Author Topic: Most suitable petrol for an M21?  (Read 474 times)

0 Members and 1 Guest are viewing this topic.

MBH

  • Star
  • *
  • Posts: 6
    • View Profile
Most suitable petrol for an M21?
« on: 10 April, 2022, 20:25:36 »
I had started my recent M21 ownership thinking that as the engine was of an old, low compression design from the days when petrol was poor, I could happily use any pump petrol in it as long as it got used up and didn't get left for extended periods. While out walking round round town today I saw someone filling his old BMW from a large plastic race can (think he had something to do with the Mercedes F1 team down the road). I asked him what fuel he used for the 1937 bike - he put BP ultimate with Valvemaster additive and he used that in his vintage Indian too. Should I use additive in an M21 and be particular about using higher grades of petrol?
I started riding with a left handed gearchange on a 3 year old bike and 32 years later have a right handed gear change and a 68 year old bike - progress ?

idie

  • Golden Flash
  • *****
  • Posts: 771
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #1 on: 10 April, 2022, 20:30:38 »
These engines were used during the war when fuel was 70 octane.

spaceman

  • Silver Star
  • ****
  • Posts: 476
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #2 on: 11 April, 2022, 06:36:23 »
The main problem with old engines is that they were designed for leaded fuel with the lead needed to stop valve seat recession - hence the use of the Valvemaster additive. For a low mileage, low revving old motorcycle, I doubt if this is anything to worry about but using an additive would certainly eliminate any concerns. In terms of fuel quality, using too low an octane fuel would be the only concern due to the potential for pre-ignition or pinking and, of course, the use of ethanol in modern fuels is a concern in fuel systems incorporating the wrong sort of plastic or rubber.

DAVE BRADY

  • Golden Flash
  • *****
  • Posts: 1982
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #3 on: 11 April, 2022, 08:33:00 »
Hi,

There have been few threads on the Forum about fuel and additives etc. and it seems like there is quite a variation as to how individual bikes, even of the same model, respond.
I have a high mileage A65 and my wife has a high mileage A10, both with well over 100,000 miles each travelled since about 1998.
From leaded to LRP to unleaded to E5 and now E10 both bikes have not suffered in any noticeable way.  No additives ever used.
No extra valve seat recession, no pinking/knocking and no gummed up carbs so far.  Maybe we have been lucky but although lead started to be put in petrol in 1922, octane was still very low and 'pool' petrol would have been the norm for several years in the 1940s and '50s.   Even the R3 is not that far removed from the pre WW2 design of our BSAs.
I would suspect that when the M20/21 was designed the fuel available was of poor and variable quality and the bikes would have been set up to take account of this.  An M20/21 would probably run on paraffin if necessary.
From what people have reported on the forum and from my own experience, I would say run it on E10 with no additives and see how it goes.  Try a tank of super E10 and see if you notice a difference.  Your bike may run cleaner because of the additives in the fuel already but it is quite a bit more expensive.  There are quite few articles on line and from what I have read, unless racing or you have a highly tuned engine, additives are a waste of money.
So try a few variations and settle on what appears to be best for your bike.

Dave.

MBH

  • Star
  • *
  • Posts: 6
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #4 on: 11 April, 2022, 08:46:00 »
Thanks for the replies - so I think I just need to make sure it does have sufficient petrol in it when I ride out to Bicester Heritage later this month for 'Drive it day'.
I started riding with a left handed gearchange on a 3 year old bike and 32 years later have a right handed gear change and a 68 year old bike - progress ?

Mike40M

  • Royal Star
  • ***
  • Posts: 105
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #5 on: 13 April, 2022, 05:37:40 »
Useless information about paraffin. I have experience of both boats and tractors running on paraffin. You have to start using the petrol tank. When sufficiently warmed up, you can switch to the paraffin tank. A couple of minutes before stopping the engine, switch back to petrol so the carburettor is filled with petrol for next start.
Don't know if modern paraffin is the same as used in the fifties.

Derek996

  • Blue Star
  • **
  • Posts: 54
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #6 on: 13 April, 2022, 09:55:59 »
Back in the '70s a friend of mine ran a plunger A10 sidecar outfit on paraffin. I kept a squeeze bottle of petrol in the outfit which he would fill the carb with to get it started then turn on the tap for the paraffin in the tank. It seemed to run ok, although perhaps a little hot and with a characteristic smell.
1948 B32 Competition
2015 Ducati Scrambler Classic
2017 Ducati Supersport 939

cee-b

  • Blue Star
  • **
  • Posts: 70
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #7 on: 20 April, 2022, 18:15:06 »
Hi,

 Try a tank of super E10 and see if you notice a difference. 
Dave.

Not wishiung to be rude, but there is no such fuel.

The super/premium/synergy/whatever else the company may choose to call it fuels are not E10. They are higher octane fuels with a MAX ethanol content of 5%. In much of the country they are 0% ethanol, but still designated E5.

I would advise the OP to disregard octane content (it is irrelevant for 'our' bikes) and read the threads regarding ethanol.

DAVE BRADY

  • Golden Flash
  • *****
  • Posts: 1982
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #8 on: 20 April, 2022, 18:27:39 »
Hi,

Yes. Intended to say Super E5.  I thought it was only Esso Synergy fuels that has no ethanol.  Which other are ethanol free?

Dave.

Derek996

  • Blue Star
  • **
  • Posts: 54
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #9 on: 21 April, 2022, 10:08:31 »
None in Scotland
1948 B32 Competition
2015 Ducati Scrambler Classic
2017 Ducati Supersport 939

MGI

  • Silver Star
  • ****
  • Posts: 288
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #10 on: 21 April, 2022, 18:22:48 »
Harvest Fuels (super) in the Midlands have no ethanol in common with Esso, there are a few others but I as I am close to both of the above I have not enquired further. Interestingly Esso in Cornwall does have ethanol added and probably it does in the North of England.

MBH

  • Star
  • *
  • Posts: 6
    • View Profile
Re: Most suitable petrol for an M21?
« Reply #11 on: 26 April, 2022, 12:44:06 »
I topped up with super (as I could get some) and did run to Bicester Heritage at the weekend and it ran fine using little fuel in the 25-30 miles gentle running. Only the tool box fastener came loose.
I started riding with a left handed gearchange on a 3 year old bike and 32 years later have a right handed gear change and a 68 year old bike - progress ?

Re: Most suitable petrol for an M21?
« Reply #12 on: 01 May, 2022, 18:33:49 »
yes i had the same worry with my M21. It ran absolutely fine until i adjusted the valve lifter and i assume must have got it wrong. It melted the gasket next to the exhaust and broke the exhaust valve seat. I think the problem was with my incorrect adjustment as it ran ok for around 1500 miles until then with no additives in the petrol. I took the opportunity of fitting a hardened valve seat and put on a solid copper gasket rather than the modern composites. No problem for the last 3000+ miles. I use E5 but have heard it will be fine on E10. Additives are prohibitively expensive and are they necessary? I would say definitely not.
john