The Service Department
THE SERVICE DEPARTMENT
JOHN DUNN
SCRAMBLER CAMS
I bought my first motorcycle three months ago (a Honda 305cc Scrambler). To put it short, it doesn't crank on like I would like it to. Can I install a Harman & Collins racing cam without rebuilding the whole thing and ending up with an unreliable piece of garbage? Don't get me wrong, I'm real fond of the bike, but I would love it if I could increase the torque range substantially.
R. L. Hampton Norwalk, Calif.
To install a camshaft in this machine, it will be necessary to remove the engine from the frame and then remove the cylinder-head. The bottom half will require no modification if in good condition. An engine that is not in first class condition should be fully dismantled, inspected and rectified before installing any high performance equipment. However, in your case, it is most unlikely that this will be necessary. It would be wise, though, to carefully inspect the condition of the rocker arm faces for wear and replace if necessary.
To obtain full performance and proper operation from the race camshaft, it is essential that high rate, high frequency valve springs are also installed. Webco Inc., can supply an S&W valve spring kit for whichever H&C camshaft you choose. It is most important that the springs are set to the correct height as instructed on the specification sheet issued with the springs.
After the head has been reassembled and replaced on the engine, but before the engine is installed in the frame, it is good practice to check the valve to piston clearance. This should be carried out with the tappets set to the normal running clearance. Turn the engine over until the crank is 10 degrees before tdc (with the valves on the overlap). Then employing a dial indicator to the top spring retainer with the needle set to zero, manually actuate the rocker arm until the valve touches the piston crown. The observed movement of the indicator needle will represent the actual valve to piston clearance at this particular crank angle. Repeat this procedure in five-degree increments from 10 before to 10 degrees after tdc for both intake and exhaust valves. A minimum valve to piston clearance of .065 is acceptable on this engine. If there is insufficient clearance, it will be necessary to provide deeper valve relief notches in the piston.
EX-ROAD RACER?
I need help! 1 bought a 1961 Honda 125, CB92, and since the day 1 got it, it has had a very annoying habit which 1 hope that you can help me eradicate. The machine is very doggish at low speeds, in any gear. Once I get going, and turning from 7000 rpm up, it doesn't act up anymore. It sounds as if it is flooded very badly, but there is no strong odor of gas either when moving or when parked. The carburetor is a Keihun, 20mm. 1 checked the points, plugs and gas lines and they are all in good operating condition. To illustrate how bad it is, it has the same pickup as that of a Honda Sport 50.
I have good reason to believe that the machine was used for road racing earlier in its life. Could these be the symbols of a hot racing cam, one that would give the power at top rpm?
Would I need a new carburetor or just what do I need to give me satisfactory performance? My Zanella 125 has a lot more pickup. The Honda, with almost three times as much horsepower should, but doesn't, equal it.
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I would appreciate your help with this problem and any information that you might have regarding this condition and the machine's performance in general.
Pedro O. Bello
West Palm Beach, Fla.
A YB racing camshaft would result in a road performance very similar to that described by you. Even when fitted with a standard sports camshaft, the CB92 does not have impressive low-speed performance, making it necessary to make full use of the gear box when accelerating from low speeds.
It is also possible that your machine is fitted with a higher overall gear ratio than standard. A ratio slightly lower than standard would give you a better all-round performance.
The CB92 is fitted with a 20mm carburetor as standard. This machine, when in good condition and proper tune, has a top speed of approximately 80 mph, which has been obtained at the sacrifice of low and mid-range torque. Therefore, even a lower overall gear ratio than standard may be better suited to your needs.
THE COMPLETE STORY
My former letter to you, which appeared in the October issue concerning my 450 Honda missing all the time, caused quite a stir in local circles. Now that I have found the cause of the missing, I would like to have you pass along the information. It should cure all, or at least it cures my 450 Honda.
I went to my local Honda dealer and asked for a new set of plugs for my 450. The salesman wanted to know if I was riding on city streets or highways. I told him most of my riding was on the streets, as I use my bike to go back and forth to work, about 35 miles each way on the streets and freeway. He then gave me a set of B-7-E plugs and my bike hasn't missed a stroke since. 1 was using a B-10-E plug and I am told this is too hot a plug for a 450 Honda when you don't travel over 70 miles per hour.
Robert H. McDaniel Reseda, Calif.
Thanks for the belated information regarding the type of spark plugs that you were using when you were experiencing the mis-firing problem. If I had been aware that you had been using an extremely cold plug for town riding I would have advised you accordingly.
Your previous letter is typical of many I receive which give insufficient information, making it difficult for me to give accurate answers when specific problems or malfunctions are being experienced.
You appear to be a little confused regarding spark plug heat ranges. The NGK B-10E is a fairly cold plug and the B-7E is a hot one. The terms hot or cold, when referred to spark plugs, relate to their operating temperature characteristics. A hot running engine, like a highly tuned racing engine which is generating high combustion temperatures in keeping with high power output, will require a relatively cold plug that is capable of operating at these high temperatures. A hot plug used in an engine of this type would overheat and fail, causing possible engine damage. A touring or sports engine operates at a much lower temperature, particularly when only part throttle is being used. This engine will require a hot plug with characteristics in keeping with the lower combustion temperature. The hot plug operating under these milder temperature conditions will, because of plug design considerations, operate at a temperature high enough to combat oiling and fouling which will occur if a spark plug of too cold a value were used.
It is best, even where a racing engine is concerned, to use the hottest plug possible to minimize the possibility of oiling or wetting.
450 CARB CURE
In reference to the letter regarding the carbs on the Honda 450 (page 2<, Oct. issue), 1 experienced the same problem and I am pleased to report that the trouble has been cured with the installation of a simple kit available from Honda which consists of some coil springs, to provide more "weight" for the vacuum-controlled slides. The kit also includes replacement needles and low-speed jets. Prior to this modification, I found it necessary to operate the machine with the choke closed halfway, winch limited the engine to about 8,000 rpm. It runs fine now at all speeds!
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Richard F. Delaney San Jose, Calif. At the time I wrote the reply to the letter to which you refer, there was no official remedy for the carburetion problems being experienced on the 450 Honda. However, as stated in your letter, there is now a kit available from Honda dealers that will solve this problem. The Honda part number for this kit is B450 3701-5. Those who may be experiencing similar problems are advised to contact the Honda dealer from whom they purchased their machine.
SOUPER HAWK
/ am currently engaged in super-tuning a 1966 Honda Super Hawk. Although 1 am familiar with the procedure and experienced in engine reworks, 1 need specific information concerning port sizes, etc. The performance I desire is this: A low-end performance at least as good as the stock bike.
A reasonable performance increase in the 5,000 to 10,000 rpm range.
A power peak in the 7,500 to 9,500 rpm range.
1 think this can be done at reasonable price, utilizing the following items: A 1966 Honda Super Hawk (305cc); Harmon and Collins racing cam: S&W competition valve springs; Webco competition clutch springs; standard mufflers with baffles; standard valves; standard 26mm carburetors with air cleaners; standard 9:5 to 1 compression ratio; careful break-in.
What size intake port will give me maximum performance with this cam in the range I desire without ruining the bottom end?
Do you recommend a tuned length intake? If so, what length should it be for a range of 7,500 to 9,000 rpm? Do you recommend opening up the exhaust port as large as is possible, keeping the same shape, of course?
Is it worth the time and effort to bring the chamber volumes to factory specs? Do you recommend careful grinding to improve gas flow over the valve seat?
Will the ignition timing remain five degrees before tdc with these modifications? If not, what should the new timing be? Would Packard 440 or similar solid copper core spark plug leads be an improvement over the stock plug leads? Would it be advisable to go one step cooler in the spark plug heat range?
In view of the fact that both valves and seat faces are bright, would a light hand lap be advisable when reinstalling valves? I would hate to have the bike down again if the valves were not sealing properly.
What is the correct torque for the head bolts and clutch plate bolts?
What is the rocker arm lift ratio and will I get valve to head or valve to valve interference with this cam?
Do you recommend opening up slightly the oil passages to the cam lobe surfaces? Will the throttle cut-away or needle require modification?
Approximately what main jet size should 1 start with when tuning?
Do you foresee any overheating problems with this setup while holding a 65 to 70 mph cruising speed for several hours?
1 realize that there is quite a bit of work involved in answering these questions, but the local dealers are unable to provide this information.
Edward J. Shaughnessy
Riverdale, N.Y. Whatever you do to effectively improve the volumetric efficiency in an effort to improve maximum power output will adversely affect low-speed performance. Your machine in standard trim is a very carefully calculated compromise between maximum power output and low to medium speed tractability.
Possibly, the most successful approach would be to consider a 350 conversion. Even when employing the standard 305 parts and components, this conversion would result in a substantial increase in mid-range torque, combined with some increase in peak output.
If a decrease in low to mid-range torque could be tolerated, an increase in peak output could be obtained by employing the race camshaft and high-frequency valve springs.
Precision rework of the valve seats and general attention to detail can only have a beneficial effect. For maximum effect at 7,500 to 9,000 rpm. an overall intake length of 11 inches should be employed. This length should be measured from the intake valve seat to the extreme end of the intake arrangement. The position of the carburetor does have some effect on the suggested length, it usually being found best to employ any extra length in the intake tract between the valve and the carburetor.
With an increase in output and capacity, jetting and carburetor adjustment will definitely require a change to ensure correct carburetion. Initially, it is best to start on the rich side and work back from this to obtain optimum results.
An increase in capacity with the existing 305 cylinder head will result in an increase in compression ratio. With this in mind, it is recommended that a total of 42 degrees of ignition be employed.
It is difficult to answer all your questions within the medium of this column Edward, but I hope the aforementioned comments will assist you in the decisions you have to make.
For specifics, such as torque values, you will find a service manual invaluable.
