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GT186 with a standard box exhaust

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GT186 with a standard box exhaust

Postby Storkfoot » Wed Aug 17, 2022 10:04 am

I am seriously thinking of rebuilding my GT186 and slowing it down by fitting a mushroomless standard box pipe.

Over the years I have run this GT186 with 24 and 30mm carbs, a 42mm Clubman, MB/bgm Clubman and latterly a Ron Moss Clubman so I have a good feel for how the GT186 performs.

My question is whether anyone has any real life experience of running the GT186 with a standard box exhaust.
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Re: GT186 with a standard box exhaust

Postby nsaints » Wed Aug 17, 2022 1:01 pm

Hmmm. What problem with the kit are you attempting to solve with a standard exhaust?

For a few years, I mistakenly thought my GT186 was over tuned as I had to thrash it everywhere
My GT has been ported
what the real problem was I was running the wrong gearing, despite setting the final drive per the manual
it is happier and a more relaxed ride with a final drive of 5 (25mm PHBL, large bore airbox, scootopia clubman)

I did try a dellorto 2/22 and a standard exhaust on an RT195. Hopeless
lasted no more than 20 miles, before I removed both carb and exhaust
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Re: GT186 with a standard box exhaust

Postby vegansydney » Wed Aug 17, 2022 2:08 pm

The Clubman dyno test performed by Darrell Taylor in Scootering (2019) was done on a GT186. It's a fantastic read with very surprising results. His personal favorite was a standard GP/dl exhaust without the mushroom baffle! :shock:

Scootering/Darrell Taylor wrote: ...my No. 1 being the standard exhaust! Not one aftermarket pipe matched the power up to 5000rpm...
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Re: GT186 with a standard box exhaust

Postby Storkfoot » Wed Aug 17, 2022 4:34 pm

nsaints wrote:Hmmm. What problem with the kit are you attempting to solve with a standard exhaust?

For a few years, I mistakenly thought my GT186 was over tuned as I had to thrash it everywhere
My GT has been ported
what the real problem was I was running the wrong gearing, despite setting the final drive per the manual
it is happier and a more relaxed ride with a final drive of 5 (25mm PHBL, large bore airbox, scootopia clubman)

I did try a dellorto 2/22 and a standard exhaust on an RT195. Hopeless
lasted no more than 20 miles, before I removed both carb and exhaust


I am not trying to solve any issues with the engine. I just want performance more like a stage 4 200, I suppose. Running a standard TV175 had made me realise how much I love more sedate engines, even if it means keeping off motorways and fast A roads.

Also, I fancy trying the Ron Moss Avanti on my TS1 225 :)
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Re: GT186 with a standard box exhaust

Postby Storkfoot » Wed Aug 17, 2022 4:53 pm

vegansydney wrote:The Clubman dyno test performed by Darrell Taylor in Scootering (2019) was done on a GT186. It's a fantastic read with very surprising results. His personal favorite was a standard GP/dl exhaust without the mushroom baffle! :shock:

Scootering/Darrell Taylor wrote: ...my No. 1 being the standard exhaust! Not one aftermarket pipe matched the power up to 5000rpm...


Thanks for that. Yes, in the back of my mind I remember that standard mushroomless box pipes perform well up to 4000/5000. I have an old dyno printout for my GT186 when it had a PHBH 30 and mark 1 MB/bgm Clubman back in April 2013. Whilst it made 16bhp, maximum revs were only just over 7000.
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Re: GT186 with a standard box exhaust

Postby nsaints » Wed Aug 17, 2022 5:11 pm

Running a standard TV175 had made me realise how much I love more sedate engines


With you on that. My standard 200 is superb
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Re: GT186 with a standard box exhaust

Postby Adam_Winstone » Sat Aug 27, 2022 8:02 pm

The early mushroom baffle-type exhausts suck!!!

As it reports in one of the period texts, the British market was initially very disappointed by the performance of Jet 200s, much of the performance loss being identified as resulting from the fitment of the earlier type exhausts. I can confirm just how restrictive these are, having real world (very, very obvious) experience of how these limit the rev range of any of the layer spec standard models.

I took a test ride on a pal's totally standard SX150, with the owner being proud as anything of his immaculate machine, only to get just beyond the end of my road before having to turn back to tell him that he obviously had the wrong free-range / gas flow limiting mushrooms baffle type exhaust fitted. Firstly, he was extremely doubtful as he had paid good money for an Innocenti stamped pipe specifically for an SX150, and from a 'respected' (more so in years past) resto expert. However, after showing him the section in the 60s tuning manual about modification to the later spec, he agreed to at least go home and look in the throat of this expensive new box.

Sure enough, mushroom baffle type and 100% wrong for an SX150! He then carried out the modification and called me back a while later to say that it had totally transformed the ride, for the better! We, in my club, still commented that his bike was ridiculously quiet, even after the modification.

I'd not fit a mushroom baffle bike to strangle as LI125S or any of the other late models... in the same way that Innocenti didn't. The further extended port durations of a GT kit give even more reason why not to fit such an exhaust.

The later spec S3/GP pipes are a completely different kettle of fish, extending the rev range enough to work correctly with the lower gearing of these models.
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Re: GT186 with a standard box exhaust

Postby Storkfoot » Sat Aug 27, 2022 10:21 pm

I have put this on hold until next year until I strip the engine anyway. I need to take the RD400 conrod (115mm) off as, otherwise, a standard exhaust won’t fit.

In the interim though, I am going back to the patched up MB/ bgm Clubman which is going to the welders yet again this week. That, like the Avanti, is adjustable and so does fit with the 115 conrod.
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