Conversation Between AbandonedBronco and Theproject

Carb jetting is based on how much air it can simultaneously pull in. Bigger carbs = more air flow, thus more fuel. They're not based on the vehicle or engine size. An 800cfm carb is going to flow the same air/fuel ratio on a 7.5l V8 as it will on a 2.2l 4cyl.

Generally, you only jet down for higher elevation, and if you want to squeeze a little mileage out of it. It's 1 size smaller for every 2000 feet above sea level, and 1 size larger for every 35° under 70° it is outside (adjusting for winter.) Then, 1 - 3 drops for gas mileage. Keep in mind that the leaner it is, the more drivability problems.

F250 jetted his WAAY down because of a modification he did, which was to put in smaller primary bores. This made it flow a lot less air, which means it required less gas. If you put 49 - 52 size jets in yours, it wouldn't even run.

Try the stock 67s and see what you think. If after a week of driving around, your spark plugs are a *dark* tan, try leaning them out to a 65 or so.

I am just now getting around to putting on my Summit carb to see if I can pass the emissions tests. That being said I noticed the conversation between you and F250 concerning the jet size that should be used. The stock jets are 67 on the Summit carb which I think are to big. Would I be better off rolling with the setup that we discussed previously, or should I look at going with the size 49-52 jets that F250 is looking at?

Thanks

I haven't gotten the throttle shafts repaired because I am moving into a city that is going to require an emissions test and I think that the Summit Carb will give me a better chance to do that. I am not going to get rid of the carter, I have my eye on another 300 project that whenever I have the time and funds to build it will go on it when it is repaired. I still have not put in a 3.25 rear yet, because I have not had the time to do so yet, so I still have the stock 2.75 unit with the SROD. Would it help to go to a 27 squirter because of this? Thanks for the help.

Josh

The link to the summit carb Summit Racing SUM-M08600VS - Summit Racing Street & Strip® Carburetors - Overview - SummitRacing.com.

I'm curious why you didn't get the throttle shafts repaired on your Carter. It sounds like it was a good combination and was working well until it started leaking.

Could you post a link to the carb you got from summit?

My setup right now, that's pretty close to ideal, is size 63 jets (stock was 66), a size 28 squirter (since I have a 3.00 rear end ratio. The lower the ratio, the lower the squirter size). The white cam is seeming to work pretty well, but still has a slight stumble off the line, so I need to mess around with a few more.

Hey AB,

I have a question about carb tuning. I have a 1980 F100 that I have converted over to a four barrel setup that is similar to yours, except that I am running a 485 cfm carter wcfb carb and Clifford manifold with headers that I picked up in a barn and rebuilt. It worked fine for the first year and even returned good mileage, 20 mpg, but now it has developed a massive vacuum leak around the throttle shaft and the mileage has dropped off to 16. I have just purchased a summit racing carb, the mv8600 which runs about 600 cfm. It is based off of the old autolite 4100 series so it has annular discharge which I think will work well on my 300 six. That being said the summit carb uses holley parts and I am looking for some advice on which jets, cams, and squirters to use so I can get close to the right tune on my vehicle and work from there. If you need more information let me know.

Thanks,
Josh

Hey Theproject,
The 3.00s and the SROD were great for stock size tires, which on this were 215 75R15s.
I now have 31s on it and it just doesn't seem to like overdrive anymore.

In OD, I'm at 1400 RPMs at 65mph and 1750 RPMs at 75. While that seems nice and low, it does dog the engine a little bit.

This might be fine with the stock 1bbl, but with a 4bbl, any dogging means the secondaries like to slip open, and my gas mileage goes down.


As for power around town though, the 3.00s are just fine. 3.5s would be fun but not really necessary.

Hey AB,
I have a 1980 F100 with 2.75 rear gears and the SROD and I am running 31 inch tires. I was wondering if you like the 3.00 gears in your Bronco, or if you had a chance to go with some deeper gears would you do it? I am toying with the idea of putting somewhere between 3.00 and 3.25 gears in my truck since I think it will allow me to get the same mileage as I had with the 235's/75 tires I previously had on the truck.

Thanks in advance,
Theproject