Mac 49 carb issues

#1
Hi everyone, I’m still not riding after a year in our local motorcycle shop . They put a new carb kit in it and said it only runs wide open throttle and won’t idle? Any ideas to help please? Thanks jim IMG_8550.jpeg IMG_8550.jpeg IMG_8546.jpeg IMG_8549.jpeg IMG_8548.jpeg IMG_8547.jpeg
 

Thepaetsguy

Well-Known Member
#2
Picture #1 shows an arrow pointing to your tillotson‘s idle screw.

Picture #2 is a standard normal tillotson idle screw.

I would (assume) what’s happening is the idle screw is set so it idles so high it just can’t idle down. Backing the idle screw down closes the butterfly and lowers the idle. Edit: it looks as if your Tillitson carb has had the idle screw replaced with a much longer then standard one. 73C36855-F8CD-4038-A1AD-1EE1B761C1A5.jpeg 36539259-E733-4D4A-B25B-F27191262A1B.jpeg
 

Thepaetsguy

Well-Known Member
#4
I‘m guessing the tillotson didn’t prime right away so you pulled and it starts but quickly shuts off and doesn’t prime the line? So someone whacked the idle screw to super high idle got the line primed and never turned the idle back down?
 
#5
Is it a Tillotson,?
Does the pulse hole in the carb align with the hole in the reed block? One picture has a black tube, one has clear plastic...does it need a remote pulse pump tube?
Don't take to a motorcycle shop. Take it to a vintage chainsaw shop.
 

Thepaetsguy

Well-Known Member
#7
Is it a Tillotson,?
Does the pulse hole in the carb align with the hole in the reed block? One picture has a black tube, one has clear plastic...does it need a remote pulse pump tube?
Don't take to a motorcycle shop. Take it to a vintage chainsaw shop.
it’s a Tillotson/walbro/zama looks Like it has one fuel line and an intake pulse hole. I bet they took the black tube off and installed clear line to see if the line is priming.

We can clearly see in this picture the throttle is wide open and not at idle. Looks like he couldn’t turn the idle up higher even if he wanted to due to the butterfly hitting the Carb body. Added a picture of the butterfly hitting the wide open throttle stop.
126D99AB-8F3E-4B64-88A3-84A7D1CCF447.jpeg
 
#9
So as I understand,? Unscrew that non stock idle . Throttle screw as far as it can go until the carb’s butterfly stops it . Then screw it back slightly?
 

Thepaetsguy

Well-Known Member
#12
So as I understand,? Unscrew that non stock idle . Throttle screw as far as it can go until the carb’s butterfly stops it . Then screw it back slightly?
Can you remove that gem air filter? THEN look into the carb itself DIRECTLY at the butterfly and see where It’s set BEFORE ADJUSTING ANYTHING. to verify where it is set at now.

IF it is set to wide open throttle yes just back the screw off and watch to make 100% sure that the butterfly is indeed closing.

I like slide carburetors BUT the slide CAN be installed backwards and installing the slide backwards will hang the throttle half way open.. Then the kart shoots off into the woods when it starts up.
 

Thepaetsguy

Well-Known Member
#13
Two screws on the right hand side? Where should they start at ? And which does what? Thanks Jim
It should be very very easy to remember from here on out..

The high side or H needle is ALWAYS closest to the air filter.

The low or L needle will ALWAYS be closest to the block

This is MOST walbro/zama/tilly big small snowmobile weed whacker it doesn’t matter the High side needle will always be closest to the air filter side of the carburetor.
92D2DB2F-6787-45A6-9D2C-468BDC2E00C1.jpeg
 
#15
Please tell us how far those screws were turned out when you turn them back in.
That manual seems to say both mixture screws should be screwed all the way in, lightly bottomed and then one turn counter clockwise for initial startup. Close the throttle and set it so it is just opened a crack before you start it
 
#16
Can you remove that gem air filter? THEN look into the carb itself DIRECTLY at the butterfly and see where It’s set BEFORE ADJUSTING ANYTHING. to verify where it is set at now.

IF it is set to wide open throttle yes just back the screw off and watch to make 100% sure that the butterfly is indeed closing.

I like slide carburetors BUT the slide CAN be installed backwards and installing the slide backwards will hang the throttle half way open.. Then the kart shoots off into the woods when it starts up.
 

Thepaetsguy

Well-Known Member
#18
The butterfly was almost shut, front screw one turn to close and LH closest to block two turns to stop / close . The throttle cable is super sticky and the lever arm is floating on the stud , just pulls off
The lever arm? I’m guessing you mean the throttle shaft? I’m surprised the throttle was closed also. IF the throttle shaft has problems it’s probably sucking in air and causing a high idle.

I have removed the choke and choke shaft entirely from carburetors like these with no problem so I doubt it’s in the choke system at all. If the throttle butterfly is closed and it’s still revving high it’s getting air from somewhere other then the throttle butterfly.
 

Triley41395

Well-Known Member
#19
I am not a carb or engine guy but I would think if you started the engine and put your hand over the intake bell it should die.
If it doesn't like others have said it's probably a gasket/air leak. Just something to try.
 
Top