Variations on a Theme by YAMAHA
FROM THE BEGINNING it should be understood that the modifications which are described in this article are not going to make a full-house scrambler or roadracer out of your Yamaha 55 or 80. They will make it go a darn sight better than the over-the-counter version and do this without a great deal of expenditure in either time or money or a loss in reliability.
First the bike should be broken in at least to the manufacturer's specs and an additional couple of hundred miles wouldn't hurt a bit. These engines are set up quite tight on assembly and it is not at all difficult to seize a piston in the early stages.
Since none of the modifications will be on the bottom end it is not necessary to remove the engine from the bike. Work is begun by removing the head and exhaust pipe. Before removing the barrel, push the piston down to BDC. On the 55 particularly you will find that the piston sticks up well into the transfer and exhaust ports. Go in through the exhaust port with a feeler gauge and measure the amount the piston rises above the bottom of the port opening. The figure will be approximately .070. On the 80 it will be much less, about .020. This projection cancels very close to 20 percent of the port opening on the 55, not to mention narrowing the port timing.
Now to correct it. The obvious way is to jack up the barrel. For the 55 make up a plate from .063 (1/16) aluminum or better yet, micarta (less heat transfer). Use a cylinder base gasket as a pattern, but cut the transfer port openings on the small side and then file to match the ports in the barrel. This .063 plate in conjunction with the extra gasket required on top of the plate makes up to .070. How's that for a simple porting job? On the 80 the same thing can be done by packing in additional base gaskets or making up a thin shim. This raising of the barrel means, of course, that the piston will not come to the top of the barrel at TDC so the amount which we have added to the bottom of the barrel must be machined from the top. Better turn this one over to a first class shop. If there is the slightest trace of a ledge at the top of the barrel due to cylinder wear it must be removed.
While we have the barrel off, a little piston modification is in order. If the wristpin is hard to remove, don't get a bigger hammer, warm the piston. Insert the piston into the barrel at its BDC location and, using the lower edge of the third transfer port as a guide, draw a line on the piston. The lower edge of the port to which I am referring is the TOP of the cutout section at the rear of the barrel spigot. Now draw a vertical line on the piston corresponding with the center of the spigot opening. The hole to be drilled should have the vertical line as its centerline, and the upper edge of the hole should just clear the horizontal line on the piston. On the 55 this hole should be 1/4" with a vertical elongation to 3/8". The 80 should be drilled 3/8" with no elongation.
When opening up the hole on the 55 make sure that all corners are neatly radiused. A word to the wise. Increasing the size of the holes results in improved performance — right up to the point where she blows. This through-the-piston porting not only assists in the transfer but serves to wash the underside of the piston and relieve some of the high thermal stresses on the crown. The drilling on the piston will probably result in some upsetting of the material around the hole and this should be taken down with a fine file or better still, a carborundum stone.
Should you feel that new rings are in order, try using two of the lower rings. These are parkerized rather than chromed, as is the standard top ring, and they seat much faster.
Compression ratios on two-stroke engines will get you an argument anywhere. Actually this is nothing more than pure math and has little bearing on the actual character of a particular engine. Performance is based on compression PRESSURE and when you start getting into this it's enough to make a fellow take up wind-up toys. An engine with poor breathing characteristics will induce a light charge into the cylinder, say 50%, whereas a good breather will go perhaps 75%. Since detonation is determined by compression pressure we can squeeze the lighter charge much harder. The Yamahas with rotary intakes and generous transfer porting are very good breathers, particularly on the bottom end.
Since these modifications are designed to increase performance in the normal operating range, our compression changes are relatively mild. On both the 55 and 80 the combustion chamber pocket is reduced by removing material from the gasket surface of the head. A mandrel should be made up, consisting of a length of steel rod brazed into an old spark plug base. With this chucked into a collet you can do a pretty accurate job. Shave off .080 using the tool feed micrometer on the lathe for your measurements. The outer edge of the combustion chamber will have to be relieved back out to the cylinder bore diameter and recessed into the head .030. The latter job is necessary to prevent rod growth from rapping the piston against the head. A usable squish area also results. Before removing the head from the mandrel, cut back on the fins about .050 to space them away from the cylinder. The head should then be lapped. A standard head gasket is used. With the work completed reassembly is straightforward. I would suggest the use of a torque wrench on the head bolts as overtightening can lead to cylinder distortion. The correct figure is 9 foot-pounds.
If you are going to run your bike away from civilization, where a little noise would not be objectionable, some exhaust modifications can be done. The slickest trick of all is to remove the baffle pipe from the muffler and make up a duplicate, but without holes or louvers. With this in place the exhaust set-up then becomes a poor man's expansion chamber. Just jerking the stock baffle out and running with the tailpipe open doesn't seem to help performance any appreciable amount, probably due to the rings inside the muffler bouncing the gases around.
Carburetion is left stock with the possible exception that you may have to go one stage richer on the main. You will find however that on both the 55 and 80 the air cleaners place quite a restriction on maximum performance. Don't run without them! Since the restriction is not so much in the filters as in the housings get out the trusty drill and start punching holes. The 55 responds very well to this treatment and the author's present bike has no less than 15 of these 1/4" holes punched in the filter case. Noisy but nice.
Ignition timing should be left at stock or retarded l/2mm depending on which end of the RPM scale you want the best performance. Advanced — top end, retarded — bottom end. Accurate timing is very important on these small-bore jobs and the old "yardstick in the sparkplug hole" bit just won't cut it. A dial gauge with a sparkplug hole adaptor is the answer. If you don't have the equipment, find a shop that does. It is a lot cheaper than pistons and plugs.
The 80 has no provisions for timing adjustment other than the point gap. About .007 is as close as you can close them down and still get reliable ignition. Sparkplugs will have to be dropped down about two stages toward the cold side. We have been having very good luck with Bosch plugs in the 260 heat range. Before we get out of the ignition system, there have been some occurrences of hard starting which were pinpointed to a dark deposit on the contacts themselves. This is most prevalent on the 55 with electric starter and is believed to be carbon dust from the generator brush. A wipe with alcohol cures the problem.
This about concludes our little series of modifications for the 3/4 hop-up. A fullhouse version is in the formative stages and when it proves out may be the subject of another article.
Appended are some random notes from the workbench:
The rotary valve from the 55 drops right in the 80 and comes on big on the top end. About 15 degrees more opening.
A really light jockey on the 55 can benefit from a 15-tooth countershaft sprocket.
A hammer driven impact wrench by Snap-On is the only answer for those blasted Phillips head screws.
Graphited oils burn dirty but ke ? the engine together.•