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1954 ES2 Engine spitting back.

Submitted by martin_oldham on Wed, 17/02/2021
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I recently bought this bike, it has original matching engine and frame numbers and original gearbox. It’s in lovely “patina” condition with little sign of mechanical abuse. The previous owner was an engineer and had just replaced the cylinder/crankcase gasket due to an oil leak, after which the bike was suffering from spitting back through the carb on tick-over. This seemed like a minor issue as, once off tick-over, it ran well so assumed it would be an ignition/magneto or carb issue.

Carb checked, magneto removed and checked, no sign of problem with ignition. Thought maybe a valve issue?

Removed head, inlet valve seat was OK, exhaust valve seat showed some carbon deposits which might be due to poor seating, possibly being held slightly open. No sign of sticking mechanism.

Valves lapped in, head replaced, engine started. Still spitting back! What next? Any ideas?

1954 ES2

Submitted by robert_tuck on Wed, 17/02/2021

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Possibly low fuel level  due to float malfunction or partial blockage of pilot drillings in jet block (which can only be checked by drifting the brass jet block out of the body.)

Submitted by ian_soady on Wed, 17/02/2021

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.. ES2, fitted witha 1 1/16" monobloc, spat back on slightly opening the throttle. I did 2 things but am not sure which cured it. One was to fit a long reach spark plug (no idea why a short reach one is specified) and the other was to fit a 3.5 slide rather than the specified 4.

It runs like a watch now.

Submitted by robert_tuck on Wed, 17/02/2021

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With this type of pre monoblock carb the pilot drillings are really tiny and can only be accessed by removing the block.   If the slide is hanging up a fraction it could cause extra air ,but would also speed up the tickover . A smaller slide cutaway would richen the mixture when coming off the pilot but does not affect the pilot mixture at tickover. I suspect when the engine was apart the muck in the base of the carb was disturbed or the float /needle clip has moved.Another faint possibility is that the valve timing has been changed on re-assembly.  A nice little puzzle for you to stave off the boredom !. And to keep us entertained.. Do the simple easy things first.Sounds like all was well before the engine strip.

Thanks, Robert!! I started on this back in December. Took everything to bits gradually and sorted one thing at a time. Thought it would be a nice lockdown project! Even bought an endoscope to inspect the inlet and exhaust valves without taking the cylinder head off. Decided to take the head off anyway and lap the valves. A bit grumpy now it's back together and still spitting back! Although I had the carb appart I guess closer inspection may be needed. What are the chances that the valve timing can have changed? You shouldn't need to take the timing gears off to remove the cylinder, surely?

Thanks for your advice. I'll let you know how I get on, once I've found the washer from the fuel tap and stopped it leaking petrol all over the floor!!

Submitted by robert_tuck on Thu, 18/02/2021

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Timing won't have altered if you did not dismantle.  Air leak at carb mounting ?. I would still put money on it being partially blocked pilot drillings, either in the jet block or in the matching body drillings. Take care knocking the block out thro the top of the carb , and watch out for the paper gasket . These old carbs are a different animal to mono or concentric, a soak and blow through will have little effect  on  blockages , nothing less than a full dismantle and a poke through with a fine wire will do.

Submitted by niall_topping on Thu, 18/02/2021

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I agree that the valve timing should not have altered unless it was disturbed in this latest strip-down, but that does not mean that it is right!

Read Mike Pemberton's piece on single cylinder valve timing for just how much variation there was in the valve events on new bikes, and that was from the factory, not withstanding replacements and bits from different bikes getting mixed together over the following 60+ years.

The timing marks on the pinions really cannot be trusted as more than a starting point, on my M50, I advanced the exhaust cam a full tooth from the marks after I checked it with a degree disc and it ran better in all respects, although it still isn't right, just quite a bit nearer.

Submitted by ian_soady on Thu, 18/02/2021

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... a long reach plug? For some reason these bikes had a short reach specified even though the thread is a full 3/4". Mine runs very well with the long reach version.

Submitted by martin_oldham on Fri, 19/02/2021

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I'm going to treat myself to a new gasket set for the carb! When it arrives I'll get the jet block out and check it properly.  I have to admit I didn't remove it when I did it before.

I'll let you know how I get on!  Cheers!

Submitted by martin_oldham on Wed, 03/03/2021

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Removing the jet block was a challenge,  plenty of heat from a hot air gun and it pushed out in the end.  Cleaned all the parts and got them ultrasonic cleaned.  Found the flange was bowed and flatted it carefully. Getting a 276 carb leak free is fun! Was careful not to over tighten its nuts. 

Finally it ticks over without spitting back,  just waiting for a chance to get out for a road test and check it's all set up properly. 

Submitted by richard_cornish on Wed, 03/03/2021

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Hi Martin,

   Perhaps your carb has been changed at some time as the ES2 went from stub fitting 276 to monobloc 376, so it wouldn't normally have a flange fitting 276. The 276 flange is prone to distortion as it is quite thin, whereas the monobloc is more substantial and has the 'O' ring seal. But it's good to hear you have solved the problem.

 


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