Topic: CB750K4 what did I get myself into?

[BrockCB750]

It's definitely the correct swingarm for this model bike.

I don't think new chains stretch anywhere close enough to make this right.

I am going to double check the sprocket size and count links on the chain. Something isn't right.

[BenelliSEI]

I agree. I used the same combo on my K6 and the adjuster bolts poke through the end lug about 1/4”

[BrockCB750]

I just confirmed 48T Rear, 18T Front, 100links.

@BenelliSEI are you saying you agree something is off? I ask because in your picture it looks pretty close to what I have going on with mine.

[PeWe]

My K6 with earlier swingarm, K2 type. K6 swingarm same lenght.
Markings at 2nd line on swingarm. See photo.
18:48 sprockets.

100 links as stock and described in manuals.

The chain my bike had at purchase end of 70's really weak. Stretched very quick so adjust wheel a must rather often.
Even the later o-ring chain D.I.D 50 HDS stretched much.

Todays chains stretch very little. Most are with rivet master link.
My K6 has the stronger D.I.D 530 ZVM-X.

It had DC chains strongest 530 chain before. It did not stretch much.
In the end rattling sideways.

My almost stock K2 has the less stronger D.I.D 530 VX 100 links. 17:48 sprockets.

[BrockCB750]

I should have been more clear. I can adjust to where there is proper slack.

This is likely my inexperience showing but I thought i would be adjusting the wheel more center of the swing arm alignment hash marks. If that makes sense.

I think if I can get proper slack I am good to go and after a bit it will require another adjustment.

[PeWe]

OK, so chain has not a proper slack despite wheel in its most forward position in the swingarm?

Really sure that chain has 100 links?
Or different swingarm?

[newday777]

Great thread.  I’m now starting to go through and read some more threads than the one I started. I’ve a few comments.

In the ultrasonic cleaner, I use a 4:1 solution of water:Simple Green.  You do have to be careful because it will blacken the aluminum carb bodies.

Acetone is good carb cleaner.  Aerosol carb cleaner is just acetone and propellant (probably propane).

I’ve cleaned stock jets in acetone for reuse with no issues.

Engine stumble or stalling at throttle tip in, can be caused by an initial lean-out condition.  All else being well, tiny passages in the carb bodies can still be restricted.  When gasoline has been allowed to sit in the carbs over time to evaporate, the resulting varnish can be extremely hard to remove.  Takes lots of ultrasonic cleaning, maybe immersed and soaked in acetone.  The lean-out condition typical of older carbs was addressed later by manufacturers by going to a pumper carb.  These tiny pumper passages in the carb bodies (I have experience with a ‘78CB750) can be difficult to clear of evaporated gasoline varnish.

Some good advice, here! I use acetone, too, and/or lacquer thinner.
In the carb bodies of the K0-K6 and F0 bikes with the 086a carbs, there is a long passage between the bell area at the back of the carb and the mainjet. This shows up as a little brass port in the back side of the carb. It's passages goes straight forward a few mm, then turns about 30 degrees downward, then turns striaght toward the top of the mainjet's emulsifier tube, entering that site at an angle near the little brass jet in the throat of the carb. This passage often plugs up with a white powder (zinc corrosion) if the carbs were exposed to the years of MTBE gasolines, then parked and left to dry out. When this passage plugs up fully or partly, the cylinder will foul plugs in less than 10 miles (usually in 4 or 5 miles). Clean it manually by removing the needle jet from the carb's body (I use an old screwdriver that I ground the tip to fit nicely into the needle jet to press or tap it out) and then run a soft wire thru from the little brass hole in the bell area of the carb, all the way to the mainjet emulsifier's passage. It will come into the passage at about a 40 degree angle. Go gently, feel your way around the 'corners' at the back as it starts in, and push it back-and-forth to push out any of the white powder you might find.

This didn't accumulate in the bikes that were run thru the MTBE years and then kept running, nor those that were parked before that nightmare was imposed on us all: it occurs in those that were parked with gas in them during those years.

I've just started cleaning a set of K2 750 657A carbs that had fuel setting in them from 2014. Results of MTBE? Ugly green in one bowl, yellow in the other 3, before ultrasonic cleaning 2 times while together, powder in the fuel inlet under the float valves. The floats and pins were solid, had to use a fine flame torch to melt the pins loose before attempting to remove them so as not to break any posts.
Next move will be to strip them down for full cleaning.

[RAFster122s]

Regular green simple green should not be used on the carbs, it is not good for aluminum. They make a version or two which are aluminum safe at reasonable dilutions...
Home Depot carries the purple version of aluminum safe simple green and there is an aircraft version that is aluminum safe.

[BenelliSEI]

I should have been more clear. I can adjust to where there is proper slack.

This is likely my inexperience showing but I thought i would be adjusting the wheel more center of the swing arm alignment hash marks. If that makes sense.

I think if I can get proper slack I am good to go and after a bit it will require another adjustment.

Sorry I missed your question. What I meant: I used the same chain length and sprockets on my K6 project. The axle is definitely well forward. I think between the 1st and 2nd hash marks and that is OK. Set the correct slack and you are fine. Check after 200 miles. It will need adjustment.

You’ll be surprised how far back it moves over the life of the sprockets/chain combo.

[BenelliSEI]

Those carbs look pretty crusty, the amount of crap above the float valves is significant. I’d say those need to come off the rack, thoroughly stripped and spend some time in an ultra sonic tank. Make sure you replace the “0” rings on the fuel tees. They will for sure leak if you don’t.

When you go to reassemble, don’t pull hard on the main emulsion tube with a long 8 mm wrench. Those towers are easy to snap off when that corroded. Just snug them up a bit!

[HondaMan]

I have seen, with chains that are pre-lubed with a heavy oil/grease, the rear wheel's adjuster marks are clear forward of the front mark when installing the chain. After about 10 miles the grease oozes out of the links and the chain becomes very loose in those situations. This varies by the chain maker, as they all have their 'secret clearance' numbers they use for their chain. When assembling the post-K2 thru K5 versions of these bikes from their crates (brand new) they chains were always very taut (RK brand chains then) and we left them alone: by the 300-mile checkup they were always very loose.

[BrockCB750]

After a boneheaded wiring mishap I have made some progress. Starter wired up and turns the bike over nicely!



https://hosting.photobucket.com/images/r798/WBlacklidge/VID20240428144841_2.mp4

[BrockCB750]

Everything wired up from coils to dyna ignition. Set timing. Wanted to start it up but realized I need the correct size fuel line.

Hopefully tomorrow!

[BrockCB750]

Going to wire up the oil pressure switch just so I can verify I have oil pressure. Also going to wire in the kill switch just in case.

I put oil in the oil tank and turned it over a bunch. Everything is really smooth.

Any other concerns I should have before attempting to start her up?

Thanks!

[BrockCB750]

Well first attempt to start it sputtered a little then gave up. Didn't realize I had run the battery down so far checking for oil pressure. Going to give it another shot here in 15 minutes and call it a night.

This week looks like I will be chasing down spark, fuel and air.

[BrockCB750]

Well... I don't believe it but...backed the choke off 1/4 from full and it started right up. Idled great! All four cylinders firing based on exhaust temp. I am in shock right now. That felt amazing.

[denward17]

Good news Brock!

[BrockCB750]

Not sure if I can share videos from photobucket...

https://photobucket.com/u/WBlacklidge/a/0974477e-fa4a-4a65-b613-66ef85a06be4/p/a8bd0fbe-8719-455a-8118-70473fdfaa04


https://www.instagram.com/reel/C6XWp5CICHX/?igsh=eWp4a2U1MzZ6cmE4

[denward17]

The instagram link worked for me, sounds great.

[BenelliSEI]

Well... I don't believe it but...backed the choke off 1/4 from full and it started right up. Idled great! All four cylinders firing based on exhaust temp. I am in shock right now. That felt amazing.

Congrats! Nothing feels more satisfying than that first start!

[BrockCB750]

Thanks!

Got some more gas and started it up again. Definitely needs some adjustment. Some common problems like idle hanging after any amount of throttle. It could be anything except the throttle linkage which snaps right back. No big deal I have literally done nothing to diagnose or resolve.

Current state is that its wired just enough to run and what is wired up isn't done well.

Trying to decide:

1) Tune it and get it running better now then wire everything up.
2) Work on wiring it up more complete, safe and asthetic. Then tackle any engine adjustments while I have the ability to also ride it.

I am leaning towards #2 because while I can tune and get it running better at idle and with some throttle I absolutely can't ride it under load and get a feel for it. Also, the janky wiring makes me nervous. Finally, the goal was to see that it can run and nothing major was wrong. That seems to be the case.