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Discussion Starter #1
Hi there folks. Just have a few questions on cold run multi-cyl carbed bikes. Dirt bike-er here and a pretty solid garage fixer-upper. But...I'm a carb-tard. I ramble on and try to hit details so skip to the last paragraph if you want my real question.

A month ago I picked up a cheap 01 ZR7S in ok condition, been laid down hadn't ran in 5 years for 300 bucks. Bought a set of carb kits with new jets, petcock rebuild, spark plugs, oil/filter change-out, air filter and then other misc stuff. I just never learned carbs when I was younger, knew the basic function (jets, air flow, vac draws fuel, float bowl) but not how they actually worked, luckily the internet exists so it was pretty easy to figure out.

So I took the carbs off, rebuilt them one at a time. New stock size jets (35 and 108 if memory serves), emulsion needles, idle jets... I blasted carb cleaner through the key ports to make sure fuel was flowing in all necessary directions. Everything seems to work fine on the carbs. I checked the pressure of the float valves (hold 3-5 psi upside down with gravity on the valve), orings are holding, diaphragm valves slide nicely etc. Idle jet ~1.67 (1.5+a hex 'flat' lol) turns out (factory in the US is about 1.25 but service manual is 1.75). This bikes center carbs have slightly longer emulsion needles and I did put them in the proper locations (not the first time though, lol). I did balance the carbs a bit, probably could do a better job though.

I am at 450 ft elevation (140 or so in meters). Bike cold (40-50 F [5-10 C]), choke on, starts right up. Idles around 1000 rpm ok. If I give it throttle it sounds like it leans and will die if I give it much. If I remove choke it dies. If I let it idle, it will slowly raise to around 2000 (still choked) over 1-2 min as it warms up. When idle is above 2000 choked (cyl head is warm but wont burn your hand yet) I can remove the choke and idle will go down to about 800-1000. Maybe 800 and idle a tad rough. It responds sluggishly to throttle at low rpm (wont die like before unless maybe I went wide open) but above 3000 it runs pretty good. Completely warmed up it seems to run great at any rpm, throttle response, etc. The intake (carb to head) boots are aged and I was wondering if this was contributing to a lean cold condition. I sprayed the **** out of them with starter fluid while the engine was running to try and identify leaks, nothing showed up. Also tried propane with no indication of leaks. Thus far I am not inclined to spend the 120$ on intake boots/orings.

MY QUESTION. Like I said I never had to do much carb tuning in the past so I don't know their quirks well. My previous dirt bike was a 2011 carbed KTM 530. Starts choke on, 10 sec later choke off, bike was fine and completely rideable, never had to mess with it. But that was a single cyl bike. Maybe it was jetted better. My current is a EFI KTM which is just killer. I just don't know if these larger multi cyl carb bikes are a bit grumpy when cold run. Because mine is in its current configuration. It needs at least a minute of idling to be ridden, probably 2. At lower rpm with throttle its quite unhappy but like said at 3000+ its ok. I just want to know if this kind of behavior is normal. The bike does have a aftermarket exhaust (jardine, popular for its time), my dad pointed out it may need different jets to run the best overall which is a valid point. Any of you have any input? If you tell me this is pretty normal I will be content but if this sounds like symptoms of incorrect jetting I will probably get new jets in the near future.

Thanks, Kyle.
 

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You did not mention the floats level......factory manual would probably tell you floats should rise about 17 mm over carb body, carbs upside down, and then do a level check with a clear tube from the drains. Level should be about 1/2 mm over the seam on all carbs.
 

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Discussion Starter #3
Yeah that one of the few things I didnt actively check. When working on them I saw no visual discrepancies. I doubt the carbs had been touched much before me, if at all. The idle jet plugs were still in, anyhow. Last week I got some tubing that should work for level checks, so I can do that this week. Thanks.
 

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It sounds absolutely normal to me too. The fact that it fires up quite easily tells me that you can’t be far off with it.

Kwaks with carbs have always raced on full choke, it’s a fine balancing act with the button to get it ticking over not too fast, whilst not killing it completely.

Also, a big, heavy, air cooled motor is going to take a lot more warming up than a competition dirt bike single.
 

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Discussion Starter #6
Thanks all. That makes me feel a bit better. I need to try balancing the cylinders again but going to wait until the gas tank is a bit lower so less to drain (petcock needs cleaning).
 

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In 1978 motorcycle emission control started for motorcycles. The first attempts were special main jets, they could be identified by a different font on the size marking. Also the fuel or air screws though visible were glued at the factory setting. As emission standards tightened the fuel/air screws were covered by plugs. The bottom line is that mixtures became very lean and produced cold blooded engines. Start up was a balance of high revving and engine stalling. Throttle response until fully up to operating temp. was terrible.Fuel injection has eliminated these problems.
 
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