I am new to the 4 stroke scene and am having some throttle issues. Hope you guys can help me out. I have a Saito 45 which after the engine is advanced passed 1/2 throttle I get a huge rpm loss. It drops back to idle again once opened all the way. I understand that the fine mixture needle adjusts the low range or idle range, while the high mixture needle controls the overall rpm range or max rpm range. I have followed the procedures on setting both needles. First by setting the high needle valve by leaning the mixture to get an appropriate high rpm. Then proceeding to the low mixture needle valve to get the low idle set. But when I try to set the high needle I don’t get anywhere near max rpm. I am getting a higher rpm response at the peak of the 1/2 way point. I have checked the valve clearances and they fall within manufactures range. Compression seems much lower than my 2 strokes but being new to the 4 strokers I am not sure what the compression should be comparatively. Any help would be much appreciated.


Ryan

Could be a multitude of items. Here is a partial list of things to check from an Engine tuning guide given to me many years ago by a fellow modeler.

Engine not broken in
Bad plug gasket
Fuel line too small
Blocked fuel vent
Pickup in tank off
Bad fuel
Crack in crankcase
Debris in fuel line
Varnish in engine
Air leak in tank
Bearings worn out
Air leak in line
Bearings defective
Kinked fuel line
Bearing misaligned
Carb leaking air
Split tubing inside tank
Clogged needle valve
Propeller too large
Loose engine
Improper carb setting
Foaming of fuel
Improper tank location
Loose head
Propeller out of balance
Loose case screws
Propeller too small
Loose plug

Ryan,

The needles on a 4 stroke are no different than on a 2. The low needle effects everything from idle to upper/high range and can effect high rpm, the high needle effects mostly mid to high range, they have to work togather. Sounds like after you adjusted the low needle it had the problem, that should tell you something. Start with the low needle rich and adjust the high end then go back to the low needle, if the motor slowly dies its probably too rich, just lean it 1/8 turn at a time till you get a reliable idle. Recheck the high speed adjustment after every 2-3 adjustments of LS needle, use a tack to check rpm as its hard to tell peak rpm by ear. Proper high speed setting is peak rpm then back off about 300-400 rpm.
Too lean on the LSNeedle will effect high rpm and will also cause midrange to suffer. If you can never get at least 9000 rpm, try a smaller prop. This should get you started in the right direction. This link should also help
http://saito-engines.info/throttle_setup.html

A pinch test is always a good way to validate the low speed. With the engine idling, pinch the fuel line shut close to the nipple. Don’t worry, the engine won't yell like your old lady. If the engine dies immediately you are to lean. If the engine picks up rpm you are to rich.