26 replies to this topic

[Midas Mk1]

Be interested how you get on, mines a non vac advanced anyway, as the dyno guy prefers them without. 

I've got a timing issue thats stopping me running, and i'm convinced its something to do with my dizzy.

[lawrie124]

you did tell him there is a vac advance unit on it didn't you.

 

as the base plate does need locking in place or it will move as the diaphragm vibrates as it no longer has "load" on it.

 

yep, but that it didn't get explained. As I said, seems a bit strange to suggest not using the vac advance, given it comes with one, but happy to try anything ahead of a trip to Germany on Saturday!

[gazza82]

Does sound a bit like a "cop-out" from Aldon ..

 

I thought having a vacuum advance was the "best" (or at least better) option for a road car?

[ACDodd]

Ok if you have 48 degrees with the vacuum connected all is good. All centrifugal advance checks are completed with the vacuum advancepipe disconnected. Re connecting after setting/checking will add in the vacuum unit advance to the total advance the centrifugal system is already delivering.

Ac

[lawrie124]

Ok if you have 48 degrees with the vacuum connected all is good. All centrifugal advance checks are completed with the vacuum advancepipe disconnected. Re connecting after setting/checking will add in the vacuum unit advance to the total advance the centrifugal system is already delivering.

Ac

 

thanks AC. I was thinking that 48 with vac on was WAY too high, but happy to be corrected.

 

I will go back to 8 no vac at idle rather than the 2ish it is currently running, then put vac back on.

 

just seems a bit strange that Mr Aldon said no need for vac advance, leave disconnected. Either he is confused or I am!

[Dusky]

you did tell him there is a vac advance unit on it didn't you.
 
as the base plate does need locking in place or it will move as the diaphragm vibrates as it no longer has "load" on it.

How do you lock it?

[nicklouse]

you did tell him there is a vac advance unit on it didn't you.
 
as the base plate does need locking in place or it will move as the diaphragm vibrates as it no longer has "load" on it.

How do you lock it?

How ever you want. It is just two discs that rotate about a common axis. If you remove the vac advance then the are free to move where ever. So a screw or a nut and bolt or some weld or you can change the base plate.

Look at one next time you pick a dizzy up they are not that complex.

[ACDodd]

The point whether to run vacuum or not is best determined using a rolling road. Some engine are happy others are not. However the engine may produce less peak power withe it fitted drivability and fuel economy more than make up for the defiency.

Ac

[carbon]

Also when setting up a distributor with vacuum advance, worth considering that not all vacuum cans are the same. For the Lucas vacuum advance there are options with:

- at what vacuum reading the vac advance starts moving

- at what vacuum reading the vac advance stops

- and the total amount of vacuum advance.

 

For info, these are marked on Lucas vac cans (at least on 25 and 45 type units) as something like 4-11-7

 

For a 4-11-7 can the advance starts at 4 inHg, ends at 11 inHg, with total 7 deg advance in dizzy degrees = 14 degrees at the crank.

 

Seems to be a bit of a lottery as to what settings are used on A-series engines. 4-11-7 is my preferred option for a fast road 1293 motor, and it does make a reall difference to fuel economy on longer runs.

[Spider]

I've found that if the Vacuum Advance is fitted but not used, timing varies very little and while best if locked out, it's not vital and I can't see it being the issue here.

 

I think I may have got confused with your opening post here lawrie;-

 

Hi.

Hopefully a simple question.

It says 8 degrees static, and I have confirmed with them that is 8 degrees at idle with vac advance removed (and blocked off at carb).

The thing is that at full revs it advances to about 48 degrees.

Surely that is way too advanced?

We usually set our cars at 4000rpm at about 38 degrees so 48 sounds dangerous to me?

Any suggestions and advice much appreciated.

Mike

 

 

 

Ok if you have 48 degrees with the vacuum connected all is good. All centrifugal advance checks are completed with the vacuum advancepipe disconnected. Re connecting after setting/checking will add in the vacuum unit advance to the total advance the centrifugal system is already delivering.

Ac

 

thanks AC. I was thinking that 48 with vac on was WAY too high, but happy to be corrected.

 

I will go back to 8 no vac at idle rather than the 2ish it is currently running, then put vac back on.

 

just seems a bit strange that Mr Aldon said no need for vac advance, leave disconnected. Either he is confused or I am!

 

 

As carbon pointed out, there's probably as many different Vacuum Advance cans as there are distributors, and as AC pointed out, what (and if) suits your engine is best done of the RR.

 

Sure, depending on the Vac Advance mech, it is quite possible to get 48 degrees, however, I took it from your opening post that this was being measured with it disconnected.

 

In general, the longer the Cam Duration, the less Vacuum Advance is needed,

 

The Cylinder Head Camber Shape and Engine Stroke also come in to play on this as well;-

 

     Longer stroke usually needs less Vac Advance (as piston velocity is higher and the Rod to Stroke ratio gets bigger)

 

     The Chamber Shape has a big bearing on the burning speed, some mods slow it down, some speed it up.

 

    (but this is probably too much information for this straight forward issue)

[lawrie124]

Thanks again for all the responses.

I have reset timing to 8 degrees with vac OFF and it now reads 42 with vac ON at about 4000rpm.

Part of the poor performance issue turned out to be a 2 year old fuel line that was perished between the van tank and fuel pump, probably sucking in air, so now pulling good and ready for a trip to Germany this weekend.

Thanks for all the advice.

Mike

[lawrie124]

.. and I will book it in for a rolling road session on my return.