Vacuum hose to PI metering unit.

[mleadbeater]

As per my recent posts on the performance on my car’s PI system, which is performing reasonably well, but still has a bogging issue on snap throttle opening, I have been pondering, .. a common activity for me, especially on cold winter evenings. 

One thing I wondered was the influence of the size of the vacuum hose from the manifold to metering unit.

My reasoning , based on a “thought experiment “, is that a very large pipe would act as a vacuum reservoir, and would slug the unit’s response, and , on the other extreme, a small bore pipe would give a more rapid response , possibly helping to reduce the bogging issue.

Before I raid my “hose box” and start trying different sizes, has anyone any experience of this issue?

Cheers

Mike

[BlueTR3A-5EKT]

53 minutes ago, mleadbeater said:

As per my recent posts on the performance on my car’s PI system, which is performing reasonably well, but still has a bogging issue on snap throttle opening, I have been pondering, .. a common activity for me, especially on cold winter evenings. 

One thing I wondered was the influence of the size of the vacuum hose from the manifold to metering unit.

My reasoning , based on a “thought experiment “, is that a very large pipe would act as a vacuum reservoir, and would slug the unit’s response, and , on the other extreme, a small bore pipe would give a more rapid response , possibly helping to reduce the bogging issue.

Before I raid my “hose box” and start trying different sizes, has anyone any experience of this issue?

Cheers

Mike

interesting.

Will you change the adaptor fittings on the mu and manifolds to accept and seal with different internal diameter vacuum hose?  Or will you make up adaptors to go with short bits of hose at each end?   
You cannot risk a vacuum leak as the knock on from Rich fuelling is **** servo brakes.

Edited December 9, 2023 by BlueTR3A-5EKT

[mleadbeater]

Peter, 

I was just thinking of using rubber hoses, nylon tubing , of which I have a selection, and jubilee clips, leaks shouldn’t be a problem if I take care, but thanks for your concern.

Mike

[BlueTR3A-5EKT]

1 minute ago, mleadbeater said:

Peter, 

I was just thinking of using rubber hoses, nylon tubing , of which I have a selection, and jubilee clips, leaks shouldn’t be a problem if I take care, but thanks for your concern.

Mike

That makes sense.  Dropping the id by inserting a smaller internal id pipe into the existing rubbers and clips

[michaelfinnis]

Not sure what the dimensions of the pipe is but say for arguments sake 1/4in bore by 1 metre length; the volume of the pipe would be 32ML. Halving the bore would drop the volume to  7.9ML, a reduction of about 24ML. These are tiny volumes in comparison to the air flow going through the manifold, thousands of litres a minute,  hard to believe that is going to make any difference. 
Besides, the system worked as Triumph designed it. The system doesn’t need redesigning, it just needs to be set up correctly and any faults to be corrected.

Mike.

[mleadbeater]

take your point, Mike, but, having tried adjusting the butterflies as well as possible, checked compressions, fuel pressure, vacuum, and installed a new distributor with electrictronic ignition, the engine still bogs. My thoughts are all about minimising vacuum release lag to the MU when the vacuum at the manifold rapidly drops on acceleration.

Otherwise the tickover is stable and can be adjusted between approximately 600 to 1200 rpm, starting easy with the enrichement lever, and it revs freely to high rpm cleanly once past the bog rpm.

I will check the injectors as well, but otherwise its still bogging.

Testing my theory won’t cost me anything, and is easily reversible, so worth a try.

Ultimately, I may need to have the MU rebuild and recalibrated. 

All part of the fun of ownership,

Mike

[Peter Cobbold]

The problem is caused by a transient lean spike in the mixture because the PI lacks acceleration enrichment.

https://supertrarged.wordpress.com/2013/06/12/the-lucas-pi-lean-spike/

The cruise mixture has to be run rich to cure the 'bogging'. But that lowers mpg.

Peter

[John L]

Which metering unit is it, CP or CR?  Have you had a look at the springs in the vacuum unit, I had to change the little spring on mine to get to run anything like.

What are the colour of the plugs after a good spirted run, you could mark the adjustment settings, on the MU and turn in all three together, (richer) like a 1/4 turn and see how it goes, you could always put it back if it didn't work.  I made a little gasket to go under the black top cap.   I was checking it will a Mighty Vacuum pump tool, shows up any leaks.

John

[marting]

If you are anywhere near St Neots, Cambs, we can hook my portable PI monitoring kit up to your car and go for a short drive. It shows AFR, fuel pressure and vacuum in real time. 

[mleadbeater]

Thanks, gents,  for your responses, the mysteries of the MU are a getting clearer.

Peter, you link is very helpful, clarifying my understanding of the PI system, which lacks any enrichment device , running rich seems a poor compromise these days.

 Martin, thanks for your kind offer, I am in York, so a bit far from you, essentially in a ‘6 not running right. I may ask you for some help on your “ analysing setup”, if thsts ok, but it won’t be until spring. The  car is currently tucked away in my garage, undergoing many tidy-up jobs.

John, your comments are most helpful in demystifying the workings of the diaphragm and spring system, I attach a cross section , the three threaded adjustments are coloured, can you tell me the individual functions of each?, also the individual function of the two springs.

You mention fitting a top gasket, is this to make it airtight to maintain the vacuum?

My car is a mid 70’s CR, the plugs looked the correct light brown last time I checked, I also checked with my colortune, the flame was a correct blue at idle.

Are there anymore writeups on t’net on the operation of the MU? , I hate manuals which state never mess with these units, words like Rag, Red, and Bull come to mind.

Thanks again 

Mike

[super6al]

Hi Mike there's loads of info here & a few other websites http://vitessesteve.co.uk

Regards

Alan

[Nigel Triumph]

On 12/9/2023 at 3:35 PM, John L said:

You could mark the adjustment settings, on the MU and turn in all three together, (richer) like a 1/4 turn and see how it goes, you could always put it back if it didn't work.

Mike,

John's suggestion above works. I've used this approach to fine tune the mixture on my CP-series 6. Better still, fit a T-piece in the MU vacuum line and adjust the three metering unit rings together to get maximum vacuum with the engine idling. A vacuum reading of around 12" Hg is normal.

I was told this by a retired Lucas service engineer, who used to tune up the 2.5PI 'jam sandwiches' used by the police back in the seventies.

It's also important to ensure you have the correct fuel pressure at the metering unit, should be 105-110 psi. Take care if inserting a pressure gauge into the high pressure fuel line. Trying to tune the metering unit without the correct fuel pressure would be a waste of time.

Nigel

[John L]

To Mike,

There is quite a good graph description of the fuel settings in that PI instructions, there are some basic settings to do first with the wedge, and to then measure the distance between the the rollers and the end of the pushrod that comes out the centre of the rotor, I know the large screw in red does the main fuelling but also takes with it the other settings as well, so screwing that in lowers the rollers and allows the large push rod to have more movement and so allows more fuel to be injected. (higher up the ramp means less fuel injected)  I know the green thread controls the mid rage fuel, light throttle and I know the blue controls the overrun fuel with high vacuum, should be very minimal fuel on overrun, but if you get a lot of popping on the overrun, screwing out that screw a little will stop it! Making it leaner ( but you do need just a little fuel at that point.

I had to make a gasket for the top hat, because I could never get the locking rings tight. The threads are sealed when the unit is setup and the locking rings tightened with a sealer.

The other part that's worth mentioning, that the 2 screws that hold the black cap on must be sealed on the inside with Arildite or epoxy to stop a vacuum leak, just make sure the screws don't push the plugs of epoxy out!

I have a Mighty vacuum hand pump, so I can make sure there is no leak at all.

I do have a wide band CO meter so I can measure just what is happening.

Hope that helps a bit?

John

[Tim D.]

On 12/9/2023 at 3:31 PM, Peter Cobbold said:

The problem is caused by a transient lean spike in the mixture because the PI lacks acceleration enrichment.

https://supertrarged.wordpress.com/2013/06/12/the-lucas-pi-lean-spike/

The cruise mixture has to be run rich to cure the 'bogging'. But that lowers mpg.

Peter

Concur with Peter. When I had PI on my car I did loads of testing on the road with an air/fuel ratio meter and data logger. The transient lean spike is very clear with snap throttle and I played for ages adjusting the MU to alter the mixture at high vacuum to get the perfect balance between idle mixture and lean stumble. There is a sweet spot where people following don't get gassed by unburnt hydrocarbons and the car accelerates smoothly. I tried this with the mu fitted to the car when I bought it and an mu rebuilt and recalibrate to triumph specs. In both cases some extra tweaking was required. Suspect fuels have changed a lot since triumph defined the mixture curves on the mu and engines have worn. In the end it worked great. And gave good fuel efficiency. All a but academic now as I am supercharged. :-)

tim

[mleadbeater]

Hi,

thanks for the further advice.

just done some experiments on the PI, trying different diameters of vacuum hose connecting it to the centre pair of inlet manifolds.

I tried a small bore hose, 2.3 mm internal dia, and then a larger diameter of 12 mm, not a lot of difference but the larger hose seemed a little better, with less bogging.  I guess its a balance between flow resistance of a small hose versus the reservoir capacity of a larger one.

Probably the best solution is a short as possible hose with a say 12 mm bore. More testing is possible in the spring when the car is back on the road.

re adjusting the fueling screws, the large on seems fully screwed in, so which other one should I try next?

cheers

Mike

 

 

Edited December 14, 2023 by mleadbeater
photo added

[DRD]

On 12/12/2023 at 12:49 PM, mleadbeater said:

Thanks, gents,  for your responses, the mysteries of the MU are a getting clearer.

Peter, you link is very helpful, clarifying my understanding of the PI system, which lacks any enrichment device , running rich seems a poor compromise these days.

 Martin, thanks for your kind offer, I am in York, so a bit far from you, essentially in a ‘6 not running right. I may ask you for some help on your “ analysing setup”, if thsts ok, but it won’t be until spring. The  car is currently tucked away in my garage, undergoing many tidy-up jobs.

John, your comments are most helpful in demystifying the workings of the diaphragm and spring system, I attach a cross section , the three threaded adjustments are coloured, can you tell me the individual functions of each?, also the individual function of the two springs.

You mention fitting a top gasket, is this to make it airtight to maintain the vacuum?

My car is a mid 70’s CR, the plugs looked the correct light brown last time I checked, I also checked with my colortune, the flame was a correct blue at idle.

Are there anymore writeups on t’net on the operation of the MU? , I hate manuals which state never mess with these units, words like Rag, Red, and Bull come to mind.

Thanks again 

Mike

If you adjusted it so the Colourtune was blue it is probably running too weak, I'd richen it up slightly. Modern fuels with ethanol have different burn characteristics to petrol 40 years ago to work with modern leaner burning engines. Our old beasts like AFR's of 12-13:1, not 15:1.

[mleadbeater]

David,

Yes, but which screw do I adjust?, large one already fully in.

Mike

[JohnG]

On 12/9/2023 at 11:27 AM, mleadbeater said:

As per my recent posts on the performance on my car’s PI system, which is performing reasonably well, but still has a bogging issue on snap throttle opening, I have been pondering, .. a common activity for me, especially on cold winter evenings. 

One thing I wondered was the influence of the size of the vacuum hose from the manifold to metering unit.

My reasoning , based on a “thought experiment “, is that a very large pipe would act as a vacuum reservoir, and would slug the unit’s response, and , on the other extreme, a small bore pipe would give a more rapid response , possibly helping to reduce the bogging issue.

Before I raid my “hose box” and start trying different sizes, has anyone any experience of this issue?

Cheers

Mike

I know that we all like to do the work on our cars, after all, that's one if the reasons we have classic cars, isn't it?

However . . . . The Lucas PI system is pernickety and needs to be set up just right.

Not right . . But, just right.

A metering unit

A plenum

6 butterflys

6 injectors

6 injector lines

The vacuum hose

To name just the primary parts

 

My '6' would never run right

It started on the button, but, was just off on tick over and lacked that pull when you buried the throttle pedal.

 

I finally bit the bullet.

Accepted that I wasn't going to cure this on my own . . . .no shame there, I have my limits in every walk of life.

 

Took the car to Clive Manvers, a man who knew and understood our cars, with a team of like minded people.

It took 2 days and a wedge of cash, BUT, from that day forward, it worked.

It just did.

Take the car to a man who knows what he's doing  then enjoy driving it.

[mleadbeater]

John,

looks like he is not around anymore, looking at the website.

I may have to resort to finding a specialist, but I’ll keep learning and trying for a while longer.

cheers

Mike

[BlueTR3A-5EKT]

Mike, are you near West London?    If so talk to Enginuity in Acton    They know the cars and have an in house rolling road to set them up.

https://enginuity.co.uk/about/

[DRD]

5 hours ago, mleadbeater said:

John,

looks like he is not around anymore, looking at the website.

I may have to resort to finding a specialist, but I’ll keep learning and trying for a while longer.

cheers

Mike

 

Sadly he passed away a few years ago and his business was wound up.

[mleadbeater]

hi, 

no, i’m near York, thanks , long trip to London, especially in a 50 year old, the car not me!

I’ll keep pondering and tweaking for now, its only a toy.

 

cheers

Mike

[stuart]

1 hour ago, mleadbeater said:

hi, 

no, i’m near York, thanks , long trip to London, especially in a 50 year old, the car not me!

I’ll keep pondering and tweaking for now, its only a toy.

 

cheers

Mike

Dennis Vessey is closer and really knows his stuff https://www.vessey-classic-car-services.co.uk/

Stuart.

[mleadbeater]

Cheers, Stuart, that’s only about 60 miles away.

 

[JohnG]

On 12/17/2023 at 2:47 PM, mleadbeater said:

John,

looks like he is not around anymore, looking at the website.

I may have to resort to finding a specialist, but I’ll keep learning and trying for a while longer.

cheers

Mike

Mike

No

Clive was based in Elmswell in Suffolk, a lovely guy

Died in his workshop, of a heart attack.

He was, I believe, just 54

 

There are a number of others in Wallingford, London and Huntingdon (they are all around here), as examples.