389 Monobloc Leaking

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Brian_Walker
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389 Monobloc Leaking

Postby Brian_Walker » Wed Jan 22, 2020 12:09 pm

Hi,
After finding my GP carb will no longer fit my VCH without getting rid of the Velocity stack I picked up a cheap 389/1 from ebay jetted for AJS single.
As the cover was a dodgy I set it up on a bit of timber bolted to my lathe faceplate with 3 cheesehead screws located to grip the cover holes and held it against hard up with a tailpost live centre to skim it up.
monocover.jpg


This solved the leaking cover but now the float needle and seat does not seal, I have removed the washer from under the seat fitting so it goes further into the float chamber and replaced the float needle (plastic one) float is brass.
It appears that to get a good seal the needle needs to be longer or the float contact point raised (most earlier floats have a tab that can be bent to suit but it appears this tab is soldered down so not adjustable.)
Are there longer needles or floats with adjustable tabs that can be used to solve this problem, or should I try and build up the tab with solder or something?
1951-52 VCH under restoration

Mick D
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Re: 389 Monobloc Leaking

Postby Mick D » Wed Jan 22, 2020 5:15 pm

Hi

Just a thought, but is the skimmed float chamber cover now restricting the float movement?

Regards Mick

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Brian_Walker
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Re: 389 Monobloc Leaking

Postby Brian_Walker » Wed Jan 22, 2020 11:12 pm

No unfortunately, it seem that with the float at its highest point the valve is only half closed, only 0.040" was removed from the cover and also the 389 has a small spacer between the float and the cover to prevent this.
Cheers Brian
1951-52 VCH under restoration

JohnnyBeckett
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Re: 389 Monobloc Leaking

Postby JohnnyBeckett » Thu Jan 23, 2020 10:06 pm

HI i think if you can not pack the needle valve down then build the float up like you say that is the best way you have got the right float and needle valve for that carb :!: :?:

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alan.moore
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Re: 389 Monobloc Leaking

Postby alan.moore » Thu Jan 23, 2020 10:21 pm

The valve being only half closed when the float is at its highest point does seem to point to a mismatch of float and needle
1939 VH Redhunter;1942 RN WNG;1951 Triumph 6T Thunderbird;1970 BSA B175 Bantam;1986 Yamaha SRX600 single
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nevhunter
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Re: 389 Monobloc Leaking

Postby nevhunter » Fri Jan 24, 2020 7:25 am

Brian, Monobloc's weren't much when they were new. The concentric is a much better thing to get right. Nev

JohnnyBeckett
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Re: 389 Monobloc Leaking

Postby JohnnyBeckett » Fri Jan 24, 2020 10:20 am

you are right nev i use concentric carbs and i have had no problems they make my ariel's run very well and not look back in buying them i think they are a lot better carbs i know some will say it not the right carb for the bike but i am not just looking at my ariels i like to ride my bikes with no carb problems :!:

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Brian_Walker
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Re: 389 Monobloc Leaking

Postby Brian_Walker » Fri Jan 24, 2020 10:32 am

Thanks guys, I will persevere with the monobloc for now, I had one on my Matchless I rode to work every day for years without any problems.
I will still refit my GP when the engine is run in and I get a shorter velocity stack and move the oil tank a little to provide more clearance.
1951-52 VCH under restoration


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