302 4 barrel idling problems

Hi guys!

This is my first post after having read a whole lot on the subject and in this forum in general.

I've got a 72' Mustang Grandé with some idling issues. It's a 302, C4 aut. trans with Holley 600 cfm, Offenhauser intake manifold, long tubes, vintage Mallory ignition and points dizzy. I've dissected the carb, replaced intake manifold gaskets and dizzy cap and rotor. I also replaced virtually every vacuum hose and naturally the spark plugs as well.

The problem I'm having occurs only at idle. When revving all seems to be fine, responsive and sounds sweet when flooring it (a bit). At Park or Neutral it will idle ok, just not perfectly steady and it sounds rough. When I put it in Reverse or Drive it immediately stalls if I don't balance it up with my foot on the gas. Even then it looses power.

The strange thing is the timing. I've had some troubles timing it and it practically only runs when it's at around 20-25 BTDC at idle with vacuum advance hose pulled and plugged. When vacuum is plugged back in it's around 30 BTDC.

However, yesterday, after changing the cap and rotor and fiddling around with curb idle, idle mixing screws and the vacuum advance I managed it to finally idle at around 15 BTDC and around 20-25 BTDC with vacuum advance hose connected. Still rough, still not steady RPM and would still not survive a gearchange from Park to Drive.

I've scanned the internet but haven't really got any clear answers, but maybe you guys could?

Thanks in advance!

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It would seem the excessive initial timing that your vacuum advance is causing is responsible for the rough idle. I suspect your vacuum advance is connected to a full manifold vacuum source which provides vacuum at idle. Try connecting it to a ported vacuum source which will only supply vacuum when the throttle blades are open.

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"When I put it in Reverse or Drive it immediately stalls if I don't balance it up with my foot on the gas. Even then it looses power."

Stalling with a load placed on the engine at idle can often be a vacuum leak. Lots of ways to check using carb cleaner spray, etc.

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Try turning the idle mixture screws out a 1/2 turn and see if that helps. You really need to get a vacuum gauge on there to see where things are at to tune the engne. As mentioned it does sound like a vacuum leak, check out the carburetor base gasket real close.

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Welcome to the board!

When you set the points are you using a dwell meter or a feeler gauge? Is it a single or dual point distributor?

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I had this problem on an FE (390), it was valve recession.

The reason your idle timing is so high is because you have to set the idle speed too high to keep it running, and the mechanical advance has started kicking in.

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Have you verified that your distributor is not installed "one cog off"? Could this be a possible cause? If the dist. is installed one cog off, the engine won't run well until the dist is rotated to what seems too far to the right. I am wondering if your symptoms indicate this could be the issue....

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Does it have a big cam?

In addition to the things already mentioned:

1. You should disconnect and plug the vacuum advance forever or until further notice.

2. You should set the gas level in the carb so it is below the inspection hole at idle or immediately after you turn it off . If it has sight windows, it needs to be between the bottom of the window and half way up.

3. If you have a pcv valve, you should plug it for now . It may be creating too much of an air leak at idle . If you let it idle then plug the valve while it is idling and the rpm drops noticeably, it is flowing too much air . You can buy adjustable ones.

4. You should get the engine tuned so there is no vacuum at the ported vacuum port at idle . This can be difficult if it has a big cam and can involve drilling holes in the throttle plates.

5. If one of your fuel mix screws has no affect when you turn it, you may have a plugged port.

6. Don't always trust the timing marks although they should be correct . I tune them by ear and performance . Your outer damper ring may have slipped some and/or your cam chain may be extremely loose . If you remove the distributor cap and rotate the engine until it is on top dead center, you can watch the distributor rotor as you turn the engine counter clockwise until the rotor starts to move, then look at the damper to see how many degrees the crank has moved . If it has moved more than around 8 degrees, I would replace the timing chain.

You can see if the damper ring has moved by locating top dead center with a piston stop.

.

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Ugh, that's not what I wanted to hear! You had the same symptoms I take it?

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Back to the points. You need to buy or borrow a dwell meter and get the dwell set correctly. This is the very next thing you should do before you mess with anything else.

The old points ignition system is mechanical. The 'rubbing block' rides on a cam in the distributor. As time goes on it will wear down and increase the amount of time the points are closed (and reduce the amount of time they are open). The spark occurs when the points open, as it wears the timing of the engine changes as well.

Since it is a mechanical system it has to compromise between being closed long enough to allow the coil to charge and open long enough to cause the coil to release the spark. As the engine RPM's increase the time for both is decreased.

It may not solve your idle issue BUT if you don't start with making sure the dwell is set correctly all of the other timing changes you do will change again when you do set the dwell correctly.

While I am an old school fan I always recommend to people to get rid of the points and go electronic. There are options that would allow you to keep the stock look.

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Ugh, that's not what I wanted to hear! You had the same symptoms I take it?

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Yes, ran rough, had to keep the idle cranked up to keep it from stalling when putting it in gear, smoothed out over 1,200 to 1,500 RPM, but was down on power. Confirmed with compression test.

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The problem I'm having occurs only at idle. When revving all seems to be fine, responsive and sounds sweet when flooring it (a bit).

At Park or Neutral it will idle ok, just not perfectly steady and it sounds rough. When I put it in Reverse or Drive it immediately stalls if I don't balance it up with my foot on the gas.

I've had some troubles timing it and it practically only runs when it's at around 20-25 BTDC at idle with vacuum advance hose pulled and plugged. When vacuum is plugged back in it's around 30 BTDC.

However, yesterday, after changing the cap and rotor and fiddling around with curb idle, idle mixing screws and the vacuum advance I managed it to finally idle at around 15 BTDC and around 20-25 BTDC with vacuum advance hose connected. Still rough, still not steady RPM and would still not survive a gearchange from Park to Drive.

Click to expand...

Back to the points. You need to buy or borrow a dwell meter and get the dwell set correctly. This is the very next thing you should do before you mess with anything else.

The old points ignition system is mechanical. The 'rubbing block' rides on a cam in the distributor. As time goes on it will wear down and increase the amount of time the points are closed (and reduce the amount of time they are open). The spark occurs when the points open, as it wears the timing of the engine changes as well.

Since it is a mechanical system it has to compromise between being closed long enough to allow the coil to charge and open long enough to cause the coil to release the spark. As the engine RPM's increase the time for both is decreased.

It may not solve your idle issue BUT if you don't start with making sure the dwell is set correctly all of the other timing changes you do will change again when you do set the dwell correctly.

While I am an old school fan I always recommend to people to get rid of the points and go electronic. There are options that would allow you to keep the stock look.

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Below is one of the things I would do.

SETTING TIMING CURVE

Before you start driving it normally, I would set the timing curve so it is optimum for your particular setup . Below is just one way to do that . In general, the goal is to run as much timing as possible without it pinging under heavy load.

The more timing you have, the more vacuum it will have until the timing reaches its optimum level.

The more vacuum it has, the smother it will idle and the more you can close the butterflies.

If the butterflies are too far open, you will be into the transition circuit of the carb which can cause idle problems . One of the indicators that the butterflies are too far open at idle is when the carb has vacuum at the ported vacuum port.

1. Disconnect and plug the vacuum hoses to the dist if you have any and leave them plugged permanently or until further notice.

2. In your particular case, start your timing at 10 degrees BTDC.

3. With the engine warm and idling, advance the timing 4 degrees . Listen for an slight but noticeable increase in rpm and irregular/rough running.

4. If the rpm increases and it still runs smoothly, reset the idle speed then increase the timing 4 more degrees and check for the same things.

5. Retard timing back to 10 degrees btdc.

6. Reset the idle speed.

7. Increase the rpm to around 2000 then advance the timing 4 degrees . Listen for an increase in rpm and irregular/rough running.

8. If the rpm increases and it still runs smoothly, reset the engine speed to 2000 rpm then increase the timing 2 more degrees and check for the same things.

POST RESULTS

TEST DRIVING

After setting the timing curve you can do the following test to see if you have too much advance.

Get the engine up to operating temp.

Drive at around 20 mph in second gear for a few seconds then floor the gas pedal as fast as you can until you reach around 30 mph and listen for even the faintest pinging sound coming from the engine . If it pings, you have too much timing for the octane gas you are using . You can either reduce the timing some or use a higher octane . The highest timing level you can run without it pinging and/or running erratic will SAFELY provide the most power.

It may ping in hot weather even if it does not in cold weather . If you find this to be the case, the easiest thing to do is reduce the timing until it stops or try higher octane gas . If it still pings with the highest octane gas, you can reduce the timing then.

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will e said:

What make and model of dwell meter?

I am also going to assume you do not have any aftermarket ignition add ons like multiple spark or ?? (It would be unlikely if you are running points). Have you pulled the cap and observed the points?

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