Topic: 1976 CB750K Running poorly, RPM issues

[saleman454]

Hello to all,
 I am new to this site so bear with me as I learn to navigate and post correctly.  Here is my bikes story.  A bit long but I think it may help in final diagnoses.  I purchased a 76 CB750K w 5000 miles 4/17. They seemed to be original miles as the bike wore its original tires, brake hoses and yellow factory bolt paint. The owner said he rebuilt the carbs but he couldn't get it running correctly.   Fast forward to 10/20 when I got time to work on it, I ultra sonic cleaned the carbs and rebuilt the carbs.  I replaced the intake and air cleaner boots with Honda parts along with a new air filter element ( Honda, no pods).
Points were set as was the timing ( static). After carbs were installed I struggled with syncing the carbs, bike would run for approx 10 minutes before it would roll smoke out of the #1 cyl and eventually load up the plug, 6-8 plugs and 10 hours later I sent it to a shop to fix the bike.  The shop said they inspected the carbs, set one float height and all was good. I road it 2 miles and the smoke came on again and the bike ran very poorly again!!
 The shop did say that I had the carbs/timing set really close. Bike went back and they pulled the motor and found a bad oil ring and cyl damage. New pistons, rings, machine work etc and the bike came back to me last week.  It ran much better but just doesn't have the snap that I remember.. Rode it 20 miles the other day and at a light the RPM's went to 3K and wouldn't go lower. Set idle and it shut off, adjusted back to normal and same thing happened at next light.  Stopped by the shop and they said it could be an issue with the timing advance springs or the unit its self. The bike was also smoking out of the crankcase breather tube after warmup. I came home pulled the plugs to find the #1 cyl plug was very tight and the point screws which were new were all stripped. A little frustrated I decided to trouble shoot again.  Below are a few things that I noticed.
1. Pulled one spark plug wire at a time and when #4 is removed it shuts the engine off, not with the others.
2. Compression was in the 150 range for all cyl
3. Plugs were NGK 7 and I installed NGK 8 plugs. Ran for 10 min and #1 smoke fairy heavily ( fuel smoke) bike seems to run rich, burns eyes in garage.
4. Bike starts cold w choke and hot without, idles at 900-1k  but doesn't seem as smooth as I remember a CB750K being
5. #1 plug is really black, #2 is slightly black, #3 a little less and #4 is gray in color.
6, checked point gap again, within specs
7, did not check timing yet,

Any help would be greatly appreciated.  I may have forgot a few details..

[goodtryer]

Other data that will be useful to know:

How many turns out are the air screws?
What jets (sizes?, original or aftermarket?) are installed?
In what condition are the airbox & intake manifold rubbers? (cracks? loose? non-original or missing clamps/bands?)
Condition of the battery?

[saleman454]

I will check air screws when I get a chance
jets are stock as far as I know and are original to the carbs when I cleaned them. I can double check but carbs would need to be removed.
New Battery ( 6 months ago with a tender installed) with a good charge.
Boots are OEM Honda and are new on both sides of the carb, all clamps are good and tight.

[goodtryer]

You've spent a lot a that shop for questionable results. Given the resources you'll find on this site, I'd take a break from those guys for a while. Stripped screws on new hardware, guesses about what it might still be... IMO They're getting paid by you to experiment on your bike.

A very good place to start is to download the service/workshop manual for your bike: http://manuals.sohc4.net/cb750k/
It's in multiple PDF's, don't forget the supplements, don't forget the wiring diagrams, etc. I have printed copies in 3-ring binders for each of my bikes and I use them frequently.

As to the problems you're experiencing, take a systematic approach, break it down into components starting at the beginning (the inputs - air, fuel & spark). Think about the system and think about the symptoms. There are inputs that affect all carbs or all cylinders one at a time, in pairs, or all at once.

You need to make sure you're in a good starting position. Refer to Part 6 of the workshop manual, page 172 according to the printed numbers, page 3 in the PDF. This is where you find the correct procedure for the compression test. Also check page 181 for valve adjustment. Basically, you need to go through the items in the 3000-mile service checklist (p. 176-177) to get yourself in a base starting position from which you can make intelligent tests and adjustments. (Obviously the things like spokes and turn signals can wait...) That lack of smooth running can definitely be a symptom of timing. You'll have to get that sorted, too (p. 180).

Then you can start making adjustments to target the sootiness in cylinder 1. Maybe you can make an extra 1/2 turn out on the #1 carb to add air to reduce the rich condition. Clean the spark plug or use one of your spares (You did check that the plug gap is set correctly, yes? page 179). Maybe the air screw changes don't have any impact. At that point start looking for air leaks, and check that the carb floats are set to the right height (p. 249 of the K1-K8 workshop supplement, 26mm. Also where you find the correct starting position for the air screws: "1 turn out +/-1/8".)

The point is, get to a good starting point, make one change at a time, and you'll make improvements. Keep a notebook, write down every change, what the symptoms were, etc. Again, I have one of these for every bike.

Plus post your progress. There are lots of people here to help.

Good luck!

[saleman454]

Checked air screws tonight
#1  2.0    turns
#2  1.25  turns
#3  1.25  turns
#4  1.0    turns

Also did an ohm test on the caps and the coils, I read that 10K caps were used on 1976 and above so these seem good???

#1  9.65 ohms
#2  9.40 ohms
#3  10.5 ohms
#4  12.5 ohms   

Coil 1/4   14.2 ohms,  Coil 2/3  13.8 ohms

Seems to be good spark when engine is turned over.

[saleman454]

Goodtryer, I was typing the last post as your message came through.

I have the manual on my laptop but printing is a much better idea.

You are correct on the shop, that is why I am looking at it myself for better or worse..lol

Tonight was spark (coils, caps and wires ). All seems to be good. 

Compression seems to be good. 

Timing will be my next check.

Thanks for the advise.. A bit crazy at work right now so it may take me a few days.

[goodtryer]

There might be something in that 2.85 kOhm gap between #1 & #4. Even the spread between #2 & #3 is over Hondaman's recommendation:

Yes. The plug caps on these bikes should be within 800 ohms of each other on any one coil, and we can get 5000 (5K and sometimes 10,000 (10K) types. These Hondas used 7500 ohm caps until 1975, then they went to 10,000 ohm caps. Today, we can use the 5000 ohm caps and the resistor-type sparkplugs (like DR8ES-L, DR8EA, DPR8EA9, XR24ES-U) to get another 2000 ohms into each plug circuit. This causes the spark to last longer, which these engines really like! The DPR8EA9 (and ND equivalent, which is slightly better) extend the tip further toward the piston, and makes the engine feel stronger in city traffic situations at lower RPM.

Did you reset the air screws to the standard 1 turn out?

Also, the exhaust hasn't been mentioned. Is it stock? Check that the header isn't loose on any of the cylinders.

Cheers

[saleman454]

Set air screws to 1 turn, exhaust is stock and is tight.

I checked the cap ohms 3 times to be sure. I could change the caps, couldn't hurt.

Thanks

[saleman454]

As of July I am happy to say the bike running as it should. 
Here's the short version...

After trying all the basics again( timing, point gaps, plugs, etc) I was advised to go to Full Throttle Powersports in Chestertown Md (should have went there before I went to the other shop)

It ended up being a defective carb body on the #1 cyl. They were badly corroded when I took them apart.

Bill also fixed the neutral oil switch leak that was supposed to be done by the first shop and made sure all systems were ready to go.

Anyway the bike is running great, starts easily, revs up nicely and sounds like a 750 should.

I want to thank all of you for your input,

[goodtryer]

excellent! glad to hear it’s all worked out.