Old school carb and ignition tuning

[serial car restorer]

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?

 

[DC_Gearhead]

Check your filters and fuel pressure first. If you’ve driven it a little id say check your plugs to see what color they are. What color is your exhaust? Also do the spray trick to check for vacuum leaks including in the body of the carb. Sometimes the bushings around linkage gets worn out and you lose vacuum there.

Have you tried increasing the timing and driving it? I ask because I run around 11 or 12. Did you try setting timing with a vacuum to see where your motor wants to be?

I’d say check the adjustments on your accelerator pump and be sure your new electric choke is getting power and adjusted right. Fuel should spray with even a little bit of movement in the accelerator. When warm are the flaps open?

Once you check the easy stuff we can talk about jet size and float levels.


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[jckkys]

I've always been able to get a lot more HP and MPGs out of a stock small block going with 20 degrees initial advance. My 351W now has 9.5 to 1 compression and still likes 20 degrees. The factory was overly concerned with potential pre-ignition. You can quickly deal with that if it happens.
The factory A/F mixture was set on a 2100 to be fairly close to ideal. A wide ban A/F gauge may improve it a little, but not much.

 

[serial car restorer]

Thanks for these ideas, guys. I was thinking about vacuum leaks a bit after I posted, and will do some additional checking on that.

I have not set timing by vacuum, and I did think that 8° was fairly conservative, but safe. I'll bump the timing up some to see if that changes anything.

 

[73azbronco]

I'd say check electric choke, works cold, works hot, its in between not so bueno, might need an adjust. I would make it less choky as first try, then too choky.

How are plugs, vacuum?

 

[El Kabong]

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 wonder if your accelerator pump isn't working. It gives an extra squirt of fuel as you open up the throttle. Vacuum drops as you open the throttle, which throws off the fuel pulling through normal circuits. That extra squirt carries you through that transition.

With the key off, remove the air filter, hold the choke open, look down into the carb, and open the throttle. Watch & see if you get a steady stream of fuel into the venturis as you move the throttle. It should squirt fuel while the throttle is moving toward open as long as there is fuel in the bowl.

If it works, your problem is elsewhere. If not, the accelerator pump is bad, the linkage to it isn't right, or a passage is blocked.

 

[bronco italiano]

What is your vacuum gauge at idle?
I am gonna go accel pump too.
I adjust carb via vacuum for highest vacuum and wideband if possible
8 degrees with non-CA fuel is a good start.
Engine leakdown is ?
If worried about ignition, pop in the orig distributor just to rule out DSII issues, but DS is quite reliable

 

[ntsqd]

8° sounds a little late to me too. No idea what spec is, but as I recall I usually ran the Ranchero at 10°-12° BTDC. I can't imagine that such a small difference would matter that much, but worth looking at.

 

[mdainsd]

Bucking under load is also a symptom of a failing condenser, bad HT leads or failing coil. I have several WW2 vehicles and I time them all with a vacuum gauge.

At idle, adjust timing for highest vacuum. At sea level a reading of 19 inches of mercury or more is great. The needle should be rock steady (if not google diagnosing with a vacuum gauge). Then see if any more can be achieved with the idle mixture screws. go back and forth a couple of times and adjust idle speed if necessary. Finally retard the timing 1/2 inch of mercury from best. Test drive listening for pinging (Spark knock).

 

[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.

 

[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?

8 degrees timing, hmm i run 18 intial timing. SBF in my experience like lots of initial advance. Are you setting your air fuel mixture with a vacuum gauge???? It makes that adjustment fool proof. I think you should try moving the timing into the teens.

 

[ntsqd]

18° is a LOT! That dist must not have much mechanical advance in it.

 

[Grey owl]

18° is a LOT! That dist must not have much mechanical advance in it.

18 is not alot for a sbf. I run 18 of initial timing and about 34 total.

 

[ntsqd]

34° Total sounds about right, but 18° is a lot in my experience.

 

[DC_Gearhead]

I think it is so strange that SBF timing is so all over the place. I searched and searched when setting my initial timing and landed on 8 since it seemed to be pretty common. My rig wouldn’t start. I decided to time by ear and vacuum per my dad’s OG advice and I ended up at 14 and it ran great. Def no science in that process, but runs great. Since then I’ve tuned it a lot and ended up at 12. Lots of folks think that is entirely too much advance, but every engine is different. It’s wild.

That being said, holy cow….18….that’s a lot. LOL.


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[DC_Gearhead]

Serial - sounds like you are onto something with your two adjustments. The more you drive that carb the better it will run, like all carbs. If it’s off just a little, I’d drive it some and then go back and check everything. In my experience newly rebuilt carbs and carbs that have sat a while take a little driving to get them functioning with all the gunk blown out. Lots of little holes and tubes to get gunk in. Floats stick and needle valves get stuck or are not quite seating. Run it and get fuel flowing.


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[Grey owl]

I think it is so strange that SBF timing is so all over the place. I searched and searched when setting my initial timing and landed on 8 since it seemed to be pretty common. My rig wouldn’t start. I decided to time by ear and vacuum per my dad’s OG advice and I ended up at 14 and it ran great. Def no science in that process, but runs great. Since then I’ve tuned it a lot and ended up at 12. Lots of folks think that is entirely too much advance, but every engine is different. It’s wild.

That being said, holy cow….18….that’s a lot. LOL.


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Maybe i am old school, i have bee n turning sbf engines for 40 years. I was always taught by some hard core Ford guys give the engine as much timing as it will take without pinging. Thise guys tuned by feel. Just keep adding timing until its running ruff and pinging uder load then back it off till yhd pknging stopps

 

[DC_Gearhead]

Maybe i am old school, i have bee n turning sbf engines for 40 years. I was always taught by some hard core Ford guys give the engine as much timing as it will take without pinging. Thise guys tuned by feel. Just keep adding timing until its running ruff and pinging uder load then back it off till yhd pknging stopps

That’s exactly how I turned mine. My uncle worked at ford for 60 years and was old old school. He and my dad used to drive the Mustangs off the Kansas City assembly line at 14 yo to park them. He stayed for 60 years.


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[ntsqd]

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.

 

[El Kabong]

Maybe i am old school, i have bee n turning sbf engines for 40 years. I was always taught by some hard core Ford guys give the engine as much timing as it will take without pinging. Thise guys tuned by feel. Just keep adding timing until its running ruff and pinging uder load then back it off till yhd pknging stopps

x3