1984 Kawasaki KZ750 high speed issues.

[tatters]

I have a high speed running issue with a 1984 Kawasaki KZ750L3 (four cylinder), which has me stumped.

No matter what l cant get the bike to go past 80mph and if l am at full throttle for more than a few mins in top gear l start to loose power followed by rolling off the throttle and back on again the bike bogs down.

1-3rd gears rev all the way to redline but 4-5th the bike is struggling to make power. Fuel eco is also awful.

Cylinder leak down test was carried out on all pots are 2-4% (excellent result).

Carbs rebuilt with stock jetting/settings and float hight is at spec (checked with clear fuel pipe on bowl drains).

Carbs balanced.

Stock exhaust.

Stock gearing.

New ignition coils and spark plugs (electronic ignition).

New carb boots and running stock airbox and filter.

Valve clearances adjusted.
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Past:NRG50,AF1125(x2),NSR125RR,ZZR250,CX500,VFR400,KR1S,ZZR600(x2),CB400N,YZF1000(x2),KH125,Z200,FX400R,CBR954RR(x2)GPZ500S,GT550,VFR750F(x2),RD350N,XR650R,CBR600F,CB250,KDX250,YZF750R,CRM250,400EXC,KLR650,TTR600RE,DR350S,R100GSPD,RGV250,VMAX1200,DL650 Present:G650XC,C12,KZ750,1190ADV

[Nobby the Bastard]

Plug chop?

What intake? Please don't say pods...

Edit: Minutes you say? I'd raise float slightly.

Edit no2 : Carb icing?
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[stinkwheel]

[tatters]

[stinkwheel]

Getting sufficient fuel flow out of the tap?
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Easy-X]

[tatters]

Took it for a run with the fuel cap open also with the fuel tape set to prime and that made no difference. I also disconnect the fuel hose going to the carbs and set the tank to prime to allow fuel to flow into a container, flow was good not restricted at all.
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Past:NRG50,AF1125(x2),NSR125RR,ZZR250,CX500,VFR400,KR1S,ZZR600(x2),CB400N,YZF1000(x2),KH125,Z200,FX400R,CBR954RR(x2)GPZ500S,GT550,VFR750F(x2),RD350N,XR650R,CBR600F,CB250,KDX250,YZF750R,CRM250,400EXC,KLR650,TTR600RE,DR350S,R100GSPD,RGV250,VMAX1200,DL650 Present:G650XC,C12,KZ750,1190ADV

[UncleFester]

Running it on 97/99 and not some nasty old E10 mix?

Last time i had an issue like this it was on an old Metro which i'd lobbed a K&N filter on and the stock carb needle wasn't the right profile. A different profile needle resolved it.
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Module 1 and 2 passed - October 2014. Happy owner of a 2017 Z1000SX, ex owner of a YBR125 / CB500 / VFR800.

[Pete.]

Pull the carbs again and inspect the needles and emulsion tubes. Check the tapers on the neeldes are straight and the hole in the mulsion tubes aren't worn out of round.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[MCN]

Does it have an automatic choke?

Does the throttle cable open the carburettors to 100%

Are you getting ghey and a wee bit scared of the beast?


There's a detailed paedofile here:

https://www.dotheton.com/index.php?attachments/mikuni-bs34-idle-circuit-tuning-pdf.176741/
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[yen_powell]

Is the vacuum fuel tap opening fully. I had problems on my GPz750 (same basic bike).
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Blackmail is a nasty word........but not as nasty as phlegm!
XT1200Z and a DR350 in bits

[stinkwheel]

Did you inspect the carb diaphragms for pinholes/tears?

I know a mate of mine was having issues with his Z900 where the ethanol had attacked the seals on the needle valve tips. Cheap and easy fix to get modern ones with viton tips.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[tatters]

Running fresh 98 Ron fuel.

The carb internals look to have been replaced not too long ago (shiny brass and new diaphrams). jets and neddle parts numbers do match stock numbers so all is correct.

I am going to get a high quailty rebulit kit for the carbs and change everything over just incase.
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Past:NRG50,AF1125(x2),NSR125RR,ZZR250,CX500,VFR400,KR1S,ZZR600(x2),CB400N,YZF1000(x2),KH125,Z200,FX400R,CBR954RR(x2)GPZ500S,GT550,VFR750F(x2),RD350N,XR650R,CBR600F,CB250,KDX250,YZF750R,CRM250,400EXC,KLR650,TTR600RE,DR350S,R100GSPD,RGV250,VMAX1200,DL650 Present:G650XC,C12,KZ750,1190ADV

[Bhud]

Bad fuel economy plus bogging down at full throttle suggest it's a fueling issue. Choke isn't stuck halfway on, is it?

[tatters]

No issues with the choke (on the BS34 carbs its a enrichment circuit.), working fine.
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Past:NRG50,AF1125(x2),NSR125RR,ZZR250,CX500,VFR400,KR1S,ZZR600(x2),CB400N,YZF1000(x2),KH125,Z200,FX400R,CBR954RR(x2)GPZ500S,GT550,VFR750F(x2),RD350N,XR650R,CBR600F,CB250,KDX250,YZF750R,CRM250,400EXC,KLR650,TTR600RE,DR350S,R100GSPD,RGV250,VMAX1200,DL650 Present:G650XC,C12,KZ750,1190ADV

[Bhud]

Lack of fuel and excess of fuel can both cause bogging. 9 times of 10 here people nail the issue by trying one thing out and then another. If not, then I think the carbs have to come off again and you'll have to go through the whole thing. Maybe a float needle is jamming up occasionally, or maybe the bowls aren't all filling up fast enough because of a bit of dirt. Many possibilities.

[stinkwheel]

Has it ever run right since you owned it?

If not check your timing (ignition and valve). Not unheard of for them to spin up a flywheel. Also not unheard of for someone to ineptly fit an electronic ignition or camchain in the wrong position.

Thing is, in the UK if you bought a 750 and it wouldn't go over 80mph, anyone would immediately think there is a problem. But you're in the US right? Most harleys won't go much over 80mph and that's totally normal and Harleys are the best bikes in the world and twice as big as a 750. So 80phm out of a 750cc Japanese bike must be normal. It might have been running like that for a long time.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[tatters]

[tatters]

The other odd things was that when l first took the carbs apart the main jet sizes were two sizes too big.

I replaced theses with the correct size as per the manual. This was odd as the bike is 100% stock.
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Past:NRG50,AF1125(x2),NSR125RR,ZZR250,CX500,VFR400,KR1S,ZZR600(x2),CB400N,YZF1000(x2),KH125,Z200,FX400R,CBR954RR(x2)GPZ500S,GT550,VFR750F(x2),RD350N,XR650R,CBR600F,CB250,KDX250,YZF750R,CRM250,400EXC,KLR650,TTR600RE,DR350S,R100GSPD,RGV250,VMAX1200,DL650 Present:G650XC,C12,KZ750,1190ADV

[stinkwheel]

No weird California compliant emissions crap?

Your description all sounds like a bike either running rich, or running out of fuel.

Does it pull better if you ease off wide open throttle? How about if you put the choke on when it starts losing power?

Umm. Crankcase breather?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robby]

Last time I came across a similar problem on a UJM, removing the inline fuel filter and running a straight line from the tank to the carbs fixed it.

[bypass2]

are the vacuum pipes allright no slits or damage. not sure but does one vacuum pipe go to one carb the the other 3 carbs are blanked off with a rubber caps are caps damaged ?

[that_impulse_guy]

on the assumption were having running our of petrol symptoms...
what the above post says...
plus
float heights. after a minute or two it bogs down...not jets..not needle....float heights or fuel starvation

float heights were mentioned earlier, so lets focus on fuel starvation (the good ole blocked fuel cap/blocked fuel filter.)

we need to talk about BSFC...brake specific fuel consumption...4stroke motors are somewhere around 20-30% efficient...yours makes...57kw?
I stick some numbers in here https://www.omnicalculator.com/everyday-life/brake-specific-fuel-consumption#how-do-i-calculate-bsfc and make some assumptions and come out with about
4g/s of fuel needed...ballpark...give or take...add a bit here and there..lets not split hairs.

take fuel pipe after fuel filter, put in beaker ,open tap, count to 10, or 20 or 30..whatever...take measurement, do some maths (4g/second is NOT 4ml/second), maybe use a scale...and you can work out if that fuel filter is crap, or the replacement ebay fuel tap is restrictive, or the fuel hose is too teeny tiny.

i post this purely for the humour:-)

you said your jets were two sizes too big...but its not clear if this bike is new to you and this problem is after cleaning the carbs and going for your first ride on this bike new to you, or if its a fault thats developed...i dunno..
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Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R

[Pete.]

#i suppose you have the air filter properly cleaned and oiled? You might try a quick run with it removed just to eliminate that.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[A100man]

+ 1 for plug chop
could thr choke/enrichmenst circuit be sticking on?

My Yam 600 (BS30s) once bogged at anything over half throttle - I pulled the carbs and found one main jet sitting in the bottom of the float bowl...
The idiot that stripped the carbs hadn't done them all up properly
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600