Topic: Plug Chop Results

[jakec]

I have two photos of plugs attached. First photo shows condition after some normal riding, probably about 20 miles or so at all RPM ranges. I thought that the plugs look OK but a little lean, with the #4 plug being mysteriously super rich.

The bike runs best at idle - 1/8 throttle, and 1/2 - WOT. At about 1/4 throttle, where I expect the needle position to be in play, the power is weak and stumbling. So I did a plug chop at this range. With brand new plugs here are the results (second photo). I'm planning to take carbs off to inspect everything once again but I'm looking for advice interpretation of the plug color in the meantime.

MY interpretation is that all needles should be raised 1 notch and the cause of the super rich #4 plug fixed (float height maybe?)

[Tim2005]

One step at a time, I'd sort out number 4 first then see how it is

[xhevi]

What's your current niddle clip position?

[jakec]

Currently raised one position.

[markreimer]

What year and model is your bike? Stock intake etc?


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[xhevi]

Currently raised one position.

Raised one from what starting point?

You lower the clip to raise the needle.

Mine were at 4th from top I was fouling all 4 plugs. I ended up with the clip at 3rd position from the top. No more fouled plugs.

[PeWe]

Sure the black plug is not oil?
If not, floading float can be one reason for individual difference.

When talking about needles, raising needles or raising the clip 2 different things.

[jakec]

We took a long break from tinkering on this bike because the symptom seemed to be resolved but has come back. I think that over time the plug has become fouled and now #4 (same cylinder as dark plug in original post) has become lukewarm when running.

It's my gf's bike which is all custom, intake exhaust etc. It is a 1977 cb550K engine with the corresponding late style 550 carbs. After this plug chop test I had lowered the needle clip position by 1, thus raising the needle. I also scoured the carbs for a possible cause of the dark plug did not find anything. Changing the needle position basically eliminated the stumble at mid range that was the reason for the plug chop. She rode about 1-200 miles more and now we are here. So the problem with the dark plug still exists.

Coils and wires and caps are all new. We have checked the cap resistance and it is good (5.03). The plan is to next check the valve clearance and ensure compression. After that we will look at carbs again. We are sure the float height is good and it has leak proof floats. So we're wondering if there are any other tricks to these carbs, like an easily blocked passage (idle) or something that is known to happen that we can check.

On her other bike which is a 350 twin, we replaced a cracked emulsion tube which was causing both a stumble at mid rpm and preventing carb bowls from draining and creating vapor lock. We will look for something similar here.

[markreimer]

I sympathize with you as these pesky jetting issues are super frustrating to narrow down.

I would suggest a couple things:
- A compression test is ok but if the results aren't great it won't help narrow down your next steps. I would suggest a leak down test as well. You can make your own tester, or buy them real cheap on amazon. I have a cb750 and did a compression and leak down test on each cylinder. Thankfully the results were all great and very close between cylinders. But I have a XR650 that showed a 30% loss of compression on the tester. I switched to a leak down tester and was able to immediately hear a huge air leak via the intake valves - not what I would have expected! I wouldn't have known that with a compression test alone.

- for carb cleaning, remove the emulsion tube behind the main jet and verify all those little holes are clean. There is also a fuel passage in there I think, depends on your carb though. Get a few cans of carb cleaner and blast all those passages clean till you're out of cleaner, then follow up with compressed air. Check your float height, but also check that the float is actually stopping fuel. If the needle/seat doesn't hold fuel back, it could be flooding that one cylinder at idle and low speeds.

- have you tried snipping your plug wire back 1/4" and attaching the boot again? you might have a weak spark from a bad connection.

- Is the black plug the one lowest to the ground when on the side stand? I wonder if leaking floats are causing that carb to overfill when parked or something. I've had two boxer-engine bikes that did that.

Hope that helps. My cb750 exhibits similar problems from time to time. After highway runs, they are all identical. But around town I always get one plug way richer than the rest. I haven't figured it out yet either...

[jakec]

To address the last point first, as the problem becomes more apparent the idle also becomes more erratic and wants to climb. Which to me points toward carb sync or any part of the idle circuit. So slow jet and the passages that feed it (including air/fuel mixture screw).

We have always blown out passages and cleaned jets with wires and been thorough. They way that we were able to fix her 350 carb issue was by finding the sectional view in the manual and just tracing every passage. The late style 550 carbs do have an addendum for disassembly but no sectional views explaining all the inner workings.

Leak down tester sounds awesome but I don't think we will be to use / don't have an air compressor where we live.

Since her bike is a chopper with a wicked lean  #1 is very low on the side stand but it running well & consistent with the others. #4 is the highest on the side stand.

[scottly]

After that we will look at carbs again. We are sure the float height is good and it has leak proof floats. So we're wondering if there are any other tricks to these carbs, like an easily blocked passage (idle) or something that is known to happen that we can check.

You have a problem with the #4 carb. Have you checked the actual fuel level with the clear tube method? Have you removed the emulsion tube and made sure the holes in the side are clear? You can't clean the emulsion tubes by spraying carb cleaner or compressed air through them when they are installed in the carbs. The 550s have an o-ring that seals the main jet in the tower, and a leaf spring that holds it in place when the float bowl is in place; are both present and in good condition? Is the tower the main jet plugs into corroded to the point the o-ring can't seal? Are the air-bleeds at the entrance to the carb open?

[jakec]

We have removed emulsion tubes and cleaned side holes. The main jets are the screw in type so no O ring or leaf spring. There is no damage to the casting. We were planning to check fuel level on this pass.

[TwoTired]

The 550s have an o-ring that seals the main jet in the tower, and a leaf spring that holds it in place when the float bowl is in place; are both present and in good condition? Is the tower the main jet plugs into corroded to the point the o-ring can't seal?

The PD carbs Main jet holder and emulsion tubes screw into the carb body and have no orings there.  And, they don’t need leaf springs.  They can be over tightened to crack the carb bodies, though.

If indeed only one carb is delivering a mixture too rich, check both the air jet passages to the pilot circuit and main emulsion cavity for full flow.  Yank the pilots out of the carb bodies and check for flow to both pilot screw throat exit and the air jet inlet.

It is hard for me to believe the slide needle taper profile will ever be ideal for an engine without the stock muffler and air induction ducts.  It will of course run ok in many throttle positions, but not all.  And do not expect any stock adjustment settings will work well with modified induction and exhaust.  The stock pilot jet may work with pilot screw turned way out,  but the main jet will certainly need to be larger.

Cheers,

[jakec]

Thanks, we will check that tonight. I will have to explore the bodies with the carb spray to locate all the circuits you mentioned.

As for the jetting we are already enlarged on both circuits although I cannot remember the numbers now. It is not running perfectly of course but it will get down the road pretty good. Like you said the needle taper isn't perfect but we have almost eliminated any stumbling in that range. Even with this rich carb situation which is mainly affecting idle.

[jakec]

Here are the results of the clear tube test. The reason the carb is angled is due to the long front end. This is how the bike sits.

I also just checked the #1 level and it is the same.

[jakec]

Slow jet (press in) fell out. Hehe 

[jakec]

We haven't had the chance to run the bike again yet but I've been thinking about the fix last night. The black plug showed up before & after the plug chop that I did back in october. But I removed carbs to adjust needles and check float height back then. And the dark #4 plug still occurred after that. So the slow jet had not fallen out at that time. I guess maybe it was loose and fell out later?

Last night in addition to reinstalling the slow jet we re checked all float heights, air screws, jet holes, and blew out the 3 passages mentioned above. Some grime did blow out through the air screw hole when carb cleaner was blown into the port in the intake. Not sure if that will have an affect or if the grime was "outside" the circuit since the screw is exposed to exterior.