Engine Konocking which Fuel

[Rock Hoppa]

I recently purchased a 2003 beta 250 rev 3, the manufacturers handbook suggests running Premium unleaded fuel with a 1.5% sysnthetic oil mix. I ran this at a trial last weekend and found I was getting an engine knock upon deceleration, some sort of detonation I think, can some sugest what the problem is and how to correct it and your reasoning behind the changes.

I have had other suggestions that I run LRP @ 70:1

[David Lahey]

Rock hoppa this is David from your trials club. It is possible that your Beta is suffering from the same thing as Ken's and Wayne's (ex Neil's) Rev 3 Betas were suffering from until recently in that the rubber carby-to-reed-case moulding may be leaking air in at times of significant crankcase vacuum and low airflow into the engine, causing a lean mixture.
Another cause may be that the fuel was not fresh. Some of the octane boosting components of premium unleaded and LRP are easily lost from vented storage (bike fuel tanks) and petrol stations with a low throughput of that fuel.
I suggest you ask Ken or Wayne from the club about their experience with that particular rubber moulding.

[aj]

I recently purchased a 2003 beta 250 rev 3, the manufacturers handbook suggests running Premium unleaded fuel with a 1.5% sysnthetic oil mix. I ran this at a trial last weekend and found I was getting an engine knock upon deceleration, some sort of detonation I think, can some sugest what the problem is and how to correct it and your reasoning behind the changes.

I have had other suggestions that I run LRP @ 70:1

Great to hear the response above does not advise to use plane fuel... Many people use 100 octane plane fuel which only masks/ cover ups the problem, all plane fuel does is resist the sound of knock by the 16 times as much lead contained and has all this other gunk that requires about 30000 ft of altitude to burn cleanly! Fuel in Australia is generally good, in Uk using 70/72 octane approx and no real prob. I would take the above suggestion, you can't beat people who have done it, also, i find the carby's very sensitive to dust, jetting etc and does require air-line to get pilot and carby holes blown out - not just the jets but the carby itself.
fuel mix 1.5% or 70:1 sounds rather similar to me, the bike would have no reason to knock if run at anywhere between 50 and 100:1 if jetted accordingly. If you do play with jetting no reason to go outside one either way of standard and generally one thing at time. For example, scottish six days only required change of one richer pilot size and that was with 50:1 fuel and much road work, like miles of it!
Keep simple and airleek above sounds like a fair option. Also, check fan working and coolant, as found here beta's seem to run very weak when hot.
aj

[Chris England]

Never ever run LRP fuels in a 2 stroke trials bike!...........the potassium used in these fuels has been known to damage crank seals! Here in the UK we can get super unleaded fuels, and this in combination with sometimes increasing size of the pilot jet slightly, often gets rid of any knocking.

Chris

[David Lahey]

Chris
Please give more info or references about the link between the potassium additive in LRP and two stroke crank seal damage. I hadn't heard of any problems with using LRP in two strokes before your message.
Thanks

[Neo]

Hey, I think Chris England was refering to sulphur.

Check some good info on Shell Optimax (right down the bottom of the page):

http://www.shell.com/home/Framework?sit ... _1602.html

All the best.

Neo.

[montyman]

Hi all
well I had a lot of problems with engine knock with my 2002 montesa and looked right into fuel, plugs, timeing ,jets etc .
Ok the run down on fuel Shell optimax is great stuff for your car but keep it away from your bike there are so many additives in it which makes it to "Thick"for better words so it will make your bike run lean. I have been told Mobil Synergy 8000 is the best at 98 octane guaranted and free of additives . But its not sold north of Newcastle so have not tried it. I use BP ultimate and find it good it's 98 octane too. I have tried 50% unleaded and 50% avgas its OK to but a lot of hassle to get Avgas. With 2 stroke oil run it at what is recomended in the manual with the montesa its 80:1 which I thought was way to lean when I first got the bike. Thinking I was doing the right thing I mixed it at 50:1 and it knocked like hell. Back to that "thick "fuel thing which makes the bike run lean. Use a good quality oil too some oils like Castrol TTS which I ran for years in my MX bikes are not recomended for less than 50:1 ratio. I guess some guys might shoot down in flames what I have just said but this is what I have found.

Monty man

[BJ]

Hi Guys

Don't know about "thick fuel" and the difference between brands, I just use my 4cent Safeway discount docket and buy premium unleaded and run a 50:1 mix in the Sherco. It never knocks, rattles or misses a beat!

I did try using regular unleaded a few weeks ago (ran out of premium) and boy did that slow things down! The differnce was not much but the slight lag in throttle response and the fluffy exhaust note was really noticable. As far as LRP goes, I have run it in bikes and chainsaws for years with no hassles whatsoever. Only now that I can no longer buy it at Safeway am I using unleaded.

By the way, has anyone got an Sherco lighting kit that they don't want???? I'm going to try and get Rec Rego on mine so I can show these boys with dirty big Yamaha enduros what trail riding is all about!!!!!

BJShercs

[Neo]

Hi Montyman,

Thanks for the run down on the "Super Fuels". You've probably saved me a ton of work (and money ). The knock is really bad on this new Sherco 290 of mine. And the kick-back can leave me with a sore foot (or two!). I've gota do something with that timing.

Must meet up with you guys in the near future - I'm spending too much time fixing rather than riding this bike


Best regards.

Neo

[Neo]

Hi all,

Just to bring this subject back to life. This is what I got back from Bogas on their Premium Petrol. Bogas are owned by Caltex.

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The premium unleaded sold at Bogas Branded outlets is not the same as Vortex or Vortex 98.

Vortex is a 95 octane premium unleaded while Vortex 98 is a 98 octane premium unleaded. As well they both contain an exclusive ingredient which cleans the car's fuel system as you drive. Neither Vortex or Vortex 98 contain ethanol.
More details can be found on http://www.caltex.com.au under fuel and oil products

The premium unleaded sold at Bogas branded outlets is a 95 octane premium unleaded, without the ingredient.

At Bogas branded outlets that contain a label on the premium unleaded pump to advise customers the product contains up to 10% ethanol, the product will be a blend of 95 octane premium unleaded plus up to 10% ethanol. The addition of 10% ethanol can raise the octane of the blended premium fuel by about 2 - 3 octane numbers.

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Best of (petrol) balance.

Neo