The Service Department
THE SERVICE DEPARTMENT
JOHN DUNN
POWERFUL SICK
My Yamaha 180 is sick. I am experiencing a severe loss of power, such that the bike is incapable of exceeding 50 mph downhill or 30 mph uphill. Acceleration is minimal, and, except in first gear, the engine bucks and gurgles from one-third to full throttle. In first gear, the bike pulls acceptably, but on shifting to second it immediately bogs.
This machine has about 8500 miles on the odometer. At 8000 miles, I had the cylinders honed, then replaced the rings and gaskets. After this, it ran almost as well as when new. But, after about two weeks, or perhaps 300 miles, the performance started to go down steadily. The problem was most noticeable on hills. At this point I replaced the plugs (NGK B7HZ), set the idle adjustment and float level on the carburetors (25/32), and cleaned air cleaners according to the manual. When this helped, but very little, I set the points by the marks at the end of the crank, with the aid of a small battery and lamp to determine when they opened. To do this, I had to use up all of the adjustment room left in the movable plate. All of this was to no avail.
What can be my problem? Strangely, though it runs so poorly under load, it revs cleanly in neutral and idles steadily and smoothly. The local shop advises me that I possibly have one or more seals leaking on the crankshaft. Can this be after only 8000 miles? If so, is this short life typical of two-strokes?
Ronald E. White De Soto, Mo.
The problem that you are experiencing could be attributable to broken or stuck piston rings. After reboring or, in some cases, just a hone job, it is necessary to chamfer the edges of the ports, particularly the exhaust port. A sharp edge could result in piston ring breakage. When you fitted the new rings did you make sure that they were the correct size? Did you check that the end gap on the rings was correct? A good rule of thumb for determining correct end gap is to allow a minimum of 0.004-in. gap for each inch of cylinder bore diameter. It is also essential when fitting new rings to clean all the carbon out of the piston ring grooves. Carbon at the bottom of the groove would cause possible ring breakage because of insufficient ring groove depth. It should be possible to push the ring at least 0.005 in. below the ring lands to insure proper operation. It is also good practice to carefully inspect the pegs in the ring grooves that stop the ring from rotating in the groove for excessive wear or looseness. If the ring is allowed to rotate, the ends will pop into the ports and cause breakage.
Short crankcase seal life is not a common complaint with two-stoke engines. However, it is a possibility. Before going any farther, I would suggest that you first remove the cylinder and check the condition of piston and rings.
FUN IN THE SUN
What do temperature and atmospheric pressure do to two-stroke tuning in competition? For example, a very sunny, warm and clear day changes to a cloudy but bright day, and it looks like rain-do I change to larger or smaller jets? Late afternoons, I notice smaller jets, two numbers lower, help; is this due to “contracting” in size of gas flow or the mixture? Relative humidity here averages 95-98 percent, temperature almost always 90 F.
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Last but not least, where can I get plans on how to build or buy a flow meter like C. R. Ax tell’s of Glendale, Calif? I need this badly for my testing equipment.
Sonny Cooper Bacolod City, P.I.
With any given engine design, the power output will vary with any change in ambient conditions. The three main elements that influence output are atmospheric air pressure (measured in inches of mercury by a barometer), air temperature and humidity (vapor pressure or moisture content of the atmosphere). A high barometric pressure with low air temperature and humidity will result in the highest output, by reasons of air density and high oxygen content.
The chemically correct air/fuel ratio for gasoline is 15:1. This ratio is required for combustion purposes. An additional quantity of fuel is also required to allow for “distribution” deficiencies within the cylinder, and also provide a cooling effect on certain critical internal components. A twocycle engine is particularly sensitive with respect to adequate internal temperature control. Most failures with racing two-cycle engines occur because of inadequate control of internal temperatures, which usually results in piston distortion and seizure. It is therefore always best to use a mixture that is slightly on the rich side.
From the contents of the first paragraph, it would appear that an increase in ambient temperature would require a decrease in jet size. However, also by reason of the increase in temperature and a possible borderline case of inadequate engine cooling (adequate cooling often can become very critical on a motocross type machine, particularly due to its relatively low forward speed, and the fact that the cooling fins are usually half full of dirt), the second paragraph would indicate a possible increase in jet size to assist internal temperature control. With all the variables, it is almost impossible to be specific in regard to correct jetting for any given condition without a vast amount of experience with a particular machine under actual racing conditions. To be safe, I would suggest that you always jet slightly on the rich side until you have the experience to jet right on the ragged edge for optimum performance.
Regarding your request for information on construction of a flow bench, please read the answer to J. Merrit’s similar request (CW, Jan. ’69).
DIPSY DUCATI
Fve run into a rather interesting tuning problem with my ’67 250 Ducati Mk III. Whenever I’m running at full throttle in fifth gear and encounter a load condition (slight uphill grade, headwind, etc.), it’s as if the throttle were slowly being closed. Engine revs just drop. I have to back off to about half to two-thirds throttle setting, where the engine will run fine. If I open the throttle up again, she’ll run up to redline, then proceed to repeat the process after a few seconds of flat-out running. The engine is stock, with the exception of a countershaft sprocket two teeth smaller than the stock sprocket. This allows me to pull redline in fifth gear, given a level run with no headwind. The bike is somewhat lighter than stock, with fiberglass tank, racing seat, no front fender, and no headlight. I’m also running the tuned racing megaphone that comes with the bike. I’ve noticed that I don’t have this problem running a muffler.
The first time this happened, I thought the engine was seizing. However, since I followed the manufacturer’s break-in procedure very closely, and have not abused the machine, I don’t believe this is the problem. I think the problem lies in carburetion. Would changing main jet size help? I run a No. 128 jet in warm weather and a No. 130 jet in cold weather with the megaphone. Standard main jet size with the muffler is No. 124. How about carburetor throat size? I’m using the stock 29-mm Dellorto now. Would the length of the intake tract have anything to do with this? I keep the engine fine-tuned as far as ignition, valves, etc., are concerned, so it must be carburetion problems.
Please don’t tell me to put the damn muffler back on and forget about it, as I’m trying to get the beast to run right in competition trim.
John Eckhardt, Jr.
Olathe, Kan.
The contents of your letter would indicate your engine will not maintain maximum power when on full load. This could be caused by malfunction that results from higher operating temperature experienced when on full load, or, as you suggest, a fuel supply or carburetion problem. I would first check the fuel supply system. Check that the fuel tank is adequately vented. If the vent is partially blocked, this would adversely affect fuel supply when the demand is high (sustained full throttle). Also, check that the fuel cock is not restricted in any way. If the main jet were just a fraction on the small size, similar problems to those described could be experienced. You may need a spark plug one grade colder in heat range. Intake tract length and carburetor choke size would not produce these symptoms.
