Topic: Ill-Advised Super Sport (CB750-F0)

[PeWe]

Sure it was not there before? I have bad dowels that will follow the cover when lifting it and jump off right over the rocker arms.
I'm sure those small bastards aim to the cam chain tunnel. 

I lift cover a up and forwards and check where I have them before lifting off the cover completely which will be backwards + sideways since I have engine in frame and frame kit with frame off to allow lifting cover, head, cylinder....

[Jessamine]

Sure it was not there before? I have bad dowels that will follow the cover when lifting it and jump off right over the rocker arms.
I'm sure those small bastards aim to the cam chain tunnel. 

I lift cover a up and forwards and check where I have them before lifting off the cover completely which will be backwards + sideways since I have engine in frame and frame kit with frame off to allow lifting cover, head, cylinder....

I was super careful each time I cracked the valve cover, since I've had a lot of experience with those squirrelly dowel pins trying to run away/getting inconsistently stuck where you least expect. I got one on order, cause I'm pretty sure whatever dumbass worked on this bike last lost it.

Presumably that was the same dumbass who stripped one of the rocker arm holder bolts too. It seems to be holding with loctite at a low torque, and it's pretty redundant and too long to get free and cause any trouble if it loosens up, so I'm not that worried... But I'll probably have to helicoil it eventually.

Still, I got the cam sprocket back on, and the timing and the valve clearances set, and everything seems alright. Not sure if anything is bent, or if a couple valves were just extra far out of adjustment. I think I'm gonna get it fired up for a compression test as-is to see what state it's in now and if it will require a full valve job.

[Jessamine]

Everything seems good with the head, so I buttoned the engine back up tonight and reinstalled it in the frame. Couple of minor setbacks, like ordering the wrong master link (non o-ring) for the drive chain and having to replace some stripped engine hanger nuts, but things went very smoothly otherwise. Having an engine hoist reeeeally helped, especially with getting the hanger bolts in solo. I anticipate starting the bike for the first time in 2 days, this Saturday.

If anyone can advise on the chain, I'd appreciate it. Comparison shots are attached. I had read that these bikes use 530s-- but the 530 master link I got seems way too small. Is that just because it's a non o-ring ML and the chain on the bike is an o-ring type? Or is the chain a different size entirely?

1975 CB750F
1986 CN250
2005 KLR650

[Jessamine]

Ah screw it, I'm just going to snag a new chain.    It's probably time anyway-- no telling how much life this one has left in it.

[scottly]

Check your sprockets before ordering a new chain. The larger link is probably a 630 (6/8" pitch, 3/8" width), while 530 is 5/8" pitch. 

[Jerry Rxman Griffin aka MuthaF'er]

530 is correct for our bikes. I'm sure you will find quite a few variations of a 530 chain. You get what you pay for. Verify tensile strength and pay for it and the chain will last a LONG time. Cheap chains won't.

[Stev-o]

Check your sprockets before ordering a new chain. The larger link is probably a 630 (6/8" pitch, 3/8" width), while 530 is 5/8" pitch.

+1...that looks like a 630 master.   Just bring your chain to the store to fit the master link...

[Jessamine]

It was a 530, the pins are just substantially longer on O-ring/X-ring chains, to accommodate for the seals. A new X-ring chain went on with no problem, and I installed the rivet link with a vice grip, copper dowel, and a hammer. Bike has an 18/48 sprocket setup.

Ran out of time at the very end today, after getting everything reinstalled... But tomorrow I should be able to fire it up and see how it runs.

1975 CB750F
1986 CN250
2005 KLR650

[Jessamine]

Got everything back together today and tried starting the bike. Only thing I could wring from it (kick-starting) was a single detonation. Would bent valves be the most likely culprit? Anything I should check/adjust before ripping back into the head?

I checked the timing, and it seems like the index mark is just past TDC when the points open (see attached photo) but idk if that would be enough on its own to cause these symptoms.

1975 CB750F
1986 CN250
2005 KLR650

[scottly]

There are 10 degrees between the T and F marks, so your timing is about 15 degrees retarded. Retarded timing will make starting more difficult, especially when kicking. Rotate the points plate counter-clockwise until you can set the timing at the F mark, and try starting it again.

[Jessamine]

Just wanted to pop in with a quick update; haven't had a lot of time to work on the bike recently, but I did get the ignition timing set right. Even so, the bike will try to run, badly, for a second or two, but tends to have small detonations and die. Very irregular. I might check the advancer first, but my best guess is that I do indeed have some bent valves (not really a surprise, given the cam sprocket jumped off). I'll be getting/making a valve spring compressor, cutting the upper frame for a kit, and pulling the head for a leakdown test soon.

1975 CB750F
1986 CN250
2005 KLR650

[scottly]

Do a compression test before taking the motor apart. Also a proper leak-down test is done with the motor assembled.

[Jessamine]

Do a compression test before taking the motor apart. Also a proper leak-down test is done with the motor assembled.

Oh, really? I've usually seen them done with the head off and a clean rag under the cam side, so you don't drain the fluid into the crankcase.

I also don't have a compression tester rn and trying to limit how many new tools I pick up for this project. I could at least finger-test it.

1975 CB750F
1986 CN250
2005 KLR650

[scottly]

A "leak-down" test is performed by feeding compressed air into the cylinder through the spark plug hole, with a set of gauges that read the percentage of leakage. It can be more specific than a compression test. For example, if you have a bent exhaust valve, you may be able to hear the air leaking at the muffler.

[Bankerdanny]

You should be able to borrow a leak down tester from O'Reilly or AutoZone

[Jessamine]

Hey y'all! Got busy with full-time work and overhauling the KLR for a xc tour, so it's been awhile since I've had time to wrench on the CB. But I finally had time today to get a proper look under the head, and...

Cylinders 1 2 and 4 have bent valves. Not sure how #3 escaped damage (doesn't leak fluid anyway), but every other valve is somewhere between slightly off and egregiously bent. At worst, it was about 30° off center on one intake and 20° on one exhaust valve. The others weren't so easy to see, but definitely weren't sitting tight in the seats. Do I have to worry about other parts being damaged as well, with that level of deformity? The seats/guides themselves don't appear to have any damage or even marring-- just some funky carbon deposition that's begging to be scrubbed.

Seems to me I should replace all eight valves, with new seals since the old ones feel pretty hard. What's the best type of these and supplier for a mechanic on a budget? Also looking for advice on what else I should replace or service while I've got the head apart. Definitely planning to finesse the ports' finish with a dremel (been reading up on it tonight)

The cylinder walls look pristine, so aside from some carbon removal on top of the pistons I think I'll leave everything below the head alone.

Please see the copious amount of attached photos (individual cylinder head pics are in order 1-4) and do let me know if you spot anything I missed!

1975 CB750F
1986 CN250
2005 KLR650

[seanbarney41]

Replace the guides. Bent valves will have damaged the guides, and the F motors were herded on guides than the Ks. Have the head surface skimmed flat and hone the bores.

it is only 77/78 that were hard on valve guides Cal but yeah, if valves are bent, guides are probably galled

[Jerry Rxman Griffin aka MuthaF'er]

Invest in stronger valve springs! I haven't dropped a valve since my ill-fated debacle in 1976 with my F.

[70CB750]

I would try to find a different head. 

My $0.02 only.

On Red I fought the whole battle with great results but it was expensive and took a while:

- new guides, valves, retainers from CycleX
- Neway cutters for three angle seat cut
- final valve seat lapping

[SOHC4 Cafe Racer Fan]

At a minimum rehone your cylinders.  They should not be smooth but have a uniform cross-hatch.  You can probably avoid an overbore.

[Jessamine]

I would try to find a different head. 

My $0.02 only.

On Red I fought the whole battle with great results but it was expensive and took a while:

- new guides, valves, retainers from CycleX
- Neway cutters for three angle seat cut
- final valve seat lapping

Can anyone chime in about the compatibility of heads between different models?  F0/F1 heads seem less common than K heads. I'm leaning towards "the devil that I know", but I will of course fully inspect the guides before making a decision. Full procedure on the manual? I'm assuming lateral play is the threat.

1975 CB750F
1986 CN250
2005 KLR650

[70CB750]

Check with Bill Benton:

http://forums.sohc4.net/index.php?action=profile;u=11731

he might have a head you could use.

[Jessamine]

At a minimum rehone your cylinders.  They should not be smooth but have a uniform cross-hatch.  You can probably avoid an overbore.

There's still some cross-hatching visible, but they are getting pretty glossy. I was mostly impressed by the lack of scoring. I've honed moped cylinders before, with a power drill honing attachment... do I have to get more technical with these?

1975 CB750F
1986 CN250
2005 KLR650

[70CB750]

No, just a honing  brush - from flex research - and power drill is enough

[Jerry Rxman Griffin aka MuthaF'er]

The 75/76 F -392 head was also used on the 77/78 K if this helps. You should be able to ID one by finding a "392" on the top fin intake side between 2 & 3. Earlier K heads will work 73/74? to 76. If you find one from a 78 K it will have the stronger F2/F3 springs but also the later style valves with the different keepers.