Topic: Question for the Carb Pros

[jimbojangles]

This questions isn't about a SOHC 4 specifically, but should apply to any bike with 4 carbs. I'm posting here because the SOHC4 community has always been so knowledgeable and helpful. I'm working on a 2001 Concours. The only answers you get from the COG forums is to send things off to have someone else fix for you. It doesn't seem like a lot of guys do their own work beyond bolting on farkles.

So here's where I'm at. The bike runs and idles nearly perfect at this point, but it takes some strange settings to achieve this. On a Concours the mixture screws on each carb adjust fuel not air, so turning them out is richer, in is leaner. All carbs should be set to about two turns out. however, to achieve a smooth idle I have to set #3 at 3-3.5 turns out. At this setting it will idle really smooth, but you can smell that it is running rich. If I set it equal with the others at 2 turns it will develop a miss at idle.

So in short. What could cause a cylinder to only want to idle under rich conditions?

What could cause this? I don't want to list what I've already done, because I don't want to eliminate an idea before someone throws it out there. I've been through just about everything multiple times, but I'm willing to retrace my steps.

Thanks,

[05c50]

I'm not a carb pro by any means, but I've been around long enough to know that if you have to adjust the mixture screw that far past the stock setting, you're compensating for something. Either that cylinder has a vacuum leak (boot, vac line, throttle shaft wear,...) or the idle circuit is partly restricted. When the engine is idling smoothly, does that plug show a rich condition compared to the other plugs? When the screw is set to the factory setting (2 turns), does the plug look lean?

....Paul

[jimbojangles]

Thanks, I know I'm definitely compensating. My father would just say to be happy and ride it.

I just had the carbs off the bike and went through the pilot circuits with compressed air and a very fine wire. Carb cleaner and compressed air blow through them all evenly. With this bike it's easy to tell when a pilot jet is clogged. The idle is immediately improved with a little choke and the mixture screw no longer has any effect. What's weird is that the mixture screw definitely affects the idle on this cylinder. So if it is the idle circuit, I feel like I can kind of rule out the jet. (the bottle neck seems to be at the mixture screw)  I plan to try swapping the mixture screw with another cylinder to see if the problem follows it. I've learned to only make one change at a time or you never really learn what the cause is.

I haven't checked the plug on #3 yet. My assertion of a rich condition is based on the smell of my exhaust and slightly worse mileage. I'll check the plug soon to confirm.

Is it at all possible for something like valve clearance or ignition (bad cap/wire/plug) to cause something like this?

[rickmoore24]

I'm no carb guru either, but are you running the correct pilot jet size for your setup? Also, bolt on farkles are good for 10hp each, so.....LOL    Good luck! Rick.


*edit- A valve adjustment can only help if not recently done.

[jimbojangles]

hmm. I assumed all the pilots are the same, but have to admit that I've never verified.

This bike was rough when I got it. I don't think a single fluid had been replaced or adjustment made in it's life. It had 20,000 miles, but spent the last couple years sitting. All hydraulics were gummed up, carbs were worse than on some barn finds I've worked on.  I think the valves must have been gunked up with years of neglect and improper running. I did a valve adjustment when I got it and then another just to be sure before a 2,000 mile trip. By the time I got back all of the valves were tight and a couple didn't have any clearance at all. The valves should good now, but I am keeping an open mind to all possibilities.

I've checked the fluid levels in all carbs with a tube connected to the drain and they all measure the same.
Carb Sync is pretty good.

[eigenvector]

I agree with everyone else and will say that if you have to turn it that far out - something's not right.

Most likely an air leak, but it doesn't sound like a big one, and it might be where you least expect it - like the mixture screw threads. 

It could be something as simple as - the rubber o-ring on the mixture screw is crushed or torn, the spring that provides pressure on the mixture screw threads is weak, or there's just a very small air leak on that intake boot.

Something you might try - get yourself some axle grease and wipe it around the ends of the intake boot to seal off any leakage between it and the carb body.  If that fixes it - you know where the problem is.  If not - don't worry about it, the grease won't hurt anything.

[RAFster122s]

Given the needle enriches fuel when turned out it is logical that either the needle circuit for fuel or fuel draw is restricted ( not highly likely)  or an air leak is causing lean running beyond possible for running...( more likely cause) So, search for air leaks.  When you enrich it for running and then spray carb cleaner at the intake boots between carb and head or manifold.  If you have a bolt on manifold check the manifold to head seam as well as the typical oring used for these often fail.
If the boots are clamped on then check to ensure boot is over carb and clamps are tight.
If the boots are molded to the aluminum manifold then you may have the problem of the rubber cracking for the joint.

[jimbojangles]

Thanks for the feedback. I did the most thorough search for vacuum leaks that I could. All came back negative. I feel confident it is isn't a vacuum leak because I did the following to confirm.  I connected the carb sync gauges and confirmed proper sync. Then tested at different RPMs and revved the bike while keeping  a close eye on the gauges. All four gauges pulled equal vacuum in unison. I feel like the offending cylinder would have lagged behind if there were indeed any hints of a leak.

My two new working theories:

1. valves/compression - can valve clearances or poor compression cause this? The valves were terrible on this bike when I got it and it had tons of carbon build up on the cylinders when looking down the spark plug holes.
2. These are vacuum slide carbs. They have a rubber diaphragm up top that pulls the needle up with increased vacuum. I don't know much about these. Is there somewhere I should check that could be causing my problem?

[eigenvector]

I'm by no means educated about the Concours - but in my experience the idle circuit cuts out the instant the needles move.

On my Nighthawk, which probably has a similar function, a sticky slide presents itself as a stumble off-idle.  It idles perfectly fine.

I don't think the vacuum would give you much information and probably not valves either.

I don't know if yours has a starter pump on the carb - possibly that is stuck partly open (or closed) and screwing with your mixture at idle.

You may consider that the difference is simply a manufacturing defect - maybe the mixture needle is too fat, maybe the idle port is too small?  In which case - turn it out as far as it needs to go and ride on.  Or if you want to chase it - swap mixture needles from one carb to another and see if the issue follows it.

[05c50]

Just throwing this out there to give you something to think about. It's easier for a rich mixture to fire than a lean mixture, until it's so rich that it starts to foul. Maybe the ignition (coil, plug, wire, resistor cap) on that cylinder is a little weaker than the others. Can you swap ignition components with another cylinder to see if the problem moves?

....Paul

[jimbojangles]

Hey Paul,

I was thinking this too, but wasn't sure if it was a reasonable lead to chase.

This bike had lots of problems with crap getting stuck in the float needles and causing flooding (no carb overflows on it) I noticed that after a cylinder flooded it wouldn't run right again until I replaced the spark plug. (I would dry the cylinder and plugs, but the cylinder would only run at higher RPMs until the plug was replaced.) I've done a lot of reading and have read that some motorcycle ignitions are weak and may even have problems pushing enough current to run a previously wet plug at idle.

I replaced the plug on this cylinder yesterday thinking it would solve my problem, but it only helped a little. I'll check the wires and caps today to make sure I'm getting as much spark as possible.

Thanks

[RAFster122s]

Pull the spark plug caps and ohm them out with your multimeter.  If they are NGK they should be in the 5K range, presuming recent purchase.  If they are off by much then replace them.  Trim the plug witre ends back about 3/4" and attach the new plug caps.

A lot of carb issues are electrical, some say 90% are...
David

[rickmoore24]

I have to agree with that, from my recent experience. I just put in the Dyna-S and green coils, static timed it with my wire rig and test light, then with a gun spot on and it ran like sh!t for some reason. Correct plugs, wires and all that. It really sounded like a carb issue with one of the cylinders not acting right. Chased it on and off for a couple days through the carbs and they looked good, I got fed up and put the points and stock coils back in and bang! Dyna stuff is in a box on the shelf, until I feel like breaking the meter out. LOL

[jimbojangles]

OK, I'm at a total loss.

Since last posting:

1. Replaced wires, caps and plugs with new parts.
2. Completely disassembled carbs and cleaned the bodies and jets with an ultrasonic cleaner (water/simple green) and blew everything out with compressed air.
3. replaced o-rings on the fuel rail (discovered they were leaking and my cause of terrible mpg.
4. replaced idle mixture screw o-rings
5. re-synced carbs.

The bike runs stronger than ever. Off idle acceleration up to red line is smooth and strong. However, #3 still requires a much richer a/f setting to idle. At first start you can feel the header pipes all heat up evenly except #3 stays cold. This is true unless I have to use choke on a cold morning. If choke is used, all 4 pipes get hot quickly.

What the heck? This is frustrating. My Cb350f idles smoother than this bike. I'm not willing to let a much more modern bike idle like crap.

I have one last remaining idea, but may be grasping.

I noticed that some of the tiny o-rings on the a/f screws were falling apart. What are the odds that a small bit of one is lodged somewhere in the idle circuit within the carb body? And the stubborn piece of crap has refused to come out despite my many attempts?
 

[RAFster122s]

Quite possible there is an obstruction in the fuel mix circuit.  Not sure what the carbs look like...  is there a plug sealing the passage made for the fuel's idle circuit.  These can be removed and a new plug lighly peened and sealed with a bit of epoxy on the outside.  If the passage is only accessible through that plug then that would be one option.  Because they are fuel tolerant there is usually nothing that can melt them out chemically and you run the risk of making matters worse.

What does a new carb off ebay run for the bike?

That would be another option.  Are you certain the coil is solid?  If it is a wasted spark design like the CBs then you could swap it's mate to the troublesome cyclinder.  But, again, how many coils?

Have you cleaned up the coil ground point and coil wiring?  Is it a points or electronic spark system?

[NobleHops]

When you enrich it for running and then spray carb cleaner at the intake boots between carb and head or manifold.

Spray carburetor or brake cleaners are very powerful solvents that can destroy paint, like that on a black-painted Concours engine. WD-40 would be a much better choice.