Ryobi ignition timing

I thought I had asked this question earlier but don’t see it.

When the flywheel magnets line up what is the approximate location of the piston.

Bill

Hi Bill,
Here is a picture of where the flywheel is at top dead center.

Dave

Many thanks.

I may be having an embarrassing oh, oh.

Bill

I believe that I was able to salvage the built in flywheel key. A real key would have been nice. Wonder it could be fitted with a real key?

The engine is a new refurbished Harbor Freight special. I took this one apart before running it. The engine seems to have very little compression relative to other two strokes. How much compression should the Ryobi have?

Bill

Hi Bill,
Three things could cause low compression. Check the ring to see if it is seated,new rings will blowby. It could have a worn ring, but souldn't since it has been renewed. The third is cruves cast in the bore above the transfer ports. These are ment to reduce compression for ease of starting. I filled them on one engine with JB Weld. I havn't run it yet. On another engine they had filled with carbon during use.

Dave

Dave,

Thanks. Found the compression issue, which was once again my stupid mistake.

I did not find curves above the transfer ports in my cylinder. Incidentally the internal cylinder temperature should be well above the 600 F limit of JB weld. It will be interesting to know what happens when you run it.

A standard electric starter doesn’t appear to be quite enough for the engine. It also does not appear that it can be hand started. However it will run but did not seem to make a lot of wind. The engine will turn 8100 RPM on a weed eater. At first run it turns 7200 RPM with a 16/8 MA, box stock carb, air box, filter and muffler.

Bill

Hi Bill,
It sounds like you got a good engine. A larger or bored out carb will in conjunction with a unrestricted muffler give good performance. This will allow you to use more pitch or prop diameter. The ideal cylinder head temp. (CHT) is usually between 180 and 220 degrees F. The charge flowing past the transfer ports will also cool this area. I am sure that of the many used engines that you have disassembled you found evidence of the cooling effect of the incoming charge on the piston. I do think that the rpm should be kept below on the Ryobi because of the stamped rod. The piston is heavy and has thick walls which doesn't help. If you do a web search on Ryobi you will find a site called teaching a weedwacker to fly. I covers boring a stock carb to 400 thousands (about 13/32"). The 11mm walbros seem to work as well also.

Dave

Dave,

Removed the air box earlier and it seemed to knock a couple hundred RPM off the top end.

Next I will tack it with open exhaust and then with a Bennett muffler with everything else stock including the air box and filter.

Would like to see 8000 RPM. Assumption is that maximum HP will be close to weed eater RPM. If it blows it blows.

Bill

Dave,

The Ryobi will turn 7600 occasionally touching 7700 RPM with open exhaust.


Bill

Dave,

The Ryobi will turn 7500 and occasionally touch 7600 RPM with small Bennett muffler. The muffler is fairly loud.

Bill

Hi,
Those are good numbers. It might do almost as well with a larger prop.

Dave

Dave,

A 16/10 is about 20% higher prop load. What prop size do you suggest?

Thanks for the information about modifying the carburetor. I usually change one thing at a time and test and prefer to do reversible things first. Often my changes will go the wrong way and need to be reversed. Eventually the carburetor will get attention. The article statement about airplane engines turning faster than weed eater engines is not consistent with my observation.

The factory torque and HP curves might be helpful is establishing a target RPM. Does anyone have a factory specification sheet?

Initial plan was to use 2 Ryobi’s on a 35# or so existing airplane. After running the engine it is unclear that two will fly the airplane. Time to rethink the plan and maybe fly it on something else.

The Ryobi is compact and well suited for model airplane use. I’m OK with the rod but the flywheel key is a horrible two-penny cost reduction. You undoubtedly noticed the #12 screws, which indicate the designer is an old fart (myself included). The rod however is unconventional indicating younger value orientated thinking. A similar design with twice the displacement would be much better. Power to weight changes drastically for the better as the displacement goes up.

Bill