Weakened mixture on new Monobloc

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56G80S
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Weakened mixture on new Monobloc

Post by 56G80S »

Just dropped the needle (D) from third from top to second from top groove on the new Monobloc on the rebuilt engine as the plug colour darker than I was used to with the worn out Concentric.

Checked the timing first.

Way I see it, if it's too weak I can make it richer temporarily with a touch of choke. As it was at indicated 65 mph putting the choke on or off made no difference to response at all.

No work tomorrow so may be out after Linda goes to work!

It's been suggested that the modern fuels mean plug colour not the indicator it was but I'll see what a plug chop shows. Also suggested that rich side better than weak and I certainly don't want to damage the "new" (1k) piston.

Johnny B
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Rob Harknett
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Re: Weakened mixture on new Monobloc

Post by Rob Harknett »

Getting this problem with a new carb asked & replied to over & over again. Usual questions. Is the carb jetted correct for your bike. Did you state which make year model you wanted ther carb for. I believe the club sends a carb with correct jets etc. Did you get a pack of jets etc with the carb.
I would not judge the plug as being correct. The old bikes sump oil, so you probably start almost oiling up the plug.
Mick D
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Re: Weakened mixture on new Monobloc

Post by Mick D »

Hi

There's some good general info here:

http://amalcarb.co.uk/ethanol-fuels

Note, their advice that adding ethanol to fuel weakens the air fuel mixture.

And I would agree, a slightly rich mixture is preferable to a slightly weak one

Regards Mick
56G80S
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Re: Weakened mixture on new Monobloc

Post by 56G80S »

Rob - Bought through Club all jetted etc as standard. I did notice that the spacer/heat block between the carb and head is is a different, slightly smaller, bore than the carb and head inlet! Been like that since I had the Matchless so that'll make a difference. The oil was changed on rebuild and then again at 900 miles. No air filter.

Mick - Yes, that's why I posted, it seems to go in the opposite direction to others experience.

Went for a run this morning. All good up to indicated 75mph. Putting choke on and off seemed to make no difference I could notice except maybe a slight reduction in performance at open throttle. Took a photo of plug chop and will post as an edit. Or more probably put it into the "Photos" .

Added into Photos section Parts Pictures - Post War singles - Monobloc 389 56G80S 3rd notch down and same but 2nd notch down. http://www.jampot.com/pic_showall.asp?c ... ar+singles but it'll be a while until they show.

These are the only pics I have and they don't really look very much different! All I can say is that I didn't take a picture of the plug between these two pics and it looked darker then. It's the same plug throughout. The only changes are that before checking the timing I found the points had closed up a bit and re-gapped them; of course reduced gap equals retarded ignition.

Ho Hum. Plug looks OK in both circumstances but the Matchless was more responsive today but maybe that's because the timing is, for the moment, as good as I can get it and I do use the full 1/2" BTDC.

Close this off really. I'll keep a check and maybe raise the needle back up if I see a problem. Maybe put the air filter on as that'll richen it a bit! I'll soon find out on the trip up to Lochearnhead later this month.

Johnny B
Mick D
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Re: Weakened mixture on new Monobloc

Post by Mick D »

Hi

Plug looks better on third notch down - but I wouldn't get too anal about it.

Regards Mick
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Ozmadman
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Re: Weakened mixture on new Monobloc

Post by Ozmadman »

I wouldn't run without an air filter on any bike despite what it says in the manual especially if you want to protect your (1K) piston. I even made a replica one for my lightweight
Paul
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Rob Harknett
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Re: Weakened mixture on new Monobloc

Post by Rob Harknett »

I have never had an air filter on a bike, except standard filters on 2 strokes. I do not use a bike in a desert. Although 5 miles from me is a cement factory, the air is full of dust. I do not think I would keep fitting a new plug just to check its colour. I would wait until the bike showed signs the plug really does need attention. Or have you been cleaning the plug ? Which for about the last 45 years have not had the central electrodes glazed. They are through away items now. ( check plug care and when to replace for your car for todays plugs ) Cleaning todays non glazed central electrode plugs will damage them. This will cause them to quickly contaminate and fail. They will not give a true colour reading if cleaned and damaged.
56G80S
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Re: Weakened mixture on new Monobloc

Post by 56G80S »

Mick - I thought it was a bit marginal too.
Rob - I cleaned that one about 2,000 or more miles ago. Meths and a toothbrush, nothing abrasive. Rarely used an air filter though.

I think we're done with this one.

Johnny B
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