficult to spot but will cause odd things like this. Oh, one more thing, make sure the nipple on your exhaust pipe is not plugged(just stick a little needle in the hole).

I just replaced all of my fuel lines - no holes.
My fuel line IS touching the engine mount as it leaves the underside of the tank. Maybe this is heating my fuel up??
Damn, you guys rock.

 

[FastEddy]

You don't want to crank the carb down. This can cause a crack in the carb or the case as well as cause warping. Check everything closely.

When tightening the carb you just want it snug enough to hold no more.

I like the rubber gasket. When you push down on the carb you can see the gasket start to compress and you know you have a seal. You want to choose a thin one that just fits around the base without stretching to much. The ones I use are about the thickness of a match book cover un-compressed.

 

[HugeGuy]

One other thing to look at, is you could be too lean. Try defaulting the needles(or at least richen up the high speed 1/4 turn or so) and see if that takes care of it. 280 degrees is on the edge of being too warm for the OS engines.. just another idea.

 

[Çh®i§tiªñ]

Last time my OS acted that way I got them to put in a new piston and sleeve. They have a great warranty!

 

[RobH]

Originally posted by DarthRacer
When I prime to restart, the fuel gets pushed back into the line from the carb by a big air bubble.

Have you checked the HSN assembly? If it's loose, that will allow air to leak in and do just as you describe. An air leak would also cause those high temps too.

If it's not the HSN assembly, take the HSN all the way out and check the O-ring at the base of the needle. If you have a chunk taken out of the O-ring, it will leak air too. It sure sounds to me you're leaking air at the carb.

Good Luck.

 

[DarthRacer]

Thanks fellas. Next chance I get I'm tearing down my engine to seal it up and check the fittings.
Hopefully I'll :sniper: the problem in the butt.

 

[Akina'sMtx-2]

Originally posted by HugeGuy
Sounds almost like vapor lock. Do you have your fuel lines running near your exhaust at all? If so, try to re-route them. Also, make sure your fuel lines aren't going up too high above the carb fuel inlet.

Yup sounds like a vapor lock to me, another sign of the engine running too hot and over heating. Vapar Lock is cause when the engine is too hot and the fuel start to form air bubble in the fuel system(line), for those who might no know what it means. Do what Hugeguy said, richin it up first, then crack down for air leak if that didn't solve the problem.