Topic: completing the rebuild of the 1972 engine, CB750 k2.

[olasek18_18@o2.pl]

Hello, I am just at the stage of completing the rebuild of the 1972 engine, CB750 k2.
New pistons +1mm have been installed in the engine,
The question is- should I set the carburetors for a larger capacity?
Can the jets stay in their original size?

 I will be grateful for any suggestions regarding new settings.

Regards

[MauiK3]

Doesn't seem to be needed, that's not much change. It depends more on how stock your airbag and exhaust are.

[olasek18_18@o2.pl]

Stock Airbox, MAC 4in1 exhaust

[Don R]

 Stock jets will be fine.

[olasek18_18@o2.pl]

Ok, first engine start behind me.
after the first start, the engine seems to work properly,
it works at 1000/1100rpm, but I hear some minimal knocking in the upper part of the head.
Maybe valves.
 during the first ride, the bike lost power after 1 mile, when it cools down it works fine, when it wants to take a ride again, the bike loses power...
I was ride on 4-5 gear 2000-3000 rpm

I put out cylinderhead cover but cam looks fine, everything on płace, valves clearance Ok.
I have No idea what happen now…

I will be grateful for hints

Regards

[denward17]

Tell us what you did to the carbs, what kind are they?

[olasek18_18@o2.pl]

Keihin 657A carb was completly rebuild before I restored the engine.
Carb was Run very good on old engine, Standard airbox main jets 105, Mac exhaust 4in1

[HondaMan]

Umm...can you tell us about the gaskets, sealants used (if any) and the O-rings under the cam bearing housings?
If the O-rings are missing (even one of them) under those cam bearings, there will not be any oiling going on at the cam (where the O-rings might be missing).
If sealants were used on the gaskets under the cylinders or at the head gasket, then it can ooze out if around the oiling passages, then block the oil jets, causing top-end damage.

[olasek18_18@o2.pl]

I used standard methods.
 I did everything together with hackaweek.
Only difference- I do not use additional silicone on oil jets under cam bearing housings.
This is the 4th engine rebuild by me, I'm not an expert but I know the basics.
I used orings and new gaskets, all 4 o-rings are in places uder cam bearing housings, dowel pins too...

[newday777]

Ok, first engine start behind me.
after the first start, the engine seems to work properly,
it works at 1000/1100rpm, but I hear some minimal knocking in the upper part of the head.
Maybe valves.
 during the first ride, the bike lost power after 1 mile, when it cools down it works fine, when it wants to take a ride again, the bike loses power...
I was ride on 4-5 gear 2000-3000 rpm

I put out cylinderhead cover but cam looks fine, everything on płace, valves clearance Ok.
I have No idea what happen now…

I will be grateful for hints

Regards

"but I hear some minimal knocking in the upper part of the head.
Maybe valves.
 during the first ride, the bike lost power after 1 mile, when it cools down it works fine, when it wants to take a ride again, the bike loses power..."

Do you have high compression pistons and high lift cam in this motor build?
Your description, knocking, loss of power, sounds of valves tagging the pistons from the cam not being degreed in properly??

[olasek18_18@o2.pl]

Standard flat pistons +1 mm, oryginal cam

[HondaMan]

A couple of questions:
1. How are the fuel lines routed from the fuel tank's petcock to the carb's tee fittings?
2. Did you replace the carb's float valves with new ones while they were apart? If so, the new ones will be stiffer (stronger internal strings) than the OEM versions. This will cause low fuel levels in the 657A type carbs. These carbs require the float setting to be staggered by 1mm, with the kickstand-side's float at 25mm and the opposite one at 26mm, as measured from the tiny slivers of raised metal next to the notches in the carb bodies. If those little reference edges are now gone, then set the depth using the bare surface of the carb's gaskets by the notch on each side of the carb, and consider that this may be a little bit lean. On some 657A carbs where the reference edge has broken off (or dissolved from use of MTBE gasolines in the 1980s) consider using 24mm and 25mm as the float heights, instead.
3. Are the floats still the brass ones as found in the 657A carbs? If they have been changed to the plastic floats then the float heights must also change.

A low float bowl level will let the bike run for a little while, then it will start acting like it is running low on fuel when it warms up.

[Jerry Rxman Griffin aka MuthaF'er]

Did you gap the rings? What gap did you use?

[olasek18_18@o2.pl]

It seems that the motorcycle was dying because it had problems with fuel, after warming up the fuel hose was bent from the temperature ...
The noise from the head is the valves.
 After the first 100 miles I'll adjust the valve clearances and it should be ok!

The engine is after regeneration: clutch chains, tensioners and bearings, and now is so quiet that I can hear everything from cylinder head

thanks for tips!!

[HondaMan]

The engine is after regeneration: clutch chains, tensioners and bearings, and now is so quiet that I can hear everything from cylinder head

That's what happened with my own K2 when I rebuilt it at 131.000+ miles. Afterward I realized that I had been listening to the rod bearings for the last 20k miles, which were the ones telling me when the oil needed to be changed. Now I have to keep track of the mileage, instead.

[olasek18_18@o2.pl]

double engine rebuild paid off!
With the flat pistons and factory settings, the bike runs perfect!
 greetings and thanks for your help

[HondaMan]

double engine rebuild paid off!
With the flat pistons and factory settings, the bike runs perfect!
 greetings and thanks for your help

Congratulations, good job!