Service
SERVICE
Len Vucci
We welcome your technical questions and comments, and will publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return any personal replies. Please limit your “Service” letters to technical subjects only, and keep them as brief as possible. Send them to: “Service, ” CYCLE WORLD. 1499 Monrovia Ave., Newport Beach, Calif. 92663.
RD350 JET
This is in regard to the article in your March, 1978 issue on the DG expansion chambers and heads for Yamahas RD350s. You stated that the RD in the test had # 105 main jets prior to the installation of the speed equipment. In order to run 105s you must be using the stock air filter which contains an air restricting baffle designed to limit air intake noise.
When I purchased my ’75 RD350, I was having trouble with plug fouling, and was told by a Yamaha service rep to replace the stock filter with one from a 1974 model, which has no baffle. In order to do this I replaced the stock jets with # 140 jets (per instruction of a Yamaha service bulletin). It also gave instructions to solder all connections on ignition wiring, thus bypassing the original male/female connectors. I am fairly certain that the plug fouling problem was related to these connectors, especially after damp weather. The plugs weren’t actually being fouled and then refusing to fire, but rather the connectors were becoming damp and reducing voltage to the plugs, causing the plugs to get oil coated and unable to fire when the voltage returned. (I found that the problem was intermittent, and the bike would run fine if a new' plug w^as used, but after a few miles the same problem would occur.)
Soldering these wires seemed to be the perfect cure, and the replacement of the restrictive air filter and installing richer jets was a great improvement—the bike ran much better and seemed noticeably faster. Since I’ve never read an article dealing with the RD plug fouling problem treated in this manner, I thought you might like to know of it. Also, try the pre-1975 air filter and 140 jets, you'll be amazed at the difference.
Francis J. Arena Yonkers, N.Y.
Thanks for the plug fouling tip.
There was indeed an air filter and jet change and it occurred during the 1975 model run. RD350Bs of 1975 and earlier, up to and including serial #308120, used the less restrictive air filter. Its part number is 2781445100, and was used with the #140 jets. The idle air screw adjustment recommendation is FA turns with this setup.
From serial #308121 on, the baffled air filter, part #3601445190, was used with # 105 jets. Adjustment of the idle air screw is F4 turns.
Because the major increase in fuel/air flow will be at high rpm, the around-town rider would probably notice little improvement in performance. But for serious pavement scratchers, the improvement should be well worth the small expenditure.
TIRE ROTATION
I own a 1973 Norton Commando with the original Dunlop K81 tires. I’d like to mount the front tire on the rear to get some additional use out of it before it weather checks. It does not have directional arrows as do the newer K81s, and I wonder whether I should reverse the rotation or not. ;
Why do the newer tires have arrows in the first place?
Keith Gillette Bridgeport, N.Y.
The construction of most motorcycle tires is such that power should he transmitted through the tire in one particular direction. The main force at the rear is driving, although it does perform some of the braking; at the front, it is solely braking, a force in the opposite direction. If a tire has arrows, their direction will be opposite for from and rear use.
If the tire is devoid of directional arrows, and has been seasoned by use at the front or rear, its position relative to (he applied forces should not be changed. If a tire is removed and replaced on the same end, be it front or rear, its original direction of rotation must be maintained. If it is swapped front to rear, or vice versa, its rotational direction should be reversed to obtain maximum tire life and performance.
FORK LOVER/HATER
A friend of ours is a racer who owns a Suzuki RM250C equipped with the latest air/coil fork from end. He came to us complaining of a love/hate relationship with the forks, in which he uses 18 psi in front and the standard level of 10 wt. oil. Seems as though that setup works fine in his garage when he bounces the front end. He said il also feels OK at the track at first, hut by the end of the second or third moto the forks get very harsh and feel like they are binding up.
He began to wonder if it was just his arms getting tired because he had checked the air settings at home the day after a race and the pressure was right.
We've had similar questions from other folks, so we thought an answer here might be appropriate.
Sounds like the problem is excessive air pressure. With the forks cool, as they are in the garage or out on the track at the beginning of the day's ride, the statie air pressure is indeed 18psi. Bui as the fork internals are heated by the rigors of a moto, the air pressure increases. This increase has several effects. One is the change in effective spring rate. With the air in the forks heated, it's as though a stijfer spring were installed.
Another, more severe effect is the increase in seal friction. With the fork tube uncapped, as is the case when we test seal friction on the dvno, the values are low, typically about 10 lb. per leg. But when the cap is replaced, thereby sealing the fork, the situation changes dramatically. As the fork is compressed, its internal air pressure increases. This increase in pressure, acting on the hack side of the fork seals, causes them to constrict tightly around the stanchion tubes. Consequently, the seal friction is no> longer the low 10 lb. component it was with zero pressure. When testing forks at 15 psi on the dyno, we have observed seal friction values approach 50 lb. as the fork was compressed to the limit of travel. And that's for just one leg.
Raise the air pressure, either by higher static pressurization or by the effects of heat, and seal friction increases even more. Compound this actual stiffness with rider fatigue and you have, literally, an uncomfortable situation. And one which isn't apparent the day after when you're rested and the forks are cool.
There are a couple of things you can do to relieve the problem. Easiest is to lower the air pressure in the forks as the day wears on and/or you wear out.
You can also install a short preload spacer in each of the fork legs. This will allow you to use less air pressure and still retain proper ride height. Start with a 1-in. spacer and 10 psi, then vary the pressure until the forks feel best for your riding style.
Experiment with different air pressures, oil viscosity and oil weight. Somewhere in the trial and error process you should find a combination which feels nearly as good at day's end as it did during practice.
WATER BIKES
I have been reading your magazine now for almost two years, and think it's really great. It has really taught me a lot that I didn’t know. But I’m confused about something.
Around here it rains heavily during the spring, the time of year when I like to ride.
I haven't been riding much, though, because I’ve always thought water contacting a hot engine could damage it in some way. But I’ve seen many pictures in your magazine of motorcycles riding through water, sometimes submerging the engine.
Is this safe? I own a Suzuki TS250; would it be damaged if I rode it through a lot of mud and water?
Mike Richards Lima. Ohio
Dunking a hot engine into a pool of cool water sure won `t help it any. But judging from the experiences we and others have had, dousing the engine doesn `t seem to have any serious effect. One would think that a cracked cylinder or head might result, but the on/v failures we've observed have been in the ignition system.
If you plan to ride the bike under extremely wet conditions, there are a few precautions which should be taken. The ignition system should be sealed to prevent failure. The cover over the magneto should have a good gasket, and the use of gasket sealer or silicone seal is insurance against water entry. Silicone seal the area where the ignition wires enter the crankcase, and any electrical connectors exposed to the elements. A waterproof spark plug cap, such as the one available from Malcolm Smith Products, should also be installed.
The carburetion system must also be waterproofed, especially if you’ll be riding the bike in deeper water. The rubber carb and air cleaner hoses should seal properly, and all vent hoses must be intact and of the proper length. The airbox must be free from leaks, and should have a small (1/16-1/8in.) drain hole at its lowest point.
Riding in water deep enough to enter through the air intake will guarantee a drowned engine. But short of that, you should have few problems with the elements.
SL175 PLUG FOULER
I own a 1971 Honda SL175 which was overhauled last summer. Since I got it back it has been fouling spark plugs regularly. It is so bad now that spark plugs will often last only a day or two. I’ve tried hotter plugs, but that didn’t work. Would using two ignition coils help? If so. how would you go about installing them? If you can help it will be most appreciated.
Lee Ryan Greeley, Neb.
We will assume the engine is in fairly good shape—no monumental oil leaks fouling the plug, or holed piston, etc. Before you try to modify the stock system, you would be wise to have the ignition and carburetion thoroughly checked.
Clean the breaker points, set the timing, and ensure that all ignition wires and connectors are in good condition. Don’t forget to inspect the coil and condenser and the spark plug lead.
With the ignition in good shape, the carburetion must be checked. The air filter element must be clean and installed properly.
Remove the slide assembly from the carb, and check that the needle has a smooth straight taper. If it is worn, replace it, installing the new one with the clip in the center notch.
Remove the carb and check floats and float level, adjusting if necessary. Make sure all jets are intact, clean, snug and of the size calledfor in the owner 's manual. The idle jet screw and passage should be clean, and the choke linkage should retract all the way.
Reassemble the carburetor, reinstall it, and you’re done. Additional help in tuning the carb can be found in the January ’77 issue of CYCLE WORLD.
TUBE OR NO TUBE
I have some comments regarding the use of tire sealant liquids (ACP, Flat-Pruf. et al) in tires, which should be of interest to touring riders.
It has been my experience with tubetype tires that, upon picking up your average nail, sealant will turn what would have been a disasterous blowout into a relatively slow leak, yielding at least time enough to move gracefully to the side of the road before air pressure vanishes altogether. This alone justifies the use of sealant: It can save your life.
While my life is being saved, however, the nail is tearing up the inner tube — especially as pressure is lost and the inner tube becomes free to move with respect to the tire (and therefore the nail). It is then seldom possible to simply remove the nail, re-inflate the tire and ride to the nearest gas station as the ads for sealant would have us believe. The hole in the tire may be small, but the inner tube is generally pretty well mutilated. I would imagine tubeless tires to be much better in this respect. The puncture hole in the tire itself is not only smaller than the havoc w'reaked on the inner tube, but the rubber is also thicker. Moreover, there appears to be no liabilities when patching a tubeless tire, w'hereas I've been advised against patching inner tubes because tube/tire movement and resulting friction-induced tube failure is likely to occur in the region of the patch.
Although the cost of a cast-w'heel/tubeless-tire retrofit to a bike currently equipped with spoke wheels is considerable, avoiding the hassle of replacing an inner tube by the side of the road in the middle of East Nowhere (especially at night and/or in the rain . . .) can make it all worthw hile.
Peter D. Wolfe
Cambridge. Mass.
We agree with you emphatically when it comes to the use of sealer in motorcycle tires. We've had several close encounters involving tire punctures at freeway speeds, and must give credit to the sealant for allowing early warning and safe recovery. The punctured inner tubes were rendered useless in more than one instance, but the cost of a new tube and a bottle of sealant is virtually painless, especially when compared to alternative consequences.
While we can and do appreciate cooler operating temperatures, lighter weight, ease of repair and changing, it is the additional safety factor over tube-type tires that make the tubeless conversion quite attractive. Unfortunately, for the owner of a spoked-wheel bike, the cost of a mag wheel conversion simply for the sake of running tubeless tires seems hard to justify. Rut for the owner who wants cast wheels, the option of being able to run tubeless is a definite incentive. S3
