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TOPIC: 78 balboa blue dt250 rebuild from NZ

78 balboa blue dt250 rebuild from NZ 03 Dec 2018 04:32 #1

  • Delcowizzid
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Evening all I just signed up thought I would share what I've been working on in my spare time.

Some years ago like 7 or so I bought a 78 1m1 dt250 for $150nz that wasn't running the flywheel was off and I was told it needed new points or condenser.so I loaded on the work Ute and dragged it home with a box of parts .
That afternoon I got the flywheel back on and got it to run badly but least I knew it went .I messed around trying to get it going ok set timing but then the point gap was way off due to the dogleg rubbing block being worn away on the points I jacked the stator plate around and set the timing again and cleaned the carb 10 times and it still wouldn't run right it would start first kick with no choke and run good for a minute till it got some heat into it then would not idle and needed half throttle to stay going but if you turned the fuel off and waited for the bowl to empty it would idle up and rev like a new bike. I bought a new needle and seat didn't help and parts were missing like the left side cover for the flywheel so I pushed it into the corner and it sat and sat and sat.
Fast forward to last month I flew out of town to go tune a race car owned by a guy who owned a motorbike shop he said he had a flatslide 35mm keihin off a kx125 k4 lying around I could have yep I'll be keen on that .
So the day after flying back I was out to dust off the old beast and give it a try.first obstical factory is 28mm time to get creative and adapt a manifold .I took the factory manifold and found it to be the same diameter on the outside as the new carb was on the outside not wanting to jump straight in and cut up the original manifold not knowing if the carb would help I joined the carb to the manifold with a clever use of tape and a cardboard toilet roll filled the bowl with some gas via a syringe and gave her a kick 3rd kick it springs to life I cut the choke and the first time ever it just sat there idling and reved up like a champ.so time to make it permenant I grabbed a 40mm long section of radiator hose the right size that fitted over the outside of the manifold and the carb trimmed the old hose clamp groove off the manifold and roughed it up with some serious sand paper and then glued it on using some 3m panel adhesive used for gluing the sides on busses and then also covered the intire outside like we used to do to repair cracked manifolds it doesn't look real pretty but it's on there and not moving lol.i also screwed 4 self tapping screws through both layers and cut the extra off inside as a failsafe .I then carefully ported inside to suit the bore size and location of the new carb which was very close anyway .
With it all bolted up and no aircleaner I was off for a ride.on the kx jetting it wasn't to bad putting around under 1/4 throttle but was a bit blubbery and wouldnt take more throttle so I lowered the needle going 1 slot higher from the middle it improved but still not great .lifted the clip to the top and it responded well rideable blasting around in the driveway.
That was it time to give it a birthday checked compression 50psi lol added some oil 135psi rings are gone.onto eBay find one set of stock size rings in England organize freight and bought them .bought the all balls crank seals kit too while I was on there also straight onto rexs speed shop for a new dual power cdi setup to get rid of the points .
Now while this was happening I thought I would get into the ports being that I can't leave anything stock for long lol.found some pictures of the late mx250 porting floating around .borrowed a mates dremel and dug out my porting burrs and into it 2mm up on the exhaust 2mm wider each side lifted the transfers 1mm and made the rear transfers square lower than bdc widened the boost port to 30mm ported the base of the barrel to a nice thin seam rolled the edges into the crankcase added a 3rd 17mm hole in the piston directed at the top end bearing and piston crown for some lube and cooling trimmed 4mm of the bottom of the piston shaped the sides to match the transfer cutout in the sleave at bdc plus major inlet port mods giving it 360 degrees of intake duration and leaving the reeds to do all the timing and sealing
I champfered all the ports and slapped it back on with the old rings as I was still waiting for the new ones.huge improvement in torque everywhere but still tops out at the same rpm with low compression and sub optimal squish but goes good enough tfor a quick blast round the streets through all the gears I'm getting excited for compression hahaha.

So time to make the porting come alive expansion chamber building time while I wait for parts obviously buying a pipe is not on the agenda if I could find one so time to sim up a design in every free program I could find to get an idea on what it needs and roll up some cones.ive got it mostly finished then ran out of gas for the mig so I'm tidying up a few little bits and pieces waiting for a new bottle then ill finish it off
For those still reading sorry its so long but pics below lol




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78 balboa blue dt250 rebuild from NZ 03 Dec 2018 04:35 #2

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More pics



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78 balboa blue dt250 rebuild from NZ 03 Dec 2018 04:37 #3

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Lots of welds with the dregs of gas



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78 balboa blue dt250 rebuild from NZ 03 Dec 2018 04:41 #4

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Slowly chipping away new headstock bearing cost 4 dollers for all new balls after I forgot they went in a race and they went everywhere.also someone painted it as you can see in the pic of the tank.i scrapped the plastics with a blade down to the sun faded blue underneath then flame polished them today look much better than before




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78 balboa blue dt250 rebuild from NZ 03 Dec 2018 13:51 #5

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video of it running after fitting the keihin flat slide
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78 balboa blue dt250 rebuild from NZ 04 Dec 2018 11:58 #6

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I like what you did with your carb, engine, and electrical. Hope the new porting works out for you in the new pipe. I am definitely interested in hearing more about what you're doing to it thank you much, Frank

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78 balboa blue dt250 rebuild from NZ 04 Dec 2018 13:25 #7

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The new pipe could of been done better I should of started bending the header end first then the big bend under the tank then it would of fitted much better than trying to get the baffle cone to fit between the carb and frame .it's kinda looking like I might have to ditch the airbox at least for a bit of testing of the chamber if I need to shorten the belly a little would be nice then I could refit the airbox its already about 1.5" shorter in the belly than the Sims suggested was just a case of having to be due to my bending strategy. Going into it I knew it wouldn't be the first and only chamber so not fussed .when I get a good design that gives me the hp curve I want I'll look at making it flash looking and better fitting.im wanting 30+hp without have to rev it to the moon so when it's all nailed down I'll get 1st oversize forged piston and bore it out retouch the ports and send it
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78 balboa blue dt250 rebuild from NZ 04 Dec 2018 16:31 #8

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Xmas has come early down here in newzealand lol time to get this bad boy installed yeehaa thanks Marcus from rexs speed shop great service .will get some better pics of the shineyness later when someone isn't asking what's arrived in the post hahaha
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78 balboa blue dt250 rebuild from NZ 05 Dec 2018 02:31 #9

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Rexs speed shop ignition is in .just need to get a plug tomorrow and lift the bike onto the ground to fire it up currently has no forks or swingarm fitted so won't be any riding just yet.

Install is easy if you can wipe your own bum after a poo you can fit this ignition .removed the flywheel and was surprised how clean the factory points stator looked.looks like its very low km .piston was very good too when I pulled it down but gudgeon pin does have wear.
So removed all the old stuff and boxed it up as a backup in case I ever need to diagnose anything .cleaned the case was a slight oil film in one spot in the bottom but no real sign of leaking crank seal but I removed the old one and fitted the new all balls seal while I was in there .
Did a quick trip to my mates house to raid his valve grinding paste thought I may as well lap the flywheel and crank it was a little filthy and had broken more than 1 woodruff key in its time preventative measures and doing it properly once .
Fitted the stator plate it came with 3 nice shiney countersink Allen key screws obviously for the none adjustable version even though I have the slotted race version so I reused the factory Phillips screws they fitted the slots perfect .set the plate in the middle of the slots and refitted the flywheel.was getting a little rubbing noise so flywheel back off and recleaned the magnets its rubbed on the old stator at some stage and had a little metal filing between the magnet and plastic magnet hoods got that out and smooth as butter so far.
The stator had one case grommet for the wiring and me being me I added the second one off the factory loom for a double layer of protection cause I could.
Mounted the coil on the factory bracket trimmed the HT lead to length fitted the new cap and connected the perfect length wiring loom that was in the kit and plugged in the cdi box and kill switch and job done bar setting timing with a light.
The kit also came with plenty of cable ties that I have not used yet as I'm not done cleaning up and painting the frame and the engine needs to come out shortly and I don't have the rear guard in to the mount the cdi on yet.

Might also pull the head before I start it tomorrow looks like I may have a bit of scouring on the inlet side of the cylinder starting to wonder if my dull top end power might be from too lean on the main not to rich even though the plug porcelain was black I better have a look in there before turds hit the fan hahaha more pics below
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78 balboa blue dt250 rebuild from NZ 05 Dec 2018 02:33 #10

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Double post
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