Topic: 1977K Rich or Lean???

[Squid2021]

I need a little help working through this one.  I can't figure if the bike is rich or lean.

1977 CB 750K, stock airbox, K&N filter, 4 into 2 exhaust.  Pamco Ignition timed correctly, valve clearances set.  Carbs are CLEAN.  Accelerator pump is working great.  Stock 35 idle jets.  The Idle Mixture Screws were missing the washers, so those are in the mail.  Any adjustment on them was not changing anything.

In order to fire it up, I need to go full choke and whack the throttle four or five times while pushing the starter.  That seems like it needs more fuel in order to fire.  It will then idle great, but it only needs the choke for like a minute.  Idle is perfect at this point, but it starts to backfire a little through the pipes after a couple minutes.

If I blip the throttle at idle it will die.  But with half choke and a blip, it revs up great.

The plugs are pretty black also.

So... the plugs and backfire makes me think it's rich.  The rev with half choke and startup makes me think it's lean.

I may switch to #42 idle jets and see what happens since I have a set.  Any Ideas?

[750K]

42 slows will be way too rich I'll bet, I had 42 in my 77 and didn't need any choke at all to start from cold. I've got 40 slows, 120 mains, needle in the middle clip position which I think is a touch rich mid range. Stock airbox and filter, 4-1 header with a baffle. Mixture screws are 1 1/2 maybe an 1/8 of a turn more, can't remember where I ended up at.

Is your fast idle cam adjusted properly? Mine was out and was a #$%* to start cold, now it fires up on the first shot at about 1/2 to 3/4 choke.

[TwoTired]

You adjust carb mixtures for an engine that is at operating temperature, not for cold starting a warm up operation.  For times when the the engine is not at operating temp, the choke enriches the mixture.  It is sort of expected that the plugs will soot up up when using the choke.  But, they should self clean when the engine is at operating temp power is being made.

[scottly]

I need a little help working through this one.  I can't figure if the bike is rich or lean.

The plugs are pretty black also.

So... the plugs and backfire makes me think it's rich.  The rev with half choke and startup makes me think it's lean.

Carbon fouled plugs can produce a weak spark. A rich mixture is easier to ignite than a lean mixture. Once the plugs have been fouled, it can cause the symptoms you are describing, even after the original cause has been corrected. Either burn the carbon off the plugs with a propane torch or replace them and run the bike again, using the minimum amount of choke and accelerator pump needed to start the motor when cold.

[flybox1]

42 slows will be way too rich I'll bet, I had 42 in my 77 and didn't need any choke at all to start from cold. I've got 40 slows, 120 mains, needle in the middle clip position which I think is a touch rich mid range. Stock airbox and filter, 4-1 header with a baffle. Mixture screws are 1 1/2 maybe an 1/8 of a turn more, can't remember where I ended up at.

Is your fast idle cam adjusted properly? Mine was out and was a #$%* to start cold, now it fires up on the first shot at about 1/2 to 3/4 choke.

+1
Sounds like you are lean.
Vacuum sync done?

[oldschoolcarbs]

You adjust carb mixtures for an engine that is at operating temperature, not for cold starting a warm up operation.  For times when the the engine is not at operating temp, the choke enriches the mixture.  It is sort of expected that the plugs will soot up up when using the choke.  But, they should self clean when the engine is at operating temp power is being made.

++!

The one thing you didn't mention is whether you've had the compression gauges on the engine. Low values mimic rich but only superficially.

Twotired, stop by sometime for the nickel tour. I'm across 880 from SJ airport. You'll find my info on the website.

Jack
http://www.oldschoolcarbs.com

[lucky]

Is the K&N filter foam or paper??

If the mixture screws did not have washers then the O rings are probably ruined.
Remove the o rings and look at them. I bet they are all wonky and chewed up.

The washer sits on top of the rubber O ring to prevent the sharp cut off end of the spring from damaging the O ring.

Also if you hold up that K&N filter and you can easily blow thru it and you have 4 into 2 aftermarket exhausts with no baffles, you WILL need that #42 idle jet.
Reply#1 750K has almost the exact same set up and is using a stock idle jet BUT he has his slide needle on middle clip position.(two steps richer) What does that tell you?
He also has the mixture screws way out.
All that can be rectified by using the #42 idle jet,leaning the mixture screws, and putting the slide needle one step richer NOT two steps richer.

The accelerator pump system must be working too. All 4 nozzles.
After you have the mixture screws all straightened out open them all to one full turn open.

With the set up you have I would set the slide needle one position richer than stock.
Clip on 2nd groove down from the top.

If you have baffles in those exhausts and the air filter is the paper type inside of the stock air box then I would go to stock idle jet and slide needle one step richer.
Main jet one size larger in both cases. Use #120's


QUOTE:
"If I blip the throttle at idle it will die.  But with half choke and a blip, it revs up great."  SOLID PROOF IT IS TOO LEAN.


IF the floats are not set correctly then all bets are off.
They must be set like the book says by MEASURING them.


You have a situation very similar to 750K REPLY#1 this thread except he does not know where his mixture screws are, or if they are all set the same. Don't make that mistake.

Set them, (mixture screws) all exactly the same.
Only adjust the linkages for the slides idle control when syncing the carbs DO NOT touch the mixture screws.
But do not sync the carbs UNTIL the mixtures seems good. (No popping on decell )
Count the turns exactly. Put a mark on the screwdriver.

[750K]

35's are stock idles Lucky, I'm using 40's. Mixture screws are all at the same turns out, just because I cant remember my exact turns out does not mean it's incorrect. Do you ride my bike? I don't think so, you sure come off sounding like a retard most of the time. Stock mixture screw setting for my bike aka a 77 cb750k is 11/2 turns out, so if I'm between 1 1/2 and 1 3/4 I'm not WAY out. you do know PD's run lean compared to early model carbs dont you?

[Gasturbine]

Sounds like you may have a fuel delivery problem. Make sure the petcock and lines are clear. If you are running a fuel filter, change it. And make sure you dont have a kinked line.