Topic: Need help with cb500 four - idle and choke problems

[roque75]

Hi all!

So I just rebuilt my engine and carbs.

When I started to drive it, the bike just runs for about 1km then the sparks seems to be failing and the bike don’t start again. After that, sparks don’t give any spark in a test.

I realized that the choke was closed (lever up) so it all makes sense. Maybe it was too rich and burning the sparks...

However when I open the choke the bike runs at 2100 rpm in idle. If I close the choke it goes to 1000rpm... is that normal? With the choke closed should the rpm be higher?

I still need to sync the carbs. I didn’t do it yet because the engine was always stalling with the choke closed. and now it have too much rpm...

I just want to ask you if you think it is a sync problem or it’s something more going on?

The mix screw doesn’t seems to do any difference and I the idle screw is out so I can’t turn the rpm down.

The bike is also making a good sound at idle...

Thanks!

[flybox1]

this ^^^

...and ffs, get the idle set screw back in.  once the bike is hot, and you can control the idle, set it...and leave the idle screw alone.
Just use choke and some light throttle to start and warm the bike...

[roque75]

Thanks for your replies!

Yes, I “bench sync” the carbs... and from the sound they look close... but of course I will need to vacuum sync them!

I replace the sparks for new ones too.

I rebuilt the carbs with the David silver kit. Aftermarket... I also measure the float.

The idle screw is in. It was a way to say that I can’t lie more the rpm from there...

[Yoshimatic]

With the idle at 2100rpm are you sure the throttle cables are adjusted correct ? The “pull” cable should have a little slack in it when in the idle position.

[roque75]

The cable is in fine. And the butterfly is already hitting the stopper.

I don’t believe I have a leak tube. It’s all new and the bike don’t make irregular sound. It’s just too high...

Calj737, what do you mean with tubing adjustments?

Cheers

[Yoshimatic]

The cable is in fine. And the butterfly is already hitting the stopper.

I don’t believe I have a leak tube. It’s all new and the bike don’t make irregular sound. It’s just too high...

Calj737, what do you mean with tubing adjustments?

Cheers

Are you talking about the choke butterfly? I mean the actual throttle crank that the throttle cables are connected to, it sounds like it can’t come back far enough for correct low idle. The other possibility is that the slide adjustments are not synced close enough and one slide is bottomed out with the other three in the air a bit. You really need gauges hooked up to check how far out the sync is.

[roque75]

Yes, I mean that too. It have a pin to stop the butterfly in the lower position and it’s hitting that pin so I can’t get it lower that that.

When synckyng the carbs I need to put all reading like the cylinder #2, right?

Is it possible that I need to change the #2 too?

I also remember that I put 100 jet fuels. The kit came with 75 too. I pick the 100 because it was what the bike had before...

[TwoTired]

Stock, the bike had 98s or 100s. The 75s would be VERY wrong for those carbs.

Er, if these are 649 carbs then 75s would be more appropriate.   It would also have a required restrictor plate over the air filter box, to choke down the air inlet.

627b carbs original to US CB500s, would have 100 mains.  Some euro CB500s had 649 carbs.

Look for the set up number stamped into the carb body mount flange top right (IIRC). 

Cheers,

[flatlander]

Some euro CB500s had 649 carbs.

... and roque75 speaks in kilometres and the name could be portuguese so you might be onto something there, sherlock

[roque75]

Well, you are all just right!

I’m Portuguese and the carbs are indeed 649A...

I tried to sync the carbs and no success... keeps high. I think I really need to lower the cylinder #2 too.

However the sound looks pretty good.

Should I try to sync #2? Or should I start to remove the carbs to change the jet fuel?

It’s strange because the bike came with jet 100 when I bought it!

Cheers

[TwoTired]

I do not have direct experience with 649a carbs.  Other set up numbers, lots.  But, not 649a.

Deltarider reported not being able to get his 649a carbs working properly without the filter cover/restrictor and the stock sized jets.

The restrictor would cause very high vacuum levels in the filter box chamber and this would present to the carb throat.  No doubt this is why they need the 75 mains.  I would also expect some other variation in carb body changes from the 627b types to make the slide needle and pilot jet mixtures to be correct.

But, I have never examined the 649 variants to confirm this is true or what other changes were done.

Cheers,

[Bankerdanny]

To answer one of your other questions, yes choking the engine would reduce the idle speed.

The 500's don't have anything that automatically boosts the idle when the choke is on, so when the bike is cold and the choke is on you should be using the throttle to keep the bike from dying.

Use the adjuster knob on the right side of the carbs to set the idle speed with the bike fully warmed up (my 550 likes around 1100 rpms)  then use the throttle to manage idle between start up and fully warm.

[roque75]

Ok, so i don’t have the restrictor...

This means that I will have more air flow than normal...
and maybe I will still need the 100jets. Right?

I don’t know what is the best option to make.

I can change the jets because it’s the stock size and revs are high now.

However I already drove it just to try and the sound look good. It was also kind of smooth, it didnt shaked a lot even with the carbs not synced...
But if i lower the #2 cylinder, I will loose the stock position.

[TwoTired]

Ok, so i don’t have the restrictor...

This means that I will have more air flow than normal...
and maybe I will still need the 100jets. Right?

It's not "more air", it's the carb throat pressure that drives the fuel volume through existing jets.
The 100 jets ought to fix the W.O.T mixture.  But, the restrictor causes the vacuum level in the carb throat to deepen.  It is this pressure that forces more fuel though all existing jets, not just the main jet.  I suspect there is something else besides jetting that allows the 649 carb to work properly with the restrictor.  I still don't have any to examine.   I think your options are to find a restrictor, or get 627b carbs which don't require the restrictor to function properly.


Cheers,

[bryanj]

If you bench synced the carbs you have adjusted them all too far on the screw adjusters, you need to totally unadjust the main tickover screw then pick a carb and lower the slide via the individual adjuster till it touches the casing on the bottom. Using the smallest drill bit shank you can find, 1/16 inch or less, as a "feeler" screw in the main tickover screw till it just fits then adjust the other 3 on the carb adjusters till they are all open the same. This should gat rid of the high tickover so you can then set the vacuum's

[roque75]

Hi!

Well, I ready bench synced them a bit lower. They are almost closed but it runs fine!
Once a drive it a bit goes to 1300 rpm. Guess I need to drop them a little more...
I still need to vacuum sync them but they looks pretty good to me!

Thanks!

[flatlander]

good stuff. i have my 550 idle at 1200rpm (fully warmed up). that's a bit higher than most set it to, but it likes it there.

[Deltarider]

The CB500 with 649A carbs had #78 main jets*, 40# slow jets, needle clip in 3^rd (= middle) position and, with stock air intake and exhaust, they run best with the noise reducing lid over the airbox. Airscrews should be out 1 turn ⁺/_- ¹/₈. In stock condition changing to #100 main jets and running without the lid does NOT improve performance. I've tested this and so did some Germans. The French and UK markets which had the 627B carbs, had the same set up except that the main jets were #100 and these models were run without the lid. The US market seemed to have the 627B with #100 main jets, the needle clip in 4^th position and the airscrews 2 turns out.
In Portugal I would trust your needles are still in 3^rd position and I would leave them there. I recommend the stock #78 main jets (inspect the little O-rings still seal well!) and run the bike with the lid on top. Please inform us what model you have. You can check this by comparing your engine- and framenumber to the ones listed in the first few pages of the various CB500 Parts Lists that you'll find here: http://www.honda4fun.com/materiale-documentazione-tecnica/parts-list/parts-list-cb500 I recommend the genuine Honda/Keihin brass parts anytime. I don't understand why people change them for aftermarket as the originals hardly wear if at all. It seems to me that when people see aftermarket sets on offer, they automatically think it's a common thing to replace.
Use the big idle knob for cold start and the first few kms or have your hand keeping the throttle somewhat open whatever suits you best. Idle should be around 1100 rpm and when throttle snapped open and closed, rpm should return to this 1100 immediately. Hesitation to return to idle usually indicates a too lean idle mixture caused by an incorrect airscrew setting or airleak, provided the ignition is timed correctly and the advancer operates as it should.
* The 75 main jets were on the CB350F.