I am new to the 4 stroke scene and am having some throttle issues. 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

Check out [link=http://saito-engines.info/throttle_setup.html]Saito Info[/link] for setup. You are getting full throttle with the radio setup?

Cheers

Actually Thats the exact info i used. First i mounted it on the plane and hooked it up to the servo and noticed the problem. No full rpm. When opened just past half it drops back down to an idle. So i pulled it off and onto the test stand to further investigate the problem. Could i be sucking in air from the manifold somewhere?

It worked OK on the test stand and then no good in the aircraft. The problem is with the aircraft fuel setup somewhere.

Cheers

no same rpm drop on the plane and stand. checked the fuel lines and pressure in the tank. Seemed to be all ok.

I was wondering what prop you are using and is the engine broke in ? Is it a new engine or one that has been around a few years? Fuel what fuel are you useing and how is the tank located to the carb on both setups.

Could it possibly be starting backwards? Just a thought.

IMO Stick40 is right about a possible solution to the problem, the biggest prop that a Saito 45 can handle is the 11 x 6, If a proper size of prop is in use and the problem persists I would suggest to clean the carburetor and change the o'rings.

Ryan, I'm assuming it is a used engine, I don't think they make the 45 anylonger. It is possible that someone had the cam off the crankcase and got it back one tooth off. Unfortunatly, you need to pull the rockers off and pull the push rods and lifters to check the timing. This woud be a last resort if you can't resolve the problem. Someone from Club Saito may have a method to measure the rocker movement and be able to check it without all the work. Maybe

Compression on a four stroke does feel a lot less than on a two stroke, especially if the throttle is closed. It varies a lot by engine Mfg. My O.S. 52 has more compression than my Magnum 52 but they are equal in performance.

Don.

!Thanks for the help gents! Yeah its an older engine that was given to me from a neighbor. I have a 10X6 zinger prop on it now. I'll check to see if it its a timing issue. wouldn’t be surprised

I would check the valve lash also before checking the timing. Seems they were set the same as the O.S. and Magnum engines. 0.002" loose and 0.004" tight on both valves


I aquired one a couple years back and it had a broken lug. I stumbled across a crank case on Ebay and rebuilt it. It is a good running little engine. I worried about not having a muffler, but without one, it wasn't any louder than my O.S. 52 four stroke.

Setting the timing looked a bit intimidating until I found the tip on using a pushrod throught the hole the intake lifter goes in. It drops into a hole in the cam locking it in place for TDC. Try very hard not to damage the gasket under the cam housing. I managed not to, but the new case was machined a couple thousands lower and if I tried to tighten the cam housing down, it locked the gears up. I cut a new gasket from a coke can and that solved that problem. Turns out I had to do the same for the back plate.

Humm, I need to put that engine on my Fun Star and save the power from the O.S. 52 for something bigger.

Don

Keep us posted.

This is what it sounds like to me also. I've had engines in the past that would start backwards and that is how they behaved. It might be worth taking a look to verify which way the prop is turning. If you're standing behind the engine (which is where you should be) and you don't feel a breeze somethin' ain't right

Please check this thread regarding the techniques to aquire the timming set right.

http://www.rcuniverse.com/forum/m_76...tm.htm#7659349

That is a good link on rebuilding the Saito. The trick with the rubber band is worth noting. Only works for the Saito though.


Using a pushrod or something else with the small point to fit the hole it the cam works for timing. Even though it is loose int the houseing, you would have to try really hard to get it a tooth off. That would include not having the crank on TDC.

I've rebuilt both a Saito and a Magnum/O.S. four stroke and other than the cam timing, the Saito is a bit eaiser to work with. I expecially like the cylinder pulling like it does. Also, unless there is s compelling reason, I leave the piston in the cylinder to prevent damage to the ring. That is much eaiser with the Saito.

Don.

The nomenclature of high speed needle valve and low speed needle valve is misleading. The low speed needle valve is acutaully a mixture adjustment. If this adjustment is way out of wack then it effects the high speed adjustment. Check the low speed adjustment compared to the recommended primary setting. If it is not even close to the intitial factory recommendeded then adjust it back to the recomemnded setting. Is this engine brand new or was it picked up used?

Right on the money Campgem!!! Runs like dream! Really appreciate all of your guys help!

You are quite welcome.

Don