Vaccuum advance

[Steven_nl]

Hi

I'm Steven from the Netherlands

I'm the proud woner of a 1974 TR6. The car was imported form the USA. The previous owner completely restored it.

He made a few improvements on the car:

- the engine was overhauled

- a different camshaft was installed. (Kent Fast Road)

- electronic ignition (Newtronic/Piranha)

- Overhauled SU Carbs.

 

After I bought the car I ususlly took it to a well known Dutch Triumph specialist. I didn't dare doing stuff to the car myself. Unfortunatly he retired and I lost my job so my budget is a little tight.

One of the first things he did was closing the vacuum advance.

A friend of mine now helps me with the Tr6. He knows quite a lot about (classic) cars. He restored a 1967 Mustang.

I couild not explain to him why the vaccuum advance was disconnected on my TR6 and some friends in France (Jaguar and Lagonda owner) were baffeled as well. Can anyone shed some light on this?

 

I often go on holliday to France in the Tr6. But I allways seem to run into engine problems. The car runs realy well, but after a week or so (small roads) the engine starts acting up. it stutters when accelerating from low revs (1500 or so). Hard to drive the twisty and hilly roads.

I'll replace the ignition with a Petronix unit. (suggestion of my friends who thought the ignition was causing the problem) and maybe put some shields between the carbs and the engine.

I sometimes have the impression the outside temperature has something to do with it but,

the temperature dial never goes past half way.

 

Oh yeah one last question. is it normal that my TR6 doesn't have a fuel pump near the petrol tank?

 

Thanks for your help!

Greets from Holland

Steven

 

[Peter Cobbold]

Hi Steven, Welcome to the forum.

There is a lot you can do yourself on a TR6 particularly with SUs ( the PI is a bit 'tricky'). Doing the work is the easy bit, diagnosing what to do takes a little longer to learn, but thats where the forum can help.

The garage may not have put a lot of thought into it - time is money. They may have decided the spark was too advanced and disconnecting the vac advance capsule will do that, at cruise. Or they may have been following a manual for the TR PI cars that did not connect it (a PI-specific issue).

 

My guess is your rough running was mainly due to over-retarded spark. This makes the combustion chambers run hotter, and kills performance.

A quick test is to run the engien at tick-over and loosen the distributor clamp until you can just slowly turn it to and fro few mm. Keep hands away from plug leads. Or ask your friends to show you how. As you slowly turn the disy the engine will speed up or slow down. Clamp it at its fastest point. See it it drives better.

 

To retime it throughly, we need decide what spark the engine will behave best with, in its present form.

That depends partly upon the compression ratio. The 1974 USA cylinder head was low compression, around 8.5:1. You need to find out if it was 'skimmed' when it was overhauled to give the standard 9.5:1 of UK cars. If it is still low compression I would set the static timing more advanced, to about 15 degrees BTDC, maybe even 18. Its easy DIY - ask your friends? or come back here.

Note that some USA manuals give the static timing as 4ATDC, because their distributors have a vacuum retard function. We need to be sure you have a vac-advance not vac-retard disy. Can you post a photo of your distributor?

A vac-advance capsule on a UK engine looks like this:

http://s1048.photobucket.com/user/Agile2011/media/IMG_9793Dystributorafterworkshopadjustnotspecificallypointingatxx.jpg.html

When vacuum is applied the diaphragm in the capsule pulls the points clockwise. Since the cam rotates anticlockwise this advances the spark ( ie makes it happen earlier).

The capsule connects to a specific take off-at the edge of the butterfly on an SU , here:

http://0.tqn.com/w/experts/MG-Car-Repair-3786/2010/07/SU-HIF44-Carb.jpg

That ensures there in only sufficient vacuum applied at cruise. It should advance the spark to about 40-45 BTDC from about 2500rpm upwards. The capsule will add to the centrifugal advance.

 

Thats a good start. How to use a timing light in the next instalment.

Peter

Edited October 22, 2014 by Peter Cobbold

[Steven_nl]

Hi Peter

Thanks for the warm welcome and the great help.

 

I'm sure my former mechanic. knew what he was doing. He was THE Triumph guy in Holland. I had the impression it had something to do with the special Camshaft or the electronic ignition. It seemed like something he allways the did with a setup like mine. (loads of ex USA cars in Holland)

I'll post a picture of my distributor asap. I'm pretty sure it's a Lucas 22D. We did reconnect the vacuum advance tube. didn't seem to make a lot of difference.

Some of your information is beyond my knowledge but my friend knows more then enough to take care of it. (He knows how to use a timing lamp) we did the trick with the loosening of the distributor and turning it.

I'm not sure about the compression. I have no specific information abotut the engine overhal. But it was a complete overhaul. My guess is it will be european compression. I guess we can measure this? What woud the best static timing be for the UK specs?

 

 

The cars often runs really well. Allthough with the specia cam you do have to rev it.

 

 

[Peter Cobbold]

Hi Steven,

Getting the correct spark timing doesn't need specialist Triumph knowledge. Because it all comes down to combustion and flame speed the spark timing is remakably similar between engines.

With the vac-advance disconnected your engine is now running without advance at cruise, which is wrong - for all si engines. The extremely poor cylinder filling at cruise greatly slows the flame so it needs more time for the flame to spread, hence the vac advance device.

 

UK spec** is static 11BTDC , crank degrees

Centrifugal advance curve is similar to these:

http://i1214.photobucket.com/albums/cc499/Ahwahnee18/TR4/Lucas_Distributor_Advance_Curves_zps03074f01.gif

NB these curves are distributor degrees and disy rpm, double them to get crank rpm and to get the crank degrees you measure with a timing light. TR instruction manuals are dreadful for hiding the 'disy degrees' in the small print.

So, from the graph, a TR6 at 3000 rpm will be timed, in crank degrees, at 11 BTDC static plus 22 on the centrifugal, total: 33 BTDC. This is at full throttle.

2 or 3 crank degrees 'out' either way from 'perfect' is not going to make a big difference to power, maybe 1%.

 

But 33BTDC is not advanced enough at cruise. Yes the engine will run quite well, but it wont be making the most of the flame. Connecting up the vac-advance capsule to the SU's throttle edge take-off will add about 10 crank degrees to bring total advance at cruise to around 45BTDC at 3000rpm.

 

You can measure the centrifugal curve with a timing light. The vac advance needs the engine to be loaded, so it cant easily be measured DIY. And the capsule max advance is fixed. The SU take-off position is critical.

 

A trick cam can add a bit more air into the cylinders, a few percent, but only at full throttle. At cruise the cam makes no difference as the engine is throttled back and the cylinders are only part-filled. So the v-a is still beneficial whatever cam is in there.

 

The thickness of the cylinder head can tell you the compression, if no-one has reshaped the combustion chambers or fitted unsual pistons (both are unlikely in a routine rebuild). See table:

http://www.goodparts.com/tech_docs/TR6_Compression_Ratio.html

 

 

Peter

 

** You have carbs so you do not suffer the inability to advance the spark at cruise that hinders fuel efficient running of the PI engines. The reason is the PI 'lean spike':

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

That 'lean spike' is why the Pi engines wont take vac advance, they would pink even worse when flooring the throttle.

Edited October 22, 2014 by Peter Cobbold

[Peter Cobbold]

Steve

Save your money on the electronic ignition. Points work fine. I suggest you may have faulty ignition parts fitted. There are a lot of appallingly bad parts around.

The rotor arm is a big suspect, so are condensers, disy caps, coils etc.

I have learned from the forum that when I need to replace my disy parts the only wholly reliable source is 'Disy Doctor' in UK.

http://www.distributordoctor.com/rotor_arms.html

Search that name on the forum and you'll soon the see the number of ignition issues his parts have solved.

Peter

Edited October 23, 2014 by Peter Cobbold

[Steven_nl]

Hi Peter

 

thanks so much!!

 

When in France we did swap the coil, the rotor arm, changed cables and disy cap, checked the color of the spark plugs 3 were white and 3 light brown (checked the carbs and found they were not set the same, changed it). Nothing worked. We used a timing lamp as well. The guys who helped me (well actually they did everyting) were retired pro's who spent their weekend working on classic Lagonda's and Jags After changing something (working 45 minutes on the car) I tried the car. the first 2 minutes it seemed the changes had worked but after a few minutes the stutering came back. Allways problems accelerating from low revs. The guys said it probably was the electronic ignition. I allready bought a Petronix so I will start there. I did order a base plate for the disy at "the disy dotor".

Obviously it's warmer in France and there are more hills. I sometimes have the impression the quality of the fuel (I only use 98) has something to do with it. In the countryside there are often no brand gas stations around. Every supermarket does offer 98 though. But the last vacation I only used brand (Total) fuel.

The problem is less in Holland. colder? different fuel? more driving in a straight line? flat? Anyway. My friend and I are determind to get it right. With some expert help from TR6 owners ;-)

 

[Peter Cobbold]

Hi Steven,

The problem is even new parts can be faulty. Even those in Lucas boxes apparently - he has moved to China.

So red rotor arms form Disy Doctor are the only ones that are recognised on here as good.

 

Heat-sensitive parts would be condenser, coil and .... that electrickery thing. Try replacing the Pertronix with points as a check.

 

The coil on that year of TR6 is I think a ballast coil. There is a resistance wire built into the loom, so in normal running the coil supply is around 6-8 volts not 12-13. If you have fitted a non-ballast coil it needs 12volts so will not be seeing enough voltage to give a decent spark. Live thread here:

http://www.tr-register.co.uk/forums/index.php?/topic/49215-hot-coil/

 

SUs are wonderful devices - I always suspect them last of all.

But its possible that the stuttering upon accleration when hot, but not cold, is their pistons not being damped fully as the oil thins with underbonnet heat. Try engine oil in them, or even axle oil. The damper gives a richer mixture transiently upon opening the throttle suddenly. If it is not damping properly the engine can suffer a 'lean stumble'.

However, I would excect the stumble to be only brief, a few seconds, not prolonged stuttering.

 

That fast road cam might be the culprit. At low rpm the overlap - when both inlet and exchaust valves are on at the same time- might be allowing exhaust gas back-pressure to upset carburation and combustion. But I would not expect that to come in only when the engine has heated up. Maybe owners with experience of that cam will advise?

 

Like the Lagonda !

Peter

Edited October 23, 2014 by Peter Cobbold

[Steven_nl]

Thnaks again Peter. My friend is reading this as well ;-)

 

"that electrickery thing".........you don't look like Catweazle

Edited October 23, 2014 by Steven_nl

[Steven_nl]

[Peter Cobbold]

That dates you. He's a bit after my time !!

 

When it comes to sparks I admit to being a Luddite.

Although I am going to give wire plugs a try:

http://www.globaldenso.com/en/products/aftermarket/plug/plus/features.html

And this if it becomes an after-market fitment:

http://articles.sae.org/12102/

 

Peter

[Steven_nl]

1958

[Peter Cobbold]

War baby - just.

[Steven_nl]

Which war would that be? Lol. I'm just a wippersnapper. ;-)

[Ragtag]

1958

 

A very good year

[Ragtag]

Hello Steven,

 

A couple of points to add from experience and too many years reading about ignition problems on this forum.

 

Your car is a 1974 so it should have a ballasted ignition wire and a matching coil. As stated by Peter, this means the ignition is running at 8 to 9 volts (12 volts on starting). The earlier cars ran on 12 volts with an unballasted coil. You also stated that the previous owner had completely restored the car and upgraded the ignition. This is often where problems occur becuse it introduces uncertainty for later owners. For example, the previous owner may have removed the ballasted ignition wire and replaced it with an unballasted version. The key point is that your coil should match the source volts.

 

A 12 volt coil with a ballasted source will run but not deliver the best spark. A 9 volt coil with a 12 volt source will run but will overheat and eventually break down. You need to determine what voltage your ignition source is delivering and make sure you have the matching coil for that level.

 

Finally, if you are planning to fit the Aldon/Pertronix unit in your distributor, you must have a 12 volt ignition source for it to work properly, according to the manufacturers instructions. This was exactly the issue I had with my car when I bought it. The PO had fitted Pertronix but at the same time had fitted a ballast resister and coil to match so the ignition was only running at 9 volts.

[Steven_nl]

Hi Ragtag.

 

Thanks for you help!

 

Allthough I don't know what a balasted source is, I'm sure my friend René will know. We will check this. I do know I have a 12V Lucas sports coil.

[Peter Cobbold]

Steven,

Photo of ballast resistor wire in the loom here:

http://www.tr-register.co.uk/forums/index.php?/topic/29748-ballast-resistor/

Peter

[Steven_nl]

Thnaks Peter.

There is a good set of Moss video's on youtube. I understand the difference now. I'll take a look. seems it is possible to measure the difference as well.Moss has a video on that too.

[Steven_nl]

@ragtag. So if you use a petronix you need to use a 12v coil. If i understand right this will leave you with a bad cold start?

[poolboy]

The Pertronix Ignitor needs to be fed a minimum of 8 volts.

This is printed on the Pertronix box:

The coil, if still being fed by a ballast resistor wire (9+ volts) once the engine is running, needs to have 1.5 ohm resistance.

If your ignition has been modified so that your coil is being fed 12 volts, your coil should have 3.0 ohm resistance...The Lucas Sports Coil is usually gold in color and is model DLB105

 

The distributor for the US 74 TR6 did not have a Vacuum Advance..It did have a Vacuum Retard module, mounted on the cockpit side of the distributor.

Quite a few people disable it..Even if not disabled, it only had the ability to retard the ignition timing when the rpms were 1100 or less, in other words, when the engine is idling.

[Ragtag]

@ragtag. So if you use a petronix you need to use a 12v coil. If i understand right this will leave you with a bad cold start?

 

Well according to Poolboy, my information is incorrect. Although I don't have the benefit of the original box, I'm pretty sure I downloaded the data sheet from the Aldon website. What I can say is that on my early car, going back to 12v made a huge difference. I too have the lucas DLB105 coil. It may have been a coincidence, of course and maybe my ballast resistor or Pertronix coil were faulty all along.

 

Having said that, you have a 12v coil (which is fine) so you need to confirm that the ignition is delivering 12v for it to be effective.

 

So far as cold start is concered, yes, the car will be harder to start (in theory) but the early cars never had this facility anyway. Do you leave your 6 outside in all weather or keep it in a garage? Modern batteries are much better than they used to be so I'm sur it will be fine.

[poolboy]

Chris, I have known of some who have the ballast resistor wire feeding the coil the 9 volts, connect the red Pertronix wire to the ignition switch. On the US cars that I'm familiar with, that would be the white wire side of the fuse for the green wire circuits.

On my 74, I ran my Pertronix off the ballast wire side of the coil, though. It never gave me any problems.

[TriumphV8]

I couild not explain to him why the vaccuum advance was disconnected on my TR6 and some friends in France (Jaguar and Lagonda owner) were baffeled as well. Can anyone shed some light on this?

 

Oh yeah one last question. is it normal that my TR6 doesn't have a fuel pump near the petrol tank?

 

 

I think Peter already pointed out perfect how to deal with the distributor.

 

I can give details why vacuum was disconnected:

The cam is not made for use with 2 carbs with the result that you get a stumbling somewhere between 1000 and 2000rpm

with very low load. That can be deleted to some extend if the vaccuum is disconnected and with rich mixture.

The reason is because you have to press the pedal a little bit more down to achieve same speed

and avoid that bad throttle opening position.

This affects cams of 280 up to 290 degrees where more is not used with two carbs normally.

 

Anyway the vaccuum of such a cam is poor, significantly less than a stock cam and will pull the vaccuum advance box

in a very different way. That is why you can get different boxes and normally must choose a box that matches the cam.

So the expert took the easy way and disconnected what might disturb only.

Hopefully he set the timing to 30 degrees at full centrifugal advance what should be checked

as the marks on the pulley should be checked, too.

I would expect with the fastroad cam the PI compression otherwise its a bad configuration.

 

But the problem that the engine runs bad after a time in my opinion is different from all these topics.

For the understanding: Too late ignition like due to missing vacadv lets the fuel burn in the exhaust

heating that up but thats it. It does no harm to the engine as too early would do. That would burn inside

the engine with hammering on the piston without doing work and heating all up.

 

My attempt to the problem would be to check wether the problem is fuel or ignition related:

Hopefully you have SUs? Drive until stumbling starts and cut off ignition immediately.

Open fuel bowl and look for fuel level. Is it like normal?

 

If it is the ignition is more suspect, else fuelpump or tank ventilation, fuel hose, etc. could be checked

 

Fuel pump has a good position at the tank but normally works properly in the front if not positioned too high.

 

Good luck!

andreas

Edited October 26, 2014 by TriumphV8

[Peter Cobbold]

Andreas,

Thats interesting: that cam is not good with twin SUs.I see you have Efi !!

You pointed out that "....Anyway the vaccuum of such a cam is poor, significantly less than a stock cam...." So I'd suggest Steve should think about the carbs too:

At low rpm the overlap on the cam leads to sudden surges in exhaust gas back-flow as inlets open when the exhaust are still open, followed by a correct air flow once the exhaust closes. So the SU piston sees rapid,widely fluctuating pressures. Others may have come across this problem so I am only guessing at answers:

Steven-nl might think of welding on a much bigger balance pipe across all six manifold stubs? - to redcue the fluctuations. And lighten the SU spring,to help the piston lift, and use much thicker oil to dampen the fluctuations. The needle will also need tuning.

As well as doing the 'plug chop', Steven should try richening the carbs to see if the stumble gets better or worse.

 

It's very difficult to be sure what mixture an SU delivers when the air flow fluctuates rapidly and the piston has too litttle time to respond. But I think it will deliver a richer mixture. ( Cant be certain on this). But it does this at the expense of air flow, because the poor piston lift obstruct flow. Which is why more throttle pedal is needed. So the engine may be starved of mixture volume. But screwing the jets several flats richer or leaner should show if the mixture is wrong. But mostly I think it's the lack of manifold depression, caused by overlap, that fails to lift the SU pistons enough at low rpm. That starves the engine of mixture volume.

 

Steven : I suggest starting a new thread about twin carbs with fast road cam - someone will know the answer to setting up the SUs. They may be able to tell you spring rate, needle profile and oil viscosity - and with that the SUs are sorted.

 

Peter

Edited October 26, 2014 by Peter Cobbold

[TriumphV8]

To ask others is a good idea and I agree, if you change cam the carb must be adapted, too.

But I am still convinced that what Steve is claiming is caused by a defect, not a bad setup.

 

You are right, I am using EFI on the CP PI-manifold with some improvements on more precise linkage.

Idle and low rev response is nice with 290 degree but on the log you can see what a real trouble is caused in the

manifold air pressure sensor for metering. It needs a lot of mechanic and electronic smoothing to

get a reliable signal to use. If cam gets hotter beyond that it would be necessary no longer to rely on MAP

but take engine revs and angle of throttle for metering (alpha-n setup).

 

I think many use the PI or hotter cam with twin carb.

First we should ask if Steve has SU or Stromberg because Stromberg is normally fitted.

As far as I know for the Stromberg different neddles are limited.

 

I did some setup with Innovate wideband controller

on similar engines with twin SU.HS6.

Not easy to understand for me was I ended with different results on

two nearly identical engines.

For both the BAM needle was close to be okay but one was at the top end too lean

the other one was too rich.........

 

So buying Innovate & BAM is a good point to start with a cam around 280-290 degrees

but it needs also some experience to do the right things to investigate.

 

Unfortunately from my experience one has to separate the inlet runners, not connect them better.

The pulsations disturb the cylinderfilling of some other cylinders:

The more are connected the more starts the trouble.

 

That is why Triumph used six butterflies in the PI.

All day drivers today have a single throttle like BMW 328.

If it comes to the M3 they had to change to 6 butterflies.

Every motorbike has individual throttles,

most modern cars with smooth cam have the single throttle.

 

So its not a metering problem of the SU or EFIs but it is caused by pulsation inside the manifold

and the only remedy at a given twin manifold to avoid rough idle is not to overcam.