Topic: 1976 CB750F Carbon Fouling Plugs

[dmc8163]

Hi, I'm a new member and I purchased a new CB750F new in 1976. I loved the bike and used it to ride cross country twice.  I recently retired and purchased a very nice red '76 750F. My problem is carbon fouling of all four plugs. The problem prevents me from riding more than 20-25 miles. With new plugs (NGK D8EA) the bike starts immediately under choke. I quickly take off the choke and it idles smoothly. I let it idle no more than two minutes. I ride a test route of nine miles at varying speeds, nte 60 mph. I pull the plugs and they are completely carbon fouled and the bike will not re-start. I tried hotter plugs (D7EA) with a similar result. I synched the carbs and there was no change. Believing the carbs were running too rich I took the bike to a Honda shop (Henderson, NC) I used to work at to have the problem diagnosed and repaired. The shop rebuilt the carbs, sealed the air box and runners, synched the carbs and installed NGK D8EA factory stock plugs. Result - $800+ and not one bit of improvement. I took it back and they installed  "stock" #105 main jets ($260). No indication they replaced the pilot jets. After spending over $1,000 I'm no longer willing to take the bike back to this Honda shop and I'm determined to repair the bike myself. I would welcome any knowledge, guidance and suggestions members might have concerning this seemingly common problem, i.e., too rich mixture.  Thank you very much.

[robvangulik]

 So with 105 jets it is still running rich?
What kind (brand AND condition ) airfilter is in there?
Without enough air it will always run rich!

[flybox1]

Its a shame you spent that much.  Lesson to be learned.....do it yourself.

Take the carbs off the bike.
Did they use Keihin or aftermarket jets?  What is the pilot jet size?
Main jets have nothing to do with idle mixture.
How many turns from seated is your idle mixture screw set at?
how old/clogged is your air filter?

[ofreen]

Welcome to the forum. How many miles on the bike? The carbs are ‘rebuilt’, were the needle jet sets replaced? What position is the needle in? After market parts? Float height correct? Air filter is recent? The ignition system is working correctly?

[69cb750]

. I ride a test route of nine miles at varying speeds, nte 60 mph. I pull the plugs and they are completely carbon fouled and the bike will not re-start.

To much gas, to little air.
Buy a new oem main and pilot jet, compare hole to what you have.
Check air filter clean, air intake clear.

[TwoTired]

What air filter do you have in the bike?  Brand?

What exhaust do you have on the bike?  Stock?

So far, from the problem description,  I'd say you need to lower the jet needle (hopefully stock Keihin) on the slide, and possibly replace the needle jet if it is worn.   But, you have to settle on what you intend to run with filter and exhaust, before fine tuning carbs.

Cheers,

[ekpent]

  My first thought was air filter also. Check that all the carb vent and overflow tubes are clear also. You probably have not been able to ride it enough to check gas mileage but should get at least around 45mpg if tuned well. Stock OEM jets are marked with the size and a stylized 'K' stamped on them.  How many miles does the bike have on it and if running really rich it may blow some black smoke when you get it on a little.

[pjlogue]

If you don't know the history of the bike I would do a complete tune up starting with adjusting the valve lash.  Then adjust the cam chain adjuster.  Then I would do a compression check.  If that is OK. (will probably be low if the bike has sat for years)  Then check the timing/points and replace the capacitors for the points.  The carbs should be the last things to start messing with because if any other items aren't OK you will be chasing your tail forever.  Be sure the float height is on spec.  Make sure all the internal passages in the carb body are spotless.  This means running a soft wire in them, flushing with lots of carb cleaner and blowing them out with high pressure air.  When the carb bodies are clean then re-install the brass making sure you have Klihin brass. Make sure your needle height is set correctly for the main jet.   If your exhaust is stock the OEM jet sizes should be fine.  Bench sync the carbs with idle air screws 2 turns out and start it up and see where you are.  Then sync the carbs and see where you are with mixture.  You should do a series of plug chops at various throttle setting runs to dial in where you stand.

The slow jet and idle air screw is your main adjustment for most of your riding at idle to ~1/2 throttle.  When I restored my K6 I found I was running very rich.  I ended up going to 38 slow jets (down from 40) and backing out my idle to 2.75 turns cleared up my plugs. 

A work of caution about bringing a 40 year old bike to a shop today.  Most of the "mechanics" in the shops today are technicians and follow a flow chart for diagnosing problems.  These bikes were build before many of them were born and they don't know how they work.  I'm not saying the shop you brought your bike to didn't preform the work they did properly, but unless you are positive they did the work properly, I would go back in and check the above. 

-P.

[Jerry Rxman Griffin aka MuthaF'er]

You are probably only running it at or off idle mostly and not getting into the upper circuits. If after market carb kits have been installed, the air screws may not be the correct ones. I did it too  I had to keep the rpms up or it'd foul plugs quickly.

[Tracksnblades1]

Hi Dmc,

Nice bike I'm fond of the F1 in red.

In addition to the already posted, engine state of tune and condition posts,
Have you had time to try the clear hose method for verifying float levels?
If your red F1 carbs are original, you'll have to follow the clear hose post for temporary modifications. If you choose this method for determining float level.

I had to install new Keihin jets in mine for a new base line. The emulsion tubes / jet holders were unique to,75-76 I think, and are no longer available. I called old Honda dealers and found nos.

Factory jetting, was 105, 40, middle clip, if I remember correctly. We're at 400-500 ft. Mine has K&N individual oval pods and 4-1 Yoshimura, so my current jetting will not be representative of yours. I'll be reinstalling my factory air box, if I decide to leave the displacement alone.

You'll know when you have it right, she'll carry the front wheel in 1st when she climbs on the pipe, even stock.

You have a very nice survivor. Spotting the tank door sticker, I wondered if yours still has the speedo sticker attached too.

Have A safe and healthy retirement

[Tracksnblades1]

For the 76F.

[BRG-BIRD]

Like TwoTired mentioned one of the many things to take a look at is the needle jets/emulsion tubes. I’m not saying this is your issue but it can be overlooked, I have had two different bikes with carb issues and black sooty plugs that ultimately ended up being the needle jets. One had the wrong needle jets and one (75 CB550F) they were worn.

Do all your checks, report back and good luck!

[scottly]

What kind (brand AND condition ) airfilter is in there?
Without enough air it will always run rich!

1+ Also, before removing the carbs do a "clear tube test" to verify the fuel levels in the float bowls. This is assuming you are operating the choke correctly.

[flybox1]

The OP's not worked on bikes before, so lets do the easy stuff first...eh?

How many turns from seated is your idle mixture screw set at?
how old/clogged is your air filter?

If you dont know what or where your IMS are? please ask.  They directly control air supplied at idle.

[dmc8163]

Thank you all so much. It seems a little overwhelming (carbs scare me) but I will follow all your suggestions/directions. There are a couple of things I don't understand and I will be asking for more clarification as I go along. I will start on the easiest check first, the air cleaner. Not sure if it is stock. Exhaust is stock. No decal on speedometer. This may take awhile so my posts may be a little sporadic. I'll keep you all posted. Thank you. dmc8163

[pjlogue]

You should also check your carb to head rubbers.  Be sure they aren't leaking.  With the bike running, spray carb cleaner around the rubber seals on both the carb side and the head side.  If it causes the engine to change rpm then you have a leak there. 

-P.