ORIGINAL: pe reivers
That narrow slit is not a big deal, and will allow better hand starting without violent kickback. At higher rpm, there is extremely little compressed gas lost, though it starts blow-down a bit earlier, and silences the engine.
I'm glad it's not as bad as I thought, but still those "advantatges" don't convince me. I don't want to trade some power for ease of start - even more if I'm planning to use an electronic ignition.
ORIGINAL: pe reivers
The porting seems very moderate, and you can raise the central 1/3rd port part to get a timing of 155 to 160 degrees.
Also widen the existing port width to at least 50% of the bore diameter, but only the part above the transfer ports. Keep 60% as max. There is nothing to support the rings past the port.
I'm sorry, but I don't really understand what you mean, specially the "widening" part. If you could post an image of pic or simple drawing it would be a great help! In fact, I didn't plan to grind anything, but if there's an important benefit I may try. This kind of motors have average power, they're made to run all day and weight little, but I'm afraid of spoiling the block while trying and have an expensive paperweight.
Also, what tools are required to do that? Won't I spoil the nickel (or whatever material it was) coating of the walls?
ORIGINAL: pe reivers
Do not bother with crankcase stuffing if you want low end grunt. Our plane engines need not run 12000 rpm in this engine class.
Improve engine breathing without going outside the time-area port windows that belong to your target rpm. Google on Jennings and Gordon Blair. Increasing carb diameters helps up to a point, after which you also need to adjust internal porting, and/or raise rpm to get more power. More rpm needs larger ports for the same time-area data, so be careful there. You may bring the engine into rpm ranges where you rather would not be.
Yes, I know, I'm not making a boat engine!
I read in several books that, basicly, the more power you get from an engine, the more spiky gets it's power delivery curve (= more critical to use and less overall torque). However, I read of many guys who had done that for plane engine, so I kind of assumed it helped too (after all, we kneed torque, but not a HD engine!)
The rpm range I thought of (being very optimistic and once converted) was of up to 10000rpm (no prop), with smaller props doing about 9000-9500rpm and ideal size prop turning at around 8000rpm. I hope those are reasonble limits!
And as for the carb, since I initially didn't thought of altering porting, I would have used any of the carbs most fellow modelers use for their Homelites. If I get to know what you described before and decide to do it, I'll run some numbers. Still, you always have to do some trial & error.
ORIGINAL: pe reivers
Edit:
I have an electronic copy of Jennings and yamaha engineering reports for the interested. These papers very much represent our tuning efforts of the sixties (when I was young). If there is demand, I can put them on my domains for reading.
If it's not an effort to you, I'd like to see them, at least by mere curiosity.
ORIGINAL: davewallace
The carb venturi is small at 11/32nds(22/64ths). An 11mm(7/16ths) carb will help a bunch. You must open the carb insulator to 5/8ths to take full advantage of the new carb. These mods will give you the most bang for the effort and money spent. These engines run great with these mods, while keeping costs down. Checkout the conversion forum. This is where the answers are. Do a search on Homelite mods.
Dave
I'll probably do that too. I will trim the screw so it's about a mm from the walls, I don't want it to contact the piston.
Regarding the carb, I'm looking at Walbro's site the
"diaphragm carburetors".
And inside there, I saw the "HDA HD" category, but they seemed to be too big (the smallest had a 11mm venturi).
Here I found the WA series (the smallest carbs), but still no sign of model numbers and such, so I don't know what to search for. Can you help me with that? I don't even know which of those it's my current carb.
So, is the venturi size the only parameter to look for? I recall reading something like size is (number on carb)/64 (so I must find one with a 28 on it, right)?
I must be doing something wrong here, but I don't know what, and I also need to find an extra plastic adaptor.[:@] I'm getting busy by moments!
Also, believe it or not, I've been searching for several days the whole conversion forum. In fact, I chose the Homelite as it seemed to be the best choice in the 30cc range (considering power, weight and budget). But I'll search again, maybe I missed something.
BTW, do you believe milling the front end will afect th stiffness of the engine? I've not seen a single Homelite with electronic ignition that wasn't milled.
Have fun while learning!