My shop is in an over-sized two-car garage attached to my home. My work is a calling, in the theological sense of the word. Ethical and moral questions arise every day – they are hard, sharp, unambiguous and unavoidable.

The Wretched Truth of How I Fix Things – An Epistolary Example

Posted: April 10th, 2014 | Author: Administrator | Filed under: Motorcycle Repair | No Comments »

Josh,

I kinda back-burnered your bike for a week or so to deal with a couple of minor crises but got back on it today – even took it out for a short test-spin.  The chassis is tight and handles very well but the motor would only run cleanly on the main jet – half to full throttle at medium or higher revs.

It’s hugely rich at/off-idle.  To  get it to fire and run, I have the throttle stop screws full-in and the pilot air screws full-out – both should lean the mixture excessively.  I have stripped both carbs and they are clean as a whistle.

In the course of double-checking my work – ignition timing, valve clearances – I did a compression test and the numbers are very good – 165 psi on both.  When the rings are fully-seated you should see 175 psi.

I’m really having a hard time getting my head around your word that it idled nicely and pulled cleanly before it blew up.  I removed and reinstalled the carbs [Mikuni VM32s] without opening them up.  All the jetting and adjustments were exactly as they came to me.

Please note well – I am not saying it did not run well – just that I am at a loss as to understand why it is not now.

What it is running like is an over-carbureted engine.  Tomorrow I am going to bolt on a stock set from a good-running CB350 and see what happens.  My guess is that it will run the way a freshly-built CB350 engine should.

If not, there is one last ugly possibility – and that is that the cam chain sprocket on the crankshaft is damaged and the cam chain has jumped a tooth.  When I was installing the cams and setting their timing relative to the crank, I had to make several tries to get the cam chain correctly engaged with this sprocket – something one does by feel as it is buried deep in the cases.  I renewed the cam chain tensioner rollers – both of which were badly chewed up and worn.

So, in your own interest – hope that I am right about the carbs.

Cheers,
Chris
Existential Motorcycles
Alexander, NC

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Joshua,

It runs smooth and strong with the stock carbs and idles like a purring puppy.

I can supply you with a cleaned and rebuilt set of carbs for $100.  I have also emailed Speedmoto that sells VM32 Mikunis allegedly correctly jetted for this bike to ask what jet sizes they use/recommend for this bike.

Give me a couple of days to put together a good set of carbs for you – install and tune them – road test and final tuning.

Cheers,
Chris
Existential Motorcycles
Alexander, NC
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Hey Chris, thanks for the update. I’m having some trouble getting my head around it too but if it’s not right it’s not right. I guess if things go smoothly with the stock carbs on then I’ll switch back, do you have anything around the shop you’d be willing to get rid of or should I start looking around for a clean set?

Joshua

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Joshua,

I knew that bragging on myself about my carb building was tempting the fates – and I was right.  It fired, idled and revved beautifully in the shop – but out on the road with the engine under load it was shit – one cyl. running way rich and the other way lean – or so the plugs were telling me.

This is an object lesson on how one can become fixated on/committed to a diagnosis and forget that there are other possible explanations for the problem at hand.  In this case I have spent most of the last two days taking the carbs out, double-checking my work, making jetting changes/adjustments, reinstalling the carbs, syncing them, taking short test-hops – probably at least ten iterations of this scene – based on the assumption that I was chasing a carburetion problem.

Late this afternoon – as despair was setting in – my brain actually started working again and it occurred to me that problems in the ignition system could cause very similar symptoms.  Turns out – I think – that the underlying problem has been electrical all along – a weak condenser that worked just fine at no load on a cold/warm engine but which began to fail intermittently under load with the engine at working temp.  Because that plug is not firing cleanly on every powerstroke, it soots up and looks just like an overly-rich mixture.   I replaced it and things improved substantially but not perfectly – the spark is still yellow-ish and kinda weak.  Tomorrow morning I am going to test the coils by swapping them out with a pair on another CB350 in my shop that is running well.  If I am actually barking up the right tree this time, I may/should have your bike ready to collect on Sat.  I’ll email you by this time tomorrow with the results.

Thanks for your patience – and welcome to the funky world of vintage motorcycles.

Cheers,
Chris
Existential Motorcycles
Alexander, NC

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Joshua,

Well, everything I have done to the ignition system has improved things – to the point where I could probably pass your bike back to you – it runs acceptably now – but I know it is still not right – and now I know the reason why.  In a kinda twisted way, the fact that it is running as well as it is, is a testament to my engine tuning skills.  I put a degree wheel on the left end of the crank – which serves as the alternator mount – and the valve timing is off – the intakes are opening too early and the exhausts closing too late.  You will recall that I earlier considered this as a possibility – but had been happy to let it go – because it would mean taking the engine out again – when I discovered other insufficiencies that could have plausibly been causing the symptoms – and was further seduced into this way of thinking when I kept uncovering genuine defects – fixing them – and improving things.

Lesson: there is always something not quite right with 30+ year-old machines – and usually many things.  The Mikunis were over-jetted, the condenser was breaking down under heat and engine load, the coils were weak.

So out comes the engine tomorrow.

The good news for you is that a.) I am not charging a farthing for all my time chasing phantoms, and b.) that when this saga is finally concluded, you will have one of the best-sorted out vintage bikes on the road. And I ordered the correct jets for the Mikunis and will try them out once the engine is back in.

Thanks for your patience/understanding.

Cheers,
Chris
Existential Motorcycles
Alexander, NC



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