Here is the continuation of the original post by av8tor1977 . . .
Here you can see where I used a carb gasket with a larger bore, and have ported out the intake manifold adapter to accommodate the larger carb bore. The manifold gets really, really thin at the impulse port slot, and near one of the mounting screws when you do this. On more extensive, high rpm builds, I make a dam and add manifold surface area near that one screw using JB Weld, and use a smaller head screw. Then, also using JB Weld, I fill in the impulse hole and slot, and use an external pulse port and hose to activate the fuel pump in the carb. By doing this I can port the manifold adapter hole larger than you see here. If you want a really nice handling engine, go with a carb with an 11mm venturi. If you want a bit more power, a 12.7mm venturi is the way to go, but will be a touch more finicky on adjustment and throttle up transition. (Not too badly though; that’s the size carb used on the Zenoah G-26 engines.)
Some people like to make a metal plate that they bolt to the back of the engine with countersunk screws, and then the plate bolts to the firewall to mount the engine. I don’t like adding the extra weight, and so I just use studs to mount the back plate, and then those same studs go back through the firewall with washers and nuts on them. Simple and light!
As to the muffler, you can purchase one from various sources that will be much better than the unmodified stock muffler. If not, you will definitely have to modify the stock muffler to go along with the other modifications here. It is much, much too restrictive and power robbing. What I do, is carefully undo the crimp that holds the muffler halves together. I then remove all the original baffling, screens, packing, or whatever else you might find in there. The only thing you want to leave is the two bolt spacers. These need to be there to keep the bolts from smashing the muffler when tightened. Then re-assemble the muffler and crimp it back together with the bolt spacer tubes located inside. You then need to make either one outlet with about a 5/8” diameter tube, or two outlets with at least ½” diameter tubes. Braze shut any other holes in the muffler, clean it with the wire wheel on your grinder, and paint it black with engine paint. Done this way, the muffler is actually about as light as an aluminum muffler, because the aluminum is much thicker than the steel. They work fine. So…
Here is the engine all assembled.
Final specs:
Squish distance = .020”
Crank pin to back plate clearance = .025”
Exhaust Open: 102 degrees ATDC
Exhaust Close: 76 degrees ABDC
Exhaust total: 154 degrees
Intake Open: 67 degrees BTDC
Intake Close: 67 degrees ATDC
Intake Total: 134 degrees
What I generally recommend is 150 to 155 degrees on the exhaust and 130 to 140 degrees on the intake for operation from around 7000 to 8000 rpms or a bit above.
For operation at 9000 and slightly above, 160 to 165 on the exhaust and about 150 on the intake seems to work well. Much more duration than 165 degrees on the exhaust is getting into tuned pipe area/timing.
If you are going for the higher rpms and the most power, definitely go with the 12.7mm venturi carb.
All done and NO paper weights created! Yes!!
This engine turned an APC 16 x 8 prop at 8400 rpms on the test stand on the first run. A bit more power and a good bit less weight could be realized by getting rid of the magneto and going to electronic ignition. This would also provide for easy hand starting and an even lower idle. It handles very nicely and is responsive throughout the rpm range. A bit more running time and a slightly larger carb would easily see 8700 rpms on the same prop, and CDI electronic ignition would help as well by eliminating the drag of the magneto flywheel, and optimizing the ignition timing. (Which should be set at 28 degrees.) This is a very nice, reliable engine. The boat guys run these engines at 10,000 rpms, so unless you run them lean or without oil, they should last forever at under 9000 rpms.
Here she is, singing away on her first run!
Source for modified Homelite 25cc piston and excellent piston rings for any engine: Frank Bowman
[email protected] Phone: 505-327-0696