17 replies to this topic

[jime17]

Hi all. Couple of simple questions if I may...

1 when checking timing at 1500 rpm with vac off, do you plug pipe and distributor connection or just the pipe? I can't see any benefit in plugging dizzy end, but what do I know?

2 how can I check that the vacuum advance is working correctly?

cheers, jim

[petehuws]

I usually just check mine by pulling the pipe out of the carb, the engine should then start to hunt 'if idling' and the carb will be sucking like a Thai...  I stop myself there, vacuum cleaner; if by returning the vacuum to the dizzy the idle levels out cleanly again then this is usually enough to tell me the vacuum is operating well, totally unscientific but a fine indicator.

 

Yes when blocking off the advance you block off the vacuum at the carb connection, however I tend to pull the vacuum off the carb, block the carb, and tape over the vacuum tube that then runs off to the distributor.  I followed Mr Peanuts advice on here recently of setting the idle to 1000, vacuum off & blocked and then setting timing at 6degree followed by an adjustment of the mixture to match, my car runs quite nicely at this and provides  a maximum advance at 3500 at 29degrees.

Edited by petehuws, 05 July 2013 - 08:13 AM.

[lrostoke]

The reason for blocking the pipe is so that you don't suck air in through the open end, this will alter the mixture.

 

If you remove the dizzy cap and undo the pipe from carb end and suck on it, you should see the dizzy backplate move

[KernowCooper]

Or if you reconnect the vac pipe at idle  with the timing light on the marks you can see the advance the vacuum unit is providing

[Captain Mainwaring]

Never stick your tongue into an inlet port with the engine running.

[jime17]

I'm very careful where I put my tongue

[Ethel]

There's no need to block the pipe just to check your advance.

[KernowCooper]

I Think the blocking of the pipe refered to the inlet take off for the vac pipe on the manifold side

[Ethel]

Either way it won't make any difference to setting the advance and won't do any harm, as long as it's refitted before you drive it. 

 

It's petty, but I'd be more worried that someone could trash the connector trying to block it.

[jime17]

Either way it won't make any difference to setting the advance and won't do any harIm, as long as it's refitted before you drive it. 
 
It's petty, but I'd be more worried that someone could trash the connector trying to block it.

what I meant was when setting timing, it's done with vac off usually at idle or just abov.... Yes?

If so, which end of the vacuum system is plugged? Dizzy manifold end?

I can't see that blocking the dizzy side would make much difference whereas not blocking the other side would I'm guessing reduce the vacuum in the intake and weaken the mixture? Is that right? I was just looking for a bit of guidance to make sure I set the timing correctly.

The second question was just so I could eliminate the vacuum advance mechanism as a reason for a persistent mid fire.

Thanks everyone so far.

[KernowCooper]

Only way you can check the timing at 1000rpm is to disconnect the vac pipe off the manifold and slip a short piece of pipe back over the pipe with a screw or something blocking the end to stop the mixture running weak and a rough idle, otherwise the vacuum at idle will pull the advance and you cant set the timing. On a road engine you ought to be running vacuum advance.

Edited by KernowCooper, 05 July 2013 - 04:37 PM.

[Ethel]

To set the timing I wouldn't bother blocking either end, just remove one or the other. It will leak a tiny bit of air at the manifold but it won't do any harm. I'm not seeing how the vac advance will cause a misfire, at least not one that can be remedied by disconnecting the pipe. As someone mentioned above, you can check its function with a timing strobe. Rev the engine  to see how it pulls the advance compared to the same revs with no vac connected. The amount of advance should be stamped on the diaphragm housing. Blipping the throttle will create overrun & maximum vacuum.

 

About the only fault disconnecting the pipe would eliminate is a punctured diaphragm.  The vac advance linkage will still be there, with potentially sticky parts, you'd have to unhook it from the baseplate. 

 

A bit more choke should get you to 1k rpm, but by all means do as KernowCooper suggests if it doesn't..

[jime17]

Ok. So could you give me a brief run down of how you'd set the timing on a standard 998 with a 59 d dizzy using a light with no dial in advance.

Everyone seems to use different methods.


Oops ethel posted at same time...

I wasn't looking to run without vac. Just to set timing correctly and to make sure vac is working.

Edited by jime17, 05 July 2013 - 04:54 PM.

[Ethel]

Just connect the strobe's sensor to the No1 plug lead and point the strobe at the timing marks. Turn the dizzy body until it lights up the mark against the appropriate graduation, clockwise to add advance. Assuming the marks are on the timing chain cover, each peak is 4 degrees, TDC is the peak nearest the back.

[lrostoke]

set tickover to a 1000rpm with vac pipe disconnected from the dizzy...

 

set timing to 8 degrees (2nd small peak on the timing marks) assuming its down on the crank pulley

Edited by lrostoke, 05 July 2013 - 06:48 PM.