Topic: Look, squirrel! Or, how bike projects shift quickly; new CB750 K6 project for me

[gearsoup]

TL/DR: I'm gong to spend the winter restoring a 750 K6 instead of my original plan of a K8.

Earlier this year I picked up a K8 for cheap. It was a basket case and has likely been outside for a decade or two. Engine wasn't locked up, so I figured "why not". So, it found its way into my shed and there it sat. With winter now here, I started to assess what was needed and it's looking more like either A) a serious cash/time outlay to get it done before spring, or B) multi-year project.
Being the impatient sort and wanting to be riding a 750 in the spring (I've got a CB400 twin), it was a stroke of "luck" when my brother sent me a FB listing for a '75.  ("luck" is in quotes because of stuff I've since discovered...)

Met up with my brother and we drove out to see the bike. The gent had it running (which I'm not keen on) so we didn't have a chance to see how much of a chore it was to get going (no surprise, it don't start very well). Being a balmy 28F on the coast, (I'm in Maine, the seller was within view of the ocean) we still took it for a test drive. It shifted clean, throttle response was decent enough, a bit of backfiring on decel and at idle.

The good:
The frame is in very good shape. No rust to speak of
The engine (externally) is also in pretty good shape, no major road rash. There is some oil seepage to attend to
New tires (yay?), seat and electrical are in very solid condition. Externally, the bike is very solid (to my eye)
The tank inside is in pretty good condition. There is some rust in the bottom, but not an insane amount. Should clean up decently.

The bad:
Gauges are wrong, I think they're off a K2-K3. Thus, mileage of 5.6k is worthless info. I've already got some eBay OEM (used) gauges on order
Front fender is incorrect, it should have two brace/stays.
Rattle can paint job and cracked side covers
Engine compression is in the toilet. So bad, that I thought my compression tester was broken. It's not...  75-110 psi across the board.

It's my fault. I brought the damn tester with me, but the bike was running and pulled nicely, I let my emotion overcome reason and failed to do due diligence.
Now, onto a leak down test to determine what needs to be replaced. I'm pulling the top end apart anyway to address the oil seepage and cam tensioner rubbers, so if I need to dump a few hundred bucks into parts, well, that's just the way it is I guess.

I know (now) that I paid too much ($1.6k) for it, live and learn I suppose 

My plans for it are to bring it back to (as close as possible) original look. That includes a paint job, any polishing needed, correct gauges. A couple exceptions would likely be a 4-1 exhaust and a short sissy bar/luggage rack for traveler comfort and storage (I'd like to find a period correct piece for that)

[gearsoup]

I've got engine gaskets, gauges, carb o-ring/gasket kit on order. After Christmas/New Year, I'll put the bike on the lift and start tearing into it.
This isn't my first bike project (but is my first 4-cyl) so I'm not super concerned about getting in over my head 

-EDIT-
Oh, and thanks to @newday777 (Stu) for suggesting I post in this forum!

[Stev-o]

Looks like a nice project, not a crazy price at $1600.   

Personally, if was running decent, I would ignore the compression numbers as long as there are within 10% of each other. Compression is known to come up after running a while. Clean fuel system and carbs, check for air leaks [that cause popping] and attempt to get it running properly before a major teardown. Good luck

[newday777]

Seth the compression tester you used may be an automotive tester and not for motorcycle and would give you bad readings. There are posts on that happening.
Post a picture of the tester.

[gearsoup]

Seth the compression tester you used may be an automotive tester and not for motorcycle and would give you bad readings. There are posts on that happening.
Post a picture of the tester.

https://www.amazon.com/gp/product/B00SKSAB8U/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

I did confirm the readings against my air compressor, fwiw.

[seanbarney41]

Steve-O +1 ride the damn thing until the engine needs to be rebuilt...that is what is so fun about these

[gearsoup]

Steve-O +1 ride the damn thing until the engine needs to be rebuilt...that is what is so fun about these

Doesn't a compression of 75-100 indicate that it needs rebuilding? There's a bit of smoke at the tailpipe, but not a serious amount.
If I'm tearing the top end apart to address the oil seepage, wouldn't it make some sense to address things while I'm there?
Don't get me wrong, but if you folks (who know a heck of a lot more about these motors than I will for several years) would just ride it, then I think that's something I should at least consider.

Then again, I'm assuming it's oil seepage, I've not done any cleaning yet to the motor to see what's going on. For all I know, it could just be road grime.

[grcamna2]

Looks like a nice project, not a crazy price at $1600.   

Personally, if was running decent, I would ignore the compression numbers as long as there are within 10% of each other. Compression is known to come up after running a while. Clean fuel system and carbs, check for air leaks [that cause popping] and attempt to get it running properly before a major teardown. Good luck

+1 on the compression numbers,for now; put some Seafoam down each cylinder of the hot engine after a ride and let it sit for a few days.
Do that a few times and run that engine.

[Mark1976]

Steve-O +1 ride the damn thing until the engine needs to be rebuilt...that is what is so fun about these

Doesn't a compression of 75-100 indicate that it needs rebuilding? There's a bit of smoke at the tailpipe, but not a serious amount.
If I'm tearing the top end apart to address the oil seepage, wouldn't it make some sense to address things while I'm there?
Don't get me wrong, but if you folks (who know a heck of a lot more about these motors than I will for several years) would just ride it, then I think that's something I should at least consider.

Then again, I'm assuming it's oil seepage, I've not done any cleaning yet to the motor to see what's going on. For all I know, it could just be road grime.

To everyone else's post, get it running right, ride it a bit, then assess the engine. Compression numbers are just that, numbers. Once its been running for a bit, then do a leak down. It may not necessarily need anything major (outside of cosmetics). It looks like its been forgotten but not unnecessarily neglected. Your bike, your project. Looks like a worthy place to start...

[HondaMan]

These engines have a way of losing compression while sitting, due to the valve faces getting a bit of rust on them. The best cure I've found is to add some oil to the gas, then ride it. Like 200-400 miles, ride it. This often removes the rust and crust and then the valves seal again. The K6 engine is otherwise good for 50k+ miles, even with the wrong oil (as in, 10w40 instead of 20w50) being used. If the oil is too light, then the rings start to give up, and sometimes the crank & rod bearings wear a little too much, causing low oil pressure.

I used to "fix" these bikes by doing the above during winters in the Midwest when I lived there: they would get parked outside by someone, unprepped, then come Spring they had low compression on one of more cylinders, while they ran fine the year before. That's when I developed this 'technique'. The 500/550 doesn't do it as readily, but still does do it over long enough parked times.

The instruments look like they are from the CB550K, 1974-5 versions.

[gearsoup]

Thanks for the feedback, guys. It certainly gives me something to think about it. Honestly, kinda glad that the consensus seems to be "run it until it needs to be rebuilt". Not that I mind tearing into the motor, but it's certainly more friendly to the wallet not to have to dig deep into the motor just yet.

@Hondaman; what's expected oil pressure on a K6 and where is "too low"? The bike doesn't have an analog gauge, just the idiot light near the gauge.  (I do have your book, probably should consult that!)

[BenelliSEI]

Another +1 to all of the above. Set valve clearances, points, timing. Really clean the carbs and do a carb sync on the bike, running, using an accurate manometer. Ride it for a few hundred miles and treat it to an oil change. By then you will really know what needs doing (and I’ll bet an engine rebuild isn’t on the list).

[HondaMan]

Thanks for the feedback, guys. It certainly gives me something to think about it. Honestly, kinda glad that the consensus seems to be "run it until it needs to be rebuilt". Not that I mind tearing into the motor, but it's certainly more friendly to the wallet not to have to dig deep into the motor just yet.

@Hondaman; what's expected oil pressure on a K6 and where is "too low"? The bike doesn't have an analog gauge, just the idiot light near the gauge.  (I do have your book, probably should consult that!)

The oil pressure: it will be about 50-60 PSI on a hot engine at speed (4K+ RPM) if using 20w50 [non-detergent] oil. The 60 PIS is when the pump's regulator opens, so it doesn't go higher. If the regulator is 'stuck' (which does happen to pumps occasionally) it can go higher, which will lead to oil weeps at places like the bottom of the oil filter housing.
It will be about 35-45 PSI with 10w40 oil at the same engine speed, getting a little higher then with revs.

At hot idle it will drop to 5-7 PSI with the heavier oil, and 3-6 PSI with the 10w40. The latter may cause the oil PSI light to flicker after a hiway run and then slowing down to an offramp with a stoplight. It shouldn't stay on then: if it does so then the bearings are worn enough that you shouldn't run the 10w40 oil.

What will likely happen: the cylinders that have lower PSI now will slowly reduce the valve clearance between the valve tip(s) and their rockers, causing "tight" valves in those positions. This indicates that the valves have 'reseated' and cleaned the light rust off their faces and the valve seats, making good seals and full compression again: the trick is to keep watch on this because it is possible, if the valve started out "loose" (like, it is 0.004" intake/0.006" exhaust) and got adjusted tighter (0.002" intake/0.003" exhaust), and then reseated itself, it also won;t seal well (or at all) for too-tight valve lash. This is the #1 cause of burnt exhaust valves over time, in my experience. While it happens rarely, I've seen 2 heads that went thru it: you can see the witness marks on the valve seat faces when it does.

[gearsoup]

Good info, thanks!

Anyone have any experience with aftermarket side covers? Looking at this set, but lack of reviews kind of worries me: https://4into1.com/complete-side-cover-kit-w-orange-honda-wing-emblems-honda-cb750k-1973-1976/

I think there's a member on here who makes some repro units, right? https://www.fb-stuff.com/fb-stuff-store.html
(I do like this one-stop-shop for all the sidecover bits and bobs)

[denward17]

^I have a set of those FB side covers, they are made very good.  After a little sanding they painted up nicely.  They are on my K8.

Also just purchased some for the 550, they look nice too, but haven't painted them yet.

[gearsoup]

@denward, I followed your restoration somewhat closely, very well done! I too, have a K8, but in far sadder condition, not sure which direction I want to take that bike in the future..


Another question (I'm sure I'll have loads) is jetting and exhaust. I'll likely be going to 4-1 Delkevic, should I expect to change my jets? I expect to be tearing into the carbs in the next week or two (waiting on kit and 30L ultrasonic tank to come in)

[HondaMan]

Another question (I'm sure I'll have loads) is jetting and exhaust. I'll likely be going to 4-1 Delkevic, should I expect to change my jets? I expect to be tearing into the carbs in the next week or two (waiting on kit and 30L ultrasonic tank to come in)

Unless you install an old (and somehow still working) Dunstall exhaust system, you won't need to change jetting. That doesn't mean others don't do it: it's just not needed. The K5/6 came with #105 mainjets, #27201 jet needles in the 4th slot, and #40 idle jets. They came with 2 different types of idle-mix adjustment screws: one type is flush with the carb's body and the other sticks out about 3/16", where it originally had a small plastic anti-tamper cap on it. The cap prevented more than 1/2 turn of adjustment, and self-destructed in about 2 years' time from engine heat cracking them. The original setting here was 1.0 turns out from full inward stop, but today, due to likely many overly-tightened previous adjustments widening the needle's seat, and the years of MTBE eating away at the seat, you'll likely find it should be more like 7/8 turn instead, lest it foul plugs.

The above mix settings presume a stock airbox is in place. All bets ore 'off' if this isn't the case.

[gearsoup]

Yes, stock airbox. While I like the look of pods, I've read too many tales of frustration in getting them to run right. I'd rather have a smooth machine that looks a bit "plain" than something cool and finicky

[Don R]

 I'm another "look squirrel!" guy, my projects seem to switch every time I go in the shop. Too many bikes bought right, and totes of parts that I drug home out of three other guys hoards.

[PeWe]

Yes, stock airbox. While I like the look of pods, I've read too many tales of frustration in getting them to run right. I'd rather have a smooth machine that looks a bit "plain" than something cool and finicky

I had no problems with pods on my K6 with stock carbs back in the 80's.
Then 836, ported head, hotter cam and 4-1 needed more air.
086A carbs. Restricting airbox not actual.

Stock engine another thing.
Stock airbox reduce the inlet sound when throttling.

Pods and problems is probably carb depending.

My K2 had the K6 carbs (086A) for a few years. They started to act like dirty inside, fuel starving..
So I assembled a set of 064A carbs I had bought cheap. Recently ultrasonic cleaned when bought a few years ago.

I found that 064A carbs have richer needles.
K2 with almost stock engine, stock airbox and Yamiya no numbers 4-4 that breathe better than stock 341.
It had ran well with 086A carbs
Air screws 7/8-1 turn out.
Needles 4:5 (1 notch richer than my K6 had stock)
Main jets 115.

The 064A carbs ran very rich on same jetting.
I had to lower the needles to 3:5 (middle)
Still 115 mains.
Still stock airbox but with K&N filter since a year back.

(I'll test 112.5mains If I can find any.)

My K6 with Mikuni TMR32 carbs have K&N oval pods. No possible alternative.
Carbs are rather wide so 1 and 4 pods are sticking out from tank.

Have to be covered when washing the bike. If not, water in pods-->inside carbs.
It will make engine to run crappy after 4000rpm. A ride will not change it. Tried 60 km rides.
Had to spray with carb cleaner, gentle compressed air thru fuel screws and into main jets with bowl plugs out.

My K6 with stock carbs and small conical pods in the 80's ran all day long on Autobahn, Germany in pouring rain without issues.

One more thing.
Stock airbox also holds the carbs. With pods, carbs will hang on rubber boots only.
Newer boots are much softer so carbs need support.
They were much stiffer before, stock and early 80's.

[gearsoup]

So...
I got very much sidetracked all winter with other chores and, honestly, just general laziness. Finally getting back to the bike and there's a few points to chat about:

1) Front fender. At some point in the past, one of the owners decided to swap out to a K7/8 front fender. While I am fond of the period-correct piece, are there any real benefits of swapping back to original intent? Is there stiffness to be gained or any other measurable improvement? On that note, are the reproduction pieces, such as is sold by https://4into1.com/reproduction-front-fender-61100-341-700xw-honda-cb550k-cb750k/

2) Seat; cover is starting to split. I can get a new cover and install it, but it's going onto 50 y/o foam. I'm tempted to just get a new seat from Texavina..

3) Paint. Holy sheesh, I was not prepared to see some of the prices that shops want for a spray of tank and side covers. One shop was estimating $1.5-2k. Looks like I might have to learn how to do painting.... And this was just a modern available blue color, nothing super fancy (as I understand it)

4) have to track down an electrical issue, as the brake lights are either stuck on, or are not coming on at all.

Hope to crack into the carbs next week. Think I'm just going to find some NOS insulator boots, these ones are hard as a rock....

[gearsoup]

And it turns out I can buy NEW pre-painted tank and covers for cheaper than a paint job on existing parts 
https://www.yamiya750.com/index.php?main_page=product_info&cPath=246_157_271_243&products_id=2846

[PeWe]

Consider the flake version before order.
They started with that soon after I ordered my Candy Sapphire Blue set.
I should have ordered the flake if I knew what to come.

https://www.yamiya750.com/index.php?main_page=product_info&cPath=246_157_271_243&products_id=2673

They have many colors that look fine.
Would be good to a have a few sets, changing like clothes.

[gearsoup]

I'm going to be diving into the carbs next weekend, no idea what the last few PO's have done to it. If one was to preemptively source replacement brass (I'm going to just assume prior owners have replaced stuff) what would I get from jetsrus.com? Or, would you suggest I wait until everything has been disassembled and go from there?
This isn't going to be my last 750K build, so if I just happen to have some OEM brass sitting in a drawer somewhere, I won't mind the minimal cost should the carbs prove to have OEM kit in it.

I was told by the PO that "oh, I had a guy rebuild the carbs completely". Seeing that it runs and starts rather poorly, I'm a tad bit skeptical of the work that was done....

[BenelliSEI]

I’d open them up and see what’s in there. Do you have a small ultra sonic tank like the one at the end of my bench in this photo? I struggled with carbs until I got one.