Hitchcocks Forum

Hi. I have recently bought a 1953 Royal Enfield Ensign 150cc two stroke. The handbook recommends a 25:1 mix of oil. I have started using Castrol GP50 oil with 97 RON super unleaded. Is this OK? When using this mix I then found it was low on HP and would only do 25mph. However, when I removed the air filter it runs really well up to 38mph. (Though the top speed for this model should be around 47mph) I have cleaned the mesh filter but when I run it with the filter back on it won't go over 25mph on full throttle. I cannot access the mesh inside so maybe the filter needs flushing through better? I have never had a 2 stroke bike before. The bike starts first time and runs smoothly and the timing is now all set up correctly. The bike has only done 4300 miles (as far as I know) It is 63 years old so am I expecting too much from this machine? Can anyone advise on what is going on here? Any help would be greatly appreciated?

I'm pretty sire that modern two stroke oils will allow you to run at more like 50-1. Have you cleaned the exhaust system out? It's as important as decoking the engine on these old stink wheels.

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It's supposed to do 100 miles to a gallon!

We have a couple of Bantam 175 engined trials bikes and I use Morris's 'Ground force' 2 stroke oil for them, it is for garden machinery and chainsaws. My reasoning behind it is that chainsaws usually get a pretty hard life, so if this oil is good enough for them, it will do for most other things and I mix it at 25:1 for the Bantams. They seem happy enough on it and don't smoke excessively or foul plugs. I also put it in the oil tank of my Suzuki X5. Hope this helps! Paul.

Personally I would not mix modern two stroke oil at 25:1. Modern two stroke oils are super efficient and can be used at much leaner ratios. 25:1 will do no harm but it is wasteful and offers no advantage at all. When I had a Suzuki X5 (great bike!) I seem to remember that the manufacturer's recommendation was NOT to use self-mixing two stroke oil in the Posi-Lube tank. The same advice was also given by Yamaha in their AutoLube system.

That should be Suzuki 'Posi-force' system.

Back again! I would say a definite NO to using GP50 in a two stroke. You need to use a self-mixing dedicated two stroke oil in a two stroke engine.

Hi Presto, I understand where you're coming from about using modern oils in smaller ratios, but a guy in our village restored a Suzuki RM 400 and the chap who rebuilt the engine told him 50:1 on modern 2 stroke oil would be fine. It wasn't and it seized after only a very short time and needed rebuilding again [top end]. Also, less oil = richer mixture [fuel / air], so it can be a bit of a lottery when deviating from the recommended ratios, even if the oil being used is 'superior' to a more basic, old fashioned type. Careful experimentation, changing the ratio by, say 5:1 at a time might be the safest way forward.

I agree Paul that care is needed - and some older engines use materials that do require a richer oil mixture. As for the oil ratio affecting fuel/air mixture, the differences are so minute that they can be ignored. I looked into this a short while ago, always having been told that mixture strength would be a factor in changing oil/fuel ratio - but, as I say, I believe the differences are so small as to be negligible. P(S Might it not be that the failure of the RM400 may have been due to other factors than simply the oil ratio strength?)

Again! One great advantage of an oil pump system on a two stroke is that the oil is regulated according to engine load/needs. At the leaner end the amount of oil delivered by the pump direct to the engine is frighteningly little - you'd imagine that such a small amount of oil would destroy the motor in moments!