If when you road test the bike you are happy with the performance then leave well alone. If not then do the job properly. Assuming that you have either MkI or Mk III cams then you can check for wear as the lift on both inlet and exhaust should be about 330 - 340 thou. Significantly less than this and the cams are worn. Then as to the timing. You do need to ascertain TDC properly, not just 'about'. Accuracy of one degree is quite reasonable. Then, at TDC, there should be more lift on the inlet than on the exhaust. The reason for this is that the 'degrees of inlet opening should be greater than the degrees of exhasut closing'. It says so in the book. Depending upon the cams and whether the followers are still flat or have worn concave you should have 10 - 20 thou more lift on the inlet than on the exhasut at TDC. The correct way to do all this is to remove the oil tank and front end and then use the DTI on the top of the valves and then take a reading all the way round each lift profile at 10 degrees of engine rotationi .e. 5 degrees of cam rotation. Check the base circles first by nipping up the tappets by about 20 thou and leave the tappets tight for doing the lift profiles. When you have the measurements plot them on a graph, a spread sheet makes it easy, and then draw a line at 5 thou lift. That is the figure at which Vincent valve timing is given. Work out what is the best compromise for your particular cam profiles as many Vincent cams are of really poor quality when it comes to the profiles. Your cam and follower hardening must have been right for them to have lasted so long. Pain in the posterior I know to have to do all this but if you are proper engineer the you know that you have to do things correctly otherwise your customers come back with the items you have made and threaten to use them as suppositories on you. Good luck.
Vincent Comet Timing
- Thread starter Matty
- Start date
Thanks Timetraveller for the informaton. I have 340thou or so lift on both valves, so this looks OK and have used a tool which screws into the plug hole to find TDC accurately.
When the rain stops I will check out the performance - but perhaps I expect too much now because even at my age (72) I ride a Honda VFR750, and my son's RD350 LC Yamaha (headbanger job) and Fireblade regularly!!!
However after 47 years as a Professional Electrical, Electronic and Mechanical Engineer (Now long retired) my approach is for pragmatism rather than perfection which is I believe a very expensive option - in other words the old Comet is probably "good enough" and cost effective for what I want.
Matty
When the rain stops I will check out the performance - but perhaps I expect too much now because even at my age (72) I ride a Honda VFR750, and my son's RD350 LC Yamaha (headbanger job) and Fireblade regularly!!!
However after 47 years as a Professional Electrical, Electronic and Mechanical Engineer (Now long retired) my approach is for pragmatism rather than perfection which is I believe a very expensive option - in other words the old Comet is probably "good enough" and cost effective for what I want.
Matty
Have tried out the Comet and it is a little improved in performance with the crank moved 15 degrees and seems to rev more easily with a bit more torque out of corners.
The main change seems to be that the engine seems smoother at higher revs with less vibration and harshness - I do not really understand this because the dynamic balance has not been changed !! Perhaps it is some complicated effect of the combustion process or gas flow. However the change is not really very great and air temperature, humidity etc. sometimes seem to produce simular effects.
Hope this little investigation may help others with their cam timing knowing that the diagram on page 120 is NOT at TDC but 15 degrees away from it.
Matty
The main change seems to be that the engine seems smoother at higher revs with less vibration and harshness - I do not really understand this because the dynamic balance has not been changed !! Perhaps it is some complicated effect of the combustion process or gas flow. However the change is not really very great and air temperature, humidity etc. sometimes seem to produce simular effects.
Hope this little investigation may help others with their cam timing knowing that the diagram on page 120 is NOT at TDC but 15 degrees away from it.
Matty