Topic: Resto for cb750k8

[letmwhirl]

Prior Post was in the wrong place. I will add a link back to it once this is posted. For now would like to get this Post in the right section to gather all the work and updates. I will not officially start until mid December.

Link to initial Post:
http://forums.sohc4.net/index.php/topic,195843.msg2300364.html#msg2300364

Yesterday I was able to hoist it up on my mobile bench. Removed Seat, gas tank (which is full and does look very nice inside), side covers and air filter box. Replaced the battery and bingo,, I have lights on the instrument panel and the starter turns the motor over.

Pulled the spark plugs, they look like they were firing ok and I do have spark on all wires/plugs. Oh no... compression is low, only around 75psi., but remember it as been sitting for 17yrs. At least that is what I am told and from the last inspection date. I dropped the #4 carb bowl and it is clean,,, no varnish. Inside the gas tank is also spotless.

Later on in the evening, I found a mouse nest in the air filter and pulled the cover over the front chain drive... loads of thick grease.. oh,, and it has around 40k km on the guage.

[denward17]

Following......

From other posts here, they say the compression has a good chance of increasing after the engine is run for a couple of hundred miles.

My K8 with about 22K miles when purchased, all the exhaust valve guides were quite worn, and the cylinders were oval in shape, which I replaced all of it.

Good Luck..

[BenelliSEI]

+1. Use your remote tank and get it running. You might be surprised how well it runs! Using an automotive compression tester? That will give you low numbers. If they are fairly even, just carry on.

[letmwhirl]

Following......

From other posts here, they say the compression has a good chance of increasing after the engine is run for a couple of hundred miles.

My K8 with about 22K miles when purchased, all the exhaust valve guides were quite worn, and the cylinders were oval in shape, which I replaced all of it.

Good Luck..

Good tip on compression.. thx

[Johnie]

I agree with John. "Carry on." I bet you will see the compression come up on all 4 cylinders. Just checking...did you open the throttle up all the way when you did the compression check? Some people forget that part. Good luck and keep us informed...

[letmwhirl]

I agree with John. "Carry on." I bet you will see the compression come up on all 4 cylinders. Just checking...did you open the throttle up all the way when you did the compression check? Some people forget that part. Good luck and keep us informed...

Thanks for verification. During the compression ck, I did open the throttle all the way. Also, I am using a compression tester for autos.

[Dime]

I have mine stripped down to the frame. Just pulled the head and cylinders this morning. My compression was around 120 psi across the four cylinders with throttle open. When I pulled the cylinders I found all four pistons scuffed and marks on the cylinders.

My thread is in the project shop called 1978 cb750k and so it begins. 

Hopefully you have better success with your engine than mine. At least I know it will be good to go when I am done it.

Following your thread here. Good fortune.

[HondaMan]

From other posts here, they say the compression has a good chance of increasing after the engine is run for a couple of hundred miles.

+1. I've seen it many times.

[letmwhirl]

 November Update …
Well guess what? It started right up after a few short shots of Brake Cleaner in each carb. Took about 3 startups after that and I let it fun for about 5 minutes. So, very happy with that result.

Couple days later removed the carbs and soaked them in the ultrasonic bath. Waiting on kits and a few hoses. The three 3.5mm ID hoses between the carbs, I just ordered a meter of it today. The breather is all cleaned up with new filter and intake hoses.

The mufflers will cleanup pretty good but I can‘t get all the rust off. I was using Mothers and 00 steel wool. And I resorted to a loose brass brush on my drill in a few places. One of the four pipes has a rattle in the muffler. The fenders are also holding onto some fine rust spots.

I pulled the wiring, tires and wheels, then worked on the rims for 2 hours each getting the heavy rust brushed down… I will have to paint them, they are pitted too bad for new chrome. The forks were also removed and disassembled. The lowers will be powder coated along with hubs and other bits and pieces.

I will buy a m-unit blue and wire up similar to Classic Octane video of a CB500. Lights and switches will remain oem, but considering led bulbs. Lithium Ion Battery and new Reg/Rec.

All of this will be done over time. And anything I do that is not oem will have to pass the strict German authorities before being able to register it for driving on their streets.

I will add a few photos from my phone.

[Dime]

November Update …
Well guess what? It started right up after a few short shots of Brake Cleaner in each carb. Took about 3 startups after that and I let it fun for about 5 minutes. So, very happy with that result.

Couple days later removed the carbs and soaked them in the ultrasonic bath. Waiting on kits and a few hoses. The three 3.5mm ID hoses between the carbs, I just ordered a meter of it today. The breather is all cleaned up with new filter and intake hoses.

The mufflers will cleanup pretty good but I can‘t get all the rust off. I was using Mothers and 00 steel wool. And I resorted to a loose brass brush on my drill in a few places. One of the four pipes has a rattle in the muffler. The fenders are also holding onto some fine rust spots.

I pulled the wiring, tires and wheels, then worked on the rims for 2 hours each getting the heavy rust brushed down… I will have to paint them, they are pitted too bad for new chrome. The forks were also removed and disassembled. The lowers will be powder coated along with hubs and other bits and pieces.

I will buy a m-unit blue and wire up similar to Classic Octane video of a CB500. Lights and switches will remain oem, but considering led bulbs. Lithium Ion Battery and new Reg/Rec.

All of this will be done over time. And anything I do that is not oem will have to pass the strict German authorities before being able to register it for driving on their streets.

I will add a few photos from my phone.

Sounds like good progress.

[BenelliSEI]

Excellent progress! Try and fish out that loose piece in the one pipe. Shake it around and see if you can get it to dropout. Take a wire coat hanger, bend a hook on a long length and fish around for it. Left in there it will clatter on every ride and make you crazy!

Why not just replace the standard wire harness with a new one?

[letmwhirl]

Thanks Dime and BenelliSEI for your responses.

Regarding the wiring, I want to clean everything up. Connections, remove rust, dirt and grime. I really enjoy the work and the satisfaction of making it look new-ish again but with up-to-date electronics. I am happy to see electronics that consolidate components logically and reliable. The m-unit does that. I have it installed on an XJ900F that I am also building. I will untape the factory loom and use some of the wires, but with replacing bulbs with leds the current draw will be less, also utilizing smaller wiring. I will open, clean and validate the switch controls. would love to update them too, but keeping it looking classic is also a goal.

Thank you so much for your tip on the exhaust. I wonder if anyone has ever removed the tip and repaired it from the inside? I think cutting the tip off and pulling the resonator(??) out and building a new one just might work. But then that would definitely involve a rechrome.

[madmtnmotors]

Waiting on kits and a few hoses. The three 3.5mm ID hoses between the carbs, I just ordered a meter of it today. The breather is all cleaned up with new filter and intake hoses.

When replacing the 3.5mm crossover tubes don't toss the old ones yet. If you are replacing the rubber crossover tubes for the accelerator pump then be sure to retrieve the little brass restrictors inside the original accel pump crossover tubes. Some of the PD carbs had two sets of rubber crossover tubes (six total) where three were a shared bowl vent and the other three for the accel pump.

[BenelliSEI]

“I wonder if anyone has ever removed the tip and repaired it from the inside? I think cutting the tip off and pulling the resonator(??) out and building a new one just might work. But then that would definitely involve a rechrome.”

On one of my K7/8 bikes, the centre pipe at the back end had rotted away (or someone pulled it out?). I cut a thin, round plate to fit nicely in the recess and welded a short length of thin wall 5/8” pipe to it. Used JB Weld and three rivets to hold it in place and added some black trim paint. The rivets didn’t look right, so a few weeks later I drilled them out, patched the holes with a smear of JB Weld and another coat of black. That was several years ago and the last time I saw the bike it still looked fine!

[letmwhirl]

Waiting on kits and a few hoses. The three 3.5mm ID hoses between the carbs, I just ordered a meter of it today. The breather is all cleaned up with new filter and intake hoses.

When replacing the 3.5mm crossover tubes don't toss the old ones yet. If you are replacing the rubber crossover tubes for the accelerator pump then be sure to retrieve the little brass restrictors inside the original accel pump crossover tubes. Some of the PD carbs had two sets of rubber crossover tubes (six total) where three were a shared bowl vent and the other three for the accel pump.

Awesome tip,,, thank you.. I only have three 3.5mm rubber tubes between my carbs. And I will look inside to see if there are restrictors. I do believe there are other metal tubes with o-ring seals on both ends which I have already replaced. Fuel was pouring out of these when I tested the motor for a start-up.

[newday777]

Waiting on kits and a few hoses. The three 3.5mm ID hoses between the carbs, I just ordered a meter of it today. The breather is all cleaned up with new filter and intake hoses.

When replacing the 3.5mm crossover tubes don't toss the old ones yet. If you are replacing the rubber crossover tubes for the accelerator pump then be sure to retrieve the little brass restrictors inside the original accel pump crossover tubes. Some of the PD carbs had two sets of rubber crossover tubes (six total) where three were a shared bowl vent and the other three for the accel pump.

Interesting about the restrictors. I wasn't aware that there was any in there

[letmwhirl]

Here are a few photos...

[letmwhirl]

“I wonder if anyone has ever removed the tip and repaired it from the inside? I think cutting the tip off and pulling the resonator(??) out and building a new one just might work. But then that would definitely involve a rechrome.”

On one of my K7/8 bikes, the centre pipe at the back end had rotted away (or someone pulled it out?). I cut a thin, round plate to fit nicely in the recess and welded a short length of thin wall 5/8” pipe to it. Used JB Weld and three rivets to hold it in place and added some black trim paint. The rivets didn’t look right, so a few weeks later I drilled them out, patched the holes with a smear of JB Weld and another coat of black. That was several years ago and the last time I saw the bike it still looked fine!

Nice job and great looking bike.   I took the rattle muffler to a cycle shop near me. They actually cut a small section out and removed the loose piece and welded the cutout back in. The piece they took out was only 2 cm long.  I will take a picture of their work either later today and post or next week. We will be out of town over the weekend.

[Stev-o]

Couple days later removed the carbs and soaked them in the ultrasonic bath. Waiting on kits...

What kits did you order?  Aftermarket jets will cause tuning issues, been there.  Confirm your existing brass are genuine Keihin, and if so, clean and reuse. 

[letmwhirl]

Couple days later removed the carbs and soaked them in the ultrasonic bath. Waiting on kits...

What kits did you order?  Aftermarket jets will cause tuning issues, been there.  Confirm your existing brass are genuine Keihin, and if so, clean and reuse.

Good tip Stev-O,  normally what we get here are not quality kits, therefore I will validate the units removed. Thanks! Oh, I hope you had a good visit in Houston.

[letmwhirl]

Here is a photo of the muffler after the cycle shop opened and then welded it.

[Stev-o]

Couple days later removed the carbs and soaked them in the ultrasonic bath. Waiting on kits...

What kits did you order?  Aftermarket jets will cause tuning issues, been there.  Confirm your existing brass are genuine Keihin, and if so, clean and reuse.

Good tip Stev-O,  normally what we get here are not quality kits, therefore I will validate the units removed. Thanks! Oh, I hope you had a good visit in Houston.

Yes, I did, thanks.  Going back tomorrow...

[newday777]

Here is a photo of the muffler after the cycle shop opened and then welded it.

Nice job on it

[madmtnmotors]

Awesome tip,,, thank you.. I only have three 3.5mm rubber tubes between my carbs. And I will look inside to see if there are restrictors. I do believe there are other metal tubes with o-ring seals on both ends which I have already replaced. Fuel was pouring out of these when I tested the motor for a start-up.

With only three rubber crossover tubes those will be the accel pump crossovers and should have little brass restrictors inside if they are original. The aluminum crossover tubes with o-rings are the primary fuel feed to the bowls. 😎

[letmwhirl]

Awesome tip,,, thank you.. I only have three 3.5mm rubber tubes between my carbs. And I will look inside to see if there are restrictors. I do believe there are other metal tubes with o-ring seals on both ends which I have already replaced. Fuel was pouring out of these when I tested the motor for a start-up.

With only three rubber crossover tubes those will be the accel pump crossovers and should have little brass restrictors inside if they are original. The aluminum crossover tubes with o-rings are the primary fuel feed to the bowls. 😎

I will definitely have a look into this and provide some feedback.