New Carbs Tested

[brownie001]

Jeez turbohead, I don't want to go into Quanten physics, thermo dynamics, aero dynamices etc, etc.   I know it's complicated with two strokes, primary and secondary comrprssions and the rest but there must be someone out there  that has gone through all of the rigmerole and can say that 'yes' 100 PSI for a 125 Rotax is in the ball park or 'No' your engine is knackered.

 

The engine is warm under normal running conditions and that's how it was tested. Due to the gearing you can't push the bike in 2nd, 3rd or probably even 4th for that matter.

 

Nevertheless, many thanks for your tips, advice, opinions and experiences.

[turbohead]

Sorry for that! But this is no rocket science of any kind, just the simple principles of the 2-stroke engine.

 

Having said that, IF you have tested with an ordinary automotive compression tester and IF you have done it kickstarting a hot engine and still got a reading around 100 psi, you have a first indication that the compression side of your piston probably can be OK. IF you have done it careful enough, your test can show a reading somewhat below what should be recorded when the test was made the proper way, with a proper instrument. 

 

Remember, leakage on the underside of the piston, through seals, crankcase and so on, is NOT properly checked with a compression test like yours.

[brownie001]

Ok so here we go again. The engine had low use, I was informed. The timing is spot on, is there much you can do wrong with CDI ?  I've renewed the crank seals, disc valve O ring, intake rubber, carb, carb to intakebox rubber filter, blackbox(coil), HT lead, plug and cap, fuel. The last resort was to check the comp ratio. Is 100 PSI too low or not ?

 

There are lots of IF's, Else's, Maybe's and trythis's in the equation, agreed. There must be someone with a clean knowledge of he ROTAX 125 TL engine who can say 100 PSI is good or not without going into the if's , but's, when's and could be's ! I mean, the things are not new on the market are they.......

[iconic558]

'Generally' low compression will cause hard starting issues not poor idle etc and even if very worn still perform surprisingly well.

I had a gasser 2010 EC250 enduro that had huge ring gaps (1.8 mm from memory!) but that bike ran sweet as and pulled like steam train....rattled like a spanner in christmas biscuit tin...but rang sweet as.

Edited December 12, 2016 by iconic558

[iconic558]

PS I'd say 100 psi was good for a small 2 stroke for what its worth

[b40rt]

'Generally' low compression will cause hard starting issues not poor idle etc and even if very worn still perform surprisingly well.

I had a gasser 2010 EC250 enduro that had huge ring gaps (1.8 mm from memory!) but that bike ran sweet as and pulled like steam train....rattled like a spanner in christmas biscuit tin...but rang sweet as.

What he said, had a rotax that was well worn and rattled as above, but started easily and ran well with plenty of power. (It was a dykes ring)

[pschrauber]

So if everything is sorted, like carb, intake rubbers (both of them), air filter, engine wise too as I understand, the pressure of the compression we might call OK too. Even timing and a CDI mounted. Then how good is the magnetism of the rotor, the engine is old and with the time the rotor can loose magnetic power if so the spark won' t be as good as it should you might compare with your 320 TL.NW. 

 

This can happen by age but too through an hard impact!

 

That would simply produce less electric energy and thus a less good spark, my advice go to the next Bosch service and let them test the rotor you can take the 320 with you for comparison. 20€ well spend.

 

[brownie001]

Well, the engine starts hot or cold first kick. Idles fine and, for a 125, pulls well but it still soots up the plug. That said, the reason I though about the compression test in the first place was because it takes little effort to kick it into life.

Maybe a visit to the Bosch Service center is not a bad idea at all. Have to have a think on what the next step should be.

 

Thanks to you all.

 

Seasons greetings.

 

Phil

[brownie001]

Took the head off yesterday and there is too much piston movement so it looks like a rebore is due. I need to get the cylinder measured up first to find out what oversize piston is required.

 

Does anybody have any reliable sources for aftermarket 125 pistons ?

[pschrauber]

Reinhard Hallat Salzhemmendorf

 

or if not available anymore:

 

Woessner Kolben for custom made, Woessner do too provide the needed piston pins, piston rings and con-rods if in need.

[brownie001]

Hi Patrik, spoke to Reinhard today. No problem if the gudgeon pin is 18mm in diaameter, 15 mm is a different story. Mine is 18 mm ]

[pschrauber]

Hi Patrik, spoke to Reinhard today. No problem if the gudgeon pin is 18mm in diaameter, 15 mm is a different story. Mine is 18 mm ]

Then everything is save as You will be now occupied for the time between x-mas and new year repairing the SWM.

[turbohead]

Hi Turbohead, I picked and fussed around with the original carbs and settings on both bikes. The 320 engine is completely rebuilt and the 125 is very low mileage but the carb was from the original high use engine. The 280 is now spot on.

As for the 125, no points involved, CDI black box. Have a new spare which made no difference.

After a long mail exchange with Martin Mathews, he told me that in his experience, the motors run best with the standard carb spec's which is what I have for both new ones and of course the old ones too but there the bodies are original of course.

So there you go, the search goes on....I'll get there one day ;o]

 

So, you just wished the 125 had very low mileage or did the previous owner tell you a nice little lie.....?

 

Anyway, have a nice Xmas rebuilding the 125 top end!

[brownie001]

The dry theory of it is as follows.

'For example, if the atmospheric pressure is 14.7 psi and the compression ratio is 11:1, the equation to solve for the psi is (14.7*11)/1. Therefore, the answer is 161.7 psi. People use this method of calculation to determine the pounds per square a piston produces'.

 

So there we have it, not the most precise and exact way but an indication that 100 PSI is way too low.

 

Whether a little lie or just sales strategy, I am happy with what I have bought.

 

Anyway, Martin Mathews of MotoSWM has told me he can supply all of the oversize pistons so I just need to find out what size I need. It won't be before Christmas though.