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Cam Timing

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
I have recently completed a full rebuild of my Comet motor. At the time it was identified that the Cam lobes were bad – having worn thru the hardening so I ordered (amongst a lot of other stuff) a new cam and cam followers. At the time the supplier did not have a Cam available so he lent me one of his that I was to return when the ordered cam arrived. So the rebuild was completed (by me) with the loaner cam and some 1,500 miles later my ordered cam arrived. After I swapped out the loaner cam for the new one I noticed that the silencer (club type with a spiral baffle) was starting to“blue” about ¼ way along its length (from the inlet side). Both cams were alleged to be Mk1’s. Today I checked the cam timing using the exhaust valve, as it’s much easier to get to. Working on 0.010” lift I measured that the exhaust is starting to open at 73 BBDC and is closing at 27 ATDC. I am using a Lucas Magneto and ATD with the timing set at 4 BTDC fully retarded and Nippondenso W22EP-U plugs gaped at 0.018” – the bike is an easy starter, though it seems to run out of puff at higher revs; maximum comfortable cruising speed is around 60 mph.

Question – what could be suddenly causing the bluing of the silencer? The Mk 1 cam timing spec for the exhaust valve is open 72-70 BBDC and close 28-33 ATDC. At 73 and 27; I am within just 1 degree of the spec – would that be a contributing factor to the bluing? Should I be stripping the timing case and retiming the cam to get that 1 degree closer so it moves to within the recommended range or is it not that important?
 

Holger

Forum Website User
VOC Member
Martyn

I very much doubt that cam timing is the cause of your problem. If you are within 1 degree of the spec then that would have to be considered close enough to be "exact" on an engine like these. Apart from anything else, pulling down the timing side to chase a 1 degree shift in valve timing could well put you on a wild goose chase with no end in sight.

From what you describe I would be inclined to suspect that part of the spiral baffle has come away at the point where the silencer is turning blue. The exhaust gases would have to be directed on to the surface of the silencer in a jet, so would I be right in guessing it is a blue patch rather than bluing all the way around?

Holger
 

vibrac

Well Known and Active Forum Website User
VOC Member
1 degree is certainly OK, to be honest in the last 2 years I have not used opening and closing points to set the timing. The 4 degrees equal opening has proved far superior method
 

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
How much valve lift is there at the points of opening and closing?
I measured at 0.010" lift but since then I have checked the instruction sheets and now know I should have used 0.005" lift - what this means is that my cam is further off the marks than I thought. Another round of checking is in order and, if as I suspect I may be more than 5 degrees out - a retiming of the cam will be required. Ho Hum. Martyn
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
That little lift may put you in a somewhat variable transition between base circle and the lobe. Using the equal lift method eliminates that possible variation. For my Emmerich/Andrews/Megacycle cams, I was told to use .050" lift for timing to eliminate any transition/quieting ramp variations.
 

Holger

Forum Website User
VOC Member
Martyn

if your bike starts easily and runs well, I wouldn't mess with it. Unless you are running on the limit using racing profiled cams, the engines are capable of running well enough within a wide tolerance on many fronts.

Just keep riding!

Holger
 

Howard

Well Known and Active Forum Website User
VOC Member
Not sure why you think blueing of the silencer is down to valve timing (especially when the timing's as close as dammit). More likely running hot due to mixture, air leaks, retarded ignition etc. But if it's just the silencer that's blueing, not the exhaust pipe, it's more likely to be a silencer problem, as Holger suggests. IMHO.

H
 
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