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spark plug sooty, adjusting carbs does nothing (k750)


thecraftdude
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hey guys, I have had some troubles with my carburetors the last couple of months.

 

When ever I have been out and riding for more then lets say 15-20min. the spark plugs always turns sooty, like totally charcoal black, I have changed the main jet from a 200 to a 150, but the soot still occurs on the spark plugs on both cylinders, they are dry and totally black.

 

I have tried with the needle bottomed out, but this only gives me around 5 more minuts or normal driving.

 

I just feel like, no matter what I do, I just can't get my old k750 to run well again.

 

Do I just have to buy a whole new sets of carbs, or is there anything I can do with my other 2 sets of carbs, to get this old lady up and running well again, or is it something completely different that I have overlooked?

 

what I have tried until now, to fix the issue:

 

- change the jet to a smaller size (the smallest i got)

- cleaned the air filter.

- no air filter

- cleaned the carbs and jets.

- installed new floats and float needle.

- installed a gas filter.

- tried new spark plugs.

- changed the gas hose to the carbs.

- tried changing the carbs for either of the 2 other Pairs I got, but I couldn't get it to run at all then.

 

(useful info if needed)

 

Bike: KMZ K750 1963 (sidevalve, 6v)

installed at the moment:

spark plugs: NGK BP7HS

K-302

main jet: 150

idle jet: 04 ?

needle hight: second from the top.

mixture screw: 1.5 turns out.

 

---------------my other 2 pairs of carbs:---------------

 

PZ28D (28mm) (PL10 stamped on the front?)

main jet: 92

needle hight:

- slide hole 2 (top hole)

- needle hole 2 ( second hole from the top)

 

---------------------------------------------------

 

K-37 (24mm)

main jet: 150

(no needle at the moment, just slide, I used the needle from the PZ28D)

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Hi, craft dude

Just a couple of ideas,

Have a look at your idle jet, see what size it is and try a smaller one.

Try running with no air filter and see what it goes like.

Try a hotter plug.

Have a look at your enrichener circuit and make sure the bike isn't being

made too rich all the time.( i.e. leaking or missing o-rings or just stuck 'on')

And make sure your floats are floating!

Just a couple of things I would look at.

Ride safe

Jim

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The idle jet says 04 on it, but it is worth looking into, no air filter, yes I just forgot to add that that to the list :P no help there either.

 

The plug is already on the hotter side for this bike, so that shouldn't be the problem, especially since It have worked fine up until the last couple of months.

 

Enrichment? there's a tickler on the float and a lever you slide over near the air filter, but that one is always fully open.

 

​i have had the thought that it might be the floats, but they do float and close off, and the needles are new and no sign of wear :/

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​i have had the thought that it might be the floats, but they do float and close off, and the needles are new and no sign of wear :/

New Chiwanese??? New from USSR... made in ???? the 60's maybe??

Not very reliable if so... New means nothing to me...as all I own is old Crap... "tested" is what I use!!!! YMMV

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  • 2 weeks later...

​i have had the thought that it might be the floats, but they do float and close off, and the needles are new and no sign of wear :/

New Chiwanese??? New from USSR... made in ???? the 60's maybe??

Not very reliable if so... New means nothing to me...as all I own is old Crap... "tested" is what I use!!!! YMMV

 

I do have the 2 sets of old brass floats from USSR, they work fine, the same with the Chinese ones, so I would say they work, last time I tested them (around 4 months ago) , I took a hose and attached a bottle of 1L gas in one end, and the carb in the other end, to see if they would leak, but they didn't over float in the 24 hours the bobble was hocked up (it did have a breather hole drilled in it, so it wouldn't make an under pressure inside the bottle, that could have effected the test

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Did you check the quality of the spark?? Good blue, snappy spark when fired?

 

I would go with RP, you have a combustion problem- too much fuel or incomplete burn...

SB#3

 

the spark is nice and bright, but my guess is also to much fuel, but I just can't figure out what the problem might be so far :/

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Did you check the quality of the spark?? Good blue, snappy spark when fired?

 

I would go with RP, you have a combustion problem- too much fuel or incomplete burn...

SB#3

 

the spark is nice and bright, but my guess is also to much fuel, but I just can't figure out what the problem might be so far :/

What Color???

If you believe the jetting and needles are even close... Lower the float height..

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Did you check the quality of the spark?? Good blue, snappy spark when fired?

 

I would go with RP, you have a combustion problem- too much fuel or incomplete burn...

SB#3

 

the spark is nice and bright, but my guess is also to much fuel, but I just can't figure out what the problem might be so far :/

What Color???

If you believe the jetting and needles are even close... Lower the float height..

 

the sparks seems to be a bright blue/white color, its decently visible in the daylight, but the floats can't be lowered, there's only one notch in the float needle.

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Did you check the quality of the spark?? Good blue, snappy spark when fired?

 

I would go with RP, you have a combustion problem- too much fuel or incomplete burn...

SB#3

 

the spark is nice and bright, but my guess is also to much fuel, but I just can't figure out what the problem might be so far :/

What Color???

If you believe the jetting and needles are even close... Lower the float height..

 

the sparks seems to be a bright blue/white color, its decently visible in the daylight, but the floats can't be lowered, there's only one notch in the float needle.

Not being familiar with your particular carburetors... I must conclude that it is a ###### poor design that does not allow float level adjustments.

I am tempted to recommend different carburetors... but, again, I know too little of the carbs listed so... without being able to adjust... I would get adjustable carbs....

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All the floats I've seen are height adjustable by gently bending their mounting arm.

But that may be a measure of my limited experience.

RussN

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Not sure why float height is the suspect. If the needle didn't shut off the fuel wouldn't there be fuel puddling on the ground?

ND

Maybe.... Float height does in fact affect Mixture...even if not allowing Overflow..

If the Slide needle is all the way down, and jets are close... I suspect Float height!

Or weak spark...or... Carburetion in general... certainly if those chiwanes replacements are in play!!!

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