Variously used for general fun, commuting or green laning, the DT175MX proved to be possibly the best of the genre. Most ended their days as winter hacks then morphed to field bikes before being literally being put out to pasture and allowed to rot. All of which makes Sheila Ambridge’s latish MX version all the more special. It’s not a restoration or an amalgamation of fresh New Old Stock parts hung on a tired and hastily fettled chassis. For once this is the real deal and, potentially, the ideal restorer’s reference.
With a remarkably low 9000 miles on the clock and even a mint front pipe to the exhaust system you probably couldn’t ask for more. However, as is the way of even the best classics, looks alone don’t necessarily make for good bike. If it doesn’t run as it should the bike is a likely to be more of an annoyance than it is an asset.
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From the day it was acquired the DT just wouldn’t run properly and even some pretty serious fettling of the carb, its jets, needle, slide etc. failed to address the issue. In fact Sheila’s husband, Paul, described the bike as running like a bag of fertiliser (or something very similar) with massive preignition issues. Factor in a collapsed piston crown and it was apparent something was seriously awry.
Enter stage left white knight Rob Pemberton, well known within the classic Yamaha world and something of a specialist in getting elderly machinery running as it should do. With all the palpable and potential problems addressed, the one area left to examine was the combustion chamber. Mr Pemberton’s front-line experience of both road and track machinery convinced him that there was a problem with the flame path, courtesy of modern unleaded fuel that was being exacerbated by the DT’s cylinder head design or, more accurately, the position of the spark plug itself. Typical of many period trail machines, various models normally run an offset plug and this was thought to be the cause of the DT’s woes.
The obvious, if potentially expensive, solution would have been to weld up the offset plug hole, drill and tap a new centrally located orifice, then accurately machine the depth of the new location to ensure that the newly sited plug didn’t simply take over the role of its late and eccentrically located cousin. Easy if you happen to have access the various pieces of hardware needed and the assorted skills to use them. Fortunately help was at hand, courtesy of Yamaha itself. The DT175MX runs a five bolt cylinder head and fortuitously so does Yamaha’s enduro series IT175. After some research a specific model year IT175 head with a central plug was identified and, after some protracted eBay trawling, obtained. The end result was a total and complete success; no pre-ignition, no pinking and most importantly no collapsed piston crowns. All of which just goes to show that even the best looking machines have the ability to confound and confuse. 
