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Adam_Winstone
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Sun Feb 03, 2019 1:45 pm

^... gear ratio, not dear ratio (LOL)!

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double h
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Sun Feb 03, 2019 1:50 pm

Adam_Winstone wrote:^... don't worry, that is a VERY common mistake to make. As power from standard motors went up, so did the rev range that they operate at, with the dear ratios being lowered (higher number) to suit:

LI125 = 5.7, GP125 = 6.1

LI150 = 5.2, GP150 = 5.7

TV200 = 4.4, SX200 = 4.8, GP200 = 5.2

... which clearly shows the changes to gear ratios over time. Also note that any other changes that you've made to the engine spec, by way of bigger carb or free flowing pipe (e.g. later SIII or clubman) will have further compounded the issue by pushing power higher up the rev range, making it even less likely to pull 4th.

Good luck getting this sorted.

Adam
cheers for the info, very helpfull.

Adam_Winstone
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Sun Feb 03, 2019 2:12 pm

FYI - Your easiest ways of resolving this so that it works as it should, in whichever order may suit your pocket best (depending on what is available), are:

a) Fit a different gearbox with lower ratio. There are lots of gearboxes that give 5.7 in 4th with the original 15/46 sprockets and 80 link chain but some have better 3-4 % jumps than others. TBH any of these would work OK but those with closer 3-4 % rise would give a slightly nicer ride.

b) Bring back enough torque so that the current 5.2 ratio is fine. The easiest way to do this is to increase the capacity to 175. Don't make the same mistake of thinking that the TV175 pulled 4.8 and that this should be your target, otherwise this is exactly the same mistake as thinking the GP150 would work with 5.2. The example given already shows that the SX200 was correct at 4.8, and many 175s will struggle to pull this ratio... remember that the GP200 runs on 5.2 so 5.2 is not under-geared for most 175s. Indeed, take it to a higher level of tune and even a 20+ BHP runs 5.2 (as per AF's gearing recommendations) when using a revvy pipe!

Note: Consider your options before buying anything. You can often pick up a good Italian LI125 gearbox for £50 as lots of people don't like the jump from 3-4, whereas you could pay £150 for a late LI150 Special gearbox (watch out for any damage to second-hand boxes; chipped teeth of wear to the bearing surfaces). Likewise, you may pick up a used iron 175 conversion for little money (beware of worn second-hand with worn out bore/rings or any porting that might result in ring breakage or reliability problems). I also wouldn't recommend the cheap iron-lined alloy kits that often have soft liners and wear out very quickly... unless you only want to use your bike for Sunday rideouts, in which case this type of barrel might be fine.

Lots of options for you to resolve the current situation and feel free to ask on here for opinions as you may get some good advice.

Best of luck.

Adam

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double h
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Sun Feb 03, 2019 2:56 pm

Adam_Winstone wrote:FYI - Your easiest ways of resolving this so that it works as it should, in whichever order may suit your pocket best (depending on what is available), are:

a) Fit a different gearbox with lower ratio. There are lots of gearboxes that give 5.7 in 4th with the original 15/46 sprockets and 80 link chain but some have better 3-4 % jumps than others. TBH any of these would work OK but those with closer 3-4 % rise would give a slightly nicer ride.

b) Bring back enough torque so that the current 5.2 ratio is fine. The easiest way to do this is to increase the capacity to 175. Don't make the same mistake of thinking that the TV175 pulled 4.8 and that this should be your target, otherwise this is exactly the same mistake as thinking the GP150 would work with 5.2. The example given already shows that the SX200 was correct at 4.8, and many 175s will struggle to pull this ratio... remember that the GP200 runs on 5.2 so 5.2 is not under-geared for most 175s. Indeed, take it to a higher level of tune and even a 20+ BHP runs 5.2 (as per AF's gearing recommendations) when using a revvy pipe!

Note: Consider your options before buying anything. You can often pick up a good Italian LI125 gearbox for £50 as lots of people don't like the jump from 3-4, whereas you could pay £150 for a late LI150 Special gearbox (watch out for any damage to second-hand boxes; chipped teeth of wear to the bearing surfaces). Likewise, you may pick up a used iron 175 conversion for little money (beware of worn second-hand with worn out bore/rings or any porting that might result in ring breakage or reliability problems). I also wouldn't recommend the cheap iron-lined alloy kits that often have soft liners and wear out very quickly... unless you only want to use your bike for Sunday rideouts, in which case this type of barrel might be fine.

Lots of options for you to resolve the current situation and feel free to ask on here for opinions as you may get some good advice.

Best of luck.

Adam
Cheers, I've got a 175 conversion piston already so will go for a rebore and take it from there reguarding gearing now I have a better understanding.

Adam_Winstone
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Sun Feb 03, 2019 3:15 pm

^... ideal.

One thing to watch out for is the compression ratio that you end up running (aim for approx. 8:1 geometric, with squish volume considered), regardless of whether you have your 150 head reprofiled or buy a 175 head already done (some off the 175 heads are too high, others too low).

Also, take any tuning advice with a pinch of salt, particularly if it suggests widening the exhaust port as this regularly leads to ring-breakage or excessive wear. It can/should be considered BUT it can ruin a barrel and cause endless grief.

Adam

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double h
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Sun Feb 03, 2019 5:06 pm

Adam_Winstone wrote:^... ideal.

One thing to watch out for is the compression ratio that you end up running (aim for approx. 8:1 geometric, with squish volume considered), regardless of whether you have your 150 head reprofiled or buy a 175 head already done (some off the 175 heads are too high, others too low).

Also, take any tuning advice with a pinch of salt, particularly if it suggests widening the exhaust port as this regularly leads to ring-breakage or excessive wear. It can/should be considered BUT it can ruin a barrel and cause endless grief.

Adam
Ok cheers, my goal is 50 cruising 55 flat out, once I'm there I'll be happy.

Adam_Winstone
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Sun Feb 03, 2019 7:55 pm

A very simple iron 175 with 5.2 top ratio should be able to do that with ease.

Let us know how you're getting on and we may be able to help with setup so that you more than realise what you're looking to achieve.

All the best.

Adam

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rosscla
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Sun Feb 03, 2019 9:57 pm

Howie, Grog had an Li 125 gearbox - might be worth asking if he still has it?
"Our dilemma is that we hate change and love it at the same time; what we really want is for things to remain the same but get better."

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double h
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Wed Feb 06, 2019 6:45 am

Adam_Winstone wrote:A very simple iron 175 with 5.2 top ratio should be able to do that with ease.

Let us know how you're getting on and we may be able to help with setup so that you more than realise what you're looking to achieve.

All the best.

Adam
Will do cheers.

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double h
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Wed Feb 06, 2019 6:46 am

rosscla wrote:Howie, Grog had an Li 125 gearbox - might be worth asking if he still has it?
I'll keep that in mind Ross cheers.

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