Engine not getting fuel - help?

[MazdaGuy]

Hey all,
I just finished putting together my first kit, and it all seems to have gone great, except one little problem - it isn't starting. After some tinkering (don't have much experience in this stuff, but I catch on quick enough) I believe I've found the problem to be that gas is not going from the carb to the engine. The spark plug is always bone dry, but the float bowl fills up fine so I know the carb is getting fuel. After reading some past threads, I tried opening the carb up and blowing through the main jet but that doesn't seem to have helped. I can't really tell if its clear or not but it seems to be, but it still isn't starting. So, anything else that could be causing the problem? Any help would be greatly appreciated. Its an NT carb by the way, on a Grubee Skyhawk 66cc.

Also, bonus question to help my understanding - how does the throttle pull gas up from the float bowl? Is it just a difference in pressure? What stops it from just going out the choke/air intake? Thanks anybody and everybody!

 

[maniac57]

This should explain how it works: http://home.everestkc.net/malsin/Motorcycle/carb/mc_carb_theory_101.htm
Have you checked for spark?

 

[MazdaGuy]

Hey - yes, forgot to mention, it is sparking no problem. Even replaced the cheap provided plug with an NGK even though the stock was getting a spark. But the plug isn't getting "wet" like I've read it's supposed to.
Oh, and thanks for the link!

 

[mew905]

suction and pressure differences suck the fuel out of the bowl straight into the engine, though without reed valves, the nature of the beast is it will actually push some gas and air back out your air filter on the downstroke (so it may get a little fuel soaked after a while). I've always found the entire combustion chamber tends to be very "dry" (loose term, the cylinder walls are so slick I cant tell if its lubed or not). Only way to really tell if the motor is sucking in the gas is by draining it. basically pushing the piston down to BDC and turning the motor upside down. It'll only drip a few drops after sitting a few minutes (watch out! if the carb's still attached, flipping it upside down will drain the bowl all over the ground) My bet is if you can blow through the jet, it's more or less likely an air leak. could be the crank seals, could be the head isn't bolted down all the way, could be the intake gasket isn't totally sealed (make sure you have a flat mounting surface on both sides of the gasket to be sure) then you can use Permetex copper gasket sealer spray, silicone, or even seal-all [I used seal all, careful though, it resembles a quick-drying glue!] to make sure its sealed. Using a 1/2" I.D (3/4" O.D) O ring can seal the carb to the manifold better. my bet is it's an air leak from the carb to manifold connection (they're terrible to be perfectly honest)., and seal-all again if you want to be absolutely sure. If the carb can move by hand, it's not sealed.

 

[MazdaGuy]

Mew- thanks for the detailed response! I'll look to try and find a possible air leak. The carb is pretty tight against where it mounts, I can't move it just by pushing on it, but it is entirely possible it's still not perfectly sealed.

 

[maniac57]

Find the leak.
Lean mixtures kill motors!

 

[crassius]

take off the air filter and look in to see if the slide is going all the way down into the carb - slide may be stuck or cable too short or binding

 

[CTripps]

I had an air leak problem on one of mine.. couldn't find it until I took the air filter off and put a small flashlight against the intake on the carb. There were four tiny dots of light at the very ends of the gooves on the carb where it slid onto the intake. To get it that little bit farther onto the intake I ended up using a scrap of 2"x3" between the seat tube and the carb. Placed one corner against the carb, and then twisted it until it was pushing against the seat tube and it pushed it together that last little bit it needed.

 

[2door]

I had an air leak problem on one of mine.. couldn't find it until I took the air filter off and put a small flashlight against the intake on the carb. There were four tiny dots of light at the very ends of the gooves on the carb where it slid onto the intake. To get it that little bit farther onto the intake I ended up using a scrap of 2"x3" between the seat tube and the carb. Placed one corner against the carb, and then twisted it until it was pushing against the seat tube and it pushed it together that last little bit it needed.

I've run across this situation on some NT carburetors. The slots in the carb neck that allows it to be tightened onto the intake were cut too deep and made it nearly impossible to get a good seal. Somewhere way back is an old thread I did with photos showing the comparison between two carbs; one with proper slots and one with slots too long.

On every installation I use a gasoline resistant sealer and I fill the slots to assure they do not leak. Squirt in a glob and wipe it around inside the carb throat making sure the slots are completely filled with sealer before installing the carb on the intake.

Some rely on an 'O' ring to seal but the trick to that is applying enough pressure while tightening the carb clamp to assure a good seal between the carb body/'O' ring/intake manifold. Simply sliding the carburetor on against the 'O' ring won't assure that it seals sufficiently.

Tom