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Running a bit too rich


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Last year, Clive Manvers sorted out the fuel system on my 1975, 6.

 

Primarily, he fitted a new PRV, a 150 bhp metering unit and refurbished venturi, with underslung throttle linkage.

 

It all works well and she pulls like a train, but, continues to run rich.

I know she needs to be on the rich side, but all 6 plugs are running black.

 

How do I weaken the mixture slightly?

The air valve is fully closed

Tick over (hot, after a 15 mile run) is 1000 rpm, give or take a needles width)

 

I would be happy if I can get tick over down to 850 rpm

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<The air valve is fully closed>

You have an air leak! So to start with air is getting through somewhere. You need to sort that first.

Make sure the throttle butterflies are not being held open. With the air valve screwed right in the engine should stall.

Check the choke lever at the FMU is fully returning.

Blank off the hose to the servo to discount a leak there and try again.

Next check the FMU diaphragm for leakage... the tongue test.

Check for leaks at the manifold to head joint, gasket failure? loose mount nuts?

Er?? is that the correct FMU for your engine or is the rest of it, cam, head etc 150 brake spec???

One other thing for rich running is the return line to the tank is not blocked or restricted.

Edited by daven
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<The air valve is fully closed>

You have an air leak! So to start with air is getting through somewhere. You need to sort that first.

Make sure the throttle butterflies are not being held open. With the air valve screwed right in the engine should stall.

Check the choke lever at the FMU is fully returning.

Blank off the hose to the servo to discount a leak there and try again.

Next check the FMU diaphragm for leakage... the tongue test.

Check for leaks at the manifold to head joint, gasket failure? loose mount nuts?

Er?? is that the correct FMU for your engine or is the rest of it, cam, head etc 150 brake spec???

One other thing for rich running is the return line to the tank is not blocked or restricted.

Dave

Hi

Air leak

This was my 1st port of call, but everything is tight

I have complete confidence in Clive's team, so have made no attempt to interfere with him work on the butterflies (it took his engineer 8 hours to get them right)

His team left the air valve closed. Why?

The inside of the venturi were blasted clean and getting the butterflies to seal perfectly was proving difficult. After 2 days, I asked Clive to leave it as best he could as the cost was rising faster than I liked.

 

The butterflies were as good as could be, but she was ticking over warm (Sept 2015) at 1500 rpm, Clive promised the tickover speed would fall.

Now, she ticks at 1000 rpm and has done for about 2 or 3 months

 

If I had an air leak, would I have black Plugs?

Black plugs are an indicator of too much fuel, not too much air Surely?

 

Mixture lever on the MU returns fully against the stop

 

The MU was new, fitted in Sept 2015, and has done less than 2500 miles

Is diaphragm damage Likely?

 

As you suggest, I will do the tongue test and check the returning fuel over the weekend, just to be sure

Edited by wjgco
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I have the same problem with all the plugs sooting up .I put it in to a workshop and it came back that two injectors were found with rubber particles holding them open.I suspect that the MU diaphragms have started to come apart so probably need to replace the MU as it seems they don't like the unleaded fuel and more than likely the amount of ethanol in the fuel.If you can find out your problem then I will be pleased if you would let me know.I will try a new set of plugs this weekend and using a colder plug to see if it sorts things out so will reply of my findings.I am getting less than 200 miles from a set of plugs!!!

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As daven says, the car should stall with the air bleed shut.

 

The plugs being black is indicating the car is running rich yes, ie too much fuel is being injected.

 

The pi system injects fuel in inverse proportion to engine vacuum.

 

Ie, throttles shut = max vacuum = little fuel needed

 

Throttles open = little vacuum = max fuel.

 

Black plus is nothing to do with the air bleed valve, unless youve left the engine idling for hours.

 

So.... sort out the air leak so the engine stalls with the air bleed valve closed.

This will allow you to set the idle.

 

Steve

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I have the same problem with all the plugs sooting up .I put it in to a workshop and it came back that two injectors were found with rubber particles holding them open.I suspect that the MU diaphragms have started to come apart so probably need to replace the MU as it seems they don't like the unleaded fuel and more than likely the amount of ethanol in the fuel.If you can find out your problem then I will be pleased if you would let me know.I will try a new set of plugs this weekend and using a colder plug to see if it sorts things out so will reply of my findings.I am getting less than 200 miles from a set of plugs!!!

Which injectors had rubber in them?

 

The bet is 2 and 5, which have banjo fittings on the MU and their o-rings fail caising your symptoms!

 

Also, running colder plugs will make fouling worse not better.

 

Steve

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Hi eric

 

No need to replace the MU, just have it refurbished.

 

Neil Fergussen is the man for this, he also does refurbed injectors all at very reasonable prices.

 

Mu is a little fiddly to extract/fit but basically easy

 

Steve

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What I was getting at is you need to sort the high idle and air leak then get the mixture sorted.

I'm guessing here the timing and valve clearances are spot on?

No need for the o/rings to fail if they are well lubed on assembly.

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