BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: MICKREEVES on 21 February, 2017, 07:55:10

Title: C15 ss Max speed
Post by: MICKREEVES on 21 February, 2017, 07:55:10
Following my previous blog-
My 1964 C15 SS from America, with 10/1 piston, has a disappointing low top speed, 60-65 MPH.
I can find nothing wrong with it.
I wonder if the original road tests 'in the day' were done with the rider  lying flat on the tank, with
the normal handlebars, not clip-ons. How much difference would that make??
I have my bike with higher than normal gearing, which I prefer. This should give more relaxed cruising,
with higher speed available in third gear.
I cant go to a heavier bike. I have not the strength to lift it when it falls over.
Mick
Title: Re: C15 ss Max speed
Post by: Arthur on 21 February, 2017, 09:10:34
Mick.

As suggested previously, it would be very interesting to see the results of putting your C15 on a dynamometer. I would expect you'll find it's producing no more than 75% of the power it had when it was new. That reduction in power would almost certainly equate to the reduced top speed you are now getting! That said, 65 mph max on a C15 is probably its safety limit given its handling and brake design!
Title: Re: C15 ss Max speed
Post by: JulianS on 21 February, 2017, 10:04:08
I think you are expecting too much from an over 50 year old C15.

Not only the engine and cycle parts but also the build and weight of rider have significant effect on performance of a small bike.

The test bikes in the magazines were probably collected by the tester from the BSA factory and would in all probability been in top condition set up properly by factory staff.

The first photo shows an extract from the road test of a standard C15 from "Motorcycling" February 22 1960. You will see that the rider got 70 mph going down hill.

The second and third photos are an extract from the road test of a C15 SS80 from Motor Cycle June 1st 1961. You will note the tester comments about cruising on the M1.

Reading the tests I think that if you can get a reliable anytime top speed of 60 - 65 then it is not doing too bad.
Title: Re: C15 ss Max speed
Post by: EDDIE SIMPSON on 21 February, 2017, 12:01:57
hi, a larger main jet may be needed at top end. if you have choke control apply flat out and feel what happens. you could raise the gearbox sprocket size and alter gearing but the engine will suffer getting up to top speed and you will hate slowing down for rounderbouts etc.
Title: Re: C15 ss Max speed
Post by: STAR TWIN on 21 February, 2017, 14:19:14
10:1! Not a good idea in a C15. Or anything these days. Consider fitting a standard piston before something catastrophic occurs.
Title: Re: C15 ss Max speed
Post by: TTJOHN on 22 February, 2017, 08:36:22
I see that you say your C15 came from the states, looking at some of their sites, it seems that a lot of US bikes had a 19T sprocket on the gear box, although the standard sprocket was 17T, I have just gone up to a 18T but have not had the chance to try it out yet, I am hoping it will give me a little longer legs, I believe Feck do a 19T but I'll wait and see how the 18 performs.

TTJohn
Title: Re: C15 ss Max speed
Post by: CRAIG12 on 23 February, 2017, 16:07:47
"I have my bike with higher than normal gearing, which I prefer. This should give more relaxed cruising,
with higher speed available in third gear" ............ sounds good IF the bike will now pull top gear , will it still rev or are you changing down unless on the flat etc
Title: Re: C15 ss Max speed
Post by: JulianS on 23 February, 2017, 16:55:09
A larger gearbox sprocket will not increase maximum speed unless the bike was undergeared for the power before the change. You will reduce the acceleration and possibly the top speed unless you reduce the weight of bike and rider significantly.

The blue top service sheet below from 1963 shows that the factory actually reduced the gearbox sprocket from 17 to 16 teeth on the SS80 and got a higher maximum speed.

Looking through the 1964 USA brochure that shows the standard gearbox sprocket as 16 teeth.

There were quite a number of engine gearbox and rear wheel sprocket variations available for the competition models some of which show in the second photo of a US bulletin.
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 17 April, 2017, 16:02:15
I found that the standard 1 1/16" new Amal carb on my bike, was feeding to 1" port in the head!
I was advised that any step in the inlet tract would affect the performance.
I filed out the head to match. I fitted the fibre heat insulating plate on to the head with the bolts. I was able to see
how the holes matched very easily.
The holes in the plate were larger than the bolts, so a step would certainly still be  possible.
I made alloy sleeves to glue into the plate, with exact fit on the bolts.
With the plate now unable to be out of line, I could cut the head to match the plate exactly.
With the cylinder head off, I mounted the carb and tightened the fixing bolts, aligning the carb to the plate.
With the head off, I could see through the intake clearly - not possible on the bike.
So, the inlet port was greatly improved. I changed the cams to the scrambles type. the piston is 10/1.
Running on a 18 tooth output sprocket, and using a satnav for checking, my top speed was now-
Top gear 60 mph, with difficulty, at 5200 rpm.
Third gear 60 mph more quickly, at 6600 rpm.
The engine did not seem to want to rev any higher. Very disappointing.
I cant think why this could be so?? All the parts seem to be in good order.
Mick
Title: Re: C15 ss Max speed
Post by: JulianS on 17 April, 2017, 17:06:43
I have no doubt that your current overall gearing is much to high for the power of your engine, max speed in top and third being the same means top is just an overdrive.

The manufacturer issued a service sheet which recommended a gearbox sprocket with 16 teeth as giving the best performance and you are using an 18 tooth sprocket .

The service sheet is attached below.
Title: Re: C15 ss Max speed
Post by: R PAWSON on 17 April, 2017, 21:03:13
With the Gold Star, according to the development history, they found that a step down from the carb diameter to the port diameter gave an increase in power and to remove the step can be a mistake.
Whilst I understand what you are trying to do getting the thing to perform to keep up with modern traffic, you also need a good set of brakes, my, admittedly limited, experience of C15 s would indicate they are pretty marginal at best and if you do get it to cruise at a speed far above it's anticipated cruising speed when it was designed in the fifties you may well be demonstrating that adrenalin is brown.
Consider that the bikes released to the press for the reports you are reading were carefully put together before being released. 
Also consider as Julian keeps saying revs mean power ( and perhaps a wrecked engine!) so overgearing doesn't help top end if the engine will not rev out.
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 18 April, 2017, 13:32:52
I removed the 'Quietener' from the rear of the silencer,. I can now get up to 65 mph. (speedo reading over 70)
About the high gearing -
with 60 mph at 6600 rpm in third gear. this would work that at 80 mph the revs would be 7270, not quite high enough for max power??
but surely near enough to give better power than I am getting??
Mick
Title: Re: C15 ss Max speed
Post by: JulianS on 18 April, 2017, 16:06:15
Mick

Maybe the best way forward is to just enjoy riding your C15 as it is. It is not going to be in anyway competative on our roads today.

There is a lot of pleasure to be had just going for a ride without all the pressure and stress of worrying about how fast it will go.

I think the question of gearing has been done to death in this thread and the similar thread on the old forum.

The only thing left to do if you want to wring maximum speed out of it would be to take the advice given by Arthur 2 months ago which was to put the bike on a dynamometer and see what that tells you about the power output and state of timing,and carburation etc.
Title: Re: C15 ss Max speed
Post by: STAR TWIN on 19 April, 2017, 17:07:49
Too often over the years I have seen people (usually keen teenagers) try and get their C15s to perform far above their design limits. It always ended in tears and blown engines.
Title: Re: C15 ss Max speed
Post by: TTJOHN on 19 April, 2017, 20:13:51
If you want more speed from a small bike buy a jap bike, stop wasting your time and effort trying to get more out of something which was not designed for speed, no matter what you do to, it it's still a C15.
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 21 April, 2017, 14:13:01
My standard 175 Bantam was faster. 65 mph normal, and with with the wind , 69.8 mph for 2 miles!
Only good news on the C15  is - one kick start every time, from dead cold, no choke, no tickle, no throttle.
Just with throttle stop set right.
Mick
Title: Re: C15 ss Max speed
Post by: STAR TWIN on 21 April, 2017, 15:37:12
If it starts first kick from cold, no choke, no tickle, your tickover and possibly cutaway are set far too rich. Set them when the engine is at operating temperature and you will notice the difference in smoother running and better mpg.
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 25 April, 2017, 18:46:42
I bought a pressure gauge to check if my C/R was really 10/1.
After a number of kicks, the gauge reached 134 PSI.
So, with atmosphere at 15 PSI the ratio would be 8.93/1.
Atmosphere at 14.7 PSI ratio would be 9.12/1 ??
My bike accelerates well, pulls away lustily from 30 mph in (overgeared) top.
Now reaches 65 MPH, on satellite speedo, at 5500 Rpm, or 7750 rpm in third.
Mick
Title: Re: C15 ss Max speed
Post by: STAR TWIN on 26 April, 2017, 20:36:30
I would hesitate to rev it to 7,750 rpm too often. Bravery or cruelty to old engines?
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 07 May, 2017, 11:53:01
Latest test this morning. Windy.
67 mph in one direction, 76 mph the other way.
Revs about 6600 with 2 teeth oversize sprocket.
I dont want to use the smaller sprocket. I prefer low revs on the engine.
This will greatly reduce stresses and (hopefully) stop it failing.
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 07 May, 2017, 11:59:31
PS.
I also have a Virago V  250 twin. This easily reaches 80 mph.
I have no liking for this 'cruiser' style bike, but it is one of the few bikes weighing about 300 lbs.
I have not the strength for anything heavier. Many of the 250 bikes are 350 lbs or more.
I will sell it soon.
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 24 May, 2017, 18:34:53
I just sold my 250 Jap bike (Easily reached 85). It did not have the appeal and nostalgic feel for the old British bikes.
My C15 Now manages about 70 MPH, perhaps  a bit more.
I measured the head volume. - 35 cc. This works out at just over 8/1 compression ratio. The piston part number makes it 10/1 ?
I find the brakes are very effective.
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 29 May, 2017, 14:32:51
I have now reverted to the original absorption silencer.With a few short runs, I find that the top speed is 70 MPH, or a little more,
measured by satellite. Speedo reads 10 mph fast. (80 top speed!). Rpm reaches 5150 at 60.That should be 5956 rpm at 70.
, and 7940 at 80. isnt that about right??
I do not want go down on gearing. I dont like to run at high revs. At 50 mph, I would prefer to gear  UP! But that would mean using third for higher speed, No.
I really aught to increase engine size for result I want, but engines are expensive and one does not know if they will perform without renovation.
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 29 May, 2017, 15:39:11
My figures were wrong. Tried again and got
6008 @ 70
6866 @ 80
So ,its about 1000 rpm too slow at 80 (if it could reach that.)
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 24 June, 2017, 13:31:30
My C15 SS came with 15 tooth sprocket. As used on C15T
the standard for C12 SS, is 16 tooth.
For C15, 17 tooth.
I have now changed to 18 tooth.
I like it!
More relaxed at higher speeds. It still moves away from rest easily, and has lusty acceleration with a handful of throttle.
And that nice bang bang sound of a big single.
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 01 July, 2017, 14:06:52
This morning I changed my rocker box gasket. Bad leaks from top end.
I tested my top speed. Now 66 mph at only 5300 rpm, not the 8000 rpm
'red line' for this engine. Much more pleasant.
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 25 July, 2017, 09:22:18
I think I have reached a plateau with my C15 SS.
With the 18 tooth sprocket, it needs clutch slip at take-off, or in slow traffic.
Did a flat out speed run. 66 mph one way. 71 mph coming back.
With the previous low gearing, it was no faster.
At only about 5400 rpm, it is more pleasant and less strain on the engine.
At full speed on full throttle, when I ease the throttle back ,there is no loss of speed until nearly down to half throttle!
This suggests that the 1 1/16" carb is bigger than needed. It would be interesting to try a 7/8" if I had one.

8.5/1 comp ratio, scrambles cam, bigger inlet valve.
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 25 July, 2017, 10:35:11
PS
80 mpg
Mick
Title: Re: C15 ss Max speed
Post by: Mark on 25 July, 2017, 19:24:25
I really can't understand what you are trying to achieve these bikes are made to enjoy & not to thrash the arse off them & probably destroy them.

SIMPLE SOLUTION IF YOU WANT MORE SPEED BUY A BIGGER BIKE! ???

Regards Mark.

Title: Re: C15 ss Max speed
Post by: TIGERJ on 29 July, 2017, 22:34:35
Mick your C15 seems to go really well I however am struggling with a 375 carb.
Am I correct in thinking you are on a 376 with 200 main jet ?
Regards
           Tim
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 09 August, 2017, 09:44:40
Mark,
I want to drive at normal road speeds - 60MPH, 70 MPH, even 80 MPH.
My bike reaches 60 OK. 65 takes awhile. 70 is rarely possible.
I would like 80 or more to be available, so overtaking is safer.
I ride FLATOUT where the road allows, that is because the bike is so slow!
There is NO problem with running at full power.  It is needed with any low powered bike.
If I had plenty of power I would not need to ride flatout!

I know a Japanese bike would give me the performance, but my desire is to have a classic British machine.
I do not have the physical strength to handle anything heavier than 300 lbs.
Cost is also a consideration. My riding is only local, its a problem thinking of anywhere to go!!
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 09 August, 2017, 10:35:06
I cant see a number on the carb.
I think the main might be 190
Mick
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 21 September, 2017, 18:38:55
I have now bought a B44. first quick rides round the block.
It easily reaches 75/80 on the clock. Clock reads about 8 mph fast.
Must try to calibrate it.
Mick
Title: Re: C15 ss Max speed
Post by: TIGERJ on 21 September, 2017, 22:19:27
My clock is also reading about 10 mph fast.
You mention recalibrate
How do you go about this ?
Title: Re: C15 ss Max speed
Post by: JulianS on 21 September, 2017, 22:53:12
Get speciaist help. Not a DIY job!
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 23 September, 2017, 07:02:55
My way is to use a satnav on the bike, or get someone in a car to drive at a set speed.
I note the speedo reading, then I stick thin adhesive strips on the speedo at each 10 mph point.
My C15 ss read 20 mph over at top speed, (85 at 65)
Mick
Title: Re: C15 ss Max speed
Post by: TIGERJ on 23 September, 2017, 07:22:41
Thanks Mick.
Good idea.
Tim
Title: Re: C15 ss Max speed
Post by: TIGERJ on 23 September, 2017, 07:26:30
Mick
You anywhere near Staffordshire for a quick meet ?
Tim
Title: Re: C15 ss Max speed
Post by: MICKREEVES on 23 September, 2017, 12:48:09
no, in march cams
mick