strange timing issue

[Nic Bowman]

Hi, I can't get the hang of this one, so your comments will be appreciated. My TR4a sits at 4 degees advance at tickover, but will suddenly jump to 12 on occasions? When it does this the tickover raises noticably (1500 rpm - that's how I know it has happened). It will sometimes go back of its own accord.

 

It happened today, and I checked the advance in general. When reset to 4 degrees, the full advance is 35 degrees (weights and vacuum connected), so the weights are not stuck (I think!). 35 degrees sounds like a lot??

 

So, questions,

 

What should the additional advance be from the weights? (it says 10 degs on the bit in the dizzy)

 

What should the additional advance be from vacuum?

 

What the heck is going on??

 

I have one of those electronic in teh dizzy ign systems (magnetronic?), all the bits are clean and shiney, vac adv is new.

 

Thanks

 

 

Nic

[67_gt6]

Hi, are you sure that something isn't causing the revs to rise - air leak in manifold etc - and so consequently you then see extra advance as a consequence of the increased RPM ?

 

10 degrees advance in the distributor is the standard.

 

Andy

Edited June 22, 2010 by 67_gt6

[Nic Bowman]

Hi, cheers. Daft additional question, if 10 degrees advance in the distributor, surely I would expect 10 degrees additional at the crank shaft? So, normal timing would be 4 degrees at tickover, plus 10 from the weights, plus 6(?) from the vacuum. If this is right, my 35 degrees seems very high?

 

 

Nic

[67_gt6]

Hi

 

The distributor turns at half the rate of the crankshaft, so 10 degrees centrifugal advance in the distributor = 20 degrees advance on the crank.

 

As it is the mechanical centrifugal advance function that you are worried about it might be an idea to plug the vacuum outlet on the carb while you are checking it, so that vacuum advance doesn't skew what you are seeing. Vacuum advance will only come into play when there is a strong vacuum eg at idle or closed throttle on the overrun.

 

All engines are slightly different so it is difficult to give a figure for maximum advance. Low to mid 30s would be the generally accepted range. Basically you want maximum advance without pinking under load.

 

I have never used a timing light on my engine. I set it the following way:

 

Warm the car up to normal temp

Use the carb idle screws to set an idle of about 700RPM ie below the point at which there is any centrifugal advance

Plug the vacuum port on the carb with a golf tee

Set the dizzy vernier adjuster to its midpoint

Loosen the dizzy

Advance it until you find the point where RPM doesn't increase any more.

Back it off slightly from that point so that RPM drops by around 100.

Tighten the dizzy back up

Road test:- find a moderate hill, approach in 4th at 1200RPM, accelerate hard in 4th. If there is no pinking then advance timing on the vernier until it just pinks under those circumstances. When you find that point, back it off a click on the vernier.

 

 

Andy

Edited June 23, 2010 by 67_gt6

[Nic Bowman]

Nice one, cheers.

 

Nic

[littlejim]

Nice one, cheers.

 

Nic

 

 

Me too Andy! Thanks.