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TR6 TR6 Vacuum Issue... some help please...

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S

Sbardon

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Vacuum Issue:
1976 Triumph TR6

I apologize for the long post leading to a simple question, I just hope to answer all the queries in one go and get an answer.

This issue is new, head done and cam just replaced by 'other', am finding many deficiencies in work unfortunately; including the distributor drive gear was in 180. (The crazy twisted installation of the distributor and offset wiring installation was the first clue.)

I corrected the distributor drive gear as per the book, reinstalled the distributor, the plug wires in the proper orientation, and the distributor looks correct with the dizzy at #1. The distributor is Vacuum retard with points.
The car previously timed right on the numbers, vacuum at 18 at idle and ran well.

Current set up:Static timing: 10deg BTDC
Plugs: 0.025”
Rockers: 0.010” cold
When running:
Idle: 750rpm
Dwell 37deg (want 36)
Dynamic: 4deg BTDC (want 4deg ATDC) (remember... vacuum retard)
Vacuum: 8.5-9.5 buzzing needle (should be 18)
Distributor Vacuum removed: Dynamic 14 deg BTDC, 1000rpm
I have checked all vacuum connections best I can, and tried this with the canister and Brake master cylinder plugged. Same.
also: for some reason, on acceleration, vacuum increases (?) to 18 at about 2500rpm

Suspect:
Lack of vacuum is causing issues with the vacuum retarded timing as it is not advancing the distributor enough. I suspect proper vacuum will rectify timing, but will not know until it is resolved.

The low vacuum indicated either a manifold leak or valve timing… a manifold leak should present itself with a steady vacuum gauge needle, the constant frequency(buzzing) of the vacuum gauge needle suggests it is a problem with the cylinders, all cylinders, ie how they are pulling air, the valve timing.

Next check:
Suspecting cam installation…. I turned the crank till #6 valves, 11 & 12 we off the cam and had 0.010” clearance. I then attached indicators to the rockers (2’ long stiff wires) and lined up the ends of the wire to each other precisely (holding the rocker clearance out). I then rotated the crank until the two indicators we passing each other on the rock, and stopped in the direction of engine rotation when they were perfectly aligned. (The two foot indicators providing precision because of their motion at the tips… about 6”.) As triumph cams are symmetrical, this indicated Cam TDC. At this point I read the degrees on the crank pulley.

The crank is 11deg BTDC. Basic 2:1 math says that when the crank continues to TDC, the Cam will be 5.5deg ATDC. Not a tooth out… but perhaps the wrong bolt holes on the cam gear.

Would this cam advance cause my vacuum issue?

I want to get the car back to spec.
Please advise.
Stephen
 
Last edited:
You say "Vacuum Advanced". Did I miss something in your post ?
The 76 distributor, in fact all since 72 lacked a vacuum advance module on the distributor...
Did you have the vacuum Retard module removed and replaced with a vacuum Advance module ?

As far as your cam timing technique ....this is how I do it... Pistons 1 & 6 at TDC with #1 on compression stroke....Set the cam timing straight up using a method you feel comfortable with, then turn the crank counterclockwise the number of degrees you want the cam advanced, then attach the timing gears and chain.
 
Poolboy, thank you for the correction, yes, retard. I will edit the post. Thanks.
The real question is.... could the cam being 5.5* advance cause the severe vacuum issue, any insight?
I just paid to have the work done, don't want to touch the cam if that cannot be the issue....
thanks.
 
When I installed the stock cam # 311399 the so called 125 hp cam, I advanced it's timing 4 degrees.
Using manifold vacuum as a guide, I set the ignition timing at 18 in-Hg....after braking in I advanced the timing to 19 in-Hg.
Sometimes I'll go as high as 20 in-Hg.
Those numbers are at an idle of 900 rpms and at very close to sealevel....vacuum Retard delete.
I once had an S2 cam which because if valve overlap created less vacuum, around 13 in-Hg with a bit higher idle 1100 rpms.
 
Poolboy, what sort of static timing did you end up at?
To be honest, my car has always run right on the factory tune numbers, 10 static, -4 dynamic, 750 rpm idle and 18-19 in-hg, and even the retard removed showed the 1300rpm. I am trying to keep it all original (yes still has oem redlines).
Unfortunately... the guy who did the work is shut down and I do not know what cam it is.... was supposed to be an improvement, but a stock TR6 number. Quite frankly, if the cam is causing all these issues.... It's gone... and I will put a stock one back in.
Just don't want to redo all the work I just paid for myself, if the cam is not the issue.
(any chance you know the crank pulley bolt diameter?, seems I may be shopping for a degree wheel to figure it out)
 
I really don't know what number is on the damper. I don't have a timing light.
I have the the distributor's advance curve set professionally then I set the timing at idle by ear and feel and use a vacuum gauge as reference.
 
Understood.... and I have tried to advance the dist till it got max Hg.... but it ended up off the chart... like 45 deg or something .... no way that is right....
I definitely have something wrong..... seems first I have to measure/see what cam I have...
 
It's not a good idea to shoot for maximum manifold vacuum and leave it there....back off about 3 in-Hg less.
 
Poolboy, thanks.... next time I am at the car I will try that and see where that leaves me... currently ordered some gear so I can measure the cam... Hoping I can figure out what I have without having to take the car apart.
thanks again for your input....
 
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