The Service Department

May 1 1963 Gordon H. Jennings

The Service Department GORDON H. JENNINGS May 1 1963

May 1 1963 Gordon H. Jennings

THE SERVICE DEPARTMENT

GORDON H. JENNINGS

I HUMBLY BEG YOUR PARDON

This is a sort of letter from me, the technical editor, to you, the readers who have sent letters, asking questions about things that fall into my domain. Many (indeed, most) of those who have done so will know that I have been sadly remiss in keeping up with my correspondence.

Much as I would like to be more helpful, there is not time to answer each letter. But, do not stop writing; the letters I get give my efforts direction, for they tell me what it is that most interests the readership. And, those letters that I consider to be of special interest will be used in this column.

OVERCARBURETION?

Trial and error has produced only error, and I am truly desperate.

The problem concerns my Zundapp 250. I equipped the machine with a Dellorto SSI 35B (1 3/8") carburetor. 1 have slides #100, 110 and 120; mainjets 100 to 180 in increments of 5. The needle appears to be marked IP1 and the needle jet is 320.

The engine runs very lean no matter what size mainjet is used. I have also tried the float chamber at various levels with no change. I am convinced the trouble lies with the needle and needle jet but I don't know where to start.

Fred Arth Canton, Ohio

The problem sounds to me very much like overcarburetion. Of course, long distance diagnosis is by no means certain, but the symptoms you describe point to a carburetor that is too big for the engine.

Such difficulties are easier to detect in a four-stroke engine: there, overcarburetion will make the engine sputter and cough back through the carburetor if the throat is too big. Two-strokes, on the other hand, will continue to run in the face of almost anything — albeit without producing much power — and that may obscure the source of the trouble.

Basically, the trouble is that with a carburetor throat that is too big, the air flow through the venturi area is so low that it does not pick up fuel properly from the spray nozzle. In such cases, the mixture would tend to get richer at high engine speeds.

I would suggest that you try a smaller carburetor. There is nothing wrong with the TT-pattern Dellorto; you have just tried for a little too much of a good thing. If the engine is otherwise stock, with regard to the porting, etc., a throat size of about 1" to 1 1/8" would seem appropriate. With modified porting, you could go up to 1 3/16"; I would not recommend anything larger for this particular engine.

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Please bear in mind, though, that twostrokes have been known to run very well on tremendously big carburetors and your Zundapp might be one such example. Before discarding the present 35mm carburetor, make absolutely certain that there are no restrictions to fuel flow between the tank and the spray nozzle other than the mainjet itself.

Do not concern yourself too much with the needle; it is effective only through the 1/2to 7/8-open throttle range. With the throttle slide fully open, only the main jet has any effect on mixture strength — or so it should be when everything else is right. The needle jet is another matter: if it were really too small (an unlikely occurrence but one that could happen), then that would make the mixture too lean under high engine speed/full throttle conditions. If a larger needle jet is not available, bore out the one you have. That sort of thing is barbaric, to be sure, but there are times (and I think this might be one of them) when extreme measures are justified. When doing this boring, however, try to duplicate any lead-in countersinks in the jet, and use good, sharp drills. Drill out in easy stages, using successively larger drills, and testing after each change, until the jet is large enough. A large selection of small, precision, numbered and lettered drills is of great value in this kind of work and if you do not have them, then they should be borrowed or bought; the success of the whole operation will depend on the care you put into this drilling.

COMPRESSION RATIO

I would like some advice concerning the Norton Atlas.

Are there any disadvantages, dangers or any other objections in a change of compression ratio from the standard 7.6:1, to 9:1?

Dennis Muenster Green Bay, Wisconsin

If you want a simple, straightforward reply, the answer would be "yes." There are disadvantages, dangers and other objections to raising the compression ratio much above its present level.

Of disadvantages, I would say that the biggest problem would be in getting the Atlas engine to run-through on a cold morning. It is a big engine, and it takes a lot of pressure to crank it over smartly enough to make it fire. This is complicated, to some extent, by a short, highgeared starter crank and when the engine is cold and the oil is thick, it takes a very determined kick to snap it through. A higher compression ratio would make this even more difficult — particularly for the small rider.

Under the heading of "dangers," I need to list only one: there is an outside chance that you would quickly ruin the engine. High compression ratios force a lot of combustion heat into the cylinder, cylinder head and pistons. This latter component is the one that would suffer most, and if the thermal loading was too high, then the piston would fail — with expensive consequences. And, there is always the possibility that the same thermal overload will also occur at the valves, with valve burning or outright mechanical failure as a result.

Other objections are the expense involved in accomplishing something that might not be successful anyway; there are difficulties in re-tuning an engine that has been fiddled (and these are more of a problem than one might think), and high compression ratios can produce serious combustion roughness, and this roughness will be felt by the rider.

ON OILS...

Most cycle dealers and mechanics I have talked to say it is unwise to use a detergent type motor oil in any cycle engine. Why? Would it be because of a lack of film strength at high operating speeds and temperatures or because of the dangers of foaming?

John C. Tegge laar Homewood, Illinois

There is an overall opinion among people in motorcycling that nothing but straight mineral oil, or perhaps degummed castor oil, should be used. This may have been correct at one time; it is not true today. Non-additive oils will do the job, but they usually can be made better by the addition of detergent and antifoaming chemicals. Detergents clean away carbon and general sludge inside the engine as soon as it forms, and keeps this sort of material from collecting in pockets that eventually breakaway in lumps — in which case they can do great damage to an engine. The little particles are far less dangerous when dispersed throughout the oil. Also, detergents clean gum and varnish from the pistons, and keep the rings free in their grooves and functioning properly. Anti-foaming agents keep the oil from turning into a froth, which can cause the sump to load and make the engine use oil. And, there are acid-neutralizers in most oil; very strong acids, mostly sulphuric and nitric are formed in the combustion process and these can, unless neutralized, do a great deal of damage. This is especially true when the engine is used mostly for short-haul work, when it seldom has time to get thoroughly warm.

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It may be true that these additives dilute the oil somewhat, and adversely effect their lubricating properties; even so, any loss is more than offset by the benefits we have outlined. Of course, racing engines are not subjected to the sort of long term service we give our touring engines, and only anti-foaming agents would seem to have much benefit there.

I would like to mention that unless a manufacturer specifically recommends a multi-grade oil for his engines, such oils should not be used. Makers of the oils might like to argue the point, but experience indicates that in some instances, troubles with high-load areas like cam followers have developed after a switch to a multi-grade oil.