Carb pushing air to the fuel tank?

[RC Addict]

Well, when he describes his issue as "carb pushing air" that tends to be an air leak issue. We all know carbs don't push air, they suck air if there is a leak. They are not pumps, so they can not "push" air. The only thing to "push" air is the exhaust pressure pushing fuel from the tank to the carb.

Gravity fed (tank above the carb inlet) systems would not require pressure to feed the fuel and therefor you would never notice an air leak in the system unless fuel was actually dripping from the source of the leak.

I know I am over-simplifying this, but basically the fuel system in an RC is a closed system. His problem MAY NOT be an air leak but I am willing to eat my Stetson if it isn't.

 

[cptvkirk]

Ha!...i say it's his lsn...but damned if i'm eating my Stetson if i'm wrong. Although, after what i used to break it in, it might just taste ok....but now there's too many chunky crunchy bits.





If you've never become intoxicated consuming the contents of your Stetson....then it should still be hanging on the wall patiently awaiting it's opportunity.

 

[frickenjoey]

Ok I have an idea.....you said Tiger Drive......the one I had on an engine back in the day had to spin the opposite direction from a roto-start. A roto-start turns counter clockwise (left) while a lot of Tiger-drives need to turn clockwise or to the right. Is it possible you are trying to start your engine in the wrong direction? Make sure that when you are starting that the flywheel is spinning clockwise (right) as viewed from behind.

 

[Djbrady]

Is it possible you are trying to start your engine in the wrong direction? Make sure that when you are starting that the flywheel is spinning clockwise (right) as viewed from behind.

2 strokes will feasibly run with the crankshaft spinning in either direction (meaning either way, they'll pull a vacuum on the carb). In my post earlier, I said if it were a 4 stroke, that might be the issue, but being a 2 stroke, it's not.

 

[RC Addict]

Precisely stated, Dj! That isn't his issue, even IF it were spinning in the wrong direction. Airplane guys know all too well how easily an engine will fire in reverse--happens all the time.

 

[dr15144]

ok u guys have gave alot of great advice, i have checked everything that ahs been stated, engine turns correct way , changed the fuel tank to one i know works fine (from my other rc) have rechecked both lsn and hsn it primes itself just fine and have even blown more fuel in to flood it! so i know the carb is getting the fuel, have put in 3 dif new plugs getting hotter each time, but once i start to crank it over the carb just pushes the fuel back to the tank along with air bubbles.... and yes when i crank put finger over exhaust it primes the engine once again, it acts like it wants to start for a sec then it just pushes the fuel and air back to the tank......... final conclusion..... buy a new carb as i dont have an extra laying around that will fit the o.s

 

[RC Addict]

That has got to be one huge carb leak. I remember olds (or Lessen?) talking about a sleeve that the carb slides down into. May not be locked into the groove and/or if it has a screw it may have come loose...

Not sure at this point, but I need to go get some salt for this Stetson...

 

[dr15144]

lol it helps to deep fry those stetsons... think i might tear down this carb one last time just to make sure I'm not missing anything...

 

[RC Addict]

Good luck, buddy!

 

[Djbrady]

alright. I have a theory.....

could it have the wrong exhaust pipe on it? Creating TOO MUCH backpressure in the expansion chamber and creating an imbalance in the engine, overcoming the normal "intake" pulse/stroke? At one point in the stroke, the piston has both ports open and NORMALLY there's just enough backpressure coming from the pipe that it creates "even" backpressure to create compression inside the cylinder... but if the pipe is creating TOO MUCH backpressure, it could be overcoming the intake side and pushing BACK through the carb and into the fuel tank.

I don't wear a stetson, for the record.

 

[RC Addict]

It could be a plugged exhaust. Maybe it collapsed inside? Good idea!

I DO wear a Stetson, for the record...

 

[dr15144]

hmmmm now thats something that i never really thought about . have a couple extra pipes and headers laying around will try that, but even with the wrong pipe on it i would think that it would still at least hit and start for a short period.... or am i wrong in that thinking..

 

[RC Addict]

Dj is correct in his thinking about backpressure. Too much restricts the flow to the tank. It HAS to go SOMEWHERE so it might be pushing it back into the carb...

 

[Djbrady]

Heh.. I have no clue.. I just like to throw random ideas into the mix for grins and giggles.

Check that collar on your carb too, like Addict said before.. an air leak can manifest itself in many strange ways.

 

[RC Addict]

It's gotta be one of the two, you have eliminated everything else.

 

[Suicide]

thats what i said check the pipe for blocks

 

[Djbrady]

That's right.. he did, long ago, in the way back..

(Just making sure he gets the credit for this one if that's the case)

Suicide, sometimes you have to write a lot, like an advertisement of your information, for people to see it... one line responses can sometimes get "read over".

pull the pipe off your header and look to see if it is blocked or check to see if your back presure line isn't on

 

[RC Addict]

thats what i said check the pipe for blocks

You did. He said he replaced all the lines and we all forgot about the pipe part. Good call, Suicide!

 

[Djbrady]

You did. He said he replaced all the lines and we all forgot about the pipe part. Good call, Suicide!

Hey.. back off.. I'M giving him the props on this one.


I gotta do SOMETHING while my wife gets ready for us to go out today.

 

[RC Addict]

Just giving credit where credit is due, as are you. Gotta get outta here and go see my kids...