Fluctuating manifold vacuum - what could be causing it?

[JohnC]

In my quest to better understand how the engine in my TR6 performs, I fitted a vacuum gauge a while ago, along with an EGO sensor. That's helped a lot in getting the PI set up properly, but it's given me some data which baffles me:

 

At low to mid RPM (say 2000-3000rpm) with a part-open throttle, the vacuum fluctuates wildly. It can fluctuate by 100%, e.g., min 5"Hg, max 10"Hg. The gauge needle flutters very fast, so that it's almost a blur. The magnitude of the flutter decreases as rpm increases, possibly as the needle simply can't respond fast enough. There is no flutter at full throttle (flat on the zero vacuum stop), nor at closed throttle on over-run (15"Hg to 20"Hg depending on rpm/road speed, gradient etc). There is some flutter at idle. Subjectively, It seems to me that the flutter is not always the same from week to week, even when I've not made any adjustments.

 

On the assumption that this flutter is undesirable, I have tried to cure it. So far I have really only done two things:

 

1. I have used a carb balancer to get the throttle butterflies set as near perfectly as possible. Previously I've set them by ensuring all open simultaneously using my fingertips against adjacent butterflies. I've tried Richard's fag paper approach, but don't have the necessary knack! FWIW the balancer I have is an Edelbrock one. This didn't seem to make any difference.

 

2. I've tried various settings for valve clearance. This also didn't seem to make a difference to the flutter, although greater clearance yielded higher manifold vacuum at idle (~9"Hg at 12 thou, up to ~11"Hg at 18 thou).

 

So I have two questions:

 

1. Is this normal, in which case I'll spend my time on something else? I'd love to hear from somebody else with a vacuum gauge!

 

2. What could be causing it?

 

In case it's relevant, the car is a CP series but with a high-overlap cam. It has a standard head (apart from hardened exhaust valve seats). The exhaust manifold is a TT1200 6:2 into single 2.5" exhaust. Inlet manifolds are reconditioned, and the throttle linkage has bronze bearings.

 

Thanks in advance for any suggestions.

Regards,

John

[robgeev]

Wouldn't pretend to know the ins and outs of this, but the vacuum in the manifold is always going to pulse-probably more defined with a longer duration cam-and more so when the engine is just coming onto the cam-with butterflies mostly closed.

 

Are you having any running problems?

 

Even if you are, I very much doubt it's a fluctuating vacuum problem!

 

Spend time on something else-barbie or Fosters possibly?

 

Rob.

[Richard Crawley]

Like Rob I don’t profess to know too much about this but I had a Lucas vacuum gauge fitted to a Ford Anglia many years ago & it didn’t flutter. The only thing I can think is that that a gauge intended for auto use should be damped so that it doesn’t respond wildly to the inlet pulses as your is doing; are you perhaps using the wrong type of gauge!

[marvmul]

I've lost the leaflet that came with Gunson's LoGauge (vacuum gauge) but I remember that a jerky movement of the needle could be caused by sticking of leaking engine valves and also by a leaking head gasket.

[JohnC]

Thanks for the replies!

 

Rob, I must admit I do wonder if I'm jumping at shadows... I'm having no running problems. Starts instantly (scary!). Possibly some pinking under heavy load (I'm playing with the timing). 113bhp at the rear wheels (I know that's not impressive, but it's not far off 'stock', so probably no cause for concern). Definite sense of coming 'on cam' at 4000rpm-ish. I have posted previously about a torque 'hole' in the 2500-4000rpm band, which I'm currently putting down to a combination of high-overlap cam and 'undesirable' 6:2 exhaust manifold. It may even be that the manifold makes the pulsing worse at mid rpm.

 

Richard, The gauge is intended for auto use, and has a restrictor inlet (i.e., a pinhole), but it is a very cheap gauge. It may be that my problem lies just there...

 

Alec, The gauge is attached to the balance pipe between the two front throttle barrels - basically in the same place as the brake servo vacuum (rear pair) and PI vacuum (centre pair). I'll try swapping the PI vacuum takeoff & the gauge takeoff to see if that makes a difference. Like you, I used to have a vacuum gauge on a previous car (a Sprite), and I don't recall fluctuation, but that was 25yrs ago. I would be interested in DrMSBs opinion, if he's willing to drop me a line. I'll pm you my email address.

 

Marvmul, I hope you're wrong, but thanks for the input. I rebuilt the engine some 11k miles ago, with new valves (& hardened exhaust valve seats). That could be good or bad I suppose. I do think that there's a bit too much valve gear noise, and that it may be more from one valve, but I can't be certain. I guess a compression check is in order. In dark moments I do wonder if there's a sticking cam follower - is that easy to detect?

 

Thanks again,

John

[robgeev]

John,

I've just overcome cam follower sticking problem (new ones!!) very definite misfire present-no noise to speak off.

 

Rob.

[JohnC]

Thanks Rob. No misfire present on my car (so far - touch wood!), so I may be OK.

 

Apologies for doubling up on a question I posted in another thread, but can the cam followers be replaced without removing the head?

Thanks,

John

[snowric]

a bit foxed.

 

i am using a gunson pressure gauge to test manifold pressure. I have stuck it into the brake servo pipe from 5 and 6 intake manifold and get a 8-9mmHG steady reading at 850 rpm. but if i attach it to the MU vacuum feed it varies wildly around 5mm HG plus or minus. of course as i have disconnected my mu pipe then i have to open the throttle to get it to idle.

 

if i screw in my idel adjuster then i get down to c 500 rpm and near stalling. so air coming in from somewhere but only just. i thought there was a balance pipe so manifold vacuum is uniform between all three inlet throttle bodies ?

 

i did a compression test and whilst not spectacular they were all 155-165. so valves unlikely to be leaking....

 

I am testing it before i send mu to neil for a re-build as my pugs are uniformly sooty = rich running.

 

i am being an idiot and should i be connecting the gauge using a t piece so that the mu is still connected and so idling nicely ?

 

 

thank you

[snowric]

i also appreciate that my last post kind of demands a "yes you are an idiot" reply but answers to my first bit would be great.

[snowric]

OK - ignore this I have discovered that if I use a t piece and keep the vacuum signal to the MU then everything is fine and the reading is 8 vs 7 normal.....