Fuel leak in odd circumstances
Discussion
Well, here's my latest little problem (ah, the joys of owning a 12 year old kit car, eh ? 
)
I have an "odd" fuel leak from my front 45DCOE.
If the engine is ticking over, or revving hard when at standstill, no leak. If I drive the car "normally", then no leak, either when running or when I turn the engine off.
If I take the car out for a damn good spanking, when I pull up and let the engine idle, there's no leak either. However as soon as I turn the engine off, after driving the car hard, the front carb positively puthers fuel out of what appears to be it's underside somewhere. Difficult to tell exactly where's it's coming from, it actually dribbles off the carb at the point where the carb bolts to the manifold, but that might just be coz it's the lowest point on the carb. It will drop about an eggcupfull of fuel, give or take.
Anyone any ideas ?!?
) I have an "odd" fuel leak from my front 45DCOE.
If the engine is ticking over, or revving hard when at standstill, no leak. If I drive the car "normally", then no leak, either when running or when I turn the engine off.
If I take the car out for a damn good spanking, when I pull up and let the engine idle, there's no leak either. However as soon as I turn the engine off, after driving the car hard, the front carb positively puthers fuel out of what appears to be it's underside somewhere. Difficult to tell exactly where's it's coming from, it actually dribbles off the carb at the point where the carb bolts to the manifold, but that might just be coz it's the lowest point on the carb. It will drop about an eggcupfull of fuel, give or take.
Anyone any ideas ?!?
Nev,
First of all thanks for that info you sent me it's been a great help!
About yer leak, take the carbs and everything off that side of the engine and check the O-rings inside!
I took mine off yesterday and my O-rings were shot to pieces which is why it's been spitting fuel out at fast idle, and the threads on the part that goes from carb to engine are mullered so I need to find a new one of them, not a clue where to look though!
Cheers
Graham
First of all thanks for that info you sent me it's been a great help!
About yer leak, take the carbs and everything off that side of the engine and check the O-rings inside!
I took mine off yesterday and my O-rings were shot to pieces which is why it's been spitting fuel out at fast idle, and the threads on the part that goes from carb to engine are mullered so I need to find a new one of them, not a clue where to look though!
Cheers
Graham
Hi Graham,
Not quite sure which part you mean, but if you look in some of the classic car mags for ads for carb specialists you should be able to get a replacement.
Seem to remember that dripping fuel was often something of a problem with DCOEs. Making sure the O rings between carbs and manifold (plus any Thackery washers or rubber equivalents) are in good condition seems to cure it.
Not quite sure which part you mean, but if you look in some of the classic car mags for ads for carb specialists you should be able to get a replacement.
Seem to remember that dripping fuel was often something of a problem with DCOEs. Making sure the O rings between carbs and manifold (plus any Thackery washers or rubber equivalents) are in good condition seems to cure it.
D!
yer that's the one, I knew it all along I was just testing to see if you knew it! 
Hi Graham,
Somebody like Burton or Vulcan should be able to provide one. So long since I bought such a thing I'd be guessing at the cost, but I reckon you'd be looking at the best part of £50, maybe more.
Why not simply make good the one you have? From what you say I assume it's the thread in the alloy manifold where the carb mounting stud screws in that's stripped.
This'll either be M8 or 5/16 UNC depending on who made the manifold (and when) and there are two ways of dealing with it.
First is to try and find a local small engineering firm that can helicoil it for you, which basically means tapping out the hole and fitting an steel insert with a suitable size thread for you to screw your existing stud into.
Alternatively you could drill and tap the hole yourself to the next sized thread (that's 3/8UNC or, slightly larger, M10) and then get a special stepped carb mounting stud made up which is 3/8 or M10 at one end and 5/16 or M8 at the other.
Either option is likely to be much cheaper than a new manifold. In fact the stud is the sort of thing turners tend to knock off between jobs, so find a friendly engineering company and you might get it done for the price of a pint.
Somebody like Burton or Vulcan should be able to provide one. So long since I bought such a thing I'd be guessing at the cost, but I reckon you'd be looking at the best part of £50, maybe more.
Why not simply make good the one you have? From what you say I assume it's the thread in the alloy manifold where the carb mounting stud screws in that's stripped.
This'll either be M8 or 5/16 UNC depending on who made the manifold (and when) and there are two ways of dealing with it.
First is to try and find a local small engineering firm that can helicoil it for you, which basically means tapping out the hole and fitting an steel insert with a suitable size thread for you to screw your existing stud into.
Alternatively you could drill and tap the hole yourself to the next sized thread (that's 3/8UNC or, slightly larger, M10) and then get a special stepped carb mounting stud made up which is 3/8 or M10 at one end and 5/16 or M8 at the other.
Either option is likely to be much cheaper than a new manifold. In fact the stud is the sort of thing turners tend to knock off between jobs, so find a friendly engineering company and you might get it done for the price of a pint.
Hi Graham,
Wouldn't go overboard on polishing the ports as it doesn't make any real difference to power. Generally thought that the most important thing is to have a smooth shape to the port with no sharp restrictions rather than having a chrome-like finish.
Other thing to aim for is matching the ports on the head to the inlet and exhaust manifolds so they have the same diameter and line up as perfectly as possible.
Not difficult to do minor work on ports, you just need a selection of grindstones and a decent electric drill. In fact Garryson I think it was used to do a head porting kit. Ask at your friendly local DIY store (not the big chains).
Electric drills are really too slow for this kind of work, but they cope. Think one of those Dremel things runs higher revs which would be better.
Oh - and don't try grindstones on the inlet manifold if you're matching the ports because ally clogs them up. You can use a half-round file for rough work if you can get it into the port ok, with small emery wheels for finer stuff.
Wouldn't go overboard on polishing the ports as it doesn't make any real difference to power. Generally thought that the most important thing is to have a smooth shape to the port with no sharp restrictions rather than having a chrome-like finish.
Other thing to aim for is matching the ports on the head to the inlet and exhaust manifolds so they have the same diameter and line up as perfectly as possible.
Not difficult to do minor work on ports, you just need a selection of grindstones and a decent electric drill. In fact Garryson I think it was used to do a head porting kit. Ask at your friendly local DIY store (not the big chains).
Electric drills are really too slow for this kind of work, but they cope. Think one of those Dremel things runs higher revs which would be better.
Oh - and don't try grindstones on the inlet manifold if you're matching the ports because ally clogs them up. You can use a half-round file for rough work if you can get it into the port ok, with small emery wheels for finer stuff.
A tuner told me recently that totally polished is actually bad .... what you are after is a slightly rough surface, akin to fine sandpaper. Apparently this helps create a tiny bit of turbulance as the fuel/air/gas travels down the ports, which is a good thing (apparently).
As mentioned above, the big thing is to get the rough edges off .... however as I understand it, this is a bit of a black art in itself, and really can't be done without a flow bench, as even small apparently innocuous modifications can ADVERSELY affect power ! (especially on cylinder head mods, rather than manifold mods)
Didn't get time to check my O rings last night, will probably look tonight.
As mentioned above, the big thing is to get the rough edges off .... however as I understand it, this is a bit of a black art in itself, and really can't be done without a flow bench, as even small apparently innocuous modifications can ADVERSELY affect power ! (especially on cylinder head mods, rather than manifold mods)
Didn't get time to check my O rings last night, will probably look tonight.
On ther subject of polishing etc, I cribbed loads of info from this very well written article. He reckons he doesn't do much on x-flows, but he certainly knows his onions.
www.pumaracing.co.uk/cflow.htm
On the subject of your leak. If it isn't your O rings, I have heard that the float chambers in both 40's and 45's can overflow under certain circumstances. I have fitted a pro-fuel regulator (and in-line filter) to combat it.
Cheers and good luck
>> Edited by DavidP on Tuesday 15th October 11:33
>> Edited by DavidP on Tuesday 15th October 11:35
www.pumaracing.co.uk/cflow.htm
On the subject of your leak. If it isn't your O rings, I have heard that the float chambers in both 40's and 45's can overflow under certain circumstances. I have fitted a pro-fuel regulator (and in-line filter) to combat it.
Cheers and good luck
>> Edited by DavidP on Tuesday 15th October 11:33
>> Edited by DavidP on Tuesday 15th October 11:35
Cheers everyone,
I was browsing the Demon Tweeks Catalogue for some stuff and found that they do inlet manifolds for a X-flow, which is good as I'm ordering plenty of stuff from them and now I can get that on one load of P&P!
Great Stuff!
As for polishing the ports, I think I'll leave it if it's more trouble than it's worth, the new one should be fine but I'll see when I get round to buying all of the stuff!
Oh and nev don't buy those mirrors from europa spares as they upped the price on mine, I've cancelled the order and am going to get them from Demon Tweeks, much cheaper!
Cheers
Graham
I was browsing the Demon Tweeks Catalogue for some stuff and found that they do inlet manifolds for a X-flow, which is good as I'm ordering plenty of stuff from them and now I can get that on one load of P&P!
Great Stuff!
As for polishing the ports, I think I'll leave it if it's more trouble than it's worth, the new one should be fine but I'll see when I get round to buying all of the stuff!
Oh and nev don't buy those mirrors from europa spares as they upped the price on mine, I've cancelled the order and am going to get them from Demon Tweeks, much cheaper!
Cheers
Graham
Graham - mirrors, the cheeky things ! Thanks for the tip, coz I was going to order them too ...
Well, I whipped of my problem carb last night ... my O rings are perfect. BTW, how do you know how hard to tighten the carbs back up again ? (I dont want to squash the rubber)
So .... what else could it be I wonder ?
Well, I whipped of my problem carb last night ... my O rings are perfect. BTW, how do you know how hard to tighten the carbs back up again ? (I dont want to squash the rubber)
So .... what else could it be I wonder ?
Hi Nev,
From memory there seemed to be two different types of rubber vibration mounts used to replace the Thackery spring washers (which you could set with feeler guages).
Some were basically just tubes which fitted between cup washers and did get squashed. Seem to think I tightened them until the cup washers were about 2-3mm apart and that seem to work OK.
Think there are also some that are a sort of twin cone design. Unless someone can say better (and if you can please do!)think these should also be tightened up until cup washers about 2mm apart.
Basically you just need enough play to allow a midges movement.
Regarding the fuel thing, might be worth checking state of the needle valves to make sure they're not worn.
From memory there seemed to be two different types of rubber vibration mounts used to replace the Thackery spring washers (which you could set with feeler guages).
Some were basically just tubes which fitted between cup washers and did get squashed. Seem to think I tightened them until the cup washers were about 2-3mm apart and that seem to work OK.
Think there are also some that are a sort of twin cone design. Unless someone can say better (and if you can please do!)think these should also be tightened up until cup washers about 2mm apart.
Basically you just need enough play to allow a midges movement.
Regarding the fuel thing, might be worth checking state of the needle valves to make sure they're not worn.
Basically throw away the 'o' ring plates and replace with 'misab'ones.These are available at Burtons.Also get the rubber cone type kit that replaces the troublesome spring washers to hold the carbs on the mainfold.Then when you put them on,it is easy to slip in a feeler gauge between the end washers to work out if the carb is on straight or not.Do not over tighten the spring/rubber mounting nuts,as this will cause fuel to foam in the float chamber causing rough running and fuel flooding.This usually causes the distributor to be washed clean regulary.It might also cause under heavy breaking with a foaming float chamber,to spill out on the front carb also
Wayhey,
Em sorry about that, just read the thread and found all of the advice SPOT ON.
However I would also like to add that you can get fuel leakage from the "fuel enrichment device" (aka the manual choke mechanism) at the very back of the carb.
If like me you have a mid engined car, you will most likely have some component of the exhaust system under this fuel leak.
This is not good.
The speedpro book about webber carbs gives a fix for this in that you make up a blanking plate from 5mm alloy to match the shape of the fuel enrichment device and bolt it in place with some 0.5mm gaskit paper to provide a fix.
Of course if you actually use the choke then you are up the swanee 'cos there is NO repair for this if you do.
On my carb I also had to use instant gaskit on the threads of the screws that hold the "fuel enrichment device" 'cos thats where the fuel was partially coming from too.
Yours etc
MoJo.
Em sorry about that, just read the thread and found all of the advice SPOT ON.
However I would also like to add that you can get fuel leakage from the "fuel enrichment device" (aka the manual choke mechanism) at the very back of the carb.
If like me you have a mid engined car, you will most likely have some component of the exhaust system under this fuel leak.
This is not good.
The speedpro book about webber carbs gives a fix for this in that you make up a blanking plate from 5mm alloy to match the shape of the fuel enrichment device and bolt it in place with some 0.5mm gaskit paper to provide a fix.
Of course if you actually use the choke then you are up the swanee 'cos there is NO repair for this if you do.
On my carb I also had to use instant gaskit on the threads of the screws that hold the "fuel enrichment device" 'cos thats where the fuel was partially coming from too.
Yours etc
MoJo.
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