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.
HONDA 750 AUTO MAGIC
I own a 1976 Honda 750A, purchased new, which has accumulated less than 8000 miles. My first problem began on day one and involved carburetion. This was repaired by Honda East in Dallas at a cost of $71. Mpg dropped to 30 from an average of over 40 mpg prior to the installation of larger jets and needles.
I would like to replace the stock #530 chain and sprockets with #630 chain with a 14-tooth countershaft sprocket and a 35tooth wheel sprocket. This would lower the numerical gear ratio from 2.67 to 2.50. How would this affect performance and fuel consumption?
I am confused on chain size interchangeability. I have been told that #630 chain cannot be installed, and also that it can be installed but it must be the endless, or staked, type. I have been informed that #630 chain can be used with the old fashioned master link. I have also been told that it can’t.
Would you recommend standard, selflubricating, or O-ring type, assuming, of course, that #630 chain can be installed?
Ernest H. True Greenville, Tex.
Before getting into the question of chain modifications, a few words on the original carburetion problem.
Because you did not elaborate upon the reasons for the change in jetting, we assume the problem was one of rough running. Many earlier 750As experienced a surging condition when run at steady speeds which was attributed to excessively lean carburetion. The common fix involved enrichening the carburetion—tractability was improved, but only at the expense of fuel consumption. The installation of bigger jets was merely a treatment of symptoms, and did not ajfect a cure.
Indeed, the actual problem was determined to be a lean condition, but not one stemming from the normal causes. Seems the carburetor float bowl overflow tubes were excessively long, and hung down into the airflow. When the bike was underway, a partial vacuum was induced in the carb float bowl, causing a decrease in fuel level and, at higher speeds, fuel starvation. Cutting 4-6 inches off the tubes would bring them up out of the air flow, and eliminate the vacuum, leaness and surging.
If larger jets have been installed as a cure, as in your case, they should be removed and replaced with the stock 44102 jets. Stock needles should be used, with the clip in the 44 3 (center) position. Fuel level in the float bowls should be set to specs, and, of course, the overflow tubes must be shortened and tucked out of the way.
Proper operation should thereby be restored, along with improved fuel consumption figures. But, if there is still a slight rough edge in the mid-range under cruise conditions, there’s one additional modification which can be undertaken. A modified jet needle 44 04-B, is available from American Honda. It has a slightly different taper. It can be obtained by your dealer through a Honda service rep, as it is not normalia necessary or stocked.
With the matter of carburetion disposed of gearing can be discussed. The stock 1976 750A utilizes a 17-tooth countershaft and 48-tooth wheel sprockets. This yields a 2.82:1 final ratio, which is nearly ideal for the 750A 's engine characteristics. If you must change gear ratios, you have several alternatives.
A smaller rear wheel sprocket could be purchased from an accessory manufacturer, and a suitable # 530 chain fitted.
Or, if the switch to #630 chain is the primary consideration, the 15/42 tooth setup from a '77 750A will bolt on, giving a nearly identical 2.80:1 ratio. The case chain guard must first be removed for needed clearance before installing the #630 countershaft sprocket.
The special method of fastening the countershaft sprocket on the 750 Auto limits you to the use of the 15-tooth 1977 item, if # 630 chain is desired. If you wish to deviate from the stock 2.80:1 ratio, a different rear sprocket is required, as is a shorter length of # 630 chain.
As to the type of chain to use, any #630 chain will fit all #630 sprockets—in this case the length is the critical factor. We'd stick with the stock #530 chain and sprockets, as the expense and hassle of a
#630 conversion would not be justified in terms of added mpg or longer life.
HONDA OIL OOZER
My 1973 Honda 350F at 13.000 miles developed a porosity in the cylinder block wall casting and now oozes oil. Its cooling fins must be wiped clean every 50 miles or so to keep the rest of the bike (and rider) clean. I can’t afford a new cylinder block and Honda won’t give me one free, so do you know of any relatively cheap fix which will halt or decrease the flow?
John Maugeroni Framingham. Mass.
Although a porous casting could be at fault, chances are you have a different, more easily repairable problem. The construction of your Four is such that some portions of the cylinder studs are exposed to the outside. Occasionally, oil can seep past the seal at the top of the stud and flow down the stud and onto the fins in the manner you describe.
Repairing the problem is relatively easy. Once you've determined which stud is leaking, remove the engine cam cover, exposing the stud. Remove the nut and sealing washer from the offending stud, and inspect the sealing surface on the head. Most likely there will be an irregularity from the original machining operation. This should be smoothed up. Clean and coat the sealing surface with some silicone sealer, install a new washer, and replace and re-torque the nut. Replace the cam cover and check the valve settings to complete the fob.
If the cause of the seepage is indeed a porous casting, the repair would be determined by the nature and location of the leak. Check with a local machine shop to determine whether heliarc may be used, or for recommendation of a suitable epoxy sealer.
KL250 FANCIER
I must say I enjoyed your test of the KL250 in the January 1978 issue of CYCLE WORLD, but I have some questions and observations. How is the cam mounted? The Yamaha XT500 cam is mounted on bearings instead of spinning directly on the cylinder-head casting like the Honda XL250. I have been told that the bearing mounting eliminates the old problem of the XL. namely seizing on the head.
In the KL250 test, the statement was made. “Your average two-stroke playbike of equal displacement will have the K L for lunch.”
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In your summary of test results I looked up the Yamaha DT250. tested in February 1977. I found some interesting statistics. According to your summary, the KL is only .39 of a sec. slower than the DT250 in a standing start '/4-mile. In the speed after '/z-mile test the KL shows up as 6 mph faster, and it is only 19 lb. heavier than the DT. It also gets 28 mpg better than the DT. If your facts are correct and were determined under similar conditions, then who ends up being lunch may be a matter of riding ability. I have never owned a fourstroke off-road bike, but this may be a first. That is. unless the Suzuki DR370 doesn't give me more for my money. However. I have not yet read any good information about the DR370 (hint. hint).
Walter H. Dean Panama City. Fla.
Like the Honda XL engines, the KL also uses the head casting for cam hearing surfaces. CW staff members are in disagreement over the relative merits of such a configuration, but statistically, the method seems to work extremely well. Only under severe load (as in racing), or by operating the engine hard when cold, or with improper valve clearances does anv problem occur, and then rarely. In any event, this would have little if any bearing on our decision to buy or not to buy a KL250.
Taken out of context, the lunch statement can be misconstrued. As you pointed out, the KL is superior in certain areas, as is the Yamaha DT in others. A long, hard run on the pavement will yield an orange winner in virtually every respect. Take the two offroad and the KL will be hard-pressed to keep up with the Yamaha DT. And that's with riders of equal ability aboard.
Which is better/ Can't say. It'll depend upon what proportion of onand off-road riding you do. They're both excellent bikes in their own rights.
So are we curious about the new Suzuki four-strokes. If they work half as well as the wooden (!) mock-ups we 've seen they 'll be fine machines. We hope to get a real one in the not-too-distant future, so stay tuned.
