1940 Cadillac

[grumpydad]

Hi all
As the title we have a 1940 Cadillac
This is 6750 cc and is a side valve V 8
It has 2 hif 44 SU carbs from a range rover
And has a open loop system
It is running on lpg at the moment
And runs quite well but very thirsty
We have a friend with a rolling road
Can some one tell us what the co2 readings should be
The engine spends most ot the time in the 400 to 2000
Rev range 2000 rpm is 70 miles a hour in top

The only problem with it at the moment
is that it wont tick over below 550 rpm on gas
it will tick over at 400 rpm on petrol

also would like to fit a flash lube system
can this go were the vacuum take of is on the carbs

I hope you have enough information to help

Thanks

[woody]

hi are you sure you need 2 carbs?? most of those old flat heads used to run on single carbs or worse case a twin choke.
it sounds like you a fuelling issue re the idle problem.
Twin Su,s are a proper pain in the ar$e to keep in tune,you could have wrong size mixers or numerouse other things that are causing problems.
You should be able to the Co to read around 1.5 / 2.5 % with hc,s under 200 providing the engine is in good nick mechanically.

hth woody

[grumpydad]

Hi woody
Thanks for coming back to me
Yes you are right it had a twin choke carter
With 1 1/8 chokes and the motor would only rev to 4000
The twin su are 1 ¾ and I love them had no problem setting them up
The motor will now rev to 5500
Yes it is a fuelling issue with the idle
It dose not seem to mater witch way I turn the idle screw on the vaporiser
It won’t go down to 400 rpm on gas on petrol its fine at 400
I do wonder if it is because the motor has not got enough suck
But I thourt that was what the idle screw was for just lets the lpg out
And no need for suck

I am a bit of a old school hot rod guy I guess
The car now has a 5 speed gear box instead of a 3 speed
Also got Austin A35 with midget motor in with twin SU
And that runs on lpg as well
Thanks

[34HF90]

It dose not seem to mater witch way I turn the idle screw on the vaporiser
It won’t go down to 400 rpm on gas on petrol its fine at 400
I do wonder if it is because the motor has not got enough suck
But I thourt that was what the idle screw was for just lets the lpg out
And no need for suck

Can I just chime in here and ask if it smells 'gassy' when idling? This might give cause to think that the gasflow isn't all it might be at that slow speed. Vacuum gauge(s) ought to tell you.
Just a thought.

[woody]

It could also be to big a mixer , what type and size do you have??

cheers Woody

[rossko]

I'm not sure I'd want to rev a seventy year old engine of that size to 5000rpm

How old is the vap? They don't last forever, a common old age fault is tiddling excessive gas at idle. That in itself wouldn't raise idle (would need more air too to generate more energy) so you might like to consider if it is off-tune on petrol, say a bit labouring, and the idle stop has been lifted to compensate for that i.e. gas reveals the true idle speed setting. Worn SU needles? Plain wrong needles for this engine??

If SU carbs suit, i would think SU carb mixers would do fine.

Set it up as best you can with care on petrol, note the CO2 - in the 2% league perhaps. Replicate on LPG, and if you are picky reduce by 10% (to e.g. ~1.8%) to account for the different chemistry. Make sure the ignition is as good as you can get it before starting this.

[grumpydad]

Thanks every one

the vap is 3 months old OMVL 90 only using one of the takeoffs
the power valve is new to it is one that has a screw for each carb
And got the pipes to the mixers the same length
The carbs have been rebuilt needles spot on took 3 go to get it right
just slightly lean above 4000 so filed the needles
ignition timing set up on rolling road to max power then backed off
just slightly
don’t know if it makes a difference the compression is only 7 to 1

I cant smell any gas at tick over so it must be being all sucked in

Thanks

[rossko]

the power valve is new to it is one that has a screw for each carb

It'd be less fiddly to have one screw and a plain Y, you don't need/want individual adjustment for twin SU.

The carbs have been rebuilt needles spot on

How do you know? By which I mean you should use the same procedure or gas analyser or whatever on the gas system.

don’t know if it makes a difference the compression is only 7 to 1

I don't know either; LPG would prefer a higher CR than petrol, so it might never make as much bhp on gas, but I wouldn't really expect it to make much difference to setting up in general. Can't think of any way for that to affect idle speed.

Do you have a dashpot lifting vacuum gizmo? (Often fitted to SU to avoid needle wear when running dry on LPG.) Air leak or misconnect in this?

Bear in mind we still don't know what you have in the way of gas mixers ; if you've just got crude spuds you might be stuck with the wrinkles.

[Sir Henry]

You don't say what flavour of SU carbs are fitted. On an engine of that size they should be HIF8 or HIF7, with the latter giving a slightly weaker mixture but still within acceptable bounds. Other than that the needles and jets have to be the correct size and your need to file the main needles indicates that they may not have been the right ones.

Also you don't mention just how 'bad' the fuel consumption is. My Silver Shadow has an engine of the same size in a body of a similar weight (2.5 tonnes) and aerodynamic qualities (a breeze block would cut through the air better). The best consumption figures I've obtained from her in mixed mode driving have been 14mpg on unleaded and 11mpg on gas. With your engine being a less fuel efficient low compression side valve I would expect even lower figures than that. Pre WWII the Yanks just weren't interested in fuel economy as 'gas' was so cheap. The low idle on unleaded indicates to me that it must be a long stroke engine which will not be as fuel efficient as a short stroke engine, but will happily idle at very low revs. I doubt that the replacement of the gearbox with a 5 speed variant will have much influence on economy (unless it's a manual) as the very earliest Caddies had a 2-speed auto box and ran well enough on that as the top ratio was the same - ie direct! Only an overdrive unit can improve on that and is very rarely found on cars of that size.

[34HF90]

Just to muddy the waters further, but might give some clue.....

Years ago I had some experience of running a gen set with a Ford V8 Pilot engine. These were flathead iron engines, much in the same vein as is being discussed. This was before the days of cheap comercially available LPG kit, and everything was home-made, with the gas fed in as vapour from a propane bottle. A home made demand valve was fitted.
There were problems (unsurprisingly).

Gas consumption was VERY heavy, and this was put down to the sidevalve configuration.

Pre-ignition and all kinds of popping and banging due to glowing carbon. Give it a shot of petrol and it ran ok for a while...

Also...although never called to do so in service, obviously, it wouldn't idle. At the time we put this down to a combination of inefficient combustion/high moving mass and lack of cylinder pressure from the propane.

I don't know if any of this is relevant....or I should get me coat....

[Clifford Pope]

Presumably the idling speed is ultimately determined by the adjustment of the throttle butterfly stop, regardless of how much gas you allow it to draw? Could it be that at such extremes of operating range - low compression, low revs, very long stroke, heavy flywheel - the idling stop just needs to be closed more on gas than on petrol?

I'm just thinking that on my Volvo the electronic idle bypass valve settles on a different point of opening for gas and petrol in order to achieve the same idling speed.

[tommo2009]

not sure about an engine this old, but most engines are struggling to run on a compression that low.