QUB ignition timing

[Stuart Baskill]

My rebuilt QUB ignition timing is set at 4mm btdc using a 2 wire red Motoplat system.I intend using it on the road only.I dont possess a flywheel extractor and I wondered if this timing would slightly soften/detune the engine probably improving it for road use.As I only ever rebuild to standard tune I would appreciate your opinions.Thanks

[druid]

QUB Ignition timing

Stuart,
You need to buy a Motoplat extractor, you'll need it when you come to renew the oil seal.

The timing depends to a certain extent on the petrol you're using. 4mm was for 5 star petrol. I tend to retard the ignition to about 3mm BTDC.

Druid

[Kim275]

Druid as usual is spot on.

I also use 3mm BTDC on my QUB - with a compresion ratio of 12:1 - and an octane rating of 98 (net of R40 etc) (i..e with octane boost to compensate for castrol R40 @25:1).

If you are going to use on the road you might want to check/reduce the compression ratio - I think standard QUB was 13:1 - which again probably worked well with 5 star and 4mm BTDC). - Druid can you confirm?
Cheers Kim

[druid]

QUB Ignition timing

I always felt that 13:1 was too high, I'd much prefer something about 11 or 11.5:1. You might lose the odd BHP but the engine was happier and lasted longer.

Stuart, if you intend to do much road work I would knock back the CR to something around 9:1. When Greeves were testing the Gauntlet army prototype they found that it suffered from something called part throttle detonation, caused by inefficient scavenging of the burnt mixture at part throttle. The only way round it was to drastically reduce the CR.

Druid

[Stuart Baskill]

I always felt that 13:1 was too high, I'd much prefer something about 11 or 11.5:1. You might lose the odd BHP but the engine was happier and lasted longer.

Stuart, if you intend to do much road work I would knock back the CR to something around 9:1. When Greeves were testing the Gauntlet army prototype they found that it suffered from something called part throttle detonation, caused by inefficient scavenging of the burnt mixture at part throttle. The only way round it was to drastically reduce the CR.

Druid

Thanks for all your info.More questions !! What size extractor do I need and is it a normal thread as many types are on Ebay but not specified for Greeves? I dont intend doing many road miles but how easy would it be to lower the CR ? Also what spark plug do you advise? Final question (for now!!!!) What wire does a kill button need connecting to? One wire must be to earth but does the other go to the blue or black wire.Thanks again to everybody offering advice

[Kim275]

Hi Stuart - not able to answer the extractor and kill switch as I run PVL not Motoplat on my griffons....hopefully druid or chris will see this as am sure they would know answers.

Re compression ratio - to reduce - you could increase the thickness of the base gasket (bottom of barrel). I would say by no more than 1mm - so as not to alter the transfer port/piston alignment too much. 1 mm = a reduction of 1.4 in compression ratio. (so if you are 13:1 will go to 11.6:1). Again Chris or Druid or other members may have other options/ideas.
Re Plug - suggest a Champion N3C should do.
Cheers Kim

[Brian Catt]

I think I have mentioned my Ossa Six Days bike on here before, but I lowered the compression on that to allow it to be a successful trail bike. It was a bit too lively for my poor riding standard, so we dropped the ratio by fitting the trials compression ring. It only did about 85 mph after that.

[druid]

QUB Ignition timing

Stuart,
I think the Greeves Motoplat puller is M26 x 1.0. They also used M27 (on the KTM I think). Let me check mine when I find it. I'll also look at my Griffon to see which wire is earthed by the kill switch.

Greeves dropped the CR on the Gauntlet by machining the combustion chamber but I would make up a ring between head and barrel like Brian's Ossa. I wouldn't advise using extra base gaskets. N3C is a good all round plug.

Druid

[Stuart Baskill]

Hi Stuart - not able to answer the extractor and kill switch as I run PVL not Motoplat on my griffons....hopefully druid or chris will see this as am sure they would know answers.

Re compression ratio - to reduce - you could increase the thickness of the base gasket (bottom of barrel). I would say by no more than 1mm - so as not to alter the transfer port/piston alignment too much. 1 mm = a reduction of 1.4 in compression ratio. (so if you are 13:1 will go to 11.6:1). Again Chris or Druid or other members may have other options/ideas.
Re Plug - suggest a Champion N3C should do.
Cheers Kim

Thanks again. If I measured the space above the piston to plug thread at TDC do you know what cc would give what CR ?

[Stuart Baskill]

I think I have mentioned my Ossa Six Days bike on here before, but I lowered the compression on that to allow it to be a successful trail bike. It was a bit too lively for my poor riding standard, so we dropped the ratio by fitting the trials compression ring. It only did about 85 mph after that.

Please excuse my obvious ignorance but is the ring like a thick head gasket?

[Stuart Baskill]

Stuart,
I think the Greeves Motoplat puller is M26 x 1.0. They also used M27 (on the KTM I think). Let me check mine when I find it. I'll also look at my Griffon to see which wire is earthed by the kill switch.

Greeves dropped the CR on the Gauntlet by machining the combustion chamber but I would make up a ring between head and barrel like Brian's Ossa. I wouldn't advise using extra base gaskets. N3C is a good all round plug.

Druid

Its a good job for me that the Romans didnt kill all the Druids !!!! Thanks again

[Brian Catt]

Stuart, yes the ring is a piece used instead of a gasket. I think there were 3 different ones for the Ossa motor at that time for scrambles, enduro and trials bikes. The first 2 were not that much different, but the trials one was thicker to drop the CR but still use a common head casting. If you want to go that route, it will need some machining to get a positive location for the ring.

[Stuart Baskill]

Stuart, yes the ring is a piece used instead of a gasket. I think there were 3 different ones for the Ossa motor at that time for scrambles, enduro and trials bikes. The first 2 were not that much different, but the trials one was thicker to drop the CR but still use a common head casting. If you want to go that route, it will need some machining to get a positive location for the ring.

I roughly measured the space above the piston at TDC and it was about 36cc which is roughly 13-1 CR. According to my maths I would need a ring above the barrel of about 3mm to drop it to about 9-1 CR.Is that roughly about what you fitted to your Ossa ? I just wondered if I was in the right area. Did you just fit a ring like a piston ring or was it like a gasket size ? What was your compression reducing ring made of ? Thanks for your help.

[Stuart Baskill]

Stuart,
I think the Greeves Motoplat puller is M26 x 1.0. They also used M27 (on the KTM I think). Let me check mine when I find it. I'll also look at my Griffon to see which wire is earthed by the kill switch.

Greeves dropped the CR on the Gauntlet by machining the combustion chamber but I would make up a ring between head and barrel like Brian's Ossa. I wouldn't advise using extra base gaskets. N3C is a good all round plug.

Druid

I,ve ordered my N3C. More questions !!! M26 x 1.0 Forgive my ignorance but M26 means 26 mm diameter but what does the 1.0 mean ?

[Brian Catt]

Stuart, yes you are just about correct with your assumptions about the Ossa head ring. It was just a plain steel ring with the inner diameter the same as the bore size. and about 10mm over bore size on the external diam. I can't remember properly, but I think the head had a small groove machined in it to accept the ring, so you would have to add that to your calcs to get the right CR.
The 26 x 1.0 means 26 mm diam but 1.0 mm pitch on the thread.