Topic: BARKER's 1974 CB750K - STUMPED!! JULY 22, 2017

[BARKER]

You call that a leak!?

HAHAHAHA

This is after I cleaned it up. It was trickling down the front of the crank case and into the little recess between the cylinder block and the starter cover.

[SOHC4 Cafe Racer Fan]

Not much of a leak if any.  It might just be a place where oil and grime accumulated.

[pipeguy]

Great job, gonna be watching this one my fellow canuk!

[BARKER]

Great job, gonna be watching this one my fellow canuk!

Nah. I had cleaned it up (anticipating some potential leaks) it definitely had fresh liquid dinosaurs in places that there was none before.

[BARKER]

Diving into this heap a little more.









I decided to take the carbs apart to see if the p.o. had messed with the jets or the position of the needle clips. To my surprise everything was stock and remarkably clean. No visible wear on any of the needles so they are all going back in.



The airbox boots were hard as rocks and deformed (the pic has 2 of the old ones and a brand new one on the right). I went to my local dealer and bought some brand new ones.



Discoreved that the #3 float bowl drain tube had a crack in it. I'm going to solder it up today.



Like that busted casting held together with mechanics wire? It looks worse that it actually is. There is no slop in the bushings. It will be replaced eventually.

So I think at this point I'm going to jam them back together with a new set of float bowl gaskets and set the float heights.

It's a heap...but it's a fun to work on heap



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[BARKER]

Not much in the way of pictures to update on.

I did take the carbs apart and cleaned them up. They were suprisingly clean though. I do believe that this bike has an original 14k miles but that it's been a hard 14k.

I set the float heights and, with some help from board members, managed to get the timing perfect.

Next up is a rebuild of the front brake master cylinder.

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[BARKER]

Updates!

Picked up a sweet new reproduction seat. The old king/queen seat has be retired!





I've also started a youtube channel to document some of the stuff I enjoy and the cb750 is going to have some videos made to hopefully help others out. Kind of in repayment for all the info I've collected from others along the way.

!! DISCLAIMER!! It's a terrible channel (at the moment) but here's a link if you're interested: http://www.youtube.com/channel/UCEtTRp79a0cYQbiOmMtMiGA

[evanphi]

Subbed your youtube channel.

[BARKER]

Sweet. Thanks! I promise that the quality will go up exponentially 

I should also mention that I completed a compression test and the results were 150 psi on every cylinder. I was pretty happy to find this out.

The final tune up item is going to be a carb sync. Hopefully I get to that this weekend.

[BARKER]

I've really been enjoying this old bike. 

I've gotten a bunch of miles on it since the start of the season and I'm discovering some quirks.

1) current fuel economy is 31 MPG. I feel like this could be a lot better.
2) on warmish/hot days the bike struggles to pull away from a stop. I find that I have to run it up to over 3000 rpms to clear it's throat and then let the clutch out. This makes me nervous because if I had to move quickly to get out of trouble's way, I don't know how safe it would be.

Recently I've checked my spark plug cap resistance (5.1k on the low end and 5.7 on the highest) but I still need to check out the coils too.

I'm also debating installing a set of Dyna coils (3 ohm) and a Dyna S ignition that came with the bike. The coils measure where they should but I don't know the status of the Dyna S. It's either buy new points, clean up current points (which I've read mixed things about) or try out this Dyna setup.

Basic things still to do:
- Sync the carbs
- Test stock coils

I love all this trouble shooting and tinkering. These bikes are so easy to work on when you follow the proper advice and procedures!

[theDQG]

Following

[BARKER]

Did a quick carb balance tonight. First time ever. Wow is the adjustment ever fine to turn the before into the after. See attached:




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[mrbreeze]

Running quite a bit better now eh!

[BARKER]

Honestly... I find it sounds much better and idles a little easier.

I had been dealing with a low rpm stumble (like when pulling away from a stop) but it turned out that I had the idle mixture screws turned out 1 3/4 turns. So I went by Hondaman's recommendation of 7/8 and the stumble is gone. It pulls away with ease. (Whoa wait!!!! It doesn't see later post in this thread)

Still should do multiple plug chops to see how the plugs are at all rpms/throttle positions
 

[MauiK3]

Looks like a good project.
Before diving into ignition mods you might want to read Hondaman's most excellent dissertation.. it's not a case of just swapping coils. I intend to use his transistorized system with my new stock coils, points and caps.
My 73 K3 also seems a bit rough off the line but I have not gone into the carbs yet. I also get the high idle when it warms up.. Hopefully it all gets  taken care of when I take it apart for freshening. I also think my clutch is tired. One mod I think I want to do is make the final gearing 17-48 instead of the current 18-48. Since we don't have big freeways on the island it may be happier spinning a bit faster. Stock exhaust always make tuning easier and they sound the best. A bit pricy at Yamiya but worth it, they are no number K0 type. Great reproductions.
Aloha
Steve

[BARKER]

So I'm still trying to find the cause of 3 hesitations that I have.

1) pulling away from a stop. - it's gotten better since the carb balance, but I'm still finding that I have to wind it up before I can pull away. Kind of un-nerving it I ever had to move in an emergency.

2) almost any gear on the highway, 3000 rpms there is a stumble if I open the throttle to accelerate. Slowly open to 1/4 ish throttle

3) 1/2 to full throttle at 6500 rpms doing 80 mph (it's testing I swear!) It stumps hard and will not accelerate past. I get some occasional popping.

I've gone through the entire tune up procedure. Point gap. Valve clearances. Timing with a strobe. Cleaned the carbs and found all factory oem sized 1974 jets. I did a compressed air with carb cleaner cleaning of all the pieces. At the time I set the float heights. I've checked the resistance of my plug caps and coil input/output. It's all good.

Plugs are almost always sooty. I typically get 30-32mpg. Sounds like rich right?

I decided to check the float heights and this is what I'm seeing (attached pics are in order from 1-4 on the bike)

They all look lean if anything. I checked them on the center stand as I was on my own.

The only other thing that I haven't found any info on is the effect of the late model pipes in combination with my 657b carbs.

I'm stumped and half thinking that because I've fiddled with so much... that maybe I need to go back over a number of tune up items again.

Thoughts?

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[mrbreeze]

I can't see your pictures but I'm wondering.....did you just set your floats to the spec in the book or did you do the clear tube method to affirm the fuel level in the bowl? The specified height of the float does not guarantee that the fuel level will be where it should be.

[BARKER]

Yup set to specs in the book with my calipers. Then did the clear tube method.

[mrbreeze]

OK. This time I see the pictures! Many years ago when I changed to pods and 4 to 1 my bike had hiccups also. I also had just moved from Wyoming to Ca. (5K ft. to 89 ft.). Getting my fuel levels right, changing needle positions and trying different main jets finally cured the problem. I can go through the throttle without hesitations. Keep at it recheck and double check all of the tune up items and experiment with different settings. It can be a bit of work but will be worth it when it starts running right.

[calj737]

Your fuel level is too low. 3-4mm below the seam is the correct height. Yours appear to be about twice that. That's why you're struggling with take off and stumble.

The sooty plugs, are they wet or dry? If dry, I'd check your needle clip position (I haven't re-read your thread) and make sure it's correct. Swapping of your pipes should not be causing these problems. Have you double checked the air filter and inlet under the seat?

[evanphi]

Clean up the spark plugs or change to new ones, also. Probably not helping things!

[BARKER]

Float heights was the ticket! Wow is this thing ever a smooth moving rocket now. Wow. I'm super pleased!

Thanks for the help guys!

For the record: I set them using a set of vernier calipers to 26mm from the gasket surface on the carb body. It turned out that they we all super low. Verified via the clear tube test!

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[SOHC4 Cafe Racer Fan]

Sweet!