GT6 Mk3 only runs well with VERY advanced timing. Any ideas why?

[johny]

No idea as Ive never done this😂

[nicrguy1966]

8 minutes ago, johny said:

One thing, did you open up both 1 and 2 tappet gaps to 40 thou as recommended in the manual? Its just that the  3 thou youre measuring on balance point is quite tight so a measuring error of 1 thou in 3 is a lot more than 1 thou in say 10... 

Second answer to same post!

Yes, 3 thou is very small, when I was turning the crank I basically had a gap in tappet 1 or 2 with the other closed, until I found the balance point and could get a 3 thou feeler gauge into both.

[nicrguy1966]

3 minutes ago, johny said:

No idea as Ive never done this😂

1st time for me too!

I've done "big jobs" like piston rings and bearings before, but this is far more delicate and fiddly than I'm used to.

[johny]

2 minutes ago, nicrguy1966 said:

Yes, 3 thou is very small, when I was turning the crank I basically had a gap in tappet 1 or 2 with the other closed, until I found the balance point and could get a 3 thou feeler gauge into both.

Thats the way and at the end of the day valve timing is never going to be absolutely perfect because of chain and sprocket wear. Of course you could always do this job every few thousand miles to keep it closer😱 On that front how worn is your chain compared with the manual measurement?

[nicrguy1966]

2 minutes ago, johny said:

Thats the way and at the end of the day valve timing is never going to be absolutely perfect because of chain and sprocket wear. Of course you could always do this job every few thousand miles to keep it closer😱 On that front how worn is your chain compared with the manual measurement?

Seems fine. The engine was reconditioned about 15 years ago, so most things are in fairly good condition. I'm assuming the guys that rebuilt the engine got the cam timing wrong.

[johny]

yes its easily done but I ask about the chain because the timing could be 1/4 tooth out but the chain wear makes it 1/2....  

[Iain T]

25 minutes ago, nicrguy1966 said:

I've done "big jobs" like piston rings and bearings before, but this is far more delicate and fiddly than I'm used to.

Finding TDC is so much easier using a simple piston stop. I bought one of tinternet and delivered next day.

Iain 

[johny]

1 hour ago, Pete Lewis said:

eed to get the cam lobes in balance and exactly mid point highest point   

best use a dial indicator to get the centre of the cam ramp 

pete

Out of interest Pete what is the procedure for doing this with a dial indicator as I cant figure out. Is the balance point, where inlet and exhaust valves are open exactly the same amount, always half way between the high and low of a cam lobe? I would have thought that depends on the design of the cam.... 

[JohnD]

Are you aware that the cam sprocket may be rotated to achieve valve timing down to less than 1/4 tooth?   Thus is  by the four bolts, of which the two pairs are slightly off set.

From WSM, p. 1.138:

"NOTE: The camshaft timing sprocket is provided with four holes which are equally 
spaced but offset from a tooth centre. Half tooth adjustment is obtained by rotating the 
sprocket 90 degrees from its original position.  A quarter tooth adjustment may be obtained 
by turning the sprocket "back to front". By rotating it 90 degrees in this reversed position, 
three-quarters of a tooth variation is obtained. After securing the sprocket, re-check the 
timing to ensure that the camshaft has not been disturbed during this operation."

The last refers to the crank being in TDC on 1 and 6.

John

 

 

[Pete Lewis]

you need a protractor ?dial  (like on the crank )  on the cam   set the DTI on inlet lobe mark the degrees 

for say 0.025 on the up and down from highest   mark the centre of the two .

do same on exhaust lobe   mark the centre of the two readings 

if on compression stroke  turn cam exactly 180 degrees (half a turn)  ft the chain with the best aligning holes 

as compression stroke uses the back of the lobe  you need to turn the crank one turn so the two lobes are uppermost 

just check both lobes give the same lift on a DTI (or feelers under the rocker )  at TDC

well something like that      easy to do not easy to write it  Ha 

bedforshire waffle 

Pete

 

[nicrguy1966]

53 minutes ago, JohnD said:

Are you aware that the cam sprocket may be rotated to achieve valve timing down to less than 1/4 tooth?   Thus is  by the four bolts, of which the two pairs are slightly off set.

From WSM, p. 1.138:

"NOTE: The camshaft timing sprocket is provided with four holes which are equally 
spaced but offset from a tooth centre. Half tooth adjustment is obtained by rotating the 
sprocket 90 degrees from its original position.  A quarter tooth adjustment may be obtained 
by turning the sprocket "back to front". By rotating it 90 degrees in this reversed position, 
three-quarters of a tooth variation is obtained. After securing the sprocket, re-check the 
timing to ensure that the camshaft has not been disturbed during this operation."

The last refers to the crank being in TDC on 1 and 6.

John

 

 

That sounds like a quarter of a tooth, how is it "less than 1/4 tooth"?

Not that I'm trying for perfection, just to make my ignition timing "normal" (and maybe increase HP a little).

[johny]

I suppose as indicated earlier there might be some play in the sprocket fixing bolts so you can rotate it slightly to get it spot on👍However then as I say in a few miles your chain stretches a bit and youve lost it😭

[Puglet1]

Well Nicrguy. Nothing for me to contribute other than to say what a fantastic forum this is.  
You have got the “Top players” supporting you throughout a very difficult problem. 

If the rest of mankind had the same attitude as these guys I think the world would be a much better place for everyone. 👍

[Iain T]

I know you don't have a piston stop but using one doesn't require feeler gauge etc. You only need a pointer ( and an easily printed 360 degree marked disc. Make sure you are on compression stroke, turn crank until the piston hits the stop then rotate back. Half way between the two angles is tdc. Spot on no messing with trying to find the rocker point. 

Google finding tdc with piston stop. 

Iain 

[johny]

Might be a bit of a mix up here Iain as the rocker point refers to getting the cam to the position it should be at when the piston is at TDC compression. Normally this is not necessary as the cam chain keeps the crank and cam in unison but if you disconnect the chain you need to find this TDC positions for both crank (eg. piston stop method) and cam (eg. rocker method).

[Iain T]

4 minutes ago, johny said:

Normally this is not necessary as the cam chain keeps the crank and cam in unison but if you disconnect the chain you need to find this TDC positions for both crank (eg. piston stop method) and cam (eg. rocker method).

Of course you need to find the cam position! Sorry

 

[nicrguy1966]

1 hour ago, Puglet1 said:

Well Nicrguy. Nothing for me to contribute other than to say what a fantastic forum this is.  
You have got the “Top players” supporting you throughout a very difficult problem. 

If the rest of mankind had the same attitude as these guys I think the world would be a much better place for everyone. 👍

I quite literally couldn't have done this job without the help I've received from this forum.

From the start, identifying that cam timing could cause off ignition timing, to providing a link to a download of the WSM to find the cam balance point, to tricks on how to removing the crank nut, and finally, how to get to the crank cog.

So many times I'd have been forced to give up and take the car to a mechanic.

I can't stress how grateful I am for all the help offered.

[Iain T]

2 hours ago, nicrguy1966 said:

I can't stress how grateful I am for all the help offered

You are not alone!

This forum is a goldmine of experienced help. I still concider myself a novice but with the forums help it gives you the at elbow confidence to tackle jobs you would never normally have done. 

Iain 

Edited October 15, 2023 by Iain T

[Pete Lewis]

as yours is still assembled you need to use the equal tappet gap to get the cam lobes at their   TDC

so feelers is all you need with a piston stop to set the piston at TDC  you must not rely on using the timing cover marks 

and hoping the crank doesnt turn while you are fiddling about .

you can make a piston stop by wrecking an old spark plug by remove the electrode and adding a bolt 

Pete

 

[Pete Lewis]

and well done for having a go  

 you have proved to yourself its not a dark art  once you have a few clues 

    a good workshop manual for bedtime reading is a mind of information   Zzzzzzz!!!!

pete

 

[JohnD]

17 hours ago, nicrguy1966 said:

That sounds like a quarter of a tooth, how is it "less than 1/4 tooth"?

Not that I'm trying for perfection, just to make my ignition timing "normal" (and maybe increase HP a little).

My misstype!  For 1/2 tooth!

But ... were you aware?

And ... that tiny degree of cam mistiming will NOT be the cause of your need for very advanced ignition timing!   You're focussing on a trivial fault.

John

Edited October 16, 2023 by JohnD

[nicrguy1966]

12 minutes ago, JohnD said:

My misstype!  For 1/2 tooth!

But ... were you aware?

And ... that tiny degree of cam mistiming will NOT be the cause of your need for very advanced ignition timing!   You're focussing on a trivial fault.

John

I only started down this route based on advice that incorrect cam timing could be the cause of my odd ignition timing.

Do you have other explanations for my need to have very advanced ignition timing? I am very open to new ideas.

[johny]

Anyway youve got clear problems one by one, testing between each and eventually youll reach perfection plus have great fun doing it😂

[nicrguy1966]

10 minutes ago, johny said:

Anyway youve got clear problems one by one, testing between each and eventually youll reach perfection plus have great fun doing it😂

So far, I'd say it was interesting more than fun. If there is any improvement at all in how the car drives, it will be very satisfying.

At the moment I'm preparing myself for the big step of actually removing the timing chain, and trusting myself to be able to put it all back together again! So far, I've been confident that everything I've done was reversible if I needed to drive to a mechanic!

[Iain T]

Best practice is take copious amounts of photos and mark where everything is now. 

Iain