A friend of mine is having problems with his MVVS 1.60 gasser.
The engine is having problems with the midrange... the top end is fine and low end is fine but mid range surges .
The needle settings --the low speed is turned out about 1.75 turns and the High speed about 2.5 turns.

running 91octane fuel with a 40:1 synthetic mix.
prop is 18x8 , the exhaust is the original box muffler but with 3 outlets drilled out (instead of one).

any help would be much appreciated.

2.5 in my opinion is high, try richen the low and leaning the high, and why would he have 3 outlets??



David Moen to the rescue?!!



Good Luck

2.5 turns seems very rich

These instructions are courtesy of Pe' Reivers:

"Both needles should be about one and a half turn open to start with. You will then be about right for standard running. Then, using the arm stop screw, set the idle to fairly fast. Using the low needle (closest to the throttle valve), find the highest idle running setting, then open the needle 1/4 turn more. The top end is next, and should be set slightly richer than the highest rpm setting by 1/8th of a turn. Repeat the procedure until satisfied. Beware: both needles influence the whole range on many Walbro carbs, and idle speed has to be adjusted in between to avoid too high an idle. For a bit more power, drill out the carby to 8mm choke diameter. Latest test data: Keep the fuel mixture after the run-in at about 1:40. You will have more power available."

I had the same problem on mine. Set the plug's electrodes gap to 0.5mm with a filler gage. It worked for me. No more missing sound at the midle range.

look at the muffler,,, it is the inverted muffler, look inside the tips of the stingers, if they are untouched they should be rolled in a bit, cut off about a 1/4 to 1/2 inch, this should help, you can cut more off if you want, the more you cut off the more the power increase !!...


Jim

Here's what I do with a 1.60 to get it adjusted properly.

Set both mixture screws to 2 turns out. This is too rich at both high and low throttle settings, but it ensures that your fine tuning is done by leaning both mixtures so that you don't get an engine that is running too fat on top in order to compensate for a lean low speed setting or vice-versa. I've used this method on several engines right out of the box and I usually get an engine that starts on the first or second flip and sets up easily after that.

Prime and start the engine. Once the engine is running, reduce your throttle to a reasonable idle. Now screw the "L" needle in slowly, RPM will rise, when you reach the max RPM, turn the "L" needle back one-quarter turn, then re-adjust the idle (using throttle trim or throttle stop on carb) as necessary.

To get the high-speed needle in the ball-park, move the throttle up to 3/4 or so, the engine will run rough as you will be ridiculously rich at this point. Turn the "H" needle in until things smooth out. Open your throttle fully, then lean out your high speed mixture by turning the "H" screw in until the engine reaches maximum RPM. Do this slowly, and use a tachometer. When you find max rpm, turn the "H" needle out (richen) about 1/8 turn, dropping your peak rpm 200-300.

Return the throttle to idle, and let it idle for a minute or so. Quickly advance the throttle to full. The engine should accellerate cleanly. If it dies right away, you are to lean, turn the L needle out SLIGHTLY and repeat. If it hesitates and "burbles" its way to full power, lean the LS needle a bit. LS needle adjustments are made in very small increments!

Once you get this engine dialed in, resist the urge to fiddle!

Hey Scoub, I'v read your post on drilling'milling out the 1.60 MVVS carb to 8mm and agree that it would help but one thing Reg. in my mind. After doing this will not your throttle butterfly plate be to small to close it off completely??? Have not looked at mine yet because if I did it would go in my mill right then. Ha. Ha. This is a wonderful little engine and would not give it up for anything. I will end up with the 35 cc in a short short!! It was a little tricky to learn how to start it at first but after finding out the combo and getting it very wet----starts like a pro everytime. Thanks for all the help you and Moen give us on these engine. Hooker53