Author Topic: Synchronising Mikuni BS38 Carburettors  (Read 4509 times)

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Offline desperado

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Synchronising Mikuni BS38 Carburettors
« on: April 02, 2013, 02:30:51 PM »
Now I have finally got my XS500C up and running I would like to synchronise the carbís properly using a vacuum gauge.

The bike is fitted with Mikuni BS38 carbs which I have statically synchronised using a small drill bit to make sure both the butterflies are opening the same amount.

The ignition timing has been set (on both cylinders) the points, plugs, condenser and HT leads replaced.

The bike starts first time and ticks over smoothly no problem, however it doesnít seem to accelerate as smoothly as I would expect, so I feel the carbs are a little out of balance.

I have 3 workshop manuals (Yamaha, Haynes & Clymer) for this bike and none of them are much use when it comes to giving the location of the screws on the carburettor you need to remove to connect the vacuum gauge.

I have looked at the carbs and can only see a screw on one of the enrichment circuitís that fit the criteria, the other carb hasnít got one!

So my question is where do I connect the gauge too or do I need to make my own opening in the carb inlet rubber?

Desperado


Offline pauli

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #1 on: April 02, 2013, 02:31:20 PM »
Best regards,

pauli

1976 Yamaha XS 500 C, 2012 Ducati Diavel Carbon Red

Offline jimmer

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #2 on: April 02, 2013, 02:32:00 PM »
Now that I think about it,I put 25,000 miles on my street bike and never synched the carbs with a vacuum gauge.Shame on me.I just made sure the butterflies physically opened the same like you did.Guess what? It ran strong and smooth and saw 105 mph about 2000 times!


Offline desperado

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #3 on: April 02, 2013, 02:32:32 PM »
Thank you for that information Pauli,

I will have another look at the carbs, I had identified that screw on the one carb but couldn't see one on the other?

I will investigate further the weekend, I think I may have inadvertantly swopped the cold start plunger assemblies around when I had the carbs stripped down, as I think one of the screws is located there.

Desperado


Offline desperado

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #4 on: April 02, 2013, 02:33:02 PM »
I had a very interesting day yesterday working on the XS getting the carbís sorted.

I found I had inadvertently swapped the cold start plungers on the carburettors which meant the right hand carb didnít have the screw in which to connect the vacuum gauge.
With this rectified, float levels adjusted, and static calibration checked I proceeded to synchronise the carbís with a vacuum gauge once I had the bike up to temperature.
What surprised me was that the carb's were indeed out of sync even though I had gone to great pains to check that the butterflies opened together. I spent the next 15 minutes adjusting the balancing screw so the vacuum levels were the same, this entailed running the bike with the tank removed until the carbís ran dry, refit the tank fill the float chambers and repeat the process.

The bike seems a lot more responsive now although I feel it is running a little rich, I checked the needle height when the carbís were in bits and they were set to the middle slot, a friend of mine is lending me his colour tune kit so I can see what the combustion looks like before making any further adjustments.

Desperado


Offline guylr

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #5 on: April 02, 2013, 02:33:30 PM »
When vacuum synching carburetors I've found that it's important to get them as close as possible between idle and off idle. So synch them at idle then check the synch at speeds above idle like 2000, 3000 and 4000 rpm. You may find that the synch goes off when above idle and if it does by a lot then you may experience surging or misfire in mid-range running. If that's the case try to split the difference between idle and off idle sync so that the idle quality is good but mid throttle running is also good. This may cause both speeds to be a little off from a perfect synch but overall the bike will be happy.

For extended running time during the synching operation where the tank has to be removed shops use a small umbilical tank to gravity feed the carbs.

Guy

Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"

Offline desperado

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #6 on: April 02, 2013, 02:33:56 PM »
I did set them first at tickover (1200rpm) and then again at 3000rpm.
I must admit I was a little disappointed as vacuum at tickover was only hovering around 140mm hg, I suppose I was expecting to much for it to be 200mm hg for a 36 year old bike which has done 21K miles!.

Desperado


Offline Muster

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #7 on: April 13, 2016, 07:28:27 AM »
Im attempting to sync the same carburetors with a homemade  manometer. But what do I need to put in the vacuum hole in the carburetors, the screw that connects the hoses? What's it called and where can I buy one?

Offline Joffa1

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #8 on: March 08, 2020, 08:22:27 AM »
I am in same position... I believe you need an M6 spigot that will thread into the carb and then the balancer hose slips over that.

Can someone confirm exactly what we need and possibly a link to where to buy them?

Thanks so much!

Offline Joffa1

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #9 on: March 08, 2020, 11:21:53 PM »
Could I use something like this and just some little band clamps to fix my tubing to the female end and screw the male end into the carb body?

https://alemlube.com.au/web/product/product_detail/0e578e20-f8c9-4798-8a17-3146853cc313

Offline Joffa1

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #10 on: March 09, 2020, 02:19:20 AM »
Or I guess a couple of these would be ideal?

https://www.z1enterprises.com/vacuum-carb-sync-adapter-kit-6mm.html

Really hoping to find something local though....

Offline Joffa1

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #11 on: March 09, 2020, 09:09:38 PM »
Surely someone else out there has made a DIY balancing setup?

Iíve made mine just need a cheap way to connect my tubes to the carb ports...

Offline lumpy252

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #12 on: March 10, 2020, 09:55:40 AM »
Hi there the 6mm connectors in the second picture you showed are the right ones. If you did buy a complete kit then the pipes and inline taps to the gauges would be included with the gauges themselves.

Offline Joffa1

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #13 on: March 10, 2020, 10:04:44 AM »
Thanks Lumpy they indeed seem to be what I need. The problem is they want like $50 in shipping! Need to find another alternative.

Offline Joffa1

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Re: Synchronising Mikuni BS38 Carburettors
« Reply #14 on: March 12, 2020, 08:54:21 PM »
Found some for much cheaper on eBay now for anyone in need.