The .61 engine I purchased has a remote needle valve, but the engine does not fit into the plane with the remote valve attached. Can I just attach the remote needle valve somewhere else in the engine compartment? Is there a maximum length for the fuel tube between the needle valve and the carb?
As advised by K & B when they were first to introduce remote needle valves, maximum length away from outlet of spraybar into throat of carb of a typical .61 - 1.00 ci (outlet into carb) is 4 inches [10mm]. The length of fuel tube between the needle valve and outlet will act as a "header" or " holding" tank. Response time to throttle input is severely compromised and will also lead to "unexplained" motor cut out in mid air - K&B advice has been well tested, even on pumped motors. (Pump pressure is on the other side of the needle valve.) Smaller motors require shorter lengths of standard diameter Du Bro tube or use tube with a smaller internal diameter but 4" still appears to be the limit with a .25 and different sized tubing. regards Alan T. Alan's Hobby, Model & RC Web Links
Though the 4" distance is probably good, response time has nothing to do with it. Once a water hose is full of water the flow stops as soon as the valve is shut no matter how long it is. However some water may spill out of it after it is shut down. The fuel in you line may also spill out into the engine when it is shut down, possibly flooding it for the next start.
I first used a remote needle on a Merco 29 because I kept breaking the normal needle. On one occasion I couldn't get a proper replacement so mounted a needle from a different make engine on a small aluminium bracket mounted on the firewall. I soldered over the broken end of the original needle and connected the fuel tube from the "remote" to the original connection. It worked well and it was some years before I returned to the "proper" arrangement by which time I was able to land the model without breaking the needle assembly - we had a very rough field at that time.
In some cases, the farther the needle is from the carb, the less precise the throttling becomes. Using a smaller diameter tube can help this. If you are only talking about a one inch change, you might not even notice the difference.
If you want to sink the money into a Jet engine from DubJet...They state you can put the needle valve up to 2 ft from the carb,as long as it's somewhat in line. Mine is bout 8 inches from the engine and have had no problems at all. Thanx