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Motorhispania RX50 issues.

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David Hayward
L Plate Warrior



Joined: 05 Dec 2018
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PostPosted: 07:58 - 06 Dec 2018    Post subject: Motorhispania RX50 issues. Reply with quote

Hi - Earlier this year I purchased a pair of Motorhispania RX50's as basket cases, and have now built one good one up out of the two.
Starting from cold is difficult, but once warm ok.
In neutral, the bike revs freely and ticks over fine, but out on the road, or on a set of rollers, the engine bogs down badly and won't pull at anything below about 5000 revs. If you put in 3rd or fourth and open the throttle, it does nothing and struggles to chug along.
The engine is AM6, with new (standard) barrel and piston, new reeds and a free flow foam air filter. Carb is standard Dellorto PHBN 12 with jets as specified in the manual. The only mod I can see from the previous owner is the expansion chamber which has "competition use only" stamped on the bottom.
On the rollers, with air filter removed, I noticed a lot of fuel mist coming back out from the intake..
I have tried swapping the ignition unit with a second one, but no difference, and even put the spare original (I think) pipe back on to get it back to standard spec as a base line.
I know the power band of these little engines is quite narrow, but riding it is impossible, other than flat out screaming away.
I can't help feeling it is a jetting issue, but my knowledge of tuning two strokes is limited and don't really know which way to go..
I bought the two bikes as a project for occasional use in the summer, but am rapidly going off the idea.
The previous owner said he stripped them down for a rebuild after the running engine over heated and blew its' head gasket..
Any help gratefully received....
Thank you,
David.
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Teflon-Mike
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Joined: 01 Jun 2010
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PostPosted: 10:43 - 06 Dec 2018    Post subject: Reply with quote

Age? Mileage?

An awful lot of carbs get completely cocked up on the presumption of a 'jetting issue'... when in fact, the problem is almost inevitably elsewhere, A-N-D that doesn't mean ion the ignition, which is the next most diddled sub-system... mostly because both are easy to identify and look a lot less scary than delving into the actual oily bits of engine.....

The thing that immediately sprung to mind from description was crank-case seals..... (but that IS little more than wild stab in the dark)

These little motors, get top-end after top-end rebuild... 'cos thats wot-yoo-doo, innit?.... often not done properly, with new bits, cos expensive, and the bottom end is ignored, cos top-end....

Race two-smokes cranks are 'lifed' in running hours; and its not many!! I think that the Honda RS125 GP bike, has a crank that's supposed to be swapped out every 100 running hours.... at an average of 60mph, that's equivalent of every 6,ooo miles... apparently they are about 'good' for one season of clubbie..... little minerelli motor isn't quite so highly strung.... but still... bottom ends are so oft avoided or ignored.

When the crank-case seals go though; they act sort of like a reed-valve; they hold pressure when the motor is not under any great load, but under load, they start to leak, and do so randomly, letting charge out depending on speed load and the way the winds-blowing.

Hint: sniff inside the magneto cover, look for petrol stain.

SO... comment in there about the PO doing a partial tear down... but, other being admission that bits bin tinkered with... its a complete unknown quantity....

Given that the thing has likely had a string of 16 year old owners who's interest in the thing will have rapidly waned as soon as a 125 or mummy's car is in a teen-foreseeable future, trying to run the thing on a pocket-money budget, that decreases with each successive owner as the resale value falls.....

Personally, I would bite the bullet, and do a complete and propa engine tear-down; I'd pay attention to the bottom end, and check prices on new cranks or crank recon, as well as spinning up the one I had in v-blocks against a DTI and humming and harring a lot....

But, bottom end rebuilt, on, if not new or recon crank, certainly new outer bearings and case seals; new piston, new cylinder/replate... and a new plug...

The ignition and carb are probably fine.... jets dont randomly change size, and there's usually bog all that can go out of tol in the ignition these days, unlike in the days of points and condenser.

But.... on know good bottom and top end, you stand some chance of being able to fault find any problems there might be... which most likely stem from 16 year old fiddle fingers 'adjusting' the carb or ignition!!!!

Trying to debug it from the outside, without eliminating the primary 'service' variables of crank-case and cylinder, is pretty much a road to madness, of presumption and assumption and never 'really' knowing anything, because its all layered on... so pin down the foundations, eliminate the variables before you start, and give yourself a known base-line of what it CANT be, before looking at what it might.

Especially on a 50cc two-smoke, that's had however many teen twit owners with more oy-dee-uz than proper spanners!
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Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'
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Monkeywrenche...
Could Be A Chat Bot



Joined: 27 Mar 2015
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PostPosted: 11:34 - 06 Dec 2018    Post subject: Reply with quote

If it's got a "free flow" air filter I'm assuming it's not factory standard, neither is the Expansion chamber, so some tuning of the jets may be necessary to get it to run right.
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David Hayward
L Plate Warrior



Joined: 05 Dec 2018
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PostPosted: 12:02 - 06 Dec 2018    Post subject: Motorhispania RX50 issues. Reply with quote

Thanks Teflon Mike for that comprehensive answer....
The boxes of bits contained a spare motor, so I will probably completely strip and re-build that one over the winter, replacing all that I can with new to give me a good starting point.....or I may just bin the whole lot....depending on enthusiasm left.
The bike is a 2000 plate, mileage a little unknown, as I had a choice of 3 speedos ranging from 6000 to 36000 miles....However many it has done, I am guessing they were hard, thrashing miles annoying all the neighbours..!
As I am in my mid fifties, I might stick to what I know and leave the screaming tiddlers to the younger generation..... Rolling Eyes
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Teflon-Mike
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Joined: 01 Jun 2010
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PostPosted: 20:42 - 06 Dec 2018    Post subject: Re: Motorhispania RX50 issues. Reply with quote

David Hayward wrote:
As I am in my mid fifties, I might stick to what I know and leave the screaming tiddlers to the younger generation..... Rolling Eyes


Thank's.. that's made my milenium.... I'm a number of months off half a century... I think that we ARE that 'younger' generation, weened on Fizzies and aRDies..... and it was my GRANDAD that got all exited when I brought home a shitheap DT50 to mess with when I was 15, and started polishing his bald spot and talking about 'primary compression ratios and filling the crank-case with araldite and warped Hilman Imp cylinder head filings i!!!!

My kids? They look for the Super-Mario cheat code on their smurf-phone to do anything!!!! All Chinky CG copies and e-bay PC cooling fan 'super-chargers' by their time and point of reference, mention a 'crank' to them and they look side-ways at you and say "This is a Mow-Tah-Bike?!?!?!" and may patiently explain that it dont have pedals to screw the wrong way to the crank!!! Lol
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'
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David Hayward
L Plate Warrior



Joined: 05 Dec 2018
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PostPosted: 07:35 - 07 Dec 2018    Post subject: Reply with quote

Mr Teflon Sir,
I know exactly where you are coming from.. Laughing
Rummaging round in the boxes of bits that the little blighters came in, I stumbled across.........a new, un-used crankcase bearing and oil seal set..mmmmm
Just a thought....and don't think too bad of me, but have you ever tried somehow extracting the oil seals from the outside in situ..? i.e motor in, or on bench but still as one lump. If this were possible I could at least see if it is indeed the seals that are the cause with minimal hassle before I do tear it all down......or chuck it in the back of the shed........ Crying or Very sad
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Teflon-Mike
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PostPosted: 10:28 - 07 Dec 2018    Post subject: Reply with quote

Funny you should mention that.... I have to say I never REALLY had a need to.... Pops did such a great job of increasing the primary compression ratio on that Dea-Tea with araldite and ground up imp.... they tended to just 'pop' out of thier own accord.... to the point that for the six months I tried to use it as an every-day to get to college.... I had a standing order at Frettons for another set of crank seals!

Problem I had was keeping the little feckers IN! not getting them out! ISTR we ended up tapping the cases and plating over the top of them!!

Howebler.... I am not familiar with the minerelli arrangement. On my Cota, the seals do actually go in from the outside and are capped over, much like the arrangement we contrived on the DT with Pops.

It does depend on whicj way the case is rebated whether its even possible... BUT, its a lip seal, like one side of a car windscreen wiper; and if you press the seal on from the outside, it can double the lip over so its like a wiper blade that only wipes one way... and basically not seal.

I have heard of it done by hook or by crook not necesserily on two-stroke crank-seals, but such places as Land-Rover transfer case seals or diff pinion seals and the like; basically, rather than completely tearing down the assembly and rebuilding as per the book; drilling a 2mm pilot hole and screwing in a wood-screw to pull the old seal out from the rebate around the shaft, then carefully, VERY carefully, drifting the new one into place absolutely square with appropriate bit of tube.....

So it IS possible.... but not necessarily the best ways about... IF the seal rebate isn't lipped so it has to come out from the inside.

Back to you.... you payz-youz-money-and-takez-youz-chansiz!
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'
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