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post #1 of 30 (permalink) Old 06-17-2019, 02:07 PM Thread Starter
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hot carbs!

I've just got everything back together after breaking all four pushrods ( yes it can be done! ). BTW this time I used a mirror and a torch to actually see they were on the tappets and a strong torch to watch them do on to the rockers- didn't want a repeat!
Anyway I'm just back from a breaking in ride ( bored out to + 0.020 and new pistons/rings ) and both carbs ( Mark 1 Amals ) were really hot. I couldn't touch them for very long at all, so not just warm. It was really hard to start after I stopped for a quick check all was well after a few miles, could the hot carbs have anything to do with that? I've only used thin gaskets between the manifolds and the engine, should I put thicker ones in to insulate the carbs a bit or is it normal for them to be so hot?

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post #2 of 30 (permalink) Old 06-17-2019, 02:43 PM
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Maybe hot because engine was breaking in? Also they are attached to the engine and it’s hot. Using a thick gasket for insulation doesn’t sound right to me but hey, it’s your bike.
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post #3 of 30 (permalink) Old 06-17-2019, 03:23 PM
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Some bikes had resin type insulators (forget the trade name) others had a thick oring. What year and model? Check head temp with IR gun type thermometer. Might be lean and hot,?
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post #4 of 30 (permalink) Old 06-17-2019, 03:29 PM
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Hi,

Quote:
Originally Posted by ploughboy View Post
both carbs ( Mark 1 Amals ) were really hot. I couldn't touch them for very long at all, so not just warm. It was really hard to start after I stopped for a quick check all was well after a few miles, could the hot carbs have anything to do with that?
Absolutely. Petrol engines work because liquid turns into vapour to mix with air. Liquid petrol vaporises at a very low ambient temperature (-43 Centigrade?) so, the higher the ambient above that, the faster it vaporises. Something like a carburettor is specifically designed to mix petrol vapour and air in the correct ratio for efficient combustion. By definition, the volume of air inside a carburettor is limited; if the ratio of petrol vapour to air is too high, the mixture won't ignite. A hot engine requires a weaker petrol-air ratio than a cold engine; a hot carb. will rapidly vaporise too much petrol for the available air, enrichening the petrol-air mix to where it won't ignite.

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Originally Posted by ploughboy View Post
I've only used thin gaskets between the manifolds and the engine,
The parts book illustrations are crap but they do show more than that ...

. (Thin?) Gasket 71-3573 should go between each mainfold and the head.

. If you enter "triumph 71-2813" (the part number of the carb. 'n' manifold mounting studs) into your preferred internet search engine, the returned images should show they're quite different from the parts book drawing ... Primarily, they're 'stepped' - most is 5/16" OD but the thread at one end is 1/4"UNF.

. Each carb. 'n' manifold fit over the 5/16" OD parts of two studs (I trust it's obvious they fit either side of the thin larger-OD ring on each stud?).

. Also between each carb. 'n' manifold is the thick O-ring 70-9711 on the parts book "Carburettor T140V" pages.

. Finally, each carb. is secured on the studs by two of "Cup", "Insulating ring" and 1/4"UNF Nut shown in the parts book on the manifold pages. The idea is each Nut can be tightened against the 'step' on the stud, which should compress the O-ring between carb. 'n' manifold but they should not touch, the compressed O-ring should form an air-tight seal. Because carb. 'n' manifold don't touch, engine heat can't heat the carb. (at least not by conduction) and there is some vibe insulation of the carb.

Hth.

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Last edited by StuartMac; 06-17-2019 at 03:33 PM.
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post #5 of 30 (permalink) Old 06-17-2019, 03:39 PM
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On the Norton Commando 850's running dual Amals, there are Phenolic insulator gaskets between manifold and head. They come in different thicknesses. When I was running a single Amal on my Commando, the float bowl would get too hot to touch a few minutes after shutdown...like at a petrol stop. This was with a two into one manifold with phenolics on the head/manifold side. I put another phenolic on the mani to carb side and this issue went away.



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post #6 of 30 (permalink) Old 06-17-2019, 05:33 PM
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I use a gasket about 6mm at a guess. the carbs stay cool. No O rings, just the thick gasket coated with a film of Hylomar.

Now,where did that bit fall off ?
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post #7 of 30 (permalink) Old 06-17-2019, 05:50 PM
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I agree with @Tornado99 the insulators are the way to go, they are rigid and with a thin gasket on each side are less likely to cause the carb to warp when tightening. Classic British Spares (CBS) and I think Klempf's British Parts has them in the states.
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post #8 of 30 (permalink) Old 06-17-2019, 07:56 PM
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Ploughboy

I have a 78 T140V

IF you use the FAT O-Rings and IF you know how much to tighten the carbs you WILL NOT distort the bodies. You WILL BE
able to start the bike easily after stopping for say fuel on a 90 degree (F) day. For lack of a better expression the carbs 'float' on the O-Rings


I know I have done it.

K
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post #9 of 30 (permalink) Old 06-18-2019, 04:27 AM
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Quote:
Originally Posted by KADUTZ View Post
Ploughboy

I have a 78 T140V

IF you use the FAT O-Rings and IF you know how much to tighten the carbs you WILL NOT distort the bodies. You WILL BE
able to start the bike easily after stopping for say fuel on a 90 degree (F) day. For lack of a better expression the carbs 'float' on the O-Rings


I know I have done it.

K
Agreed!
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post #10 of 30 (permalink) Old 06-18-2019, 04:58 AM
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Hi,

Quote:
Originally Posted by rweb View Post
I agree with @Tornado99 the insulators are the way to go
Mmmm ... but, as it shows beside the OP's posts, he has a T140V and, as "Caulky"'s picture shows, where would the "Phenolic insulator gaskets" fit on a T140V's standard carb.-mounting studs?

Another reason Meriden devised the T140's carb. mounting arrangement is the previous rigid mounting allowed engine vibration to affect the carbs. ...

Hth.

Regards,
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