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Cleaning carbs FZ750

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A100man
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Joined: 19 Aug 2013
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PostPosted: 23:56 - 07 Jan 2020    Post subject: Cleaning carbs FZ750 Reply with quote
Hi all

Trying to eliminate a a mid-range stumble (about 6k rpm) on the FZ so I've got the carbs off for a clean.> All pretty fair inside to be honest but I have a question about the pilot air jet(s) on the intake mouths.. I have no idea where they lead to. will a quick blast with the compressor sort them? I'd like to know they're not blocked although I doubt its the cause of the stumble..

Cheers
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Easy-X
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Joined: 08 Mar 2019
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PostPosted: 01:44 - 08 Jan 2020    Post subject: Reply with quote
I would have thought air only sorts loose debris. Any reason not to give 'em a squirt with decent carb cleaner?

This something that's built up over time or just suddenly happened?
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jaffa90
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PostPosted: 02:19 - 08 Jan 2020    Post subject: Reply with quote
FZ 750!!!! Adam had one of them?
It`s a simple engine with only 20 valves and the carbs having more pilot feeds than others.
Pilot jets work mostly up to 3,000rpm so what about the ignition module / timing acting daft at 6,000rpm?
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Nobby the Bastard
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Joined: 16 Aug 2013
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PostPosted: 09:21 - 08 Jan 2020    Post subject: Reply with quote
Does it have the original unmolested airbox? If not put one in.

They're renowned for being fuckers with a midrange stumble without one.
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A100man
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PostPosted: 10:43 - 08 Jan 2020    Post subject: Reply with quote
Nobby the Bastard wrote:
Does it have the original unmolested airbox? If not put one in.

They're renowned for being fuckers with a midrange stumble without one.


Yes it does, although I made some silicone hose connectors to replace the dried up old rubbers..
I think it started when I ran the tank down well into reserve, that said I also had a spot of bother in the summer when #1 plug cap gave up the ghost (after 2 days outside in rain and damp) - forcing it on to 2 cylinders. The strange thing is it will rev through it which 'feels' to me more like a fuelling issue..
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Fizzer Thou
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PostPosted: 13:04 - 08 Jan 2020    Post subject: Re: Cleaning carbs FZ750 Reply with quote
A100man wrote:
Trying to eliminate a a mid-range stumble (about 6k rpm) on the FZ so I've got the carbs off for a clean.> All pretty fair inside to be honest but I have a question about the pilot air jet(s) on the intake mouths.. I have no idea where they lead to. will a quick blast with the compressor sort them? I'd like to know they're not blocked although I doubt its the cause of the stumble.


My Exup-R had a problem on the way home from a breakfast ride to the coast a few years ago.It would go onto three cylinders then chime back onto all four when I was least expecting it.Once home I tried some resonably good spark plugs,then removed the float chambers and checked all of the jets.In the end I rechecked the original plug caps that I had checked over the Winter layoff.I found that one of the internal resistive elements had gone open circuit so I replaced all four with suitable lengths of brazing rod.This cured the problem until such time as one of the plug caps split along the length of the cap which opened up when the engine got hot.This was cured by replacing all four caps with new NGK items.

As for the air jets,I use carb cleaner to check that they are clear.When applied via the intake to the carb then the cleaner 'should' exit into where the jet holder bolts into place.If you have not already done so I would suggest checking the emulsion tubes for elongation as they are a common fault where the needle has worn the tube.They are available from the likes of Allens Performance,which is where I bought several carb parts for my Exup-R Thumbs Up
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A100man
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Joined: 19 Aug 2013
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PostPosted: 13:15 - 08 Jan 2020    Post subject: Re: Cleaning carbs FZ750 Reply with quote
Fizzer Thou wrote:


My Exup-R had a problem on the way home from a breakfast ride to the coast a few years ago.It would go onto three cylinders then chime back onto all four when I was least expecting it.Once home I tried some resonably good spark plugs,then removed the float chambers and checked all of the jets.In the end I rechecked the original plug caps that I had checked over the Winter layoff.I found that one of the internal resistive elements had gone open circuit so I replaced all four with suitable lengths of brazing rod.This cured the problem until such time as one of the plug caps split along the length of the cap which opened up when the engine got hot.This was cured by replacing all four caps with new NGK items.

As for the air jets,I use carb cleaner to check that they are clear.When applied via the intake to the carb then the cleaner 'should' exit into where the jet holder bolts into place.If you have not already done so I would suggest checking the emulsion tubes for elongation as they are a common fault where the needle has worn the tube.They are available from the likes of Allens Performance,which is where I bought several carb parts for my Exup-R Thumbs Up


Thanks for the comprehensive reply.. Indeed I replaced #1 plug cap with an 'red' NGK long reach item. I tried to dismantle and clean the guts of the OEM items but they have a tiny 'shim' between the spring and the resistor that corrodes. You can try and clean it but it will fly off into the darkest recess of the garage never to be found again!

When you say 'jet holder' would that be pilot jet, main jet or the domed nut thingy that holds in the needle jet?

Cheers

J
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600
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Fizzer Thou
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Joined: 06 Aug 2011
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PostPosted: 11:16 - 09 Jan 2020    Post subject: Reply with quote
There is no reason to believe that the carbs on the FZ750 are that much different to those on my Exup-R so the domed nut holds the jet holder into place on the carb body.This gives access to the strangely shaped seal between the two and also allows the needle jet to be extracted from the top of the carb body for inspection and cleaning.The seals are not available seperately from Yamaha but I bought my new ones from Allens Performance as my originals had become affected by petrol and shrunk to a degree.

As for the tiny shim or washer inside of the OEM plug cap,I left those out when I originally cleaned the slight corrosion on the internal components.
Years ago I fitted Dyna high output coils and this makes for better throttle response when used with better quality spark plugs.I use D8EVX plugs and the colour of the plug is up there with that which is of the ideal fuel/air mixture.

What is the state of your plugs?How many miles on them?
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A100man
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Joined: 19 Aug 2013
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PostPosted: 15:20 - 09 Jan 2020    Post subject: Reply with quote
Fizzer Thou wrote:
There is no reason to believe that the carbs on the FZ750 are that much different to those on my Exup-R so the domed nut holds the jet holder into place on the carb body.This gives access to the strangely shaped seal..


Strangely shaped seal? Hmm not spotted that baby. I'll take a pic later and post what my carbs look like..

Fizzer Thou wrote:

What is the state of your plugs?How many miles on them?


Ah, cough, yes. A plug swap is on my to do list.. Embarassed
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