Topic: Plug chop photos

[emlupi]

Hello all,

OK, so I did a plug chop today on my 1972 CB750K. Here are the photos and brief description of my process;

The first set of photos is from an idle chop. Warmed the bike up. Swapped in a fresh set of plugs. Ran the bike at idle for about 5 minutes or so. No throttle blipping, just let it idle. Shut it down and took the photos.

The second set of photos was done at 1/4 throttle. Warmed up the bike. Took it for a short ride to a local school parking lot. Swapped in the new plugs. Ran it up to third gear and at 1/4 throttle, while pulling up a small incline, shut the kill switch and pulled in the clutch. Coasted to a stop and pulled the plugs out.

The plugs show the bike is running lean, to my untrained eye. What say ye, and what suggestions go you have?

As an aside, the plugs while I was tuning the bike always looked sooty to me, ( see the 3rd photo of the single plug) so I did drop the pilot jets to #38 (from #40). I guess I should go back to the stock 40's.

[evinrude7]

which carbs do you have? 

[emlupi]

Round top 657A

[PeWe]

The single plug look too rich
Needles clip in middle position? Can be set lower wich will lift them and cause rich.
I had 40 pilot even with tuned engine, 836 +ported head + cam. Needles one stuo up (clip 1 step down from middle)

[emlupi]

Pewe,

The needle clips are in the 4th position down from top, which is on the rich side.

I was really paying attention today and most of my riding, mainly back roads cruising, was done at right around 1/4 throttle.

Dropping the pilots down to #38 seems to lean it out too much. According to "the chart", the needle clip position is a major contributor to fuel mixture at these throttle positions. I guess the next tweak should be a change in clip position up to the middle notch ( 1 step leaner.)

Sigh.........that means pulling off the carbs again to change needle clips.

[PeWe]

Change needle clip is PITY on many carbs and it can affect the carb sync too.

Newer Mikuni has a separate plug to remove covering the needle and not affecting sync. Remove tank and open the carb top covers.... everything done within 30 minutes.

Maybe a PO had changed the carbs jetting and thought it should give more power?
Or carbs have served a modified engine?

[emlupi]

According to the Hondaman book, the k2's came this way from the factory. #40 pilot jets, #110 main jets, and needle clips at 4 notches down from the top. For some reason, this bike seems to be running very rich in the idle to 1/4 throttle area. The plugs are sooty and I get a lot of black sooty residue in the exhaust as well. Maybe I will send him a PM and ask for advice.

[PeWe]

Float level is correct? Only seen with clear tube routine. Fuel level 2-4 mm under the float bowl gasket.

[emlupi]

Yes, I did this test and made the proper adjustments.

[zerodaydave]

Hello all,

OK, so I did a plug chop today on my 1972 CB750K. Here are the photos and brief description of my process;

The first set of photos is from an idle chop. Warmed the bike up. Swapped in a fresh set of plugs. Ran the bike at idle for about 5 minutes or so. No throttle blipping, just let it idle. Shut it down and took the photos.

The second set of photos was done at 1/4 throttle. Warmed up the bike. Took it for a short ride to a local school parking lot. Swapped in the new plugs. Ran it up to third gear and at 1/4 throttle, while pulling up a small incline, shut the kill switch and pulled in the clutch. Coasted to a stop and pulled the plugs out.

The plugs show the bike is running lean, to my untrained eye. What say ye, and what suggestions go you have?

As an aside, the plugs while I was tuning the bike always looked sooty to me, ( see the 3rd photo of the single plug) so I did drop the pilot jets to #38 (from #40). I guess I should go back to the stock 40's.

Can you tell us more about the bike?

Carbs?
Air Filter or Pods?
Exhaust?
Verified fuel level and float height?
Needle position?
Jet Size?
Air Screws?

Valves done?
Timing correct?
Vaccum synced?

Those plugs look really lean to me.

[flybox1]

put back in your 40s and do another idle plug chop.  reuse those lean plugs.
report back w a picture

[emlupi]

put back in your 40s and do another idle plug chop.  reuse those lean plugs.
report back w a picture

Will try to do this over the weekend.

I put the 40's back in and did some backroads cruising. Majority of the time throttle was between idle and 1/4 position. Pulled the plugs and they are now back to dark and sooty. I contacted Hondaman and he gave me a few leads to go after. Some of it involves removing the carbs so I may wait before getting that deep. I'm having too much fun riding.

[flybox1]

with 40s, you will have to adjust your IMS leaner, but only a idle plug chop will show you roughly how much....

[emlupi]

Mark suspects that the emulsifier air passage may need cleaning. That and several other checks including the insulator clamps. I installed new hardware on the clamps and the they seem to be nice and tight but maybe not tight enough? When I have some time I will dig into these things.

[flybox1]

might be so.
if you ever pulled out your brass emulsion tubes/jets and they had that green/grey discoloration/oxidation, then yes, those passages need attention.

[emlupi]

He says the needle jets should be removed for cleaning the passages with a soft wire. Never removed these, how do they come out?

[flybox1]

You'll have to remove the top caps and slides, and with a wood dowel, tap them down and out.
Nothing harder than wood, or you'll cause all sorts of issues.

IIWM, I wouldnt remove them.  Just stick a flashlight into the area the slide sits, and then view from the bowl side, into the main jet holder to see if the needle jet is clean, or out of round.
Being in a dark or dimly lit garage will help.

[emlupi]

I think he is referring to removing them before sliding a wire into the emulsifier air passage. The end of the wire will make its way out into that chamber where the needle jet lives.

[PeWe]

More stuff about carbs here: http://forums.sohc4.net/index.php/topic,5410.0.html