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T120-T140 HP differences

7K views 5 replies 4 participants last post by  Adam M. 
#1 · (Edited)
T120/TR6-T140 HP differences

When checking stats on the different bikes I see the T140 listed as being 49hp, while the T120 shows 34-46hp. What would cause such a spread in the T120/TR6 motors? Surely the single carb is not a 12hp difference. If the 46hp figure is correct is there really only 3hp difference between a 650 and a 750?
 
#3 ·
Hi Marc, Over the years the 650 come with different heads, cams, valve sizes & carbs, compression, plus very different flywheel wieght.

It has been said the 750 was de tuned to help reliability.

The 750 has 1/2" shorter rods. The short rod 750 feels like a totally different motor while riding. The way it revs & pulls is just different, even the vibration feels different in a way. Of course both vibrate like mad, but different.

On the road the short rod 750 is strong runner & puller. More than a 650. Period magazine test show this. There seems more to it than just 100cc. I think it's the rod length, but that's only a guess.

I've ridden my 750 & other's 650 back to back many times so I have feel for both.

A big bore 650 conversion to 750 used to popular. I've never ridden one so I can't speak to that.

My dream motor would be single carb early 1970 650 with low compression piston & late 5 speed gear box, 20x46 sprockets.
Don
 
#5 ·
Hi Marc, A few reasons for lower compression. My motor was made for 98 octane leaded fuel. Best we have in California is 91. These bikes want to ping on 91. When a bike pings it's not really making proper power. Plus the damage it does. Remember it's already started pinging before you can hear it.

Changing head gasket from .050 to .080" thick lowered cranking compression warmed up motor lower from 180 to 150#. Ping was dramatically reduced. I also rounded all surfaces in combustion chamber & valve pockets on pistions. My head had been skimmed aprox. .010". Richer jets reduced ping further as well as slightly retarded timing.

Net result is bike ran much stronger & better, especially 2 up. On a side note vibration is less with lower compression. Surprisingly less. Even my wife noticed as a passenger without me bringing it up. Motor kicks easier also which I feel (know) gives higher cranking speed & better starting.

I had expected a noticeable drop in power. However it ended up with more power. I used to have to buy special race gas. That had me on a 100 mile leash. I hated that. Now I just go to a regular gas station. Recently did a 322 mile ride to the mountians. Could never do that before.

I want 7:4 pistons from the Tiger Trail motor. I was so afraid to lower from 8.6 but now I want lower.

Even if I did loose power with 7.4 pistons, not a problem the way I ride. As long as I can cruse 60-70 mph that's good for my use. What I want is a very reliable, safe, comfortable bike. Probably to me reliablility is the most important.

I was told lowering compression will produce more exhaust pipe heat & blueing. I didn't notice that during winter, but after a few 200 mile rides on 100f days the pipes did indeed blue more. About 1" more blue down pipe. Now dark blue ends about 3/4" past front edge of crossover pipe. That's ok with me.

There certainly is a place for race motors on the street, but not my cup of tea.
Don
 
#6 ·
With a help from Mr Pete on britbikes I build BSA A65 low compression motor ( 7.5 : 1 ) which is stronger and faster than all the rest A65 I know. Compression is overrated :).
No tuning was involved - just different BSA parts developed and used in different models of A65.
 
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