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MG Midget and Sprite Technical - Stumbling Dellortos (NMC)

Forgive me fellows. Some of you may know that I have a Frogeye, but right, now I'm strugging with my Elan +2 and I know that there are some knowledgeable fellows on here who may be able to give me a few pointers.

Car has twin Dellorto DHLA45s and has always been a bit stumbly below about 3000rpm. i.e. you floor the throttle and the car will kangaroo for a a second or two before it picks and and pulls hard.

Last week after 5 or so miles of driving, I floored the throttle and it was as if I'd turned the ignition off. Backing off the throttle and I was able to proceed but had to be very delicate on the throttle to nurse the car home.

So here's what I have done: blown out all the jets and emulsion tubes, checked the fuel filters, float levels, and fuel supply. Fitted a Powerspark electronic ignition, new coil, cap & rotor arm. These ignition changes have helped but I still have this serious stumbling issue when the engine is warm and I've done a few miles.

For what it's worth, when it goes in to "stumble mode", I get a quite a bit of exhaust popping on the overrun - weak mixture perhaps?

Bear in mind that up until last week, I didn't have this problem, so something has changed, got blocked-up or broken.

Apologies for the long post, but if anyone has any ideas and recognises these symptoms, I'd be grateful for their input.

Simon
Simon Wood

Sounds like an air leak on inlet manifold.

Rob
Rob aka MG Moneypit

Weak fuel pump - fuel may be delivered but perhaps not enough.
Blocked fuel filter - as above.
Blocked jets - Blowing through may not have removed hard debris - rod them through carefully with a piece of copper wire.
Fouled spark plugs
Yes - could be an air leak somewhere weakening the mix - does the engine run hot?




Oggers

Rob and Oggers,

Thanks very much for your replies. I don't think it's manifold. Lotus twin-cams don't have a manifold as such, each inlet port is cast in to the head directly and each port is fed by a single barrel from the carbs, in isolation to the others. What I'm experiencing doesn't feel like a misfire, more of a total power loss. I don't think that it's likely that all 4 ports have an air leak, but I see where you are coming from.

Each carb has a filter in it's inlet. These are clear.

I did rod each of the jets out with fuse wire - not totally conlusive but they all seemed to be clear blowing through either end.

Plugs are brownish - trending to white electrodes.

Engine not running hot - all seems normal in this respect.

I too suspect fuel supply, but how to test? Fuel pump is mechanical, not unlike original frogeye. Cranking the engine yields what looks like a plentiful amount of fuel. Fuel pump has a glass bowl at the top which is full up all of the time. There don't seem to be any leaks. Furthermore, if I cane the car to the point where the stumbling occurs and then back off - stop the car for for half an hour it doesn't seem to recover until it has cooled down. There must be a clue there....

I'm a bit stumped and frustrated....
Simon
Simon Wood

Put some dry gas in it...sounds like water contamination.
S

Water in fuel? That's interesting, and I did fill the car up from my usual source just before the problem appeared. Forgive my curiosity but why would the problem only appear when the car is hot and on full throttle? Wouldn't if be there all the time?

Just trying to understand the mechanism. Think I will dismantle the fuel pump tomorrow
Simon
Simon Wood

I'm not familiar with Dellortos,but I wonder if they have an accelerator pump jet? If so, it will be controlled by a lever and a diaphragm, which can become porous, giving the symptoms you describe.
Bernie Higginson

I'm with Steve -- Fuel
Maybe might not be water but stale fuel , which is hard to diagnose
Check by spilling a bit of your fuel onto your workshop floor
Good petrol will evaporate off after a couple of minuites ----- If it's gone off it will take ages to evaporate and leaves a cement floor slippery to feel

willy
William Revit

Another source of a vacuum leak could be where the manifolds mate with the cylinder head. Check the nuts that hold the manifolds as well as the carburetor to the manifold.

Had a similar issue with my 79 MGB and that was the cure of my vacuum leak problem.

The old 10/10 repair. 10 hours of diagnostics and 10 minutes to fix it.

Cheers

Gary
79 MGB
gary hansen

More suggestions

Vacuum advance blocked or disconneced
LT connections at coil come adrift/poor
Loose HT/plug leads
Plug gap set incorrectly
Incorrect plugs even

It still sounds like it is running a bit weak to me - plug colour and the stumbling. Enrich it and see what happens.

Oggers

If water is more dense than fuel then could water be pooling in the float chamber and only be sucked up at high power settings? Fuel pipe off and run it into a milk jug to see how clear it is? I had the same problem with a lawnmower this year and a similar issue with a homebuilt aeroplane some years past. At low power it just settles the droplets to the bottom of the chamber and filter.
r thomas

Float level?
The delorto on my midget stumbles a bit when cornering hard right slightly down hill.
These carbs are verry picky about the float level so that could be it
O K

Because no o eme tioned it yet

check that the fuel cap is venting properly... take the cap off andshove a rag in and drive it for a bit and see what happens

id also have a look at the spark plugs and moisture inside the dissy cap plus make sure you got good wire connetions

prop
Prop and the Blackhole Midget

Thank you all for your replies - I appreciate your thoughts.
I don't think it's stale fuel. The car is regularly used and refuelled. Lotus twin-cams have no manifold as such. Each barrel of the carburettor has it's own MISAB o-ring type mounting direcrly to the cylinder head which has the inlet ports directly cast into the head. If it was a misfire and one of these was leaking then yes that would be a likely cause but I don't buy the fact that all 4 could be leaking.
No vacuum advance on +2.
LT connections all good, new electronic ignition, coil, cap, rotor. Newish plugs (NGKBP6 - as used by all twin-cam owners - again can't be all 4 of them) Plugs brownish with whiteish electrodes. I have richened the mixture slightly but this doesn't seem to have made any difference.
The "stumble" isn't a misfire, it's like a complete loss of power which only returns when I lift OFF the throttle, then I have to very gently use the throttle to progress.
So yesterday I dismantled the carbs: accelerator pumps all working OK, diaphrams sound, all jets blown and rodded out. Dismantled fuel pump, bit of crud in the filter, cleaned all that out. Ran out of time so haven't had time to test things out. Will try and get out tonight.

I quite like Prop's tank venting suggestion. If it starts to stumble, I'll undo the fuel cap and see if there's an obvious inrush of air to the tank.

There's got to be a clue in the fact that the car will run for several miles before I hit this issue, so it could be either heat or (I still like Prop's suggestion) some sort of fuel starvation issue.

I did wonder whether I could rig my strobe timing light up and have it inside the car to see whether I'm still getting spark energy in the plug leads at the critical time, i.e. under full power - would that work as a credible test?
Simon
Simon Wood

Simon the usual way of detecting loss of sparks is observing if the rev counter flutters or momentarily drops back to zero.
David Smith

Simon,
similar symptoms to my Midget, turned out to be a faulty (newish) rotor arm. You've replaced yours but is at similar to the Midget and if so have you used a genuine red one?
J Tickle

David - no rev counter flutters. Rev counter has Spida electronic replacement module so it's not like the old impulse ones fitted to midgets/sprites. Also a flutter would only show low tension failure and not high tension wouldn't it?

Jeremy - yes red rotor arm now fitted in place of original (no rivets).
Simon Wood

Can you borrow and fit a fuel pressure gauge?
David Smith

OK an update - just been out and done 10 miles or so in the car - it's much better. Whatever I did yesterday has had a significant effect, just a couple of stumbles towards the end of my run but I didn't have to nurse the throttle at all.

Prop - at the end of the run I opened the fuel filler - there was no sharp intake of air from the tank so I guess it's venting correctly.

David - I don't have access to a fuel pressure gauge, but what little I do know about Dellortos is that they are very sensitive to fuel pressure - if it's too high then the needle valves don't shut off and a whole raft of other issues ensue.

I suspect that it could be a lack of fuel volume rather than pressure. After my dismantling exercise yesterday, the carbs were completely empty and it took a lot of engine cranking - perhaps 1 minute - (whilst the float chambers were filling I would guess) before the engine would fire. I will source a pump rebuild kit as they are cheap and readily available.

Thanks again for everyone's input.

Simon
Simon Wood

Re Prop's suggestion: You don't always hear that intake of air on releasing the cap. If it stumbles again the best way of checking the filler cap isn't causing a vacuum in the tank is either drive it with the filler cap off, or find a plastic cap of some sort (rattle can type sometimes fit) drill a couple f holes in it to ensure it "breathes" and use that for a while.
GuyW

Guy - thanks for that suggestion. Next time I go out, I will go "capless". But the weather is too nice to not be driving the Frog right now....
Simon Wood

To use a rattle can cap.. wrap some duck tape around the filler untill the cap fits snug
Prop and the Blackhole Midget

Simon
What material are your fuel lines from the pump to the carbs
If your Elan is like mine it will have a solid plastic line from the tank to the pump then a metal pipe from the pump up to a Y piece and then 'rubber' lines to each carb
It could be that the fuel has attacked the soft 'rubber' and it has swollen inside restricting fuel supply--------------like swollen brake hoses do to brake fluid supply

If your 'rubber'fuel lines are the originals the material isn't a good mix with unleaded fuel and they need replacing anyway--AND have a good look at the 'Y' piece, as it's a well documented cause of under bonnet fires with Elans

Also on Dellorto's there is a filter inside the banjo fitting where the fuel line bolts to the carb, They need to be very clean as they are super fine and restrict flow easily if dirty at all--

willy
William Revit

Willy,
Thanks for that.
Yes I replaced the fuel line between pump and carbs a year or so ago with "modern-fuel-compatible" fuel hose all British Standard kite-marked.
There was no feeling that there was any sort of restriction when blowing through this pipe.
I have no 'Y' piece as the front carb banjo has 2 push-on connectors, one from the pump, one to the rear carb.
I did clean out the banjo filters - they were spotless in any case.

Simon
Simon Wood

OK, I got there in the end. It was as simple a problem as a blocked tank-pump fuel line. The intermittant way in which it presented itself made diagnosis rather confusing. Anyway, for my efforts, I now have a car which starts and runs far better than it did before, largely, I suspect, due to the fitment of electronic ignition and the other new ignition bits.

Thanks for all your contributions
Simon
Simon Wood

Simon
That reminds me---
On my elan- an s4,----but I think plus 2s are the same
Where the fuel line comes from the tank and through the floor, it passes over the top or the diff and then heads to the right hand side of the car's chassis - Just in front of the diff ,before it runs forward along the centre section of the chassis section there is a short 'rubber' joiner connecting the two sections of plastic fuel line. If it hasn't been replaced ,it needs to be. Mine got bad and was sucking air for ,I don't know how long. I didn't even know it was there until I went off hunting for a fuel smell
Sorry,-- I forgot all about it
willy
William Revit

This thread was discussed between 29/10/2016 and 07/11/2016

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