28cc McCulloch
 

I have a 28cc McCulloch with a zama carb and JEH ignition. Can anyone tell mehow to modify the carb and set the timing with the hall effect sensor. Also is the stock muffler satisfactory. Heres a picture of it.

RE: 28cc McCulloch
 

The carb is only a 3/8" bore carb. The engine will do a lot better with a larger one not to exceed 1/2" The Zenoah 23 carb is readily available but they cost around $40 (Walbro WA167A) . However, your original carb is usable. Disconnect the throttle arm spring but don't take it off. Most people remove the choke.



If you have a fixed advance type electronic ignition, the Timing should be set at 28 degrees. You will need a protractor fastened to the crankshaf. an old plug with a dowell glued in the center to act as a piston stop. And a piece of wire and a paper clamp to fasten the wire to the fins to act as a pointer.

Someone more familiar with your ignition design may know if it is the fixed, advancing, or the type that retards when the engine is turned slowly but advances back to the 28 degree point as the engine gets back up to speed?

For the fixed type, Install the piston stop plug. Bring the piston up to the stop and take note of the the degrees on the protractor. Move the crank in the other direction to where it stops without moving your pointer. Take note of the degrees. Top Dead Center is at a point half way between (beyond) the upper limits of what the piston traveled. Remove the piston stop and place the crank at top dead center.

Then you want to get your spark to trigger at 28 degrees before top dead center.

Enjoy.

Jim

RE: 28cc McCulloch
 

Thanks for the help. Can I open up the bore of the carb to 1/2'', the throttle plate is bigger than the bore itself. Its about 9/16''. The ignition is fixed timing. Should I set the timing by when the plug fires or when the timing light comes on.

RE: 28cc McCulloch
 

When the plug fires. Some of these fire when the Hall effect makes and some when it breaks. It's probably better to use the situation when the plug snaps.

Some people have been somewhat successful boring out carbs a small amount. But there are so many things at play, you could cause a dead spot in the mixture if it would run at all. At this point in your conversion hobby, its best that you obtain a larger carb. Walbro and Zama run aabout the same if they are the same size carb. But the carb on your McCulloch has a odd shapped choke area that makes it questionable for modification.

Look around at a chain saw shop and see what you can hustle up? Call Carr Precision and get a Zenoah 23 type carb. They are 7/16" and will run real nice on your 28 cc Mac. The carb will be a Walbro WA167A and have a little 28 cast into the venturi on the choke side. They are a little over $40. The Macs with a walbro have a 24 cast into the venturi. The 16" Mac 32 saw has a nice carb on it.

I have several 32cc ones. One was a 28 but I put the bigger jug and piston on it that i got from TKG. I have the 1/2" (32) Walbro carbs on them that I got at a good price from a model boat place in FL. I don't think the carry them anymore? Warehouse Hobbies? cannot remember. Its been 4 or 5 yrs.

Enjoy,

Jim

RE: 28cc McCulloch
 

Look up the CH ignition site and look at the pictures and nose around and see what you can learn?

Jim