The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: sheffield steel on 12.04. 2020 20:02
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Hi All
I’m sure this must have been discussed previously but I can’t seem to find any advice. I have a 1960 A7 Shooting Star, it has a rebuild engine with many uprated parts which runs great, but performance wise is pretty much standard. I’m running what I believe to be standard size sprockets (19/42) which seems to give great performance from a standing start but at 50mph or so it’s all out of puff. I’ve tried a 20T front sprocket which appeared to make things very sluggish (unless I failed to give it adequate chance to prove itself). Is there a better gear ratio available or am I just expecting too much from the old girl?
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confused, is timing set up correctly, what size crank sprocket is on it, mine fly's with 20 engine 19 or 20 gearbox, didn't fly with a 23 engine *whistle*
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Bergs. You and me both. '54-62 SS as standard has 18 teeth on the crank, 43 on the standard clutch drum. Usual 19 on the box, rear cog is 42 teeth.
SS..... We don't know enough of the build spec of this bike to offer an answer, but as a first step checking the basics is a must. "Out of puff" is a bit subjective, but the wrong carb set up and a simple blocked air filter is a first place to look.
Swarfy.
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Thanks for you responses.
I fairly confident that the timing is correct and there’s no air filter fitted so that definitely isn’t the problem!
There seems to be some confusion on the standard sprocket size in the primary which I’m now suspecting is the issue. I’m running 18 engine, 43 clutch, 19 front sprocket and 42 rear sprocket. The general consensus seems to be that a 19, 20 or 21 might be more suitable on the crank though?
Bike is a 1960 A7 S/S swing arm.
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Your sprockets are as the book says. Below from the owners manual.
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The book I have also shows those sizes which are what I’m running. However the general opinion in a long discussion on the BSA Singles & Twins page on Facebook is that I should be running at least 20T on the engine if not 21T. They all seem to think that 18T is for sidecar work.
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You say the bike out of puff at 50 mph.
50 equates to about 3500 rpm on the BSA specified sprockets. So it seems that something is inhibiting the bike from reving. Fitting a larger sprocket will not help it rev.
Fit too large a sprocket and the bike will be faster in third than in top will be much less flexible and accelerstion will be slower.
You can see from the page I posted earlier what the factory recommended sprockets for sidecar are.
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Hi All,
My opinion is that the original gearing was suitable for the road speeds of the 50's and 60's
For today's higher road speeds the gearing can be upped to provide a more relaxed ride
Its been a very long time since I rode an A7SS, but remember winding it on and seeing just shy of the magic "ton" on the speedo *eek* (I have no idea how correct it was though )
My own SR 650 will rev out with a 23/43, 19 /42 gearing *smile*
I think there must be something up with Sheffield's A7 *????*
John
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How restrictive are those mufflers?
gearing a bike up doesnt make it faster, and can result in a lot more use of third gear and the bike not ever getting on the cam in top.
As mentioned we have no idea whats in this motor but through the gears does it appear eager and rev or feel flat?
On my (race) RR I keep it to 6500 but have to watch the tach as it'll keep revving well into the 7's, these engines if not constipated should be very willing.
I have a stock cooking plunger B33 and that will never go past 5000 but is strong as an ox up to near 80mph (runs a19T engine sprocket)
Perhaps the motors still tight?
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I have to agree with RR about a tight motor , I knew with mine after rebuild when it had reached its loose stage, it proper took off and I hit 7000 in first and second then thought don't be a pillock *bash*
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Thanks all for the suggestions
The bottom end was rebuilt by SRM a few years back but I can’t be doing more than a few hundred miles a year. Bottom engine has bearing conversion and billet rods etc but essentially is standard performance wise. Head has been overhauled but again it’s standard. When I’m riding it the engine feels great, revs nice and freely and pulls well but just seems to rev out at about 50/60 tops. It certainly feels like a gearing problem to me rather than an engine or timing issue but I’m no expert with these engines.
I do want to strip the carb and check jet sizes etc though.
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Hi Sheffield,
I'm very new to motorbike engines. But, in times past I spent many years with performance car engines. I saw a great many that were over ported on the inlet side. Chuck away the standard air filters, fit twin Webbers and air horns. They looked good, sounded good, but bogged down at speed. There was not enough depression on the carbs to drag the fuel through at speed. You say you're not running an air filter. Just might be worth a try.
Regards
Len
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When I’m riding it the engine feels great, revs nice and freely and pulls well but just seems to rev out at about 50/60 tops. It certainly feels like a gearing problem to me ...
When you say rev out, is she fair howling? you want to take pity on the motor, she cant give any more... I'm more familiar with the larger motors although have ridden a mates star twin and that is a real smoothie, purrs along at the speeds you mention effortlessly all day without showing a sweat. When I had a super rocket in road trim that was happy at 60/70mph open road cruising and not revved out but was on song with more at hand for passing. For road use these bikes are not naturally tapped out as we want to get home, they do like revs and eddie dow once quoted the safe continuous red line at 6800.
I certainly am not a fan of over gearing, you put more loads through the engine and reduce the oil pressure. Once they are warmed up they are happy to buzz along all day. It doesnt do them any harm and the hotter cams like more than 4000 on the clock and sing at six.
There are many reasons this engine could be a bit constipated, tightness, too big a carb or incorrect jetting, too restrictive mufflers. But not I would expect gearing.
I would expect you would need to be on sidecar gearing to rev out at 60mph or be on a c11 (they do run out of steam at 50!).
My zb33 absolutely barks (the exhaust note hardens up) at speeds over 60mph but its practically still only doing around 4000 odd (guess) as can be wound out to close to 80mph at which point she has gone flat powerwise (being around 5000) being on cooking cams.
But if you think its buzzing too hard and has nothing left, try amending the gearing by all means, its easy to do. Might though be worth doing some maths on what your current 60mph is in RPM. Its easy enough to do. You should be able to do that speed in third.
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with an 18 T you should be able to pull around 90mph
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that's interesting RR what is it with 20 -43 19 -42 I think mine hit the ton at about 6000, busy listening to engine flat on tank and watching for the magic signal
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happy to oblige, if anyone wants the spreadsheet because they have different wheel sizes or gearbox ratios, pm me your email address. I work out my circumference from height to axle centre then use the old 2xpi calc. my formula is metric then I convert to mph using the 5/8 miles/km ratio.
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cheers RR *beer* the glance I got was about right, thanks for that
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Nice to know a clubmans trim cafe A7ss can hit the ton, I'm quietly collecting bits to make an A7ss race bike and have visions of matching the 47hp they claim the daytona race A7ss bikes managed, beating the goldstars entered ;)
and we shouldnt forget the great Chris vincent won the 62 TT on a humble A7ss
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Just a though, could the valve timing be out by one tooth, could be worth a check. also the long stroke crank timing gear will fit but is marked one tooth out, a trap for young players *eek*
Also my 54 A7 I got when it was 6 months old would make the ton, those were the days *smile*
Peter
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Just a though, could the valve timing be out by one tooth, could be worth a check. also the long stroke crank timing gear will fit but is marked one tooth out, a trap for young players *eek*
Also my 54 A7 I got when it was 6 months old would make the ton, those were the days *smile*
Peter
yes there's something not right other than gearing if a shooting star can only just get to 50-60mph. You'd hope SRM could align the dots but you never know. If the longstroke ones are marked one differently it should be possible to count the number of teeth between dots for a later one... as a quick test. cam timing would certainly also explain it.
I count 49 teeth with a 29/20 split between marks
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Shooting Star road test from 1961, interesting but take with a pinch of salt.
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RR lost lots of weight on mine and some inlet porting I did years ago, head and block top skimmed and run with tetraboost rocket fuel added in generous doses . the berger build has a head ported [gas flowed etc] by cylinder head shop with super rocket inlets which they say use a 34mm mikuni on it. I don't think I will be getting 47hp but will be putting some ideas and effort into the engine. many ££££££s worth of goodies waiting in spare bedroom then dropping it in a featherbed, it could be interesting, I will have to wait until brother has built his workshop because he is the engineer and I am on the beer haha, when the pubs open that is
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berger - why drop it in a featherbed when a BSA frame handles better?
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RD I have betsy in one piece and the other frame is lots lighter and came cheap as chip paper with the rrt2
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Thanks again everyone for your input *smiley4*
A few people are suggesting cam timing but they have been in and out a few times and it’s always been the same performance wise, I’m confident that the dots are aligned but couldn’t be sure that all the parts are correct for the year / model. I will check valve openings manually but not sure I’d notice if things were one tooth out due to incorrect parts. I’d like to think SRM would have picked up on this though...
I will check carb jetting and restrictions in silences next.
I’ve ordered a cheap 20T engine sprocket so no harm in trying it.
Just a thought....aside from the risk of getting my jeans caught in the chain! Is there any danger running the primary exposed with no oil whilst I trial the new sprocket to save having to seal the cases and recking another gasket.
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yeah Sheffield steel run it no harm in a few miles like that, wipe primary chain with a rag or you get oil thrown at you
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Some oil will blow from the 3 front chaincase fixing screw holes.
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oooops forgot about them , but don't put them in the holes you will lock the crank, so just shove the case on without sealed gasket and run it, jobs a good en
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Hi All,
In recent months I have viewed two bottom end assemblies that were put together by SRM
For some reason they do not fit the cam pinion or idler,
So unless they built the full engine it would have been the customer who finished the build
John
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There is a four teeth difference between the gears. Here is a link to an article I posted some time ago. It deals with the longstroke engine but points out the difference between the gears.
https://www.a7a10.net/forum/index.php?topic=7342.msg51278#msg51278
If you have any doubts about the cam timing put a degree disc on and check it.
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Brian.. Thanks for that reminder. Longstroke timing gears look the same as shortstroke gears, but all are different.
Crank pinion...timing dot to keyway position differ from shortstroke pinion.
Intermediate gear....marks different as above.
Camshaft gear....peg for breather sleeve in different position.
All this is somewhere on the Forum, a good few folks have had this problem.
Timing disc check will tell if the basic cam timing is correct, but also confirm the bike has a 356 cam.
Worth checking valve springs look correct, as weak or missing (inner) springs can limit performance.
Cheers.
Swarfy.
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This may help, it is on the forum some where but this a copy I made of it
Peter
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This is possibly a stupid question but here goes, I don’t mind looking stupid...
A few people have mentioned a tight engine could be contributing to the problem which seems logical.....but how tight is tight????
I’m turning the engine over using a ratchet about 8” long on the end of the engine sprocket (I have a nut securing the sprocket on the cush drive not C-Spanner thing), there’s no primary chain or rocker box fitted and the plugs are out. All the timing gears, oil pump, mag and dynamo are fitted. The effort required isn’t huge but it doesn’t exactly spin freely. I certainly can’t turn it by hand on the cush drive.
It’s probably about 4 years since all the bottom end was rebuild by SRM, but I reckon I’ll have only done something like 1000 to 1500 miles in that time.
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I would expect that to turn by hand with a reasonable grip of the cush drive (far less grip than opening a jam jar).
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well what can I say , apart from if you were me I would find a nice hard block of wood and smack the end of the crank nut and see if it loosens any when turning, I would say that is tight but wether it is bore to piston or crank end float who knows without investigating. you could drop the timing gears off to make sure it isn't tight on the bushes. the crank pistons and rings on there own should turn easily by hand.
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Ok thanks, further investigation required....
Next stupid question....
I’ve removed the carb (which is an Amal 376/208) which is 1” bore, however the inlet manifold is approx 1” 1/16 diameter (28mm). Is this correct?
Bike is a 1960 A7 Shooting Star
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Sheffield steel yes 1" is correct
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Thanks
All the info I can find says it should have a 376/289 mono lock fitted not a 376/208. Both are 1” bore so any idea what the difference is and if it would be worth fitting a 376/289?
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not sure on that one , think it is to do with jetting but any 376 jetted to shooting star spec will do 270 main jet 30pilot needle106 needle position 3 from top. carb slide 3/1/2, I also think that having the head a little bit bigger than the carb is good for turbulence and was done on purpose