'69 351W Holley Carb Jet Sizing - Vintage Mustang Forums
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post #1 of 40 (permalink) Old 08-05-2019, 12:25 PM Thread Starter
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'69 351W Holley Carb Jet Sizing

I have a relatively stock 351W in my my car. The only mods to it are a Holley 750cfm vac secondary carb, Edelbrock dual plane intake, Ignitor II igntion and dual exhaust. I was, and to some degree am still having problems with it heating up on the highway. I have replaced the t-stat, upgraded to 3-core ACP radiator, has good fan clutch, has fan shroud, etc. A guy was trying to help me and recommended I pull the plugs to see how it was running thinking that it running lean could be causing it to get hot. I pulled the plugs and the porcelain was white, no color at all. The carb had 70 jets in it. I've since upped the jets to 75s and the plugs are still pretty much white; they have a little color to them but not much. The car is getting kind of crappy gas mileage I think, anywhere from 8-14mpg and to get the 14mpg I had to baby it pretty good. The 8 was from when tuning, setting timing, and accelerating moderately aggressively on the highway, running 65-80.

For some reason it seems this engine just loves to drink fuel despite it not being built up much. When I put the carb on initially it would stumble, pop up through the carb, or stall if you stabbed the throttle from a stop. It ended up needing the 2nd biggest accelerator pump to correct this.

I was thinking about swapping the Edelbrock 600 off my Jeep onto the car to see if mileage increased but I almost hate to do that since the driveability is great on the Holley and it's dialed in for the most part.

My question is: do you think I need to step up to the 77 jets based off my spark plug readings or is there something else I should be looking at to determine if it's running too rich or lean?
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post #2 of 40 (permalink) Old 08-05-2019, 12:32 PM
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Where is you initial timing and when and how much full advance do you have? What are the vacuum readings? These are other components of the equation that need to be checked. Relatively speaking, my suspicion is the engine is way over carb’ed.
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Patrick
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post #3 of 40 (permalink) Old 08-05-2019, 12:45 PM Thread Starter
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I just replaced the distibutor with a rebuilt factory unit. I mainly paid attention to total timing when I set the new distributor. I set total timing at 35 degrees and I think that put initial timing right around 7 degrees. I don't know about vacuum readings but can check. Where should I check and what range should I be looking for?

I thought maybe it was over carb'd also but if that was the case wouldn't it blow black, spark plugs be black, bog, poor driveability, etc? The car runs and drives like a dream
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post #4 of 40 (permalink) Old 08-05-2019, 12:50 PM
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Depends on the other factors. Vacuum should be checked at the manifold. Shop manual will give you ranges...depending on the cam you have installed.

7 seems like a low initial to me. Shop manual will prescribe that value as well, but I would advance it until it pings under loaded conditions (acceleration) then back it down a couple degrees. Readjust curb idle after that.

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Patrick
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post #5 of 40 (permalink) Old 08-05-2019, 01:02 PM Thread Starter
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Ok, I'll look at the factory manual I have and see what it says for vacuum reading. The engine was rebuilt by someone previous to me owning it but I think it's a stock cam or at least it sounds that way. I did just have the intake manifold off last year. I checked it to make sure it wasn't warped and that it sealed well.

I believe the shop manual says 8 degrees but I will double check. I had it set at 10 initial timing but that put total timing past 36 which I was told not to go past that's why I set it off of total timing. It did cool down after pulling some timing out.

It might even be just how these old cars run but when I'm on the highway it runs about 200 degrees and when I get off the highway, depending on how hot it is, it gets up to 210 then eventually comes back down to 195-200 after driving for a couple minutes.
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post #6 of 40 (permalink) Old 08-05-2019, 02:49 PM
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Absolutely nothing wrong with those temps.
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post #7 of 40 (permalink) Old 08-05-2019, 03:00 PM Thread Starter
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Originally Posted by patrickstapler View Post
Absolutely nothing wrong with those temps.
I guess it's just in my head then; I hate seeing the temp gauge go above 1/2. What seems weird but might not be weird for an older car like this is that it runs cooler around town but when I get on the highway it gets warmer, even after I put my trans cooler on, and when I get off the highway if I get stopped at a light or in traffic the temp rises above 1/2 on the gauge and then cools back down after I drive for a bit. It's probably more of me being paranoid. Does that sound like normal behavior and I guess normal temps if running a factory thermostat?
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post #8 of 40 (permalink) Old 08-05-2019, 06:54 PM
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The way I see it, you have two options here. 1. Drive it and pay for the gas. 2. Change the carb to something more manageable...maybe a 650...maybe even a 600 and retune it for that. Summit has a couple different sizes of similar design to the Autolite 4100 that are supposedly very easy to tune.
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post #9 of 40 (permalink) Old 08-05-2019, 07:20 PM Thread Starter
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Thanks for the replies and info!

What kind of ballpark mileage do you think I should see out of my setup with a mix of highway, around town, and back road cruising? It has the stock highway gear in the rearend. It got 14 last fill up which I think is decent and I could probably improve on that a little with easier driving.

I will have to look into those carbs and see how much they cost and decide if I feel like fiddling with setting up a new carb. My first thought is to leave the carb that is on it for the rest of this season and look into swapping it over the winter.
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post #10 of 40 (permalink) Old 08-05-2019, 07:30 PM
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Well IDK, but I can give you a reference. My car is a ‘66GT A Code. However, I rebuilt the engine to roughly 289 K-Code spec, solid lifter cam, right at 10.5:1 compression, Toploader, 3.55 Traction-Lok rear. I would assume it was very close to stock K-Code HP. I drove the car from Huntsville, AL to the NPD show in Silver Springs, Fl and got almost 17MPG.

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post #11 of 40 (permalink) Old 08-05-2019, 08:04 PM
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seems like you are going the wrong way with your jets if you are worried about fuel mileage.
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post #12 of 40 (permalink) Old 08-05-2019, 08:22 PM Thread Starter
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Well IDK, but I can give you a reference. My car is a ‘66GT A Code. However, I rebuilt the engine to roughly 289 K-Code spec, solid lifter cam, right at 10.5:1 compression, Toploader, 3.55 Traction-Lok rear. I would assume it was very close to stock K-Code HP. I drove the car from Huntsville, AL to the NPD show in Silver Springs, Fl and got almost 17MPG.
Wow, that's pretty dang good gas mileage, especially for the rear gear and no overdrive trans. I would be very happy if mine got that. My build is far from a performance build so maybe I should downsize the carb.

I took a look over at Summit, is this the carb you were talking about? https://www.summitracing.com/parts/sum-m08600vs If so people go gaga over it, say it's better than Edelbrock 1406s and Holley carbs. I went through the reviews and questions, found a few people with 351 stock-ish setups like mine and say the carb is great.
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post #13 of 40 (permalink) Old 08-05-2019, 08:32 PM Thread Starter
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seems like you are going the wrong way with your jets if you are worried about fuel mileage.
I thought so also but the car was getting hot and this is what the plugs looked like with the 72 main jets in it.
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post #14 of 40 (permalink) Old 08-05-2019, 08:52 PM
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PatrickStapler is right on when he says there's nothing wrong with those temps. You want your coolant temp to run between 200-210 in order to have your oil temp running between 230-240.

As far as the carb goes... one question: Where are you pulling your PCV from?

Bart

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post #15 of 40 (permalink) Old 08-05-2019, 08:55 PM
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Originally Posted by 69CougarDropTop View Post
I thought so also but the car was getting hot and this is what the plugs looked like with the 72 main jets in it.
ignore the plugs when looking for lean for cruise/idle. lean out the carb until it surges/bucks or isn't responsive and then back it off from there.

i would rather see you decrease the main jet, and increase the pvcr to keep total jet area as the carb was stock for a safer WOT but leaner cruise/idle. also play around with the idle jet....
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