Topic: Rich 1 and 4

[M 750K6]

Hi all,

Could do with some help on a persistent problem. CB750K6, standard air filter and exhausts. Rubbers look good and sealed. Plugs 2 and 3 are about right. Plugs 1 and 4 are sooty, insulator and electrodes. 1 slightly more than 4, by a tiny amount. The soot is dry and rubs off, so fuel. No sign of rich through the exhaust gas.

I swapped Plugs 1 and 2 and the sooty plug cleaned up now it was in cylinder 2, the clean plug turned sooty in no.1. I've double checked the points condition, gap and static timing is spot on. The advance mechanism is free, not seized. Last time I removed and cleaned the carbs, I took the bike for a run then did a carb balance with a Carbtune vacuum tool. No. 1 was quite a bit higher vacuum to the other 3, until adjusted. Valves are in spec, done cold when the engine was out and then re-checked (cold) 400 miles later.

The bike is running really great, so i could leave it alone, but ive not left the plugs more than 150 miles before checking and cleaning. So, i don't know if they will soot up and give in beyond that.

The idle mixture screws are at 1 turn out on all 4.  I turned in the idle mixture screw on 1 and 4 about 1/8th and it made no difference to the plugs, but the bike was less happy. A slight abruptness at low throttle opening. This went away when I reset the screws to 1 full turn. IIRC, the needle is on the middle notch, the same for all four carbs. I'm on the original brass, all clean. I tried keyster replacements, but all 4 needles were slimmer and I was running too rich on all 4. All 4 float heights have been double checked to spec, adjusted with the carbs at an angle so they are touching the valve top, not pressing the valve down.

My thinking is it's probably fuel not ignition. Possibly wear. I think idle is OK, as leaning 1 and 2 mixture screws did not reduce the soot on the plugs, but changed the low throttle pick up. So, I'm thinking it may be 1 and 2 are too rich as the needle comes into play? Is it a cardinal sin to lower the needle one notch on just the 2 outer carbs, then re balance the carbs?

Anyone else solved a similar issue? But, as I said, the bike is really running great on a ride!

[Finnigan]

You said the soot was dry, any chance its oil getting past the valve seals?

Compression good on all 4?

[scottly]

All 4 float heights have been double checked to spec, adjusted with the carbs at an angle so they are touching the valve top, not pressing the valve down.

That method assumes the spring-loaded pins in the needle valves are a certain stiffness; it takes some pressure on the needles for them to seal. Check the actual level in the float bowls with the "clear tube" method.

[M 750K6]

You said the soot was dry, any chance its oil getting past the valve seals?

Compression good on all 4?

I don't think it's oil. It's dry when I pull the plugs and I think I did everything well on my recent head rebuild.  replaced the valve stem seals. Valves were lapped to a nice even dull finish and no play on inlets, very minor play on exhausts, but I think nothing to worry about. The cylinder bores were worn, but I had them checked and was assured I wasn't ready for a rebore. They were honed for me and rings gapped.

I can’t say about compression. My compression tester has daft rubber stems that don't allow me tighten them into the plug holes. Should upgrade, but budget was exceeded on my rebuild!! It feels like good compression on the kick-start. But I doubt that's a good test. But it runs strongly on the lanes I ride, sprightly right up to 85mph, which is where I draw the line, to keep my licence!

[M 750K6]

That method assumes the spring-loaded pins in the needle valves are a certain stiffness; it takes some pressure on the needles for them to seal. Check the actual level in the float bowls with the "clear tube" method.

I'll have to look into that, never tried it.

[M 750K6]

As recommended, I'm planning to check the fuel levels in my carbs. On a CB750K6, if the floats and valve are working correctly, what is the height I need to check for with the clear tube method, please? Is it c.3mm below the bowl to carb joint?

Thanks, Martin

[scottly]

Yes.

[M 750K6]

Thanks 👍

[HondaMan]

On the K6: if you have the needles #27201 they should be set with the clip in the #4 position.
If your needles have one more digit, like #27130n (n can be 1, 2, or 4) they can be in the middle notch. These later needles run leaner below 4000 RPM than the earlier ones, and were used with the late-opening intake valves of the "F0" cam. The K6 engines were mixed with K5 and F0 parts, so this sort of thing happened quite a bit, and not always correctly(!) I have discovered, over the years.

This almost sounds like a weaker spark on the #1 - #4 coil? It only takes one plug cap out-of-range (ohms) to make both act up. The OEM plug caps were 10k ohms on the K6, modern ones are 5k ohms for the most part. They should be 9.5k to 10.5k ohms or so, and within 500 ohms of each other: otherwise the spark discharges unequally between the 2 plugs and they both act up.

[M 750K6]

Helpful, thank you. I will look at this after float levels, if that doesn't help. The cam and tappets are original,  but the head and valves (valve springs etc..) are not, probably a K4. My head's gasket surfaces had been badly damaged and leaked and this was all I could get late last year.