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Offline wade the k

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Where to start
« on: February 29, 2024, 07:18:09 am »
New to the board, beginning my restoration and need some advice on where to start.
1973 CB750k  7,500mi.  All original and stock.  Hasn't been on the road since 1983.
So to begin I changed the oil, filter, carbs were locked up so removed them and had them cleaned up.  New battery
Reinstalled carbs and connected my dummy bottle and it fired right up.  I have fire on all 4 cylinders and #'s 2,3,4 running.  #1 not running but has spark and has fuel in the bowl.  Needle valve appears to be working properly.  #1 plug is completely dry, so must not be getting gas.  Good vacuum on #1 carb (it's sucking) so valves must be opening and closing. I haven't dug into the ignition, wanted it to run first before I started making adjustments.
Any suggestions on where to go next?
thanks guys. 

Offline BenelliSEI

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Re: Where to start
« Reply #1 on: February 29, 2024, 07:24:34 am »
If the float bowl is full and you have spark in #1, that carb needs to come apart. Jets or passageways are plugged……

Offline Stev-o

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Re: Where to start
« Reply #2 on: February 29, 2024, 07:34:22 am »
First off, welcome to the forum.  I agree with John, the pilot jet may be clogged.  Did you only clean the outside of the carbs?

If doing a restoration, I suggest starting a thread in the "Project Shop" area so we can follow along and answer any other questions you may have.
'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline seanbarney41

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Re: Where to start
« Reply #3 on: February 29, 2024, 07:45:47 am »
Yup, all it takes is a drop of water to plug these pilot jets
If it works good, it looks good...

Offline willbird

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Re: Where to start
« Reply #4 on: February 29, 2024, 08:36:13 am »
Yup, all it takes is a drop of water to plug these pilot jets

Not too bad to just swap the pilot too and see if the problem follows the pilot jet. While your down there you might also pull the main jet holders out and see if they have "stuff" on them, does not take long, and while pilots are out and main jet is out you can blow a little something through those passages. Can take idle screw and the spring under it out too and blow that passage out too, or rinse with carb cleaner.

Offline wade the k

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Re: Where to start
« Reply #5 on: February 29, 2024, 08:55:59 am »
Hey thanks everyone, great advice.
The carbs were completely disassembled and cleaned by a guy I know and trust.  I know a guy! ;D.  I have had him work on several of my dad's old 70's bikes I got back on the road and sold.  CL 350, yamaha rd350.... he really is pretty dang good with them.

With that being said, something not right with this one.  I like the idea of switching out the main jets on #1 and #4 and seeing if the problem follows it.  I should be able to do that with the carbs on the bike!  Not looking forward to the removal and install again, that was a chore for sure.

I will do that and report back in with my findings.  Also, move the post to the other board for the future and post a few pics.

thanks!
wade

Offline willbird

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Re: Where to start
« Reply #6 on: February 29, 2024, 12:15:06 pm »
Hey thanks everyone, great advice.
The carbs were completely disassembled and cleaned by a guy I know and trust.  I know a guy! ;D.  I have had him work on several of my dad's old 70's bikes I got back on the road and sold.  CL 350, yamaha rd350.... he really is pretty dang good with them.

With that being said, something not right with this one.  I like the idea of switching out the main jets on #1 and #4 and seeing if the problem follows it.  I should be able to do that with the carbs on the bike!  Not looking forward to the removal and install again, that was a chore for sure.

I will do that and report back in with my findings.  Also, move the post to the other board for the future and post a few pics.

thanks!
wade

Yea I would not pull them off, I'd suggest finding a screwdriver the right length to tighten and loosen the pilot jets with the carbs on the bike, it requires one on the short side. "stuff" can migrate into places after a bike runs a bit too. Looking at the petcock bowl will tell you if you are getting any fine sediment out of the tank.

Bill

Offline wade the k

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Re: Where to start
« Reply #7 on: April 21, 2024, 01:53:13 pm »
Hey guys reporting back in!  Got the carb straightened out, and yes as simple as a clogged main jet. Thanks for the advice.
Ordered and installed a new Delkevic exhaust, the old one just too rusted out to salvage.
Now I need advice on where to head next.
It is running, just not great.  Won’t idle on its on (attached video). This is first time running since 1983!
What I have done so far:
Oil and filter change.
New plugs
Carbs cleaned and bench synched
Checked compression-165 across the board.


Things I know I need to do:
New points and condenser. (Switch to pointless at this time?). Set timing. Not a valid %s URLCam chain adjustment
Vacuum sync carbs.

Suggestions on where to go next?

I want to get it running properly prior to going to phase two:
New brakes front and back (everything stuck)
Unstick (replace?) clutch
New tires.

Sorry for the rambling post, just want to do one thing at a time so not to get overwhelmed.  Thanks in advance for all of your advice!

Offline HondaMan

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Re: Where to start
« Reply #8 on: April 21, 2024, 06:15:38 pm »
Don't be surprised if it runs a little rough for a time. When these bikes get parked that long, the open valve(s) in the head are (slowly) exposed to moisture and they frequently will build a thin layer of rust on the valve faces and seats. The good news is: running the engine a while will make them seal again, in most cases. Add a little bit of oil to the gas (one or two ounces is enough) to help this process along. I also recommend putting about an ounce of oil into the gas anyway, because modern gas doesn't have lead, so these top ends don't get any lubrication from the gas anymore.

It takes about 100-250 miles to make the rusty valve seats/faces 'heal', so just take it easy for a while, then recheck everything.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline gearsoup

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Re: Where to start
« Reply #9 on: April 22, 2024, 03:27:23 am »
Add a little bit of oil to the gas (one or two ounces is enough) to help this process along. I also recommend putting about an ounce of oil into the gas anyway, because modern gas doesn't have lead, so these top ends don't get any lubrication from the gas anymore.
... so just take it easy for a while, then recheck everything.

Would any oil do, or something specific? And define "take it easy"...  ;D
Current Project: CB750-K6 (hoping for OEM look)
Future Project: CB750-K8  (likely go wild on this build)

Offline BenelliSEI

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Re: Where to start
« Reply #10 on: April 22, 2024, 05:20:44 am »
Have you done a carb sync on the bike. The “bench sync” is a good starting point but using a proper manometer on the bike makes a huge improvement.

Offline wade the k

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Re: Where to start
« Reply #11 on: April 22, 2024, 06:35:04 am »
Good advice on adding the oil. Assuming a good quality two stroke oil like yamalube would do the trick.

Next thing will be the vacuum sync then. I don’t own the manometer if someone could provide a link that would be appreciated.

After that on to the ignition.  I have struggled with old coils in the past may go ahead and replace.  Thoughts on switching over to electronic ignition at this point?  Or stick with points for now?

Thanks

Offline Stev-o

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Re: Where to start
« Reply #12 on: April 22, 2024, 07:05:56 am »
I don’t own the manometer if someone could provide a link that would be appreciated.

 I have struggled with old coils in the past may go ahead and replace.

Thanks

The Morgan Carbtune is prolly the best tool out there to sync your carbs...

https://www.carbtune.com/

But a lot of guys also use the Motion Pro...

https://www.partzilla.com/product/aftermarket/2XU7-MOTION-PRO-08-0411?gad_source=1&gclid=Cj0KCQjwlZixBhCoARIsAIC745AnlyK7uFyRQ8ukwltrBVMVmaOdklBtGQGAviGHHnG-XXvmYb__4E0aAq3REALw_wcB

I'm old school and have the Hondaline vac gauges.

As far as coils, they rarely go bad on these bikes. However, the spark plug caps do, test and replace if necessary.
'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline wade the k

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Re: Where to start
« Reply #13 on: April 22, 2024, 07:38:14 am »
Thanks for the links Stev-O.

Everything is still Original stuff on the bike, so thanks for the tip on the plug caps. 



Offline HondaMan

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Re: Where to start
« Reply #15 on: April 22, 2024, 07:38:02 pm »
Add a little bit of oil to the gas (one or two ounces is enough) to help this process along. I also recommend putting about an ounce of oil into the gas anyway, because modern gas doesn't have lead, so these top ends don't get any lubrication from the gas anymore.
... so just take it easy for a while, then recheck everything.

Would any oil do, or something specific? And define "take it easy"...  ;D
I use oil that I took out of the bike, sort of 'recycling' it. If I don't have that, I use 2-stroke synthetic oil (it won't smoke). Anything that will lube the valves and valve seat faces while it re-breaks in works fine.
'Take it easy' is: don't be running to redline, stay in the sub-7000 RPM range until the engine wakes up and smoothes out.

The carb balance will likely be 'off' while this is going on, so unless you just want to learn how to do it (over and over for a while), wait until it idles better and the carbs have cleared their throats  from some use.

I wouldn't be concerned with the points, condensors or coils: the plugs might need to be replaced as they may have rusted during the long sojourn. The sparkplug caps can be checked for moisture damage, which would show up as high resistance: they should be in the 7500 ohm range if they are the OEM caps - this was K3 standard resistance caps.

I offer a Transistor Ignition that is a drop-in unit made expressly for this bike: it will stop the points from wearing for a LONG time (like, decades) and won't require changing anything else on the bike. It also offers 100% 'backup' in that if it should die on the road (like maybe lightning hit you?) you can just unplug it and plug in the points directly, and ride on again.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com