The Service Department

THE SERVICE DEPARTMENT

GARY BRAY

HORSEPOWER INCREASE

I would like information on raising the horsepower on my 1959 Zundapp 250. 1 have already fitted a Webco high compression head. I would like details on porting and stuffing the crankcase. Bill Mark Berkeley, Calif.

The problems involved in porting a two-stroke can many times cause the inexperienced tuner a loss in performance. I would suggest in this case that the ports be smoothed up and all rough edges removed. This would include intake, transfer and exhaust ports. Care must be taken not to change the shape or size of the transfer or exhaust ports. If this machine is to be used in competition I would recommend a 1-3/16" Amal GP carburetor. If this carburetor is used, then the intake port must be matched to it. Porting under these conditions is best accomplished by using a small high-speed hand grinder.

Packing the crankcase for an increase in power will help, but only on top end. Contact Webco for information on crankcase padding. Ignition timing is also very important. If a magneto is being used, 1/4" BTDC is correct. If a battery system is used, then .157 or 23 degrees BTDC should be employed.

The exhaust system is of equal importance. I think the Bultaco Scrambler type exhaust system will give you the best results. In other words, the size, length of pipe and shape of expansion chamber should be as close to that of a Bultaco as possible.

COOL RUNNING

I own a Yamaha YDS-2. I will be touring the Southern part of the U.S. this summer and I expect to encounter 120degree heat on the road in Arizona. Do you have any tips on keeping the engine running cool in that kind of heat? William B. Butler Hawthorne, Calif.

Raising the needle position and main jet size may help, although the increase ir."altitude in that area could make this unnecessary. An increase in fuel consumption will naturally result from richening the mixture, however. I would suggest using a colder plug on the road, say an NGK 8H or any brand of comparable heat range. Moderate speeds will also be easier on your machine. One obvious way around the heat of the day is to schedule your trip so that you cross the desert at night. A complete tune-up, with much emphasis on ignition timing, should be a must just before you leave. Incorrect timing is in many cases responsible for seizing, burning holes in pistons, etc. The exhaust system should also be inspected, and exhaust ports and mufflers cleared of carbon.

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HONDA 350 CONVERSION KIT

I have some questions about the 350 conversion kit for the Honda 305. Since the Forgedtrue pistons are lighter should the engine be balanced? Do the pistons come in oversizes? The stock Honda cam comes in about 6000 rpm; when does the #7164 Harman & Collins cam come in? What combination of jetting and spark plugs should be used? Gordon O. Kennedy El Granada, Calif.

Usually when a product is manufactured to increase performance, many hours go into both design and testing. In this instance, the 350 Honda kit has been designed to give an increase in performance with the least amount of trouble.

The fact that the pistons are lighter is actually an advantage. Lighter weight means less reciprocating weight, tending to favor a better balance factor. Unfortunately, pistons are available in only one size (64mm) at this time.

The #7164 cam begins to become active between 4000 and 4500 rpm. Jetting and spark plugs used in this article on the 350cc conversion kit (CW Oct. 1963) were stock. I feel that stock jets and plugs are suitable for touring, but if any high speed work is to be done, then colder plugs would be advisable. As far as jetting is concerned, one usually has to go up one or two sizes, but location, brand of gas and atmospheric conditions are critical factors in the final selection of jets.

PISTON SWITCH?

1 own a Ducati Bronco 125cc and wonder if it is possible to raise the compression from 6.8:1 to 8:1? Is this possible by using the piston from a 125cc Ducati Super Sport as both engines have the same bore? Will this increase in compression harm the engine? P. Scott Foster Knoxville, Tenn.

Unfortunately this piston change is not practical. Although these machines have the same bore, the shape of the piston domes are entirely different. The 125 Bronco piston has a flat top whereas the 125 Super Sport has a high, curved dome. Also, the valve pockets in the Super Sport piston are not located to accommodate the shallow valve angle of the Bronco. In many cases the valve pockets can be altered but, due to the fact that the Super Sport piston is a very light casting, this is of course impossible.

The main problem we are dealing with here is the difference in combustion chambers. The Super Sport has a deep hemispherical chamber and the Bronco a flatter chamber with a shallow valve angle. There are two ways that I know of to increase the compression ratio: you can weld the top of the piston and re-machine it; or mill the cylinder head, or cylinder itself. These methods are many times used in building racing engines and the job should be done only by someone who is experienced in this type of work. As for an increase in compression being harmful, the Bronco 125 has an extremely sturdy lower end and could certainly stand more compression. If an increase in performance is desired and the methods I have outlined are impractical for the owner to undertake, I would suggest installing a 22mm carburetor. If this is done, the intake port should be smoothed and matched to the larger carburetor. Installing a 1/16" larger intake valve will also give a definite advantage.

HAWK TALK

I own a 1962 Honda Super Hawk 305. It has a tremendous 1st gear but will only wind up to 7000 rpm before it starts screaming. My 2nd gear goes fairly quickly to 8000 rpm but it takes forever to get to 9000 rpm and is just wound out at 9000 in 2nd. My 3rd gear is worn out some (1 can tell by the tinkling noise it makes in 3rd) and it will not wind over 7000 rpm. High gear seems to be in pretty good shape but, running flat out at top speeds, it will only reach 7000 rpm and a bit over 95 mph. I know it should wind much farther, but it won't. Bob Nelson Fort Lauderdale, Fla.

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It sounds like your machine is very much in need of work, possibly a ring and valve job. You stated that your CB77 was a 1962 model but mentioned no information on total mileage, mileage between valve jobs or length of time between tune-ups, if any.

If this machine has 10,000 miles or more on it and has not had a tune-up and the valves refaced and seated, it definitely should be done. A motorcycle tends to lose its edge quicker than, say, a car. This is due to higher rpm, greater heat, and the fact that a motorcycle engine develops a tremendous amount of power for its size, proportionately more than any other type of piston engine.

FLYING HIGH Having seen the picture of Sonny Nutter sailing gallantly through the air at Ascot astride his G reeves, 1 decided to learn the basics of jumping. I used the lip of a dry floodwater reservoir which was about 12 ft. long and 7 or 8 feet high. 1 hit it at about 40-45 mph with my 45 cu. in. 1947 Indian Scout. The results were nearly disastrous.

The take-off was smooth, but the landing left something to be desired. When the rear wheel touched down, the front wheel was still over 2 feet in the air - the makings of a real jolting experience. My fellow riders tell me it's my bike's fault becase it is too heavy, the foot risers are not well positioned and because of the total lack of rear shocks or springs. Personally 1 think it's my speed and form that are off. What's your view? Roger Patton Baker, Oregon

I feel your analysis of the problem is correct. Form and experience contribute to the end result and jumping a machine takes practice. If you are going to continue this as a form of recreation I would suggest using a smaller jump. Every jump has its own peculiarities; some can be attempted at full throttle, some at halfthrottle, etc. Usually on a long jump, as the rear wheel touches, the throttle should be brought back on. This will tend to ease the front end down.

As far as the weight of your machine and lack of springing is concerned, several years ago riders had to use rigid Indians and Harley 45s in T.T. racing complete with jumps. These machines were probably lighter than yours, however, due to the fact that lights and non-essential accessories had been removed for racing. Positioning of pegs and controls are important, as well as the distribution of your own weight, but all of this comes with experience.

You said in your letter that the rear wheel came down first. The rear wheel touching first is the accepted method; for heaven's sake don't come down front end first. •