Topic: Problems after carb rebuild

[janne_83]

Hi all. I have a small problem after carb rebuild. I started with a fully fuctioning Honda CB 500 K3 1978 last winter and here's the work I've done to it during winter:

- Fully cleaned and rebuilt carbs
- Slide height adjustment by bench sync (2 & 3 was originally a bit higher)
- Mixture screw adjustment to 1,5 turns (nbr. 1 was opened 1 turn and the others about 1,5 turns)
- Boots mounted properly (the intake boots were a little displaced 'cause they weren't in a correct position at the "air chamber")
- Clamps are really tight (all 8 of 'em were a lot more loose before and in fact I don't actually know how tight they should be!!)
- New air filter (old one was incorrect type yet same kinda model)
- Air filter box seal fixed (it was loose and hanging partly off)
- Air filter box and "air chamber" fully cleaned
- Breather tube replaced since the old one was not correct size (too small)
- Old breather tube returned to it's original place (the air chamber.. Yes the previous owner took it off the chamber and used it as a breather tube)
- New fuel tube from tank to carbs
- Valve tappet clearance adjustment

I'm running a stock airbox and 4-1 exhaust. All the changes I made were really small (valve adjustment, slide height, only one mixture screw was off from 1,5 turns etc..."). No big changes whatsoever.

I thought I'd end up with a perfect running bike but I have issues with throttle. The bike starts nicely right away and the idle seems okay so far. When applying throttle the rpm:s grow slowly and "unsurely" and sometimes the reaction to throttle is nowhere to be found if you do it quickly. If I want to get the rpm:s higher I have to apply throttle very carefully but even then I can't get the high revs out at all. The bike may also stall when applying throttle quickly.

It seems to me that I may actually have a really small problem with an easy fix but I can't say which way I should go not to mess with wrong things.. Okay, the whole package is much more sealed and tight now than before so I guess there would be a lot less air flow through the carbs.. should I mess with the mixture screws? I guess they adjust the fuel so a little tighter would mean less fuel and more air?

I guess the timing can't be an issue 'cause the bike ran all good last year before I started the work.

[goodtryer]

Just think of it as a process of elimination. You've worked on the carbs, and airbox, etc. Now move on to the other things. Don't assume that because it worked before that it was correct then or now. Go ahead & check the timing; that way you'll know for sure it's not the problem.

Spark plugs: are they the right type, set to the right gap?
Plug caps: good resistance? relatively even across all 4?
Carb needles all set to the same slot?
Jets are correct?
Choke is moving correctly?
Battery condition?
Voltage to the coils?
Fuel tank vent is not clogged?
Petcock screen and/or in-tank screen is clear?
etc.
etc.

Make a notebook & keep careful track of your findings & changes. It's very useful to be able to go back & know with certainty.

Good luck!

[Nurse Julie]

All the clamps on the boots/ rubbers only need to be nipped up to form a tight air seal. If you have them done up too tight, not only can the clamps split and damage the boots but can distort the rubber boots giving access for air to get in. Are you using the correct clips, jubilee clips are not very good, the ridges can cut in to the rubber.
Also make sure the throttle cable is sitting correctly in the grove on the big butterfly on the top of the carbs.

[seanbarney41]

Be careful of seemingly unrelated stuff that may have been disturbed in the pursuit of carb cleaning...for example wiring?  fuel flow? vacuum leaks of any kind?

[Deltarider]

Nurse Julie is right. I was always told to fasten the rubbers juuust enough, so they seal OK and no more. Further I'd concentrate on what you've done since the bike performed well, in particular the air intake. If I were you, I'd abandon that breather system altogether and just plug its entrance tube on the air filter case. It will have no negative effect on the running of the engine and the air filter element will love it. You can reroute the breather tube as shown in Fig. K2-1 on p.170 of the Shop Manual Honda CB500-550, http://manuals.sohc4.net/cb500/service_manual/HSM500550_15.pdf . Actually I recommend this rerouting for all CB500-550 models.
'Rebuilt carbs', does that mean you've changed OEM for aftermarket parts? Yours has the PD carbs, right? You didn't mess with the float height, did you?
Have you verified fuel flow is sufficient and did you have a look at the sparkplug noses?
Last but not least: is the gasoline still OK?

[janne_83]

During the winter I already decided to check the timing purely out of interest so that's gonna be checked, though I'm pretty sure the solution is not going to be found from there. I didn't check it yet because I enjoy working outside so i decided to leave the examination-type-of-work to be done later on.

I know I should not assume, but yes, spark plugs are pretty new, changed them to right type last year. And it's running on all four. Correct jets, choke is correctly set-up working better than ever, did run it with fuel cap open to make sure it's not clogged (had this issue on couple of occasions before), battery condition not checked but should be ok, I've had bad batteries before and I know how it affects... still have to measure the battery, it might be part of the problem. Petcock should be okay but has to be checked. I cleaned the screen as the tank was empty, so it should be ok.

I have pretty careful working methods so basically nothing else besides the fuel flow rings a bell here. How could you confirm a descent fuel flow? Should I try the old fuel line tube from tank to carbs?

I had an inline fuel filter attached last week on the bike and I could see the gas flow but I couldn't say if it's enough or too little. The first thing I did was to get rid of the filter because the engine was running super-hot and I had somewhat the same problem so I was sure that's the problem. I could say the flow was insufficient with the inline filter. Without the inline fuel filter the situation is definitely better with idle but with throttle the problem is the same. Lot's of things to be checked but I really cannot think of anything special.

PD carbs yes and I used Keyster aftermarket parts.. so no OEM but quality parts, I guess. I carefully compared each part before replacing and for example with emulsion tube I kept the original since the 500 and 550 have slightly different ones and Keyster included 550 -version. I didn't mess with the float heights, but I checked them. Three of them were on the same height and it was the correct, reasonable height. One was few millimeters off comparing to the other three so I trimmed it a bit; to the same height. Definitely not a big change. Gas tank was empty so now the gasoline is fresh, new gas.

One more work popped into my mind; one of the "air chamber" boots was broken and now I repaired it. Nice big 4cm cut so earlier there definitely was a possible air-leak there. The boots were also not in correct position so it was impossible them to be placed perfectly. All of this I fixed. I expected the bike running better with all this fixed but the result was the opposite.

[goodtryer]

i found this post that briefly explains one possible process of measuring the fuel flow. i believe he wrote a separate post late with the full process explained but you will probably get the idea from this one, too:

http://forums.sohc4.net/index.php/topic,185073.msg2150199.html#msg2150199

[jaytee-nz]

"Keyster aftermarket parts" - are generally not regarded as quality replacements and have caused many running issues.
Do have the original Keihin jets ? If so, make sure that are really clean with no blocked passages and re-use those instead.

[Motorcycle jones]

Did you remove the 4 mixture screws and make sure the o rings and washer
 and spring was in the correct order, and that the o rings were undamaged?

Remove the air box and then open the throttle and make sure that when you turn the throttle that all 4 slides are opening at the exact same time and that the slides go all the way down. Put your finger in the opening of the carb and test each one.

Re install the air filter system.

You did not even mention the floats or float needles. If any of the float needles are not concentric (rubber ones) or if they are metal and they have a shiny ring on them they could be damage, run your your finger on the pointed part od the float needle and that shiny ring could actually be indented. Just get 4 new float needles if you really want to be sure.
Set the float height as instructed by the Clymers' workshop manual book. Not internet.

Float NEEDLES all need to be on the same notch.

[HondaMan]

PD carbs yes and I used Keyster aftermarket parts.. so no OEM but quality parts, I guess. I carefully compared each part before replacing and for example with emulsion tube I kept the original since the 500 and 550 have slightly different ones and Keyster included 550 -version.

In my experience, Keyster needles are junk and seldom (if ever) match the taper of the Keihin needles. This seriously messes up the midrange fuel mixture.Compare your new needles side-by-side with the old ones (or better jet, instal the old ones) to see how much they differ: in the CB750 kits they are unusable if you want normal performance at all.

Are your mainjets the screw-in type, or the press-in types? If press-in, the Keyster O-rings are too skinny and will not seal the emulsifier holder, making the midrange (1500+ RPM) uncontrollably rich. These O-rings need to be (hard to find( 1.2x39mm (1.2 x 4.0 will also work). If you have the thin ones, you will have noticed they barely stay in place when pressed in, and they leak fuel past them like they are not even there.

[Deltarider]

i found this post that briefly explains one possible process of measuring the fuel flow. i believe he wrote a separate post late with the full process explained but you will probably get the idea from this one, too:

http://forums.sohc4.net/index.php/topic,185073.msg2150199.html#msg2150199

Warning: the method in the link concerns the oldstyle carbs, not the newer PD carbs.
Janne, I would refit the old fuel tube just to experiment. Your model has only one fuel line to feed all 4 carbs, so everything needs to be exactly right, both supply as well as vent. You haven't accidentely connected the fuelline to the vent tube, have you? I'm asking this, because on the CB550K4 the two are next to eachother. I don't know what the situation is on the CB500K3.

[janne_83]

"Keyster aftermarket parts" - are generally not regarded as quality replacements and have caused many running issues.
Do have the original Keihin jets ? If so, make sure that are really clean with no blocked passages and re-use those instead.

Yes I do have the orig. Keihin Jets kept in storage. The specs are the same and after comparison they seemed identical so I decided to go with the replacement. I have to keep in mind this option as well.

[janne_83]

Did you remove the 4 mixture screws and make sure the o rings and washer
 and spring was in the correct order, and that the o rings were undamaged?

Remove the air box and then open the throttle and make sure that when you turn the throttle that all 4 slides are opening at the exact same time and that the slides go all the way down. Put your finger in the opening of the carb and test each one.

Re install the air filter system.

You did not even mention the floats or float needles. If any of the float needles are not concentric (rubber ones) or if they are metal and they have a shiny ring on them they could be damage, run your your finger on the pointed part od the float needle and that shiny ring could actually be indented. Just get 4 new float needles if you really want to be sure.
Set the float height as instructed by the Clymers' workshop manual book. Not internet.

Float NEEDLES all need to be on the same notch.

I did remove the mixture screws; originally one was missing the O-ring and one was missing the spacer.. the other two, well the O-ring had become one with the throttle body, completely smashed so after carefully cleaning those bodys I inserted a new set in a correct order -> screw -> spring -> spacer -> O-ring. One would think that missing parts would cause trouble but once again; now that it's all perfectly done the situation is worse.

The float needles were identical in the replacement set so I did use 'em. Kept the old also so it's a possibility to return 'em if needed. Both are indented the same way.

[bryanj]

Have you had the press in pilot jets out to clean jets and passageways? They block very easy

[janne_83]

PD carbs yes and I used Keyster aftermarket parts.. so no OEM but quality parts, I guess. I carefully compared each part before replacing and for example with emulsion tube I kept the original since the 500 and 550 have slightly different ones and Keyster included 550 -version.

In my experience, Keyster needles are junk and seldom (if ever) match the taper of the Keihin needles. This seriously messes up the midrange fuel mixture.Compare your new needles side-by-side with the old ones (or better jet, instal the old ones) to see how much they differ: in the CB750 kits they are unusable if you want normal performance at all.

Are your mainjets the screw-in type, or the press-in types? If press-in, the Keyster O-rings are too skinny and will not seal the emulsifier holder, making the midrange (1500+ RPM) uncontrollably rich. These O-rings need to be (hard to find( 1.2x39mm (1.2 x 4.0 will also work). If you have the thin ones, you will have noticed they barely stay in place when pressed in, and they leak fuel past them like they are not even there.

I did compare the needles side-by-side; they were indentical and I couldn't tell any difference between em'. Even though the replacement needle seemed like a quality part I decided to keep the old ones 'cause -> if it seems as good as the replacement and there's no wear, why change it? In addition, in the parts book there's a different part nbr. for 500 and 550 so I was a bit worried about the replacement needle though I couldn't tell any difference. Just to make sure, I kept the old one.

Main jets are the screw in type. The specs are the same and after comparison they seemed identical so I decided to go with the replacement.

[janne_83]

Have you had the press in pilot jets out to clean jets and passageways? They block very easy

Yes, pressed in pilot jets, kept the old ones since this was not included in the set. The only part I didn't even remove as I went along with the work as I was worried I would damage 'em while removing them. Passageways and pilot jet cleaned carefully and I ensured they are open.

[bryanj]

They need to come out, grip with vice grips then twist and pull at same time, refit by tapping with nylon hammer

[janne_83]

I did some research on my bike today after few days of break and I made some progress: As long as I remember the bike has made occasional "whiz":ing sound when running cold. I didn't pay attention to it partly because, as I said, the bike was working well before.. on the other hand I didn't have any idea what the small occasional sound was.

Today I found the origin; the carb intake rubber boots spits gas every now and then between the rubber seam. As I mentioned before the rubbers are now super-tight and I was going to loose them a little but now it seems it's impossible 'cause the gas-spray cannot be blocked even when the clamps are tight as possible. They are not broken but can it be that the rubber is hardened over time and therefore cannot function? I dunno.

This is obviously a problem but I can't say if it's the whole problem; the bike made the sound also last summer before the carb rebuild and I ran the bike pretty normally although the problem was already there.

The other thing I noticed is that the battery is not that good. The voltage is 12,5V when the bike is turned off, but as soon as I turn the key the voltage drops to 11,9V. The bike starts well and when idling normally (1500rpm-1800rpm) the voltage is 13,5V-14V.

I also ran the bike without the air filter but that didn't bring me any closer to solution. The bike stalls when trying to open the gas and there's troubles to even idle.

[Duke McDukiedook]

You for sure need new carb boots if gas is spraying out of them.
Be sure the boot clamps are original or a smooth aftermarket that fits properly.
There was a thread on the board where people sourced good aftermarket clamps that are smooth on the inside.
 

[Deltarider]

On the French forum it was pointed out that on the K3 models the orientation of the rubber boots between the carbs and the head matter. I myself have no experience with it; I just pass it on. You may want to check this. Have a look at the pic. As you can see, the end with the two thin lines should face the head, the one with the one thin line should face the carbs.
Your battery does not like multiple starts followed by just some idling. I'd recharge the battery regularly.
For the battery on our bikes there are two simple tests. When the battery has been recharged and left alone for some hours, voltage should still read 12,6 V minimum.
When during electrical starts, voltage drops to < 9,8 V, battery is near its end.

[kerryb]

I read where you ruled out timimg as a source/cause, but when I experienced the same symptoms last year with a 76 550f my problem was fixed by ignition improvements.  After very though carb work and rework, the fix was getting the points gap set better, timing with a strobe and new plug caps. 

[janne_83]

On the French forum it was pointed out that on the K3 models the orientation of the rubber boots between the carbs and the head matter. I myself have no experience with it; I just pass it on. You may want to check this. Unfortunately I have a problem uploading pics here, so the link directs you to the UK site. As you can see, the end with the two thin lines should face the head, the one with the one thin line should face the carbs. http://www.sohc.co.uk/index.php/topic,19929.msg177149.html#msg177149
Your battery does not like multiple starts followed by just some idling. I'd recharge the battery regularly.
For the battery on our bikes there are two simple tests. When the battery has been recharged and left alone for some hours, voltage should still read 12,6 V minimum.
When during electrical starts, voltage drops to < 9,8 V, battery is near its end.

Thanks! I checked this and they are set up the correct way but I haven't noticed that they are hard... hard as a rock from the end of the head. Already ordered 4 pieces to replace them.. It would've been an easy change two weeks ago when I had the carbs still on my table Well, it's like that.

[janne_83]

I read where you ruled out timimg as a source/cause, but when I experienced the same symptoms last year with a 76 550f my problem was fixed by ignition improvements.  After very though carb work and rework, the fix was getting the points gap set better, timing with a strobe and new plug caps.

Yes, this is definitely something I have to check also. I can't get over of the fact that I had a pretty nice running bike before the carb-work and therefore it seems very weird to find the solution from timing which has not been changed (over the winter) but it's definitely worth checking.

Now that I have to remove the carbs once again to change the carb boots I also have to consider if I'll change some of the Keihin parts back to carbs... main jet probably.

What do you think about the role of the needle seat? I changed that also to a new one while working with the carbs.

[janne_83]

i found this post that briefly explains one possible process of measuring the fuel flow. i believe he wrote a separate post late with the full process explained but you will probably get the idea from this one, too:

http://forums.sohc4.net/index.php/topic,185073.msg2150199.html#msg2150199

Warning: the method in the link concerns the oldstyle carbs, not the newer PD carbs.
Janne, I would refit the old fuel tube just to experiment. Your model has only one fuel line to feed all 4 carbs, so everything needs to be exactly right, both supply as well as vent. You haven't accidentely connected the fuelline to the vent tube, have you? I'm asking this, because on the CB550K4 the two are next to eachother. I don't know what the situation is on the CB500K3.

Thanks Deltarider, I'm positive and this is actually impossible to miss on K3, 'cause there's only fuel 'input'. I'm gonna experiment with the old fuel tube also.

btw. suddenly we can see your pics again! Yay!

At least now I have a theory: when I started this topic I didn't have any clue where to go or what to do, but what I think now has happened:

The sound of gas spraying in between and out of the intake is familiar, I remember it from the past but earlier I didn't have a clue what caused it. When I assembled the bike after the carb rebuild, by tightening the old, hard rubber boots (that I didn't pay attention to) I actually made the existing vacuum leak even worse. And now when the situation at the air intake -section is a lot better, the combination of better air intake and worse vacuum leak is the symptom/result. This brings me back to my original analysis that the problem must be at the air/gas/mixture -section.

I will check the timing / old fuel line / plugs etc. also but this is what I think the main problem is. Please feel free to express your opinion about the analysis

[HondaMan]

One thing that hasn't been mentioned here but is a problem on nearly all of the 500/550 bikes I see today (50 years old) is the O-rings in thoe castings that bolt the carb hoses to the head. They are hard as plastic now, and don't seal well, if at all. If they were unbolted and then remounted with the old O-rings still in place, then they certainly will not be sealed now.

The good news is: they are the same o-rings as found in the valve caps, so it is easy to find a set.