View Full Version : Engine Trouble

29-Jun-09, 08:22 PM
Hi everyone,

I havent been on this forum in ages due to being at uni and basically substituting petrol and a spanner for beer and a pot noodle, since then a lot has happened!

My craft is now complete barr the pain job and ive even started designing a new one.

But ive recently had a lot of trouble with the engine, those vets out there know what im talking about when i say the rd250lc, i know its a bad choice but 8 years ago i didnt know any better!

Well i have posted a vid of youtube highlighting the problem.


Basically the engine is not running well, its had its timing checked, carbs cleaned and jets increased, still no improvement. It seems to not be able to hold up the RPM despite the blades being moved from 45degs to only 20degs. Ive tried them all!

It was only the other week when i accidentaly forgot to turn the fuel line on when the craft suddely became a bit more alive, the revs picked up the fan sounded good and i crashed into my fence............the question is why? and how do i keep it like that......

I am out of ideas now......the only thing i can think of is that the pulse pump rated for 50BHP is pumping too much into the carbs which is causing a massive over ritch mixture?

Any help apprecitated alot.

Many thanks

29-Jun-09, 08:36 PM

A quick look at the plug should tell you if you're getting too much fuel.


29-Jun-09, 08:57 PM
Hi Nick,
Take off both air filters make sure both slides are moving and opening together adjust on the carb adjusters till they are in sequence which means balenced and check full throttle is achieved.
(Slides can be in back to front which prevents full travel)
Then check air filters are clean before replacing and fuel filter is not gelled up or blocked up and tubing is not breaking down on the inside preventing flow of fuel.
Over fuelling would be seen by the carbs dripping from the overflows which can be created temporarily with a fuel primer bulb.
40:1 fuel mix 240 - 260 sized jets should give digestive biscuit coloured plugs too dark is too rich.
Many years of LC use in the 80's in F3!

Derek Sweetman

30-Jun-09, 11:42 AM
Hi andy and Derek,

I made sure the carbs were openeing together and i set up the correct idle speed using the slide stop adjustment, the air filters are new K+N's and seems fine, a small pin in the side prevents the slides from going in back to front.

Fuel does spit out of the carb overflow pipe I have attached a fuel recovery container so it doesnt go all over the floor.

Im running at 40:1 and the jets are 270 i think, i cant remember but it is around 260....

The colour of the plugs are black oil so i know its running ritch.

Is there anything else that could be making my engine run so ritch?

Im interested to know if the airscrew has much effect on performance?


30-Jun-09, 10:48 PM
if fuel is spilling out of the carb overflows thens its probably also spilling into the engine as well. this would make sence with the apparntly correct running when the fuel was rescrited and the dark plug colour when misbehaving.
Bike engine carbs are designed to take 4 to 12" head of fuel gravity fed from the tank. pumping directly into the inlets will over come the float chamber inlet valve system causing the fuel level to be far too high resulting in flooding or very rich mixture. the classic ways of overcomming this are to fit a restrictor (dangerous) fit smaller float valves (fiddly and difficult to get hold of) or use a float chamber mounted on the gaurd that can withstand the high fuel pressure from the pump as a header and gravity feed to the carbs from 10 to 12" up. SU carb float bowl of any old english car (min, mogy thou etc) does the job nicly.

1-Jul-09, 08:48 PM
You should also check that the floats are correctly set - if at the wrong level they could be letting fuel past. Check that the floats do actually float, some of the old style hollow ones have been known to let fuel inside and so float lower than they should - take them out and give them a shake; you will hear and feel if anything is inside that shouldn't be. The other possibility is that the float needles are worn out or deformed - if I remember correctly they are tipped with a rubbery material which can deform or get damaged, especially if the carbs are left empty for a long time - this stops them sealing properly. I had similar problems with old carbs on a TZR, new float needles cured it of incontinence. Unfortunately they aren't cheap - about 30 a pair when I last bought some and that was a few years ago.

Good luck


1-Jul-09, 10:16 PM
Is the engine mounted at the same angle that it was in the bike? If not the carbs may not be horizontal so the floats will not be operating correctly. If this is the case you may have to put a bend in the inlet track.


2-Jul-09, 11:39 AM
The angle of the carbs is not far off to that on the bike, i doubt it would make much difference as surely they are disigned to cope with inclines i.e hills.

I will check the seals, and the gravity feed system seems the best way to move on this i will give it a go..

Thanks for the response because i was at the end of my tether with this engine.


2-Jul-09, 11:50 AM
buy an lc haynes manual
set the carbs as described and that should be that.
That is what i did on my lc bike and it starts and runs a treat.
following the set in the book you will only need a tweak here and there.
you can pick the book up on ebay cheap
good luck.