Topic: weird carburetor behaviour

[mazingerzeca]

Hello all.
The bike is a CB750 F2. I've rebuilt the engine, cleaned the carbs and so on. I can't set the idle under 1500 rpm. Synchronized the carbs, put the air screw 1 and 3/4 turns out and these are the sympthoms:
- The idle is set at 1500 rpm, but  if I stop on a traffic light it gets lower and lower until it stalls, so I have to keep revving it.
- If the engine stalls, I try to start it again and I can't, until I turn in the idle screw and it starts, with the idle set at 2000-3000 rpm, and when it starts, I have to set the idle lower again.

I turned in the air screw 1/2 turn in and it seemed to improve a litle, but the problem remains. It looks like the idle circuit was not working and I was achieving the idle only with the main circuit?

[TwoTired]

Tune up history?

Air filter age?  Stock air filter?

Stock exhaust?

Spark plug deposits look like?

All head pipes heat the same?

Carb jets and settings are?

- The idle is set at 1500 rpm, but  if I stop on a traffic light it gets lower and lower until it stalls, so I have to keep revving it.

This behavior is generally associated with too rich a pilot mixture, as the spark plug soot up and foul.  (Too lean an it would speed up and quit).
Maybe a a jet or two fell out.  Or, something related to the questions above.

[lucky]

Many important facts are missing.

Like Two Tired already mentioned.

The fact that so many important facts are missing means that
you did not think they were important. But they were important.

Quote: " cleaned the carbs and so on" 

[bjbuchanan]

Stop playing with your knob. After you put up more info on specifics we can help more. What I can tell you right off the bat is to ride the bike and set the idle on a hot engine, then leave it alone. It stalls out because the idle is set too low because you keep moving the knob to make it idle for the cold engine. Problem is that makes it too low for a warm engine

That year should have a fast idle cam anyway so you don't have to do that when it is cold. Once set up properly those carbs work nicely

Post up the carb numbers stamped on the body, like o69a, near the manifold boots.

[mazingerzeca]

- Carbs completely disassembled, cleaned and new o-rings
- Float height adjusted at 14,5 mm
- New air filter, stock
- Stock exhaust
- All exhaust heat the same
- Jets are standard
- pilot screw set at 1-3/4 turns in, changed to 1-1/4 later
- These carbs are PD41B
- I've done it on a warm engine, the problem with the idle falling happens on a warm engine, and I find it hard to start it again, until I turn the knob on.

As far as I know, turning the pilot screw in would lean the mixture in this carbs, and turning the pilot screw out would richen it, isn't it

[harisuluv]

Very similar problem was posted a week ago.  Turns out the choke was not opening/closing fully cause it was binding.  After fixing the binding problem went away.

on your bike turning them in will lean the mixture yes.

[lucky]

What year is this F2???  It could mean a big difference in the float level.
You said PD41B carbs. So it is a 1977?

Does it have the accelerator pump system?
No mention of that.

[mazingerzeca]

What year is this F2???  It could mean a big difference in the float level.
You said PD41B carbs. So it is a 1977?

Does it have the accelerator pump system?
No mention of that.

Yes, it is a 1977, and yes, it has the accelerator pump system.

[lucky]

What year is this F2???  It could mean a big difference in the float level.
You said PD41B carbs. So it is a 1977?

Does it have the accelerator pump system?
No mention of that.

Yes, it is a 1977, and yes, it has the accelerator pump system.

You need to check that the accelerator pump system is working.
Remove the air filter and with the engine off look into the intake with a bright LED pocket flashlight, of each carb, and twist the throttle a few times ,you should see a squirt of gas.
Make sure all 4 carbs work.

[mazingerzeca]

The acceleretor pump is working on the four carbs. Anyway, this pump only works while accelerating, and the problem was at idle.
This is what I've done today. I've checked the clearance between the needle and its jet and it was a bit loose, so I changed it for a new set (the four of them) that I had. I've done this because on another bike I rebuilt the engine was always running too rich, and that was the cause, so I thought that these carburetors could suffer the same problem.
The result is that the idle circuit is not working at all. The bike only works on acceleration, or if I use the choke.
So, this is what I think: before changing the needles and jets the bike was working (badly) at idle due to the amount of gas passing through the needle and the jet, even without accelerating. After changing the needles and jets, no gas enters at idle, so the idle circuit must be clogged. What do you think?

[mazingerzeca]

Hello all.
This is becoming really frustrating. I've cleaned the idle jets, which seemed to be clean, but anyway...
It's getting impossible to set the idle. It seems like the idle circuit is not working at all. I don't know where else to look at.

[mazingerzeca]

One more thing: I set the float height to 14,5 mm, according to the manual. They were set at 16,5 mm. The funny thing is that on another set of carbs that I have the float height was set to 16,5 mm also. Should  I set it back to 16,5 mm?

[Maurice]

How's the charge on your battery? What voltage do you get below 2k RPM? Those are symptoms of a battery not charging properly or too much riding at low rpms.

[flybox1]

One more thing: I set the float height to 14,5 mm, according to the manual. They were set at 16,5 mm. The funny thing is that on another set of carbs that I have the float height was set to 16,5 mm also. Should  I set it back to 16,5 mm?

@16.5mm, the fuel level in the bowls might have been TOO LOW to actually cover the tip of the pilot jet, and only cover the end of the main jet.  You might not have a blocked idle circuit...but maybe one starved for fuel 
Set the floats to 14.5mm.

[mazingerzeca]

The battery is a bit old. I didn't think this could have any influence. In fact I start the bike with crocodiles attached to my car. So, should I change the battery?

[Maurice]

I would start there. Does the headlight get dim also when that happens?

In fact, yesterday I was on the same boat, took the bike out at lunch break at work, and stopped in the middle of nowhere to adjust idle mix when the bike died. Wouldn't start back up with kick until you waited a bit. Limped back to work with people yelling at me when it died again at a red light, and puling into the underground parking the headlight was very dim. Bingo.

So on the ride back home in the evening I kept the idle at 2.5k and made it ok. The whole afternoon I was worried it wouldn't have enough juice to fire back up

Now need to go over the charging system...

[flybox1]

The battery is a bit old. I didn't think this could have any influence. In fact I start the bike with crocodiles attached to my car. So, should I change the battery?

yes, probably, but do the battery voltage checks at the various RPM's and post your findings.
(battery voltage at: rest, idle, 2000rpm, 3000, 4000, 5000)

[mazingerzeca]

I have the carbs on the table, checking everything again.
About the air screw. I've installed it in the following order: screw, spring, metal washer, o-ring. But maybe that's the wrong order and I'm blocking the gas flowing with the o-ring and it should be screw, spring, o-ring, metal washer?

[bjbuchanan]

If you have the PD carbs, here it goes

http://www.cmsnl.com/honda-cb750k-750-four-k-1978-usa_model7244/partslist/E++1901.html#results

Either way cmsnl is a great site that is super easy to navigate, check it out

It figures that this is one of the less detailed fisches up, but it seems that it goes screw, spring, washer, o-ring. The screw gets sealed on those models. That is the usual order for any other carbs that use the o-ring too. You want to block an air leak with that o-ring on those models

[flybox1]

If you have the PD carbs, here it goes

http://www.cmsnl.com/honda-cb750k-750-four-k-1978-usa_model7244/partslist/E++1901.html#results

Either way cmsnl is a great site that is super easy to navigate, check it out

It figures that this is one of the less detailed fisches up, but it seems that it goes screw, spring, washer, o-ring. The screw gets sealed on those models. That is the usual order for any other carbs that use the o-ring too. You want to block an air leak with that o-ring on those models

correct!

[mazingerzeca]

I just wanted to hear that the air screw was in the wrong order and that would solve everything.
So, as I said, the carbs are on the table. This is what I checked:
- New air filter.
- Charged battery.
- New needle and needle jet.
- Float height set to 14,5 mm.
- Pilot air screw set to 1 3/4 turns out.
- Cleaned jets and air passages.

Again, the story was as follows:
It had an inconstant idle, so I changed the needle and needle jet, and set the float height. After it, everything went worst, no having idle at all. The bike only works under acceleration or using the choke.
The only thing I can do is to put the old needle and needle jet, which would result in an inconstant idle, but at least it would idle. As I mentioned before, I think that the problem resides in the idle circuit. It was not working with the old needle and jet, but as it was loose, it allowed to enter the gas through it.
I am really desperate with these carbs, and I don't have any idea about what to do next.

[harisuluv]

I haven't really been following this, but if you had problems at idle, why not start with the idle jets?  The needle jet doesn't involve throttle ranges til 1/4-3/4 throttle.

Have you pulled the idle jets and cleand them?

[mazingerzeca]

I've removed the idle jets, cleaned them and nothing. I mentioned the float height was set at 16,5 mm. I changed it to 14,5 mm, as the manual says. I think I will change it back to 16,5, just for desperation or superstition. I don't know if that will affect the idle.

[harisuluv]

"cleaned them" -- this could mean nothing, I can't analyze this method. 

What did you do to them?  exactly.  and how did you verify they were clean.

[lucky]

You previously said the idle jets "seemed" to be clear.

That means you did not actually remove them and hold each one up to a bright light to see if you could actually see the hole in the jet.

They are either clear or clogged.  The word "seemed" means nothing in the world of professional mechanics. You need a .010 guitar string sometimes to unclog them and some carb spray and compressed air. The holes are about .013 thousandths .

For instance if i said, "hand me those carbs and if I can find a clogged idle jet you owe me $100" IF you hesitate at all, I know you are not doing it correctly.