Topic: CB750 K2 seems rich at idle

[Scott]

Hi there
I have put a rebuilt set of carbs on my K2.
These carbs were set up for a "race engine"
At idle black smoke pours out of the exhaust and the engine runs "lumpy" but form 3000 RPM upwards the bike runs strong.
I'm only trying these carbs as my bike hesitates when twisting the throttle from idle with the carbs that I have been running for the last 3 or so years.

I have swapped the idle jets for a second hand set out of some spare carbs and they are #40

The main jets are #108 I think and these were taken out of the carbs that I had running on the bike.

I've had them on and off 3 times yesterday to make these changes and I am about to pull them off again now, but not sure what to look for.

A question about the clip on the needle though. Taking the clip higher or lower = what at idle.
Or does clip position affect mixture the whole way through carb operation from idle to full open.

Thanks
Scott

[flybox1]

what do your plug insulator deposits tell you?
stock pilot jets for a K2 are #35's (IIRC) so yeah, if you switched out the pilot jets to 40's, you are making the idle circuit MORE rich.  but this is only an assumption because we dont know what intake you are running.  stock box/pods/stacks, etc.
You change idle mixture with pilot jet and A/F mixture screws adjustments.
clip position only effects mixture above 1/4 throttle.  adjustment makes no difference at idle.

[PeWe]

Floats OK or allow overflow causing very rich at Idle?

Check ignition with ign strobe lamp.
Risk that advancer springs are sloppy that cause too early full advance way before 2500rpm.

[Scott]

Thanks for the replys
I have read a few threads I should have know better but it slipped my mind.

The standard air box is in place and it has a standard paper air filter in it.
The bike has a 4 into 1 exhaust on it.

When I bought the bike 3 or so years ago I pulled the carbys off to clean them and put carby kits through them which included new jets from memory.
I am still looking to try and find the parts that I removed from the carbs so that I can check what the idle jets # was before I replaced them 3 years ago.

At this point in time I have replaced the #40 idle jets that came with the "race" carbs with #40 idle jets that where already on my bike. I only did this to see if for some reason it made a difference due to the jets being made by different manufactures.

The mixture screw adjustment is set at 3/4 turn out.

Float levels are OK but I only checked this with a gauge and not clear hose with carbs on bike.

Ignition timing I set not long ago after I straightened the points shaft and I have Marks ignition module on the bike so I think that will still be OK.
The bike has been OK before swapping the carbs to these new ones except it had a flat spot getting it off the mark at a set of traffic lights

Thanks for trying to help

Scott

[flybox1]

please tell us about these 'race' carbs...
what makes them race carbs?
why does your bike need race carbs if you have a stock airbox and paper filter?

smells fishy 

[Nic]

stock pilot jets for a K2 are #35's (IIRC)

Stock idle jets on KO to K6 is #40. Some KOs had 38s but were cold blooded on cold startup.

[Scott]

I bought an engine that was built by a bloke to introduce his son to historic bike racing. The engine has never been used because his son likes to drag race cars and is not interested in bikes.
The engine is a competitive engine for a beginner racer. The guy who built this engine has had a bit of success himself in racing CB750's and has built a number of successful engines.

I was going to put this engine into a Rickman frame (still will one day) and thought that I needed carbs to suit this engine. I didn't have time to find a set of carbs and rebuild them so I bought a set of reconditioned carbs that were set up to be OK with this race engine.

I was pretty sure that my current engine in the K2 would not run correctly with these new carbs, but I thought I would try anyway and by changing a few things they may run OK

The main reason I'm trying these carbs is to get rid of the problem that I currently have and have had for the last 3 years which is the flat spot when accelerating from a standing start

Thanks
Scott

[Nic]

Dunno if this will fix your problem but I had a hesitation off idle that drove me nuts, I fixed it by soldering up the hole in the mixture screws, or, just get solid ones, this is of course if yours have the holes, it fixed mine perfectly. Set screws 1 turn out to start with.

[Scott]

I just looked at the needle on the slide.
The needle on the carbs that is running rich at idle has #27307 on it.
A needle from a spare set of parts carbs has needle with #27201 on them.

Does this make the needle a different size or is it just a batch number type thing

[Nic]

Use the 201s with clip on 4th slot from the top, ie, second from bottom.

[Scott]

Gday Nic
I just had a look at the carbs that came off my bike and the mixture screw has got the hole in it.
As I have a set of spare carbs that I am robbing for parts I think I will try what you did with soldering up the hole and give it a go.

I have to duck out for a couple of hours but will do it later today and chuck those carbs back on.

So far no frustration but quite enjoying this.

Scott

[Scott]

Hopping for a celebration by the end of the day

[Nic]

After 3 years let's hope for success You'll probably need to do some light sanding to tidy up the solder, use 600 and try not to deform the taper too much.