Topic: Carb Rich Issue

[wjustinleigh]

I know this has been discussed a billion times on here but I'm struggling with the carbs for a few months and it's the only thing holding this rebuild back from the road but I can't make any improvement to even go the right direction. 

Symptoms:
Overall:
Plugs black (means rich I believe)

When cold
Idles at low idle fine, blips to advance and back properly.  Some backfiring through carbs but sounds pretty good.

When hot
S#$t hits the fan.  Barely can hold low idle, constant fussing with idle screw and manual throttle adjustments to hold low idle.  Starting from a stop sometimes engine nearly dies when given 1/2 throttle.  When given 3/4 throttle after a delay engine roars to life and has all the power in the world.  If I stayed above 4K RPMs you'd never know there was an issue.  When cruising at 1/4 throttle bike is lumpy, like I'm learning to drive a clutch.  Will not come down off advance unless I take idle way down or I put it in gear and force the engine into low RPMs.

The bike:
1972 cb500, stock airbox, 4into1 exhaust.  Recently rebuilt head, dyna ignition and coils (tried with the points, runs the same).  I rebuilt the carbs with some kits off ebay, first carb rebuild so I didn't pay attention/know there were jet options and I don't know what jets I have in there.  After I got the bike running I took it to a reputable shop in Seattle and they did an ultrasonic carb clean and assured me they got all the tiny orifices.
Screws from the shop carb sync were pretty far out - about 4.5 turns, but it had pods on it at the time.  I turned the screws into the factory 1.5 turn setting, but I don't notice any performance difference between the two settings.
The advance mechanism springs were weak so I tightened them per a thread here and they feel pretty good now.

Thoughts on where I should go from here?  And if possible, ways to do it without taking the carbs off the bike as the stock airbox is a b#%$h to remove.

[wjustinleigh]

Here's a video of it trying to idle while hot, at low idle it can barely stay going a small blip sends it to advance where it stays unless the idle is turned way down.
http://youtu.be/7Esn7mj6YWA

[harisuluv]

Well, if I were you, first thing I would do is not resign yourself to trying to do this without taking the carbs off the bike.

Because you want this next time you take them off and fix them to be your last time doing so.  Trying to do it on the bike isn't going to help reach that goal.

You don't know what jets are in there, so you have no starting point or reference to adjust from or to.

Also, did you rebuild them yourself the first time?  Then they were cleaned a second time by the shop yes?  What kits did you use, and did you use the new slide needles from the kits?

What did you set the float height to?  Did you bench sync them?  Did you vacuum sync them?

I should tell you that people like me will come in here and chime in and tell you EXACTLY what to do.  I or others can tell you systematically what to do to ensure you fix your carbs.  However, most of the time these threads seem to be people getting advice that is good, and correct, and then the OP says "yeah, well I'm gonna try X" which was never even mentioned. 

Probably need to put the oem needles back in (if you swapped them) set the float height properly, and then get a good sync.

[wjustinleigh]

Ok, carbs coming off.  The carbs were pretty ugly, not sure that I saved any stock needles/jets but will order stock replacements if they aren't the stock size (not sure if jets have markings to indicate their number, but I'm sure I'll find out soon).

On the carbs, yep I built them with the kit's slide needles.  The shop then disassembled, cleaned and reassembled them.  I bench sync'd them and the shop vacuum sync'd them.  But then I dumped the pods to try to help solve the issue which would have necessitated adjustment.

[TwoTired]

I'll add to what Harisuluv typed.  You have to know what you have before you plan on a change.

Also, once the plug's center electrode insulator becomes fouled with carbon, spark becomes more difficult to jump only within the spark gap.  And, this changes combustion efficiency, leading to erratic unreliable firing of the mixture.

Easiest way to bypass this problem is to replace the plugs each time they get a carbon buildup.  But, using abrasive powder cleaner and an air compressor can also clean them to work nearly as well as new.  I didn't have much luck with the blow torch method, but, in theory that can work also.

Anyway, it becomes a chicken and egg situation, where a rich mixture fouls the plugs and it runs badly even if the mixture is corrected, until BOTH the plugs and the carburetion is corrected.


Lastly, did you ask (and pay) the shop to fix the plug fouling/carb rich issue?
If they didn't fix it, why don't you hold the shop responsible?

[Duanob]

I'm curious to know which shop you used?

If you have a rich condition in the low RPM range it's either your needles aren't seating properly or in the wrong clip postition, or your pilots jets are wrong or the pilot circuit is plugged some how. Make sure you get plenty of airflow into the idle air screw hole and out the pilot jet hole with jets removed. If not you have a plug. Did you replace the needle jets as well as the needles? They are usually matched set, either as new or they wear together. Or it could be the float height is letting the bowls flood with gas. just go through and check everything as you go since the carbs are off.

BTW I've seen your bike at backfire moto, very unique paintjob.

D

[lucky]

I could not tell from the video if you have stock exhausts.

"When cold
Idles at low idle fine, blips to advance and back properly.  Some backfiring through carbs but sounds pretty good."

If you try to rev it up while it is cold and it pops or back fires it can mean the choke is not open far enough.
Did you have the choke on? All the way"

I agree that you should put the stock needles and jets back into the carbs.
Make sure the needle clips are in the stock position. Absolutely must be .

Don't go to that shop anymore.


You will get expert advice from TWO TIRED.

Checklist on carbs.
Idle jets clean and you can see daylight through them.
Floats set correctly.
Slide needles with clip in the stock position.
When you open the throttle all slides start to open at the same time.
Pilot air screws all set to stock position (same number of turns).
Stock air filter.
Stock exhausts.

[wjustinleigh]

The shop is Seattle Used Bikes, they usually do a good job and the head mechanic is a great guy who will tell you a lot about these engines over beers next door at the Fusebox.  I can't put it all on them, when I brought it over there it was the first I had fired it up after putting it back together (first engine I had done significant work on) and it had been down to the frame with a complete head rebuild.  Valves were way off, carbs were way off, now I know the coils were suspect, etc., it's a miracle that it could drive me over there to drop it off.  They got it running well enough to drive for a reasonable cost with some advice of things to work on next.  I got through all that stuff and arrived where I'm at.

Responses to questions/replies:
-Exhaust:  aftermarket 4 into 1.
-Revving while cold - sounds great, no popping or hesitation.  Some backfiring through carb while at idle - not a pop, just the sound of air backwards through the carb.  Started with 1/2 choke then going to wide open, no change.
-Paint job/Bfire Moto:  Sadly I sold that 750 to an Australian guy.  I have two 500s and a bandit in the garage, something had to go so I have to get this bike going so I have something to ride to the start of backfire moto next Wednesday!

I'm hearing loud and clear that the carbs need to come off and have stock jets/needles inserted.  I saw somewhere the adjustment I have to make for the 4 into 1 so I'll look that up and adjust and report my findings.  Hopefully the manual has a good schematic showing which is the pilot,idle,etc.  Also, to any future readers of this post, apparently you can't just buy some carb kits off ebay without knowing what you're doing.

If anyone has a line on the stock airfilters I'm all ears.

[Duanob]

Here go here:

http://forums.sohc4.net/index.php?action=downloads;cat=2

Download what you need or the whole thing.

there are cutaways showing how the different systems work in the slide carb. For the idle, the air goes into slide cut out and draws air into the idle air screw where it is metered, then into a passage to the pilot jet where it gets mixed/atomized with fuel then into the the venturi on the motor side of the carb. The main and needle do nothing at this point up to about 1/4 throttle. About 1/4 throttle the needle and pilot system work together so if your pilot system isn't working it will be a rough go until almost 1/2 throttle. The mains kick in after 1/2 throttle. the needle jets take up the middle mostly but overlap with the pilot and the main jet systems. Once you figure that out then you can figure out which system is funked up. My guess is most issues in the lower RPM range tend to be a pilot jet problem. They have tiny holes and get clogged easy. the mid range is usually a needle jet problem, dirty or worn needle jets, wrong clip position, etc.

I've never heard of Seattle used Bikes might have to stop by sometime. I've been to Jim's Southend and stuck my head into Twinline only once. I figure at $450 to clean and rebuild carbs I learned to do it myself!

And yes first BFM is April 16th!

[wjustinleigh]

Thanks for the downloads, those are great!

[lucky]

You need to get a Clymers workshop manual.

The only way to understand it.
OR you can check You Tube where kids show you how to fix your carb, but do not even know the name of the parts?

[wjustinleigh]

Odd the pilot/slow jet from the kit was a '40' which is the same number as the original, but they look far different.  The stock jet had much larger holes and was much longer.  Have the clymers for it, invaluable.  http://oldmanhonda.com/MC/Rcarbs.html is great too.  In the pic the one on the left is stock, right is from the kit.

[TwoTired]

Odd the pilot/slow jet from the kit was a '40' which is the same number as the original, but they look far different.  The stock jet had much larger holes and was much longer.  Have the clymers for it, invaluable.  http://oldmanhonda.com/MC/Rcarbs.html is great too.  In the pic the one on the left is stock, right is from the kit.

Well, there you have it.  The aftermarket part has less brass to save money for the manufacturer.  Further, see the outlet tube made as part of it.  The emulsion tube mixes air with the fuel on it's way to the carb throat.That is the emulsion tube for the slow jet.  Note the original jet has larger and more holes in the emulsion tube which would allow more air to mix in?  Now you know why the carbs run rich.  I wonder what changes they made to the slide needle and the main jet?
You don't show the length of the new jet compared to the original.  The length matters as it extends into the fuel bowl gas supply to pick up fuel.
The "new" pilot jet would also explain why the Shop turned the pilot screws outward in an attempt to bring the pilot fuel mix leaner.  But, 4.5 turns is pretty near the limit of effective adjustment range on that screw.

Here is a picture of a pilot jet that has been sectioned.

[wjustinleigh]

A visual inspection of the needles and main jets (100s) they look the same.  I'll post photos tomorrow.  The stock pilot jet is significantly longer than the aftermarket part.

Just saw this thread, http://forums.sohc4.net/index.php?topic=99596.0, seems to be a very similar issue.  They have a photo of the difference in length in stock and aftermarket jets for anyone with this problem who stumbles on this thread in the future.

[Duanob]

Stock pilot jets are 28mm long. You will notice the emulsion air holes are a lot bigger than the aftermarket, hence a possible rich condition at idle with the aftermarkets. You can get what you need a jets-r-us.

http://www.jetsrus.com/a_jet_kit_street/honda_550_CB550F_1976.html

Also here:

http://www.partzilla.com/parts/detail/honda/HP-99124-076-0400.html

One of the best tools I ever bought is the K&L wire carb cleaning tool.

http://www.motorcycle-superstore.com/10115/i/k-l-carb-cleaner-wire-set

2 reasons: 1) for having the right wire size for cleaning and 2) I notice that #38 pilot jets will only take the smallest wire in the kit. Anything bigger without forcing it I assume has been drilled out.

I went through the same issues as you the last couple of days, a very rich condition at idle and low RPMs. Taking the carbs apart again, I found two after market pilot jets and found all four could take the 3rd smallest wire. Luckliy I have a spare set of carbs I bought from a swap meet a couple of years ago, the best $20 I ever spent on SOHC4 stuff. The pilots were in good shape so i swapped all 4 of them. That and a new set of spark plugs and she runs like a top. I will recheck the plugs after a test ride but no more stumbling or blowing black smoke out of the exhaust.

[wjustinleigh]

Thanks all the advice and info it was useful and I know much more about how carbs work now.  Stock pilots back on and so far so good.  Everything was still clean in there, I just touched up the stock pilots that were in the used parts box, checked the passages and float level and put them back together.  Was a beast to get started - probably because the brand new plugs were now toast, but once started and warmed had no problem rolling from no throttle all the way up.  Still sticks on advance a little longer than it should, but unless I had just gone through this I probably wouldn't even notice.  I'll do a little bit of tweaking the mixture, replace the plugs, and expect this to be behind me.  Just in time for bfire next week!

Without ebay it'd be near impossible to get this bike running, but some of the stuff I got there has caused the most trouble (clutch cable, carb kits).  Seems like a post about thoughts on what you can get from ebay and what needs to be genuine parts would be helpful to someone like me.

For future reference if someone has a similar problem and stumbles on this:
Current (working) set-up - 100 main jets, 40 pilot jets, needles in third position, air screws 1 turn, 4-1 exhaust, stock airbox, dyno ignition and coils.  First, pull off the bowls and see if you have the aftermarket pilot jets that are smaller with very small holes.  If so, replace them with the stock version, put your bowls back on and you might not have to wrestle the air box.

[Airborne 82nd]

You need to get a Clymers workshop manual.

The only way to understand it.
OR you can check You Tube where kids show you how to fix your carb, but do not even know the name of the parts?

Why the Clymers instead of the Honda factory service manual?

[flybox1]

I think of the Clymer book as a cliff notes version of the shop manual 
I kept mine on the back of the ishtter.  Helped pass the time, and easily understand how systems worked.
In the garage, I have the shop manual to get the deets if needed.
I did find questionable info in the 350F/400F manual.

[Duanob]

Still sticks on advance a little longer than it should, but unless I had just gone through this I probably wouldn't even notice.  I'll do a little bit of tweaking the mixture,

Good to know it's back to running! Considering the 500 is stock, 2 turns out you still might be running a bit rich.

Without ebay it'd be near impossible to get this bike running, but some of the stuff I got there has caused the most trouble (clutch cable, carb kits).  Seems like a post about thoughts on what you can get from ebay and what needs to be genuine parts would be helpful to someone like me.

Yes it's a lot of trial and error as far as parts go. I found out about after market headgaskets the hard way. 3 months after installing one, my oil leaks were back and worse than ever. And yes the same with control cables, after market quality kinda sucks.

your bike is pretty unique, I'll look for you if I decide to head to BFM night next week. As of today, the weather doesn't look promising.