Which vacuum port on Edelbrock carb?

[Rog's troubleshooter]

A little help/advice, please? I'm trying to troubleshoot a MkII which is developing no real power at all. I'm all set to do compression tests etc, but while checking the timing I noticed that the vacuum advance pipe is connected to the "manifold" port on the carb, rather than the "timed" port, which the Edelbrock manual says is correct for a non- emissions controlled engine. That does mean a lot of advance at idle, though - about 35deg including the 12deg initial timing without the vac connected.
Any advice on whether that's good advice on the 383 when used in a Jensen?
(I have tried it via the other port, and it seems to make little or no difference to the running of the engine- which might suggest I've not yet found the real problem!)

The car starts easily and fairly evenly, runs without overheating or any smoke, I had it on a rolling road and the mixture seems reasonable, but it seems to be developing (if that's the right word!) about 175bhp..........

If this was one of the engine types I'm more familiar with I'd be checking the cam timing now, as the symptoms suggest the cam's timed late........

As always, any helpful thoughts greatly appreciated!

John

[thomaslk]

maybe the vacuum canister is not working at all - therefore no real difference which port you use. But a defunct vacuum canister (most common failure is leaking/broken diaphragm) will not have influence on max power output as it is designed to work under part throttle conditions only.
As you said next step is checking timing / timin curve. Carb check recommended as well (incl fuel filter, fuel pump/pressure, float height) check also if butterflies are fully open when pedal is down - sometimes the adjustment is slipping.
BTW i tried both ports and my car likes the full vacuum (not timed). Carb tuning for both ports is different of cause.
have fun
Thomas

[grottenolm]

Maybe an easy approach to sort out a vacuum issue with the brake reservoir: connect the carb directly with the brake booster through one pipe. This can tell you if the brake reservoir is leaking and therefore not enough vacuum is available.

[Philip Lochner]

Vacuum can not (and should not) add any power to an engine - certainly not at large throttle openings.

Vacuum can only "add" power at partial throttle, by starting the fire earlier and thus allowing more time for the fire to achieve a good burn and thus extract the most energy from it. Vacuum thus only makes the engine feel livelier and reduces fuel consumption AT PARTIAL THROTTLE.

You thus need to connect the distributor's advance diaphragm to the point that offers "ported vacuum", ie vacuum that only appears at the port when the throttle is slightly opened.

The first thing to check is if the distributor's centrifugal advance mechanism is working correctly. This advances timing ONLY as a function of RPM.

Another aspect you should also consider is cam wear which is more likely than incorrect cam timing. Many flat tappet engines (such as our Chrysler V8's) suffer this problem. From what I read, it seems that accelerated cam wear results from using (modern) oils that has insufficient "ZDDP" (Zink and Phosphor), molecules which act as sacrificial lubrication between the cam and the lifters. Cam wear can also result from using a cam with no or insufficient "Nitridiing" (hardening). If a cam replacement is required, the lifters should also be replaced with new. Our engines need SF or SG spec oils - "old" oils that has lots of ZDDP.

[Rog's troubleshooter]

Many thanks for the replies, everyone, looks like I need to get to the real problem with the lack of power before I worry about this.

Checks so far that haven't helped:
Vacuum unit is fine, I can apply vacuum and watch the timing change;
No vacuum leak that I can detect;
Centrifugal advance works fine, and it has an advance-adjustable distributor with electronic ignition- I've set it to give 35deg max at 2500rpm;
Carb is clean, adjusted correctly and the mixture shows OK on a gas analyzer;
Iridium plugs are fitted, with silicon leads;

The engine doesn't misfire, though it does feel to be "missing" one (or more!) cylinders. It idles smoothly enough, and cruises OK - it just won't "pull" under acceleration!

Checks for the weekend are Compression test, valve timing checks, and if they show nothing, individual plug shorting tests.
Any other useful checks I could do?

Might I be better starting a new topic on "poor power from 383?" or similar?

Thanks again for the help,
John

[Philip Lochner]

The engine doesn't misfire, though it does feel to be "missing" one (or more!) cylinders. It idles smoothly enough, and cruises OK - it just won't "pull" under acceleration!

What you are describing sounds like an engine that does not breathe. Either worn cam or excessive build-up of carbon behind the plugs.

It runs fine when small amounts of air is needed, but when a lot of air 8s needed, its not there.

[taximan]

When I replaced my cam and followers it released a good 100 plus horses, that's only a guess but it wheelspan up the drive for the first time in my ownership. Before the swap it was adequate and ran ok, the cylinder compression rose by 30% across all 8 (heads being reconned may have helped this)
Shaun.

[Philip Lochner]

One more thing, it is possible for a cam to be badly worn but compression test is still ok, because the engine turns at cranking speed only . Only when the cam wears to the point where the inlet valve hardly opens, will the compression test start showing poor readings.

Its quite easy and quick to visually inspect the cam on these cars. Remove intake manifold, remove valley pan, inspect cam.

[thomaslk]

... you are definitely on the right track - do not forget to check fuel availibility/pressure at high loads/WOT.
Everything might be ok at idle/low load - but if you really ask for lots of fuel (and that is what a 383 does) there might be "undersupply" because of fuel pump/line/filter issues. I would recommend to disconnect the fuel line from the carb and check if the supply is within specs during cranking. Cold engine department recommended during the test
rgds
Thomas

[AndrewP]

John,

Before you delve in too deep, what is your total advance at 3500 rpm without the vac unit disconnected?
If you do some searching the general number mentioned is 36 degrees.
On my 383 with standard cam and dissy weights settles at around 16 at idle. Any less and with the Shell 98 we get here it's a bugger to start and knocks when giving it the beans.
I ended up fitting lighter dissy springs which makes a huge difference to the 'peppi-ness' of the engine. Worth doing in my opinion.

Cheers

[Philip Lochner]

I ended up fitting lighter dissy springs which makes a huge difference to the 'peppi-ness' of the engine.

Its helpful to explain WHY the lighter centrifugal springs made the engine more "peppi" Andrew?

[thomaslk]

IF we look to the original question - missing some 100 horsepowers - i am really sceptical if we will find them "in the vacuum" or related to lighter/more aggressive advance. John wrote his total ignition at high revs is about 35 degr and this should get all ponies up to speed ignitionwise.
Optimization of advance curve via centrifugal or vacuum advance could definitely improve engine behaviour/characteristic but will not locate where the ponies are hidden.

rgds
Thomas

[Steve Payne]

When was the gearbox last refurbished? A worn or tired gearbox and torque convertor can consume a lot of the engines power.

I have fitted refurbished gearboxes to cars that were feeling flat and had a new lease of life as soon as the refurbished gearbox and convertor were installed.

Just because the fluid is the correct colour and it changed gear does not mean it is in good order.

Steve

[AndrewP]

I ended up fitting lighter dissy springs which makes a huge difference to the 'peppi-ness' of the engine.

Its helpful to explain WHY the lighter centrifugal springs made the engine more "peppi" Andrew?

Hi Philip,

Ive tried to sit and think here at work why it does what it does, and concluded that I dont really know THE answer. I'm assuming that the engine characteristics, fuel RON being used and the OEM mapping/curve of the distributor have been designed to give the best all-round acceleration, driveability and fuel consumption balance rather than all-out performance.
By putting in different springs all you are doing is effectively re-curving the distributor (the centrifugal or mechanical advance part) to bring in the advance sooner in the rev range the by-product of which improves the responsiveness of the engine.
To explain why it made it more responsive I assume Id need to go into Stoichiometric AFR to explain why having more advance at high engine loads means more power though?
All I can say is that by adding more advance, means the spark occurs sooner before TDC. Why this then equates to more power I cant answer when in my head, the best scenario would be a number CLOSER to TDC at max compression?

Perhaps someone can explain the science behind it?

[Philip Lochner]

By putting in different springs all you are doing is effectively re-curving the distributor (the centrifugal or mechanical advance part) to bring in the advance sooner in the rev range the by-product of which improves the responsiveness of the engine.

IMHO, I recon you've nailed it there. In other words, the engine will tolerate getting to full advance earlier in the RPM range but it will not tolerate more advance (more degrees before TDC).

Manufacturers always have to design for the lowest common denominator out there which is why they seems to have implemented a cautious advance curve.

Why this then equates to more power I cant answer when in my head, the best scenario would be a number CLOSER to TDC at max compression.

Thing is Andew, it takes TIME for the fire in the combustion chamber to reach peak intensity (where the most heat is generated = most pressure). The whole idea is to start the fire early enough so that the fire will burn nicely when the piston is at the right place AFTER TDC to extract maximum effort from that fire.