Another Sloper question

[Paul.W]

The saga continues..
1928 350cc OHV Sloper.  It's all back together with rebuilt bottom end (courtesy of Alpha), starts first or second kick and runs and rides, but;

...its completely gutless.  Ignition timing is pretty well OK (if I fully advance it kicks back, fully retarded it starts OK).  The valve timing is set according to the marks on the gears, the piston reaches the top of the barrel so compression should be right (the piston is pretty well new, fitted by previous owner).  I've checked the carb jet (sign of desperation), the valve clearances are according to the manual (it says about 4 thou).

On the road flat out with fully open throttle is about 43mph.  Its only a 350cc but the literature says it shouldn't be much slower than the 500cc, and should cruise at about 50 with a top speed nearer 65.  It just lacks any go and I've run out of ideas, so any suggestions of what I might try would be gratefully received.  It has been suggested to me I might try altering the valve timing by a tooth - check the inlet starts opening just before TDC for example. Its easy to do so may be worth trying.

Its very disappointing. If I can't get it to cruise comfortably at about 50 it will have to go, but I'd rather keep it - but only if I can enjoy riding it.

[royblackburn1@btinternet.]

Hi Paul is there any slack in the throttle cable and you think its flat out and its not, just a thought. Roy

[Paul.W]

A good thought, but no.  I stuck my finger up the inlet and the carb and choke are fully open when the throttle is fully opened.  Thanks for suggestion though.

[BEVANC]

Does it have standard gearing ? I found my 500 Sloper was geared way lower than standard for running with a sidecar. Happy enough solo at 60 mph now I've upped the gearing,  but usually cruise at 50 mph. Cheers, Bevan

[Paul.W]

That's a good point.  Would a 350cc normally be used for sidecar work?  It doesn't feel particularly low geared when I set off in first, but I've never ridden another to compare it with.  Any chance, when you have a moment, that you could count teeth on the sprocket so I can compare - unless you happen to have it written down.  Which sprocket did you change, and where did you get it from?

The more I think about it, the more I think that's a plausible explanation - I haven't managed to find or think of any other.  Although flat out at 45 would seem to be slow even for a sidecar.

[Paul.W]

That's a good point.  Would a 350cc normally be used for sidecar work?  It doesn't feel particularly low geared when I set off in first, but I've never ridden another to compare it with.  Any chance, when you have a moment, that you could count teeth on the sprocket so I can compare - unless you happen to have it written down.  Which sprocket did you change (I assume the rear one, the drive sprocket would be difficult to change), and where did you get it from?

The more I think about it, the more I think that's a plausible explanation - I haven't managed to find or think of any other.  Although flat out at 45 would seem to be slow even for a sidecar.

[BEVANC]

Hi Paul,
The standard solo gearing for a 500 Sloper is engine: 21, gearbox 19, rear wheel 42 teeth.  I bought a 21 tooth A10 4 lobe engine sprocket which fitted fine. My rear wheel sprocket has 46 teeth.  Although the bike performs well and seems happy at 50 mph, I am intending to get a 42 tooth one made at some point (doubtless will need to take out a mortgage !).

I do not know if this info also applies to the 350, but I can find out from a contact who has one and let you know in a day or two.
Cheers, Bevan.

[BEVANC]

Hi Paul, an update from my contact with a 350 Sloper. His has engine 19; clutch 43; gearbox 15; rear wheel 42 teeth. He cruises at 45 mph, is unsure of top speed as he doesn't want it to self-destruct, but he's guessing 70 mph under ideal conditions. Might be useful to know your carb details and jets etc for comparison, also how you have set your timing. Cheers, Bevan.

[Paul.W]

Thanks for that Bevan,

Things are getting clearer.  I've counted teeth on all the sprockets and done the sums and also found a table of ratios in the handbook.  My ratios are,
engine             -  22
clutch              - 48
Drive sprocket  - 19
rear wheel       -  42
From the handbook, the conclusion is that I have 500cc gearing on the bike, rather than the 350cc ratios it should have.  The result is that in top gear it is 16.0mph/k revs instead of the 14.1mph/k revs that is should be (assuming a 3.50x19 tyre)  i.e, it is over-geared by about 15%. I think that means that it is running out of puff at about 3k revs = 43mph.  With the lower gearing is should have, 50mph would be about 3.5k rpm which doesn't seem unreasonable - I think the engine should manage those revs.  I think my fundamental problem is the bike is over-geared.

I have the opposite dilemma to you.  My rear sprocket has 42 teeth, to get the equivalent gearing to standard for a 350cc I need a 48 tooth sprocket.  I don't fancy the likely cost of having one made so I'm looking at alternatives. 48 teeth (and other) sprockets are available off the shelf for modern bikes but the PCD will be different and the BSA sprocket is dished and not flat so a flat sprocket may foul something.  The bigger sprocket is about (its an estimate) 1" larger in diameter so it should fit (the chain guard might need moving/modifying slightly).

One solution I've considered (which would also work for you) is to buy an unmachined 48 tooth platewheel (see www.simplybearings.co.uk) and machine out the centre (or possibly use the Suzuki one), then get a flange machined up that will fit on the wheel and carry the machined platewheel.  This will provide the necessary offset for the sprocket, be an easily reversible mod and give me the correct overall gearing.

Interesting, the handbook says that 350cc gearing should typically be (there are variations);
engine       -  22
clutch        -  43
gearbox     -  15
rear wheel  -  42
so your contact's bike appears to have a lower engine gear at 19 than original, the others match the table. Those ratios would give him a gearing of 12.21mph/k revs. so 45mph would be 3.6k.

Sorry about length of essay  -  I got a bit carried away. I think all the figures are right, but I might check them. Tomorrow I plan to do a high(-ish) speed run in 2nd and then work out what the revs are to see what it  seems happy to rev to. It would be nice to know what max revs are safe/sustainable. Also, I wonder if the larger engine capacity is done by increasing bore or stroke.  If stroke then the 350c might be happy revving slightly more.

[BEVANC]

Hi Paul,
It certainly does look as if your bike is geared up too much. The fact that your clutch has 48 teeth must surely mean you have a different clutch fitted than David's 350 ? He said his was different to my 500, but I don't recall in what way. 

If our rear sprockets have similar offset, maybe we could do a swap - my 46 tooth for your 42 ?

Someone told me that an A7/A10 42 tooth sprocket could be used as a donor if I wanted to machine it to a ring and attach to the original dished part. I don't know if those go up to 46 or 48 teeth. 

I have my gearbox mainshaft with a gear making company who are rebuilding a missing tooth. They did say they could make me a new rear sprocket, so I might take mine in to see how scary the cost would be.

If you like, I could ask David what his carb details are, in case you've got something radically different which is also limiting its performance.

I will be interested to hear about your progress.
Cheers, Bevan

[Paul.W]

I took it for a run today with a friend following in a car.  The conclusion was (once I'd sorted the slipping clutch) a top speed of 40-ish in 2nd (slightly down hill and downwind), and a top speed of 43 in top (into the same wind) so gearing does seem to be the (a?) problem.  I've gambled £12 on a 48 tooth sprocket (for a Kawasaki).  When it arrives I'll see if I can make an adapter plate and fit it.   Swapping is an idea, but I don't want to commit myself until I know what I'm doing, fitting the Kawasaki one will tell me if that's the problem.  If adapting the new sprocket to fit (and I can't see why not) that would be a simple solution for you as well.

In the next day or two I'll look at the carb and identify the jet size.  I'll let you know what I discover.

[Paul.W]

Update......

I machined up a carrier to take the Kawasaki sprocket.  Once painted black you'd never know it was there.  This has changed rear from 42 teeth to 48 which is about 15% lower, so it now has the correct gearing for a 350cc.

Result is that it is better.  I can now get about 34 max in 2nd and have maxed out on the latest test at 47 mph in top and it doesn't feel stressed at those revs.  That is OK-ish as a cruising speed I guess - but it is effectively flat out and I still feel it should be able to do better than that.  What reviews I've managed to find suggest that flat out should be closer to 60.

What the ratios tell me is that in top 47mph is about 3.3k rpm and 34mph in 2nd is also about 3.3k revs. I will tinker a bit more, but I have a feeling that is all it has to give.

[BEVANC]

Hi Paul,
Some progress, but still a bit disappointing as you suggest.

To eliminate another possibility, if you wanted to give me your carb details/settings/jets etc I can find out from David if they are the same as his 350.
Cheers,
Bevan

[Paul.W]

Thanks. I'll do that.  I need to replace the head gasket.  As soon as the piece of copper I've ordered arrives I'll pull it to bits (again!)

[Y13 Alan G]

Hi Paul,
I have a 1929 500 cc BSA Sloper with sidecar and flat out with or without a passenger is about 45 mph.
Regards
Alan