Author Topic: Sooty Plugs again- (Read 6021 times)

78whiteorbs · « **on:** December 19, 2011, 09:14:16 AM »

73 CB750 running stock airbox with D8's . Fouling plugs again.....

Repaired my cracked main jet towers and installed 110 mains- runs great ,doesnt smell rich at all. Carb floats set at 26mm, nice thick spark on all fours.....
points are pitted and can not be filed anymore. Replacing with some new points soon as they come in - could this be the problem? Other than that it runs beast-

nancy · « **Reply #1 on:** December 19, 2011, 12:14:08 PM »

Sooty on all 4 - 1st I'd eliminate electrics as a common denominator.
Points - OK you're on to that. Sounds like they're worn...so replace 'em. Correct gap set?
Coils - you have a thick spark - so I assume these are OK...and unlikely both are failing. Unless your "thick" and what is really "thick" are not the same? Maybe the spark is not that great?
Ignition timing - check and adjust while you're in and around the points.
Plug leads and caps - make sure u have good clean ends on your leads. Unscrew caps, snip off 2mm of lead clean. Test caps with multimeter..should be 5ohm...mine were shat and were showing 10ohm Screw caps back onto cleanly cuts leads.
New set of plugs.
And check/adjust valve clearances.
If it is still sooting..move onto fuel system.
Idle jet set one turn out from full closed. Even if idle is crappy...leave it at not more than 1 turn out.
Jet needle clip might be set wrong. Clip in too high a slot. Standard clip position is middle. I moved mine DOWN one clip position to reduce excess fuel in my K1 engine.
Anyway - the above is what I have followed to sort out mine - and it stil has some issues...but I'm awaiting a new ignition coil which hopefully will fatten up the weak spark I had on #3 - as that remained sooty...in spite of me trying all the above. With a new coil(s) I may review my changes to carbs - if my spark thickens I may need more fuel again.
Regards

Mark

78whiteorbs · « **Reply #2 on:** December 19, 2011, 01:43:47 PM »

thanks for the reply - I was eyeballin those PAMCO ignitions but that isnt really a proper fix- after installing new points I am gonna do as you suggested. My spark is as thick as I have ever seen on any other older bike than ran proper tan plugs-

TwoTired · « **Reply #3 on:** December 19, 2011, 02:07:42 PM »

Spark doesn't cause a rich mixture. (At least, not without noticeable misfires.)

I think Mark once mentioned that the air passageways could get corroded and block air passage to the emulsion tubes. This would make a rich mixture. But, if all the plugs display the same deposit patterns, the odds that all the carbs have corroded or blocked air passageways in exactly the same way seems remote. (I'd still check them anyway.)

Otherwise, check the fuel jets for alteration/wear (measure the orifice size) As well as the slide needle and slide needle jet orifice.
Have you tried dropping the slide needle a notch?

Air filter bears scrutiny, as well. (Take out any extra rags.

)

78whiteorbs · « **Reply #4 on:** December 19, 2011, 05:37:13 PM »

no clogged passageways , checked - ulatrsonic'd them 3 times and they are clean as a whistle.

brand new jets with not even a nick from a flathead screw driver on them .

new air filter-

will try dropping the needles a notch after I double check that I am not having weak ignition.

turned all air screws 1 extra turn out -

Also finally installed my speedo cable so I can tell how fast I am going !

I feel I am getting closer somehow-

nancy · « **Reply #5 on:** December 19, 2011, 05:49:33 PM »

U shouldn't be turning the airscrews out an EXTRA turn at all. Should be 1 turn max..maybe 1.25 turns at extreme. That is idle air screw. Your sootiness is not likely (possible though) related to idle jet - unless all you do is idle. Likely your needle jet and/or mains if jets are at fault.

78whiteorbs · « **Reply #6 on:** December 19, 2011, 06:46:57 PM »

I havent ridden the bike very much( I have another daily driver) but I start it up and let it get to temp everyday - go on about a 1 mile blast around my neighbor hood.
It starts easily with no choke . Just a bump of the button or one kick if I am feeling manly.
So a majority of the "running" of this bike has been at idle speeds and revving it up and such . Before I repaired the cracked towers It didnt really do much past 6k rpms .The power just kinda feel off. Now it is smooth all the way to the top and pulls nice and hard. So it was seeming to reason to my mind that if it was running rich it was running rich from the idle circuit . I'm no Sherlock Holmes though -

If the pilot jets are nice and clean then the only other way I can reason a rich condition is clogged air jet passage? Are those air jet screws suppose to have little rubber o-rings on them > Mine do not . Man , after installing the new points this weekend , I guess the carbs are coming off yet again and take a look see at the needles-

Surely I know not a tenth of what you fellas must but what is the harm in leaning out your pilot circuit with the air screws ? PLease , I am sure this is a stupid question but I must ask it anyway so have mercy

I have read that some people have them out even further because of "carb wear" and other instances

nancy · « **Reply #7 on:** December 19, 2011, 07:33:06 PM »

Increasing your air to the fuel mix is the wrong way to solve a problem with excess fuel being burnt. You need to reduce the fuel..not increase the air. The 1 turn out on the airscrew is a benchmark, a standard, a place to start from knowing that is goooood to go. That is the recommended amount. Now you need to reduce your fuel.
Of greater importance - too much air makes for leanness which makes for overheating and engine failure. So avoid the temptation to screw with your airscrew. TwoTired would be the man to chime in her and elaborate on air/fuel mixes - he has the knowledge you seek..while I am just a student who has passed "SOHC 101" and heading into year 3 of my studies!

But - this is a path I have been down and indeed am still travelling.
Yep - pull off the rack AGAIN. Hey..mine has bene off maybe 20 times...
Check those clip positions on the needle. If u haven't been into these - the retaining screws are accessed from the the throttle slide - but they get stuck and the heads are easily wrecked. SO,...spray with lots of penetrant and leave it a while. DON"T use a screwdriver that just sort of fits...find one that fits snug inot those Philips head screws and push down and give a good twist and hope they come. One of mine didn't and I used a drill to drill away the head and drilled out the rest oversize, used a tap and re-threaded with a screw size slightly bigger - not much room in there.
No I don't think there are O-rings in there...on the idle screw...not on mine anyhoo.

Spanner 1 · « **Reply #8 on:** December 19, 2011, 07:44:13 PM »

Dude, you answered your own question when you said '

Quote from: 78whiteorbs on December 19, 2011, 06:46:57 PM

I havent ridden the bike very much( I have another daily driver) but I start it up and let it get to temp everyday - go on about a 1 mile blast around my neighbor hood.

Nothing will blacken you plugs better than the above practice.... please ride your bike at least 50 miles ( in one go ) and report your plug colors

78whiteorbs · « **Reply #9 on:** December 19, 2011, 07:46:57 PM »

hmmm, perhaps with a spare set of plugs I will give it a try-

thanks Spanner

nancy · « **Reply #10 on:** December 19, 2011, 09:11:48 PM »

Yes, Spanner1 makes a good point there..and you may indeed be worsening your situation with those shortie rides. However, you stated that it needs no choke to start from cold - so to me that indicates too rich a mixture.

kpier883 · « **Reply #11 on:** December 19, 2011, 09:56:28 PM »

I changed to #35 idle jets and 105 mains because I was forever fouling plugs. I can't remember if I changed the idle first or the mains, but I think I did idle jets first because it would start without the choke all the time, hot or cold. That made me believe it was rich on the idle circuit. I think I changed to 105 mains later due to some other issues.

Anyway, it stopped the black sooty plug problem. Mine is a '74 CB750.

dave500 · « **Reply #12 on:** December 19, 2011, 10:02:15 PM »

use d7ea plugs maybe?one heat range wont hole a piston.

TwoTired · « **Reply #13 on:** December 19, 2011, 10:31:00 PM »

Your carbs don't have accelerator pumps to enrich the mixture while the slides are mechanically opened.
The opening of the slides causes sudden lost vacuum, and the vacuum is what pulls the fuel in. If the mixture were ideal before the throttle was opened, the engine would stumble due to lean mixture when the throttle was opened.
These early carbs were set to run over rich at idle so there would be enough excess fuel for the air increase event as the throttle was opened. Prolonged idle and low speed operation WILL allow soot to build on the spark plugs. The air screws are adjusted to provide this over rich idle mixture. Turning them outward, normally causes the engine to stumble with sudden throttle opening from low speed under load. This is measured by how much throttle causes this to occur. Normally, you can twist the throttle under any load condition up to one half total travel, and the engine will accelerate smoothly (if not quickly) in any gear from near idle RPM. If the the idle mix is too rich, you may be able to whack the throttle full open and get smooth engine response, but the likelihood that the plugs will soot, increases. If the idle air bleed screws are opened too far, very little throttle travel under load will create engine stumble (I call it wheeze) and it won't recover unless the throttle is returned to a very low position.

You can think of the carbs as providing three operation modes for engine operation. Idle, where the pilot jets dominate the mixture up to about 1/4 throttle, The throttle valve (slide needle) which dominates the fuel mixture between 1/4 and 3/4 throttle position, and the main jet, which dominates the fuel mix for 3/4 to WOT.

If, for example, the idle mix is set rich, and the throttle valve is set lean, then operating at mid throttle (like on the freeway for 15-20 minutes) could allow the plugs to self clean and the engine to get to operating temp. (Running at idle does not get the engine to full operating temp.) The same is true for other rich/ lean combinations of the three fuel metering devices.

Having said this, it is not normal for the plugs on a stock bike to soot foul while idling, though they do have that tendency. Operating with choke, even partial, certainly will. As will an engine with carbs that have problems.

Hope this helps,

78whiteorbs · « **Reply #14 on:** December 20, 2011, 06:49:14 AM »

Very informative Two Tired , thanks for the explanation. Now I have some more knowledge/ammo .

There is some low speed stumble but I thought it was because the carbs havent been synced yet and they were "fighting " each other ( dont have a gauge, but a friend does so I am working on that ) Low speed "parking lot " acceleration has been bumbly but If I get on the needle and take off swiftly she is smooth as butter to the top of my RPM gauge . Guess I am gonna ride her for a coupla days to work and see what the plugs look like .

Mainer4 · « **Reply #15 on:** December 20, 2011, 07:14:16 AM »

Position of the clips on the main metering rods? Should be center (#3)
Wear on the main metering rods? If they are original and bike has more than 50K miles think about this as these were known to wear and your will see a richer fuel mixture. This could account for sooty plugs particularly in low speed runs (35-45MPH). Converting to 105's may be a bandaid for worn metering rods.
I would be willing to bet you are seeing black on the exhaust ends as well and probably black smoke if you rev er up.

78whiteorbs · « **Reply #16 on:** December 20, 2011, 07:16:49 AM »

Had 117.5 mains and it was blow a little wisp of black smoke not much , and the pipes were sooty too. Changed to 110 mains repaired my cracked carb towers and no no more black smoke or sooty pipes

nancy · « **Reply #17 on:** December 20, 2011, 01:37:21 PM »

Give it a decent 50mile thrashing as Spanner1 suggested; reassess with a view to going standard on pilot jet size and idle screw adjustment - following a change to your needle clip position. That's my view on your business...hope you get tanned and not too sooty!
Regards
Mark

78whiteorbs · « **Reply #18 on:** December 20, 2011, 01:43:00 PM »

Thanks, that sounds like a proper plan there

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Author Topic: Sooty Plugs again- (Read 6021 times)