Mopar Muscle Association UK

We're looking at ways to reduce / eliminate the likelyhood of the dreaded 'vapour lock' returning on our Plymouth.
There's been a couple of occasions when the weather is really hot, after allowing the car to sit at idle, it's just cut out and will not restart. Despite having disconnected fuel hoses at each end to vent the system.
Both times it's resulted in recovery as even after 6 hours we couldn't get it started. Fine the next day, mind.

We already have the inline filter with a return line, fitted before the mech pump. So in theory this should prevent any vapour blocking the line. But this doesn't eliminate the problem.
So a couple of initial thoughts -
Firstly adding a leccy fuel pump in addition to the Carter mech pump, that could be turned on when needed to push the fuel through. If we do have an issue the mech pump inlet hose could be detached so the leccy pump can clear the line? i suppose it depends on where the vapour is most likely to be stopping the fuel. We don't really want to go solely electric if it can be helped.
Secondly fitting a 'Phenolic' carb spacer to isolate some heat from the carb. We have a bit of underhood clearance so could probably fit a 1/2" spacer, but if a 1/4" would do the job it's less likely to upset the carb linkage. If these are worth fitting, would a 4 hole spacer be better than an open spacer? We would prefer to retain decent drivability over any top end improvements.
Found one here..
https://www.cantonracingproducts.com/ca ... iomXzqDQNY

any sage advice gratefully accepted.

When you have vented the lines was it at the carb and you witnessed fuel coming out of the inlet to the carb by turning the engine over and fuel coming out the line... if you did i am guessing the fuel lock be occurring in the carb itself...which i would imagine the spacer will probably help?

Bryan

Hi Bryan,
No, i disconnected it first at the carb, nothing came thru when cranking. Then disconnected at the mech pump - can't remember which side but still nowt. I also disconnected the vent tube from the sender that goes up thru the trunk.
T'was a tad frustrating.

i have used phenolic spacers to good effect with the webers on mine

i took the attitude that they were standard fitment on so many cars equipped with webers as OEM, (fiat alfa etc short aluminium manifold on top of engine) that it made sense. A pot metal carb body on an aluminium intake on a hot engine is the same regardless, doesn't matter what carb it is.

guess it depends if your vapour problem is due to carb heat or line heat


electric pumps do heat up the fuel as well..... and they are not very good suckers, if they end up full of gas rather than liquid, they won't move things on at all, so it would need to be out back below the tank to assist.
depending on your vented filter you may just circulate the fuel up to the front and back via the return, heating it up in the process

spacer is cheapest and least hassle, i'd be trying that first
remember you will need 2 gaskets
remember to check clearance before closing the hood :~)

you'll need to choose/buy/ make a 4 hole spacer or an open spacer.

My basic view is that
-4 hole spacer for street, makes each carb barrel longer so results in faster flow at the exit into the plenum , better atomisation at lower rpms
- Open spacer for track, increases the size of the plenum so more volume of pre mixed fuel and air to meet demand instantly

i.e both can have an impact by changing the plenum characteristics slightly, alongside the fact that the carb stays cooler when at low rpm or motor just switched off....

this may mean a tweak to idle settings a tweak to advance settings.

in essence from an insulation point of view they are there to cover heat soak at slow running and switch off
as soon as you get fuel through the carb again, the vacuum caused by the motor and the associated fuel vaporization (if you can get any out of the carb), brings the inlet and carb surface temperatures back down to a sensible really quickly. i.e fuel evaporation in the inlet works like a fridge which is why they implemented the heat crossover for the standard cast iron inlets.

i found having the vacuum advance connected also helped, I have it plumbed into manifold vacuum via a pulse smoother, 1940s/50s style, i never had a problem with overheating, i plumbed it in for a bit more get up and go.... but i have noticed it runs a little cooler at idle now it is connected up. it also starts easier possibly due to an advance spike the second it fires

Dave

What about your tank cap? reason I ask is I bought a new vented cap before we went to IOM but it seemed to be for venting vapour build up out, not letting air in to prevent a vacuum, luckily I had the original with us and just switched till we got home. Had to drill a small hole in the new one in the end.

Thanks for the input Dave(s).
Scotty - Yes a vented cap was another item on my list which i forgot to mention. Is it ok to just drill a small hole to let the system breathe? Though the tank vents anyway through the pipe from the sender, through the trunk and then out under the car. Can't help thinking a vented cap wouldn't hurt.

Dave999 - a couple of questions.
Yes i have at least 3/4" clearance so in theory a 1/2" spacer is ok. 1/2" spacers seem to be much more readily available. I presume the thicker the better for insulation? I'll check throttle operation to see if the extra height will upset anything. Chris Goodale has a 1/2 kit including gaskets & studs for a very reasonable sum on ebay.
And regarding an in line leccy pump. I can't remember whether it's best positioned high as possible ie above the tank, or below as you said. I'm sure when i had one fitted before it put it up on the cross member above the axle, based on some instructions i read. Though mounting it low would make sense to prevent it running dry. (all the instructions i have, just say to have the outlet inclined slightly)

Stu Twin wrote: Mon Feb 24, 25 5:23 pm Thanks for the input Dave(s).
Scotty - Yes a vented cap was another item on my list which i forgot to mention. Is it ok to just drill a small hole to let the system breathe? Though the tank vents anyway through the pipe from the sender, through the trunk and then out under the car. Can't help thinking a vented cap wouldn't hurt.

That's all I did since my original cap only had a small hole, IIRC I dismantled the inside before I drilled it.

4 hole phenolic spacer is going to help, even a thin one. Have you blocked the heat crossover in the manifold? If not I would. I don’t understand the return filter before the pump, to me that’s just making the mechanical pump work harder trying to suck the fuel up. If the return line is on the suction side of the pump then isn’t the pump trying to draw fuel down that line as well? I don’t understand why it wouldn’t.

yes thinking about it aren't those filters for between pump and carb?
fuel pressure keeps vent closed.
vapour allows it to open, and vent the vapour back to tank, until fuel hits the valve and it closes again, to maintain pressure to the carb

1970s mopar thing...

thicker the better.... well in theory yes, but as i say, the volume of the HOLE(s) in the spacer impacts the flow and volume of the plenum.
Mine are 1/8 inch, my carbs don't get very hot, but mine is port on port induction, and that causes the inlet manifold to get very cold at mid to WOT.

1/4 or 1/2 would do it i'm pretty sure... you have put a none conductive barrier made of card gaskets and phenolic resin in the way of the heat flow, metal does not touch metal apart from 4 studs and nuts so conduction is reduced dramatically.

pump positioning is dependent on the style of pump, but most decent electric ones are very good at pumping and absolutely dire at sucking. can they lift the fuel from the tank to the pump height in order to PUMP it?... probably, but take recommendation from supplier of pump,
if they suck air they tend to overheat so keeping them FULL and the fuel moving all the time helps with pump life... hence usual solution is pump from tank through fuel rail to regulator and back via return the fuel just goes round and round the system is never dead headed. but pressure is maintained by the regulator where it matters

in theory an In tank electric pump is the proper way for FI, being submerged they stay cool, a carb pump is less powerful, so inline pumps survive longer in that kind of application and have done since the ticking SU types on BL cars of the 60s/ 70s, but i see no reason why a decent carter mechanical pump with the correct pushrod and gasket thickness can't do the whole job.
they "suck" and "blow" well and self regulate .

an electric pump needs to be on a key switched relay supply with a rollover shut off...

other option is a swirl tank on the front inner wing
this is a 3 connector tank with a bosch or or similar pump submerged in it

pipe from your mechanical pump to fill the tank via inlet 1
and return from swirl tank back to the rear of the car outlet 2
The exit from the pump to the input of the carb via a regulator, in line, with pressure set for carb,
the regulator should have a return line to the 3rd connection on the swirl tank, inlet 3, or again back to the main tank, and cap off inlet 3 up front, either will do.

you are however driving around with a litre of extra fuel in the front of the car, its not under pressure in the tank as there is a return line, but you are in essence just heating up a load of fuel because the submerged pump uses the surroundings as a heat sink.

Its a lot of hassle

try the spacer first.

and try the vented filter in the line to the carb not the line from the tank, if its a mopar one you would be using it as per original 1975 mopar,
the back pressure of fuel seen from the carb keeps the vent valve shut until there is vapour behind it when you stop. Pump pressure then vents the vapour and closes the retun valve when you start.

If its aftermarket, follow the instructions i don't know what the intended purpose for it was originally. a petrol fountain is not fun, wrap it in a plastic bag when you experiment to stop inadvertent spray all over.

id be doing this work outside

Dave

Handy info again chaps, thanks.

From memory (a bit fuzzy), when we used to have the stock 5/16 line from tank to mech pump, we just had the plain in/out cannister filter between the pump and carb, tucked up under the alternator. Twas a tight fit.
And when we uprated things to go 3/8 line, that meant the only sender available had a return line, so we got the filter with the extra spout for return. (Wix 33041, now discontinued in 3/8" size which is a pain) At the time I think it was JC who fitted the filter in line before the pump, probably because it was easier. Position was discussed though i guess no one thought it was that important. Now we know

So yes i guess we ought to relocate it, after all as Dave says, Chrysler fitted them after the pump when they had the Vapour Separator. I presume it will work ok vertically as opposed to horizontally as it now, with the return line uppermost. If mounted vertical the return line will point down.

Clearance looks ok for a 1/2 spacer and shouldn't upset the linkage too much so will go for the one Chris has for sale.
https://www.ebay.co.uk/itm/226479167093

I'll see if the pump can be as low as possible.

some of those filters do need to be installed the correct way up which can mean vertical or horizontal or just having the vent port at the top side

Dave

There are always loads of carb spacers for sale at the NSRA swappie (this sunday) if you fancy a day out and they are cheap.

Just a quick thought Stu.....I have put a fuel cuttoff in the fuel line just before the pump. The idea was so that I can switch it off, and run the pump, filter and carb dry before leaving her unused for any length of time (ethanol damage prevention primarily).

If I leave my car for more than a day or so, it takes quite a bit of turning over to get the fuel through to fire her up, except when I switch the cutoff on. I can leave the car a few days and the fuel stays in the isolated section including the carb and she fires up straight away. I either jump out when shes running and switch the cutoff off, or just do it a few seconds before I fire her up. I also have a spacer to help with percolation.

Just wondering if you fitted one it may help??? When you stop, just flick the tap, do what you need to, then before you restart, flick it off. Just a thought and I may have missed the exact nature of your issue. If I stop at a petrol station or shop for a few mins, she starts instantly, but if Im more than 15 mins or so, I have to pump fuel through on the pedal a few times until she fires up. I also fitted a one way valve at the tank outlet but it doesnt seem to stop the fuel draining back for some reason.

Just food for thought, good luck

Thanks Steve, I was thinking a non-return valve might be handy for keeping fuel up near the carb. As you say if it’s sat for more than a few days it can take a while to get fuel up to the carb. Which it what I was hoping a leccy pump would help with.
And hoping it might force some fuel through if we do get a vapour issue.
So that along with a carb spacer/insulator and sorting out the position and orientation of the filter/separator, we’ll hopefully improve things.

But I think moving the filter after the pump means it needs to be vertical and the return will therefore be pointing down ??

Eletcric pump just after the tank to help with starting if it's been standing fpr a while, fuel vapour separator with a return to the tank after the mechanical pump before a fuel filter, that way the fuel never sits there and gets hot. I've run my Charger like that for at least 15 years and never had a problem, you could put a spacer beweet carb and manifold if you have the room.