Author Topic: piston change ...new jets ?  (Read 520 times)

Offline Elmondo

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piston change ...new jets ?
« on: 18.09. 2018 18:54 »
In my infinite wisdom I decided to put 10>1 pistons in my refurbished gold flash 1961
I have a concentric carb.
Will the jets need to be changed.If so what do you suggest!

Online Triton Thrasher

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Re: piston change ...new jets ?
« Reply #1 on: 18.09. 2018 19:17 »
If you don’t already know how to tune the carb,  your go-faster escapade isn’t starting from the best position.

The Amal tuning procedure is in most of the manuals and is no doubt in a BSA service sheet somewhere.

http://amalcarb.co.uk/downloadfiles/amal/amal_tuning_guide.pdf

Online muskrat

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Re: piston change ...new jets ?
« Reply #2 on: 18.09. 2018 21:23 »
G'day Elmondo.
An increase in compression shouldn't require different jets but I go up one size just due to the extra heat created.
You may need to retard the timing a tad or higher octane fuel to combat pinging if it occurs.
Cheers
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
Australia
Muskys Plunger A7

Offline Black Sheep

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Re: piston change ...new jets ?
« Reply #3 on: 18.09. 2018 22:21 »
10:1 pistons, iron head, present day modest octane petrol. Hmm, not a course I would recommend. Yes, up the main jet a size just to try and reduce temperatures a bit, retard the ignition a few degrees too. Build up some hefty calf muscles (right side only required).
2 twins, 2 singles, lots of sheep

Online RichardL

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Re: piston change ...new jets ?
« Reply #4 on: 18.09. 2018 23:07 »
If I'm correct, there are a couple of folks here with 10:1 pistons who have through-bolted from the crankcase up through the head to keep from blowing the barrels off the flange. I believe Muskrat is one of those types.

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Online muskrat

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Re: piston change ...new jets ?
« Reply #5 on: 19.09. 2018 08:42 »
G'day Richard.
Yes I needed that when running 14:1 and revving to 8 grand! Blew the top off a few times before I developed the through bolt system.
High compression = faster burn = retard ignition. High octane = slower burn = advance ignition. High compression and high (100+) octane = just right. More is good, lots is better and toooo much is just enough.  *whistle*
Cheers
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
Australia
Muskys Plunger A7

Online RichardL

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Re: piston change ...new jets ?
« Reply #6 on: 19.09. 2018 13:18 »
G'day Richard.
Yes I needed that when running 14:1 and revving to 8 grand! Blew the top off a few times before I developed the through bolt system.
High compression = faster burn = retard ignition. High octane = slower burn = advance ignition. High compression and high (100+) octane = just right. More is good, lots is better and toooo much is just enough.  *whistle*
Cheers

So, since you're at 14:1, maybe Elmondo needs no warning about being at 10:1.

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline Black Sheep

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Re: piston change ...new jets ?
« Reply #7 on: 19.09. 2018 14:41 »
Not so easy to get hold of AVGAS 100LL in the UK unless you are near a friendly flying club. 14:1. Brave or what?
2 twins, 2 singles, lots of sheep

Offline RDfella

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Re: piston change ...new jets ?
« Reply #8 on: 19.09. 2018 17:27 »
I'd be surprised if it was on petrol at 14:1. Maybe the old 130 octane avgas, but I believe that was withdrawn decades ago. Most likely on alcohol, which cools the engine. In fact, cast iron head / barrel is ideal for an alcohol motor.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

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Re: piston change ...new jets ?
« Reply #9 on: 19.09. 2018 20:47 »
G'day Elmondo.
Are you using the std 356 cam or the 357? Hhat size concentric? Sorry for getting a little sidetracked.

G'day RD. Yes running methanol in the A7SS. T160 pistons, A10 rods shortened 2.5mm, twin 932's with 640 main jets. 12 MILES per GALLON  *eek*
Cheers
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
Australia
Muskys Plunger A7

Offline Minto

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Re: piston change ...new jets ?
« Reply #10 on: 29.09. 2019 18:07 »
Musky

More is good, lots is better and toooo much is just enough.  *whistle*

I’m getting that on a T-shirt!
Lol
52 A10 plunger

Online chaterlea25

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Re: piston change ...new jets ?
« Reply #11 on: 29.09. 2019 21:37 »
Hi All
In my infinite wisdom I decided to put 10>1 pistons in my refurbished gold flash 1961

I'm not sure that wisdom i the correct term?
*conf2* *conf2*

With todays crap fuel I think you are on a hiding to nowhere especially with an iron head *????*

To minimise your problems without buying new lower comp pistons I would get an 80 thou thick head gasket from
https://coppergaskets.us

John
1961 Super Rocket
1963 RGS (ongoing)

Offline Gerry

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Re: piston change ...new jets ?
« Reply #12 on: 30.09. 2019 02:14 »
Jesus......10 to 1 and 14 to 1 the mind boggles. I have the standard 7.5 to 1 in my Flash with twin carb alloy head (with manifold and single carb, solid copper head gasket) and its just about right for me to kick over after having my weeties! My mate lent me his Rocket Gold Star with 10 to 1 comp' pistons and I'm buggered if I could kick it over, had hell of a job but got there in the end. Took it back to him and told him to stick it.......no not really but wouldn't borrow it again. Can't kick anything over at the moment, fell over over two months ago on my right hip and am hobbling around like peg leg pete. Getting cortisone injection today. Wish me luck. Gerry

Online bsa-bill

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Re: piston change ...new jets ?
« Reply #13 on: 30.09. 2019 12:20 »
I tarted my last A10 up to rocket plus spec 9.5 /1 piston, 357 cam, Alloy head, Goldie silencers and so on, then called it a Rocket Gold Flash.
Then found it was a pig to start, tried various ignition and carb settings to some but not a lot of betterment, when it did start ( had to be within three or four kicks or the plugs fouled) then it pinked on hills worryingly,  Pinking and starting were improved massively with Pazon electronic ignition and 8.5 / 1 pistons, it is now a good ride and was a pleasurable and interesting exercise.
With hindsight, I now prefer a more standard Flash and intend to bring it back to that in the future as I do not use the extra oomph that it has but still do not regret the experience and knowledge gained.
One thing the bike has are down tubes cut and shut, probably means it has a lower sell on value but it turns quicker and more confidently than my other Flash.
We are fortunate that spares are available still that allow builds to personal taste (and back if required)
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Online edboy

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Re: piston change ...new jets ?
« Reply #14 on: 30.09. 2019 20:54 »
10/1 may require new dental fillings when cruising the queens highway