Fluctuating Vacuum

[jojim65]

Hi,

 

Looking for some help here please,

 

My PI TR6 which was always smelly at tick over was noticeably worse on a 50 mile run last Sunday.

 

Today I had an opportunity to remove the plugs which were sooted up and I have changed. Valve clearances checked and a couple tweaked back to 10 thou but they weren't far out. Fuel pressure is a steady 102 psi.

 

But, with a vacuum gauge fitted the needle is fluctuating between 8 and 12 Inches at a warm idle which would appear wrong.

 

I'm assuming I have a vacuum leak and hope it's throttle body related rather than sticking valves but would like your advice and observations please.

 

I have attached links to a short video to try and illustrate the problem.

 

Thanks

 

Jim

 

https://youtu.be/LDJ2x_OfTlw

 

https://youtu.be/TRCQjP8r0MU

[John L]

I don't think you have a problem there, That gauge needs a snubber, you need to use a pair of plyers and slowly close the tube, until the fluctuations stop on the gauge, but not so much that no vacuum is shown.

Where exactly did you plug into the manifold?

Do a suck test on the line to the MU and see if it holds a vacuum.

John

[jojim65]

Vacuum gauge was connected into the servo pipe, have to say it never occurred to me to damp the gauge which I shall do tomorrow.

 

I also try a suck test.

 

Thanks

 

Jim

[poolboy]

• Do any of these scenario examples look like what you are seeing. ?

http://www.secondchancegarage.com/public/186.cfm

[jojim65]

 

Hi,

 

'Suck' test held vacuum without a problem although the petrol taste wasn't very nice.

 

I've clamped up the vacuum tube quite tightly and I hope you can now see the video.

Vacuum is now a steady 10 and a smidge at 800rpm idle.

 

Any observations would be most welcome.

 

Thanks

 

Jim

 

[John L]

Don't really see a problem now.

Can you put the brake vacuum line back on and tee in your vacuum gauge in the MU line and see if you get the same vacuum as in your last test, that should then isolate the servo. This will make sure you are getting the required vacuum at the MU.

Are you still getting your sooty plugs?

What you could do now is to take off the black cap on the metering unit, there are 3 ring nuts, if you could loosen the bottom one (the largest one), mark the position and turn clockwise all the above rings as a unit, this will weaken the mixture over the whole range, try 1/4 of a turn, and try on a run and see what colour the plugs are, may need another 1/4 turn.

If no improvement go back to the original setting.

Is the choke cable on the metering unit stopping the lever from returning to the fully rearward position, push it back with your finger to confirm.

John

Edited July 25, 2015 by John L

[jojim65]

An update....

 

Vacuum tee'd into MU and it's reading just over 9ins, I took off the servo pipe and sealed it, no difference so I'm assuming the servo is fine, just my clamping of the vacuum gauge pipe was a little less enthusiastic so a slightly lower reading than before but still respectable?

 

Plugs replaced and the car hasn't been out of the garage so little open throttle and none under load, but they do show signs of blackening up. I can't make a judgement on them till the car has been for a run.

 

Choke cable is fine and not fouling, timing checked and at 11 degrees.

 

regarding adjusting the concentric rings I think it's anti-clock to lean the mixture.

 

I do know my throttle bodies need some work, although not long sine they were refurbed by Prestige they have been played about with by a third party and need to be reset....I've just bought a jig so hopefully I can do that off the car.

 

It has been suggested that the return pipe from the MU to the tank could be blocked....not sure how to measure that though.

 

And so it goes on...

 

Jim