The Service Dept

THE SERVICE DEPT

WALT FULTON

WEBCO CYLINDER HEAD FOR YAMAHA 400 MX

As we pointed out in last month’s test of the Yamaha 400 MX, the bike suffers from an almost critical level of engine heat. Webco is aware of this problem and is in the process of affecting a cure. The dyno indicates that all two-strokes suffer from heat. Because of this, most horsepower readings are obtained by holding maximum power for only a few seconds. These are different from flash readings in that the power has stabilized.

Yamaha’s off-road bikes with the high pipes tend to run hotter than most other two-strokes. The three reasons for this are obvious. The pipe, which runs directly across the head, retains a tremendous amount of heat. In addition, placement of the pipe also restricts the flow of air across the head; and the fin area must be cut down for the pipe to fit in position.

So far it sounds as though Yamaha is in trouble and lucky to finish anything it starts. Nothing can be farther from the truth, as a look at the record books will bear out. But, wouldn’t it be nice if Joe Average could go to his local dealer and for $49.95 bolt on a part that would improve the cooling and at the same time pump up the horsepower over the entire operating range?

Webco has such a part, a trick cylinder head. Unfortunately it won’t be available until after you read this, sometime toward the end of summer. This 356 aluminum casting with a T-6 heat treat will come in anodized black only and does what Webco claims. Take a look at the comparison charts of the same bike run on the same day at the dyno. As you can see there is a substantial increase in the horsepower column and a 10 percent improvement in cooling.

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You weight conscious riders will be disappointed to learn there is an increase in weight, but that is the price you have to pay. Fin area has been increased by 35 percent. Combustion chamber volume has been reduced and shape has been altered. This aids the flow of gases in the chamber. The spark plug location remains the same as with the standard head. There is an additional boss cast into the head that can be drilled and tapped to accept a compression release.

Horsepower means heat. The more horses the more heat; but Webco has managed not only to increase hp, but also to aid engine cooling with this inexpensive part. The average person can install the head with nothing more than a torque wrench. CW feels that this mod is a definite advantage for the serious racer or weekend gladiator.

DYNAMOMETER TESTS HORSEPOWER AND TORQUE

RESPONSE TO END RESPONSES

I am writing this letter in response to Peter Holmes’ letter in your Feb. 1975 issue. I agree that there are many people who own Yamahas and other two-stroke bikes with nagging plug-fouling problems. I am no exception; Fve had my share of them too. Fear not, good fellows, I have solved most of these problems. Being an avid reader of CYCLE WORLD and other magazines, I have discovered the secret of making RD350 Yamahas run some distance without fouling plugs or covering several counties with an oily fog. Through the adept use of proper adjustment procedures and pure wizardry, you too can have a fogand foul-free Yamaha.

First, do not run high-test gasoline. The high lead content only serves to foul plugs prematurely. Besides, with a corrected compression ratio of only 7:1 you don’t need it. Use low-lead or Amoco.

Secondly, set your oil pump (not the cable) to .004 or .005. Most are set between .008-. 012, since new engines run hot. Many dealers may do this on purpose to new machines to keep them from seizing while new. Yes, you’ll still get plenty of oil; I took the barrels off after 2000 miles to decarbon and polish the innards and the pistons were practically soaking in oil. (My 1973 RD350 is set at .004, and my wife’s 1974 is set at .005). The little spacers can be had from any dealer for pennies and come in various thicknesses. Floyd Clymer’s manual tells you how to do it on pages 34 and 35.

Thirdly, dealers sometimes put richer main jets in when new; again, to keep them cool. Having the needle raised a notch will have the same effect. An overly rich condition will foul plugs within 25-50 miles (as mine did). Do not cut or alter the exhaust baffles and then raise the needles to get more power. . .It doesn’t work!

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Four, I read somewhere that RD350s have slightly weak ignition coils with resistor-type spark plug caps. These conditions may cause it to misfire after some low-speed running, like when you’re cruising in fifth for awhile at about 4000 rpm and then go to pass a truck. It usually clears out, though.

The above procedure, assuming all else is tuned and in synchronization, will cure your ills. My bike and my wife’s bike both give us no trouble whatsoever, and-get this-we both get 65 mpg when hotdogging and 85-90 mpg when cruising at 55-60 mph! These feats have been witnessed by friends on several occasions while filling up at the gas pumps.

We ride with several other people who have similar Yamahas and they get results almost as good. They, too, have no problems at all with their bikes.

I must agree with you, though, that there are bikers who bought RDs that were improperly set up; they had nothing but headaches with them. An acquaintance of mine had an experience similar to yours. The dealer told him all sorts of lies. They installed plugs that were too cold, the carbs were set all wrong, the oil pump was way too high, and he ran high-test gas. I spent four or five hours on it and he hasn’t had a problem since. He has a 1973 RD250. If the damn things are set up right, they’ll run right just like any other bike.

I haven’t done the non-resistor plug caps or new coils yet, but I might. I do use Champion plugs (either L78s or L81s), they’re cheaper and just as good as NGKs. I run regular Gulf gasoline (usually) and Castrol two-stroke oil. I get about 600 miles per quart of oil.

I hope I’ve helped some of you frustrated plug-foulers out there. I’ve been through it myself, and it’s a real hassle.

Rick Buck Glenside, Pa.

You offer both dealers and riders good advice, Rick. If more individuals set up and maintained their bikes to factory specs, the majority of service problems would be nonexistent.

Your points on fuel are well-taken. In fact, Yamaha recommends a low-lead fuel with a minimum octane rating of 91. The lead adds a certain amount of stability to the fuel as the octane is increased and also aids in lubrication. It does, however, foul plugs. Because of the low compression and small bore of the RD350, leaded fuels are not necessary.

I am a little skeptical of your claimed mileage figures. As you know we were able to average only 35 mpg. This figure was arrived at after several weeks of testing under all conditions.

Proper service is the key to a smoothrunning bike or car, or to anything mechanical for that matter. Too bad more people don’t see things this way.

Now, let’s just hope this puts an end, once and for all, to the stacks of mail we get complaining about or offering solutions to the RD350 plug-fouling problem!

Z1 CARBURETION

I own a 1973 Kawasaki ZI. I am very happy with it except for one problem that myself and several friends with Zls have experienced. There is an annoying hesitation just off of idle that is bothersome, especially on such a heavy bike whose low-speed control should be precise. I understand that the carburetors have been redesigned on later Zls to alleviate this problem, but can anything be done on the old carburetors to improve their performance. Kawasaki also changed the spark advance unit, which I have done, but the improvement with this change alone is minimal.

Bob Unger New York, N. Y.

I know just what you are talking about. I have a 1973 Z1 also, but the modifications you mention haven’t been carried out on my bike yet. It’s like the plumber with stopped-up drains.

The new advance was to be used in addition to the carburetion changes. In effect, the initial advance is changed from five degrees to 20 degrees; total advance remains at 40 degrees. The bike idles a little rougher, but smooths out just above this point. This was necessary because of the richer settings needed to get rid of the flat spot you refer to. Needle jet, pilot jet and slides are richer. Your dealer can give you the exact numbers, but good luck in finding the pieces. It has been our experience that these parts are not available even through Mikuni America. The only other alternative you have is to replace all four carbs with the updated parts— and that gets expensive.