The V8 Owners Forum -

I have a Dax Rush kit car fitted with a 3.9 Rover V8, fast road cam, Rhoads lifters and 1404 Performer carburettor. I have also fitted Air Fuel Ratio (AFR) sensors to both exhaust systems and a vacuum gauge on the inlet manifold.

The issue is that the car runs rich with an AFR of 10 to 10.5 when at cruise (12 in/hg) light throttle. I have checked the float level, changed jets and rod combinations, adjusted idle mixture screws, changed step up springs and confirmed the fuel pressure is 5.5 psi

If I apply steady moderate acceleration (metering rods still down) the vacuum drops to 8 to 10 in/hg and AFR is appropriate to the jet / rod set up (12.5 AFR) but as soon as the engine catches up to cruise the AFR drops back to 10. Any suggestions as I feel I have tried everything and completely flummoxed!

What sort of air filter is it running ?

I have a 14" x 3" pan filter but no difference with or without the filter which is new.

From the Edelbrock set up manual, I'm using a No5 set up (.083 main and .062/.052 metering rod) and the car runs appears to run very well but as described very rich and drinks fuel.

Previously I used a No 11 set up (.065/.052 metering rod). Had the same rich cruise issue but ran too week when touching the throttle. This was some time ago and the mind grows fuzzy as to the exact detail.

So, I'm now waiting for another selection of metering rods to arrive and will update with any progress and who knows, perhaps a solution!! (I can dream).

Many Thanks
Allan

I've done loads of work on these carbs but I've only setup one 3.9 lump.

You are definiately going about things the right way using an AFR gauge, any other way is just total guess work!

The edlebrock chart is full of errors, what's more it does not even list all of the jet and rod combos that are possible. I now use a spreadsheet which will compare a combo against the base settings, it then give a percentage leaner or richer for both the cruise and power up settings.

The 3.9 that I did was I guess what you would call stage 2 tuned, flowed heads and performance cam but standard size valves.
It is setup with 83 primaries, 62-52 rods and get this 104 secondaries!

The AFR are as follows:-

tickover 13:1
cruise 14-14.5:1
Accel 12.5:1
WOT 12.5-12.8:1

At some point in time I will make a 63-52 set of rods (I make my own or modify Edelbrock ones) and give them a go because in my experiance the RV8 will run at 14.5-15:1 on cruise.

Currently the setup is 3.9% leaner than the base setup on cruise and 10.8% leaner on the powerstep.

Something seems fundmentally wrong with your carb because I can not understand how the ARF goes leaner when the rods lift, after all when they lift the thiiner part of the rod is then the only thing poking down into the primary jets, its almost like the tip of your rods is thicker than the middle bit!!

Other things to look out for when setting up the primary side is to make sure that the secondaries don't start to open. You can even stick a block of wood under the throttle to stop this. Then accelerate with the throttle open as much as the wooden block will allow, make a note of the vac reading then use rod springs that will lift the rods at this level of vacuum or at a slight higher level of vacuum. Then with the wood still in place you can accelerate and make a note of the ARF. I have read loads of good books on this sort of stuff, Vizard reckons that engines work best when the AFR on acceleration is around 12.5:1, WOT is always best at 12.5-12.8:1. 13:1 is right on the borderline an any leaner is too lean. Now you engine won't melt at 13.5-14:1 but it will be losing BHP, whats more that drop off in BHP on the lean side is really steep, the drop off on the rich side is much less.

I used to run my WOT at 13.5:1, when I richened it up to 12.8:1 it felt like I had gained another 10-15 BHP and 500RPM at the top end!

I don't have any of my info to hand, I can post it up later in the week ?

Many thanks for your indepth and considered response.

From the Edelbrock manual I have tried combinations 5, 6, 7, 11 and 10. All suffered with this same problem.

The best performance combination I found was 5 (.083 main and 62/52 rod) where on light to moderate accelerate I get an AFR of 12.5. The other combinations were too weak and the car ran horrid. It actually runs very well on 5 but as with all other combinations drops back to 10ish on cruise. I'm using a blue step up spring 3"hg and have even tried with no step up spring to ensure the rod is in the down position but no change.

As suggested I double checked the manual jet-rod area and is in the right ball park in terms of rich to weak.

As this issue exists with all jet/rod combinations I'm coming round to thinking I am sucking fuel from somewhere else. I'll have a play with the secondary set up to see if anything changes. (perhaps tape the inlet over?).

I'll update with any progress

Many Thanks
Allan

Thanks Sidecar and JSF55 for your input that I have taken onboard while continuing to resolve this issue. The principle of fault finding is ‘make one change at a time then test’. Pity I didn’t stick to it! I tried lots of changes and have overcome the rich running on cruise and the AFR now responds to main jet / metering rod changes. I am pretty sure this was due to the following:

The books says “set the float to 7/16” at the outer edge of the float”. This in my view is a bit ambiguous as the float seam is slightly curved and depending on where you actually place the gage (7/16” drill) will alter the actual float level. So I placed the centre of the drill drill approx 10mm in from the end which reduced the fuel level by approx 1/16”. Now when I make changes to the jet/rod set up the AFR follows that change.

You mentioned you made your own rods. I too tried this last year to even up left and right hand AFR. It sounds a bit unprofessional but the best way I found was to use a larger metering rod and use fine wet & Dry sandwiched between rubber then placed in a vice with a twisting action. Have you a better suggestion?

Using a .083 jet I now have the following result.
Rod 6552
65=AFR 11 to 12 slightly rich for cruise but liveable.
52= AFR less than 10 (off scale) far too rich

Rod 6755
67= AFR 16 too weak
55= AFR 10.1 – too rich still

From the above it looks like I could do with a 6657 rod. What do you think?
There is a stock 6557 metering rod that perhaps I will order and try first.

Many thanks
Allan

I have a Westfield Seight with 4.6 V8 and I have been fine tuning the carb ever since I bought it !
Your engine seems to need a very lean jet and rod set up compared with stock. Do you have a bonnet bulge with air intake or is the filter sticking out of the bonnet ?
When I bought my car, the carb had 80 primaries, 62-52 rods and 95 secondaries. This set up may actually work for your car as it leans off cruise 19% and power 21%.
On advice from the guys on this forum, I have increased the filter height and removed the choke flap. I now run 86 primaries, 68-57 rods and 89 secondaries. You haven't mentioned what secondaries your carb has ?

I guess that’s what owning a Kit Car is all about. Even when its running well you just can’t resist the temptation to tinker. Like you I have been playing with various enhancements and have revisited the carburettor set up on many occasions, each time thinking I’ve got it right at that time.

3 years ago I put the Dax on a rolling road. The result was that timing was set at 15 BTDC at idle and ran with no vacuum advance. The 1404 was richened with .083 main and 6252 metering rods and put out 270 BHP which is how it been ever since.

Having just returned from a trip to France it ran early well but also conscious that often ran at an AFR below 10. Fuel consumption had historically been 25 MPG where is now less than 20 MPG. Hence I’m once again on a mission! I have therefore reset the timing to 10 BTDC @ 1200 rpm and no vac. Then reintroduced the vac advance as I understand this keeps the plugs hot and better for economy.

I agree, it does look like I’m heading in the too weak direction but will see it through. I will probably settle with a compromise of target AFR and drivability.

For the purpose of setting up the Cruise and power I have disconnected the secondary link. Once I’ve sorted the lower end I will move on to the secondary side which at present is .098

With regards to the air filter, it does protrude out of the bonnet ( 1.5” above and 1.5” below). Have you found this to be an issue and if so what? Does your whole filter protrude out of the bonnet? Would fitting a deeper filter element have the same effect?

Many Thanks
Allan

That's a good output from a 3.9. I took mine to Aldon Automotive in March to install their Amethyst mapable ignition box and my 4.6 was showing 270bhp and 300 lbft at 3100rpm. He said it was running lean so didn't want to take it any higher than 5500rpm. Went for a richer set up after that. Shortly after the exhaust manifold gasket let go. After replacing it, the AFR gauge shows it is now running too rich. I suspect the gasket had been leaking for a while, which would explain the leaner readings.
So just when I thought I had got the right set up, I find myself having to start the process again !
As for the air filter, mine was originally only a 2" high filter with a drop base, so the top of the filter was only around 1" above the top of the carb. The guys on this forum said this can cause problems, so purchased a higher filter but was constrained by the bonnet bulge from going to a 3" one. I guess airflow is not an issue for your carb if the filter is out of the bonnet.
I note that you have reintroduced the vacuum advance, which is not supposed to work well with the 1404 carb. When John Eales did some work on my engine, he disconnected it and set the timing to 14' BTDC. The fuel economy did suffer though as a result, although it is not a great concern of mine !
The Amethyst does now allow me to map the ignition according the engine speed and vacuum. Do you have a Lucas DLM8 dizzy with the module on the side ?
Keep us posted of your trials.
John

If it helps, playing with my MegaSquirt with EDIS I've found ignition timing probably has more effect on economy at cruise than mixture - assuming it's not silly rich.
Of course the more advance you use, the weaker the mixture can be - within reason.

It seems trivially easy to get these setups running really well. Getting good economy too is the difficult part.

Johnny,
Now that’s interesting. I wasn’t aware that the 1404 was not meant to run without a vac advance; however, I can confirm it definitely does run better without it. So now that I have solved the cause of the very rich running perhaps I’ll return to the 15 BTDC at idle with no vac advance and go through the carb set up again. Like yourself I’m not too perturbed by the MPG but running at 10ish AFR was clearly not good

The distributor is a Mallory twin point but replaced the points with Pertronixs ignitor The ignition is MSD6AL so by modern day standards no real control of the ignition system .

Out of interest do you know where you heard about the 1404 and not being good with a vac advance?

Dave,
I have been quite surprised at the range of AFR that the RV8 will quite happily run at. If it wasn't for the AFR sensors I would be none the wiser.

I'll update once another set of metering rods have arrived and fully tested. AGAIN!

Many Thanks
Allan

I've 'banged on' about the Eddy carb not working well with the standard Lucas vac system on this forum several times. Basically the vacuum signal that the Eddy gives causes the vac advance to kick in quite hard just as the throttle is being cracked open and this can cause the engine to kick back if you are exiting a turn at high-ish revs (when all of the mechanical advance is in). The Mallory vac system is adjustable in that you can control how much extra timing it adds, you can not adjust the vacuum levels that it operates at.

I don't think that your issues are ignition related anyway, well as long as you are running something from 15-20 degrees at tickover and 28-32 all in at 2750-3000 RPM with the vac system perminantly disconnected then that will be good enough to eliminate the ignition as the cause of your problem. I run a programable MSD system with a locked out Lucas dizzy, the MSD systems are very good and give a healthy performance increase over the standard systems in my experiance.

Thanks all, some really useful information and your input has been greatly appreciated. From what you are all saying I feel happy to revert back and continue running without a Vac Advance. From my personal experience the car feels slightly unsettled / surges with the vac advance connected.

However, apart with continuing with the journey above you have now planted the seed re a programmable ignition system. Is this easily achieved within a reasonable budget? Does it require an Engine Management System with lots of inputs? Perhaps a question for another Forum Subject?

Many Thanks
Allan

Allan Robinson wrote:Thanks all, some really useful information and your input has been greatly appreciated. From what you are all saying I feel happy to revert back and continue running without a Vac Advance. From my personal experience the car feels slightly unsettled / surges with the vac advance connected.

However, apart with continuing with the journey above you have now planted the seed re a programmable ignition system. Is this easily achieved within a reasonable budget? Does it require an Engine Management System with lots of inputs? Perhaps a question for another Forum Subject?

Many Thanks
Allan

You could replace your system with the programable MSD without too much trouble, you would need to lock your dizzy up so that the advance curve is controlled by the unit and not the bob weights in the dizzy. My dizzy was a standard Lucas one but I have removed all the internals, I have made a new base plate and fitted an MSD pickup, this is triggered by the Lucas trigger wheel. It took a fair bit of work to sort out the position of the pickup inside the dizzy. From the outside my setup looks standard!

http://www.msdignition.com/Products/Ign ... 6AL-2.aspx

One of the reasons that I went for this system is that I can retard the ignition when my NOS system is activated. It also has a MAP system which is programable, this means that I can run extra ignition advance under high vacuum, I have carefully set this up so that it won't kick in just as the throttle is being cracked open.

Here is some gumph....Mine is the last one that they talk about...

http://www.youtube.com/watch?v=k0ig-VRVMuE