velo mac compresion

[wetdog]

i have a velo mac in which will not start (even on the rollers ) cc volumed the head and comp is just under 12 to 1 on petrol ? turns out its fitted with a triumph 750 bonnie piston 76mm and the barrel hight has been skimed , anyone seen this done before ? is anyone making barrel packing plates i need one about 3/16

[Rex]

12:1? Was that done by someone in error, or did they believe that such a high CR would make a really fast bike?
Velo's aren't the easiest bike to start at the best of times (with the low ratio K/S) but one that high would be just painful.
Think I'd chuck the piston up in the lathe and attempt to remove enough metal (possibly using a Dremel on the bumpy bits) to bring the CR down to something realistic.

[wetdog]

dont think he worked it out , i need to lift the barrel , or start again , new barrel piston etc , the mac is a nice soft smooth bike but as you say this one will not start and im not surprised . the piston is standard bonnie and i would not like to turn anything off it thanks

[rogerwilko]

Still should start? Check all the  other obvious stuff first. Lots of race bikes use H.C. When you do get it started take it for a ride and see if it pings. 350 Matchy and BSA B31's use these pistons without any trouble.

[Revband]

Yes I have one in my B31 together with a pair of Goldie clubmans cams starts first kick every time and goes like a good un.

EDIT
I forgot to say when I first did it I made a compression plate to fit under the barrel from 3/16 alloy sheet, I have now removed this plate and can't really notice any difference. If you want to drop the compression it only took about an hour to cut drill and file the plate to shape not a big job at all.

[wetdog]

what compresion are you up to ? on petrol
the standard B31 350cc 71 by 88mm with a comp of 6.5/1 is this what you started with ? total volume @ 415cc this includes head chamber

this is only @ but your bike is now 400cc plus head chamber @65cc so comp is now still only 7.15/1 . im not surprised its working and running sweet at that , this things 12/1 and will not run on petrol

gets better , when I asked the owner what he did about the small end bush , turns out he hand reamerd it in situ , I have a mac rod and pushed the bush out , and worked out to reamer this bush to accept the bonnie wrist pin the bush wall is now 1/32 in thick , just as well it don't run as this would fail

[Revband]

Hi Wetdog

I haven't measured the compression ratio on this bike but  with the bigger piston and the dome top I would estimate it to be nearer 8-1 than 7.15-1 but certainly wouldn't argue about it.

The bike you have is as you say far too high a compression for petrol and needs dropping down, also the problem you have with the little end bush makes me wonder what else the dopey PO has done to the bike, personally I would do a complete strip and carefully check everything in there, as I said previously a compression plate is easy to make, get a piece of 3/16 soft alloy sheet from your local supplier and spend about an hour making one. I used a cylinder base gasket as a template.

[rogerwilko]

Original Goldy little end bushes are only 20 thou thick. Don't worry about it.

[Bomber]

He might need some 'Dope' to make it run at 12/1 possibly

[R]

Lots of race Nortons and Triumphs etc had race pistons at 12:1
Might need race fuel to run well, but it should at least start....

You'd want to check that the valves are well clear of the pistons though, in case the PO didn't ...