View Full Version : Mac 72 hp engine problems

PR Gyro Pilot
06-09-2005, 06:44 AM
I have a mac 72hp, engine with a 50-26 prop on it. the problem that I have is that Im only getting around 3200rpm. and thats not enoght power for takeoff and climb, that a good RPM for mantaining altitude. I have a holley 1920 carb with a 65 jet. the engine has 3ring pistons and good compresion. I notice a small leak on the rear cranckshaft seal, that is been replace this weekend. other than that I dont see anything else. I have friends that have the same engine with 50-30 props that get more RPM than me.
What would be an optimum RPM for that engine?
what should be the fuel consumption of that engine per hr?

Im new to this engines, I used to fly a gyro with a subaru engine and RFI reduction drive that produced 425+pnds of thrust. and I sold that gyro like 2yrs ago because I wanted a lighter weight machine.

Now I have a bensen with mac 72hp, but I feel that the machine will not climb as fast as it should.

C. Beaty
06-09-2005, 05:17 PM
The original, chrome plated 72 cylinders were permanent mold castings.

Where the transfer passages turn the corner to enter the cylinder, there are frequently aluminum flashes where the mold pieces separated.

Removing these flashes can make a large difference.

Also, make sure the cylinder base gaskets aren’t partially obstructing the transfer passages.

Make sure the ignition timing is correct. Remove the carburetor and you can spot the timing mark on the rotary valve. The points should just begin to open when the timing marks are lined up. We used to always to retard ignition by about 1/8” at the mag mounting ears. Rotating the mag in the same direction as crankshaft rotation retards ignition.

PR Gyro Pilot
06-11-2005, 05:48 PM
well today I replaced the rear seal of the crankshaft remove the carburator and setup the CDI timing of the engine. Now Im getting 3400rpm. its about 300more RPM than what I was getting. I will fly tomorrow and see how it feels , the temp. went down to 375-400deg.

PR Gyro Pilot
06-28-2005, 04:51 PM
I was making a mistake when I mentioned that compresion was good, last week I tested the cylinder compresion and I got 90psi on 2 cyl. and 70psi on another and 60psi on the last one. I took the engine apart and ordered the new rings and front seal, I will put the engine together this friday and tested on saturday. hopefully I will get the 3900-4000 RPM that I need.

06-28-2005, 06:35 PM

One thing to check that can cause low RPM's, is that the carb is not centered over the throat in the case. If this happens, the butterfly may not open completely.

Just something to check before tightening the two nuts on the carb base.
When I have to remount a carb, I hold the butterfly open all the way while tightening the nuts.

Also check to see that there is no excess slack in the throttle cable at idle.
This can also cause the butterfly not to open fully.
I adjusted mine awhile back and gained almost 100 RPM.

C. Beaty
06-29-2005, 10:34 AM
If you have original chrome plated cylinders, nearly forever is required for the rings to seat.

Until they do, compression will be low and cylinder head temperature will be high.

If you have modified cylinders with cast iron sleeves, use a glaze breaker to insure ring seating.