The Service Department

THE SERVICE DEPARTMENT

GORDON H.JENNINGS

A TWO-CARBURETOR "74"

I have a 1953 Harley-Davidson 74 on Which I would like to install dual carburetors. However, I 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 up. George McCluskey Milknocket, Maine

You present me with rather a tough question. Actually, I would like to know the kind of riding you had in mind. Un less you like to flog it along pretty hard, the stock setup will be best. For absolute power output, it has been well demonstra ted that an all-separate system is best-in which case you could make up a pair of stub pipes that would carry two carbure tors. The middle-of-the-road alternative wouJc~ be to mount a pair of stock car buretors on a single manifold, and make up a throttle linkage that would open one of the throttle valves about half-way be fore the second throttle valve opened at all. Then, from that half-way point, the second carburetor would come into action and, opening rapidly, would reach the fullopen position with the first carburetor. This type of "progressive" linkage has been used with great success on automo biles, and it would provide smooth low speed running for your engine and a level of power that, while not as high as that of the all-separate carburetion system, would still be a fairly satisfying improve ment.

ROTARY VS. POPPET VALVES - AGAIN

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Your October issue says that the rotary valve had no advantage over the poppet valve. It seems to me that the opening could be made much larger in a rotary valve? I was very interested to see that Yamaha has introduced a rotary-valve 125.

John Kenny Jr.

Bristol, Pennsylvania

It is true that the rotary valve can be made with a larger area than the poppet valve, but there is not the advantage in flow that one might expect. The edges of the valve body block the port, in varying degrees, all but a brief portion of the time, and the end result is no better, and can be worse, than is obtainable with the conventional poppet valve. The most successful examples of the “rotary valve” have been those that are commonly referred to as “sleeve-valve” engines, and

Thanks for the tip; Puch owners everywhere will be glad to know this, we are sure. As for the tapering-off of technical articles, it is simply unavoidable. With all of the sundry demands on my time, it will not always be possible to do the articles. Incidentally, we are always glad to see good technical manuscripts come to us from outside our office. We take expert opinion wherever we find it, and are always glad to have the material in the magazine. Perhaps the article on bore/ stroke ratios in this issue will be sufficiently technical to ease your disappointments with previous issues.

SUPERCHARGED TWO-STROKES

I read your series on two-stroke engines, which presented 1/ic various methods of improving performance. The article brought a question to mind: how about supercharging? Supercharging is one of tile most ef ficient methods of improving breathing, and is now seen quite frequently in the automotive field and has also been em ployed on several ,notorcycles. Super chargers from foreign cars are commonly used to boost cycle dragsters, but I can not recall ever seeing one on a two-stroke. It seems to me that a supercharger mounted on a two-stroke engine would give an even higher power boost than on a four-stroke because it is giving pressure underneath the piston while it is on the compression (upward) stroke. Would a supercharger work on a two-stroke? How much power would be gained? Also, if supercharging is practical, what type would be best? Some engines will turn 10,000 rpm; the vane~ype super charger is unsafe above 6000 rpm. Would the Rootes or centrifugal type be better? Mike Gerald Edmond, Oklahoma Supercharging is, indeed, one of the best ways of getting more power; but not from the conventional, crankcase-scaveng ed two-stroke. In such engines, with their piston-controlled transfer and exhaust ports, the exhaust is always open for some time after the transfer port closes, and any pressure that has been built up will escape. Therefore, no supercharging of the cylinder is, in fact, possible. As for the boost given the upward motions of the piston by the pressure from the super charger: this would be exactly balanced by the increase in under piston pressure when the piston came back down again. In the final analysis, it must be said that, while it is possible to add a supercharger to two-stroke engines of the type we have been discussing, any slight increase in per formance that might be realized from the better scavenging of the cylinder will just about be balanced by the added power needed to drive the supercharger. On the other hand, the two-stroke en gine can be supercharged, profitably, whenever the valving system permits the closing of the exhaust before the transfer ports. This is true of the twin-piston lay out like that used by Puch, and of such engines as the GMC two-stroke diesel, which has its scavenging air supplied by a Rootes-type supercharger. You are cor rect about the limitations of the vane type blower. Its parts are not entirely in

balance and it cannot be run too fast. The centrifugal supercharger is good at very high rotational speeds, but has poor lowspeed output characteristics. In the end, the only two-stroke engines that perform satisfactorily in supercharged form are those that are designed specifi cally for that purpose. Then, the proper steps can be taken to insure that the supercharge is not lost. out the exhaust, and the rather bothersome feature of crankcase scavenging can be eliminated altogether. they have never shown any advantage beyond the ability to run satisfactorily with a slightly higher compression ratio than their poppet-valved counterparts. The Yamaha engine has a rotary valve, as you say, but it is a different thing al together. In that engine, the rotary valve is used to admit the mixture to the crank case (it being a two-stroke engine) and in that kind of duty the rotary valve is not subjected to the extremes of heat and pressure that make it so grossly impracti cal as a replacement for the poppet valves in a four-stroke engine.

AND YET MORE PUCH HOP-UP

I would like to give you a little informa tion about Puch motorcycles, which I have had some experience with. A light-alloy barrel is available from the factory in Austria. The barrel has hard-chromed bores and is available for both the 250 and 175, and both enable two carburetors to be fitted. Both Emil Jerinkitsch and I road-race our Puchs and neither of us has ever ex perienced overheating, which is an ad vantage over other two-strokes. A nice letter to the factory (Steyr Daimler - Puch AG, Werke Graz, Aus tria) will be greeted with enthusiasm and the return mail will bring tuning informa tion and possibly the exact output of the 250 and 175 with the light-alloy barrels installed. I don't know the exact prices, but the barrels and pistons for the 250 and 175 are about $90 and $65 respectively, and a close-ratio gear cluster is also available for about $25. The tuning manual that is sent free on request says that the compression ratio may be raised, and gives dimensions for a tuned exhaust system. I have noticed that your technical articles have tapered off to nothingness. How about an article on frame, steering and suspension design? Fraser A Ian McA ninch Hamilton, Ontario