Topic: Why are my plugs STILL black??!! Help

[nino400F]

Gday people,

So, here is the scoop.

Before reading here is current setup:
657B carbs
105 main, 40 slow, high positioning on needle
4-1 exhaust
Stock air box from 750F (mine went missing)

Bought a 1972 CB750, went through the whole thing, (valve adjustment, cam chain adjustment, set timing properly, sonicleaned carbs, vacuum synced carbs, cleaned air box and put new filter, changed oil and filter)

Upon starting it I ran it for about 25km then the plugs fouled (D8ES), went to parts store and bought DPR7ES-L plugs as they are bit of a hotter plug thinking this will correct the issue. NOPE!

Ran another 30km and she fouled them again. So pulled the main jets and they were a 102 that someone Hogged out to probably a 115 or so. Way too rich, threw some 110,s in and still running way too rich and black.

I just put 105’s in it, cleaned the plugs up and rode it for 7km, plugs are still too black, though I am getting some small view of tan on 1,3 but not enough to be proper. I have some 102.5’s that I could toss in but don’t know if it will make a difference.

Exhaust is popping on the off throttle after accelerating.

What to do? Drill holes in bottom of airbox??

Any and all help is appreciated.

[ekpent]

 Won't cure your problem probably but I don't waste my time trying to clean dirty/fouled plugs. I like new ones,D8EA, especially if I would be making tuning changes. If stock K2 then the main jet would have been 110 I think with a 40 slow. If you can find real Kehin stamped jets those would be best to use. They are marked with a stylized 'K'  on them. Might want to check yours as some aftermarket ones can be pretty bad. I had some aftermarket #40 slows once that were big enough to drive a truck through !  Good Luck on cleaning up your ride.

[TwoTired]

You are aware that the main provide fuel mixture at 3/4 and above throttle positions?  And the slide needle controls mixture in the 1/4 to 3/4 throttle position settings?

If you chose the change and try method over the Dyno or test track procedure, you could try to change the slide needle position to correct your over rich settings to see if that satisfies.


Cheers,

[nino400F]

So I dropped down to the 102.5 and it is still getting the plugs black. Do I drop the needle clip or raise it. To lean it out? @twotired

CalJ737 when Choke is on all the way it doesn’t want to stay running without me revving and still runs like crap, choke off it runs ok until warms up, then runs good until plugs start fouling

[Robbo]

What needle position are the carbs set to?

If we use a number scale with #1 position being the notch at the bottom (closest to the bowl) and counting up from there.


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[Progas]

You also might want to check how much voltage is getting to your coils, older bikes corrode the connections over the years and you would be surprised how little voltage ends up at your coils!

[seanbarney41]

CalJ737 when Choke is on all the way it doesn’t want to stay running without me revving and still runs like crap, choke off it runs ok until warms up, then runs good until plugs start fouling

The lever is operated in orientation to the choke butterfly. Lever level to the ground, choke is OFF. Lever down, choke is fully applied. How are you operating it? Perhaps a picture of your setup-

this is a 750 cal, you got it backwards

[nino400F]

What needle position are the carbs set to?

If we use a number scale with #1 position being the notch at the bottom (closest to the bowl) and counting up from there.


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The needle is the 3rd notch from the bottom notch. I have raised it to the 5th notch. Will this resolve or should I increase jetting?

[STLrocker]

first things last. have you confirmed that your ignition system is operating properly? do you have good hot spark? all the jetting in the world wont fix weak spark or other tuning issues.

has compression been checked?

i also would second the comment about confirming that your pilots are actually 40. also, have you checked float height?

if you move the clip up to the top notch on the needle, that will lean out the mixture at mid throttle. middle is stock.

[TwoTired]

Do I drop the needle clip or raise it. To lean it out? @twotired

Raising the clip on the needle moves the needle downward toward the orifice, which is leaner.
Probably only need to move it one clip position.  Just a guess, though.

You should look at both the needle and the needle jet to inspect for wear and/or alterations.  If they drilled the mains, they may also have drilled the needle jet orifice.

Cheers,

[Robbo]

When you stated above that the plugs were fouled:

Are they

1. dry black carbon fouled

Or

2.  Oil fouled (wet tip) ?

When this happened, were you running primarily on side roads (40-50 km/h), main roads (60 to 80 km/h) or highway (100 km/h)?

What do you have your air screws set to (# of turns out from seated).

Is the battery new and fully charged?


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[nino400F]

When you stated above that the plugs were fouled:

Are they

1. dry black carbon fouled

Or

2.  Oil fouled (wet tip) ?

When this happened, were you running primarily on side roads (40-50 km/h), main roads (60 to 80 km/h) or highway (100 km/h)?

What do you have your air screws set to (# of turns out from seated).

Is the battery new and fully charged?


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So to answer your question yes they are typical dry sooty fouled plugs.

I put needles as lean as they could go, dropped jets to 102.5, and now it runs worse. I assume because it’s not getting enough fuel when I go half throttle and up. Yet the plugs are still sooty but with some signs of tan and white just at the very tops of the electrode, inside the plug is still sooty. At this point I am thinking spark. Points and condensers are fresh. But the coils are older with some obvious age.

Please describe the best way to check the coils voltage and the range I should be looking for. I imagine I would take a digital voltage meter but where would I touch the probe to get an accurate reading? Can I do this with the tank on?

Again thanks for all the great input

[Robbo]

What do you have the air screws set to?

I ask because this is important to understand.  The idle circuit still plays a roll in the overall air/fuel mixture while the bike is moving at low speeds.

It has an effect on how the bike operates.

Btw, if you have previously leaned out the needle position and then change to smaller jets, put the clip back to the middle position.  Otherwise, you run the risk of leaning out the motor to the point of catastrophic failure.
If it’s still not right, take small steps and move the clip one position at a time and test again.


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[STLrocker]

the primary side of the coils should have 12v to ground with the key on. to test you would use a meter with one lead on a good ground and the other to the terminal on the coils. there are two terminals on each coil. i dont remember wire colors. just test both terminals on both coils. on each coil, one should have 12v, the other will have nothing. if you dont have 12v at each coil, find out why. you do need to remove tank to do this test.

the other terminal at the coils, the one that dosent have voltage, is coming from your points. you say points are new. double check adjustment and make sure they are clean.

if it were my bike, before i do all of that, i would just pull all of the plugs and attach them to the plug wires one by one and ground them to the engine and manually operate the points to check for spark. what you need to do is rotate the engine with the kickstarter until a set of points is closed, then ground a plug from the corresponding coil (one coil is for cyl 1&4 the other s for 2&3) and manually open that set of points. as you open the points, you should see a big fat blue spark from the plug. key should be on for this. then rotate engine to close other set of points and repeat test for that coil.

if you look here , you can find electrical diagram for your bike.

http://forums.sohc4.net/index.php/topic,17788.0.html

[STLrocker]

if you dont have good spark when doing the test above, you need to look at your points, plug caps at the ends of the spark plug wires, check 12v+ supply to coils, and make sure wire connections are clean and tight. you can also repeat test with new plugs in case old ones are fouled.

and to be clear on coil testing, you can check the primary side with a meter for 12v supply. the secondary side is the high voltage output to the coils. this cant be checked practically with a meter. the test for the secondary side of the coil is the spark test that i outlined above.

i would baseline everything. check compression, check spark, check timing, set carbs back to factory specs verify float height... there shouldnt be a reason to lean out the carbs beyond the factory spec if everything is close to stock and working properly. unless you live at a very high altitude.

[nino400F]

What do you have the air screws set to?

I ask because this is important to understand.  The idle circuit still plays a roll in the overall air/fuel mixture while the bike is moving at low speeds.

It has an effect on how the bike operates.

Btw, if you have previously leaned out the needle position and then change to smaller jets, put the clip back to the middle position.  Otherwise, you run the risk of leaning out the motor to the point of catastrophic failure.
If it’s still not right, take small steps and move the clip one position at a time and test again.


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Air screws are to stock 1 turn out, I am going to check spark in the morning. It does fire but maybe weak. Need to determine. I will set everything back to stock and work on the spark side of things. Again, it is so lean at this point with small jets and deep needle I am almost certain it will be spark. 1-4 plugs look better than 2-3 plugs. But all are fouled.... (sigh)

[TwoTired]

to determine if your coils are ok, Measure the primaries and secondaries with an ohmmeter.
The primaries should be near 5 ohms.
The secondaries should be in 15K ohms range.

Cheers,

[Robbo]

Assuming your electrical is found to be working okay, your float heights are set correctly and your pilot jets are true 40s, put in the 105s, set needles in 4th position and go 1 1/8 turns on the air screws.

If it still runs poorly, try going 1 1/4 on the air screws.

Note: you may need to start it on choke as turning out the air screws will lean out the idle mixture.


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[scottly]

So to answer your question yes they are typical dry sooty fouled plugs.

Dry sooty plugs ARE NOT caused by ignition problems! They are caused by too much fuel or too little air. With no spark, there is no fire, and with no fire, there is no soot. Fouled plugs, on the other hand, will show as ignition problems. You can fix a carb problem, but if the plugs are fouled, the motor still won't run properly. Either replace the spark plugs, or burn the black off the fouled ones with a propane torch.

[flybox1]

So to answer your question yes they are typical dry sooty fouled plugs.

Dry sooty plugs ARE NOT caused by ignition problems! They are caused by too much fuel or too little air. With no spark, there is no fire, and with no fire, there is no soot. Fouled plugs, on the other hand, will show as ignition problems. You can fix a carb problem, but if the plugs are fouled, the motor still won't run properly. Either replace the spark plugs, or burn the black off the fouled ones with a propane torch.

+1
this^^^^

[nino400F]

Plugs were cleaned off then soaked in contact cleaner then burned and blown. I have three sets I keep cycling through. I agree with no spark. But what about weak spark? Wouldn’t this cause the effect or am I still at a carberation problem?


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[scottly]

No, a weak spark won't cause heavy soot.
Have you done a "clear tube" test to determine the actual fuel level in the float bowls?

[nino400F]

No clear tube test. All bowls have good fuel in them. What is the clear tube test? How to do it? What result am I looking for? What’s the point?


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[PeWe]

Floats might overflow. Have you checked float heights?
Best method is clear tube when it cover how the float valves work too.
Old CB750 carbs should have 26mm, 25 with later aftermarket valves with stiffer springs.

I have a bunch of velvet black plugs due to floading floats, 1mm over gasket instead of 2-4 mm under.

[nino400F]

I will re gap the floats, have ordered new #40 slow jets (not sure if PO hogged those out too) when those arrive will set all back to stock. Put new DPR7ES-9 plugs in and see where I am at...

Will post again with results and possible picks of plugs.


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