TR6 vacuum

[Ken F]

How common is it for tr6 manifolds to suck in air from below?

ken

 

[stuart]

1 hour ago, Ken F said:

How common is it for tr6 manifolds to suck in air from below?

ken

 

Well they shouldnt so somethings wrong.

Stuart.

[Ken F]

6 minutes ago, stuart said:

Well they shouldnt so somethings wrong.

Stuart.

Thanks Stewart, I’m not saying they are but I’m tracking down my low vacuum on my cp tr6, I’ve read it’s common but if I take the rubber pipe off that feeds the air valve and put my finger over it the engine dies, that surely proves I don’t have a vacuum leak. 
gauge connected to manifold so rules out brake servo. 
i had 8”hg yesterday then 5 now its dropped back to 2 “hg. 
I checked timing again and I’ve checked valve clearances. 
I set the butterflies previously, Martin Giles advised me to let butterflies touch the bodies and forget the 2 thou”. 
since I rebuilt the metering unit ( without touching under black cap) I’m wondering if the over fuel screw needs adjustment. Would rich fuel delivery cause low vacuum?

I did compression test btw. 
 

my gast has never been so flabbered

 

Ken. 

[stuart]

Over rich would wash the bores and could contribute to lower vacuum eventually, What does a compression test show? Im also surprised that Fred says let the plates touch as that contributes to wear. You should be able to stall the engine by screwing the air valve right in if the butterflies are set correctly. Have you tried spraying carb cleaner round the manifold to see if engine idle note changes. Have you tried the tongue test too to make sure the metering unit diaphragm is OK?

Stuart.

[Ken F]

33 minutes ago, stuart said:

Over rich would wash the bores and could contribute to lower vacuum eventually, What does a compression test show? Im also surprised that Fred says let the plates touch as that contributes to wear. You should be able to stall the engine by screwing the air valve right in if the butterflies are set correctly. Have you tried spraying carb cleaner round the manifold to see if engine idle note changes. Have you tried the tongue test too to make sure the metering unit diaphragm is OK?

Stuart.

I just did a compression test on #2 It went up to 120 then the hose blew off my tester. Don’t laugh. The readings were all good when I checked recently. 
I did the tongue test. Ok. 
screwing in the air valve the rpm goes right down, if I pull the pipe off the plenum and put my finger over engine dies. 
I’ve tried spraying wd40 where I can reach but I’ve got aircraft grade metal heat shield around the exhaust manifold and it’s like a razor if I touch it. Surely if i putt my finger on the pipe and engine dies there can’t be a leak. 
The M U worries me with my rebuild. When I put it on the car I could get a small crescent moon or nearly a complete hole on the rotor so I opted for the small hole. 
The screw that’s impossible to get at whatever it’s called I wonder if that’s giving too much fuel giving bore wash but as I said I just got 120 psi on one cylinder and the oil doesn’t smell of petrol. 
Thanks Stewart. 

[stuart]

I take it the enrichment cable returns to rest on the metering unit? 120 is a bit low, did you do it with throttle wide open?

Stuart.

[Ken F]

7 minutes ago, stuart said:

I take it the enrichment cable returns to rest on the metering unit? 120 is a bit low, did you do it with throttle wide open?

Stuart.

Yes cable returns to rest. 
might have gone higher if the pipe stayed on. 
I didn’t touch the throttle, I just took off the dizzy lead and span it over. 
 

Ken

[Ken F]

11 minutes ago, stuart said:

I take it the enrichment cable returns to rest on the metering unit? 120 is a bit low, did you do it with throttle wide open?

Stuart.

I rebuilt the injectors as I was told dribbling injectors could affect vacuum. 
when I put them back and got them bled it started immediately, I then had 8”hg it later dropped to just over 5 then later still dropped to about 2hg . 
 

I tweaked the dizzy and increased the rpm then I was able to adjust the idle with the air screw. When I did timing light test it was at about 23 degrees before so I set it ant 11 before.  After resetting the timing the air  screw had  little effect and I couldn’t get it to idle at 850. About 600rpm was all I could get. 
 

my gauge is new, I’d like to try another or find a way to test against a known vacuum to rule that out. 
 

Ken

[Ken F]

23 hours ago, stuart said:

I take it the enrichment cable returns to rest on the metering unit? 120 is a bit low, did you do it with throttle wide open?

Stuart.

Got 150 once I’d fixed the tester pipe. 
Ken

[Ken F]

On 6/17/2026 at 2:02 PM, Ken F said:

How common is it for tr6 manifolds to suck in air from below?

ken

 

Hi,

Does anyone agree with my theory that if I take the air pipe to the air valve off the plenum and put my thumb over it and the engine dies that I CAN’T have an air leak?…. like; from the manifolds where they bolt to the head?

Screwing in the air valve completely the revs drop considerably but the engine still just about runs.
 

I’ve only got about 2 or 3”HG of vacuum. On a CP tr6 I should have at least 8”hg. 
 

Ken

[Chris Hale]

1 hour ago, Ken F said:

Screwing in the air valve completely the revs drop considerably but the engine still just about runs.

If this happens then you do have an air leak.

Have you checked the rubber pipes connecting the throttle bodies?

[Ken F]

43 minutes ago, Chris Hale said:

If this happens then you do have an air leak.

Have you checked the rubber pipes connecting the throttle bodies?

It stops with thumb over the pipe, doesn’t that point to air valve not sealing? 
Screwing in the valve is gradually reducing air whereas thumb over the pipe is instantly stopping air and any leak isn’t enough to keep it just ticking over but maybe a sudden reduction stops the engine. If all cars stop the engine by screwing in the valve then that could be what’s happening I suppose. 

I’ve checked those pipes yes. 
 

Thanks Ken. 

[trry]

I don't think the thumb-over-the-pipe test completely rules out an air leak. It just proves that blocking that air supply stalls the engine.

The fact it will still run with the air valve fully screwed in suggests it's getting air from somewhere else. With 150 psi compression, I'd be looking for an induction leak, throttle spindle wear, butterfly adjustment, or even checking the vacuum gauge against another one before touching the MU again.

Cheers.

[Ken F]

5 hours ago, trry said:

I don't think the thumb-over-the-pipe test completely rules out an air leak. It just proves that blocking that air supply stalls the engine.

The fact it will still run with the air valve fully screwed in suggests it's getting air from somewhere else. With 150 psi compression, I'd be looking for an induction leak, throttle spindle wear, butterfly adjustment, or even checking the vacuum gauge against another one before touching the MU again.

Cheers.

Thank you,

You obviously took the time to read back into the post, not everyone does, including me I’m sorry to say. 
if I had another gauge I would like to try a comparison. If anyone nearby has one to lend me I’d be grateful, if I bought another new one ( this one is new) there’s no guarantee that would be accurate. 
I will have to take my heat shield off to spray under the bodies, to illuminate the possibility of them leaking. 
I will have to take the plenum off again and recheck the butterflies, I will put some sort of grease around the spinles where they  go into the bodies to see what difference that might make, if it’s big I’ll buy the Fred Milturn kit. 
if nothing makes a difference I’ll retime the MU and put a piece of tube on the max fuel screw so it can be adjusted in situ. Apparently it works. 🤔

thanks for all answers much appreciated. 
Ken. 

[TR NIALL]

45 minutes ago, Ken F said:

Thank you,

You obviously took the time to read back into the post, not everyone does, including me I’m sorry to say. 
if I had another gauge I would like to try a comparison. If anyone nearby has one to lend me I’d be grateful, if I bought another new one ( this one is new) there’s no guarantee that would be accurate. 
I will have to take my heat shield off to spray under the bodies, to illuminate the possibility of them leaking. 
I will have to take the plenum off again and recheck the butterflies, I will put some sort of grease around the spinles where they  go into the bodies to see what difference that might make, if it’s big I’ll buy the Fred Milturn kit. 
if nothing makes a difference I’ll retime the MU and put a piece of tube on the max fuel screw so it can be adjusted in situ. Apparently it works. 🤔

thanks for all answers much appreciated. 
Ken. 

If you turning that Max Fuel Screw it only needs the slightest bit of movement.

[Ken F]

2 hours ago, TR NIALL said:

If you turning that Max Fuel Screw it only needs the slightest bit of movement.

Thanks Niall that’s good info. 
 

I got a a bit on a couple of manifolds clamp nuts and a bit on the centre body top nut. I sprayed under the bodies, I don’t think the rpm changed because of the spray. There was an increase in revs but I suspect that is the MU delivering more fuel because it thinks 1”hg is the throttle being floored. 
At 15k to 20k rpm I have about 8”hg. 
I can’t get a smooth idle, it goes down to 2”hg at 650 to 850rpm and it’s not stable. 
It sounds horrible, the exhaust even sounds weird now, might have pin holes in or the baffle loose. 
I will take the air box off and recheck the butterflies next. 
 

Spare a thought for me (or I suspect not) while you’re out in your cars with the wind in your… er… I better say wooly hats I don’t want upset anyone, it might be a sore point. Hairing along and I’m sweating over a hot engine. 
 

enjoy the summer 

Ken
 

[Steve-B]

Ken,

Do we need to get a couple of us TR Chilterns 6 owners around yours to help? Chalfont is like center of our universe 

[Ken F]

19 minutes ago, Steve-B said:

Ken,

Do we need to get a couple of us TR Chilterns 6 owners around yours to help? Chalfont is like center of our universe 

Wow! That’s not an offer I was expecting. Thank you. 
I have a few things to check or recheck and if there’s no improvement I will take you up on that offer. 
 

Kind regards Ken. 

[Steve-B]

Ok, I’m no genius but there’s some helpful guys in the group. I’m sure a call to Peter might get him to send out a sos 🤞

[Andy Moltu]

If you have a manifold air leak the tick over tends to be high. Check the seals around the injectors.

I Have you just fitted new butterflies?

They need to be centred - loosen the screws slightly and then let the throttle snap shut - this tends to centre them. Nip up the screws. Worn or twisted spindles? Whilst running put the heel of your hand over each inlet in turn and check it gets sucked - should be fairly minimal if the throttles are closed (or use a air flow meter). The marginally open (2 thou of fag paper) stems from the early days of the injected 6 cylinder TRs (5s and very early CP 6s) but most had the air bleed valve fitted by Triumph dealers which serves the same purpose as the tiny gap. This then allowed the throttles to be set fully closed and set to open synchronously. 

New butterflies often take a bit of time to bed in (or gum in) to make a good seal. 

[Ken F]

20 hours ago, Andy Moltu said:

If you have a manifold air leak the tick over tends to be high. Check the seals around the injectors.

I Have you just fitted new butterflies?

They need to be centred - loosen the screws slightly and then let the throttle snap shut - this tends to centre them. Nip up the screws. Worn or twisted spindles? Whilst running put the heel of your hand over each inlet in turn and check it gets sucked - should be fairly minimal if the throttles are closed (or use a air flow meter). The marginally open (2 thou of fag paper) stems from the early days of the injected 6 cylinder TRs (5s and very early CP 6s) but most had the air bleed valve fitted by Triumph dealers which serves the same purpose as the tiny gap. This then allowed the throttles to be set fully closed and set to open synchronously. 

New butterflies often take a bit of time to bed in (or gum in) to make a good seal. 

Thank you,

not new butterflies but I previously had set them to 2thou” which I found out was for pre air valve cars as you say. When I synchronised the throttles with them closed by adjusting the vertical rods I got the vacuum up to 10 “hg, I put the air plenum back on and the vacuum dropped to 5 at 900rpm and down to 3 ate 600rpm. 
I really thought I’d fixed it, why refitting the air box should make a difference I don’t know. 
I set everything up using the white manual. 
I can’t get it to idle now, if I push the choke in when it’s warmed up it dies. 
 

Thanks Andy

 

Ken

[stuart]

20 minutes ago, Ken F said:

I can’t get it to idle now, if I push the choke in when it’s warmed up it dies. 
 

Thanks Andy

 

Ken

Then you need to open the idle air valve a bit.

Stuart.

[Ken F]

2 hours ago, stuart said:

Then you need to open the idle air valve a bit.

Stuart.

Thanks Stewart, I was referring to with the plenum off. The valve didn’t make much difference and after 1 1/2 turns no difference. 
 

Ken

[Ken F]

1 minute ago, Ken F said:

Thanks Stewart, I was referring to with the plenum off. The valve didn’t make much difference and after 1 1/2 turns no difference. 
 

Ken

I found #4 manifold metal sleeve was a bit loose when I put the rubber sleeve joint on. 
I wiggled it and it came out, I cleaned and degreased it and will put back with locktite. 
I wonder if this could be the cause of the vacuum reduction when I put the heavy plenum on. I’ll report back after I’ve let it harden for 24 hours. 
 

ken

[Ken F]

On 6/25/2026 at 12:11 PM, Ken F said:

I found #4 manifold metal sleeve was a bit loose when I put the rubber sleeve joint on. 
I wiggled it and it came out, I cleaned and degreased it and will put back with locktite. 
I wonder if this could be the cause of the vacuum reduction when I put the heavy plenum on. I’ll report back after I’ve let it harden for 24 hours. 
 

ken

Bought a bottle of 638 loctite for £13, cleaned up the sleeve and stuck it back. Not a jot of improvement, Ai said that will be  your problem.. I might as well have asked some bloke in the pub who knows naff all.

It needed doing anyway but the search for my missing vacuum goes on, when I can be arsed. 

Had a few things to do today, firstly, recheck the timing... my new timing strobe light refused to work.... Fuel pump pressure next....my 6500psi hose sprang a leak on my pump pressure gauge kit, must be its not petrol resistant, I made it up 35 years ago and it hangs in my garage used about 3 times. It chose today to leak. I couldn't have tested the fuel pressure anyway as it ran out of petrol, I only put in 5 litres  last week and only ran the engine up to temp a few times.

I gave up wasting my time in the garage and wasted the rest of the day watching F1 on Now tv, even the shouty bloke didn't stop me nodding off.

I am loosing interest in F1, Brundle said Adrian Newey is on the grid in the grid walk, great! I thought find out what's going on with the Aston's, but he chose to interview a load of celebs who unlike the F1 fraternity weren't trying to avoid him they were hanging on his coat tails to be interviewed.

Ken