376 monoblock advice.

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Simonjones
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376 monoblock advice.

Postby Simonjones » Mon Sep 03, 2018 9:09 pm

Hi all. I’ve got about 130 miles of running in done on my NH.
I decided to buy a brand new carb for it but it seems too rich. So here’s what I’m running..

58 cylinder and ally head
WNG+20 piston
Straight through exhaust
New Amal 376/8 jetted 106, 25, 200
Conical air filter.

Ran a bit woolly and the plug is sooty Black.
I tried lowering the needle by one notch.
This made it run better and lightened the colour of the plug but it would spit from the carb when blipping the throttle coming up to road junctions etc. A sign of weakness?
So I put the needle back on the middle notch, took the filter off and off I went for a 20 mile test ride.
Feels just like it did before... Woolly, not running very cleanly at high ish revs. Not good with hills. Pilot air screw is at one and a half turns out although I have tried it another half turn out with the current set up and tried it another half turn in with the previous set up, made no difference. Not sure where to go from here. It surely doesn’t want jetting down does it?
1950’s NH350 bitsa 1949 Triumph Speedtwin

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dave.owen
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Re: 376 monoblock advice.

Postby dave.owen » Mon Sep 03, 2018 10:00 pm

I think a 160 main jet would be more suitable.
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Simonjones
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Re: 376 monoblock advice.

Postby Simonjones » Tue Sep 04, 2018 6:38 am

Hmm.. Thanks Dave. I agree, It would appear to need jetting down but the 376 with a 200 main is listed as correct for a late NH.
The chart on Drags website doesn’t show a 376 with anything less than that in fact.
276 are listed with a 150 for earlier NH’s
Maybe using an early low compression piston has caused me this problem?
1950’s NH350 bitsa 1949 Triumph Speedtwin

john.whiting
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Re: 376 monoblock advice.

Postby john.whiting » Tue Sep 04, 2018 12:29 pm

monoblocs are notorious for the float binding or sticking on the pivot......the spacer being the usual cause...and also of intermittent flooding from the needle sticking...The 200 jet is pretty right,the numbers dont correspond with the earlier carbs,forgotten the reason...The downdraft angle may also need to be compensated for,by lowering the float level.

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Re: 376 monoblock advice.

Postby Simonjones » Tue Sep 04, 2018 6:41 pm

Thanks John. Yes, good point. I’ll check the float level. I have to admit, i don’t fully understand how to do it yet.
1950’s NH350 bitsa 1949 Triumph Speedtwin

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paul.jameson
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Re: 376 monoblock advice.

Postby paul.jameson » Tue Sep 04, 2018 8:45 pm

I would be looking at your throttle slide cutaway. Although only responsible totally for a small part of the throttle opening, it does have considerable influence. If you are running a 376/4, try a 376/3. The 3 or the 4 indicate the height of the cutaway in 1/16ths of an inch. Obviously, the lower the cutaway, the richer the mixture you will get.
Paul Jameson
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Simonjones
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Re: 376 monoblock advice.

Postby Simonjones » Tue Sep 04, 2018 9:45 pm

Yes.. Another good point.
Wouldn’t it be nice to have a range of different jets, slides etc
I gave Amal a call today and described what I’d tried with needle height etc. The guy I spoke to kindly offered to send me a smaller pilot jet free of charge as he thought it sounded like it wouldn’t be a main jet or needle problem so I’ll give that a go. At least it’ll eliminate one possibility.
1950’s NH350 bitsa 1949 Triumph Speedtwin

john.whiting
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Re: 376 monoblock advice.

Postby john.whiting » Wed Sep 05, 2018 2:08 am

You can easily increase the slide cutaway by rubbing on sandpaper,or with a fine file.....best to alter your old carby slide,not the new one. Mark the slide before you file,and just take a smidgin off and try......See if there is any improvement.......as little as a 1/32" is half a size in slides.....You can also reverse the effect,but be aware the needle will then be a bit lower.

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Re: 376 monoblock advice.

Postby Simonjones » Wed Sep 05, 2018 12:47 pm

Could be really interesting to have a go at that.

Thanks again John :D
1950’s NH350 bitsa 1949 Triumph Speedtwin

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Re: 376 monoblock advice.

Postby nevhunter » Wed Sep 05, 2018 2:16 pm

Monobloc's always had a problem with the float It's all a bit compacted. The concentric was an improvement. The later Amal looks Japanese. Nev


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