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The Carburetor Debate

carburetorfuel upgrade k65 k68 k301

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#1 Fletch

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Posted 02 December 2014 - 08:40 AM

Hi,
im after some options on the age old debate/opinion trade off about carburetor upgrades and which models are best for reliability and consistent performance.

i currently have k301 carbs on my  '65 k750 dnepr at present, (i guess it could be agreed that's the correct model for my year of bike) they are a bit rubbish and i doubt that what ever fuel that does make it to the cylinders is seldom in the right mixture, so for me an update is in order.

i have heard positive things about k65 / 68 carbs. (compared to the k301 i guess anything is better)

it seems the k65 is a good carb, but needs setting often to remain in peak condition. and the k68 is also a good carb but tends to run a bit rich.

also is there really much difference in fuel mixture delivery due to the shape of the slides? the k65 has the flat slide, where the 68 has the round one.

any thoughts?

cheers.
1965 Dnepr K750

#2 MotoJ

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Posted 02 December 2014 - 09:04 AM

I have Chinese PZ28s on my M72 and they have been great. They bolt right on, no flange conversion baloney, and I have hardly touched them since I got them dialed in.
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#3 racepres

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Posted 02 December 2014 - 10:40 AM

I feel that a Flat slide is superior to a Round one...Hence Mikuni waiting for the Lectron Design patent to expire, and then going from round to square!!
Having said that...the round slide Mikuni is hard to beat..for the $$
I'll keep my K-65's thank you...with the walboro needle, and a strategic drop of lictite here and there...They are working well enough for me!

#4 guzzidude

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Posted 02 December 2014 - 11:54 AM

:smileypopcorn:
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#5 yatesfly

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Posted 02 December 2014 - 05:52 PM

I have pekar 65s on my mt-16 I tried pekar 68s but like 65s better don't have much fuss with them in16000 Klms
dnepr MT16,64 hd xlch,71hd xlh, 82 honda gl500

#6 SafetyBob

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Posted 02 December 2014 - 07:11 PM

What they said...

Plus, I have had great results with K-65's.  The not staying tune line is really not true.  Issues with the float needles are.  I have the K-68's as well, I have had issues with slides, plastic top caps and mixtures.. I have removed them and replaced the with Kaptex K68's..  They are a Ukraine copy but have metal top caps and better float needles.
I have the 301's on my K-750.. I really have no issues with them at all, but I do not ride the bike much due to its age and 6v system.

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#7 Ken Ulrich

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Posted 03 December 2014 - 01:53 PM

About Carbs; They are all an imprecise device that mixesair and fuel. Lets start with air: air has a standard pressure of 29.92 mb, however that never happens, high and low pressure areas are constantly on the move, resulting in pressures from approimately 29.60 to 31.5 mb of pressure, but hold that thought, now our mighty rider leaves LA for the mountains, traveling far and wide and ends up in Wolf creek pass, or Leadville Co., about 11,000 and 10,000 feet, lets say the temp is up to 105F at those locations, so now in reality , the pressure altitude is about 13,500 ( referenceing the LOL critic's in UK, all number are approximate). If you are useing some 301's on your  K750,you are operating really  far out of the design range of the carbs. The air weight is about 1/3 of the sea level weight, while the fuel is still standard weight. The weight  and volume of air passing the venturi now must lift the fuel up to the venturi to mix it, and it is now all out of kilter. In aircraft you have the luxury of mixture controls, that lets you reduce the amount of fuel aidded to the air. In the 301's, you can lower the needle and the same thing will occur, you will start to match the fuel to air ratio. It is  some times a daunting task...... The things that change carb tuning are many, off the top of my head;
carb gaskets, do they fit...do they leak air?
air cleaners
exhaust pipe lenght
mufflers
are all passages  in the carbs clean?
have they been boogered?
is the float level properly adjusted?
is the ambient temp greatly above 70F or far below, like -20F? That looks like a 90 Deg differental to me.
Is the air dry, like desert air at 4% humidity, or 99% humidity at 35 deg F?
All carbs mentioned above, were designed by carb designers, most are pretty good or they would not have survived to this point in time. The 301 was designed in WW ll, with alife expectancy of maybe a couple of weeks.

All Carbs will wear, from dust, water, corrosion,acids boil off , of a nearby battery, ( Yup I saw that happen)
and plain old getting beat up. There is an urge to tune up a bike by adjusting the carbs, and or cleaning them up with out the proper tools and or knowledge. Google is full of info , look it up! first, it will save you time and money
Then think about all the above and how it impacts your bike and its operation.....Happy riding   Ken :smileywaving:

I am sure there is other opinions out there, have at it

#8 Mike Goldthorpe

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Posted 03 December 2014 - 08:09 PM

I am going to use the Chinese carbs on my M66.  Mostly because they were $40 for the pair (I was in Shanghai at the time so could pick them up).  They also really look like the Bings on my R50 :-)
I do, however, have later heads on my bike so I do the need the adapter conversion baloney.  However, I am hoping the adapter acts as a heat sink so I am hoping they turn out to be baloney with mustard and mayo ;-)

#9 lobofuego

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Posted 05 December 2014 - 07:59 PM

Noticed one day...that the smaller of the SU carbs have an inlet tract at about the same as a Ural...fittings which almost fit, and are aligned on a verticle plane, which means they could be used on older urals...Just sayin!...

#10 ivar

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Posted 26 December 2014 - 06:28 AM

View Postracepres, on 02 December 2014 - 10:40 AM, said:

I feel that a Flat slide is superior to a Round one...Hence Mikuni waiting for the Lectron Design patent to expire, and then going from round to square!!
Having said that...the round slide Mikuni is hard to beat..for the $$
I'll keep my K-65's thank you...with the walboro needle, and a strategic drop of lictite here and there...They are working well enough for me!

was not aware of that patent - Mikuni has been making flatslides for years (TM / TMX carbs) - are these under licence from Lectron then?  

Have been messing with flatslides on my 2-stroke snowmobiles for the last 15 years.  The TM's work great and doesnt require much tuning at all once I've fount the correct setup.  But there's one drawback with them compared to VM roundslides - air leaking around the carb slide, causing an erratic idle at times.

I'm thinking VM carbs should be easy to find cheap from a snowmobile wrecker, only I suspect the most common size , 34mm is a bit too big for a 650 Ural/Dnepr?

and one thing in favor of the VM's , there's a plethora of tuning components available for them
Dnepr MT-16

#11 TassieChris

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Posted 27 December 2014 - 02:56 AM

Another vote for the chinese PZ28D's - I have them on my M72 and have had no issues - Once they are tuned in run like clockwork and seem to stay in tune just fine.
Plus as Mike said cheap as chips and easy to get hold of.

#12 ivar

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Posted 27 December 2014 - 05:57 AM

does the pz carbs share tuning parts with other common carbs?  (e.g. mikuni?)   did a quick ebay search, looks like the pz30's are easier to find
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#13 racepres

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Posted 30 December 2014 - 09:59 AM

View Postivar, on 26 December 2014 - 06:28 AM, said:

View Postracepres, on 02 December 2014 - 10:40 AM, said:

I feel that a Flat slide is superior to a Round one...Hence Mikuni waiting for the Lectron Design patent to expire, and then going from round to square!!
Having said that...the round slide Mikuni is hard to beat..for the $$
I'll keep my K-65's thank you...with the walboro needle, and a strategic drop of lictite here and there...They are working well enough for me!

was not aware of that patent - Mikuni has been making flatslides for years (TM / TMX carbs) - are these under licence from Lectron then?  

Have been messing with flatslides on my 2-stroke snowmobiles for the last 15 years.  The TM's work great and doesnt require much tuning at all once I've fount the correct setup.  But there's one drawback with them compared to VM roundslides - air leaking around the carb slide, causing an erratic idle at times.

I'm thinking VM carbs should be easy to find cheap from a snowmobile wrecker, only I suspect the most common size , 34mm is a bit too big for a 650 Ural/Dnepr?

and one thing in favor of the VM's , there's a plethora of tuning components available for them

The patent expired long ago...no licensing required..especially if you happen to be in a Foreign country...Look at the outrageous infringements by some Chinese companies!!!
Anyway 15 years is Not a long time in the scheme of things...I ran a Lectron more than 25 years ago!!! When all Mikunis were round, unless they also were unscrupulous!!!
Yes...VM's are seemingly everywhere...I should think that a set off of a 350 or so twin would be perfect...
But, I'll keep my K-65's thanks...Just won't look like a Dnepr with a Diaphram on top of my carbs!!!

#14 Serious Black

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Posted 07 January 2015 - 04:08 PM

I fitted 2x mikuni VM24 carbs bought from India meant for a 350 Enfield Bullet to my Chang Jiang - more or less the same motor. I will eventual get round to going down a jet size but for the past 3 years they have performed better than ANY other carb I've tried, PZ24, PZ28 or Chinese Keihin PK28 copies. They are about £25 each and well worth it. You will also need the rubber mounts which you can rotate 90 degrees and use a bit of creativity to rig up a hose to the filter. Oh and you can throw away the strangler choke..

....Oh yeah, the fuel consumption is really good!
The devil is in the detail.

#15 ivar

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Posted 11 January 2015 - 10:40 AM

these should work?
http://www.ebay.com/...2aa5ba1&vxp=mtr
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