Deeteemx

Definitely worth doing the "radiator cap" test that James has described before taking the head off, Andy. A lot of head gasket leaks on DTR's are relatively small and may not require head skimming or even a new gasket. I have repaired 4 or 5 bikes just by refitting the old (genuine Yamaha) head gasket covered with a thin layer of jointing compound (such as blue hylomar) on the inner and outer circumference. So far I've had a 100% sucess rate using this technique, although other people I know have had less luck. You won't find this repair recommended in any manual, but it may be worth considering.

Obviously it is best to check that the problem lies with the head gasket before dismantling the engine. One of the best ways to do this has already been mentioned by James, ie remove the radiator cap after the engine has cooled down and see if there is a release of built up pressure. The black carbon on the head gasket, head and barrel has clearly shown where the gasket's have been blowing on the 15 or so cases that I have witnessed. And a jet size that "is way too small" (why would anyone fit one of these?!!) would probably melt the piston before the coolant had time to overheat!

I'm well aware of the cooling and anti corrosive properties of coolant. I'm also aware that if a bike has an underlying overheating problem, or is ranted hard offroad, the extra few degrees boiling point provided by coolant could be critical. However, DTR's have a large capacity cooling system and large radiator for a 125, and are very unlikely to overheat in normal road use (irrespective of whether coolant or tap water is used) unless there is an underlying problem. And as for posting back after I've run my bike with tapwater - well brace yourself, or look away now! In more than 30 years of riding on the road, and competing in trials, motocross and enduros, I've used neat tap water on numerous occasions, particularly in my old YZ250. Yes, it did boil up occasionally when the going got really tough - the Weston Beach Race for instance, but so did a lot of other bikes. The only long term damage caused was to the drain bolt which went rusty! I've also used tap water in lots of road going DTR's that I've rebuilt, with no problems. Maybe I was lucky, and I'm not recommending that others follow my example, but tapwater has never caused me the kind of problems experienced by James and Andy.

It should be easy to find whether the problem is electrical - simply take out the spark plug when the engine has cut out and see if it has a spark. As OG says, you can take readings for the 2 coils behind the flywheel (on the engine) with a multimeter and compare them with the resistances shown in the Haynes manual. The wiring loom is unlikely to be the problem as the ignition system bypasses most of it. The wires from the stator plate connect almost directly to the CDI. A loom from a post 1998 DTR is completely different....and loose connections on your battery will not affect the running of the engine.

Oh dear, sounds like another blowing head gasket! Losing the top off the expansion bottle is one of the first signs, as pressure builds up in the radiator. Its also a sign that the vent from the expansion bottle (where it connects to the sub frame) is blocked. This is a common problem on pre 2004 models, as the inside of the subframe corodes where overflowing coolant runs down inside it. Using tap water to mix with antifreeze will not cause this problem...and neither will a slightly incorrect sized main jet.

If your other options fall through, I have a standard DT125RE tailpipe available in vgc (but has been given a coat of what looks like black high temperature paint). Paint is flaking a bit and silencer has a few specks of surface rust breaking through, but is very solid. £25 including postage, or possibly swap for one of your 1988-2003 DEP tailpipes? Jerry

I'm with OG here! DTR's are very prone to blowing head gaskets, even new head gaskets! It is very important to thoroughly clean the surfaces of the cylinder head and barrel before fitting the new gasket. I know some "amateur" mechanics who've taken up to 3 attempts to do the job properly! Make sure there is no damage to the mating surfaces, and tighten the head nuts in a cris cross pattern so that you don't warp the head. Sorry, but I reckon you've wasted your £12!

I've not examined the operation of the DT125RE servo in detail, so don't know if they all operate backwards. It's certainly strange, and the bike's I've come across all seem to perform too well (without crossed cables) to have powervalves operating this way. Have you earthed the green/black wire yet? I'm sure this effects the powervalve operation in some way. And yes, the numbers on the servo are under the rubber cover. As for reversing the power supply to the servo......I've got no idea if this would work! May be worth a try (if you've got a spare servo in case it goes bang!)

A lot of confused owners have asked me how the cables should be fitted because there are numbers ("1" and "2") where the cables attach to the servo and the powervalve housing. People often think the cable that connects to the number "1" position on the servo must connect to "1" on the powervalve housing whereas they should connect "1" to "2" and "2" to "1". If a powervalve is incorrectly set up (eg where the powervalve is rotated anticlockwise 45 degrees or more beyond the open position), crossing the cables over will improve performance as this will cause the powervalve to rotate clockwise as revs increase, back towards the fully open position.

Yep, that servo is on drugs! A lot of owners get confused about which way to connect the powervalve cables, and some connect them the wrong way round by mistake. I don't think Mr Yamaha crosses the cables over at his factory.

Sorry Vez, I haven't got any photos of the pcb at the moment, and don't know how to attach them to the thread (as you've already noticed, I'm still learning how this forum works!) I discovered the pcb in a speedo I have that broke apart in an accident. Its quite an elaborate affair with lots of micro electronic gubbins attached to it. It may just be there to operate the temp warning light....or it may be something more sinister?!! The Haynes manual gives detailed specifications for the 1989-98 and 1999-2003 carburettors, and I don't doubt their accuracy (although I haven't checked them). The manual says the power jet increases from a '60' on the earlier carb to a '95' on the later one. I think this jet is probably in the float bowl, near the bottom of the vertical passage that runs down the side of the carb.

Some carbs do have the ID mark printed (in black) just above the tickover adjuster screw. Older carbs don't seem to have it (or maybe its just faded away with age) but many of the later carbs that I have seen do have it. Carbs from 1999 to 2003 (3RME models) do indeed have a 240 main jet as standard, instead of the earlier models 210. According to the Haynes manual they also have a different pilot jet, power jet, starter jet and float height. Many people upgrade to a 240 main jet (pre 1999 models) when they fit a full performance pipe, but most don't bother when they simply upgrade the tailpipe. I think all 1999 - 2003 models have frame numbers starting JYA4BL... and engine numbers starting 4BL...irrespective of whether they are an import or a UK model.

The DT125RE from 2004 wasn't fitted with a reed switch....although they do have a very strange printed circuit board contraption inside the speedo!

I've seen a similar article refering to this mysterious "silver/grey box with 4 wires" that is apparently fitted to some 1997-99 models....but I've never come across a bike that actually had one fitted (and I've worked on a LOT of DTR's!). ALL UK DTR's were originally sold with restricted exhausts (to comply with the 12hp learner laws), the earlier models had the cone at the manifold, the later "3RM" models have a restrictor in the expansion chamber. Although the DT125RE has a cat, their exhaust seems to be generally less restricted as this model normally performs well with just the green/black wire being earthed.

The restictor bottle on the side of the carb to airbox hose started being fitted to some DT125R's from 1998/99 but I agree that not all DTR's had them for the first 2 or 3 years. I've seen some that just had the mounting lug (for the top of the bottle) welded to the frame but no actual restrictor bottle. Not sure if the bottle had been removed by previous owners and the intake hose replaced with an old type version, or if the bikes were supplied like that from new.