Topic: CB750 K7 No compression mystery

[M.Henry]

I have a super old cycleserv brand service manual, my page 91-92 is on the carbs.
What service manual are you referencing in particular?

[dave500]

you are holding the throttle wide open I hope?

[M.Henry]

Well the carb sitting on the work bench wasn't at wide open throttle. That's my problem.

Sarcasm aside, thanks for that reminder.

[dave500]

ha ha,no worries,if you have set a gap anywhere on the lobe even if in the wrong position you should have valves closing fully,if you have no gap on the base of the lobe the valves wont be closing.

[M.Henry]

Well we had enough of it, pulled the motor, took the head off, and I have a bent valve in cyl 4, the tip of the valve stem on cyl 2 in was damaged, and cyl 1 had a valve that wasn't seating.

Guess I'm gonna be focusing more on that performance engine a little more

[PeWe]

Why make it complicated?
First time I adjusted the valves  on my CB750 K6 was with Honda user manual that I got with the bike.  Just read and do.

Next adjustment was ignition. Same there except I got help by a friend that was car mechanic and CB750 owner. He showed me how to use a test screwdriver with an inbuilt lamp when setting the ignition. Max advance found when giving full throttle on 5:th gear at around 120kph and small pinging to be heard. Also possible to retard just a little from this setting.

You have to know when ’T’ and ’F’ on advancer plate is to be used.

Compression tester: Run engine on the starter, carbs fully open. 5-10 sec is enough.

[PeWe]

Very important to rotate the crank several turns after cam installation. No plugs and feel it will turn over easy. Check that the notch on cam end is up, verify T @ 1:4. If notch is down continue to rotate to next 1:4 T on advancer plate.
It easy to make a mistake setting chain 1-2 cogs wrong with cam chain slack on wrong side (front).
I guess the cam was installed completely wrong here.
Let’s hope your guides are OK.
Time for a complete head job, cut seats with new valves.
Look for a refurbished head.

[M.Henry]

Yeah. I've got 3 heads total, but 2 are 74 and this is my only 77. I don't have a working set of the earlier carbs, just my pd carbs for the 77. So I'll probably be doing some restoration work on this head, buying new valves, and possibly doing the port and polish job I was gonna do anyways.

We did rotate the crank many times by hand, I'm surprised we didn't feel anything like the valve breaking, maybe it was from further back in this working. I didn't know there was enough give in the chain to have a full tooth or two or slack on the wrong side. I'll have to verify that when installing from now on.

[dave500]

maybe for practise remove the damaged valves and reassemble the top end and check your work?

[754]

 All 3 heads will work, just pull the exhaust studs out and use them in the other heads.
 You will have higher compression i think.

[M.Henry]

are the intake ports and exhaust ports the same for a K4 and K7 head?

i.e. could I still use my same pd carbs and carb boots.

Oh and why did I forget to remove the exhaust studs when pulling the engine in the last few times. That makes it so much easier to wiggle it out. (I use the hack a day method of a wooden frame ratchet strapped to the engine.)

Yeah I was already thinking of taking the valve assembly apart. I've done it a billion times on a cbr motor, but never on a cb engine.

[M.Henry]

Also what aspect of the k7 to k4 head swap would make my compression go up?

[754]

 K7 has smaller chambers, diffferent knock pins or o rings maybe, a few things to fiddle with.

[PeWe]

A hotter cam with longer duration and higher lift need to get the piston and valve clearance verified. I do not remember the exact degrees where I check by tighten the valve adjuster screws until they hit piston or cannot be tightened more. Min 2mm further down from 0 lash is OK as I have understood it.

[M.Henry]

Great news:

I remembered today that I have a parted out K4 engine in storage. I bought it way back when I first got roxann and totally forgot about it. It has a head with full valvetrain installed that Should just swap over nice and easily.