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Idiots guide to setting up carbs
Posted: Sat Jun 02, 2018 8:33 pm
Author: oceantools
Evening All
A 'newby' here - so kindly be gentle! I always wanted a Z900 from when I was a nipper and, finally, I bought one a couple of months ago at the tender age of 54. It's a lovely looking bike - a 1976 KZ900-A4 that has been nicely restored. The only minor (perhaps major!) issue is the carbs. I won't bore you all with the issues (bit tricky to start; cuts out on the overrun; sooty etc) other than to ask this: Does anybody have a fairly definitive guide to setting up the carbs they could send me, please?
I'll try to put some pics on the forum when I work out how.
Many thanks.
Re: Idiots guide to setting up carbs
Posted: Sat Jun 02, 2018 9:16 pm
Author: warren3200gt
Welcome to the fold. Is the bike standard with 4 into 4 exhaust and stock airbox? Do you know which carbs you have? Are they just airscrew, just fuel screw, or both? Assume they are Mikuni vm 26? Close up pics of the carbs and details of exhaust and air filter type will help us give you the correct info.
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 9:42 am
Author: oceantools
Hi there. Thanks for getting back to me. As far as I'm aware she's pretty bog standard. It's a 4:4 exhaust, stock airbox and (I think) standard 26mm Mikuni carbs. The thing that is most obvious with the issue is that if I gently open the throttle she almost cuts out. Give it a big handful is no problem but that's not great on, say, a roundabout.
I've ordered a K&N replacement air filter. Any experience of those?
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 9:56 am
Author: ZedHead
Are you sure it is the carbs? Lots of time and money is spent tinkering with carbs only to find out that there is something else going on.
No amount of carb afjustment will solve a low compression issue, so it is worth eliminating this possibility sooner, rather than later.
One of the most common causes of sooty plugs and poor running on these old bikes is low voltage to the coils. Read about the famous "wired george" relay cure.
https://www.wgcarbs.com/index.php/using ... s/89-coilsIf after all this, you still can't set the carbs up as per the manual. You probably need to fully overhaul them, which means a thorough clean, ideally ultrasonically, and fitting good quality carb kits. Common fueling problems are blocked pilot jets, worn needles/needle jets, wrong float bowl fuel height, sticking choke plungers and not forgetting the obvious clogged air filter.
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 4:15 pm
Author: oceantools
That makes for interesting reading - thanks for the input. I'll buy a compression tester and check the compression. The air filter is slightly grubby so I'll see how the K&N works out.
The Dyna S upgrade is quite interesting. I did something similar to my 1964 Volvo P1800S a couple fo years ago - made quite a difference.
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 4:37 pm
Author: Pigford
If the pipes are "repro" - they are restrictive and can upset mixture.
A common problem with std A4's is a stumble just off throttle...... but check spark/compression/filter first.
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 6:43 pm
Author: Simonic
ZedHead wrote:Are you sure it is the carbs? Lots of time and money is spent tinkering with carbs only to find out that there is something else going on.
No amount of carb afjustment will solve a low compression issue, so it is worth eliminating this possibility sooner, rather than later.
One of the most common causes of sooty plugs and poor running on these old bikes is low voltage to the coils. Read about the famous "wired george" relay cure.
https://www.wgcarbs.com/index.php/using ... s/89-coils.
This 'wired george' page is very interesting. Is this a common mod ?
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 7:12 pm
Author: warren3200gt
Common, effective and simple.
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 8:09 pm
Author: Taffus
Simonic wrote:ZedHead wrote:Are you sure it is the carbs? Lots of time and money is spent tinkering with carbs only to find out that there is something else going on.
No amount of carb afjustment will solve a low compression issue, so it is worth eliminating this possibility sooner, rather than later.
One of the most common causes of sooty plugs and poor running on these old bikes is low voltage to the coils. Read about the famous "wired george" relay cure.
https://www.wgcarbs.com/index.php/using ... s/89-coils.
This 'wired george' page is very interesting. Is this a common mod ?
Oh yes!
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 9:53 pm
Author: Zedorset
Taffus wrote:Simonic wrote:ZedHead wrote:Are you sure it is the carbs? Lots of time and money is spent tinkering with carbs only to find out that there is something else going on.
No amount of carb afjustment will solve a low compression issue, so it is worth eliminating this possibility sooner, rather than later.
One of the most common causes of sooty plugs and poor running on these old bikes is low voltage to the coils. Read about the famous "wired george" relay cure.
https://www.wgcarbs.com/index.php/using ... s/89-coils.
It worked for me
This 'wired george' page is very interesting. Is this a common mod ?
Oh yes!
Re: Idiots guide to setting up carbs
Posted: Sun Jun 03, 2018 11:16 pm
Author: chrisNI
What’s in the carbs at the moment? With a standard setup the first place to start is with the stock jets and needle settings, then checking float levels and balance. If all that’s where it should be and it’s still off then as has been said there are other areas to check. Including not having a layer of crud round the fuel tap. They run best when everything’s right
Re: Idiots guide to setting up carbs
Posted: Mon Jun 04, 2018 7:10 am
Author: kev edwards
I am not a fan of the weird George mod as it just bypass's an already existing problem, all electrical connections should be clean and have good flow. Have never used the mod, just kept harness's working right.
Re: Idiots guide to setting up carbs
Posted: Mon Jun 04, 2018 9:04 am
Author: ZedHead
kev edwards wrote:I am not a fan of the weird George mod as it just bypass's an already existing problem, all electrical connections should be clean and have good flow. Have never used the mod, just kept harness's working right.
Of course, in principle you are completely right and with new loom and switch gear the mod may not be needed, although electronic ignition, if fitted, does create additional load that could expose weak spots in electrical components that were originally designed for points, melted fuse holders are a fairly common example of how old design can fail.
Low voltage to the coils is known to cause the sort of running issues that oceantools describes. A quick measument for good 12v at the coils should eliminate any suspicions here.
Re: Idiots guide to setting up carbs
Posted: Wed Jun 06, 2018 6:28 am
Author: oceantools
Thanks for the various bits of advice so far. With a little bit of judicious fiddling of the air mixture screws the bike is actually starting, ticking over & running quite nicely. The plugs are a bit sootier than I'd like them to be but perhaps that's a result of riding the last couple of miles home through the stop/start of the city? I'll take her for a thrash on open roads and whip the plugs out when I get chance.
What is the general opinion on electronic ignition? Worth doing?
Re: Idiots guide to setting up carbs
Posted: Wed Jun 06, 2018 7:55 am
Author: kev edwards
Dyna s electronic ignition replaces the old points plate, plug and play, around £150, works fine with stock coils, done mine as a matter of course.