Vacuum advance on a 1500

[bigjones]

Yeah, vacuum advance - a mysterious topic.

This 1500 has a Pertronix ignition and I replaced the ZS with a single carb meant for an MGB. Other than that it is stock.

OK, so I'm trying to tune it up. I'm, following Barney Gaylord's article:

<https://mgaguru.com/mgtech/care/cf120.htm>


With the vac adv line plugged with a golf tee, I set the timing so that it is about 20 deg advanced at idle. It goes to about 34 deg when I apply max throttle.
So far so good.
However, when I re-connect the vac advance it goes to about 44 deg with max throttle. This is bad because Barney says there will be some advance but it will go away with max throttle.

Now then, the ZS had a port on the carb for the vac adv. The MGB carb doesn't, so I hooked up the vac line to a port (approx 1/8" pipe) on the intake manifold (the one that originally was hooked up to the anti-run solenoid). Didn't I read anyways that manifold vacuum is preferred over carb vacuum?

I was just going to leave the dang thing plugged but I thought I'd ask you guys.

Cheers!

 

[dklawson]

Hi Adrian.

As you have found out, there is ported and manifold vacuum. The distributor will be designed for one or the other and the vacuum advance won't work properly connected to the wrong one. Chances are your distributor was designed for ported vacuum while the connection you made was for to manifold vacuum. Your connection to the manifold will be causing the excessive advance. It is probably possible to drill the SU for ported vacuum... it probably has a boss or recess for a brass tube that would normally be for that purpose. Perhaps the guys on the MGB part of the board could tell you how and where to drill. If not, maybe one of them has a carb body that is already set up for ported vacuum that you could switch for yours.

However, the vacuum advance is really there for fuel economy during part throttle cruising. If your goal is not better fuel economy you could just plug the vacuum connection and set the maximum dynamic advance to about 32 BTDC. Just a thought.

 

[bigjones]

Hi Adrian.

...The distributor will be designed for one or the other and the vacuum advance won't work properly connected to the wrong one...

Ah, hah!
That's the ticket!
Thanks Doug for clearing up that mystery.

You are right again in that this carb does have a boss that looks like it could be drilled for a pipe.
I'll probably leave it alone though!

I'll leave the vac line disconnected and plugged.

Cheers!

 

[JPSmit]

According to Jeff Schlemmer (Advanced Distributer) 1500's had Vacuum Retard rather than advance due to emissions requirements. He suggested I disconnect mine - which I have. I think it runs better. Certainly doesn't run worse.

 

[bigjones]

...1500's had Vacuum Retard rather than advance...

Well, that certainly puts the cat amongst the pigeons.

I'm pretty sure that on one of my dizzies the vacuum advances the timing but that might not be an original dizzie.

I'll report back after some investigation

Thanks for the post!

 

[dklawson]

Did the MG version of the 1500 have a Delco or Lucas distributor?

 

[Morris]

I want to clear up something Doug said (sorry Doug). Your distributer does not care if your vacuum signal is ported or not. The only difference between ported and non ported is whether vacuum advance is applied at idle or not. Ported vacuum is an emissions thing. It's purpose is to reduce NOX (IIRC) at idle by allowing a lower idle advance.

20° at idle and 32° at WOT is a touch high for a 1500 IMHO. Also when testing WOT on a parked car, you only get a fraction of a second of true WOT before you redline and have to close the throttle a bit, so your test results may be inaccurate. You cannot test WOT advance without a dyno.

According to your tests, you are getting 14° of mechanical advance. If you connect the vacuum advance to the manifold and shoot your timing at idle again, this will give you your total vac advance. Your potential total advance will be vac + mechanical advance.

Where it me, I would set the static idle advance to 15°. This will give you max drivability, economy and engine safety. More advance can mean more umph, but without precision control, you can reach the law of diminishing returns (and holes in pistons) real quickly.

 

[bigjones]

Morris,
Thanks for the added information.
Let me ponder it awhile.
I'll take your suggestion and set the timing to 15 deg at idle (800 rpm) with the vac line connected, Is that what you meant? I'm thrown by your use of the word "static". Doesn't that mean without the engine running? This dizzie has a Pertronix unit so I can't do a "static" timing adjustment.
Cheers!

 

[bigjones]

Doug,

The 1500 Midget had a Lucas distributor:
45D4 for UK (points ignition)
45DE4 for US (electronic)

Cheers!

 

[bigjones]

OK, I've changed my mind.
I'm leaving the dang vacuum line plugged.

This is the reason:
Engine fully warmed up and idling at the recommended speed of 800 rpm the vac line pulls a rapidly fluctuating 17-20 psi (according to my trusty MityVac).
Seems a bit high to me though I have zero basis for that
I set the timing to 10 deg BTDC. Then I connected the vacuum line and it shot up to 22 deg BTDC.
Seems wrong to me.

Anyways, I set it to 15 deg at idle, as per Morris's suggestion, with my super-duper timing light (thanks for the recommendation Doug). Went for a test drive and it has improved dramatically. Still have a bit of a "miss" now and again but overall I'm super chuffed. To prove it I just made a modest contribution to Basil's efforts - are you proud of me?

Cheers!

 

[dklawson]

Morris, no offense and no problem, my mistake.

 

[Morris]

Sorry... I should not have used the term "static." What I meant was timing without the vacuum line connected.

If your vacuum is fluctuating that much, you may want to consider a leak-down test. Google it if you are curious.
https://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/
https://www.harborfreight.com/cylinder-leak-down-tester-94190.html

There are some pretty helpful comments on the harbor freight reviews.

 

[Morris]

Also... according to your experiment, your total vacuum advance is 12°. So your max potential advance is 26°. With a base timing of 15°, that will give you a partial throttle cruise advance of 41° from roughly 3000 rpm up. As a frame of reference, I run mine at 45° under those conditions.

In short, you should be fine with or without the vacuum advance. Just keep a sharp ear out for pinging. It will sound like someone tapping your engine with a small hammer. I think pinging is unlikely for you, but if you hear pinging, just remove the vacuum advance.

 

[bigjones]

Morris,
Thanks for the additional information on the timing. I'll just keep it at 15 without the vacuum attached.
I'll being doing a leakdown test and I'll report the result, in another thread.
Many thanks!