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PostPosted: Tue Sep 14, 2021 6:31 pm 
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Joined: Tue Oct 15, 2013 6:28 pm
Posts: 1423
Location: East Fife.
Car Models: SAAB 96/MGB/Classic Twingo/Niss Figaro
heli-saaber wrote:
Andy - a few comments, if they might help. You said that you'd fitted a new mechanical fuel pump a while ago - it could be that it was "old stock" and the diaphragm is weak/perforated - your proposed flow test should bowl that out. As it seems a lot better with the electric pump in series, it could well be that your engine-mounted pump is not 100% effective. I've had occasions in the past when I've bought a "new" pump and found the rubber parts to be damaged/perished. I think I was able to buy a new diaphragm kit for it (but can't remember where - it was a very long time ago!). I'm not convinced that lagging the pipe from the pump to the carb will sort out the apparent fuel starvation/evaporation problem but it might help. Carbs and fuel pipes on V-engines do tend to get hot - might be worth looking at a few detailed V6 and V8 diagrams to see how other manufacturers cope with it. I assume that you've got the non-metallic heat-isolation "gasket" between the manifold and the carb. You seem to have checked everything else thoroughly that needs to be checked - it certainly is a bit of a mystery! Good luck with sorting it out - see you in Sweden next year!


David
Yes, I have the heat insulation plate fitted, the order from the bottom up is, - gasket - vacuum pick up plate for servo - gasket - insulation plate - gasket - and then the carb.
Lagging the fuel pipe had no effect, I was "grasping at straws"........


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PostPosted: Tue Sep 14, 2021 6:37 pm 
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Location: East Fife.
Car Models: SAAB 96/MGB/Classic Twingo/Niss Figaro
Doive wrote:
I'm not sure the electric pump, pumping into the mechanical pump, which is itself pumping, is going to give the best performance. I'd be inclined to bypass the mechanical pump completely to eliminate that. Also worth checking your float is free to move, the levels are all correct, and the needle valve doesn't have any crap in it or sticking. Had it before where sediment in the tank was getting caught by the inline filters, but the fine silt deposits were making it all the way through into the carb and causing issues. In our V4 I have a mesh type inline filter at the tank pickup, a paper element inline filter before the electric pump at the front (sucking type), and a Filter King bowl type filter after the pump to trap any leftover sediment that manages to get past both inline filters. Perhaps slightly over the top, but never had a problem with fuel starvation since. Another consideration is carb heating as has been mentioned - we are running a Weber 34ICH, and the previous owner of our car had a thick wooden spacer block turned up to sit the carb on, and extended studs. I've tried it with and without, and the spacer block definitely makes a difference to hot running. One last job I want to do is to lag the exhausts with heat wrap, to try and keep under bonnet temps down in traffic.

Dave
I did, at one point, in the proceedings, have the electric fuel pump only. The mechanical fuel removed and blanked off. The issues were still there.
Now maybe the MGB pump is not the best pump to use, but I had it one the shelf, for my MGB, new in the box, so seemed daft not to use it.
If it can feed twin 1 1/4" SUs, I'm sure it can feed a twin choke Weber.
On an MGB it sits beside the tank, and pumps to the engine compartment, so that's why I put it under the back seat.

I've had the carb top off, almost no sediment in the bowl.
Float is at 41 mm, as well as I can measure it.
Needle valve seemed fine, though I didn't strip it.

This slightly odd ball set up of the electric pump feeding the mechanical pump seems to work fine, though the suction side of the mechanical pump might not like it, and I suppose it could cause other problems down the line.
I'm sure it's not designed to have 3 or 4 psi of pressure on it.

Andy


Last edited by The Troll on Tue Sep 14, 2021 6:48 pm, edited 4 times in total.

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PostPosted: Tue Sep 14, 2021 6:43 pm 
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Location: East Fife.
Car Models: SAAB 96/MGB/Classic Twingo/Niss Figaro
Steve Briance wrote:
Don't rule out the new pump... My 99 had a new aftermarket pump for a few months but, it was rubbish. If the car stood for a couple if weeks, the pump suction wasn't enough to pull fuel through to the carb. I got a priming bulbs inline with it to help. It did but then a new ac delco pump went on and it starts on the button every time. I can't help wonder if your issues are related.. Maybe e10 fuel has played its part.

Why not fit an inline pressure gauge next to inlet on carb
That way you can see what is happening and what difference your plumbing changes make. Get a fluid filled gauge or it will be difficult to read with needle bouncing around.


Steve
This is where I now am. I'm thinking this relatively new pump is at fault.

I'm wondering if on it's own, it's not sucking hard enough at certain times, (like when going up hill), but the electric pump feeding the mechanical pump gives it the boost it needs.

One things is clear, as soon as I re-connected the electric pump, to feed the mechanical pump, at Washington Services, the problem immediately vanished.
Not a single hesitation from there to Buxton, Stratford, and back to Fife, or running round the county today.

It's nothing to do with E10 fuel.
I've been having this issue off and on for years. It used to be perhaps once a year, twice at the most, what's new now is that it's happened many times in the last 9 or 10 days.

I actually managed to avoid using any E10 fuel, on the trip to Englandshire this weekend


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PostPosted: Tue Sep 14, 2021 8:48 pm 
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Location: East Fife.
Car Models: SAAB 96/MGB/Classic Twingo/Niss Figaro
Doive wrote:
This is the fuel pump I have fitted to four of our fleet thus far, with a fifth planned once a carb rebuild kit arrives for the Allegro. Very reliable and not had a problem with any of them.

https://www.dellorto.co.uk/shop/car-acc ... mp-133010/


Doive
Does this pump chatter away all the time, or, like the SU type pump, does it only pump, when the pressure drops, ie it's silent most of the time, just stroking now again to maintain pressure?


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PostPosted: Wed Sep 15, 2021 7:54 am 
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Location: Glenrothes, Fife
Car Models: 95, 99, 900, 9000, 93, Sonett
The Troll wrote:
Doive wrote:
This is the fuel pump I have fitted to four of our fleet thus far, with a fifth planned once a carb rebuild kit arrives for the Allegro. Very reliable and not had a problem with any of them.

https://www.dellorto.co.uk/shop/car-acc ... mp-133010/


Doive
Does this pump chatter away all the time, or, like the SU type pump, does it only pump, when the pressure drops, ie it's silent most of the time, just stroking now again to maintain pressure?

To start off with, it clicks rapidly to source and provide fuel to the carb. Once it's built pressure, it only clicks occasionally. Having said that, it's relatively quiet and can only be heard if you're listening to it above the engine. Good reliable pump. The version I have linked above is designed to be fitted within the engine bay (sucker pump), and there is another (133000) that is meant to be fitted next to the tank (pusher type).

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PostPosted: Wed Sep 15, 2021 12:27 pm 
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Joined: Tue Oct 15, 2013 6:28 pm
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Location: East Fife.
Car Models: SAAB 96/MGB/Classic Twingo/Niss Figaro
Thanks Doive. I think if I go down this road, I'd go for the one by the tank. There's enough clutter already under the bonnet.
I'm still thinking of first of all replacing the mechanical pump.
I can get a Quinton Hazel branded one (good brand?) for 30 quid from a motor factor in Penicuik.

Andy


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