Trying to set timing!

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paulsv8manta
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Re: Trying to set timing!

Post by paulsv8manta »

Hi Sidecar,
So what I will do this weekend is, disconnect the vac hose and measure what the timing is at 4000 rpm. I'll get back with the results asap.

Hi SuperV8,
I did remove the water jacket plate under the throttle plate and two of the four holes used to bolt this plate did go through to the inlet this has been filled some months back. Good call.

In regards to the throttle plate gap, I've read from previous posts some disagreements on this subject, whether the throttle plate should be closed or a slight gap, mine is actually closed.



SuperV8
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Re: Trying to set timing!

Post by SuperV8 »

In regards to the throttle plate gap, I've read from previous posts some disagreements on this subject, whether the throttle plate should be closed or a slight gap, mine is actually closed.
It was just a thought - if the throttle gap is too big, might be harder to get a lower idle? but if yours is closed then it's something else!
I would think you should have some (very small gap) though, as you want the throttle plate to stop on the throttle stop screw - not on the plate/wall of the inlet.

when its idling and warm maybe try squeezing off the hose from the warm up extra air valve - to the plenum. This should close when warm, and if it isn't closing fully then that would increase your idle.

If your still adjusting the timing this will also effect the idle speed -

Tom.
Dax Rush 4.6 supercharged V8 MSII

DEVONMAN
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Re: Trying to set timing!

Post by DEVONMAN »

paulsv8manta wrote:
Wed Nov 28, 2018 5:33 pm
Hi Sidecar,
So what I will do this weekend is, disconnect the vac hose and measure what the timing is at 4000 rpm. I'll get back with the results asap.
It will be interesting to see what advance you will get at 4000rpm. With the vacuum disconnected I would expect 28 degrees given 2 degrees at idle and 26 degrees added by the mechanical advance in the distributor. The advance curve in a Rover distributor is usually set to be used with a timed port and using manifold vacuum will I suggest give all sorts of advance values at different loads and revs. One example is that keen off idle throttle movement will kill the manifold vacuum and the timing will effectively tend to only 2 degrees which will give poor engine response.

I believe the idle setting for this engine is about 10 degrees which is conservative due to the manufacturers concerns about drivers who slog the engine at low revs and to allow for bad fuel and towing loads. 12 -14 degrees gives a much better engine response when used in conjunction with ported vacuum but the max advance at wot would then exceed the usual 36 degrees normally aimed at for this engine. The way this is usually corrected is to modify the end stop in the distributor to limit the movement of the advance weights. There are various kit available also to allow change to the advance curve springs if you are willing to experiment but this is very time consuming if you want to get the optimum performance.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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sidecar
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Re: Trying to set timing!

Post by sidecar »

How have you worked out that the mechanical advance is giving 26 degrees?

DEVONMAN
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Re: Trying to set timing!

Post by DEVONMAN »

sidecar wrote:
Thu Nov 29, 2018 8:51 pm
How have you worked out that the mechanical advance is giving 26 degrees?
Based on what I have seen in various Rover Distributors. The 3.5 distributors tend to have 26 degrees mechanical advance giving 36 all in based on 10 degrees at idle. The 3.9 distributors have less mechanical advance giving about 32 all in.
When I have needed to use a distributor on a 4.6 rover I have used a distributor with 18 degrees mechanical advance and set the idle to 12/13 degrees with good results. Most lucas distributors have their max advance stamped on the plate below the advance weights. The stamped value is distributor advance which of course is half the engine advance.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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sidecar
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Re: Trying to set timing!

Post by sidecar »

I guess that is fair enough, I have a shed load of old Lucas dizzies kicking around and have seen that the mechanical advance can differ between them, I don't fully trust the numbers that are stamped on them, they seem to be out by a few degrees quite often.


I must admit that my acknowledgement of 36 degrees (which is more or less what you want on a 3.5) is based on the fact that when the OP revved the engine the 36 was being supplied by a combination of the static setting and the mechanical advance but having said that there may well have still been enough vacuum in the manifold to cause the vac canister to still be operation. This is why the canister should be disconnected when trying to determine the all-in at low vac levels. In this case if it is not 36 degrees if it was my car I would modify the dizzy to give 36 degrees. If after that once the vac canister is reconnected it is found that at 4k the timing is more than 36 I would not be worried because the timing would back off if the engine was at 4k AND under a heavy load.


The problem with recommending any figures over the forum is always going to be that they are just general figures, for example nobody would believe the idle timing figure that I'm running on my 4.6 which is around 32 degrees from memory, its gets more confusing when I state that my all in is 32 because on the face of it it looks like I only have a 0 degree advance sweep! This is not the case because I actually have a 16 degree sweep and a 16 degree vac system sweep. I came to the conclusion that my engine wants 32 at idle after many hours fiddling with it. I have worked on several other engines that also need far more idle advance than the 'normal' accepted figures.

DEVONMAN
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Re: Trying to set timing!

Post by DEVONMAN »

Hi Sidecar,
I totally agree that the factory/Lucas distributors are frequently inaccurate and likely to have +/- 2 degree error when new.
The reason I chipped in on this post was to clarify whether the OP was intending to use full manifold vacuum or timed port vacuum. I believe timed port vacuum is more suited to the Lucas distributor. Of course many American V8's use manifold vacuum but no doubt will be used in combination with a distributor which has a suitable advance curve. I look forward to getting some feedback from the OP this weekend.

PS. My forged pistons rattle like crazy if I exceed 15 degrees idle timing.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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sidecar
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Re: Trying to set timing!

Post by sidecar »

I actually think that the non-timed port should always be used if possible but hey ho, that what forums are all about!

The other thing to throw in the mix is that there are at least 4 different vac canisters that Rover/Lucas used, they differ in many respects as to how they operate when a vacuum is applied to them. If you try to use an unsuitable canister then it will never work well with the non-timed port.

One reason that I like the non-timed port is that you are much more likely to be able to get the idle advance that the engine wants, you can find what the engine wants just be twisting the dizzy and listening to how how the engine repsonds, it can end up being quite a high figure. The problem is that you can not leave the dizzy setup to give the required idle timing just by statically setting it to the required figure because the centrifugal advance would end up giving far too much total (all-in) advance.

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