Topic: Carb tuning - one more time please

[mycb750k6]

So I'm a bit frustrated with the carbs on my 76 CB750. Let me say the bike is completely tuned from points/timing through valves/carb synching and it still won't idle until it's been running for 15 minutes or more. I've read all the carb posts here over and over and just can't get it. I'm used to my 71 and my buddies 70 just starting up and idling. And if I set the idle screw so that it will idle semi-cold, it will race at 1600-1800 once it's warmed up. The lowest I can get it to idle even warmed up is 1200. The carbs are clean and the jets and are clean. Air screws are about 3/4 turn out. Can someone please point me in the right direction as to what to try next. I know some have posted that that's the way these bikes are but my other 750s don't do this and I want to fix it if I can. I have a set of 71 carbs my friend just rebult for me and I may try those but I'd rather know why I can't get these to work.

[andy750]

Ah sounds frustrating and it can be. You are correct though that its wrong to think "thats the way these bikes are" - it can be fixed!

Ok where to start:

1. What size are your main jets? pilot jets? For a K6 should be 110/40. Is that correct?
2. Needle settings - is it 3rd position from top? If not where is it?
3. Check pilot jet again as this is source of the idle - also check your air screws are seating properly as 1 turn out is the norm but will depend on needle setting.
4. Have you changed parts recently? New rebuild kit perhaps?
5. Check for air leaks at carb-engine rubbers? If not spray wd-40 and see if idle changes. If yes you have an air leak.
6. how is throttle cable routed? Is it binding? Is this the cause of the high idle?
7. I your choke working correctly? Take off the airbox and look in the back of the carbs for starters.

Just a few things to start with.

cheers
Andy

[mycb750k6]

Thanks Andy - this is a good start. BTW, I just sent you a PM on the pit bull topic.

Anyway,
1. My mains are 120 and I'll have to check the pilots. I changed them from 110 for no good reason but the problem has been there since I bought the bike a year or so ago with the 110s. So I will check the pilot size.
2. I don't know the needle setting either but I'll get that today a well. Carbs need to come off for that???
3. I'll check the pilot jets. Frankly I started with one turn out and turned them in from there but I can't really say I noticed a difference when I was doing that.
4. No - no rebuild kit by me. When I bought the bike, Carpy replaced the carbs so I don't know the status other than they were 76 carbs so they may have been rebuilt but I really doubt it.
5. OK I will check for an air leak at the engine rubbers. I will let you know but not till later today.
6. I'll check the routing of the throttle cable. I will say that when it is idling high, I can turn the throttle handle forward, pushing the cable in, and the idle does reduce.
7 No, the choke is not working correctly in my opinion. I need full choke to start the engine and I need to shut the choke off once it starts to warm but beyond that there's no intermediate setting or choke adjustment setting. It's either off or on.

So I will pull the airbox and examine all the things you've mentioned and get back to you. Anything else while I'm at it?

[Spanner 1]

While your at it !, with the slides bottomed-out ( at idle ) the motor depends on air sucked thru' the idle air port to mix with fuel available in the idle/slow jet. I would squirt carb cleaner thru' on each carb to verify clear.......it's the tube on the same side as the idle air screw on each carb ( viewed from airbox end ). With the air screw set at , say, 1 turn out you must see carb cleaner squirting into the carb throat if clear........ good luck.

[Yoshi823]

On my 750F2 carbs there was a cam lever that raised the choke rpm. This was adjustable. Maybe the lever system on your carbs are the same or similar and the cam system has seized...?

[mycb750k6]

OK I'm back with my report.
My pilots are 40s. I pulled them out and blew them out with and air compressor and cleaned out the idle screws and holes as well and sprayed carb cleaner into all the appropriate holes and put it back together and cartuned it. No change. Still won't idle cold and after warming up for temn mnutes or more will not idle below 1200-1400. Keep in mind it accelerates great, doesn't back fire or smoke, hads new plugs and tune. So must be the idle jets I thought but Noooo. So any suggestions what to do next. I set the air screws at 1 turn out and tried +/- 1/4 but it had little to no effect. My tank is clean because I por-15'd it last summer.

[TwoTired]

And the spark plug deposits look like?...

And all the head pipes have the same heat?...

And all the float bowls are getting fuel?...

And none of the intake runners are leaking?...

And you checked that each of the four entrys into the pilot circuit carb body were capable of flowing to each of the others?...

And your mechanical spark advance is working freely?...

[mycb750k6]

And the spark plug deposits look like?... They are tan but the 2-3s are darker but not rich darker
And all the head pipes have the same heat?...Yes
And all the float bowls are getting fuel?...Yes
And none of the intake runners are leaking?...No - however they did take quite a bit of tightening but I don't want to over do it.
And you checked that each of the four entrys into the pilot circuit carb body were capable of flowing to each of the others?... No I didn't - How do I check that?
And your mechanical spark advance is working freely?... It appears to be OK using the timing light. Advance is in the correct range.

[BobbyR]

Long shot but which way are you slides facing?

[TwoTired]

And you checked that each of the four entrys into the pilot circuit carb body were capable of flowing to each of the others?... No I didn't - How do I check that?

The pilot circuit has a fuel jet, an air jet, a metering screw, and an exit point into the carbs main air channel.  I check that each of these holes, where they reside in the carb body, can flow to each other by forcing carb cleaner from an aerosol can into one opening, blocking two and observing full flow from the one that remains open.  Then, do the the same with each hole in turn, thus proving an unblocked, unrestricted passage between each of the 4 entry points in the carb body.

[BobbyR]

OK I'm back with my report.
My pilots are 40s. I pulled them out and blew them out with and air compressor and cleaned out the idle screws and holes as well and sprayed carb cleaner into all the appropriate holes and put it back together and cartuned it. No change. Still won't idle cold and after warming up for temn mnutes or more will not idle below 1200-1400. Keep in mind it accelerates great, doesn't back fire or smoke, hads new plugs and tune.[/color] So must be the idle jets I thought but Noooo. So any suggestions what to do next. I set the air screws at 1 turn out and tried +/- 1/4 but it had little to no effect. My tank is clean because I por-15'd it last summer.

According to reports here over the years 1200 is not an unusual idle speed. Considering the tachs are not precise, it could be lower. These are very cold blooded bikes so 10 minutes is not an unreasonable warm up time. You report good performance, maybe your expectations are fairly high.

[TwoTired]

Bobby brought up some good points.
Do not expect the engine to idle well when cold.  That is just what the choke is for.  76 carbs do not have a fast idle cam, to hold the slides open during warm up. YOU are expected to keep the idle speed up until after it reaches operating temp.  After it warms up is also where you should be setting the idle speed with the big knob.

Further, when you say it won't idle, what are the specific symptoms.  Describe what happens when it "won't idle".  Sputter?  Engine changes speed before quitting.  Gradually slows down? or ends abruptly.

What does it take to restart?

Have you vacuum balanced the carbs?  If not, then you don't get to complain about idle issues.

When it is warmed up, how far can you twist the throttle at low RPM under load before the engine will "wheeze" and hesitate until the throttle is backed off?

You nay also wish to try D7EA plugs during cold weather.  They will allow better running during warm up.  But, they are not the best plug for running in the summer heat.