I have a new engine with a rear mount carb. My first gasser has a side mount, so I could simply make my spacers to what I needed. The new engines carb hits the wall of the engine box. I cant get the engine back far enough. Do I need to take carb off & put engine thru the hole & remount carb? I hope this makes sense ......

What you need to do is measure the the length from the firewall to the prop hub. That will determine your spacers for the motor so that you have the proper distance between prop and cowl.

Next if the carb is hitting the firewall you neeed to drill a hole in the firewall to allow the carb to stick through. Make it large enough that you don't have to remove the carb.

I've had to do this with all my 3W55's because the carb extends back much farther than most motors.

Just spend the time to fuel proof the inside of the firewall as well as the carb will spit fuel (even if its a small amount) and over time it can weaken the wood.

As Sweetpea said - you can make some simple measurements, make the standoffs to suit and if necessary simply make a hole in the firewall if required.

Another alternative is that you can move the firewall back to suit the additional length. It depends on the airframe. Many ARF's on the market are pretty weak in the firewall area anyway and it can be a simple job with simple tools and a little patience and care to to remove it and locate it further back.

It depends a lot on how much the carb protrudes into the firewall and how much "meat" is left between the engine mounts and the carb hole. I recently had the experience on this exact dilemna on a 27% plane where if I cut the hole out to suit the carb, the high speed needle would have ended up in line with the firewall and one standoff would have been very close to the cutout for the carb. an hour or so later, I had the firewall removed, moved back and epoxied into place so I didn't need to have the hassles associated with the carb in the firewall.