Main Jet Question for you FE guys

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SPLINEHEAD
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Main Jet Question for you FE guys

#1

Post by SPLINEHEAD » 20 Mar 2018, 05:00

I have been tweaking the carb and "stuff" on Pole Cat and the car is running the best I have ever had it. Pulls good, runs cool, 19" of vacuum at idle I am very happy. Butt, being an anal engineer I like to have everything at its best. So, this 390, C6 car with an Autolite 4300 carb was driven on some hot laps around the country side Saturday lighting up at most stop signs and even doing a doughnut in an empty parking lot. So then I drove home and put her away.

Last night I decided to look at the spark plugs and see how she did on my run. I pulled four plugs and they all looked the same, very clean with half of the exposed insulator light tan-ish . The other side of the insulator is very clean with no tan-ish coloring. In other words the plugs are tan-ish on one side and clean with no color on the other side.

So my question is, would it be ideal to put one size larger main jets in? Should I leave it alone? I'm thinking change the jets up one size, opinions?

Thanks in advance, Gary

Gary Hill
Pole Cat
1969 XR7 390 S code 4bbl C6 3.0 TrueTrac rear end

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BossElim69
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Re: Main Jet Question for you FE guys

#2

Post by BossElim69 » 20 Mar 2018, 10:13

if your seeing tan Gary I would say you have it jetted correct. Now if you really want to get anal. Buy a air/fuel ratio kit and install 02 sensors in the exhaust then you can monitor you exact ratio and jet accordingly. Around 13.0 is optimum air/fuel ratio.
Brian Carpenter, President CATS classic Cougar club
67 Cougar road course racer
67 Cougar project car
67 Cougar GT
68 XR7-G trailer queen restoration.
68 Restomod
69 Boss 302 Eliminator
69 XR7 390 survivor
69 XR7 428CJ 4spd
70 Boss 302 Mustang
1993 Harley Davidson
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SPLINEHEAD
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Re: Main Jet Question for you FE guys

#3

Post by SPLINEHEAD » 20 Mar 2018, 11:05

Ha, I actually thought that when I retire and build this engine that I would probably do that. When I retire I do plan on pulling the engine and do as Royce suggested in another thread "bolt in some CJ heads and hold on" is how I think he put it. Retirement is 3-5 years away so I'm going to get everything I can out of her for now.

Thanks Brian

Gary Hill
Pole Cat
1969 XR7 390 S code 4bbl C6 3.0 TrueTrac rear end

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xr7g428
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Re: Main Jet Question for you FE guys

#4

Post by xr7g428 » 20 Mar 2018, 11:32

With a Holley, the main jets are for cruising. I like to go down one jet size at a time and then see if it lean surges at about 45 to 50 mph on level ground. When I find the "too lean" point I go up two sizes and it usually runs really well. It is absolutely essential that the ignition (including vacuum advance) is spot on before you do any carb adjustments.
Bill Basore, Editor / Publisher
Legendary Cougar Magazine
Currently in the Cat House
'67 XR7 GT 390 4 speed, AC, AM FM, Lime Frost Green
'68 XR7-G 428CJ C6, Tilt-Away, AM, Black Cherry
'68 XR7-G 390 4 speed, Sunroof, Cardinal Red
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93RGTE
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Re: Main Jet Question for you FE guys

#5

Post by 93RGTE » 20 Mar 2018, 11:41

Butt, being an anal engineer
Pun intended?
Gary

SPLINEHEAD
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Re: Main Jet Question for you FE guys

#6

Post by SPLINEHEAD » 20 Mar 2018, 12:09

93RGTE wrote:
20 Mar 2018, 11:41
Butt, being an anal engineer
Pun intended?
Absolutely! They kind of go hand in hand or......never mind.

Gary Hill
Pole Cat
1969 XR7 390 S code 4bbl C6 3.0 TrueTrac rear end

SPLINEHEAD
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Re: Main Jet Question for you FE guys

#7

Post by SPLINEHEAD » 20 Mar 2018, 12:26

xr7g428 wrote:
20 Mar 2018, 11:32
With a Holley, the main jets are for cruising. I like to go down one jet size at a time and then see if it lean surges at about 45 to 50 mph on level ground. When I find the "too lean" point I go up two sizes and it usually runs really well. It is absolutely essential that the ignition (including vacuum advance) is spot on before you do any carb adjustments.
I got one of my old books out a year or so ago where this author discusses reading plugs. He speaks of getting timing set first as well and that you can tell proper timing by where the heat color stops at on the ground strap of the plug. Ideally, as he writes, if timing is set properly the heat will stop basically in the apex of the 90* bend. I never tried it back in the day and did so before I put the car away for winter. If he is correct, I have that and some of the other color areas that he discusses correct. However, he doesn't discuss the situation in my original post.

Right now it is at 16* and 36* and does not seem to have any spark knock or ping. The sad thing is that I'm loosing my hearing and need to get a friend in the car to listen for me. So right now I think I have the timing dialed in.

Gary Hill
Pole Cat
1969 XR7 390 S code 4bbl C6 3.0 TrueTrac rear end

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