Mopar Muscle Association UK

The Holley (4160) I'm using has main jets stamped '65H'. Runs fine around town but acording to my home-made lambda sensor gadget on a motorway run it's a bit on the rich side. The plugs are dark/black - slightly sooty.
I was going to change the jets but the local shop only has 64 or 60 in stock, which sounds like not enough or too much...

Any advice welcome.

Thanks.

Main jets are usually size 60. With some mods my engine runs slightly lean on these. Light brown plug.

Prothed wrote:Runs fine around town but acording to my home-made lambda sensor gadget on a motorway run it's a bit on the rich side.

If it is fine around town your primary jets must be about the right size. If it is RICH on the motorway your secondary jets must be too BIG. In your case the secondary jets are two holes drilled in the bottom of a plate. The plate is a cast alloy so it will NOT solder!

Steve, i'll get me coat. Shouldnt you be in court or something!

Drilling jets is fine by me! They don't flow the same as the real thing though as you are removing the taper at each end.

You are right that with vacuum secondaries they should be closed at light throttle and low rpms but if the gears are low and the rpms higher than stock they will be open enough to effect mixture. Sometimes you can get a low manifold vacuum at part throttle cruise that will open a power valve too. So a vacuum gauge would be useful to see if manifold vacuum is high or low when driving.

I think what you suggest is right. He should wire shut the secondaries to rule them out and get it running right on the primaries first as anyone should when tuning a carb.

OK, I got my coat and went to garage. I meant to say main jet in many Holley 600 applications is 66 (which I use) and I have upped it to 68 for a better plug colour. Over to steve n' Dave!

Prothed wrote:The Holley (4160) I'm using has main jets stamped '65H'. Runs fine around town but acording to my home-made lambda sensor gadget on a motorway run it's a bit on the rich side. The plugs are dark/black - slightly sooty.

Now i am awake and thinking more clearly....

I don't think a 65 jet would make the plugs THAT dirty. You do need to start from scratch. Make sure you have no vacuum at idle on the dizzy port. Measure the mixture and vacuum at idle. Check you have the correct power valve. Run on the primaries only. Then finally set the secondaries.

Excellent. Thanks for all this. I'll do some more measurements and post the results. (You have to drive a fair way from here to get away from the speed cameras and the traffic...)
I was wondering if changing the jets one size would make any practical difference or whether the tolerances were such that you might not see anything?

One or two jet size changes is only 'fine tuning'. For the plugs to be sooty you have a lot of extra fule you don't need. Make sure there is no fuel dripping into the carb or anything like that.

Here are the results from the test run -
speed rpm vacuum sensor o/p
tickover 400 17" 863 mv
40 mph 1800 16" 880 mv
60 mph 2500 15" 940 mv
80 mph 3200 10" 900 mv
xx mph 4000 5" 930 mv
I wanted to repeat the readings because it was difficult to look at everything and write down the results and try to watch where I was going, but then the sensor fell out of the exhaust and after being dragged down the road it didn't work anymore.
So, the mixture looks pretty constant but a bit rich.
(The output curve for the sensor should be shown below).
I'm inclined to try the 60 jets, but all supporting/contrary views welcome...

Remind me what engine this is?

400rpm is too low for an idle speed on a Mopar V8. You want 600rpm on a manual and 800 on an automatic.

Where in the exhaust was the sensor? It needs to be as close as possible to where all 4 branches of the manifold or header meet i.e. just behind the collector or in the downpipe close to a cast manifold.

The reason for this is that the output of the sensor is greatly dependant on it's temprature.

As advised you need to disconnect the secondaries and see how the primaries only run. Is that what you did? I am not clear from what you write if you did that or not.

Its a standard 383 with a Mallory distributor (no vacuum advance), Holley 600 cfm and Holley street dominator manifold.
The 400rpm is in drive, its higher in neutral - not sure what but it pulls away no problem at any throttle.
I used a heated (4-wire) sensor. The sensor was mounted in a bit of tube and stuck up the end of the exhaust. (The commercial ones use this method if there is no existing adaptor).
I haven't disconnected the secondaries yet as I lost the sensor first and also I wasn't sure how to do it! (I didn't want to risk damaging the carb by doing something I shouldn't).

OK. I was just worried about exhaust temp changes with rpm at the tail end of the pipe. But actually as you can see from the graph it would indicate leaner as it got hotter and not richer (doh!) so lets take the readings as realistic for now.

If it pulls the rpms down to 400 in gear it doesn't sound very "standard" to me!

There should be very little if any rpm drop from neutral. It sounds like you have way too little stall speed in your converter. But lets worry about tuning it first.

If adjusting the throttle to get 800rpm at idle in neutral produces any vacuum on the dizzy advance port that confirms that you have a bigger duration cam than standard and is also the reason for the rpm drop in gear. You have to get this bit on the carb right first.

So if 800 rpm gives you vacuum on that dizzy port you either need to adjust the secondaries so they are open slightly (only some carbs have this adjustment and it is a screw under the carb that works like the idle speed screw but on the secondaries only) OR if you don't have this adjustment you need to drill a hole in each PRIMARY throttle blade (starting at 1mm).
Either of these will give you more rpm at idle without you having to open the primary blades so you should be able to set the idle speed so that you do not have any vacuum on the dizzy port.

Then plug the dizzy port because you are not using it.

Next disconnect and plug the vacuum line that opens the secondaries. You can't do any harm to the carb or engine so don't worry about it.
This will prevent the engine opening the secondaries. However there is a chance the throttle blades might open a bit by themselves so twist a bit of wire around the linkage so that they can't open but make sure your primaries will still open OK.

THEN do your test runs.

You will find that no matter what rpm you drive at (over 1500rpm) the mixture will be the same. You have two fixed jets on the primary side. The amount of fuel they draw into the engine depends on airflow through the carb. The ratio remains fixed no matter what the rpm. So it should be the same mixture at 2000rpm as it is at 5000rpm.

If it is not then you are getting extra fuel from somewhere else. One possible source could be the fuel pump pressure if you are not running a regulator. If the pressure rises with rpm you could be overfilling the fuel bowl. Fuel bowl level is critical on Holleys.

If everything seems hunky dory and you have managed to get your mixture set OK at all speeds you can now make the secondaries work again. On your carb the secondary jets are usually non-adjustable holes drilled in a plate. If these holes are too big it could explain why it gets richer as it goes faster. You will need to fit a plate with smaller holes so that the mixture stays the same at all rpms as it did before on the primaries only. You can drill these holes bigger to make then richer but you cannot make them smaller to make them leaner.

There is a company making secondary plates that take normal jets. However these are very expensive.