Topic: Need Help with Carb Sync

[VerdantGuardener]

Alright, y'all, I need help. I am at my wits end with this. I think I am having a carb tuning issue.

1972 CB500k

Stock Airbox with new filter
Set points gap and points, gets 12v right at the F marks.
Set valve clearance to spec, rotated engine to confirm.
Synced carbs, they are all reading roughly the same (300mm ish?)
Completely clean carbs, new jets and o-rings
Set float height to spec
New Reg/rec and am battery is staying at 12.7v between runs
Battery reads about 13.2v at 3k rpm
Throttle tube has about 1/8 inch of play before engaging the throttle body
I have experimented with all sorts of variations on air mix screw, from all the way in to all the way out. Bike runs best with them almost all the way out.
4 into 1 exhaust
Jets are 100 and 38
Checked for leaks around intake with spraying some fluid
After all this I pulled the spark plugs and they look fine (tan, no oil or gas)
Runs great at higher RPM (I can get it to red line and I feel like it has a good amount of power)

THE ISSUE:

Bike will start and idle great at 2k rpm indefinitely. However, when I blip the throttle it'll spike up to 3-4k rpm, then slowly climb back down. If I decrease the idle screw to below 2k rpm, it'll slowly bog down and die. Any Ideas?

I am thinking I want to pull the carbs and check the idle air circuit to make sure there are no blockages.

[Floshenbarnical]

What bike is this, what are the stock jet sizes, do you have stock airbox

[VerdantGuardener]

Edited main post, s1972 CB500k with stock airbox and 4into1 jets with stock sizing (100 and 38).

I should have all the OEM kehien jets, if I have to pull the carbs again I think I will try putting those back in. They were gnarly when I pulled them out (bike sat for 15 years).

Also should mention that I rebuilt the top end with new o-rings and gaskets. No leaks. I am wondering though if compression (or lack thereof) could cause such an issue. I would think that lack of compression would cause a single, constant symptom like not firing, not one that makes the idle wander like my bike is doing.

[Floshenbarnical]

I’m really just repeating what others have said in other threads but:

- 4:1 jets are dog mess. I found this out the hard way too. You want OEM jets, so if you can clean them great, if not there should hopefully be OEM or Almost OEM available from JetsRUs.
- You have a 4:1 exhaust. If the cleaned OEM slow jet doesn’t fix your problem, you may need to rejet, at a guess. Try some #40 OEM slow jets and see what happens
- Did you verify your float height using clear tube method? This seems to be the only real way to guarantee your fuel level is not too rich or too lean. Not all floats are the same.

I’ve only been wrenching on bikes for like 2-3 months, so hopefully a vet will come in here, tell me I’m an idiot, and tell you what the actual problem is. But the above is my $0.02

[VerdantGuardener]

Well I appreciate your comments no matter how long you have been wrenching on bikes.

I should also add that when I close down the choke very, very slowly (so the butterfly is closed, cutting off air), the bike slowly dies. This tells me the bike is not running lean, as it should pick up as the fuel mixture richens. At the same time, the spark plugs look fine, tan / dark brown, so I don't think it's rich either. I am hesitant to rejet because of these factors.

I just went out and restarted it, the creeping idle seems to come and go (perhaps as it warms up?). I ran it for about 3 minutes and the revs did not creep / hang. It just really didn't want to idle below 1800 rpm, which I think is strange. I may take it out for a spin again before the night ends and see if i can get more info.

[Kevin]

Sounds to me like you have a vacuum leak.
Spray a mist of water around your intake boots (between carbs and head) and see if the idle changes.

Sent from my SM-S908U1 using Tapatalk

[smee]

Alright, y'all, I need help. I am at my wits end with this. I think I am having a carb tuning issue.

1972 CB500k

Stock Airbox with new filter
Set points gap and points, gets 12v right at the F marks.
Set valve clearance to spec, rotated engine to confirm.
Synced carbs, they are all reading roughly the same (300mm ish?)
Completely clean carbs, new jets and o-rings
Set float height to spec
New Reg/rec and am battery is staying at 12.7v between runs
Battery reads about 13.2v at 3k rpm
Throttle tube has about 1/8 inch of play before engaging the throttle body
I have experimented with all sorts of variations on air mix screw, from all the way in to all the way out. Bike runs best with them almost all the way out.
4 into 1 exhaust
Jets are 100 and 38 -> OEM brass? If you have the OEM, clean them well and put them back in.
Checked for leaks around intake with spraying some fluid
After all this I pulled the spark plugs and they look fine (tan, no oil or gas)
Runs great at higher RPM (I can get it to red line and I feel like it has a good amount of power)

THE ISSUE:

Bike will start and idle great at 2k rpm indefinitely. However, when I blip the throttle it'll spike up to 3-4k rpm, then slowly climb back down. If I decrease the idle screw to below 2k rpm, it'll slowly bog down and die. Any Ideas?

I am thinking I want to pull the carbs and check the idle air circuit to make sure there are no blockages.

Sounds like your lean on the pilot. Your comment on taking it off choke further adds evidence to this. If OEM brass, most likely it's clogged. You'll need to pull the carbs to clean and verify. I like using high e guitar strings for this. If it's a jet you got from a refurb carb kit, you will most likely need to go up a size or two.

[RAFster122s]

If your original brass in the carb is not Keihin, stamped with stylized K, then you will need to get new brass parts from Jets-R-Us as they are about the only source for correct tly sized brass if it is not orderable from Honda directly. Do not try to use aftermarket brass, it is so incorrectly sized it is extremely difficult to find replacement aftermarket brass that will work. It will not run correctly if it runs.

The float needles often are longer messing up the fuel bowl setting which uses a measurement to set the fuel level. Be aware of that gotcha, the clear tube method of measuring your fuel level allows you to set the height correctly usually using and aftermarket float needle assembly.
Getting the original brass clean and then ensuring you don't have air leaks will allow you to get the bike running right. HondaMan mentioned the cb750 intake boots on the carbs are sometimes not sized quite right and the clamps can cause and air leak if not sized or tightening down correctly. Overtightened can cause the clamp to cut into the boots or to have a small leak if I remember correctly what Mark mentioned.

[newday777]

Edited main post, s1972 CB500k with stock airbox and 4into1 jets with stock sizing (100 and 38).

I should have all the OEM kehien jets, if I have to pull the carbs again I think I will try putting those back in. They were gnarly when I pulled them out (bike sat for 15 years).

Also should mention that I rebuilt the top end with new o-rings and gaskets. No leaks. I am wondering though if compression (or lack thereof) could cause such an issue. I would think that lack of compression would cause a single, constant symptom like not firing, not one that makes the idle wander like my bike is doing.

Welcome aboard the forum VerdantGuardener
Definitely remove the aftermarket brass. Aftermarket brass is never sized to tolerances of the original Keihin brass.
To clean the original brass put them in a jar of lacquer thinner a couple of days to loosen the hardened gas deposits then blow the jets out with air gun. Sometimes heat is helpful too to loosen the tar, Heating in an ultrasonic cleaner.
As mentioned use a high E guitar string to clean out the holes if needed, don't try to force steel in the holes of brass as you will change the hole size and will have more problems than you have now.
Reply back with updates here.

BTW
Go to the new members introduction section and introduce yourself and your bike, how long you have had it and what you have done to it. Add some pictures as we always like pictures. 👍