Help on a '68 Bonneville T120R sound after rebuilding carbs - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 6 (permalink) Old 03-25-2018, 09:45 AM Thread Starter
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Help on a '68 Bonneville T120R sound after rebuilding carbs

Hello. I recently purchased a 1968 Triumph Bonneville T120R. It had been sitting for about 2 years before I got it, but the previous owner took great care of it. Over the past month or so, I have been able to get it started, but it would die when given throttle. Yesterday, I rebuilt the Amal 930 carbs and gave them a good cleaning. After reassembly, I started the bike and after a few minutes of kicking it started up. I made the adjustments to the pilot air screws and idle screws and got it running pretty smoothly. I took it down the street and it ran perfectly...for about 2 minutes. I then started hearing a whooshing sound from the left side. I immediately turned it off and coasted home. Now when I attempt to kick it over, I get a loud whooshing sound like I am pushing air. I know that is pretty vague, but before I tear the engine apart diagnosing, I was hoping someone could help me diagnose the problem if they too have experienced the same thing.

One thing I did notice is that the gasket between the head and carbs was much thicker than the replacement in the kit. Another thing is when I was tickling the right carb, it appeared gas was coming out right above the float bowl as well as out of the tickler like normal. Doubt that is what caused the issue, but just thought I would provide the info.

Thanks so much for any help you can give!
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post #2 of 6 (permalink) Old 03-25-2018, 10:08 AM
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Possibly blown head gasket
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post #3 of 6 (permalink) Old 03-25-2018, 01:02 PM
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Originally Posted by ekaplan1020 View Post
gasket between the head and carbs was much thicker than the replacement in the kit.
If you haven't already, you should check the parts book for your bike - pages 70/71 show both a gasket and what Triumph called an "Insulating block" between the cylinder head and each carburettor manifold.

The "much thicker" part you're looking at is likely the "Insulating block", ~1/8" thick as standard; if you want to confirm with pictures, enter "triumph 70-2968" into your preferred internet search engine and it should return images of and links to.

Afaik, the "Insulating block" is Tufnol, an industrial laminate that dramatically cuts the amount of heat that would otherwise be conducted along the manifold from the cylinder head to the carburettor; without the "block", the heat causes additional fuel vaporisation, enrichening the fuel-air mixture until it's too rich for combustion.



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post #4 of 6 (permalink) Old 03-25-2018, 06:16 PM
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Like Dave, i would suspect the head gasket has blown out.Could be the carb gasket has broken up a little and letting in air but check the head gasket first.A little oil squirted in the plug holes will give a good smoke signal out of the gasket.

Now,where did that bit fall off ?
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post #5 of 6 (permalink) Old 03-25-2018, 11:27 PM
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Stick a 2ft length of garden hose in your ear then get someone to kick the engine over whilst you move the hose around listening for the noise.

Enn skal lytte, når en gammel hund gjø.
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post #6 of 6 (permalink) Old 03-26-2018, 02:02 PM
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I don't think a blown head gasket would give you a loud wooshing sound, but it might give you a quiet one !
Start by looking at the top end, it could be something as simple as a loose or stripped spark plug, or maybe a valve stuck open.

Good Luck !

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