The Service Department

THE SERVICE DEPARTMENT

GORDON H. JENNINGS

COMPRESSION AND CARBURETORS

I read and liked your articles on two-strokes. I would like to see more. I have a question about my Yamaha YDS2: As you mentioned, compression ratio is an important factor. According to the factory, my bike has a compression ratio of 8:1. How high can I raise this and where and how much should I mill to get this compression ratio? I want to use the bike on the road; no racing.

In addition to this, what do you think of using 2 McCulloch MC 20 carburetors? (I would like to bore the intakes.)

Jefferey Flaherty F.P.O. San Francisco, Calif.

Using good, premium-grade American fuels, I can see no reason why you could not use a 10:1 compression ratio without suffering any immediate ill effects. Of course, the increase will require a change in ignition advance, for best performance, and you will have to experiment until you find the right setting. And, some adjustment will also have to be made in the mixture, although not very much.

The best place to make the modifications necessary to increase the compression ratio is in the cylinder head. The best procedure is to partially fill in the combustion chamber by welding, and then machine away until the chamber has a volume that will give the 10:1 compression ratio. You can, at the same time, arrange for a slightly larger “squish” area, which is usually a good thing to do when making a modification of this sort. We have covered squishtype combustion chambers, their workings and purpose, in previously published technical articles and there is an illustration of one such chamber in the Jawa hop-up article in this issue, so I will not go into that any further.

Now, about the method of determining compression ratio: it is all very simple if one remembers that we are interested in the ratio between the total volume of both cylinder and combustion chamber when the piston is at bottom center, and when it is at top center. These volumes may be obtained by pouring light oil, from a graduated cylinder (which may be purchased at any of the large photo supply stores) into the spark-plug hole, with the engine upright and with the piston at top center, until the clearance volume is full. Then, one may add the piston displacement to the figure obtained to get the total cylinder volume.

In your engine, the piston displacement, for one cylinder, is 125 cubic centimeters. At an 8:1 compression ratio, the combustion chamber volume, as measured by the poured-oil method, should be about 18 cubic centimeters. A 10:1 ratio would re quire a combustion chamber volume of about 14 cubic centimeters.

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As to the use of McCulloch carburetors: I would not recommend them for the Yamaha engine. The carburetors that are supplied with the Yamaha are excellent, and very sophisticated in design - better, in some respects, than any other type of motorcycle carburetor. Actually, the best advice I can offer is that you forget the whole thing. You are not, by your own admission, interested in racing, and your bike is, in its standard form, fast enough to be interesting. Some motorcycle engines produce such a limited amount of power in standard that a great deal of room is left for the experimenter; that is not the case with the YDS2.

CURIOSITY

Does the same person ride all your lest bikes; what position is taken on high-speed runs; what are your tester's dimensions (height, width and weight)?

D. W. Wood

Oakland, Calif.

The answer to the first part `of your question is "yes and no." Yes, with a few exceptions - which we note in the test re port - all of the actual performance tests are made by one person; No, we do not restrict the riding of our test machines to one person - all of us try to spend enough time on the bikes to get a good idea of their qualities, and our written reports represent our collective opinion. Our performance test rider has personal likes and prejudices, as anyone will, and it would be grossly unfair to put only his opinion into our reports - even though whatever he gets out of the machine is exactly what is given as its performance in the data panel. The position taken on high-speed runs is simply as tight a crouch as can be managed. Perhaps some of the bikes would be a trifle faster with the rider stretched out prone, but that is not possible on all bikes and we therefore do not try it at all. Our test rider is, I must admit, much too large for getting the best possible results. He is six-feet, one-inch in height, his width varies a good deal but reaches a maximum at the shoulders, where he is just under 20 inches wide, and he weighs 185 pounds dressed to go (helmet, leathers, etc.). However, despite his bulk and in seeming contradiction to the fact that he is scandalously inexpert in a scrambles or hare-and-hounds, and leaves more foot prints than a centipede while negotiating a trials course, he can do a creditable job at the drag-strip.

A TWO-CARBURETOR "74"

I have a 1953 Harley-Davidson 74 on which I would like to install dual carbure tors. However, / am not sure just how to go about it. Would it be better to have a separate manifold for each cylinder, or to install both carburetors on a single manifold? It would seem to me that separate manifolds would work best, but I want to be sure before having the manifold (s) made un.

George McCluskey

Milknocket, Main

DUCATI FORKS

I am the proud possessor of a new 250 Ducatj Scrambler. After having this ma chine for about 30 days prior to your article in the A ugust issue of CYCLE WORLD, I did not realize it was hard starting because it started on tile first kick, in most instances, whether hot or cold. Also, if there is something wrong wit/i tile fork angle, would you make a sug gestion as to what will cure this? I found it to forgive almost every mistake I made.

Ken Lewis

Glendale, Calif.

We are delighted to hear that your Ducati starts on the first kick, in most instances; our test machine was rather reluctant, in most instances. Only the fact that the bike ran so nicely, and vigorously, after it started kept us from getting peevish. Per haps ours was just maladjusted in some manner.

As for the fork angle: we think that a lot of riders would like the handling better if the forks were a few degrees nearer the horizontal. Of course, this is not to say that all riders would agree. Anyway, as we went to some lengths to point out in the article, the Ducati scram bler handles superbly on most surfaces; it is only when the going got very rough that we did not care for the quick steering. .