Advice needed on new Top End Spec

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chris2470
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Fri Aug 24, 2018 8:44 am

Hiya, I'm looking for some advice on the way to go with a top end rebuild, I'm looking for 23-25hp with good torque.
Currently I'm running a Rapido race that was purchased second hand as it had a seize, this was re-bored to 70.4, and fitted a new piston (Indian jobby with Asso rings) along with a LTH reed and a boost port cut into it, but, due to the ring peg spacing this was quite narrow, around 15mm. I'm now thinking of fitting a better quality 71mm piston a which would allow a wider boost port and some extra tuning to get the spec required.
The dilemma i have it that i can help thinking that rather than throw more money at this project, just go straight for a TS1 or a Casa SS225, when you add all the costs originally paid plus the extra work required, it doesn't seem worthy, plus I could always sell what I have to fund this new kit. Also I'm wondering no matter what i do the LTH will always restrict the top end power and performance, would this be correct ?
Any thoughts would be greatly appreciated, thx Chris.
Best Regards

Chris

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Burnside
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Fri Aug 24, 2018 9:51 am

What about the GT240 kit? Buckets of torque

Warkton Tornado No.1
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Fri Aug 24, 2018 10:22 am

Hi Chris.

Despite the reservations that were stated about the Rapido from the outset, it has gone on to become a very useable cylinder kit, despite the actual flaws such as exhaust port symmetry & other port durations.

I have been involved with several, & I reckon the best way to make them tuneable is to 'stroke' them with a 60 mm crank or larger.

In that way they can be tamed to better port timings.

The addition of the reed valve makes them even more driveable & economical by preventing spit back, & won't really knock much off top end. The boost port (presumably a finger from the inlet) will gain very little but does have the advantage of cooling/lubricating that side of the piston as well as contributing to the transfer process being spread over a wider area. I wonder if a couple more either side of the peg positions might be worth a try.....

Dependent upon how well the top end has been set up, you should be realising the kind of power you seek, but so many factors come into play, particuarly the exhaust. Optimising an engine is very, very time intensive!

I would persevere with what you have, because if you do buy into the longer crank idea, that can be utilised in other engine formats, were you to go that route.

Good Luck!

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paulnobodyimportant
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Fri Aug 24, 2018 1:08 pm

Hi Warkton,
I've a classic 200 Rapido kit that I used on an earlier scooter, with a LTH reed valve and standard 58 crank. Wasn't really impressed with it, so stuck it on the shelf and put in a spare TS1 engine. However, later next year, I will be building a GP200 up, which is missing its top end, so I thought that i would re-use the Rapido on this to keep it relative standard. That being so and considering that I still have to buy a crank, what set up would you recommend with this classic Rapido?

Thanks

Paul

Warkton Tornado No.1
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Fri Aug 24, 2018 2:34 pm

Hi Paul.

I'd go for a 70 mm bore to suit a TS 250 piston, & add a window on the inlet side, or, the DT 250 piston may suit (but that confounds me as it surely would have been the obvious choice for the GT 240 kit, although I have questioned the two people I believe to be most closely involved & never had a response.....)

The crank (just webs if you can get them!) should be 60 mm or greater (I use a 61 mm) so that the rod can be selected from Grampian's catalogue to best suit the piston (compression height, wrist pin diameter,) & any likely base packer. The webs can be 'basic', generic GP unless you subscribe to HPC (which I don't)

BTW I simply never, ever consider head gaskets in even basic engines as there is always a way around their existence, especially for any ambitious leaks....

As said previously, optimising any engine is very, very time consuming as there are so many factors that combine.

I would just concentrate on getting the squish, centralisation of the head & it's CR, porting (crankcase & barrel) all as optimum as they can be & dependent upon exhaust, the jetting (too many tuned scooters are set up too rich IMHO) & to a lesser extent, choice of carburation, the result should be a more civilised engine.

You can add finger port(s) @ a later date, hopefully encouraged by what has been achieved with a few, simple changes that only require patience & being very methodical.

I hope that helps.

mick1
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Fri Aug 24, 2018 8:54 pm

back to the original question........an Avanti with a decent exhaust and carb, will get close to the 23-25 hp required with very little tweeking.

Warkton Tornado No.1
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Mon Aug 27, 2018 6:45 pm

mick1 wrote:
Fri Aug 24, 2018 8:54 pm
back to the original question........an Avanti with a decent exhaust and carb, will get close to the 23-25 hp required with very little tweeking.
I'm not sure what point is being made there, but my own responses remain the same & just as valid.

The Original Post has key words, such: "advice", "dilemma" & "wondering" but summarises with "Any thoughts would be greatly appreciated"

wintermod65
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Thu Aug 30, 2018 1:22 pm

Burnside wrote:
Fri Aug 24, 2018 9:51 am
What about the GT240 kit? Buckets of torque
i second that , get over to discodez scooters and have a go on the demonstrator pulls through the gears even with Clubman pipe or read the write ups in scooternova issue 3 i think or on SULK ,

i took it for a test ride , most fun ive had since i parted with my fireblade !! bought the kit and GT engine case straight after .

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