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Weber 45MCHH Tuning/Rich Mixture

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Shelby1115 Avatar
Shelby1115 Robert Kaylor
SHELBY, NC, USA   USA
Finally going to address the rich mixture on my '78 B and realized unlike most who have the usual Weber 32/36 down draft carb. Mine has a Weber 45MCHH (WK065) so it tunes different. Ive looked through the forums and online and not had much luck finding any info on a good tuning process for this carb. I was able to locate a schematic to help familiarize myself with it and the carb. Does anybody have any good suggestions on how to go about tuning the carb since its only got the single idle mixture screw, so that I can hopefully resolve the rich mixture and have the car running good as well. Thx...

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EricR Eric Ritzman
Crossville, Tennessee, USA   USA
1978 MG MGB "Fionna"
I have the same carb, (now a temporary situation since I have a downdraft ready to install) they are difficult to tune, even using an AFR meter.
Is it rich all the time or just at cruise?

Shelby1115 Avatar
Shelby1115 Robert Kaylor
SHELBY, NC, USA   USA
Pretty much all the time. At idle you can definitely smell the mixture is rich from the exhaust. And the MPG definitely suffer.

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EricR Eric Ritzman
Crossville, Tennessee, USA   USA
1978 MG MGB "Fionna"
What jets are in it?
What's your idle speed?
Do you have an Air/Fuel Ratio Meter?
Have you checked your timing? They like a lot of advance.
How many turns out is the mixture screw set?


The tuning issues with that carb is the reason that I'm replacing mine with a dgv. Even using an AFR I've been unable to get mine tuned to what I
consider an acceptable condition.

GMB3 Avatar
GMB3 George Badger
Soquel, CA, USA   USA
1968 MG MGB "1QUICK B"
Check out this thread and email Keith for his carb settings.
http://www.mgexp.com/phorum/read.php?1,3569484



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GEO



Edited 1 time(s). Last edit at 2017-08-10 04:33 PM by GMB3.

Shelby1115 Avatar
Shelby1115 Robert Kaylor
SHELBY, NC, USA   USA
Not sure about the jets as the PO owner had a shop install the Carb for him

Idle speed is around 1000 rpm

Currently set at 6.5 turns to bottom running smoothly...Idle drops when turned in to around 4.5 to 5

Have not checked timing

Dont have a meter



Edited 1 time(s). Last edit at 2017-08-10 08:02 PM by Shelby1115.

EricR Eric Ritzman
Crossville, Tennessee, USA   USA
1978 MG MGB "Fionna"
That's the reason it's running so rich, your going to need to get that turned in to between 3 and 4 1/2 turns.
There is round cover on top of the carb with a single bolt in it, if you take that off you can unscrew the jets and take a look at the size.
If it hasn't been changed from the factory it will be a 45, even though that's too small you should still be able to get it to run ok.

Turn the mixture screw in and use the idle speed screw to compensate for the idle dropping. You'll need to check your timing with the idle
at 800 or below (to take out mechanical advance, you can set it back up when your finished) and the vacuum line disconnected from the distributor advance, (make sure you plug the manifold where you took the line off).
It will most likely need to be between 16 and 22 degrees for best performance. There's a lot of information here on setting the timing when using a weber, even though it isn't for the same model carb the procedure holds true.

The goal is to get the smoothest running engine at idle with the mixture screw set in as far as possible.
That carb was designed for a Harley Motorcycle and refitted to use in LBC's, it is at best an exercise in compromise.

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riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
Robert
A bit of reading that may serve ................

http://www.mgexp.com/phorum/read.php?1,729992

http://www.mgexp.com/phorum/read.php?1,1285847

x2 contact Keith, he has his carburetor dialed in fine and is happy

B



Check your ego Amigo!

Shelby1115 Avatar
Shelby1115 Robert Kaylor
SHELBY, NC, USA   USA
Ok before I go tinkering too much I want to make sure I'm speaking the same language.

Eric,

You reference a mixture screw which this Carb clearly has and idle speed which I'm not sure of. I've gone over the carb and the schematic multiple times. The only other adjustment screw that I'm finding is what's labeled as the "Throttle Stop Screw" on the schematic.

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EricR Eric Ritzman
Crossville, Tennessee, USA   USA
1978 MG MGB "Fionna"
That would be the one. Right side of the carb if you are standing at the driver's side fender.

Shelby1115 Avatar
Shelby1115 Robert Kaylor
SHELBY, NC, USA   USA
Got into the Carb this morning.

The jets are as follows:
Idle: F8 50
Main: 170

EricR Eric Ritzman
Crossville, Tennessee, USA   USA
1978 MG MGB "Fionna"
You should be able to make it run fine with that combination.

Shelby1115 Avatar
Shelby1115 Robert Kaylor
SHELBY, NC, USA   USA
Thanks for the help guys...I dove into it this morning and was able to get it tuned to around 4.0 turns and running pretty well. Took it out a couple times after for a couple short rides and did well. So I'm gonna give it a go for a few days and see how it runs and see how it does with the MPG to see if takes care of the rich mixture.

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