Main jet tuning... ...in France

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Nico383

Nico383

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Hi Guys,

I'm now dealing with the carburation tuning on my 1970 Plymouth duster.

I first ordered a full set of main jets from quick fuel ($94) but it turned out I need smaller primary jets than the 68 stock ones (kit range is 75 to 110) so I ordered 57, 58, 59 and 60 (regarding the AFR gauge reading I had).

Unfortunatly it turns out I need something between 60 and 65 (regarding the AFR gauge reading I now have)...

The thing is I have to pay $47 just for shipping to France whereas a pair of jets costs $5.

So I'm looking for a charitable souls who, in the name of what I would call "international mopar solidarity" :) , would agree in buying some jets for me and ship it via regular mail to France... Of course I plan paying this nice guy via paypal before the parcel is shipped...

Please help!

Thanks,

Nico
 
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I see Jegs offers main jets in your specified sizes at $7.70 each. I doubt they would take the time to ship it International First Class (least expensive) at about $23 for less than a pound (I estimated 12 oz for 4 jets. Maybe someone has the jets you need and will step up to the plate to help you out!
 
What size engine and what altitude?and what cam?
I'm guessing your pretty close to sealevel, and since you didn't say 340 or 360, I'm guessing 318. And since you didn't brag on the 268* in it, I'm guessing cam is smaller.

60 to 64 is pretty small, even for a teener, with just a small cam.
Could it maybe be rich for another reason? like a too-early PV calibration,or a too-high fuel level,or an overly rich low-speed circuit,engine sucking hot-underhood air,or a poor timing curve, or engine running hot, or something else? Like headers over-scavenging on a big overlap cam,or an AFR gauge installed in a bad spot. Just asking.....
 
Hi AJ,

Thank you for the interest and sorry for the lack of information.
Engine is a 383 with a 650 cfm double pumper carburator. Camshaft exhaust duration is 228*, and intake is 223* but I don't really know what does it involves..
Here are the camshaft spec :
http://i1354.photobucket.com/albums/q692/nlangebe/IMG_2845_zpsmkxz08lm.jpg
Power valve is stock (6.5) like all the other tunable things on the carburator, fuel level is fine...

So you think I should have bigger main jets?

Cheers mate,

Nico
 
No, I don't think you should have bigger main jets.
I'm saying 64 and less is pretty small, and I'm wondering if your A/F gauge is giving you an erroneous reading. Where exactly have you installed it, and what brand is it?And at what rpm and throttle setting is it telling you that you are rich?

There are several fuel circuits in that carb, and each of them begins working at a different manifold vacuum level.
I assume you know that the required MJ is determined with the engine working as hard as it possibly can; as in WOT in third gear.
But, you can sometimes balance the MJ against the PV
A 383 with a 650, and a 223/[email protected] cam won't need much for fuel. But it probably idles with 16ish inches of vacuum at 750 rpm with 14* of timing. In which case a 6.5-PV is not the right one. By the book they say the PV should be 1/2 the idle vacuum. So for you, IF it idles at 16, then the PV should be16x1/2=8, And that would then be an 8.5, as the PVs are only available to the nearest .5 IIRC.
With the 8.5, the PV will start flowing earlier. Sometimes this allows a 1 or 2 MJ smaller for a streeter. Smaller from IDEAL, not smaller from your 68 or whatever. I like operating like this cuz most of the time you are on the low-speed circuit or the mains and only occasionally on the PV. So a little lean on the mains around town makes it snappy, and it doesn't want to pull in to every filling station.
 
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Ok, well, I'm going to check the vacuum pressure at idle in drive and I will first change the PV accordingly.
I keep you in touch.
 
yellow rose said:
I may have some 64's I'll send you if you think you need them. I'll check today.
Click to expand...

Hi Yellow rose, thank you for your help :) I'll let you know as soon as I'm sure I need them.

@AJ : I checked the vacuum at idle, transmission in D : it's between 12 and 13 with vacuum advance plugged in.

I guess I should keep the 6.5 PV?

Nico
 
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I did some tests with a 4.5 PV I found. First feeling : I get "holes" in acceleration when applying load so I guess 4.5 is too small.

Keeping the 60 main jets I get an AFR around 15.6 when cruising at 3000rpm in 3rd gear.
with 68 main jets I get an AFR around 12.0 when cruising at 3000rpm in 3rd gear.

Correct me if I'm wrong but it seems I should keep the 6.5 PV and play with the main jets size to come closer to 14.7?

@yellow rose : If you have some main Jets you could sell, I would be glad to relieve you from this burden ;)
 
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PV has ZERO to do with cruise and A/F ratios.

The way holley suggest choosing a PV is so wrong it's not even funny. That method only gets you something that MAY work. You have an A/F gauge, use that to choose a PV, not some formula that has been proven wrong way too many times. Same as the carb sizing formula... RUBBISH.
 
crackedback said:
PV has ZERO to do with cruise and A/F ratios.

The way holley suggest choosing a PV is so wrong it's not even funny. That method only gets you something that MAY work. You have an A/F gauge, use that to choose a PV, not some formula that has been proven wrong way too many times. Same as the carb sizing formula... RUBBISH.
Click to expand...

Ok... And what's the process I should follow using the AFR gauge to find the good PV?
 
Put the 6.5 back in it, and figure out your main jetting. Concentrate on ONE thing at a time. You go changing a bunch of things at once it has the potential to create confusion.

The number on the PV is the approximate vacuum reading that it will open. It has nothing to do with the amount of fuel provided.

Cruise A/F is controlled by the IFR/IAB at low rpm and the main jet at higher rpm. Find out the point where the transition occurs from IFR to main jet. You should see a change in A/F when that transition occurs. Otherwise you may be working on the wrong.

The PV is a load/WOT enrichment circuit. Have a Vacuum gauge and A/F hooked up. Get the car on cruise going up a slight grade and feed it throttle. Watch the A/F and vacuum gauge. The A/F number should go leaner as the vacuum gauge drops in value. You want to find a tipping point where the PV opens. If the engine gets to the 15-16's as you are loading it, the PV likely needs to be a higher number. If A/F stays low, the later the PV should open.
 
Or just swap it out for a PV plug, Get your transfer fuel sorted, and then your mains, and then finally put the PV back in and get it's timing set right.
And then you can work on the secondaries.
Focus on one thing atta time. Spend hours on that one thing if you have to.Or days lol.
 
make your secondary part of the carb non functional when Tuning the carb. Take of this rod which connects Primary and secondary Butterflys. tune the Primary circuit first with your AF gauge. I bet your jets will be in the 75 range. if this circuit is ok connect the rod and tune the secondary circuit.
 
and drive the car. RPM arround 2500. This is above idle circuit and below PV opening. I also unplugged the accerlerator pumps to have a true reading on the A/F meter
 
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