Belt Drive Conversion and Electric Start Questions - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 11 (permalink) Old 06-24-2019, 05:06 PM Thread Starter
Minitwins
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Belt Drive Conversion and Electric Start Questions

I bought this '82 T140ES (electric start) in little pieces in many boxes. Somehow I got it all together. It came with a belt drive conversion that seemed to be in perfect condition. Regardless, I bought a new Gates belt, a name I have learned to trust.
It ran great for 100 miles around town, then after cruising on the freeway for 8 miles the engine bogged down and stalled. It was too stiff to kick over, and of course the electric starter had long before broken the sprag clutch. I loaded the bike on my pickup and went home with my tail between my legs. I had always had misgivings about the belt drive so that's the first thing I investigated.
1. The belt drive has a flange only on the outboard side of the crankshaft wheel. Why only one side? When I exposed the primary drive the belt was offset 5/16" inboard meaning that it was not fully engaged with the wheels. The belt and everything are in perfect condition. The belt doesn't have any directional arrows. There is printing on the belt. Is that an indication of direction of rotation? There's no sign of wear on either edge of the belt and no sign that the belt has been rubbing on the aluminum castings on the inboard side. But still it just isn't right. Does someone have advice about a guide surface that could be improvised (or that might be missing) on the inboard side of the belt?
2. Now that I've opened up the primary drive and inspected it the engine turns over quite freely and has incredible compression. When I bored the cylinders I fitted the new pistons and rings with the minimum recommended clearances. I suspect that the engine tightened up simply due to thermal expansion of the pistons. Maybe the engine could stand several hundred more miles of gentle break-in before I push it too hard. The compression is so good that I'm not sure my 190lbs can kick it over. I'll try the trick of bringing one piston to TDC then kicking it.
3. Has anyone yet solved the problem of the freekin' sprag clutches breaking? My electric starter worked well for maybe 20 starts before it broke.
Thanks everyone for your help and advice. I've easily spent 100 hours getting this beast together. It's time to start enjoying riding it.
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post #2 of 11 (permalink) Old 06-24-2019, 05:20 PM
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If it has had a partial seizure, you will see damage on the piston skirts and bores when you remove the cylinder.

You said “minimum clearance.” What clearance was that?

Were you labouring the engine at under 3,000 rpm? That will make it seize.
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post #3 of 11 (permalink) Old 06-24-2019, 05:22 PM
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And no, the electric start will never be reliable.
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post #4 of 11 (permalink) Old 06-24-2019, 06:26 PM
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I find my T120 to have an area around 75mph on constant throttle, to have a weak mixture and seized twice in the past when going up a slope. i vary the speed now with some lower, then higher speeds and no more seizures. As TT suggests, labouring the engine with a loaded bike will also have that effect.

Now,where did that bit fall off ?
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post #5 of 11 (permalink) Old 06-24-2019, 07:58 PM
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Quote:
Originally Posted by rambo View Post
I find my T120 to have an area around 75mph on constant throttle, to have a weak mixture and seized twice in the past when going up a slope. i vary the speed now with some lower, then higher speeds and no more seizures. As TT suggests, labouring the engine with a loaded bike will also have that effect.
it shouldn't seize at constant throttle.

a needle with the same length and diameter straight section but with a steeper taper will deliver more fuel to that throttle position.

i be kevin
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post #6 of 11 (permalink) Old 06-25-2019, 06:38 AM
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If you are running your belt drive in oil then make sure that your gates belt is made for it. Not sure about Gates these days [I haven't kept up with the times], but 20 years ago when Gates belts were being used in primary belt drive conversions the Gates conversion was a dry one ie no oil, the belt was a rubberised Kevlar construction. AT10 type belts [stainless braided wire encapsulated in urethane] were designed for running in oil.

And my Hyde/Hayward belt primary on my Trident only has one guide on it. Its on the outer rim of the drive 'sprocket'. I've never had an issue with it and its on a 1000cc kit.

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post #7 of 11 (permalink) Old 06-26-2019, 08:17 AM Thread Starter
Minitwins
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Thanks for the input, guys. I think I'll improvise an inboard belt guide, some kind of smooth surface on the slack side of the belt that will prevent it from drifting inboard.
By "minimum clearances" I mean the smallest clearances of the range that is given in the original Triumph manual.
And actually, since the compression is so good I don't intend to pull the barrels. If the piston skirts are scuffed a bit I think it really doesn't matter, except esthetically.
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post #8 of 11 (permalink) Old 06-29-2019, 10:01 PM
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Did you actually measure compression or are you just assuming you have good compression because the engine doesn’t want to turn over easily ?
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post #9 of 11 (permalink) Old 06-30-2019, 02:19 PM Thread Starter
Minitwins
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The gates belt is in perfect condition despite being immersed in oil for over a year.
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post #10 of 11 (permalink) Old 06-30-2019, 02:23 PM Thread Starter
Minitwins
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Latest development:
I fabricated a 16ga. sheet metal guide for the inboard side of the belt. It's located where the belt leaves the large wheel. It's secured by the hollow bolt that carries the alternator wires out of the primary case. It's stabilized by the case above the bolt. It has a smooth approach for the belt and a 1" flat surface that the belt can touch. I think that will solve the problem and my worries.
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