The Service Department

THE SERVICE DEPARTMENT

BY GORDON H. JENNINGS

INLET/OUTLET PLUMBING

How do siamesed pipes work, both for vertical and Vee twins?

Also, does the position of the carburetor along the length of the tuned intake pipe make any difference?

Dick Cormany Sherman Oaks, California

Not knowing which siamesed exhaust system you have in mind, it is difficult to give you an exact answer. Generally speaking, however, the siamesed systems used on motorcycles are intended solely as space and weight saving efforts. On the other hand, there are circumstances where the siamesed exhaust system might be used to improve engine performance.

It is possible, by using pipes of the proper diameter and length, and with the correct angle where they join (about 30 degrees), to induce the out-rushing flow of exhaust gases from one cylinder to extract exhaust gases from» the other cylinder. To get this effect, the pipes must be relatively small in diameter and the junction of the pipes should be at some distance from the port.

Due to the wide spacing of exhaust pulses from a twin (further complicated by the unevenness of the pulses in a Vtwin) it is very difficult to get a true extractor effect. In most cases, it would not be feasible to arrange for any extractor effect with fewer than four cylinders. And, in the final analysis, there is probably more power to be had from the megaphone-type system anyway.

The position of the carburetor along the length of the entire induction tract will most often be determined by convenience. Usually, the correct tuned length will be just long enough to accommodate the port, from valve-head to the face of the cylinder head, the carburetor body, and a short air-horn to smooth the flow of intake-air.

When the tuned length calls for more than that, the extra pipe should be placed between the carburetor and the cylinder head — and it is advisable to use a short piece of rubber hose for this purpose as it dampens some of the vibrations that can upset carburetion. Too much pipe out in front of the carburetor is not good practice: the induction pulses get stronger down toward the valve-end of the induction pipe, and these pulses can, when the pipe is in a state of resonance, create some very peculiar fuel-flow behavior in the carburetor.

HESITATIN’ HONDA

ƒ own a Honda “50” which has about 2600 miles on it. For the last 700 miles it has cut out at 40 mph and above. It does not always miss exactly, but hesitates as though there were a loss of power. Also, occasionally there is a sound like the cylinder was being hit with a tack hammer.

The Honda has never failed to run or start and generally runs good at lower speeds. Sometimes the difficulty will clear up for a while.

I removed the oil line which goes to the valves and oil flowed while the engine was at low rpm, but stopped when the engine speed was increased. Could you tell me what could be the difficulty? 1 have always kept an adequate level of clean oil (Castrol 30 wt.) in the engine.

A. C. Hollister Jr. Amite, La.

In all probability, the problem you have with your Honda is caused by some kind of malfunction in the engine’s oil pump. The power loss and hammering you describe are symptoms of valve-gear oil starvation, which causes the valves to stick. The loss of power is produced by the failure of the valves to seat properly, and the sound you hear is the pistoncrown battering against the stuck valves.

The reason this problem only appears at speeds above 40 mph is that the engine’s pump can push enough oil to keep everything lubricated at low speeds, but the oil-pumping effects of a fast-turning crankshaft is pulling away all of the pres sure when you go faster. Centrifugal force, acting on the oil in the crankshaft's drilled passages, increases the flow to the connecting rod a great deal as the crank speed increases. The crankshaft acts as a secondary oil-pump, and in this instance, draws off the oil that should be going to the valves. The solution to the problem consists of finding the fault in the oil pump.

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ERRORS AGAIN

Here now! You numbskulls in the tech. nical department have done it again. You made a stupid, blundering remark about the two-stroke engine being invented in Germany (in last month's S1aico road test) when you know very well that it was the brain-child of Sir Dugald Clerk, an En glishman. What will you tell all of the people who write to inform you of your error?

Joseph C. Parkhurst

Editor, Cycle World Magazine I will tell them that we were wrong (exceedingly) and that we are contrite and humbled (but not very). •