Car fails on full throttle

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politeperson
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#1 Car fails on full throttle

Post by politeperson » Thu Apr 08, 2021 6:03 pm

Evening Gents,

I was testing a 4.2 on Sunday and it kept packing up!

Looking for opinions prior to digging in at the weekend.

I starts and drives well, smooth and powerful. Sounds perfect.

However when large throttle openings are used, it goes onto 5 cylinders, then 2, then stops.

So you pull over, turn the ignition back on, then it starts normally and goes for a mile or two then all this repeats.

I put on a fuel pressure gauge today. 4psi on with the engine stationary, ignition on. 3.5 psi at idle. However, take the revs up and psi drops to nothing.

So I assume it is possible to have an SU fuel pump that makes pressure but doesn't provide enough volume?
James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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mgcjag
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#2 Re: Car fails on full throttle

Post by mgcjag » Thu Apr 08, 2021 6:42 pm

Hi James...fuel pressure will drop becaus when the floats drop you open circuit so drop the pressure....what you need to do is measure the flow rate that the pump is delivering..cant remember it but just look it upon Burlan site....just disconnect pipe from filter to carb...switch pump on and time how long it takes to pump known quantity..Steve ....edit think its pint every 15-20sec.....could just be a blocked fuel filter
Steve
69 S2 2+2 (just sold) & Building a C type replica

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#3 Re: Car fails on full throttle

Post by steve3.8 » Thu Apr 08, 2021 7:59 pm

My guess is a badly dented / creased fuel line would give those symptoms , cannot think of much else if the pump is ok and you have 4psi in the line initially.Where did you place the gauge ? between the filter and carbs?
Steve3.8

64 3.8 fhc, 67 4.2 fhc

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#4 Re: Car fails on full throttle

Post by politeperson » Thu Apr 08, 2021 8:08 pm

I put the gauge between the filter and the carbs. The filter is new, the tank is new, the carbs are too!

Might take the filter out of the bowl.

The line looks OK and is new. Nothing I can see. Might have to blow down it see if their is resistance.

I get 4psi then, with a few revs it drops right down to zilch, which would explain why the engine is starving I suppose.
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James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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Simonpfhc
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#5 Re: Car fails on full throttle

Post by Simonpfhc » Fri Apr 09, 2021 6:44 am

You could eliminate all those fuel line items by just using a temporary gravity fed system using a suitable container above carb height.

Simon.
Simon
62 3.8 FHC
52 XK120 OTS
91 Porsche 928GT

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Tom W
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#6 Re: Car fails on full throttle

Post by Tom W » Fri Apr 09, 2021 7:13 am

A few things come to mind in addition to what’s already been suggested.

Are the floats able to drop fully?
Are the float levers able to drop fully?
Are the needle valves the correct size?

Worth checking the above as it’s straightforward. Although if you’re seeing zero pressure on the gauge with the car on the drive and the engine at wide open throttle, it suggests a fuel supply problem more than a restriction to the float bowls filling.

Beyond that, I’d be looking for a wiring fault that caused the fuel pump to cut out under full throttle. Maybe the wiring to the fuel pump is being moved when you use full throttle? Driver’s footwell would be the only place the two come close I would think.
Tom
1970 S2 FHC

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#7 Re: Car fails on full throttle

Post by mgcjag » Fri Apr 09, 2021 7:43 am

Why would you not expect the fuel pressure to drop?.....if youve tee,d the guage between filter and carb then with pump on you will see pressure.....but as soon as you open the end of the pipe the pressure will drop....which is whats happening when the needle valves are open almost continually on full throttle.....you should however still have plenty of flow from the pump to supply fuel....so long as the flow rate is ok.....Steve
Steve
69 S2 2+2 (just sold) & Building a C type replica

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Series1 Stu
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#8 Re: Car fails on full throttle

Post by Series1 Stu » Fri Apr 09, 2021 8:45 am

:yeahthat:

Also, as Steve has already suggested, check for fuel flow, not pressure. I think the spec for the pump is 8 gallons per hour or some such - its in the manual.

Regards
Stuart

If you can't make it work, make it complicated!

'62 FHC - Work In (slow) Progress
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#9 Re: Car fails on full throttle

Post by christopher storey » Fri Apr 09, 2021 8:54 am

You need at least 15 gallons per hour i.e. 4 pints in 2 minutes ( and preferably rather more than that up to 20 gph). The symptoms are precisely those of inadequate fuel flow, not pressure although of course as the flow is diminished, the pressure will drop as well. As has already been suggested, either a kinked line or a blocked filter could be the cause, but an inadequate pump would also eventually give the symptoms

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#10 Re: Car fails on full throttle

Post by Tom W » Fri Apr 09, 2021 10:03 am

I assume the car is on SUs, and the float levels are set correctly.
Tom
1970 S2 FHC

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#11 Re: Car fails on full throttle

Post by politeperson » Fri Apr 09, 2021 5:56 pm

Hello Boys,
Very kind responses as always, thank-you for your time.
Yes, triple SUs.
One call yesterday at 4.15pm and look what I have now.
Do you like my diorama?
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Superb service from Julian Barrat as always.
I shall fit it tomorrow and report on the findings.
Might even do a silly video!
James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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madjack4
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#12 Re: Car fails on full throttle

Post by madjack4 » Fri Apr 09, 2021 10:02 pm

Hi had same problem a year or so ago it was faulty fuel filter.Filter was only about 6 weeks old ,by all accounts there have been a batch of dodgy filters about .My supplier had a car they had just done a 90k restro on that broke down after a few days with customer not good

Regards

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#13 Re: Car fails on full throttle

Post by politeperson » Sat Apr 10, 2021 5:10 pm

Well now its all sorted.

I swapped pumps which improved fuel delivery. The car would now maintain 1psi when the motor was working, however the fuel volume from the filter seemed poor for a 4.2l car.
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So I unplugged the line to the body mount in the rear direct from the pump and was met by a positive torrent in comparison to that reaching the front.

I bit the bullet and severed the solid line in front of the irs, removing it completely to the filter, seperating the 5/8 union near the sump.

Off the car it was apparent that although it looked in good condition from the out side, it was partially clogged, causing the issue, along with a slightly weak pump.

A call to my friend elicited a length of 8mm copper line this afternoon, which i rejoined to the remains of the original line with clips and braided fuel line.

I was not going to remove the IRS to change the line completely, That can wait!

As a bonus, whilst I was at my friends garage, I fixed a knock from the rear of my Porsche which has been bugging me for the last 8 months. My dealer had failed!
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Turned out to be a slightly bent heat shield that needed to be pulled back with my finger! Sorted.

Anyway, refitting the solid line was challenging in one piece, however once struck up, the engine sounded much happier and was operating at 3.5 psi fuel pressure, even when working.
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So all done.
James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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