Hi Andy, I thought Id post up here rather than mail you as other people can see the out come, if we can cure the fault.

Breifly, I have changed the standard SU setup for a modified manifold and weber 40 DFI 5 carb reputed to come from a landrover V8 .

I experiance a "jerkiness" from the engine at over 3k rpm under load, with a light load I can get the revs up to 4.3 or so.

I have changed the fuel filter and raised the float hight to its highest level, but although I have a slight improvment over the initial fault, it is still displaying hunting @ 3.5/4.3 krpm.

The jet sizes on the 40 are 190 mains(bottom of float bowl) and 115 air correctors(at top of joint face)

the 38 DGAS jet sizes are 145 mains and 185 air correctors.


I could just try to use the DGAS... but it will need mods to the throttle linkage, and i'd rather try jets first as it involves less work. ;)

RIK

The 190 /115's seem massively rich as a first look especially when considered with the 115 air correctors - the norm is to have the air correctors .30 or so bigger than the jets

Hence the Standard DGAS at - 145 and 185

Also look at the inlet needle valve that should be a 2.5 - 3.0 size - The 190's are 1.9mm diam and add two of those to a 2.0mm or 2.5mm inlet and you can see the carbs capability to empty the float bowl quicker than it can be filled. Especially at the top end with the air correctors doing very little.

The Weber selection material I have says for the 30mm chokes - a 1.5 to a 1.8 main is right coupled with a 2.0 air corrector

I would put the 38 DGAS air correctors in as a fiurst step to see what the difference is - I would then put the 38 mains in and see.

It may be that 1.55's with the 185's would be the way to go - I am sure I have some smaller mains that could be drilled 1.5mm

Or use the 1.45 mains and drill the 115 air correctors to 1.50

The main jets and emulsion tubes fuel for the air flow through the size of venturi and the air correctors richen and lean the mixture at the very top end.

So at a first glance the jets are massively oversize for the 30mm venturi and the air correctors are massively small. - all in all lke running with the choke pulled on - and we know what that feels like - sound familiar......

Make some sense from that.

Rules to note

Chokes/venturis are sized for capacity/power/rpm
Mains are sized for the venturis
Air correctors are sized to trim for top end power based on set at 2.00 for stock.

A 30mm choke on a 1600 will have the same jets as a 30mm choke on a 3000. within .2mm

drip a drill in the air correctors and make sure they are about the size stamped on them and have not been drilled.

Cheers

Andrew

OK here goes....

I fitted both dgas jets to the dfi.

IE mains @ 145 and air corrector @ 185

Result was it spluttered and refused a throttle opening of more than about 1/2 and
nearly didnt go up hills. :)

Fitted mains @190 with 185 air corrector.

result it will pull to 5.5 ish through the gears thenthe float bowl funs dry!

in top i can get it to pull up to about 4k before the hunting kicks in.

A point to note here is that the 38 DGAS carb has a significantly larger float bowl.

Next step is to try the dgas which i think will prolly be ok, and /or fitt new pump as I currently have an mG su one and IM not convinced that it can flow enough for the carb.
RIK

Right then - it's the lack of the power valve in the DFI thats causing the leaning out with the smaller jets and why it needs such large mains to compensate but I would say that the air correctors are far too small - However it also depends on how rich the emulsions are as well.

What are the choke sizes on the DFI? I am assuming they are 30mm from memory

Check the filter in the top of the carb as well to make sure its not blocked and then a flow test on the pump to see just how much petrol it is pumping.

Cheers

Andrew

I tested pump delivery at the same hight as the carb and came up with this :-

1Pt = 1min 30 secs

3600 (1hr) / 90 (1min 30sec ) =40 pints per hour

40 / 8 = 5Gals per hour .

That just doesnt sound enough to me, If the engine was averaging 15 mpg

Id be going at 75/80 ish (3.5/4Krpm) at max delivery and not allowing for the 5 mpg "draw"
at max acceleration.
I think i need a 18gal p/h pump.

Suggestions? or is it a "nail on the head" moment :)

Info for you Andy

Venturi sizes 32mm
emulsion tubes F50 (4x4 holes about 1mm)
Pilot jets (?) 65
Air correctors 185
Mains 190

Right then the 32mm's will have jets from - 160 to 195's :D based on a 200 air corrector so the combination of 190/185 should work pretty well - the F50's are a mid range emulsion so should work well enough.

Get the fuel pressure/capacity sorted. - should be looking at a pump capacity of 100L/hr

Cheers

Andy

NOw then now then,
I installed the mitsubishi pump you lent me today, tested its through put@ 36secs for 1 pint.
Which is neat because 1 hour = 3600 seconds.
So 100 pints per hour divided by 8 = 12.5 gals .
Anyhoos armed with that info i trundled off (ahem) up the hill Ive been using to test it, @5.5k in third i bottled and changed gear, and im afraid my anal sphincter
was in danger of proving what colour adrenalin is.... so I bottled out at 4.8 in top.

Net result... sorted B)

Now i need to take it for a longish run to make sure that nothing is going to fall off before Stonleigh... oH,.. and a plug chop would come in handy as well..;)
Cheers , Ill try to get a new pump asap.
RIK

So you don't have to dig it out I have bounced this thread to the top Rik

Having gone through the carb details on the phone - check the pump delivery again just incase its got a bit blocked or that the hoses are collapsing somewhere as you have some base figures already its easy to check if there has been any reduction in the fuel delivery

Everything we checked elsewhere seems to tie in - there is another little trick I have read about to test the progression richness and that is to cut a piece of thin wire that is long enough to drop into the air corrector and poke out of the top effectively reducing the diameter of the air corrector and the air mixing of the progression tube.

It will be interesting to see whats going on with those screws and lock nuts and if they are to do with the progression circuit.

Cheers

Andrew

Hi Andy and thanks for the chat yesterday,somtimes its just a case of not being able to see the wood for the trees.

If you look to the right and just above the tick over udjustment scew in the bottom pic, you'll see a screw and lock nut.

I think this is progression adjustment, I have taken both sides out by 2.5 turns and she runs fine with no throttle cable adjustment at all.
The top pic , just to the left of the choke linkage hole , you can see a screw head.
This is another jet, on both sides they were partialy blocked with oxide deposits. both cleaned.

Took the car out for a 25 mile run today, and its absolutley fine, no hessitation at any speed and any throttle opening, it didn't missfire at all either, and considering that I am using the origional ignition systems 1 ohm primary coil and not a 3 ohm 12v item thats quite good.

I checked #1 & #2 plugs just before I put her tobed for the weekend , and theyre both sooty.
I guess I've gone from too lean a setting to too rich.
I'll be trying to sort that next week, whne I'll have more time as I'm on hol.
Thanks again,
RIK

Those adjustable jet thingies dont come up on any of the standard Webers in my books - so it looks like a special for a specific manufacturer.

Interestingly in my Vizard Pinto book he tests this carb abd states that its good for 330CFM - not too far short of the 390CFM Holley so capable of flowing plenty of air.

The slotted head brass screws that air inline with the carb mounting bolt and sit just under the air filter flange are the idle jet holder screws - all the fuel for the idle and progression circuits pass through those jets - you can follow the porting route via those cast "pipes" - If they had corrosion in them then the progression would be effected.

I would still be tempted to try the 38DGAS - its power valve circuit makes it more driveable whilst giving better economy and you wont lose out on power - or look out on e-bay for a 500CFM twin choke Holley.

Still - good that its sorted

Oh - we now have a copier in the office that scans straight to PDF so I will copy the relevant bits of the Weber manual for you for future reference.

Cheers

Andrew