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Discussion Starter · #1 ·
Hey all, I've been having trouble getting my engine to run smoothly. Falls flat and has some pretty severe stumbling going on when I try to accelerate. Idles fine and revs smoothly when in neutral, but sucks under load. Wondering if I'm running rich?

I'm wondering if it has something to do with my choke not working properly. I did some research today and I have questions.

I have the automatic heat-operated choke. It runs off of the heater hose, and seems to work fine--the choke plate opens as the engine gets warm, my fast idle seems to be working.

Anyway--can anybody tell me what to do with the circled ports? I've seen pictures of them plugged/capped and the heat tube going to an exhaust manifold. I have aftermarket exhaust, so that's not an option for me unless I get one of those heat stove setups.

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What do these ports do? What's the difference between the heat tube and the heater-hose powered choke? Do I need both? Can they cause problems if left the way they are?

Any and all general/specific information is very welcome. Thanks!
 

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The places you have circled are indeed ports for the automatic (hot air) choke tubes. They run from the passenger exhaust manifold to the carb and heat the choke spring at start up. If they are missing your choke might not functioning (opening and closing) the way its supposed to. The heater hose is probably the only thing making it open.

That said if the choke is open during driving this should not in anyway affect the cars operation as you described. Have you verified the plate is all the way open when you are driving?
804974
804975

If you still have the stock manifolds this is what those connections would look like. Since you have after market exhaust just cap the ports and maybe think about an electric or manual choke
 

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If you don’t plan to use the correct chock tubes, you need to plug the fresh air tube on the carb. (The upper port you circled) Otherwise you are pulling unfiltered air into the carb.
 
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Discussion Starter · #4 ·
The places you have circled are indeed ports for the automatic (hot air) choke tubes. They run from the passenger exhaust manifold to the carb and heat the choke spring at start up. If they are missing your choke might not functioning (opening and closing) the way its supposed to. The heater hose is probably the only thing making it open.

That said if the choke is open during driving this should not in anyway affect the cars operation as you described. Have you verified the plate is all the way open when you are driving? View attachment 804974 View attachment 804975
If you still have the stock manifolds this is what those connections would look like. Since you have after market exhaust just cap the ports and maybe think about an electric or manual choke
Thanks, this is very helpful. I can see the hot air tubes going off the manifold to the fitting by the choke. It sounds like they need to be used in addition to the heater hose to operate the choke? If I have JBA shorties, am I basically forced to convert to manual or electric choke?

I've seen things like this:

804980


Do they work well?

And to answer your question, yes--I have verified the choke plate is opening properly. I did this when messing with the fast idle cam. It's so strange: engine starts up fine, idles at set fast-idle speed until warm, blipping the throttle brings it down to normal idle RPMs, and it even revs up smoothly. All seems great until you go to drive; giving it gas (even applying gas gently) make the engine stumble, shake, etc. Feels a lot like when you let the clutch out without giving it enough gas. It sucks hearing that you don't think this could be the cause of my problem.

I've checked and rechecked and rechecked again my timing. My mixture screws are 1.5 turns out. The strange thing is that I was taking the car to work for a week and it was running just fine. All of the sudden, it falls on its face when trying to accelerate. I didn't change much. I've checked the accelerator diaphragm in the carb by looking at the squirters while pushing on the throttle. They shoot fuel just fine, for the entirety of the throttle stroke. That, and if my timing is somehow off (how common is it for the timing ring around the harmonic balancer to slip?), is the only thing I can think of. I'm wondering if my fuel pump bit the dust?
 

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Discussion Starter · #5 ·
If you don’t plan to use the correct chock tubes, you need to plug the fresh air tube on the carb. (The upper port you circled) Otherwise you are pulling unfiltered air into the carb.
Thanks, good info. I figured as much; I'll find a cap for the time being while I figure out what I'm going to do with my choke. Cheers!
 

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Discussion Starter · #6 ·
Also--what's the purpose of the fresh air tube that goes from the manifold to the fitting on the bottom of the air horn (the upper of the two I circled in original post) and pulls filtered air? It just gets hot and does what exactly?
 

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If you want to keep the hot air choke with your headers, get some copper tubing, use it to make 4 or 5 loops around the #3 primary header tube, and connect it to the OE choke heat tubes with compression unions.
 

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Choke really should have little to do with this. Your engine runs after cold start and coming off of fast idle. Choke had done it most important function. Other than maybe keeping the butterfly open.
You can check it for 12v after warmup, but what you’re experiencing sound like either a vacuum leak, a blown accelerator pump (diaphragm) or a low fuel pressure condition. This can be caused by either a booggered up fuel filter, stuck float (general carb issues!) it cold be so many things. Don’t forget to check for correct spark/cap/rotor operation…esp if you’re using those god awful points 😉 then get out the dwell meter.
 

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It sounds like they need to be used in addition to the heater hose to operate the choke?
Not in addition, that is the primary way the choke was designed to operate. The heater hose is the supplement not the other way around.

But I think we've already determined the choke is not causing your problem. Others already gave some great input as to what to check. I would add that rechecking your idle mix and making sure you don't have a lean or rich condition would be a good idea. You said your mixture screws were 1.5 turns out, but is that giving you the smoothest idle? Most go by ear or vacuum gauge not the amount of turns.
 

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Discussion Starter · #11 ·
Not in addition, that is the primary way the choke was designed to operate. The heater hose is the supplement not the other way around.

But I think we've already determined the choke is not causing your problem. Others already gave some great input as to what to check. I would add that rechecking your idle mix and making sure you don't have a lean or rich condition would be a good idea. You said your mixture screws were 1.5 turns out, but is that giving you the smoothest idle? Most go by ear or vacuum gauge not the amount of turns.
I've spent a fair amount of time messing with the mixture screws. I have both a tachometer and a vacuum gauge that I have used while turning the mixture screws. The vacuum does not show much variation when tuning with the mixture screws, other than when I turn them in too far--the engine gets rough and dies, obviously. Turning them out doesn't seem to make a difference, but they seem happiest (highest idle) about 2.75 turns out. That seems like a lot to me. My carburetor was rebuilt at sea level and I'm at 6,000 feet, so it seems like that would be way too rich?

I think it could be a timing thing. I've disconnected and plugged the vacuum advance from carb to dizzy and then set timing to 12 degrees. Again, idles and revs fine, but when under load, stumbles and is extremely weak. How common is a slipped timing ring? Can I set timing via vacuum? I've read all over the internet that people have been doing their timing by adjusting for max vacuum and then backing off one or two hg for today's fuel. I've also read that this is a terrible way to do it; best vacuum at idle will grenade your engine when driving. What gives?

Pro-Vacuum Timing threads:

Anti-Vacuum Timing:
That article appears to be written by somebody who knows their stuff. I want to trust it more than the other threads above. See the "Ignition Timing" section about 2/3 of the way down.

I guess my next step is to turn the engine over to TDC and then pull plug #1 to verify the piston is actually there? Any other thoughts? How can I test my mechanical fuel pump? My vacuum gauge also does positive PSI but wouldn't fuel get into the gauge if I just plumbed it into the fuel line?
 

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Discussion Starter · #12 · (Edited)
If you want to keep the hot air choke with your headers, get some copper tubing, use it to make 4 or 5 loops around the #3 primary header tube, and connect it to the OE choke heat tubes with compression unions.
Thanks Woodchuck, I appreciate your consistent (and knowledgeable) feedback/advice. If I were to go this path, what would I do for the fresh air tube?

And would a choke stove kit work instead of wrapping copper tubing around the header as well? I will if I have to, but I think the choke stove would look cleaner.
 

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Discussion Starter · #13 ·
Choke really should have little to do with this. Your engine runs after cold start and coming off of fast idle. Choke had done it most important function. Other than maybe keeping the butterfly open.
You can check it for 12v after warmup, but what you’re experiencing sound like either a vacuum leak, a blown accelerator pump (diaphragm) or a low fuel pressure condition. This can be caused by either a booggered up fuel filter, stuck float (general carb issues!) it cold be so many things. Don’t forget to check for correct spark/cap/rotor operation…esp if you’re using those god awful points 😉 then get out the dwell meter.
Thanks, some great overall advice here. It does somehow seem like a fuel supply issue to me more than a timing problem, because I think the chances of the timing ring on the balancer slipping are very low. On a low pressure application like the mechanical fuel pump, how can I test it? Pull a line and have it squirt into a bag? My vacuum gauge doubles as a positive PSI measurement tool, but wouldn't fuel just go into the gauge if I hooked it up?

I really hope it isn't anything significant with the carburetor. The darn thing was rebuilt by Dan Nolan at Mustang Barn when he recurved my distributor. It has probably about 100 miles + engine break in on it. A vacuum leak might be more likely, but I have thoroughly sprayed carb cleaner around the carburetor spacer, intake runners, all over--and nothing changes vacuum. I suppose it could be an internal leak somewhere, but I've seen no signs of fluid cross-contamination and the engine was recently rebuilt with new gaskets and all.

It's got Pertronix II electronic ignition. I find it unlikely this could be the culprit: is it possible that it could be causing problems only when under load? I feel like a problem there would show up at idle and/or when revving in neutral as well?

When you say it could be a clogged fuel filter, which filter are you referring to? I have no fuel filters in my system other than the one in the fuel pump. It's got the canister style with the integrated filter.

Thanks for the input. Much appreciated as I try to figure this out!
 

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Discussion Starter · #14 ·
Thanks Bob, love seeing pictures like this. Why the heck can't I find these in my shop manual? Where are those from? Is that from a different year than the '65 shop manual?

Regardless, I appreciate it. I'm far from stock on this car so I'll have to think something up, but those are a great help to me in figuring out what I need to accomplish with my setup.

Cheers!
Alex
 

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Alex you've gone from being sure its a timing problem to being sure its a fuel problem in the span of 30 min lol. Lets slow down a bit and let the knowledgeable people respond (definitely not me lol).

In the mean time tell us what kind of car and motor you have. All we know is you have a autolite 4100. Is this a new problem or has the car been sitting? Any recent work done? How old is that fuel pump and filter? What vacuum readings are you getting?

You can get a shop manual online. I got mine at the motor bookstore for $41.95

 

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Discussion Starter · #16 ·
Alex you've gone from being sure its a timing problem to being sure its a fuel problem in the span of 30 min lol. Lets slow down a bit and let the knowledgeable people respond (definitely not me lol).

In the mean time tell us what kind of car and motor you have. All we know is you have a autolite 4100. Is this a new problem or has the car been sitting? Any recent work done? How old is that fuel pump and filter? What vacuum readings are you getting?

You can get a shop manual online. I got mine at the motor bookstore for $41.95

I'm not sure of anything, nor do I think I declared I was. I've been exploring the possibilities of timing and fuel causing my problems is all.

The car is a '65 coupe with a mildly built 289. Engine has been rebuilt, as well as the carburetor and distributor. This is a new problem for me, but a part of the break-in/tuning process; there's only about a hundred miles on the engine. Vacuum sits at a solid 15hg and needle is steady, but more vacuum can be attained by reducing the advance. As stated earlier, not sure if I should be doing timing by vacuum or by timing light... currently, it has been set with my timing light. I posted some links earlier that iterate my confusion and uncertainty about whether or not it's okay to adjust timing via vacuum. I obviously don't want to grenade my engine 😁

All the engine work is generally very recent. I got it broken in last fall, it sat over the winter without any action, then I've been slowly getting it running and working out the kinks this summer. Countless little things to do on the car but engine work has been priority. All components were either rebuilt or replaced; there's nothing old in this engine. I installed the fuel pump when building the engine about three years ago, but again, it was broken in more recently and it should only have about 100 miles on it.

Cam specs for reference:
805116
 

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I'm not sure of anything, nor do I think I declared I was. I've been exploring the possibilities of timing and fuel causing my problems is all.
I meant no offense! Its just that your were rapid posting and jumping around a bit. ;)
 

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Discussion Starter · #18 ·
I meant no offense! Its just that your were rapid posting and jumping around a bit. ;)
No offense taken! Didn't mean to sound like I was offended, lol. I try to give a good response to every little bit of feedback I get because of how much I value people taking the time to pitch in. I do rapid post a fair amount in an attempt to provide as much information and address as many people as possible.
 

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Try this - remove air cleaner from carb. Start engine and have an assistant put car in drive with their foot firmly on brake. You are under the hood to operate the throttle at carb. Slowly give it some throttle and when it starts to spit and sputter as you described give it a little spray of carburetor cleaner and see if it smooth's out. If it does, then you have a lean condition on acceleration.
If not, pull a couple spark plugs and see if they are black and carbon fouled. If so, then it is an over fueling problem. ( I doubt this is the case or it wouldn't idle good with fouled plugs).
It also sounds like there could be an ignition problem ( plugs or wires arcing to ground, moisture in distributor cap). These things some times don't show them selves unless engine is under a load.
Doesn't sound like a timing problem even if it where to far advanced or retarded. Under acceleration, incorrect timing would cause low power (retarded) or spark knock (advanced).
 

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Discussion Starter · #20 ·
Try this - remove air cleaner from carb. Start engine and have an assistant put car in drive with their foot firmly on brake. You are under the hood to operate the throttle at carb. Slowly give it some throttle and when it starts to spit and sputter as you described give it a little spray of carburetor cleaner and see if it smooth's out. If it does, then you have a lean condition on acceleration.
If not, pull a couple spark plugs and see if they are black and carbon fouled. If so, then it is an over fueling problem. ( I doubt this is the case or it wouldn't idle good with fouled plugs).
It also sounds like there could be an ignition problem ( plugs or wires arcing to ground, moisture in distributor cap). These things some times don't show them selves unless engine is under a load.
Doesn't sound like a timing problem even if it where to far advanced or retarded. Under acceleration, incorrect timing would cause low power (retarded) or spark knock (advanced).
Thanks, this is some good guidance. Feeding it carb cleaner when sputtering is an interesting suggestion, and I'll have to give it a try.

After fixing up my choke per @Woodchuck's suggestion (wrapping copper wire around header), the car runs slightly better now. There is still a consistent miss/stumble under load, but it's not nearly as bad. I'm not sure it's the choke, or the timing--I messed with both today. I adjusted the timing to where it seemed happiest, then backed off a little bit. This is giving me a lot of advance--timing light measures about 20* of timing--but I'm not sure I trust the timing ring on my harmonic balancer.

I have another question. When giving the car a little throttle and holding it there, are the squirters in the carburetor supposed to shoot a continuous stream of fuel? When I push and hold the throttle linkage to bring the RPMs up to about 2,000 there is only a squirt of fuel at the beginning and then nothing else as the engine continues to rev.

Thanks in advance,
Alex
 
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