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Sparkplugs 1 month 2 weeks ago #30176

  • dcarver220b
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Kawboy, those 3 transition ports kicked my ass! Took forever to find them...

"The Low-speed Circuit only functions between Idle and around 2,000 RPM. The Main circuit operates between 1,500 RPM and top end, so there’s a cross over point between the 2 circuits."

To clarify my limited carb knowledge, not a challenge, but on other Japanese carbs I've worked on, there's also a transition from the needle exiting the needle jet, where needle diameter and taper, height in the jet, and the jet provide a another 'transition' plateau? Idle circuit, mid range (needle and jet), and main.

Just wondering, honest question.. Never really fully learned carb theory. Need to mentor under my bud Mark who is freaking genius! 

 

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Last edit: by dcarver220b.

Sparkplugs 1 month 2 weeks ago #30179

  • Kawboy
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Kawboy, those 3 transition ports kicked my ass! Took forever to find them...

"The Low-speed Circuit only functions between Idle and around 2,000 RPM. The Main circuit operates between 1,500 RPM and top end, so there’s a cross over point between the 2 circuits."

To clarify my limited carb knowledge, not a challenge, but on other Japanese carbs I've worked on, there's also a transition from the needle exiting the needle jet, where needle diameter and taper, height in the jet, and the jet provide a another 'transition' plateau? Idle circuit, mid range (needle and jet), and main.

Just wondering, honest question.. Never really fully learned carb theory. Need to mentor under my bud Mark who is freaking genius! 
 

To your question- The low speed circuit only functions from idle to around 2,000 rpm because there's not enough velocity in the throat of the carb to draw fuel up from the main jet. At around 1500 rpm, the velocity of air in the throat will be enough to initiate the flow of fuel in the main and as the rpm increases and the airflow increases, the fuel demand "transitions" from the Low Speed Circuit to the Main Circuit.
What your referring to regarding the tapered pin in the Main Circuit and its position to the Main Jet alters the flow of fuel coming through the Main Jet. In a constant velocity carb where a diaphragm is used to raise the throttle piston, the flow of air is always at a constant ratio to the flow of fuel from the Main Jet. In carbs where the throttle piston is raised/lowered by a mechanical action like throttle cables or a linkage set up, it is possible to crack open the throttle and have full flow of fuel through the Main Jet and due to the rpm of the engine, the airflow may not be at full flow. so there would be a sort of enrichening until the airflow/fuel flow balanced out.
It's an arguable point because the fuel flow coming from the Main Jet is a result of the airflow through the throttle throat, but I'm sure you can see that , for example, if you're cruising down the road at 3500 rpm and then decide to pass a car, on a mechanically operated throttle piston, your action to open the throttle pulls the throttle piston up which raises the needle in the jet, but the airflow going through the carb hasn't changed until the rpm changed, so more fuel flows than air until you stop accelerating. In a CV carb (constant velocity) as you open the butterfly plates, you're allowing more air to pass through the throat but the throttle piston only rises proportionately to the airflow going through the throat so the throttle piston needle only rises proportionately to the airflow so the fuel to air ratio remains constant.
Fascinating science especially when you start getting in to 4 barrel carbs with accelerator pumps, or flat slide carbs , or diaphragm carbs which are mechanical fuel injection carbs so to speak. I'll take open loop fuel injection with a programmable ECU any day over carbs because you can have complete and total control over the fuel under any and all conditions.

 

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Last edit: by Kawboy.

Sparkplugs 1 month 2 weeks ago #30180

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The reasoning behind going one grade hotter on the plugs is the different formulation of fuel now in the UK (not sure if your fuel is the same in the US?). Yes the plug grade was recommended when the 1300 was released but fuel has changed (at least here) in the meantime. It's common practice among 1300 owners in the UK to go to a grade hotter plug (with everything else healthy & in tune) with noticeable improvements in running & no reports of detonation or signs of plug overheating. I've had to tweak the fuel injection on my Z1300 too because of modern fuels, it ran too rich. 
Only dead fish go with the flow

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Sparkplugs 1 month 2 weeks ago #30181

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The reasoning behind going one grade hotter on the plugs is the different formulation of fuel now in the UK (not sure if your fuel is the same in the US?). Yes the plug grade was recommended when the 1300 was released but fuel has changed (at least here) in the meantime. It's common practice among 1300 owners in the UK to go to a grade hotter plug (with everything else healthy & in tune) with noticeable improvements in running & no reports of detonation or signs of plug overheating. I've had to tweak the fuel injection on my Z1300 too because of modern fuels, it ran too rich. 

Interesting points Phil. I change the spark plugs on our vehicles every 50K and they always come out snow white. On a fuel injected vehicle with O2 feedback and knock sensors, I'm good with that. What this tells me is that with 9:1 compression, 100C thermostats, it's possible to run lean enough for the spark plugs to run white hot. So maybe the gasoline standards between here in Canada and Europe are different.

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Sparkplugs 1 month 2 weeks ago #30186

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The  point to my suggestion is that every time you back off the throttle - especially from higher RPM's  - the higher vacuum created in the intake manifold closes off a metered portion of "Mixing-Air" supplied by the enricher system.... proportional to the level of vacuum..  Less mixing-air.....richer fuel mixture. Under normal riding the enricher spring keeps the piston open and the correct proportion of air is maintained. 
So in a simple and relatively easy process of elimination of helping determine the cause of the sooty plugs, I make the suggestion. 
I did this two years ago and haven't had any backfiring and got a (albeit modest) but otherwise noticeable and recordable increase in K.P.L. (M.P.G.)  

 
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !

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Last edit: by scotch.

Sparkplugs 1 month 2 weeks ago #30187

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For what it's worth.......  I.M.H.O.:  I doubt changing out spark-plugs will have any lasting affect.
Instead:  Remove the diaphragm/piston from the enrichers (3) and make a solid gasket for under the cover.  That will help stop the enriching system from sooting the plugs.  This system is redundant anyway if like most  - you've disabled the  "Clean Air System" - typically by blocking-off the reed-valves and disconnecting the "Vacuum Switch valve".  Page #180 in my manual explains how this system was "supposed" to work.


 

Thanks Scotch,
I agree, the whole enricher system is unneeded. I did much the same thing some years ago while I was pulling my hair out trying to figure out how to lean it out. I undid it by identifying the opening in the carb throat and plugging it with JB Weld. I recently used your carb tool so I may have blasted them clean. I took care to clean in the inside of that port before I plugged it, so unless the JB Weld just dissolved, they're likely still plugged.
Your idea about a solid gasket sounds like a better idea.
Bill
1947 Indian Chief, 1968 BSA Shooting Star, 1970 BSA 650 Lightning, 1974 Kawasaki W3, 1976 KZ900 A4, 1979 KZ750 B4, 1979 KZ750 B4 Trike, 1980 KZ550, 1981 KZ1300, 1982 KZ1100 Spectre, 1985 Kawasaki ZN1300, 1987 Yamaha Trail Way, 2000 Honda Valkyrie Tourer, 1981 GL 1100, 2009 Yamaha RoadLiner S

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