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Thu Nov 05, 2009 5:19 am

Can't say I know much about timing, and have searched the forums and not found the information I need. I have a '77 bus with vacuum advance distributor (dual carbs, know now to look for the distributor model on the front of the unit?).

Bus running a little less well than usual especially pulling up hills in high gear.

Followed the Idiot Book and did the valves, and then set the point gap and timed it statically (7.5*BTDC). Sucked on the hose attached to the vacuum canister and the points didn't move at all. Attached a strobe light through the #1 spark and revved the engine. The notch in the pulley advanced perhaps 20-25* but didn't have tach meter to note rpm. Disconnected and plugged the vacuum hose and did same again: notch in pulley advanced same as before with vacuum hose attached.

Pushed the points round (as suggested in Muir's book) and plugged the port to the vacuum canister with my finger but points returned though with audible sucking noise from vacuum canister.

Took bus for a run through the hills. Starts and idles and runs well, and has fine acceleration except when going up the hill in high gear when engine begins making ugly rattling sound (like a rattle-snake, is this 'pinging'?).

A few questions: I'm assuming the distributor is a vacuum-advance only model. Why would the timing advance with rpm when the vacuum hose is disconnected and plugged? Is the ugly rattling sound (pinging?) going up hills a possible result of a leaking/ruptured vacuum diaphragm? How can I be sure I've diagnosed a faulty vacuum canister correctly? What are my options if the vacuum diaphragm is bu**ered?

Thanks for any help.
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Thu Nov 05, 2009 7:07 am

The distributor is a machanical/vacuum advance. If you are sucking on the hose and the plate is not moving your vacuum canister is shot. The reason it advances when it is reved is the machanical portion of the dist is still in operation. The pinging is the result of the dist. not advancing as needed under load. :)

Thu Nov 05, 2009 7:21 am

if it holds a vacuum it is still good and the breaker plate may need oil and cleaning. If it doesn't hold a vacuum then it is bad.

Thu Nov 05, 2009 7:21 am

Are you assuming the distributor is a mechanical-vacuum advance based on what I said about the timing being advanced with the vacuum hosed disconnected? Or are mechanical-vacuum advance distributors more usual than a vacuum-advance only distributor?

Is the 'rattle' I heard 'pinging'? It didn't sound anything like a 'ping' to me.

In the last post, you say if it holds a vacuum then it may be ok. I pushed the points round with my finger and squished my finger over the port to the vacuum canister. The points returned immediately to their usual position.

From what I've read, repairs to the vacuum canister aren't an option. How difficult would it be, do you suppose, to find a replacement part?

Thanks for the help, by the way.

Thu Nov 05, 2009 7:21 am

Wow, so many little things in this post that could add up to big problems.

First of all don't use the Muir book as your sole bible for maintenance, you need a Bentley manual too.
You need the numbers off the side of your distributor before you can find out anything regarding parts or timing specs.
Timing to +7.5 may have worked fine 30 years ago when all the parts were new but today the only reliable method is timing to +28 degrees at 3000+ RPM with the hoses off. STATIC TIMING WILL NOT WORK ON A TYPE 4 !!
To test the diaphram suck on the port and watch for plate movement, then stick the tip of your tounge on the end of the port and see if it holds a vacuum. If it won't move or bleeds off your diaphram is toast.

Thu Nov 05, 2009 7:36 am

does your dist have one hose or two?
two hoses=DVDA one hose=SVDA

here's a new SVDA can: http://www.aircooled.net/new-bin/viewproductdetail.php?keyword2=IGD0007&cartid=1105200945462403

Thu Nov 05, 2009 8:01 am

I was able to bring a vacuum can back to life. I bought a used SVDA dist. for a spare. This dist. was sitting on a shelf for a while. When I tested the vacuum can with a vacuum pump, it would bleed back to zero in less than one second. I squirted some Armor All in the vacuum port, shook it up, and let it set over night. The next day it held vacuum. I just tested it again and it still holds vacuum. I don't know if it will work for you, but it saved me $50.

Thu Nov 05, 2009 8:25 am

Althea78 wrote: I was able to bring a vacuum can back to life. I bought a used SVDA dist. for a spare. This dist. was sitting on a shelf for a while. When I tested the vacuum can with a vacuum pump, it would bleed back to zero in less than one second. I squirted some Armor All in the vacuum port, shook it up, and let it set over night. The next day it held vacuum. I just tested it again and it still holds vacuum. I don't know if it will work for you, but it saved me $50.

That's good for you, but if it was me I'd be worried about the longevity of said "repair".

I'd spend the $50.

Part number 021905271F from bus depot, distributor vacuum unit.

Edit: just noticed the link posted above. $30 from aircooled.net

Thu Nov 05, 2009 10:25 am

busdaddy wrote: Wow, so many little things in this post that could add up to big problems.

First of all don't use the Muir book as your sole bible for maintenance, you need a Bentley manual too.
You need the numbers off the side of your distributor before you can find out anything regarding parts or timing specs.
Timing to +7.5 may have worked fine 30 years ago when all the parts were new but today the only reliable method is timing to +28 degrees at 3000+ RPM with the hoses off. STATIC TIMING WILL NOT WORK ON A TYPE 4 !!
To test the diaphram suck on the port and watch for plate movement, then stick the tip of your tounge on the end of the port and see if it holds a vacuum. If it won't move or bleeds off your diaphram is toast.

To add to Busdaddy's post here, the exact RPM isn't important so you do not need a tachometer, what you want is your timing to be at full mechanical advance (hoses off). Just rev the engine up until the timing doesn't advance any more with higher RPM, this is full mechanical advance.

It does sound like you may be getting pinging or worse knocking. Do you have the stock timing scale and are you sure you are using the correct notch on your pulley? If you have an engine that is pinging or knocking you will greatly reduce its life, maybe down to a few days or even hours of driving.

Thu Nov 05, 2009 4:40 pm

Didn't realize you can simply replace the vacuum canister. I'll have to look into that. The distributor has one hose, by the way.

To quote busdaddy - static timing will not work on a type 4. What about a type 2?

Thu Nov 05, 2009 5:18 pm

Become one with your motor. Do the timing by ear. The bus should still run pretty decent without the vacuum advance. Well at least on flat ground.

Thu Nov 05, 2009 5:20 pm

Quote: Become one with your motor. Do the timing by ear.

do not follow this advice. you cannot time your engine by ear, it is quite possible/likely it will be wrong and this can damage the engine.

Thu Nov 05, 2009 6:06 pm

tomfreo wrote: To quote busdaddy - static timing will not work on a type 4. What about a type 2?

WTF? He's talkin' bout an engine, not a body style.

Thu Nov 05, 2009 6:41 pm

73kombi wrote: tomfreo wrote: To quote busdaddy - static timing will not work on a type 4. What about a type 2?

WTF? He's talkin' bout an engine, not a body style.

Yeah what he said, most Aussie busses come with a 1700/1800 or 2000 after 1972 don't they? Those big aluminum cased engines with an oil filter, down exhaust and the fan on the end of the crankshaft are called type 4's, even if they are installed in a bus. Even if it is a type 1 static timing ot timing by ear will kill your engine in very short order, don't do it.
Static timing is OK for an initial startup after distributor replacement but you'll want to do it properly with a light before you leave the driveway.

Thu Nov 05, 2009 6:42 pm

Be nice Kombi...

The bus is a type 2. It has a type 4 motor in it. Busdaddy's advice applies to your bus.

Thu Nov 05, 2009 10:41 pm

Quote: do not follow this advice. you cannot time your engine by ear, it is quite possible/likely it will be wrong and this can damage the engine.

Alright! You have a point, most people cannot time their engine with their ear. I can though, every time I have done it I have checked it with a timing light and been spot on 7.5 BTDC. Luck? I dont think so I just have a good ear for timing, I know exactly what I should be hearing.

Thu Nov 05, 2009 10:51 pm

Nica -you are just a lucky kind of person. Go buy a mega-lottery ticket next time you do it sucessfully.

Thu Nov 05, 2009 10:56 pm

Hahahaha :lol: Last week after putting new points in I should have!!

Thu Nov 05, 2009 11:22 pm

I will make it sound more logical, yet I now will not condone this procedure because I do not feel like getting the rap when someone blows their engine because they followed my advice. Anyways...
The valves "clack" and they "clack" in a different tone or volume depending on where the timing is. When the timing is too advanced they "clack" louder in idle than normal and if the timing is to retarded the motor sounds weak in the tone coming from the carbs when you rev it. Now its the musical balance of "clack", the tone from the carbs and meticulous attention to the overall tone of the motor that makes timing with the ear possible if you memorize all these details when the timing is properly adjusted. Then a test drive is involved, because proper timing is also felt on the gas pedal. :wink: I perfected this by having way to much time on my hands.

Thu Nov 05, 2009 11:46 pm

you sure have the Zen thing down to a science. OOOOOOOOOOOOMMMMMMMMMMMMMMMMMMmmmmmmmmmmm ... I hear the valves talking to me .... they are saying .... wait, .....wait ... we are at 7.345698 BTDC, crank us up some more.

Amazing. I will listen next time I set the timing.