Topic: Timing with Engine Mods

[MattFreeman]

I'm going to post this in a separate thread but while I'm here...

It was pointed out to me (by a motor guy, not a sohc guy)that the plugs look like they may be intruding into the head too far.

I didn't think much of it but it occurred to me last night that it could be a result of the porting. Should I look for shorter plugs?

[TurboD]

To answer your last question and post. NO..

[70CB750]

Matt, do you have this diagram?

I just went around on jetting my F3 engine:

>>>>The F3 engine came with 657A caburetors, for rebuilt I bought kits with 40 pilot and 120 main jets.

She started right away and  idles beautifully right from the beginning, Pamco with stock airbox is a powerful combination.

But with needle in third position she would not move, I opened the throttle and she promptly fell on her face. Spark plug white like snow.

Needle in 4th position she would move some, but still felt weak.

Today I moved the needle all the way to the bottom and she finally woke up. Every needle change meant taking out airbox and the whole carb rack - compare to this, the K0 is amazingly easy as far as setting needles.

I remember from somewhere, needle notch is about 1.5 in jet size, means she is now running 126 or so jet.  She feels good, but future will show if 125 jets are needed. <<<

I have some shifting problem, did not take her really to the road yet, but from what I can feel she goes.  Did you try to move the needle?  It is major PITA, but changing clip, taking it for a spin, changing clip again is the way to do it.

[PeWe]

I do not want to start a discussion about pods or stock airbox.
Facts about my old CB750 K6 -76 on the photo direct out of the barn and 19 years coma. It looked like this since restore to 1984 except some small details.
836cc, ported head, hotter cam. OEM K6 carbs with pods. I had VM29 smoothbores 1984, back to OEM 1985 when VM29 had very bad carb boots, to0 flexible and prone to crack.
Jets: Pilot std 40, needles 1 step higher (clip lower), mains 147.5 (stock with air box 105) Carb synched by bench method.

I'm convinced that a tuned engine need more airflow than a std airbox with std filter can provide. If airbox help, the carbs must be jetted too lean

The engine on the photo ran very smooth and nice. Most as a tourer with 2 persons and bags down to the Mediterranean Sea 4 times, each holiday 5000-6000km during 3 weeks to Jugoslavia, Italy and Spain. Top speed 210km/h according to the speedometer and friends bikes keeping same speed. Up to 200 rather quick, the last 10 has to be squeezed even with 2 persons and lots of luggage, tent sleeping bags.... Gearing was 17/48. I have today back to std 18/48.

Only problem was 1 broken point(bakelite part against rotor, bad quality, propeller logo), intermittent contact problems with one of the coils, yellow or blue wire oxidized connection. Bad battery in Italy, no headlight under 4000rpm.

[MattFreeman]

After raising my needles,  I re-visited the balance for my carbs. I had balanced them awhile ago and I found that they were not particularly well balanced now.

After struggling to get them balanced again I found that once I have them close and I blip the throttle they get all wonky again.

I believe that the strange, looks like it's running lean and rich at the same time, is a result of the carbs not being balanced.

I am going to try again with the airbox on and if that clears all my issues I will be glad to have it fixed and will stand tall to the chorus of "I told ya so's". 

Will report back with results.