The V8 Owners Forum -

Hi guys,

I’m currently carrying out a few initial drives with my rebuilt Rover 3.9 (Weber 500 with RPi 3.5l set-up, JWR dual port manifold, viper hurricane cam, 4 branch tubular exh, lucas dizzy) and am after a few pointers and advice if that’s ok regarding getting things set-up.

Ultimately I plan to take the car for a rolling road session to get the right rods/jets etc specified (as far as I know there’s a good chance the RPi set-up will be too lean), but before I do that I’m wanting to try and get some of the other bits and pieces in order first to hopefully minimise how much needs sorting during the RR session.

I’ve been reading on here about the ignition timing, static, mech advance and vac advance. For sure I’m going to look at what mech advance I’m getting over the weekend and see whether I need to change the rate of mechanical advance.
I expect I will need to change it because as far as I know the dizzy is a standard item, but I’ll double check. If so I’ll get the real steel kit and have a play.

If I get the mech advance right, I’m then thinking I’ll need to look at the vac advance. From what I’ve read here it looks like the Weber carb has too much vacuum for the lucas dizzy vac canister. Is there a good way around this? Is putting a physical stopper inside the dizzy to limit the vac advance an effective approach? Can you get different vac canisters?

I can see disconnecting the vac system altogether has been suggested by a few of you, but I’d like to see if there’s a way of making the vac system work if I can.

So, am I right in thinking that if I get my mechanical and vacuum advance about right then it’d be a good time to then look into getting a rolling road session booked?

Thanks guys.

Gary.

The Lucas distributors and their vacuum modules as fitted to the Rover V8 are not designed to run with full manifold vacuum. So you could change the distributor to say a Mallory item or leave the vacuum module off altogether. It makes no difference as far as performance goes, rather it assists in reducing fuel consumption while cruising along a freeway with minimal throttle opening.

Ron.

The more I hear about RPI the more I believe that they don't know their 4rse from their elbow! According to them the cure for everything including a flat tyre is their ignition amp box.

Do you know what random selection of jets and rods they have fitted to your carb?

With regards to your ignition then good figures to go for would be 14 degrees static advancing to 36 all in at 2700 - 3000 RPM. Of course you must check that your timing pointer really does point to zero at TDC otherwise it will all be a waste of time. You may well find that your dizzy can be setup to give these figures without too much work apart from changing the springs, it just depends on which flavour of Lucas dizzy that you have.

I did try a mechanical stop on my vac system to limit the total amount of extra advance that it could supply, it worked OK but as MPG is not high on my list and BHP is I ditched the whole system for a programable MSD. I understand that MPG is important to some people.

I got the carb 2nd hand, done very few miles and was told it had RPI's set-up for 3.5l V8 - but the exact rods etc are not know.

I've checked the TDC pointer for the crank and it's actually correct much to my surprise.

I suppose if I were to put a stopper on the vac advance then although it limits the maximum advance it can have it also means it'd be quite switch like really, a stopper isn't going to make it any more progressive. Not sure if that would matter all that much though?

I guess there might be another location somewhere on the carb or manifold that might have a better vac signature for the lucas, but I suspect that'd take a long time to find and you'd end up with a swiss cheese carb/manifold at the end of it!!

Some sort of vacuum multiplier/reducer device is what's needed... surely something like that must exist... I can picture how it'd work with a second diaphragm but making one would also probably be a lifetime's work!

mk1storm wrote:I got the carb 2nd hand, done very few miles and was told it had RPI's set-up for 3.5l V8 - but the exact rods etc are not know.

I've checked the TDC pointer for the crank and it's actually correct much to my surprise.

I suppose if I were to put a stopper on the vac advance then although it limits the maximum advance it can have it also means it'd be quite switch like really, a stopper isn't going to make it any more progressive. Not sure if that would matter all that much though?

I guess there might be another location somewhere on the carb or manifold that might have a better vac signature for the lucas, but I suspect that'd take a long time to find and you'd end up with a swiss cheese carb/manifold at the end of it!!

Some sort of vacuum multiplier/reducer device is what's needed... surely something like that must exist... I can picture how it'd work with a second diaphragm but making one would also probably be a lifetime's work!

There is a good book on ebay which gives alot of info on setting these carbs up, its one of the Speed Pro range of books.

The book suggests that it is possible to have a tee piece in the vac pipe so that it leaks air into the system, this will reduce the 'signal' that goes to the vac canister. You can then control the leak by sticking jets or a plug with a very small hole drilled into it into the tee. (You could feed the leak from the base of the air filter so that no crap gets into the engine).


With regards to rolling roads you need to be careful, if the road is an inertia dyno it can only allow the engine to accelerate which is no good for setting up the cruise part of the carb. Really they are no good for anything less that WOT acceleration. A brake dyno can hold the engine against a steady load so you can setup the part throttle circuits. AJMHO!

mk1storm wrote:I got the carb 2nd hand, done very few miles and was told it had RPI's set-up for 3.5l V8 - but the exact rods etc are not know.

I've checked the TDC pointer for the crank and it's actually correct much to my surprise.

I suppose if I were to put a stopper on the vac advance then although it limits the maximum advance it can have it also means it'd be quite switch like really, a stopper isn't going to make it any more progressive. Not sure if that would matter all that much though?

I guess there might be another location somewhere on the carb or manifold that might have a better vac signature for the lucas, but I suspect that'd take a long time to find and you'd end up with a swiss cheese carb/manifold at the end of it!!

Some sort of vacuum multiplier/reducer device is what's needed... surely something like that must exist... I can picture how it'd work with a second diaphragm but making one would also probably be a lifetime's work!

I've done a good few MG v8 conversions for folk having to use the old dissy with the weber 500. Blanking off the vac line will not limit the total advance on the dissy. Max advance is all mechanical as long as the vac signal drops off at the right rpm.
I have never bothered checking manifold vac vs vac signal at the port on the 500 carb. My guess is that the manifold vac might be lower at part throttle. I dont need to worry about the vac or mech advance curve bcause my engine either sits at idle or runs flat out when I drag race.

On other engines I build for MG mates I normally blank off the vac line and then set the idle advance to 10 degree's for carb engines as a baseline. Then I run the motor up through the rev range and log the advance at 1500, 2000, 2500, 3000 until the dissy wont advance anymore. Most of the old dissy's I've worked on are either stuck at an advance setting cos the weights are rusted or are as loose as hell because a spring has come off. You can fiddle with the dissy to find the max advance and lock it or fit a Mallory dissy with no vac module which comes with a tuning kit to set the max advance.

The old dissy mech advance mechanism needs to be free moving if you are going to have a chance at a smooth running engine on part throttle.

I had an MGB V8 here last year that was an RPI setup with Weber etc..
The carb and dissy were all over the place. He had something like 20 degree's of idle advance and it topped out at nearly 50 degree's at 3000 rpm as the part throttle vac was too much for the dissy ?

Re the dyno logging, the guy I use near me will do whatever I ask him to do. Thats how I set my engines up. I ask him to get into test gear and then take his time to bring the revs up while we log AFR and manifold pressure from idle to flat out. This gives me some info on how the fuelling does throughout the rev range. And he will let me look into the carb so i can see when the vac secondaries start to open so I can note rpm and manifold pressure at this point.

Sidecar - If I were to bother with fiddling with a tee piece on the vac line and play around with some hole sizes (I guess it wouldn't actually take that long to mock stuff up) can I check my advance curve first without vac to set the baseline mech advance curve and then reconnect the vac, run the engine back up (at no load of course) to see what the vac addition is?

mgbv8 - For the engine's you've done for the MG mates, do you mean you've perm disconnected the vac system or just did that for the timing set-up? Are these running webers too?

Cheers for the info so far guys.... keep it coming!

mk1storm wrote:Sidecar - If I were to bother with fiddling with a tee piece on the vac line and play around with some hole sizes (I guess it wouldn't actually take that long to mock stuff up) can I check my advance curve first without vac to set the baseline mech advance curve and then reconnect the vac, run the engine back up (at no load of course) to see what the vac addition is? mgbv8 - For the engine's you've done for the MG mates, do you mean you've perm disconnected the vac system or just did that for the timing set-up? Are these running webers too? Cheers for the info so far guys.... keep it coming!

With regards to checking what the vac system does to the timing then yes there would be a couple of ways of doing this, you could just plug the canister into the non-timed port whilst you have a strobe pointing at the crank damper, you will then see how much extra timing the canister can give, it will be around 14 degrees at a guess. The problem is working out how much vacuum is required to start the canister moving and then how much vac is required for any given advance in the timing. (I think that the canister has a code number on it which can help but I would not believe it anyway!)

You could use the engine as the source of the vacuum then cobble up some system that allows you to bleed some of this vacuum into the canister whilst you have a vac gauge connected up at the point that the pipe goes into the canister. You could then bleed various levels of vacuum into the canister and note the effect on the timing, you could write all this stuff down and produce a graph. I think that you can buy a vac pump with a gauge which would be an easier method of doing the tests but it would cost money!

When I was messing about with this stuff I never went this far, basically I wanted to run a decent amount of static/idle timing. (idle timing and static not always being the same). I also wanted a decent mechanical curve and a decent all in figure. Once that alot was sorted I then noticed that the vac system if connected would cause a big kick back when just coming onto the throttle when applying the power when the revs were up over 3k RPM. The problem was that the mechanical system was all in and then the vac system would dump another 14 degrees of advance on the top. The kick back was not good for the engine at all. I have no idea how much vacuum was being generated at the time. After a while I just thought sod it and disconnected the vac system.

If you spend long enough messing around and possibly if you are prepared to not run the optimum mechanical curve then you may get the vac system to work. I could not be bothered!

In the end it all became irrelevant anyway; I have now locked up my dizzy and made a new base plate for it. It has an MSD pickup fitted which triggers a programmable MSD system. The same thing has been done for Muscle Manta’s car although we have gone one stage further on his car.
We have fitted a MSD MAP sensor which adds another 14 degrees of timing when the manifold is at cruise vacuum. The MSD unit allows for very precise control of when the 14 degrees is added or removed. The MAP sensor is connected to the non-timed port on the carb.

I could not do this on my engine because annoyingly the idle vacuum is exactly the same as the cruise vacuum. Using the timed port on the face of it would have been the way round the problem but in the end it would not have worked either. (For other reasons that I won’t bore you with!)

Mr. Muscle can get 27 MPG out of his stage III 4.6 which is very nice!

Hmmmm ok, definitely doesn’t seem to be an easy way to get the lucas vac working with the weber on the face of it then.

Little surprised there hasn’t been an aftermarket vac canister produced to work with the weber but then again I suppose the cost of developing vs the number of people still trying to run it must not make it worth it.

Ok, so does seem like I should perhaps first sort the mech advance curve and then try without vac and see how I go with that.
If it’s not looking like working well then I guess ditch the lucas system and go down the Mallory dizzy/programmable MSD route…

So does the Mallory vac system work OK with the Weber vac signature?

Nothing lost by trying the no vac option first I guess, about £8 for some springs and all reversible. I might not be running with the right carb set-up at this stage though – It’s a job to know if it’s way out or not so bad, the plugs look OK I think, no soot and a reasonably nice colour as far as I can tell. Get a little bit of an unburn fuel smell from the exh on full load accel, not sure if this is uncommon or not?

The Lucas vac relies on *where* the vacuum feed is derived. Ie, on top of the closed butterfly, so at idle there is little to no advance, as it effectively changes the pick-up point from post to pre butterfly. In other words from an area of high vacuum to near none.

Frankly, as petrol has changed dramatically since our dizzies were designed - and of course many engines are modified - it makes sense to use programmable ignition to set an exact curve to what's needed. Rather than faff about with ancient technology which never worked that well when new.

mk1storm wrote:Hmmmm ok, definitely doesn’t seem to be an easy way to get the lucas vac working with the weber on the face of it then.

Little surprised there hasn’t been an aftermarket vac canister produced to work with the weber but then again I suppose the cost of developing vs the number of people still trying to run it must not make it worth it.

Ok, so does seem like I should perhaps first sort the mech advance curve and then try without vac and see how I go with that.
If it’s not looking like working well then I guess ditch the lucas system and go down the Mallory dizzy/programmable MSD route…

So does the Mallory vac system work OK with the Weber vac signature?

Nothing lost by trying the no vac option first I guess, about £8 for some springs and all reversible. I might not be running with the right carb set-up at this stage though – It’s a job to know if it’s way out or not so bad, the plugs look OK I think, no soot and a reasonably nice colour as far as I can tell. Get a little bit of an unburn fuel smell from the exh on full load accel, not sure if this is uncommon or not?

One of the carb setups that RPI tried was way too lean on cruise, the engine would run but it was not nice to drive! It's not a big job to check out the jets and rods in the carb, post up what you find and I'll give you my opinion of it! (remove the air filter stud before you take the top off the carb, it will rip the gasket if you leave it in place)

I think that the mallory vac canister (which can only be used on their dizzy) is adjustable for the amount of advance that it can supply but it is not adjustable for the amount of vacuum required to operate it.

If your mechanical system is setup as per standard with hardly any static advance and a low total advance then you will be in for a nice suprise when you re-time it, it makes a big difference!

I wrote a load of old gumph on the dizzy ages ago....

http://how-to-build-a-pilgrim-sumo.wiki ... by-members

I also wrote a load more gumph on the Eddy 500 carb...

http://how-to-build-a-pilgrim-sumo.wiki ... ing-system

(I now think that the larger RV8 lumps do need a slightly different setup to the 3.5.) Basically the 3.5 does run well with 86 primaries, 67-55 rods, 83 secondaries and silver rod springs.

Excuse my ignorance, but are the sizes of the jets and rods stamped on them somewhere? Yeah I’d really appreciate it if you were able to give me your opinion on my set-up, thanks. Driveability really is quite poor at low load on re-accel at low-ish RPM but at the moment I’m attributing that to the vac advance issues…

Definitely have 8deg static at the mo (well that’s 700rpm ish with no vac connected), not actually checked what the idle timing is (ie with vac connected), but shouldn’t be any different as I’m connected to the timed port, but need to see what the total advance is, won’t be until the weekend before I can check it. Would be very cool if I can get a chunk more power out of it!

Right, I’ll check out what rods/jets I have, post that up. Will also look at the mech advance and see what I’ve got there too and post that up as well… Just as well have a play getting the mech advance right (if needs sorting) and also perhaps throw some other rods/jets in and see what I get.

Thanks for the great info and advice!

mk1storm wrote:Excuse my ignorance, but are the sizes of the jets and rods stamped on them somewhere? Yeah I’d really appreciate it if you were able to give me your opinion on my set-up

The sizes of the jets and rods that are fitted in your carb are stamped on the jets and rods. They can be a right git to read!

The rods will have something like 6552 stamped on them.
The jets will have something like 383 stamped on them. (83 thou series 3 jet)

"Excuse my ignorance, but are the sizes of the jets and rods stamped on them somewhere? Yeah I’d really appreciate it if you were able to give me your opinion on my set-up, thanks. Driveability really is quite poor at low load on re-accel at low-ish RPM but at the moment I’m attributing that to the vac advance issues…"

Hi
this is more likley to be transition from idle circute to primary main jets issue rather than vac advance and probably not enough accelerator pump added into the pot. Try and sort out the transition issue before adding any more accelerator pump. You can mask the immediate problem by adding accelerator pump but you will just end up with a problem elsewhere in the rev range.
Best regards
Mike

Ok, cool. Will take a look in the next day or two and let you know what I’ve got in there.

Mike – To try and sort the transition between the idle circuit to the primary main jets (without fiddling with the accel pump – currently on outer most of the three holes by the way, was initially on middle hole) would I tweak the idle mixture screws or would the change of primary main jet (or both) sort this?
For additional info, the idle CO figure measured recently during the IVA test for my car (kit car) measured 2.7% ish on both banks I think it was (limit of 3.5% due to engine age)… I went for the rule of thumb 2 turns out on the idle mixture screws I think it was. Sound like they’re set too lean?