solex - poor mpg

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MARK.WALSH
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solex - poor mpg

Postby MARK.WALSH » Sat Jan 12, 2019 7:54 pm

I took the square 4 for a blast on new years day covering over 70 miles at speeds between 50 and 70mph
On the way home I had to hit reserve despite having a good few gallons above the reserve line prior to the journey. I estimate I was getting around 35MPG or so.
No leaks - not an overly rich mixture and nothing daft like brakes binding and so on.
I know the solex carbs aren't great but would those mpg figures be acceptable?

Cheers
Mark

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Re: solex - poor mpg

Postby Bob.Murphy » Sun Jan 13, 2019 11:13 am

That seems a bit low to me.

It was a long time ago but when I ran my '53 Square with its MkII motor with a Solex Carb I was averaging 50mpg. I didn't hang about either (less traffic and no electronic surveillance in 1970 !).

On one occasion I 'chummed' a friend home to Somerset from the IOM Ferry in Liverpool - he was on a 1930s AJS, we went down through Wales and over the (solitary) Severn Bridge - I got 90mpg (but man, it was boring).

Once, in the Isle of Man, running two-up, I found it used a lot more fuel - slow twisty roads and a lot of hills. I guess it really depends on the type of terrain you're on.

On a straight main road without hold-ups I would expect 50mpg.

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Re: solex - poor mpg

Postby MARK.WALSH » Sun Jan 13, 2019 1:02 pm

Thanks Bob
I was hoping for 40+ mpg at least.
Not sure how these carbs respond to ultrasound cleaning and the like - wondered if that might help
Mark

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Re: solex - poor mpg

Postby paul.jameson » Sun Jan 13, 2019 8:33 pm

Like Bob, I think that feels a bit on the low side. I have most certainly been down to that sort of figure but with a sidecar fitted and someone in it. I do believe that the change to the SU carb was an attempt to improve mpg but the thing which seems to affect mpg most on a Square is compression ratio. Assuming that you have the 5.8 to 1 pistons which are standard for a 4G or MK I, I don't think you will ever achieve high mpg. But you will have a much smoother, more flexible engine which to me is worth the poorer mpg.
I suspect the only useful thing you can do is to check the usual suspects such as ignition and valve timing and distributor wear.
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simon.holyfield
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Re: solex - poor mpg

Postby simon.holyfield » Sun Jan 13, 2019 10:16 pm

Mark, I wouldn't bother with ultrasonic cleaning on the Solex unless you have a bath and it doesn't cost anything. They are very easy to clean manually.
cheers

Simes

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Re: solex - poor mpg

Postby nevhunter » Sun Jan 13, 2019 11:09 pm

Put your finger in the end of the exhaust pipe(s) and feel how much soft carbon is there. (not when it's still hot of course) It will be pretty evident if it's rich... Then check float height and jet sizes etc. if it's sooty. An individual "type" of carb is rarely the culprit on it's own because of it's particular design.
. A plain bearing 4 cyl engine has a lot more friction than a roller bearing single of half the capacity and extra fuel at higher speeds is not unusual for larger bikes.. Some types of oil ring have a high friction contribution. It all adds up.
When you close the throttle does it have a fair bit of engine braking ? Nev

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Re: solex - poor mpg

Postby MARK.WALSH » Sun Jan 13, 2019 11:17 pm

thanks for the replies.
I will have to check engine braking next time im out on it but I don't think it is overly noticeable.
One thing I must pay attention to is the float height though, as the contact point on the float to the shut off valve is a bit concave from wear. I had thought of applying a bit of solder to ease the dimple in it - might be part of the problem?

Mark

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Re: solex - poor mpg

Postby nevhunter » Mon Jan 14, 2019 12:20 am

IF you add weight it won't work well and will make it richer. Vibration can affect this function.( Higher frequency). The carby might be getting hot and venting a bit off or if bubbles form, the float sinks and allows more fuel in. Nev

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Re: solex - poor mpg

Postby john.whiting » Mon Jan 14, 2019 4:18 am

Its also worth checking that the starter circuit isnt leaking ,and its fully off.

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Re: solex - poor mpg

Postby simon.holyfield » Mon Jan 14, 2019 7:52 am

john.whiting wrote:Its also worth checking that the starter circuit isnt leaking ,and its fully off.


Absolutely. The HT leads on my bike are very close to the bistarter and have prevented the mechanism returning to normal running position in the past. That area has had a lot of attention recently.

Do you need both bistarter positions on cold start?
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
http://ariel-square-four.blogspot.com


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