Having trouble with mid range. Idles good, transitions to full good, and runs at full throttle good. Mid range though is a mess. sputters and sounds bad rich. Have found that the low end doesn't effect the mid range at all. High end needle does, but if you get Mid to sound good, then full is bad lean. Using a brand new Walbro WJ71 carb, and brand new CH ignition with 3s Life pack.

New plug as well ?

Does the carb see any propwash or rushes of air from cowl??? Could be diaphram vent holes need to be shielded from that air hitting them just a thought outside the obvious stuff

Yes, new plug, and carb is situated that diaphragm cover is on back of carb, toward firewall, so no propwash at all

Originally Posted by voodoodb

Yes, new plug, and carb is situated that diaphragm cover is on back of carb, toward firewall, so no propwash at all

Does it run any different in the air than it does on the ground? I've seen where these side intake engines had issues with the air rushing past the intake. Sometimes fixed by using a velocity stack or putting an air defector in front of the carb and intake opening.

Runs the same. I have been told that it may be that it is a chainsaw engine. They were not designed to run other than idle and wide open. Not sure if that is true, but does make since.

Originally Posted by voodoodb

Runs the same. I have been told that it may be that it is a chainsaw engine. They were not designed to run other than idle and wide open. Not sure if that is true, but does make since.

A chainsaw based, piston ported engine (conversion or custom) can run and throttle every bit as good as a purpose built, usually reed valve engine given a good carb and ignition and if the engine is in good condition. The differences are mainly that a newer reed valve engine will have a bit more lower end torque and will produce its best power in an RPM range where a prop is happy. Chainsaw engines and conversions were designed for higher RPM operation and are at their best when propped for a bit higher RPM than what might be considered normal today .... which means they like less prop load.

The problem with conversion (such as Sachs, etc.) and custom built engines that only use selected parts from a saw (cylinder, piston and sometimes the crank and a few other parts) is that most of them are getting old now as few are produced these days. Old means that ignition components can be suspect as well as carbs and bearing seals, gaskets, etc. You've replaced the carb and the WJ71 is well proven for a wide range of engines including the 4.2 but how about the rest of the engine. Is it in good mechanical condition? Are the crank seals and gaskets still good? A minor leak can cause all sorts of problems including rough running. How about the ignition? How old is it and is it working properly. Too much timing advance can cause rough running. A gentleman here on FG sells a pressure test kit for gas engines and you might need this .... or fabricate one.

Is this engine a real Sachs (most original ones had orange cases) or is it a custom engine like Brison, FPE, Fox, Taurus and quite a few others. This would be helpful info to get to the problem. Mag or Electronic ignition? If electronic, mechanical or electronic advance?
What kind of muffler? Lots of help here but more info is needed.

Edit: Rereading the first post, I see the ignition was also changed. Retimed? Did the engine originally have mechanical or electronic advance and what is it now.

Openning up your spark plug gap in most cases will help to clean up a mid range burble.



Info from a post by Antique,,, Spark plug gaps are VASTLY overrated...Any gap from .020 tp .060 will work with a GOOD ignition...Any spark plug, CM6 included...Don't believe it ? Try it yourself...



BV

Another thing..... you mentioned 3s life pack. Are you sure its not 2s? If so you would likely want a 2s (6.6v) life on there not 3s...I would think that could cause all sorts of problems

Thanks for the info. Will try opening the gap.
So Antique did the conversion to electronic ignition as well as rotated the carb for straight shot from servo a few years ago. I started having problems with the module a couple of weeks ago so I bought a brand new unit from CH. Voltage says 6-12v on module. I had a 3s Life pack sitting around so thought it would work perfect. It has a red anodized case with original Sachs piston, rod, crank, and cylinder. Antique said they could be considered new with very little wear if any. I put a new cylinder gasket on it last week, OEM new old stock. Overall engine looks great and in very good condition. Also replaced manifold and carb gaskets last week.

Originally Posted by Bill Vargas

Openning up your spark plug gap in most cases will help to clean up a mid range burble.



Info from a post by Antique,,, Spark plug gaps are VASTLY overrated...Any gap from .020 tp .060 will work with a GOOD ignition...Any spark plug, CM6 included...Don't believe it ? Try it yourself...



BV

I believe Ralph was exaggerating to make a point that gap isn't ciritical and that a larger gap is OK. There are limits though and I wouldn't go over about .026" or so on most of the plugs we use. A too large gap is hard on the ignition coil or plug cap as they may arc internally rather than across the plug gap. Certainly a larger than normal gap does seem to help midrange.

Originally Posted by voodoodb

Thanks for the info. Will try opening the gap.
So Antique did the conversion to electronic ignition as well as rotated the carb for straight shot from servo a few years ago. I started having problems with the module a couple of weeks ago so I bought a brand new unit from CH. Voltage says 6-12v on module. I had a 3s Life pack sitting around so thought it would work perfect. It has a red anodized case with original Sachs piston, rod, crank, and cylinder. Antique said they could be considered new with very little wear if any. I put a new cylinder gasket on it last week, OEM new old stock. Overall engine looks great and in very good condition. Also replaced manifold and carb gaskets last week.

Have you flown this engine before? Was there a problem with the original carb .... probably an SDC80 or something similar. Regardless of who did the ignition conversion, I'd still check the timing. Ralph is the best but any number of things could happen to the engine since he did the conversion.

I've flown several 4.2 Sachs before and always found them touchy to tune but achievable with patience. What exhaust are you using on the engine? One muffler I used caused the engine to be very difficult to tune.

Yes I have flown. Original carb was a tillotson. Couldn't find rebuild kit for it so after researching, bought the Walbro. Muffler is a Jtec Pitts.

Originally Posted by voodoodb

Yes I have flown. Original carb was a tillotson. Couldn't find rebuild kit for it so after researching, bought the Walbro. Muffler is a Jtec Pitts.

I was lucky, none of my engines had the Tillotson carb. My engines tolerated a Pitts type of muffler well. One had a side mount box muffler and it is the one that was difficult to tune.

One more question .... what airframe is the engine on?

Originally Posted by voodoodb

Thanks for the info. Will try opening the gap.
So Antique did the conversion to electronic ignition as well as rotated the carb for straight shot from servo a few years ago. I started having problems with the module a couple of weeks ago so I bought a brand new unit from CH. Voltage says 6-12v on module. I had a 3s Life pack sitting around so thought it would work perfect. It has a red anodized case with original Sachs piston, rod, crank, and cylinder. Antique said they could be considered new with very little wear if any. I put a new cylinder gasket on it last week, OEM new old stock. Overall engine looks great and in very good condition. Also replaced manifold and carb gaskets last week.

Wow Ive never heard of that! Well easy enough to plug in something closer to 6v and check another thing off the list of possibilities.

My sachs ripped!, But it was on a p51 so I held it WFO most of the time never used the midrange!