Don't want these Ads? Why not sign up as a Trials Central Supporter.

Sign in to follow this  
Followers 0
beamish owners club

Bsa C15 Conversion Advice

23 posts in this topic

For my sins, I have just bought a 1962 C15 (distributor model) which looks the part of a trials bike but has a standard road version engine. It has a 375 monobloc Amal carb. I am going to change the engine duplex sprocket for a 18T and the gearbox sprocket for a 13T with a 64T on the rear wheel. Once I have taken off the lights, got it to run smoothly at low revs and got used to Pirelli tyres I hope to trial it. Am I on the right track to have a usable machine or do I need to do anything else?

I had heard that lengthening the inlet manifold helps with low revs and torque - does anyone know if this is true?

I would like to find a kick start that clears the footpegs - any suggestions?

Any advice on engine/gearbox oil would be gratefully received too!

Share this post


Link to post
Share on other sites

Don't want these Ads? Why not sign up as a Trials Central Supporter.

For my sins, I have just bought a 1962 C15 (distributor model) which looks the part of a trials bike but has a standard road version engine. It has a 375 monobloc Amal carb. I am going to change the engine duplex sprocket for a 18T and the gearbox sprocket for a 13T with a 64T on the rear wheel. Once I have taken off the lights, got it to run smoothly at low revs and got used to Pirelli tyres I hope to trial it. Am I on the right track to have a usable machine or do I need to do anything else?

I had heard that lengthening the inlet manifold helps with low revs and torque - does anyone know if this is true?

I would like to find a kick start that clears the footpegs - any suggestions?

Any advice on engine/gearbox oil would be gratefully received too!

1. Junk the ignition fit a pvl and blank off the distributor hole or fit a breather to it.

2. Amal concentric 622 is good as in another post although i prefer the 624 . I run a 627 on the B40 but i like big carbs.

3. Junk the tyres use a Michelin front and IRC rear. Pirellis are err.... crap.

4. Terry Weedy for the kickstart. Same as Millers but cheaper.

5. Lengthening the inlet does help. I made one on the lathe but if you dont have acess ring Surry Cycles who do a thick tufnol spacer. This is essential to stop the carb and fuel vaporising.

6. you didnt mention this but the forks will need to be changed at some point as will the yokes along with the hubs but 1 - 5 should get you started.

good luck and enjoy.

Share this post


Link to post
Share on other sites

Thanks for the advice! I agree about the Pirellis by the way! The PVL is

Share this post


Link to post
Share on other sites
Thanks for the advice! I agree about the Pirellis by the way! The PVL is

Share this post


Link to post
Share on other sites

Something to be aware of if you do remove the distributor for PVL ignition. The distributor drive shaft also drives the oil pump from its bottom end. When the distributor is removed there is nothing to hold the driveshaft in place (as far as I am aware - there isn't on mine anyway) therefore there is nothing to stop the drive shaft riding up in its housing and disengaging from the oil pump which means the oil pump is no longer driven - not good

Mine had a blanking plug which I never gave a second thought to but last time I put the engine back together the driveshaft wouldn't stay located in the oil pump, it kept disengaging when the engine was started as there was nothing to hold it in place. Why this has never happened before I have no idea but I've been very lucky. I refashioned the blanking plug and made it longer so that it rests upon the top of the drive shaft and holds it in place now.

Maybe there was/is something missing on my bike to locate the shaft but I can't see anything on parts diagrams. Just something to be aware of.

I've heard that some people also use B25 pistons in the C15 as they are flat topped and reduce compression giving a softer bottom end - combined with moving the carb backwards. Not sure if moving the carb back improves torque but it will soften the power delivery by slowing up the gas flow.

22mm Amal will increase response off the bottom end over a 24mm as it increases gas flow due to smaller venturi. So using a 24mm and moving the carb back should provide the best combination for a softer power delivery - in theory anyway...

The pre65 brigade in this area reckon that new Amals can come with casting flaws in the airways. Without question, the one I bought had very poor airflow out of the pilot jet housing when an airline was stuck on the pilot inlet at the rear of the carb (with the pilot screw removed and a finger blocking the hole. One of the boys that knows drilled out the pilot screw air passage, tapped out the swarf and when we repeated the airline test there was a much stronger blast of air out of the pilot jet housing. Still however, the bike used to cough stall a lot when opening the throttle (and if anyone else says 'you're trying to ride it like a 2 stroke' I'll swing for them...) unless the pilot screw was right in, or quarter turn out at most. I put the carb in an ultrasonic cleaner last week which should remove any obstruction from the airways and now it will pick up cleanly with the pilot screw just over 1 turn out so maybe there is something in what they are saying about the Amals having flawed casting in some units.

I'd also fitted a new PVL too, so it could also be that which made the difference - I'll never know now. I was convinced the PVL that was fitted when I bought the bike was faulty as the swings in performance were to radical to be carburation. When I removed the old one (having bought the replacement already) I found that the earth to the frame from the mounting bracket was not too clever so it could have been that too. However, I cleaned that up, fitted the new PVL, re-fitted my nice clean carb and it runs ok now - should know by now to only change one thing at a time.....

I can also recommend the book on C15 by Rupert Ratio (yes, that is the author...) as it has lots of little tips on running and maintaining a C15 - usually find one on ebay but Merlin books have them I think.

Share this post


Link to post
Share on other sites
Something to be aware of if you do remove the distributor for PVL ignition. The distributor drive shaft also drives the oil pump from its bottom end. When the distributor is removed there is nothing to hold the driveshaft in place (as far as I am aware - there isn't on mine anyway) therefore there is nothing to stop the drive shaft riding up in its housing and disengaging from the oil pump which means the oil pump is no longer driven - not good

Mine had a blanking plug which I never gave a second thought to but last time I put the engine back together the driveshaft wouldn't stay located in the oil pump, it kept disengaging when the engine was started as there was nothing to hold it in place. Why this has never happened before I have no idea but I've been very lucky. I refashioned the blanking plug and made it longer so that it rests upon the top of the drive shaft and holds it in place now.

Maybe there was/is something missing on my bike to locate the shaft but I can't see anything on parts diagrams. Just something to be aware of.

I've heard that some people also use B25 pistons in the C15 as they are flat topped and reduce compression giving a softer bottom end - combined with moving the carb backwards. Not sure if moving the carb back improves torque but it will soften the power delivery by slowing up the gas flow.

22mm Amal will increase response off the bottom end over a 24mm as it increases gas flow due to smaller venturi. So using a 24mm and moving the carb back should provide the best combination for a softer power delivery - in theory anyway...

The pre65 brigade in this area reckon that new Amals can come with casting flaws in the airways. Without question, the one I bought had very poor airflow out of the pilot jet housing when an airline was stuck on the pilot inlet at the rear of the carb (with the pilot screw removed and a finger blocking the hole. One of the boys that knows drilled out the pilot screw air passage, tapped out the swarf and when we repeated the airline test there was a much stronger blast of air out of the pilot jet housing. Still however, the bike used to cough stall a lot when opening the throttle (and if anyone else says 'you're trying to ride it like a 2 stroke' I'll swing for them...) unless the pilot screw was right in, or quarter turn out at most. I put the carb in an ultrasonic cleaner last week which should remove any obstruction from the airways and now it will pick up cleanly with the pilot screw just over 1 turn out so maybe there is something in what they are saying about the Amals having flawed casting in some units.

I'd also fitted a new PVL too, so it could also be that which made the difference - I'll never know now. I was convinced the PVL that was fitted when I bought the bike was faulty as the swings in performance were to radical to be carburation. When I removed the old one (having bought the replacement already) I found that the earth to the frame from the mounting bracket was not too clever so it could have been that too. However, I cleaned that up, fitted the new PVL, re-fitted my nice clean carb and it runs ok now - should know by now to only change one thing at a time.....

I can also recommend the book on C15 by Rupert Ratio (yes, that is the author...) as it has lots of little tips on running and maintaining a C15 - usually find one on ebay but Merlin books have them I think.

Totally agree woodie which is why i prefer the 27mm carb on the B40 combined with the extended manifold. Gives me the best of both worlds.

You may well be right about the distributor shaft. It's a long time since i worked on a distributor model forgot about that.

Rupert Ratios bible is again money well spent.

So there you go B O C the forum agrees for once ;)

Share this post


Link to post
Share on other sites

Hi guys.

Just sat the F type C15 engine on me lap (the one for the Scott Ellis Bike), and found a small set screw half way up the distributor shaft tunnel that holds the shaft in place, guess yours must be missing, or this is a mod for the "f" type engine.

hope this helps, and good luck with the build.

Ps, the PVL is a must I agree but shop around for a better price IE, Karting direct. or the new UK distributor in Cambridge, Or Alan Whitton.

Regards Charlie.

Share this post


Link to post
Share on other sites

be aware with the PVL that they are not all the same

make sure you get a dedicated trials one from someone like alan whitton, may not be the cheapest but it will be the pucker stuff..

Share this post


Link to post
Share on other sites

Yes, be very careful with PVL'S. Those from Martyn Adams and Alan Whitton are fine, but some of the others who have jumped on the bandwagon selling them have ended up with the wrong type for our needs.....

Share this post


Link to post
Share on other sites

Totally agree, from experience it is cheaper to get the right one first time.

The kart ones are designed for running flat out at high revs, the HT coil has fewer windings (lower voltage spark) and often little or no built in advance.

Various HT coils are available with varying degress of advance, guys like Alan Whitton will make sure you get the right one for the job.

At the risk of sounding like a socialist, you may pay a few quid more but you will also get a lot of free sound advice thrown in and a least you are supporting a fellow enthusiast.

Go on you no it makes sense!

Share this post


Link to post
Share on other sites

Well it's all been said. Told you a pvl is the way to go. If you now buy from someone other than Alan Whitton then dont say we didnt tell you ;)

Been there done it got the tee shirt. Low price isnt always the best deal.

Edited by Old trials fanatic

Share this post


Link to post
Share on other sites

Hi Guys,

I agree with what you say, buy your PVL from a man that knows, and can machine the plate and bore the rotor. IE Alan Whitton,

The trouble with me is I alway like to do the job the hard way, because it gives me more time in the shed. But thats anouther story.

I bet you'll have your's fired up before mine as well, But I am working on Six bikes at the moment And I can't remember if I have got one I can ride. are well.

Regards Charlie.

Share this post


Link to post
Share on other sites

so are we saying here that the PVL sold by Sammy Miller for a C15 is not suitable for trials?

Share this post


Link to post
Share on other sites

Sam's may be fine, but i know the two sources i mentioned are good. I've never used one from Sam, but he must have sold plenty and i've heard no complaints.

Share this post


Link to post
Share on other sites

dont forget that a bit of machining is required too so you may as well use alan or somebody like him and get the whole lot done at once

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0