Won't rev above 7k, carbs or coils?

[VTRDark]

If the cam timing was out, he would have noticed it on the first ride out of the shop and it would have been corrected.

I'm not saying it is this or not, but the problem here is that it's not something that can physically be seen ie timing marks. The timing marks and cam lobes will all look correct. It's the relationship between one cylinder firing and the other and the amount of turns on the crank getting from one to another. So if the cams are out and someone does not turn the crank the right number of degrees whether starting from the front or rear it can end up looking correct but wrong.

Don't rule out an over-oiled airfilter though as this is an easy fix. Stick a standard one in and it will run much better. A k&N will only give you tuning grief with these bikes. And no sticking a dyno jet kit in wont help. It takes a bit more than a different needle profile and sticking bigger mains in. Even if once you have spent a small fortune of dyno runs trying to get thing right you will have only gained a few extra BHP on top with a standard engine. It really isn't worth it.

[Wicky]

Didn't Marty run around for a while with his 180 out and not notice till it was dynode?

[VTRDark]

He did indeed and the carbs had been tuned (disguised) to work around it so it went up to 9000 just not at the responsive speed it should have done.

[bigtwinthing]

If the cam timing was out, he would have noticed it on the first ride out of the shop and it would have been corrected.

I'm not saying it is this or not, but the problem here is that it's not something that can physically be seen ie timing marks. The timing marks and cam lobes will all look correct. It's the relationship between one cylinder firing and the other and the amount of turns on the crank getting from one to another. So if the cams are out and someone does not turn the crank the right number of degrees whether starting from the front or rear it can end up looking correct but wrong.

Don't rule out an over-oiled airfilter though as this is an easy fix. Stick a standard one in and it will run much better. A k&N will only give you tuning grief with these bikes. And no sticking a dyno jet kit in wont help. It takes a bit more than a different needle profile and sticking bigger mains in. Even if once you have spent a small fortune of dyno runs trying to get thing right you will have only gained a few extra BHP on top with a standard engine. It really isn't worth it.

i agree, like changing bolts for Ti when you are overweight, waste of time lol. if you want a faster twin, buy an SP2 or a bloody Penagally thingy KTM,s new 1290 is bloody mental and comfy.

[Stratman]

one other possibility is a tear in the diaphragm of the petcock, take it apart and check, just remember how to put the bugger back together again

+1

[Alex.]

Ok, you have worn me down... I'll check the cam timing next.
The main explanation that has been playing on my mind is that of the intake not coping with a 'big bang' engine. That is the first time I have read anything that tries to explain why the timing being 180 degrees out still allows the engine to run and run well, apart from up top when the demands for air are greater. It also goes some way to explaining why it is worse with the race cans, which will demand more air.

Is there a definitive guide to checking the timing, with pictures and everything?
Any chance of a link to it, to save me searching too much at work...

Oh, and I have checked the fuel tap diaphragm and the vacuum pipe, all looks good.

Alex.

[sirch345]

Is there a definitive guide to checking the timing, with pictures and everything?
Any chance of a link to it, to save me searching too much at work...



Alex.

Yes there is thanks to Cybercarl, this is for fitting manual CCT's, but it has the INFO to check the valve timing:-
http://www.vtr1000.org/phpBB3/viewtopic ... 31&t=28583

Chris.

[AMCQ46]

have a look in the workshop section on the fitting instructions for CCTs. the relationship between rear TDC [compression] and front TDC [compression] is explained there.

[Miztaziggy]

There have been a few of these recently haven't there.

I'm confused though. Can someone confirm for me, I thought that if someone did the CCTs wrong and put the timing 180 out, it was just the valve timing that was out.

I.e. the inlet / exhaust valves were reversed on one (or both?) cylinders.
Therefore as the piston falls, the inlet valve should be open, but it would be the exhaust valve that would be open.
When the exhaust valve should open after detonation, the inlet value opens.

The way people are describing it on here it sounds more like both cylinders are detonating at the same time rather than opposite, but that would involve changing the crank shaft around?

[Alex.]

The camshafts are rotating at half the speed of the crankshaft, so 180 degrees of the cams is 360 degrees of the crank.
I would assume that these bikes use a wasted spark ignition system, where the coils fire each time the piston is at about TDC.

If the cam is 180 degrees out, the inlet would be open when the exhaust should and the exhaust open when the inlet should. But due to the wasted spark, it all lines up and the engine works.

Alex.

[AMCQ46]

ziggy,
with the 180deg error, the cams are correctly aligned to the piston stroke [if you look at each cylinder in isolation], but as there is 2 TDC's on a 4 stroke cycle, the problem is the relation ship between when the back cyl fires and when the front cyl fires [or sucks]

there should be a 450degree relationship from rear cyl firing TDC till the front one fires, but as the crank FT mark occurs every rev, it is possible to put the engine back together at 90deg relationship as well.

if the induction phase is only 90deg apart, then the airbox cant flow enough air to deal with the combined back and front cyl overlapping demands.

hope that makes sense

[Alex.]

ziggy,
with the 180deg error, the cams are correctly aligned to the piston stroke [if you look at each cylinder in isolation], but as there is 2 TDC's on a 4 stroke cycle, the problem is the relation ship between when the back cyl fires and when the front cyl fires [or sucks]

180 degrees of the camshaft.

there should be a 450degree relationship from rear cyl firing TDC till the front one fires, but as the crank FT mark occurs every rev, it is possible to put the engine back together at 90deg relationship as well.

if the induction phase is only 90deg apart, then the airbox cant flow enough air to deal with the combined back and front cyl overlapping demands.

hope that makes sense

450 & 90 degrees of the crankshaft.

Just a little clarification, as the two rotational speeds can get a little confusing with just numbers...

Alex.

[AMCQ46]

yup, that makes it more clear.

the 180 naming confuses a few people

[Alex.]

While I'm thinking about this....

When all is right, the firing of the pistons in relation to the crank is:-

Rear fires - 1 1/4 crank rotations - front files - 3/4 crank rotation - rear fires - 1 1/4 crank rotation.....
So the airbox has at least 3/4 of a crank rotation to refill after the gobs of air have been sucked through the carb.

When the camshafts are 180 degrees out:-

Rear fires - 1/4 crank rotation - front fires - 1 3/4 crank rotation - rear fires - 1/4 crank rotation....
The airbox only has 1/4 of a crank rotation to recover and refill. At low engine speeds this is enough time, but as soon as revs rise the time is not enough and the air is restricted.

To me, this sounds like a plausible reason. I may be wrong, it has happened before!

Alex.

[AMCQ46]

you got it