My classic thumper Suzuki DR600 was ailing; it’s been about 30% down on power, 30% down on compression and thus 100% in need of a top end overhaul. So what’s the issue?
Well, ask any comedian, and they’ll tell you the secret to good comedy is all about the... timing. I wouldn’t have expected to find the answer to the Suzuki DR600’s running problems in the first rule of how to tell a joke, but by golly – if I’d looked, I would have. The DR’s issues all pointed to tiredness in the bits that squeeze the mixture inside the cylinder.
I was convinced giving the rings, bore and valve seats some love would cure its low compression, oddly coked up inlet tract and general lethargy all in one go: well, once the top was off, it was clear they weren’t the reason this old thumper wasn’t pulling like a train.
Checking the top end out revealed things weren’t as wrong as I’d expected with my compression culprits. The valves and seats held fluid in a wet test, the bore not only looked fine but measured up within tolerance, the same for the rings and gaps. True the piston looked a little tired, but again, everything measured up okay. I had been looking for something inside that we could blame for the 30% plus loss of compression (110psi tested against an expectation nearer 150), and I was disappointed – there wasn’t anything obvious.
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| With the head off, initially no dramas visible. |
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| Is this the most coked up inlet tract you’ve ever seen? |
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| Wet leak test – valve seats are all spot on. |
What was also unsettling was just how heavy the carbon deposits in the inlet tract were. I already knew we had a problem in that area, but with the DR600 head in my hands it was much worse than expected. I knew the valve clearances were perfectly set, the valve seats hadn’t leaked at all in the wet test and yet still the inlet tract was dirtier than the exhaust! Hmmm, this one was a real head scratcher.
The second opinion came from Steve Mann of MTS who quickly surmised that the cam timing was very likely out. Bingo. I hadn’t even thought of this, and yet retracing all the issues we’ve had, that diagnosis stood sound. The real shame is that I hadn’t checked the timing when I’d disassembled the bike to prove it, but I’m 99.5% confident that he’s spot-on.
The Suzuki DR600 has a single overhead camshaft which operates both sets of rockers, so skipping one of its 42 teeth when timing it up would set both the inlet and the exhaust out by just over 8.5º. The coking up inside the inlet port suggests that the cam timing was set up advanced, and thus the inlet side was opening at the end of the exhaust stroke and collecting the dirtiest deposits from the combustion. Is it any wonder it didn’t want to go up hills very fast?
So, how does this happen? Getting cam timing a tooth out is easy; lining up the timing marks on the camshaft and the flywheel is simple enough, but sticking the camchain tensioner back into the barrel inevitably tightens the chain. This pulls the cam sprocket out of its correct alignment with the crankshaft, leaving big consequences. On an engine with double overhead camshafts poor timing can make the inlet and exhaust valves interfere with each other, causing massive damage.
Thankfully our DR’s simple single-cam meant that was never a problem for us, as no valve overlap damage could be caused. True that really bad timing could still have hit the piston, but ours hadn’t. Plain lucky again!
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| Bore micrometer checks proves we’re within tolerance. |
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| Piston ring gaps also measure up within tolerance. |
So what to do next? Well, the time served racer in me detests opening engines without making them faster. As it became clear a while back that this motor definitely needed the lid taking off, I started to plan cramming a few more ponies into it. A late night eBay session bore fruit with a second-hand barrel for just £15, and I’ll be reboring this to home a 1mm bigger piston (Suzuki’s maximum recommended oversize).
After deciding on using standard Suzuki parts for reliability, I ordered a stock 1990 DR650 piston for the 1mm oversize I was after, with the added benefit of it having a Teflon coated skirt creating lower heat and friction. Although neither the DR600 or DR650 are models among those in their excellent vintage parts initiative (which offers special listings and deals on parts for specific models) they still benefit from Suzuki’s across-the-board high support for classic models. A DR600 top end gasket set and a piston from the later DR650 were available in just two days. Not bad for a 30-year-old bike that was sold in tiny numbers in the UK.
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| The rings may measure up okay, but the colour of the piston raises the question how well were they sealing? |
Although fitting this DR650 piston into the 600 will only add 13cc (the extra 50cc of the DR650 was from an increase in stroke as well as bore) it should on paper add around 1bhp as well as eradicate any tiredness from the old rings and bore.
Along with the cam timing, this little pep-up means the bike’s previous asthmatic best of 27bhp on MSG Racing’s dyno should be blown away – Suzuki claimed 45bhp back in 1985 which should see about 40bhp on a reasonable reading dyno – if we can secure that after this refresh, I’d be a very happy chap indeed! Next time out I get the re-bore nailed and build the engine ready for a dyno test.