Mopar Muscle Association UK

Hello all,

I need to pick some brains please. (the exposition will be long but please stay with it)

I have a 750 VS Holley on the 428 C*br*J*t. It appeared to be running rich as it smelled rich at idle. However, our jaunt around the lochs revealed about 10-12 mpg, not bad considering the roads were twisty, there was some spirited driving, and Linda did most of the driving.

Running the advised tests on the Holley revealed:-

a: I had vacuum at the ported port on the carb.

b: I had vacuum at the manifold port on the carb.

c: Adjusting the idle screws made very little difference to how the car appeared to idle. One screw could be fully bottomed out, and the car would still idle. The other screw would be within half a turn of bottoming before the idle quality headed downwards towards a stall.

d: With the idle speed screw set at a nice idle speed (according to the dash tach, which is likely to be wrong) I still had vacuum at the carb ported port. The vacuum would only drop to zero just as the engine stalled out.

e: Vacuum at the manifold port wasn't high. Best was 12, idling as best I could get it was 8-ish. Flicking some revs into the engine had the vacuum rise to about 20.

I consulted my Holley books and on here and through PM's one or two folk on this message board. The general consensus was that the car was running on the main jets as the throttle plates had to be so far open to get an idle speed, hence the vacuum at the ported port.

Off came the Holley and following the rebuild book I put a Trick Kit into it. Jets were 70's, and PV was 6.5 in the carb. No jets with the Trick Kit (but 70 is about on the mark) and the PV in the Trick Kit was 6.5 also.

I reassembled the Holley.

Based on small sections of the books, and advice from MMA members, I set the primary throttle openings so that a small ammount of the idle transition slots were visible (in actual fact, so that a 25 thou feeler gauge just obscured the slot). I then cracked the little set screw for the secondary plates and wound them out until the secondary transition slots were just visible. (Strangely, one was slightly more visible than the other)

I also set the accelerator pump as per the books. When removed, there was approx 50-60 thou of clearance before the cam arm moved the pump arm. On rebuild, it was now lightly touching the pump arm. Any movement of the throttle shaft gave almost instant movement of the pump arm.

Is this correct ?

I then put the Holley back on the car. After cranking to fill the chambers, a couple of stabs on the accelerator and it fired right up. A couple of leaking issues, but they were easily sorted. Car seemed happy-ish to idle at the settings I had set when re-assembling.

Hmm, car still looked and smelled rich, though.

Retreived my vacuum gauge.

a: No vacuum at the ported port above the throttle plates. Good. As it should be. Hence no vacuum at idle going to the distributor.

b: Vacuum at the manifold port on the carb revealed not an awful lot of vacuum at all. Tweaking the idle speed a bit got the vacuum up to about 8 inches. Vacuum readings were nice and steady.

c: Adjusting the idle screws revealed absolutely no change or apparent improvement or degredation of idle quality. Again, one(either) screw could be bottomed out and the car barely missed a beat. I set them for one turn out as that seemed 'best'.

Got out timing light. Set timing as best I could (it seemed reluctant to remain steady at idle, and flipping the throttle got the timing to rise, but it came back down slowly) to 12-14 degrees. Giving it some revs saw it rise happily, although I was on my own and have no idea of how much revs I got it up to, but I would say it struggled a bit to hit 30 degrees of advance.

I took the car for a couple of short runs and fiddled again with it today.

It now has the nasty habit of dieseling on when I switch it off, and it coughs back with a large cloud of, presumably, fuel out of the carb. What causes this run on ?

If I chopped the timing back to 6 degrees (as per the service manual), it doesn't diesel, just stops.

I then checked the carb. The primary float was too high now for some reason, so I pulled it back to the odd dribble. Secondary was okay.

Ported vacuum was off, manifold vacuum was 8 ish still.

With the car idling I put my gloved hand over the secondaries, and the engine almost immediately stalled. Does this mean that the engine is idling pretty much through the secondaries ?

Putting my hand over the primaries makes the engine stall, but not as instantly.

Still no change to idle by adjusting the idle screws.

So, I am confused of Darnley here ....

Any thoughts appreciated.

1. Why does the car run on ? Is that a timing thing or an over fuelling issue as it seems to have appeared after the carb rebuild, before I started adjusting the timing ?

2. The idle speed screw needs 3/4's of a turn before vacuum appears on the ported port. Have I not opened the primary throttle blades enough on initial setting, and conversely, have I opened the secondary throttle blades too much ? Surely, uncovering the secondary transition slots now means that not only are the secondaries supplying more idle air but they are also supplying some idle fuel. By way of further information, winding the primary idle speed screw in to close the primaries, causes the engine to stall, but it also sees the manifold vacuum drop. Not sure about this, I would have thought it should have risen. Should I set the primaries open a bit more ( I have opened them about 1/8th to 1/4 of a turn since initial rebuild setting to get the car to idle better and pull up some vacuum (the brakes are deadly at low revs)) and close the secondaries off. If I close them to cover the transition slots, they will still be a bit open from the 'burn' mark that is on there from before.

3. When idling, any slight movement of the throttle see a small shot of fuel from the squirter. Is this correct ? Should this happen instantaneously like that ?

As ever, help welcome. I wouldn't say the car is getting me down, but every trip is throwing up snags when this was supposed to be a fun car to carry me through to the Falcon

All the best

When I have a spare hour i will read that.

Adjust the fule level so it is not running out at all. Just for now to make sure the fuel bowl valve is not leaking.

It will run on a bit if you have a fuel leak, good compression, and some carbon in the cylinders.

Sandy , my money says you have a vac leak somewhere, maybe brake booster.

Run a blow lamp round the intake & all vac lines. (not lit )

A lean condition will cause run on.

Carry out a vac reading @ constant vac port, then do again with all vac take offs plugged , see if theres a difference.

BTW , how did you manage to suffocate the primarys/secondaries seperately?

With the motor ideling, put your hand over the carb , if it still runs (lumpy) , you got a vac leak.

The rich stink out of the pipes is a classic sign of not enough idle timing, the fact that you cannot get rid of this with the idle screws also says it's timing.

You say that with 12/14* it's running on, here's what I would do....

Do a starter kickback test on the motor, this will determine where idle timing needs to be, if you let me have that figure I will be able to tell you what the rest of the timing events also need to be.

Lets say it kicks at 16*, static timing needs to be 14*, this gives the starter a nice easy life, now on a stock distributor you won't be able to run this figure because the total will be way too high causing detonation so the mechanical timing will need to be restricted to acheive the 2 figures.

I would then use the vac can (adjustable) connected to manifold vac to further increase the timing at idle, maybe as much as 10*

What you find though is in stock distributors the mechanical curve can start way too low (rpm) to do this as you get vac and mechanical timing stacking up together, (not good), so the mechanical curve needs to start a little late but come in rather fast so as the can is dropping the mechanical is taking over.

Once the correct timing events are happening for the motor, regardless of whether it's still dieseling or not then tune the carb, look at it this way, too little idle timing will give the appearance of a rich idle, stink out of the pipes and a general bad idle, no matter what the idle circuits are set at this will not change much because it's not the motor that is actually rich it's the exhaust valve opening before the fuel has had time to fully burn due to the low initial timing figure. Increase the idle timing and the pipes will then be clean

So the running on is due to the motor being lean, once the idle timing is high enough then the idle can be fattened up to resolve the running on.

You cannot tune a carb until the timing is done for the motor as timing will change the whole perception of what the motor is doing, now on the other hand so will changes to carb tune (changes how the motor appears to run)
BUT timing is for the motor, cam spec, compression, combustion chamber design, stall, gearing, etc and has nothing to do fuelling, so always timing first, then fuelling to suit the timing the MOTOR needs.

Hope thats not too confusing, if you want our tuning guide let me have your email address and i'll fire it off to you

If it was lean then putting his hand over the carb would increase the rpm not decrease it.

It is running rich.

Dave wrote:If it was lean then putting his hand over the carb would increase the rpm not decrease it.

It is running rich.

A vac leak will cause the motor to hunt when putting your hand over the top,(depending how bad) if no vac leak the motor will cut out straight away.

Don't worry about the running on for now Sandy.

Concentrate on getting it to idle. When the mixture is right at idle the running on will take care of itself.

If it is a stock camshaft then 12 degrees advance is fine to start with. But increase it to 14 if you want just to rule timing out and get a decent idle rpm and smoothness.

If it has a hot cam in it then increase timing the way Wil has stated.

If the idle mixture screws are not working even though there is absolutely NO vacuum at the distrbutor port then you probably have a fuel leak of some sort.

The idle screws are a very rich mixture of air and fuel. More fuel than air actually. So a fuel leak into the carb will make them redundant.

If the fuel bowl level is OK (leave the sight screws out while testing) your vacuum level might be too low for the power valve. if that opens it will richen the carb BUT when that happens just below the valve opening point the rpms will go up and down and not stay steady.

Make sure the squirters are not dribbling fuel in as well. Get your head over the carb with a good torch and have a look while it is running.

Sandy, it sounds like you've opened the secondaries too far. Are you sure you were running on the main primary circuit at idle before? If the idle screws weren't having much effect because of a vacuum leak, opening the secondaries to compensate will only make things worse.

Adam68 wrote:

Dave wrote:If it was lean then putting his hand over the carb would increase the rpm not decrease it.

It is running rich.

A vac leak will cause the motor to hunt when putting your hand over the top,(depending how bad) if no vac leak the motor will cut out straight away.

That is true if it was a bad leak I agree. But I think the idle mixture screws would work and it would not smell rich?

Gents,

Thanks.

To take things in turn.

Adam:- I will check the brake booster. I know this is a Ford, but when I had the booster off (as you need to do to get at the rockers) I did notice that the hose was tight to the right angled fitting, but the grommet that goes inot the booster which supports the other end of the fitting wasn't really in the best of conditions. I tried refitting it with some goo, but, I suspect that it isn't too good. Removing the pipe and blocking it will show if that is a leak. Not sure if MoPars are the same or the fitting/grommet should be tight. Unfortunately, the Monaco isn't here so I have nothing for comparison purposes. Thanks for that one.

I will block off everything and try it.

Using a glove (and glad I had it on as fidlling with the distributor clamp bolt involved some hand/fan intrface which the glove saved ) I can put my hand over the secondary barrels of the carb quite easily and the engine dies almost instantly. Too quick almost to whip my hand off to allow it to recover. Doing the same over the primary barrels and the engine hunts down, relatively quickly, but I can still get my hand off enough to get to the throttle to keep it going.

Adam wrote:Sandy, it sounds like you've opened the secondaries too far. Are you sure you were running on the main primary circuit at idle before? If the idle screws weren't having much effect because of a vacuum leak, opening the secondaries to compensate will only make things worse.

Adam You might be right there. i just re-read his post. I had missed that.

Sandy you need to close them and start again. Open the secondaries a TINY bit at a time to increase idle speed and close the primary throttle at the same time to decrease the speed again. Keep doing that until the vacuum on the dizzy port JUST drops to zero.

If the secondaires are opened too far the jets in there will come on and dump fuel in.

If you cannot get this ballance right then drill a 1mm hole in each primary throttle blade and try again.

Dave wrote:

Adam wrote:Sandy, it sounds like you've opened the secondaries too far. Are you sure you were running on the main primary circuit at idle before? If the idle screws weren't having much effect because of a vacuum leak, opening the secondaries to compensate will only make things worse.

Adam You might be right there. i just re-read his post. I had missed that.

Sandy you need to close them and start again. Open the secondaries a TINY bit at a time to increase idle speed and close the primary throttle at the same time to decrease the speed again. Keep doing that until the vacuum on the dizzy port JUST drops to zero.

If the secondaires are opened too far the jets in there will come on and dump fuel in.

If you cannot get this ballance right then drill a 1mm hole in each primary throttle blade and try again.

Don't need to drill holes with a demon (idle eeze)

Only messin Dave agree,depending on the cam specs , don't need to on the stocker 428 cobras.

Adam68 wrote:Don't need to drill holes with a demon (idle eeze)

As long as you take the swarf out first.

Dave

hand over either primaries or secondaries stops the motor which it will on any 4bbl, the difference in how long the motor takes to stop is down to the amount each blade is open and what air it's drawing.
Pull the choke and see what happens.... motor stops, no difference to a hand over the top,

How can it be rich if the idle screws make NO difference wherever they are,

also 12/14* isn't especially high, thats a bone stock recurve figure, so how come it's running on, if it was rich then it would love 14.

Like I said, timing is for the motor, fuelling is for the timing that the motor needs, not the other way around.
Right at this moment it is unimportant whether it's rich/lean whatever, 6* is too low, get this right for the motor then address the carb

Eek, keep up, Sandy ...

Hi Neil,

Thanks for that. I 'think' I see what you mean. When I bought the car, it did have a starting problem (which was due to a bad earth to the chassis and not going to the engine block). Starting involved several attempts, mostly met with a click, and then boom, away it went. However, when I took the car to show a friend and he could watch while the car started it did kick back, badly. We set up a timing light (a good Snap On one) and the idle timing was (and my memory may be playing tricks here) about 26 degrees. We disconnected all the vacuum lines to the distributor (I will come back to that) and I 'remembered' that the initial timing should be 6 degrees. Which we set. Kickback disappeared, as you would expect.

Investigation of the distributor shows it is one of Fords' 'Better Ideas'. It has both vacuum advance and vacuum retard. The vacuum advance goes to carb ported vacuum. Vacuum retard goes to a port on the inlet manifold. If I understand the theory correctly. No ported vacuum advance at idle. Highish vacuum retard at idle. Distributor timing at idle set by distributor alone.

Accelerate. Ported vacuum rises dialling in some advance to distributor. Manifold vacuum drops dialling out the retard on the distributor. As revs rise, ported advance stabilises and may in fact drop, but mech advance takes over.

De-accelerate. Throttle plates close. Mech advance decays, but the manifold vacuum rises which in turn pulls the retard portion of the vac canister against any ported vacuum advance, and perhaps also works against the residual mech advance, so the timing should decay quickly.

I don't seem to have this quick decay, but, just typing it has given me something else to check (how quickly advance decays with reatrd connected or disconnected). I had run the car without the retard and there didn't seem to have been much effect.

Returning to my initial 'tuning' attempt with my friend. I then noticed that the vacuum pipes were not correctly connected. The ported vacuum was blocked at the carb, and the distributor was using manifold vacuum for advance. Retard was connected properly.

This may explain why the car had so much initial timing with the vacuum still connected.

But also seems to be what you are implying by using manifold vacuum to pull in even more idle advance. Maybe the guy who had the car knew a bit more than I gave him credit for ? Perhaps I just had too much in there and hence the kickback ?

Anyway, set for 6 degrees and connected the vac advance to the correct ported port on the carb. However, at the time, this port was still getting several inches of vacuum so I was still getting advance on there. The car seemed to run okay, but it 'hunts' on steady throttle, fine on acceleration and de-acceleration.

Anyway, how do I do the kickback test ?

Timing doesn't seem to be steady, unfortunately. It wanders a bit as the engine idles, but the idle note changes slightly as well. However, not enough to say that the it is fluctuating.

Recommended initial timing from US forums is 12 degrees. I don't seem to be getting much mech advance. Never saw more than 30 total.

My email address is: sandy.mercer1@ntlworld.com

Thanks.[/i]