Topic: Runaway idle, carbs or ignition?

[grcamna2]

Would be nice to know what type of material Honda uses when they source their carburetor o-rings.

[jakec]

The O rings I just received based on sizing info I posted above. New O ring at bottom. Little smaller but it went onto the jet perfectly.

[bryanj]

The theory is that a larger cross section on a smaller id should stretch to be about same but i cant handle the math for that

[dunnp2]

http://www.sohc4.net/carb-o-ring

That's where I got my original numbers. Seems they're approximations.

To summarize, we don't know what OEM material was, but Nitrile (Buna-N) or FKM Fluoroelastomer (Viton) would both work great as they only will contact gasoline, and at low pressure.

For those of us looking for a readily available replacement for the following...
-1.1mm x 4.2mm (6.4mm OD) - Main Jet
-1.3mm x 5.7mm (8.3mm OD) - Inlet AKA Float Valve

I'd propose the following:
-1.2mm x 4.0mm (6.4mm OD) - Main Jet: https://www.mcmaster.com/9262K249
-1.5mm x 5.5mm (8.5mm OD) - Inlet: https://www.mcmaster.com/9262K519
OR, if you can't handle metric units and need 'standard' dash number sizes...
-901 measures .056" x .185" (.297" OD) [1.4mm x 4.7mm (7.5mm OD)] - Main Jet: https://www.mcmaster.com/9751K111
-009 measures .070" x .208" (.348" OD) [1.8mm x 5.3mm (8.8mm OD)] - Inlet: https://www.mcmaster.com/9452K17

I haven't tried these myself, just trying to provide a readily available alternative to OEM if you can't find them. They'll be a tighter fit so you'll need to grease them and carefully install to avoid damage, but I they should work just fine. This isn't rocket science.

For the dash number options, I wouldn't recommend them but US hardware stores are a lot more likely to carry at least the -009 size than anything metric.

My floats are leaking and the o-rings are loose, so I'm going to shove in some -009 o-rings that I have already.

[grcamna2]

Thanks for this info

[dunnp2]

I'm really curious about this as well.

 I've had a BUNCH of 500/550's and the last 3 or 4 have all had that "hanging idle" issue. I can slip the clutch and make it stop, so I suspected the advance springs. Trim those and, in some cases, it helped. Never completely cured it. Some better than others.

 ALL have had fully rebuilt carbs; as in, COMPLETELY disassembled and ultrasonically cleaned, new O-rings, etc. All have had Keihin brass, except for a larger main in one or two cases. Emulsion tubes and needles stock.
 
 Stock intake, air box, etc. Most have had new O-rings on the intakes. All have been bench and then vacuum sync'ed.

 Exhaust systems aren't stock, but all are the "no re-jetting required" systems; MAC, Delkovic, etc.

 I'm getting pretty frustrated with it. I can't understand why it's happening. I've even suspected today's gas, even though I pay a little extra for ethanol free 89 octane.

This might help: the phenomenon that causes the 'hanging idle' is: too much non-aerated gas in the intake system. The 550 and the Baby fours suffer from it more than the 750 just because the intake runners are longer, relative to the size of a sucked-in charge. Once it gets "wet" it tends to ignore the throttle setting when shut off until the 'wetness' gets used up.

In most of the situations I have 'fixed' that were not just from the spark advancer, the problem was excessive fuel from one of 2 things:
1. Too big main jet, usually after someone "rejetted for [fill in the blank]" situation. The truth is: there are no pipe changes on the bike that need this, because the pipes that would need it would have to be longer than the bike (750 is similar).
2. Leaking O-rings around the mainjets. This has become a HUGE problem in the last 10 years or so for those carbs with the push-in jets that are held in place with the little spring clip. All of the kits I have seen have a too-thin O-ring cross-section, so unmetered gas slips past the jet, imitating a [much] larger mainjet (like in #1 above).

To try to help with #2, I just had a batch of custom-made O-rings done in the same size as the last good mainjet O-rings I have found. These worked in Mooshie's carbs, and I am just now rebuilding a set of 022A (550) carbs that have suffered the same problem: the mainjets on 2 of the carbs simply fell out when I took off their bowls(!), now THAT's too loose...

I first came across this situation in the CB (twins) carbs, back when. They had push-in jets, too, in 2 different sizes. Owners who "rebuilt" them using cheap kits in the 1990s suffered the same too-small O-ring cross-section problems, and those poor bikes wouldn't even start!

These custom O-rings seem to be fitting both the mainjet and the fuel valve (different sizes). The O-rings in some of the kits are also too thin for the fuel valve, and this also makes the bowls too deep while letting the bike dribble all the time. Messy!

Back to my original issue...
1. I replaced the intake runner (large) o-rings that might have been causing a vacuum leak
2. I reverted to the stock needles and emusion tubes, stamped 273004 - stock for my 069A carbs. 2nd clip.
3. O-rings on main jet and fuel valve are tight and properly seated
4. Bench sync completed
5. Main jet is questionable (from my cheap rebuild kit) and I have new ones on order.
6. Slow jet is Keihin #38, stock for my carbs.
7. Rubber boots are new and clamped properly.
8. Floats are set to exactly 22mm
9. All other maintenance items have been done: cam chain adjust, tappets, new correct heat range plugs, spark timing, new filter (with the exception of my advance springs which have 3 coils cut off and I think advance too late now)

Result: Latched idle problem persists.
When I blip the throttle at idle, it runs away to 4k-5k rpm and won't come back without slipping the clutch. But it does idle happily below the 'tipping point' rpm.
Riding it is a bit sketchy because it keeps pulling while I'm coasting through a corner (0 throttle) and I have to ride the brakes.
There is a large dead spot in acceleration from 1/4 - 1/2 throttle for a second or two, then it pulls hard but is a handful because I have to quick-shift to keep the rpms under control.

Does anyone else have any ideas? I've replaced every part on the carbs and I'm very sure there are no vacuum leaks. The main jet is the last part I haven't gotten a new stock replacement for, but this problem seems too extreme for that to be the cause. The kit said it was a #105, #98 is stock. Going to 98 either way...but not confident that it'll do anything.

What tests could I do?

[grcamna2]

I think,if it was my job,I would remove the advance mechanism and give it whatever love it needs;what else could it be,you've done all the other things you should.

[dunnp2]

I do wish I could find replacement spring specs for that mechanism. $60 for a replacement is hard to go for when I don't know if it's the issue.

[grcamna2]

There are threads here on the forum(I apologize,I don't know where now)about rebuilding the advancer.

[dunnp2]

Here's a good one: http://forums.sohc4.net/index.php/topic,34454.75.html

But seems like it's a custom spring. Probably just worth it to buy the replacement advancer in my individual case.

[grcamna2]

Here's a good one: http://forums.sohc4.net/index.php/topic,34454.75.html

But seems like it's a custom spring. Probably just worth it to buy the replacement advancer in my individual case.

Have you disassembled it and smoothed everything off and lubed all the pivot points? a bit of work.
I wonder if you can find a pr. of NOS springs using the part #'s on a couple internet searches ? I prefer fresh springs rather than clipping coils,myself.
I always look to the advance and ignition first(electrical is the most troublesome and the weakest,imo)then go for the carbs.The replacement would be simple enough,new everything.

[dunnp2]

Coming back to this with some thoughts, been just riding the bike for a while and enjoying the weather. I'm still having issues from two sources:
-Idle advance springs are cut way too short, doesn't advance until pretty high rpm
-Very rich idle

I bought a new advancer assembly but can't swap it in because it brings back the runaway idle, which I think is from the rich condition.

I believe it's actually not getting enough air so I have to run with the idle screw pretty far open, which dumps extra fuel in and also makes it idle high. It'll either idle high or die out. Gas coming out the exhaust.

Size 35 slow jets, downsized from 38 (stock for the 069A) by the previous owner.

New o-rings on both jets, they seal nicely. Floats are set dead on and verified with the clear tube test.

It runs great everywhere except idle.

Is there an air channel for idle air to bypass the closed slides? I'm thinking that might be blocked. Looking for ideas before I take the carbs off again to stare at them...

[dunnp2]

Update, I found a nice drawing of the carbs from HondaMan, post #2 here: http://forums.sohc4.net/index.php/topic,182280.0.html

Seems I need to investigate the idle air passage. The idle air screws don't seem to do anything on any of the carbs. My idle air screws are the stock type with no cross-drilled hole, though I also have the other type and tried them, also with no effect.

It's just weird that I'm having the issue on all 4 carbs at once...at least I think I am.

[dunnp2]

Finally got annoyed enough to take the carbs off the bike again, there's no issue with the idle air passage. Does anyone have advice on this subject? To recap:
-Idle is stupidly rich with stock 38 keihin slow jets (069A carbs). Black smoke out the exhaust.
-Swapped to 35 slows and still very rich but at least starts (with a struggle) reliably and idles without running away. Will not start with choke.
-Idle adjustment screw is far open to get enough air to idle. This is the cause of the runaway but it won't idle with less opening. I know this adds fuel as well....
-Mains are stock in stock position. Everything else (floats, etc) is stock and triple checked. No vacuum leaks. Good compression. New plugs. Went through ignition, functioning great. Stock airbox with no lid, it was missing when I got the bike. New o-rings that fit tight everywhere in the carbs.
-Feels great outside of idle, over 1/4 throttle has great pickup, power, response.

Currently I can get the main idle adjustment screw set so it will just barely keep idling when cold and usually doesn't run away when it's warm, but I have to slip the clutch sometimes to keep the engine under control at stop lights. Scary in corners how it sometimes keeps pulling when I close the throttle.

I had thought the idle air passage was the culprit because the idle air screws don't do anything, but I guess that's because it's so rich. 4into1 doesn't carry a size smaller than 35 for the slow jet, what else can I do to lean out the idle?

[Scott S]

 While I, too, have struggled with a high idle condition on the last few CB550's, what you are describing reminds me of an issue I had a few bikes ago. I was worn emulsion tubes.

 This was a stock engine, carbs, exhaust, etc. The needles had worn the inside of the emulsion tubes and it was rich no matter what. New, N.O.S. emulsion tubes cured it. Good luck finding a set now, though!

[dunnp2]

Oh, that's a good point! My emulsion tubes fall out when I open up the carbs, all of them, both the old ones and the new off-brand ones...I'll try a new set from 4into1.

At one point I debated shoving an o-ring on that outer shoulder because that's where it feels loose. Has anyone done that or similar?

[jonda500]

screwing out the idle mixture screws adds air making the idle mixture leaner - screwing them out cannot add fuel!

I had a similar problem with my 550F when it was going - I didn't have the hanging idle but had the same ridiculously rich mixture at idle and above too. I also have #35's, #98's and needle clips at the second slot.

The initial rich mid range problem it had at first I cured by fitting OEM emulsion tubes and needles - these took years to get, I had to buy another set of 069A carbs which fortunately had good condition OEM needles and emulsion tubes in them!

My 500 also had a very rich idle problem (combined with a lean at full throttle issue)- I have actually all but fixed it, but now have a lean mixture issue at very small throttle openings (that point between slowing down and speeding up). Being reluctant to pull the carbs off for the umpteenth time, I have just learnt to live with it. To get it to this point took a lot of trial and error - currently it has new oem emulsion tubes and needles clipped #4 slot, new main jet o rings and #105 main jets, oem #35 slow jets, mixture screws 7/8ths of a turn out and I have lowered the float levels to 4mm below seam. Lowering the fuel level was one of the earlier things I did so I would now like to try putting it back to 1 or 2mm below to see what difference it makes if any.
John   

[dunnp2]

Oh, that's a good point! My emulsion tubes fall out when I open up the carbs, all of them, both the old ones and the new off-brand ones...I'll try a new set from 4into1.

At one point I debated shoving an o-ring on that outer shoulder because that's where it feels loose. Has anyone done that or similar?

I have two complete sets of emulsion tubes that fall right out of the carb body, in every hole. Everything I've read makes it sound like they should be so snug that you might need to tap them out from the top by removing the slide. Am I misunderstanding this? Maybe my carb bodies were modified to enlarge the emulsion tube hole?

Jonda500 - I'm at similar settings to you. 98 main jet, OEM needle/emulsion tube in #2 (from the top, stock) clip location, new main jet o-rings, 35 slow jets. Mixture screw doesn't do anything because it's way too rich. Black smoke when I rev it.

I'm reluctant to lower the needle to the first clip because it runs great on the highway at higher throttle openings. Any other ideas?

Also, roughly how much air gap should there be underneath the slide at idle? I've heard of bench syncing using a 1/8" drill bit/pin. Does that mean it should be that far open at idle?

[Deltarider]

There is nothing wrong with needle jets (AKA emulsion tubes) falling down, after the main jets have been removed. Mine have done so from the beginning, be it that sometimes I have to twist the throttle grip in order to raise the slides and with them the needles.

[Scott S]

 When the emulsion tubes are clean, they'll drop out easily. Any that I've had to pry out were pretty gummed up with varnish or deposits. The main jet and the spring hold them in place.

 I'm still not sure this is 100% what you're experiencing, but this thread is worth a read:

 http://forums.sohc4.net/index.php/topic,184299.0.html

[dunnp2]

When the emulsion tubes are clean, they'll drop out easily. Any that I've had to pry out were pretty gummed up with varnish or deposits. The main jet and the spring hold them in place.

 I'm still not sure this is 100% what you're experiencing, but this thread is worth a read:

 http://forums.sohc4.net/index.php/topic,184299.0.html

VERY interesting! I was just looking over the carbs sitting on the bench for the umpteenth time and noticed that my tuning screws were backed way out. If I set them to 3-4 turns exposed above the lock nut as I've seen in pictures, the main idle adjust screw is a few millimeters away from making contact. I was considering what else could be causing that...I really need to look closer at that bell crank. Thanks for the suggestion!

[Scott S]

 Adjust the bell crank first, using the German method, then set slides using a small drill bit as a feeler gauge.  I bet you'll end up closer to the 4 to 5 threads out.

[dunnp2]

Adjust the bell crank first, using the German method, then set slides using a small drill bit as a feeler gauge.  I bet you'll end up closer to the 4 to 5 threads out.

Hoping to have time to try this soon. I'm sure my 49mm measurement is way off, because the throttle return cable ('push' cable) barely engages the bracket and the idle screw doesn't make contact.

Could someone tell me about how far the slides should be open at idle? I know mine are open about 4 or 5 mm. That's the very fine point where it struggles to keep running but doesn't run away so stays in a somewhat reasonable rpm range. I suspect that's way too far open. I'm also running my advancer with 2 full coils cut from both springs to keep it from advancing for a very long time. Sucks to drive but also prevents the runaway idle for the most part.

[HondaMan]

When I set these up after cleaning them for others, I set the individual adjuster screws as low as possible. This ensures the slides push hard to close all the way to the minimum. I 'bench sync' then by looking thru each one toward a bright light (bulb) and adjusting the screws of the 3 that are not closed to the most-closed one, until all are closed. They usually vary less than 1/2 turn of the screws across the whole set, and end up with just a couple or 3 threads of the adjusters above the nuts. Then I turn the idle setting screw inward to just barely open the set and ship them that way. No one that I know of has ever had the [carb-induced] high-idle problem from this setup.

The spark advance CAN (and on the 500/550 often DOES) cause a poor return-to-idle situation when the springs have softened to the point where they cannot pull the weights all the way in below 1200 RPM. The fix for this problem is to cut off at least 1/2 turn from each advancer spring and bend the last coil out 90 degrees to re-capture the weight. Often I have cut more than that.

[dunnp2]

When I set these up after cleaning them for others, I set the individual adjuster screws as low as possible. This ensures the slides push hard to close all the way to the minimum. I 'bench sync' then by looking thru each one toward a bright light (bulb) and adjusting the screws of the 3 that are not closed to the most-closed one, until all are closed. They usually vary less than 1/2 turn of the screws across the whole set, and end up with just a couple or 3 threads of the adjusters above the nuts. Then I turn the idle setting screw inward to just barely open the set and ship them that way. No one that I know of has ever had the [carb-induced] high-idle problem from this setup.

The spark advance CAN (and on the 500/550 often DOES) cause a poor return-to-idle situation when the springs have softened to the point where they cannot pull the weights all the way in below 1200 RPM. The fix for this problem is to cut off at least 1/2 turn from each advancer spring and bend the last coil out 90 degrees to re-capture the weight. Often I have cut more than that.

So the slides should be essentially closed at idle? Not 4-5mm open?

I'm running my springs with 2 full coils cut off. Way too much for normal driving, feels sluggish until it advances and almost throws me off the back.

I took a look last night, realized my idle screw bracket was bent. I bent it straight and it still only has 1 or 2 turns of contact before it bottoms the spring out. Is that normal? That's with 4 threads showing on all of the adjusters - in other words I can bottom the idle screw out on the spring and the slides are only about 1-2mm open. Something seems wrong here.