Yamaha TRX850 Enthusiasts Forum

I am rebuilding a VFR800 from pieces at the moment, I've just done the valve clearances & put it all back together, but I am some weeks or possibly months away from starting it, so I thought I better get this right before commencing proper reassembly

Anyway, it's a V4 800 cc motor, like 2 V twins side by side. Front & rear left cylinders cylinders share a crank pin, same on the right.

There is a pickup on the rear inlet cam & an ignition pulse sensor on the crank, now, the crux of the problem is, assuming I've got the timing right from the front cylinders to the rear cylinders (which I do), it won't matter if the crank was on the exhaust or compression stroke... right?

(I asked this question on the American VFR site, but considering they advocate not bothering to check valve clearances pretty strongly, I didn't get much info... )

Firing order is dictated by the cams as far as I can work out, it goes Cylinder 1, 180 degrees, C3, 270 degrees, Cylinder 2, 180 degrees, Cylinder 4. (630 degrees)

Then another 90 degrees & we're back to C1 again.

I, uhh..

M.V. wrote:it won't matter if the crank was on the exhaust or compression stroke... right?

Hmm? Wha?

The cam moves at half the speed of the crank so they're not automatically matched up, if that's what you're asking? Though you have a 50/50 chance of getting it right! But if the pickup is in the right place at the right time, how could the crank pulser thing know any different?

Snod Blatter wrote: But if the pickup is in the right place at the right time, how could the crank pulser thing know any different?

That's my theory basically. I don't know if it's right though...

Lets say I lined the timing up at TDC on Cylinder 1, removed all the cams, rotated the crank 360 degrees, Cylinder 1 is now at TDC again, but at what would have been the exhaust stroke.

Now I put all the cams back in as per the manual, the timing should be right. Right?

As long as it all goes round 360 degrees when the crank goes round 360 degrees (as in, front and back rotates at the same speed, no weird gearing - never assume with a Honda!) then there is no difference, the pistons just go up and down. There is no exhaust or compression stroke if there is no valve actuation going on, only up and down, or if you want to be technical TDC and BDC. All timing starts with #1 at TDC, nothing more complicated than that.. That is, as long as you can line up the marks on the cams properly with whatever they are supposed to line up with. If the cams are still gear driven you may have to turn the motor over many times to get the marks in the right place on the cam-driving gears because they often spin at a different rate to the crank. I don't know enough about what you are dealing with to help you with that side of it..

Snod Blatter wrote:As long as it all goes round 360 degrees when the crank goes round 360 degrees (as in, front and back rotates at the same speed, no weird gearing - never assume with a Honda!) then there is no difference, the pistons just go up and down. There is no exhaust or compression stroke if there is no valve actuation going on, only up and down, or if you want to be technical TDC and BDC. All timing starts with #1 at TDC, nothing more complicated than that.. That is, as long as you can line up the marks on the cams properly with whatever they are supposed to line up with. If the cams are still gear driven you may have to turn the motor over many times to get the marks in the right place on the cam-driving gears because they often spin at a different rate to the crank. I don't know enough about what you are dealing with to help you with that side of it..

Thanks, that's very helpful! Timing is right as far as crank & cam marks are concerned, which means I'm good to go!

Thanks mate.

M.V. wrote:Timing is right as far as crank & cam marks are concerned, which means I'm good to go!

I don't know how it could be wrong, honestly.. But I have no experience of V4s, too many cylinders! Just be sure to turn it over by hand when it all goes together, make sure nothing gets in the way of anything else.. If it spins okay then have a go at starting it, once you've put the rest in place!