Old school carb and ignition tuning

[bronco italiano]

x3

Funny and my Meathead Uncle tuned that way. Despite his personality, he was a great old school Ford tuner

 

[serial car restorer]

Checked the accel pump first, as that seemed the most likely. I found that while they did squirt, the streams were weak and small.

I had set the linkage choices per the instructions with the rebuild kit. But to be honest, I can't recall which set of specs I used - '68 289 (current engine), '69 302 (original engine), or '72 302 (year of carb). In any case, the pump lever was in the shortest throw (of 2) and the throttle shaft side was in the second shortest throw (of 4). I switched to the longest throw on both ends and the difference was significant. Maybe not perfect yet, but drivable now.

I can't find my vacuum gauge anywhere, I haven't used it in many years. Might have given it away, so vacuum tuning will need to wait. Once I got the accel pump closer, I bumped the timing to 12° as well. I don't think that's going to help a lot, but it sounds like a better number.

OK, still not good. Stumbles and coughs under load at small throttle openings - say, off idle to maybe 1/3 throttle. If I get past that, call it ½ throttle or more, it pulls better but pops and bangs loudly (from exhaust, not carb) on overrun or deceleration. And while it's hard to be certain from the plugs (since I'm not driving it any distance or at speed), they are very dark brown (not sooty or oily, though). The pops on deceleration, dark plugs and the exhaust smell lead me to think it is rich.

The jets in it now are 48. I have another set of jets from a scrap carb, they are 49. Also looking at the float level. When I reassembled it I set the dry float level at ~15/32" as that seemed a pretty safe number based on specs for a number of different years of 302 carbs. But now I see that the late 289s and some of the early 302s spec ⅜" for the float level. If I'm envisioning this correctly, changing the float level to ⅜" from almost ½" would bring the fuel level up in the bowl, which is probably going to make it even richer. Am I thinking correctly on this? Or does the float level on this type of carb have anything to do with the mixture at all?

 

[Grey owl]

The Dist in my Ranchero's 302 gave 34° total with the initial set at 12° BTDC. 18° BTDC initial would have put the total at way too much timing. Which tells me that the mech advance in that dist. was very, very low. An unusual thing IME.

Its not too much initial timing. Don't know where you guys get your info but its wrong . I use an after market performance HEI distributor and my total final advance is 34 - 36 degrees. It has a mild comp cam and runs 15 inches of vacuum at idle. If I set the timing with a vacuum gauge which takes the initial timing to the engines absolute mechanical maximum its 23 -24 degrees initial. So don't tell me 18 is too high because it isn't. All your doing by running low initial timing is robbing yourself of horse power. My engine runs perfect it starts with the touch of the key no enless cranking even when hot. So i have no problem telling you, you don't know what your talking about.

 

[Grey owl]

It's been many decades since I've played with any carb more complex than a slide-valve 2-stroke dirt bike carb. And shoot, my issue might not be fuel related. So I'm going to ask for advice before I start mucking around and maybe make things worse.

Here's what I have. The subject is a '69 Bronco w/3 speed.

• 289 (not the original 302) that seems mechanically healthy.
• 2100 carb from a '72 302. Definitely a Bronco carb, 1.08 venturies. Main jets are 48. (Elevation 800 ft.) Completely disassembled and rebuilt by me, parts from Mike's carbs.
• Fully electric choke conversion installed instead of the '72 hot air choke.
• New small-cap, single vacuum Duraspark II ignition. Vacuum connected to carb port on right side, just forward of the throttle shaft.
• New standard copper plugs, gap opened to .035".
• Air screws, float level and accelerator pump adjusted per instructions from Mike's Carbs.
• Timing at idle 8° BTDC.

The engine starts easily cold, or when fully warm. In between still starts OK with a little throttle. The engine spits and coughs a bit if you blip the throttle during warm up. Once fully warm, blipping the throttle no-load revs cleanly. But trying to rev with any load, like trying to start out from a stop in first or reverse it coughs and bucks. If I baby it or over-rev and slip the clutch I can get it going, but any further throttle under load causes more coughs and bucking.

I have a feeling that this is carb-related, not ignition. But since this is a brand new ignition (new dizzy, DS box, etc.) I don't want to rule out something there, either.

Thoughts?

Sounds like a problem in the primary idle circut. Unplug a small vacuum port on your intake, has to be manifold vacuum. If the engine runs smoother with the port unblocked its your carb.

 

[ntsqd]

I don't know where you're getting your info, but it is exceptional to my experience. Much beyond 34° total advance and it would ping. This is consistent with what other's have reported over years, not just this thread. I was also told this by the owner of the racing engines machine shop that I worked in as an undergrad. SBF's do not need or want as much total timing as an SBC wants by ~2° As an aside, I also keep in mind what Smokey said decades ago, "2° too much timing will hurt the engine, 2° too little timing won't matter." I forget if that was in his "Power Secrets" book or one of his Circle Track columns. I have read that the better the combustion chamber design, the less total advance an engine needs.

With the mechanical advance in most Ford distributors that puts the initial in the 10°-12° range. Push the initial much past that and hot starts begin to fight the starter. I tried setting the dist. to it's lower mech. advance setting and running more initial. The engine didn't like it. Hard hot starts and detonation at lower rpm were the results that I got. Too much initial timing.

If your engine likes more than 10°-12° and doesn't detonate at higher rpm then run more, but it's an exceptional thing. Would suggest to me that perhaps the cam timing is late, but whatever.

 

[EPB72]

Grey owl no liking your #S.. so your base timing is 24DEG and total34=36 that leaves 12Degress mechanical .... It would be nice to Know What RPM your all in at?? I'm guessing well above 3000 more like 3500,, maybe,, Thatwould be pretty common on alot of aftermarket performance dist.. {MSD| mallory.. not sure asbout the ready to run pertronics or these chinnede knock offs,,, but they regularly have overly conservative advance curves , most need adjustments ......both advance travel and spring tension to get all in at or slightly below 3k,, 22-24 degress ...

 

[ntsqd]

Should not always follow "the way we've always done it" without question, but the way that we've always done it may be rooted in that's what works the best in most situations.

 

[Grey owl]

Grey owl no liking your #S.. so your base timing is 24DEG and total34=36 that leaves 12Degress mechanical .... It would be nice to Know What RPM your all in at?? I'm guessing well above 3000 more like 3500,, maybe,, Thatwould be pretty common on alot of aftermarket performance dist.. {MSD| mallory.. not sure asbout the ready to run pertronics or these chinnede knock offs,,, but they regularly have overly conservative advance curves , most need adjustments ......both advance travel andacce spring tension to get all in at or slightly below 3k,, 22-24 degress ...um

My timing method is simple but it is effective. First of all with the engine modifications i have make the stock specs are completely useless. I use a vacuum gauge to determine the the absolute maximum initial timing the engine will take before it noses over. Then i check that with a light. On this engine that is 24-26 degrees of initial timing. That is not what i am going to run it just gives me a baseline of the max. I then use the light to set the amount of initial i will run. The max total timing for this engine is 36 degree at 3500 rpm. So 18 of initial gives me 18 of mechanical. The engine stops making power at 5500 pretty common for a street engine. 18 gives me the best drivability it starts instantly when hot lets the engine pull hard all the way to 5500 without pinging on 87 pump gas.
Maybe 12 is all your engine can handle i dont know they are all different. But don't try and tell me that 18 is too high bc it isn't for this particular set up. In fact its on the low side it doesn't like anything lower.
It does make putting the right pieces a little more challenging . For example useing a 600 vacuum secondary carb created a dead spot just off idle and the lower engine vacuum created by the cam made tuning it out difficult. It could ould been done but the simple solution was putting on a Holley Braller series 650 double pumper. That carb is far more adjustable in terms of air fuel mixture. Look making broad base claims that 18 of initial timing for a sbf is too much is just ridiculous, it really is. Forget stock specs and tune for what you have.

 

[ntsqd]

I wasn't telling you that 18° is too high for your engine. I was telling you that 18° is far higher than decades of many people's combined experience with these engines has shown to be the normal max and for you to suggest someone else use that is irresponsible. You don't know their engine's configuration and by your own admission yours is so far from stock that what works on your engine is very unlikely to work for anyone else.

I've NO idea what the stock spec is, I've never owned an all stock SBF so I've never bothered to look it up. If it wants more than 10° initial then give it more, but don't start at more than that.

 

[Grey owl]

I wasn't telling you that 18° is too high for your engine. I was telling you that 18° is far higher than decades of many people's combined experience with these engines has shown to the normal max and for you to suggestr someone else use that is irresponsible. You don't knowars their engine's configuration and by your own admission yours is so far from stock that what works on your engine is very unlikely to work for anyone else.

I've NO idea what the stock spec is, I've never owned an all stock SBF so I've never bothered to look it up. If it wants more than 10° initial then give it more, but don't start at more than that.

I was putting 18 initial timing in sbf sìnce i was 16 today i am 66. I have probably had 20 of them in various vehicles including broncos. Mustangs cougars etc. Stock spec is 8 and any stock 289, 302, 351 windsor could easily run 14 to 16. Its an engine the just seems to love higher initial timing.

 

[ntsqd]

And based on my 40 odd years of messing with SBF's you're the outlier.

 

[serial car restorer]

Wanted to circle back and close out this thread. With some advice and suggestions from Mike at Mike's Carbs, I was able to find and correct the issue. The root cause was a vacuum leak at the power valve cover, which lead to it not enrichening the mixture properly when vacuum dropped. The leak was caused by a slight warp of the power valve cover. Sanding it flat on a machined surface corrected the issue, and the rig now (finally) runs as it should, pulling smoothly under all conditions. Something it hasn't done in about 20 years...

Can't recommend Mike's enough. He had quality parts, and the knowledge to assist via email.

 

[DC_Gearhead]

Awesome! Glad you got it fixed


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[serial car restorer]

Awesome! Glad you got it fixed


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You and me both! Felt like I was beating my head against the wall for a while on this. I really wanted to make this carb work, I don't really care for most aftermarket carbs.

Did one last change after dinner tonight. During the step-by-step troubleshooting I had installed the slightly richer #49 main jets. While it seemed good on the test drive, it still smelled a touch rich, and the plugs were a little dark. So I reinstalled the #48 jets I started with. I'll take it for another spin tomorrow, but I expect similar performance to today, with maybe a touch less exhaust smell.

 

[73azbronco]

i recall 12deg as the sweat spot. n YMMV. Considerably.

 

[serial car restorer]

Today's test drive with the leaner main jets was successful. Starts great, hot or cold. Idles smoothly at about 650 rpm. Pulls nicely, no stumbles or hesitations. And the exhaust smell is cleaner.

Final config (for my local 400-800' elevation):

• Carb - Motorcraft 2100, D2AF, 1.08" venturies, '72 Bronco, auto choke converted to full-electric
  • Main jets - 48
  • Air screws - 1.75 turns out
  • Accelerator pump linkage
    • Hole 2 on throttle shaft lever (hole 1 nearest shaft, hole 4 at end of lever)
    • Inner hole on pump lever
• Duraspark II ignition, small cap, single vacuum
  • Initial timing 14° BTDC
  • Plugs gapped at .035"