I thought some of you who are long time owners might find some of my experiences interesting (and some worth avoiding!)
I am the original owner of a 1976 CB750F. I bought it in Maine while living there; I now live in Massachusetts. It has been across country, to Florida in the winter, and to Canada many times. Had an 836 kit for a few years and alot of other experiments. It now has about 80,000 miles and about 3 years ago I decided to rebuild the motor.
All tensioners were like concrete as were intake manifolds. It has long had Manley SS valves so those were OK to regrind. The original pistons had good diameter but the compression ring grooves were way too wide. So I hunted down new pistons and rings and came up with a new set of cylinders on Ebay. Visions of an essentially new motor danced before my eyes.
Put it all together and drove it carefully for a month or so. Over the next couple of years, it ran well but always used some oil (about a quart every 800 to 1000) miles. Plugs showed oil deposits and the pipe was sooty. I was not happy. I can recall going the first 15000 miles and never using any oil
A compression test showed readings in the mid 150s which would jump to 175 with some motor oil squirted in (screw in style tester with about an 8” neck). Back in the 70s I would get 175 or thereabouts out of the orginal motor with a hand held tester.
Also oil leaks developed around the head and the motor looked like hell. As I ride it to work when possible, a decent pair of pants might end up with oily knees.
Finally this summer I got my act together and decided to see what was wrong – fix the motor inside and take care of the oil leaks.
I took it to Harry’s Machined Parts in Northboro MA. The owner, Rick Stetson, is one of the most savvy internal combustion people in the country, having built pro stock dragsters and almost everything else for over 30 years. In fact he ported the head on my bike in 1978.
Essentially he told me the rings had simply never seated or broken in properly. However it should have been driven, I did not. I was probably far too easy on it, and also used 20W-50 oil. So new rings were in order.
The surprise (see another post of mine) was that 6 of the cam tower bolts had pulled their ancient HeliCoils out of the head so that had to be fixed. The 6 hockey pucks underneath were leaking oil as were bolts. All fixable.
So we got the motor back together with threadwork, fresh gaskets and fresh rings.
Now how to break it in?? I took the motor home and assembled the bike, then loaded it in a truck and took it back to Rick at Harry’s. Yesterday we spent 4 hours with it on his Dynojet1 dynamometer. A picture is (hopefully ) attached.
The dyno has monitoring for RPM, load and Air/Fuel ratio. Also powerful air blowers to cool it. All this is run through an interface to Windows based software. He also has an infrared temp gun.
So I was surprised to learn three things would happen, with work:
1) it would get broken in properly. Varying loads and varying temperatures.
2) we would adjust the carburetion in all areas: idle, needles and main.
3) we would get horsepower and torque curves
also the carbs would get synched (last done in 1986)
To break it in, we used Castrol 10W-40 GTX. He ran it with a 20 to 25% load between 4 and 5000 RPM stopping when the external, rear cylinder temps got to about 260 degrees. The blowers would quickly lower them back down to about 100 degrees and then in a bit he would repeat.
In the process we got to see how the carbs were, and in all areas they were on the lean side. The dyno records and displays this for the entire RPM range.
I worked summers in the late 70s at a Honda dealership and used to watch people fuss over carb synching. Rick had it spot on in about 3 minutes.
After awhile, he ran it up to 9000 RPM with load in 3rd and 4th gears to get some horsepower curves. The shape of the curve appears identical to what Cycle Magazine got back in ’75. What was a surprise was getting a peak of 65 HP and not much less than that to 9000 RPM. Cycle got 58 HP. I will scan in and show the actual graph generated next week when I get near a scanner. Bear in mind, my head has been ported, I have a K&N filter and my airbox has been drilled. Also I have the stock 4-1 header with a MAC muffler on it (needed for the emissions probe the dyno has). By way of comparison, in his database is a run for a 970cc CB750 based motor which generated 84 HP. Proportionally, mine is doing a bit better.
I just took it for a ride on the street; tomorrow it gets 20W-50 oil and leads a moderately easy life for about 500 miles.
The plugs showed all signs of a good mix. The pipe showed no signs of oil. It runs super and sounds wonderful. At 80,000 miles it has another life. Old but very healthy and mean.
If you live in MA and want outstanding service of any kind for your Honda motor, Harry’s Machined Parts is your place.
John
