24" frame is a little tight. need help

A couple of things you can try:
-Buy an offset intake manifold from one of the engine vendors. It has a bend to allow the carb to be mounted beside the seat tube.
-The classic ghetto method is to saw your existing manifold into two pieces and connect them with flexible automotive heater hose and clamps.
 
I would cut the rear motor mount saddle crossways in at least half or less. If fact I always cut them down to be thinner.

It allows the motor to drop lower in the frame. I don't know if it will solve your issue but it will help.

It for sure will give you more clearance to the cylinder head. The offset intake manifold is always a good option no matter what you end up doing with rear mount.

After looking at your attachment a second time you are limited to how low you can lower the motor without interfering with your derailer. Will make a difference on how much you can cut the rear mount down.
 
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All the previous is right, I just wanted to add one possible option. If you are going to build for power in the future one of the billet manifolds and a Dellorto style carb might just squeeze in there. They aren't a ton of money and if you are going to add power later it might help you spend money and tune once instead of twice.
 
The 'aftermarket' intake Z-pipe that is commonly available, may or may not work. It doesn't work well with mine - still too close to the centerline of the bicycle that with the carb mounted, has hard-interference contact with the oversize tubed frame. Once i get my motor mounts finalized (finally), i will take a 2nd look at seeing if the intake Z-pipe can be modded, or just get something else fabbed up instead. The earlier commenters are also correct - try to drop the engine as low as you can go.

I'm working with an 18" Miele MTB/Rock-hopper frame - inside frame tube measurement from crank to top-tube, 13".

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Mine is a tight fit too, I had to remove the manifold to put my uni filter on then squeeze it back on, it's a tight fit but it works
 
I could be wrong on this but I believe the intake side relies somewhat on the resonance like the exhaust side. If this is true you might try some different lengths of tubing to get the power where you want it.
 
With the flexible intake manifold, you will need to attach the carb to the frame. Otherwise, twisting the throttle will actually move the entire carb up and down instead of just the slide.
 
Don't mount that engine with that front saddle inthe case just touching the frame lime it is in the pic, that is recipe for broke studs.

Build a T shaped steel plate that will mount to enginewith bolts and then the lo g part that would be the bottom part of the T needs to be bent to follow the angle of the frame and then secured with a small muffler type clampthat will give you a very strong mount and much less chance of breaking a stud.
 
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