The Service Department

THE SERVICE DEPARTMENT

JOHN DUNN

A7 ASTHMA

I am writing to you concerning a Service Department letter from Ernest Zimmerman (CW, Oct. ’68). I had the same trouble with my Kawasaki'350 A7 just after I got it. The trouble was really embarrassing, because I was the motorcycle mechanic at the marina I worked for. We sold Chrysler boats and outboard motors and Kawasaki motorcycles. When I first got the bike, it ran fine, but after I had some miles on it and started revving it up higher, it developed a high speed miss. I did all the things that Mr. Zimmerman did and then some. It still missed. Finally I tried running it with the rubber covers off the carburetor covers. This allows the carburetors fresh air with no restriction. It was like a new bike! The miss was gone and I must have gained 5 hp.

Now that I knew it was an intake restriction, my first thoughts were of the air cleaner. It was completely cleaned out and still the engine missed. I finally tried a rather expensive experiment. When building Kawasaki engines for racing, I had always cut off about 0.25 to 0.375 in. of the carburetor bell to allow more air flow. It seems that the cover is simply too close to the carburetor mouth, because after the modifications the engine ran perfectly. The miss is gone and it will run up past 10,000 rpm without a trace of a problem.

When cutting off the bell on the carburetors, I took off all the metal that seemed to be in the way. Also the main jet metering needle had to be raised up one notch (the clip is in the middle groove). The float level should be set at 1 in., measured with the carburetor upside down, from the gasket surface to the “bottom” (now up) of the float. The idle mixture had to be richened out slightly, too.

As to the problem of fuel leakage, check the needle valves and seats and the carburetor cover vents (on the outside of the covers, below the carburetor). Make sure they are clean or the carburetors have a tendency to leak a little.

Bruce Bash forth Burbank, Calif.

Thank you for your very informative letter, Bruce. Your diagnosis and modification of the intake bell length is quite feasible. It is recommended in the Yamaha tuning manual to pay particular attention to this point when preparing a machine for racing purposes. I cannot understand why Kawasaki did not pick up this apparent (according to your experiences) fault during the design and development phases. Perhaps the fault is borderline and does not occur in all cases. To my knowledge there are many 350 A7s operating without adverse effect. However, your experience is worth mention and further investigation particularly by others who experience a similar problem. A small reduction in intake tract length (0.25 to 0.375 in.) would not cause any noticeable adverse effects on general engine performance.

STREET-STRIP TRIUMPH HOP-UP

I am rebuilding the top end on a 1968 Triumph Bonneville, As I would like to make the engine a little warmer, I plan to use Mahle 10:1 compression ratio pistons, larger intake valves, light alloy pushrods, heavy duty valve springs, lightweight collars and keepers, racing tappets, etc. (All the parts listed with the exception of the pushrods are cataloged by Triumph.) The rest of the engine including carburetors and cams will remain stock.

Now for my questions. Is the compression ratio I have chosen too high? I use the bike mainly for transportation, mixed with an occasional highway jaunt and a few runs a summer at my local dragstrip. I am mainly concerned with overheating problems that might arise with these pistons. Second, how should I rejet my carburetors? I am using stock main jets, and the needle position is in the middle notch. Is there anything else I can do to lighten up the top end? As I plan to use the stock exhaust system, will I realize any appreciable power gain with these modifications? I have been considering a set of TT pipes for the dragstrip. I will appreciate any help you offer.

Mike Stoll St. Paul, Minn.

Providing that you use a premium grade of gasoline, a compression ratio of 10:1 is not unreasonable, even for street use. However, it is advisable not to let the engine labor under load at low engine speeds. A little more use of the gearbox will be necessary to allow the engine to run freely and smoothly.

The operating temperature of the engine will be higher because of the higher expansion ratio and the increased output. To combat possible deficiencies in the cooling capacity of the engine, it may be necessary to go slightly richer on carburetion, thereby using the latent heat of evaporation of the additional fuel to act as an extra cooling medium. No more than a small change in main jet size should be required. It is usual for an engine with an increase in compression ratio to respond to a reduction in spark advance. A reduction of two degrees would be well worth trying.

When installing the larger intake valves, enlarge the valve throat and pocket by an equal amount, being careful not to destroy the existing port shape in this area. Blend the modified valve pocket gradually back into the port, retaining the same size at the carburetor flange. This modification, even when employed with the standard camshaft, will result in a useful increase in volumetric efficiency. A small change in jet size will be required to compensate for this change. It also may be necessary to raise the needle position to improve mid-range response.

Before final assembly and balance, make sure that the piston crown-tocylinder head clearance is adequate and that the intake valve relief is of sufficient diameter and depth to accommodate the larger intake valve.

The standard rocker arms can stand some lightening and polishing. It is best to remove most of the excess material from each end of the rocker arm, because the ends move at the highest speeds and therefore have great inertia. Do not remove large quantities of material from the heavily loaded structural areas. A highly polished surface in the latter areas is good because it reduces the chance of breakage resulting from fatigue.

(Continued on page 18)

Con tinued from page 15

X-6 VOLTAGE LEAK

I own a very nice Suzuki X-6 250 motorcycle. It’s been very reliable, but I’ve had a few problems. One is that the spark plug wires from the coil are old and I can’t keep the spark from grounding on the frame. I’ve tried wrapping it with electrical tape many times, but it still manages to leak. I’ve sprayed the wires with sealer, but it still leaks. I find if I put a couple of pieces of wood on the wires it helps, but still leaks a little. I’m desperate because I don’t want to shell out 20 bills for a couple of coils and wires because I need just the wires and the coils are still good. What do you suggest?

Mark Gigas Camarillo, Calif.

Normal insulation tape is not good in areas where high voltage is concerned. It mainly is intended for wiring where the voltage is relatively low (110 volt). I cannot name a make or source off hand, but I am sure there are several types of plastic tubing available that would slip snugly over the high-tension wires and provide adequate insulation. Try some of the electronic supply houses and inform them what the application is. On some Hondas, the high tension wires are clipped into recesses in the cylinder barrel. The wires have been sleeved with plastic tubing to protect them from the heat. Perhaps this tubing would do the trick for you.

A CLUTCH IN A PINCH

I am the owner of a 1967 Hodaka Ace 90 equipped with Barnett heavy duty clutch plates. My problem is they drag terribly when idling in gear. Especially when the engine is cold, I can squeeze the clutch and put it in gear and it will jerk forward and die. Adjusting the control cable did not help at all.

Jim Janssen Sepulveda, Calif.

The clutch drag that you are experiencing is due to the clutch not fully disengaging. The clutch cable is intended for operating the clutch and should not be used as a means of adjustment. However, the clutch cable should be adjusted to provide the correct amount of free play at the handlebar lever. Measured at the tip or ball end of the handlebar lever, the Hodaka should have 0.25 in. of free play. If the cable is adjusted so that no free play exists, this will result in rapid clutch wear and damage. The other end of the cable is attached to the clutch actuating lever which is at the front of the clutch side cover. Checking the free play of this lever will show if the clutch is properly adjusted internally.

(Continued on page 20)

Continued from page 18

Remove the cable from the clutch actuating lever and check its free play. If the clutch is in proper adjustment, there will be 0.093 in. of free play when measured at the tip of the lever. If the amount of free play is incorrect, it will be necessary to remove the clutch side cover to make the necessary adjustment. Before removing the cover, fully depress the kick start lever and lock it in this position by inserting a screw driver in a convenient hole. Remove the transmission drain plug and drain the transmission oil. The clutch side cover is secured by four Phillips head screws. These should be removed with a hammer drive tool. If the cover is stuck, tap around its edges with a soft mallet. Remove clutch cover making sure that the clutch operating cam does not fall out. Put it in a safe place until reassembly. In the center of the clutch you will notice the clutch operating disc. Behind this disc you will find one or more clutch adjusting shims. By adding more of the shims you will reduce the amount of free play at the clutch actuating lever and conversely by removing shims you will increase the free play.

If incorrect adjustment was the cause of your problem, it will be necessary to add shims to obtain the correct free play. Shims are available in 0.2, 0.3 and 0.5 mm for this purpose. To reassemble, install estimated thickness of shims, replace operating disc, cam and cover. Check free play of actuating lever (with clutch cable still disconnected). If the free play is still incorrect it will be necessary to remove the cover and add or take away shims until the desired free play is obtained. On completion of this adjustment, replace drain plug and refill transmission with the correct grade of oil. The manufacturer recommends that SAE 30 W oil be used when the temperature is above 32 F and SAE 20 W when it is below 32 F. Using a thicker oil than recommended would also cause the clutch to drag.

Another thing that could be causing the problem is that the new Barnett clutch plates were not checked for correct thickness. The correct thickness for new friction plates is 0.078 in. If the plates were too thick, the clutch would drag even if properly adjusted. [g]