Measure Compression Ratio

‘ONE OF THINGS THAT REALLY APPEALS TO ME ABOUT WORKING ON PRE-WAR BIKES IS THE EASE OF ACCESS’

Should it really be this fiddly to do an oil change?

Should it really be this fiddly to do an oil change?

Complaining about fitting oil pipes to his Norton Commando the other day, my mate Paul said, ‘You need a vice-like grip in child-size hands!’ He’s right: one of the things that really appeals to me about pre-War bikes is the ease of access. It seems that the newer the bike, the less space there is to work. In my youth, the only old bikes I could afford were, er, ‘specials’, a flattering term that truthfully meant they had all been bodged and blown up so many times that very little of the original factory spec remained. Sundry electrical components were jubilee-clipped into corners beneath the seat and tank, petrol taps snuggled between carburetter tops and contorted oil pipes struggled to find their unions. But my nearly-new T140 Bonneville wasn’t much better – it took a safe cracker’s feel to locate the tank rubbers to prevent disastrous metal-tometal contact. Topping up generally meant pouring oil all over the electrics and (famously) new Bonneville purchasers were handed the toolkit with the keys and paperwork because when it reposed in the tool-tray the seat wouldn’t shut... After that, my 1932 Sunbeam was a revelation. Things seemed to have been thought out, and the two leather boxes on the rear carrier held enough tools to do any job. What went wrong? I believe as the factories updated existing designs, they created issues that needed compromises. On today’s bikes, these extras have taken so much space so that no mechanic larger than an earwig can get in without partial dismantling. Rigid frames and sprung saddles? I tell you, it’s the future...

 

TOP TIP

BE ORIGINAL

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Can I be controversial? I don’t like stainless spokes. The idea of spokes that never rust was appealing but they just don’t look quite right on a bike that is supposed to be restored to original. As a lad I used to get fed up hearing my BSA wasn’t ‘the right maroon’, but then my knowledge had come from black and white photos.

Now I see Hondas in not quite the right metallic blue... so apologies to all those ‘silly old gits’ of the past. I see now that when you’ve grown up around something, while it doesn’t matter if it’s not reproduced exactly, you can’t help notice it and feel compelled to inform the perpetrator. Pipe and trilby for Mr Parkington...

 
This hole in an early Amal mustn’t be covered

This hole in an early Amal mustn’t be covered

CRANKY CARBS

So you’ve had the carb and ignition reconditioned and the engine top-end overhauled, probably at considerable expense. How annoying, then, when the bike is a poor starter and near impossible when hot. That’s what Vaughan Miles is experiencing with his 1936 BSA 350cc model R6. Vaughan contacted my blog site (rickparkington.co.uk) to say he is suffering this trouble despite the bike going really well once started and plug readings indicating a good mixture. He adds that the bike also won’t tick over, unless you wind in the adjuster... then it revs its head off. Problems like this are difficult to diagnose – especially remotely – but a few thoughts come to mind. Engines will usually start from cold however rich the mixture unless the plugs get wet – it’s weakness that is the most likely problem. Vaughan says the throttle needs to be closed for starting, which may mean that the slide cutaway is too large and a lower number slide would help richen things up. But given that it really doesn’t want to go hot I wonder if the pilot system is blocked; cold flooding the carb gives an artificial fuel supply swishing about that may mask the absence of fuel coming through the pilot jet. This would explain the lack of tickover until the throttle is adjusted so high that it is well past the tickover range. On these early carbs the jet is a fine drilling inside the central jet block; if the carb has been overhauled it should be clear. An examination with some fuse wire may be in order – being copper it’s soft enough not to damage the jetway. There is another big pitfall I’d check: the fuel enters by a hole in the base of the block and using the wrong fibre washer can block it. It’s a mistake I’ve made myself and easy to check.

TUBE TROUBLE

While wheel sizes of 19 and 21” inch are common, 20” is less so. The Vincent front wheel is the best known but, in fact, 20” pops up on all sorts of things, from the rear wheel on pre- Featherbed Norton Internationals to the front of an ex-Army BSA B40. Nonetheless it’s still a limited demand and there was a time when 20” tyres were so hard to obtain that many chose to fit alternative rims. Fortunately Avon filled the breach with limited-production batches of both front ribbed and rear universal, but what still seems to be lacking is a 20” inner tube. I bought a 20” front tyre at the Stafford Classic Show and was advised by the retailer that stretching a 19” tube to fit was a better bet than trying to squeeze in a 21”; this pleased me because I’ve always thought the same, suspecting that it might led to creases that would chafe and puncture. When I went to change the tyre, at first I thought the old tube must be full of tyre sealant because after removing the valve no air came out. Poking about with a twist drill soon produced deflation but it was only when I removed the tube that I found the cause. A 21” tube had been fitted and had creased so badly around the valve that it was strangling it – I’d had to puncture it to release the air. A 19” tube went onto the rim without much difficulty so that settles it for me. If you can’t get the right diameter, smaller is better than larger.

SUZUKI SEALS

After sitting in the garage for two years, Keith Hett’s Suzuki T500 has started smoking on the left cylinder. Concerned that it is burning gearbox oil, Keith asks if it’s likely to be the crankshaft centre seal or one of the end seals that seems to be causing the problem. I have no practical experience of the big Suzuki but with twostroke twins generally, the centre crankshaft seal is there to separate the two cylinders’ crankcases – preventing the side that is compressing from affecting the one that is on induction. In the case of many two-stroke twins the centre seal isn’t a normal rubber ‘garter’ type; Yamaha use an aluminium labyrinth type and Villiers a sort of piston ring, because durability is more important than a perfect seal. The centre seal is there to resist sudden changes of pressure: just like the piston rings, it’s a different situation to a seal that is trying to keep oil out. There are conventional seals on either end of the crank and if gearbox oil is getting into the crankcase, it’s likely to be the clutch-side seal that is the problem – assuming the alternator doesn’t run in oil. The usual problem with changing seals on Japanese bikes is that many have a lip around the outside of the seal that fits into a groove in the housing, meaning that the cases have to be split to fit it. I don’t have a manual for the T500 but looking at an illustration on the internet it looks like the seal is held in place by a circlip, so it should be possible to change it relatively easily. There’s also an ‘O’ ring seal in there – that might be worth replacing too.

Creases in the old tube show how crumpled it was in the tyre

Creases in the old tube show how crumpled it was in the tyre

DAMPER DISAPPEARANCE

Tom Cutler has been working on his Triton forks. Taking off the top nuts he found that a damper rod had come unscrewed and was unsure how to get it back. The end of the damper rod screws up into a threaded hole in the underside of the fork top nut, secured with a locknut. Well, it should do... if it works loose, the rod telescopes down into the fork leg and becomes very difficult to retrieve. Deft fishing about with a wire hook or magnet will fetch it up, but you need to pull gently and slowly. It’s a damper after all – the harder you pull, the stronger it resists. Once you’ve got it back out of the top and screwed safely into the top nut, there is one other thing to bear in mind. The reason for the long thread and the locknut is to adjust the length of the rod; I think the easiest way to get it right is to take out the front wheel and adjust the rod until the wheel spindle pushes easily through the fork ends. That way, you know the extended length of both legs is the same.

 

How To

MEASURE COMPRESSION RATIO

I wanted to check the Martinsyde’s compression ratio, and it turned out to be pretty simple. Here’s what I did…

1. With the top-end off, I was able to set the piston to a measured TDC and do the job on the bench. On the bike you’d have to lean it so that the spark plug hole is uppermos

1. With the top-end off, I was able to set the piston to a measured TDC and do the job on the bench. On the bike you’d have to lean it so that the spark plug hole is uppermos

3. Pour the oil in, piston still at TDC on compression with the valves shut, until it brims the bottom of the plug hole. With the plug hole uppermost, the chamber is completely full.

3. Pour the oil in, piston still at TDC on compression with the valves shut, until it brims the bottom of the plug hole. With the plug hole uppermost, the chamber is completely full.

2. Fill a measuring jug up to a clear level mark (here 600cc) with light oil; you’ll need to tip it out after and a teaspoon of 10/40 left will seep past the rings easier than SAE 50...

2. Fill a measuring jug up to a clear level mark (here 600cc) with light oil; you’ll need to tip it out after and a teaspoon of 10/40 left will seep past the rings easier than SAE 50...

4. 525cc left, making combustion chamber volume 75cc. Adding the 339cc swept volume gives a theoretical BDC volume of 414cc. Dividing 414 by 75 gives a ratio of 5.52 to 1.

4. 525cc left, making combustion chamber volume 75cc. Adding the 339cc swept volume gives a theoretical BDC volume of 414cc. Dividing 414 by 75 gives a ratio of 5.52 to 1.

In