The Big DR350 Project. *Updated*

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oxfordmoto
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Re: The Big DR350 Project. *Updated*

Post by oxfordmoto » Sat Apr 20, 2013 9:04 pm

hi Stev fired up my dr on thursday,carb pissed fuel when first turned tap on but sharp tap with hammer soon put pay to that. Bit of a shake down today but at about 4 fired her up properly set air jet best as i could and then idle. Set off to petrol station, much better throttle response for sure. Snappier acceleration and picks frony wheel up with ease not done full speed run yet back brake need more bleeding plus gonna go over bike tomorrow check boklts extr. Your jets and settings on fcr 37 seem bloody good, will play some more but so far so good. My gutted crf 450 pipe is noisey but sounds really cool...mot may not be easy without some kinda temp baffle....will let you know more soon...thanks for your post saved me a lot of time...regards Rich

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Thedktor
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Re: The Big DR350 Project. *Updated*

Post by Thedktor » Sun May 26, 2019 8:27 pm

ENGINE REBUILD 2016

This story started back in 2012 when I took over ownership of a very neglected DR350 that had been stood outside for 4 or 5 years and was in quite a sorry state.

After a lot of work the bike was brought back to a usable condition and was ridden solidly for two years on the trails covering about 13k miles. Having had previous bikes with more modern suspension the old DR needed a lot more rider input to keep it in order over the rougher stuff but somehow the challenge of that was fun, and the refined, punchy engine just never failed to deliver the goods.

The years sat outside had not been good for the engine, the valve seals, rings, bore or all three were past their best - oil consumption was evident from the word go, around 100ml per 1-200 miles and DR350’s do not normally use much oil.

The relevance of the oil consumption coming up, but the bike got progressively harder to start from cold (it's a kicker) until I had to admit defeat when there was just no compression.

Finally got round to stripping it down and expected to find the rings either gummed up or completely worn out. Well, surprisingly they weren't! And no pictures cos I forgot. :oops:

However, some careful measuring determined the oil control rings were a little out of spec, and presumably contributing to the oil burning, but the main compression rings were clean and well within tolerance. Interestingly the bore showed no signs of wear at all, despite the mileage.

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Not a very good picture, wtih too much shadow, but there really was no wear evident. So what was causing the lack of compression? A look at the cylinder head gives a clue - the result of all that oil being burnt over thousands of miles.

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The valves and seats had a lumpy layer of carbon on them, and hence were not sealing properly! :idea:


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A load of parts ordered up, gaskets, rings, valve seals, and a couple of exhaust valves for good measure. You could argue the case for a rebore and new piston, but I think the piston will outlive the rest of the bike anyway...

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A new cam sprocket was also ordered to go with a new cam chain, but I didn’t notice until reassembling some weeks later that it was the wrong sprocket! :? Some investigation and the older kick start model uses a different sprocket to later DR's, and for some reason I had missed it on CMSNL’s website.

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Because of the return costs the sprocket was kept for the other DR, which it would fit, and then the right one could be sourced.

However, on comparing how the new cam chain sat on the teeth of the old and new sprockets I could not detect even the slightest bit of wear on the original sprocket, even after 27k miles of use, surprising.

So the old one went back on, after lapping the valves in, fitting new seals etc.

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The last picture is for those that have worked on valves who will know the fun of a little collet pinging out under high spring tension and disappearing for hours, until you finally find it hiding in the darkest most inaccessible part of the garage! :shock: :roll:


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Head bolts going in:

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After torquing up the head bolts, then carefully applying sealant to the valve cover, then doing up the many bolts holding it on.... I was most annoyed when one of the bolts spun in the head, with a stripped thread! :shock: :x

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There should be a thread in that there hole....

This is not an unusual problem on DRs as the thread is quite short due to the oilway, but stripping threads is not something I do! Regardless, this needed fixing, which meant taking the head off again. Or did it?

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Using a long bolt screwed into an adjacent thread the alignment of the drill could be checked in all directions, and jamming the drill's body hard against the frame resulted in a perfectly straight hole. (Mock photo without all the rag to catch the bits!)

Cut a new thread:

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And put in a helicoil:

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This all had to be done very carefully being right next to the main oil feed for the cam.


Now, in addition to this top-end rebuild was a load more work that needed doing as during the time the bike was out of action I had borrowed many parts for the other DR!

A pair of old wheel had to be dug out and new bearings and tyres fitted, both front and back callipers were gone and needed some old “stock” ones rebuilt. Chain, sprockets, seat, exhaust, hand guards, mudguard – the bike had been stripped bare! :?

Anyway, got there in the end over some weeks, and a few easy prods on the kick start and she fired up sweet as a nut. Next stop MoT test, and no problem there as pretty much every moving and braking part had been renewed!

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Ran it in over a few hundred miles, oil and filter change, then back into service.


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I know I've said this before but with the FCR carb and other mods the engine's flexibility, instant response, and pull through the gears really is quite remarkable for its size and age - you would never think it was a 26 years old.

Well, you *can* tell as soon as you hit anything remotely bumpy - if only that engine could be easily fitted to a more modern chassis! :P
- Steve



Southampton area

DR350

Montesa 349

Richard Simpson Mark II
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Re: The Big DR350 Project. *Updated*

Post by Richard Simpson Mark II » Sun May 26, 2019 9:31 pm

if only that engine could be easily fitted to a more modern chassis!

Ain't that the truth.

Beta has access to the DR350S engine for the Alp 4.0.

If they stuck the engine in a modded version of the X-trainer chassis, I'd have to go and buy one.

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