both ways work Panzer

at idle the throttle plate restricts the air flow and you have the maximum vacuum you will achieve in the engine (other than full deceleration from speed). as the throttle plate opens (accelerator pedal is depressed) then the vacuum drops - at full throttle - theoretically - you have no vacuum in a normally aspirated engine. turbo engines start boost at about 2k revs and build up a positive pressure in the manifold.

In reality however the vacuum in most engines is still present at WOT (wide open throttle) as there is a parasitic flow effect from poorly machined and rough surfaced intake castings. Also most cars are built so that the throttle body aperture restricts the air flow to limit the engine so even at WOT there is still a slight vacuum. you can get to an almost break even point by opening up the throttle body and polishing the inside runners of the intake but you would then have to limit the revs in some way unless you start to move to forged rods - moly rings etc.

try Grahams site for vacuum diagnosis - I used mileposts garage

http://automotivemileposts.com/garage/v2n8.html
and the live link to the diagnostics is halfway down the page, just click and it opens a new page with symptom and diagnosis

cheers
barri

__________________
61 Austin mini
67 Lotus 7
74 450sl
76 Cadillac 8.2l (501 ci)

some new cars

megasquirt conversion on:
djet 74 450sl http://www.mercdjetmegasquirt.britautorepair.com/
cis 76 450sl http://www.merccismegasquirt.britautorepair.com/

the best view is always from the point of no return

just thought I would post a quick update. the mechanics seem to be fine, no valve damage, valve timing where is should be the exhaust flows free from the manifold joint back. came in for my lunch so I will put her back together this afternoon and recheck the ignition timing and the pertronix and if none of that gets me anywhere plan to start ripping into her next weekend. The 117 on the trolley is the one I'm just starting to convert to EDIS - will go into my 78 450slc

the left (driver side cam mark US) the angle makes it look a bit off but its on the mark


and the rhs cam mark (passenger side US)


and the crank showing maybe 2 to 3 degrees off TDC (the camera angle may make it look better or worse - its difficult to get the camera close enough)


and then just for interest the 117 ready for conversion (when I have time)


will keep you updated
cheers
Barri

__________________
61 Austin mini
67 Lotus 7
74 450sl
76 Cadillac 8.2l (501 ci)

some new cars

megasquirt conversion on:
djet 74 450sl http://www.mercdjetmegasquirt.britautorepair.com/
cis 76 450sl http://www.merccismegasquirt.britautorepair.com/

the best view is always from the point of no return

Sure, vacuum drops sharply as the throttle is opened (that's why you toss in a bunch of fuel with a carb accelerator pump or an extra FI shot) but rises back to the normal working values listed for a healthy engine when engine speeds rise and stabilize during acceleration...whole system won't work if it doesn't. Floor board it and vacuum will stay low, but if it just drops steadily and the engine dies out, you have a problem with a constricted exhaust...if it can't get out of the cylinder, nothing can get in either.

The vacuum modulator is designed to operate around the normal parameters displayed by a health engine; vacuum drops as throttle opens during acceleration and in response the trans shift stiffens up to allow firm shifts at the higher RPM. This reduces slippage and raises shift points to optimize acceleration of the car.

and now for a completely different question

has anyone ever heard of the distributor slipping one tooth whilst the rest of the timing stays in place.

I checked everything this weekend and everything is perfect but the distributor seems to be one tooth out of alignment with the rest of the components. If I run the dist 30 degrees off (right at the current end stop) at idle the engine starts to build vacuum and runs fairly well right up to 5K - not perfect but drivable - I have never heard of one of these distributors skipping a tooth though.

any thoughts welcome - here is a picture of the set up in case you dont remember what it looks like but the chain drives a cog just below the driver side (US) cam that drives a shaft to run the spiral gear that runs the distributor.



I pulled the manifold on sunday though and physically checked for vacuum leaks, checked compression etc etc nothing is wrong other than the distributor appearing to be 1 tooth out, even the little mark on the side of the rim of the dist body no longer lines up with the #1 position of the rotor arm.
cheers
barri

__________________
61 Austin mini
67 Lotus 7
74 450sl
76 Cadillac 8.2l (501 ci)

some new cars

megasquirt conversion on:
djet 74 450sl http://www.mercdjetmegasquirt.britautorepair.com/
cis 76 450sl http://www.merccismegasquirt.britautorepair.com/

the best view is always from the point of no return

barri,
In order to get to close to 30deg BTDC at 3000rpm, I had to rotate the distributor to the end of the slot (others with same engine have had to do same). If I recall correctly, I achieved max vacuum just before the end of the slot. I think I currently have it set back slightly from end.

I doubt you skipped a tooth. Can't see that happening. But perhaps the distributor clamp slipped causing the timing to retard?

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5

It probably did not slip...someone probably removed it in the past for some reason and then failed to get it re-installed properly...happens all the time.

Check it: Slowly rotate the engine to the 5-7 degree advanced of TDC mark on the crank pulley (I believe this is the approximate spot for your stock timing setting when checking it W/O vacuum connected) and then check the position of the rotor with respect to #1 plug wire in the cap. Do this with the distributor set at the mid-point in its adjustment range. Rotor should point approximately at #1 plug when it is correct. In order to get to the ideal 28-30 advance position @ 3,000 RPM, you would normally need to rotate a properly installed distributor all the way to its farthest advance position...against the end of the slot...when all is correct. If your check does not show this, then someone did R&R the distributor and then re-installed it wrong. Just loosen it, lift it up to dis-engage from the lower gear and carefully rotate it one tooth advanced to re-align it properly. Set it back down solidly and clamp it and then check/set the timing you need.

May work to solve your problem...as they say, stranger things have happened.

I notice that there is a driven gear that the distributor worm gear is attatched to, any chance that there was a timing chain change and that gear was dragged a tooth out by the chain coming thru?

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

Hi 230 and Panzer
I rebuilt the engine myself about 15k miles ago. Its been running just fine since then with no problems. Whatever happened - happened suddenly after a 70mph run down the freeway. But your both right in that it does look like something has moved in the timing.
I'm now busy disassembling the distributor to see if anything is wrong inside it. It will keep me young and handsome at the very least!
cheers
barri

__________________
61 Austin mini
67 Lotus 7
74 450sl
76 Cadillac 8.2l (501 ci)

some new cars

megasquirt conversion on:
djet 74 450sl http://www.mercdjetmegasquirt.britautorepair.com/
cis 76 450sl http://www.merccismegasquirt.britautorepair.com/

the best view is always from the point of no return

Is it a solid state ignition system? I wonder if the rotor may have shifted over time, or during a sudden acceleration and deceleration of that shaft...like free spinning a drill chuck open or closed through centrifugal force

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

I have a pertronix on this car and from what I can see so far it looks like the reluctor, which has a spring clip on the top to secure it, has slowly slid up the shaft and pushed the rotor upwards. its still all apart as I removed the plates to check the advance weights while it was out. I will post pictures this weekend when I put it all back together.
hopefully thats the problem but will have to get it back together to see. it would explain the movement Ive been seeing in the timing above 2500 rpm.
cheers
Barri

__________________
61 Austin mini
67 Lotus 7
74 450sl
76 Cadillac 8.2l (501 ci)

some new cars

megasquirt conversion on:
djet 74 450sl http://www.mercdjetmegasquirt.britautorepair.com/
cis 76 450sl http://www.merccismegasquirt.britautorepair.com/

the best view is always from the point of no return