LCGB Forums

Not really a tuning question but it is related to performance. I have a S2 with a 175 iron top end fitted with a SH1/20 carb and a clubman exhaust. It manages 45mph with a best of 50 downhill with a tail wind. I am thinking of swapping the exhaust for a casa S2 one as the clubman has seen far better days. Will the change rob me of any of my modest top speed? Only doing about 3000 miles a year but don't want to take any longer doing them

Cheers

Tony

I suspect you can do what you like to the exhaust, but you will see little change to the top speed with such a small carb.

With an engine that is flowing well, then a change of exhaust might make a difference to the performance, though I can't predict whether the change will be for the better or worse.

I trust someone will come along with some stories about good performance with small carbs. I am happy to be proved wrong.

maximum of 50 downhill with a 175???? I'd be doing more than thinking of an exhaust change!

Thanks for the replies. I did fit a 22mm Delly carb but struggled to set it up. It would only start with choke and throttle wide open. It then died as if flooded. Float was free on the pin and fiddled with jetting but to be honest I lost the plot with it and went back to what I knew worked. I was told to expect better top speed and even changed the front sprocket and chain with the 125 gearbox. I was thinking of a 26mm PHBL carb to get a bit more out of it but again wasn't sure if I would see much benefit after my issues with the SH2/22.

Any suggestions appreciated Donnie

The PHBL will certainly give you more to fiddle with than the sh2/22 and more fuel in

Was your 22 a Jetex? They can be very hit and miss...

What's your final ratio with that box? Are you wringing its neck to get to 50 or does it feel over geared?

The 22mm was a Dellorto that I was told came off a working GP 200. Bought at Wickstead and re-jetted. Not sure about the final ratio but with the 125 box I was advised to fit a 17t front sprocket with an 81 link chain. When I change up to fourth gear it seems to struggle at first - as if I changed up too early but it was screaming in third. Sits comfortably at 45 when at speed but it is wot.

Tony you should be getting a lot more than an indicated 45mph with a 175cc engine. Even a tired one

I would suggest that the first thing you do is set your points gap really accurately and using a timing wheel set your timing to 19 degrees BTDC .

The number of different atomiser, slide and main jet combinations you could try in any carb are considerable and setting up a carb properly is challenging to say the least.
The chances of you bolting on any sized carb and having it start and run properly with the existing jetting slide and air flow are very unlikely.

I'll give you an example . I have a 1964 Li150 series 3 with original engine to which I added a new 22mm Jetex carb supposedly jetted for 150cc . The scoot has an indicated top speed of 55mph !with perhaps more to come if I pushed it. I weigh 18 stone !
My 22mm carb ,like yours, came supplied with the wrong slide (200cc) and an unmarked atomiser which turned out to also be incorrect. Even though it still performs well on a standard 150 engine.

Your 175cc should be capable of 55-60mph with a standard sh1/20 carb on it provided your timing is correctly set and your points gap is correct and you are using the correct spark plug and have a decent air flow etc etc .

Start with the timing and points and be really accurate and then we can advise on atomisers slides and jets for your existing carb or a 22mm jetex which is a fine carb once the thing has been sorted properly.Then look at exhausts to compliment that for best extraction

Tony Hart wrote:Not really a tuning question but it is related to performance. I have a S2 with a 175 iron top end fitted with a SH1/20 carb and a clubman exhaust. It manages 45mph with a best of 50 downhill with a tail wind. I am thinking of swapping the exhaust for a casa S2 one as the clubman has seen far better days. Will the change rob me of any of my modest top speed? Only doing about 3000 miles a year but don't want to take any longer doing them

Cheers

Tony

45mph? your series 2 should do that with a standard engine, yep think the carb might need a change and fine tune to get the best out of your 175 top end. Do you know if your barrel has had any tuning done to it? or has it just been bored out to 62mm and the LI ports left standard size? Paul

Thanks for the replies, info and advice. It has an electronic ignition which is set at 19° BTDC and checked with a strobe. The 20mm carb is jetted for a S3 TV175 with a slightly larger main. The top end is a 175 iron barrel which I have ran in and has done about 2000 miles now (at 45mph ). To be honest there is no noticeable difference in top speed from the 150 top end I had fitted previously. I have drilled out the swan neck in the airbox and the filter is relatively new. More fiddling needed.

Tony

Any guidance for ball park jetting for the 22mm carb? I will give that another go this weekend. Can't remember what I tried before but it will have been a sophisticated wild arsed guess.

Tony Hart wrote:Any guidance for ball park jetting for the 22mm carb? I will give that another go this weekend. Can't remember what I tried before but it will have been a sophisticated wild arsed guess.

Try Starter - 50, Pilot - 50, Atomiser 5899-2, Main - 120.

Cheers. I will give these a go and let you know how I get on.

Standard Dellorto SH1/20 was a perfect carb on the TV175, TV 200, and SX 200 and still sought after. Cleaned and jetted correctly you should easily achieve 55mph on the flat. Bigger carb is not going to deliver more so don't waste time and money messing about and achieving little or nothing.
Something else wrong; possibly internally collapsed exhaust; over geared, or poorly machined and matched cylinder and piston. My opinion after over 40 years of hands on oily fingernails and wasted pennies !

Hi Chris. The SH1/20 carb is clean and plug colour is good so I think I'm in the parish jetting wise. Could be gearing - on advice I changed the front sprocket to 17t but kept the rear at 46t in an early 125 box. Further work needed.

I used this gearing on a 175 kitted engine, 20mm carb and a clubman. It pulled really well. In fact this was one of the nicest bikes to ride. There is something else that needs sorting as others have said.

Paul

Okay, I think I've got this. The carb is too small but is also the perfect size; however it could be dirty and incorrectly jetted. My engine is over-geared but should should pull great. The exhaust should have little effect on my top speed but could also be the source of the problem due to collapsing or being blocked. Apart from that it could be something else

One thing at a time I think. Will try original gearing first and let you know how I get on. (Cheapest first as I still have the old parts)

Thanks to all for the input-every day is a school day

Tony

17tooth front sprocket seems too tall to me on that set up, be better with a 16 to be honest.

Hi Donnie. I can't remember who suggested the change but it pulls great through the gears to third but doesn't seem to have any guts when I shift up to fourth. I am going to try the original 16t sprocket and 80 pitch chain. If no improvement I will try the exhaust. Might end up with a large shopping list for Wickstead next week. Hopefully swap things over tomorrow.

Cheers all

Tony

Original front sprocket on an S2 should be a 15 tooth, not 16, the rear should be 46.
Try the original set up first, with an 80 link chain. My guess is that this should provide a noticeable improvement.
If you have been running a 17 on the front ( and a standard 46 on the rear) you will have an 81 link chain in the motor, so just double check all your components first.
Often tuning your motor requires a raising of the final gear ratio, not a lowering, as the useable power is often pushed further up the Rev range
Hope that helps

Cheers Soullad. Definitely 17t with 81 pitch chain in there at the moment as I fitted them. Probably mistaken with what I took out but no doubt was a 15t front sprocket. I will be putting the original back in.