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Discussion Starter #1 (Edited)
Hi All-

I have a new-to-me 96 T-Bird. I bought it from a guy who had carbs done and a lot of other work--$2k worth--by the local triumph dealer 4 years and 20 miles ago (you read that right). 10k miles total, so it's sat a lot over its lifetime. it ran, but only with the choke full on, and then with a lot of backfires. I coaxed it home like that, and removed and went through the carbs, re-using all of the original gaskets. Everything seems to be in good order, and all the jets were cleared and cleaned. The floats were equal and 15mm. There was some varnish, but it wasn't horrible. Air screws at 2.5 turns on reassembly. New NGK plugs and an oil change. I cleaned the tank as well. Put it all back together and it (amazingly, to me) fired right up. But.

The backfires are possibly worse than before. Only on cylinder #1 based on the exhaust--I don't know how you could tell if it were on the shared pipe. I believe everything is stock, including the exhaust which is Triumph but labeled "not for road use". Side quest: are these TORs? [edit: yes]

So...I guess my question is, what could it be? Presumably it ran a treat after the PO dumped a ton of cash into it, and that was no miles ago so I wouldn't think coils. Rubbers were replaced then and they show some slight cracking but they're THICK, and I doubt they're leaking air through. I tried the starter fluid trick and thought I heard a slight uptick in RPM but it wasn't dramatic.

If I go through the carbs again, I'd replace the gaskets. Can anyone recommend a kit that includes the basics? And I'm open to replacing the rubbers on either side of the carb and I heard about the Speed Triple trick with bigger jets. Is that a thing and is there a well-known formula for the jets?

Thanks for any input.
 

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If the rpms changed with the starter fluid the air leak needs to be addressed before doing anything else. Which cylinder did the rpms uptick on? A shop red assed me when I brought in my 96 to have some tuning done and there was a clamp or two loose (this was many years ago) and he said you cannot get a bike to run properly with any air leaks.

The other items to check are the o-rings for the floats presuming you have Mikuni carbs. They can degrade and leak, do you have any fuel leaking anywhere? The vacuum caps need an o-ring under the carb tops, make sure they are in place as they are easily lost.

Have you bench synced the carbs using the drill bit method?

Your float heights appear to be good if you have Mikuni carbs.
 

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Hi,
During last two years I have passed througout Mikuni carbs rebuilt and Keihin as well. Stock components and almost all new inner parts, ultrasonic cleaning, exact float level setting, pilot screw 2.5 turns out- set the screw to get hightest rpm on idle( checked by notebook with ignitech ignition unit).. What a surprise :AFR on idle 10.5-11!.. No way for me to figure out just by engine sound...No surprise so far still not 100% satisfied -even if it run nicely, but usually black spark colour...So finally I got AFR gauge, got welded lambda connectors on manifolds and set pilot jet screw to get idle 13.5 AFR -..in each carb different position -I guess orifice in carb (where tip of screw is coming out) is just a little ( not visible)wider as the years/ previous settings went by..

To confirm theory: I tried older pilot screw with normal tip appearance and what a huge difference in AFR ( usually just a little respons of screwing in on AFR and finally very lean as the tip closed the orifice, so cone shape of the tip must have been worn out) makes with just a small invisible worn out of the tip and carb orificium. I can not imagine to set precisely all 3 carbs without AFR gauge
 

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Why is that every time someone has a problem with a carburetted bike they assume it must be the carburetor's fault ? Backfiring out of the exhaust under load is unburned fuel being ignited by the hot exhaust. That does NOT mean too much fuel is being supplied by that carburetor. You did the carburetors. Actually they have been done TWICE. Look somewhere else. I strongly suspect a bad coil. [Maybe a bad plug wire]. Something that is only happening in that cylinder. You only need to go through a set of carbs once. Why waste your time doing it for the third time ? ...J.D.
 

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Too lean a mixture can also cause backfiring (actually, afterfiring; back firing is back, out the carb). Lean mixtures burn slower and are still burning when the exhaust valve opens, causing the noise.

Which begs the question: are we talking backfiring or afterfiring?
 

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Discussion Starter #6
Too lean a mixture can also cause backfiring (actually, afterfiring; back firing is back, out the carb). Lean mixtures burn slower and are still burning when the exhaust valve opens, causing the noise.

Which begs the question: are we talking backfiring or afterfiring?
I'll be honest, I have no idea. I would have said backfiring but your statement above makes me question everything I know about internal combustion engines (admittedly, not all that much). The pop is heard and felt out the exhaust. I've posted a video, which may or not be helpful in answering this question. YouTube link

Apologies in advance for changing to landscape after starting off in portrait...
 

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First thing I would do is move that ignition coil to a different cylinder and see if the problems moves with it. I have a long history with these engines. They are known for having crappy ignition coils. Doing that will eliminate any guesswork about the coil. ...J.D.
 

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if you swap coils over and it still does it then it may be worth checking valve clearances too, intake might not be fully closing on that cylinder, it is low mileage but i think they are supposed to be checked at about 12000 so might be worth a look
 

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CannuckTBS and Wire-Wheels made some very good suggestions. The coils always seem to be a lurking issue especially with the Gill coils but I have had a PVL coil fail that is rumored to be more reliable. I have had an 70’s Kawasaki make your popping sound which did turn out to be a tight valve.

If this was my bike I would do two things first: try moving the coils around like Wire-Wheels recommended to see if the popping changes. If this doesn’t change anything you need to look for an air leak since you feel it pops worse than before. It’s possible a clamp isn’t sealing properly or rubber seated properly. Did you take the tops off the carbs where the rubber diaphragms are located? Did they look ok? Is it possible one of the tiny o-rings under the vacuum caps is missing? Are the vacuum caps tight? The reason I say this is with the vacuum operated petcock on my 96 if I pull the vacuum hose off the fuel valve which is tapped into #2 carb creating an air leak it will immediately start popping similarly to yours.

After doing those and it still acts up you may need to check valve clearances like CannuckTBS suggests :)
 

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While I can buy in to the idea of a tight valve, I am thinking not. Won't get in to why just yet. I like to add the ingredients to the diagnostic stew, one ingredient at a time. It is just a little easier to digest. If moving the coil does not shed light on what tbe problem is, move the plug wire next. ...J.D.
 

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Is the popping random, or does it happen, e.g. when you open the throttle, or accelerate? If it's random, I'd lean more towards ignition. If it happens in response to throttle then maybe carburetion.

Have you pulled the plug and looked at it?
 

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Discussion Starter #12
Is the popping random, or does it happen, e.g. when you open the throttle, or accelerate? If it's random, I'd lean more towards ignition. If it happens in response to throttle then maybe carburetion.

Have you pulled the plug and looked at it?
It's actually pretty constant. The backfire (I'm sticking with that term until I understand better what "afterfire" is) is actually pretty sharp/loud and I wanted to make sure I wasn't doing any damage. So I'm not sure I'll be able to tell a lot from the plug.

I'm going to try to drop by the local shop today and pick up a new set of rubbers. My money is on an air leak somewhere. the vacuum cap on that carb doesn't match and looks a little sketch, but it keeps suction on my tongue so I assume it's OK. But I'll move it over to another cylinder because it's easy. And I'll move the coil and/or wire over to eliminate that. Again, because it ran well 25 miles ago and then sat for a while, I doubt it's something electro-mechanical but if it's easy to rule out, I'll do that first.

I did put the o-ring around the vacuum port back, but used the original gaskets for everything so it's conceivable that lets in air. I did the same with the petcock and now that leaks pretty good so it's definitely possible.

I'll shoot out an update with new info when I get it but it'll probably be the weekend. Thanks for all the input guys, I absolutely appreciate it.
 

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Hi All-

I have a new-to-me 96 T-Bird. I bought it from a guy who had carbs done and a lot of other work--$2k worth--by the local triumph dealer 4 years and 20 miles ago (you read that right). 10k miles total, so it's sat a lot over its lifetime. it ran, but only with the choke full on, and then with a lot of backfires. I coaxed it home like that, and removed and went through the carbs, re-using all of the original gaskets. Everything seems to be in good order, and all the jets were cleared and cleaned. The floats were equal and 15mm. There was some varnish, but it wasn't horrible. Air screws at 2.5 turns on reassembly. New NGK plugs and an oil change. I cleaned the tank as well. Put it all back together and it (amazingly, to me) fired right up. But.

The backfires are possibly worse than before. Only on cylinder #1 based on the exhaust--I don't know how you could tell if it were on the shared pipe. I believe everything is stock, including the exhaust which is Triumph but labeled "not for road use". Side quest: are these TORs? [edit: yes]

So...I guess my question is, what could it be? Presumably it ran a treat after the PO dumped a ton of cash into it, and that was no miles ago so I wouldn't think coils. Rubbers were replaced then and they show some slight cracking but they're THICK, and I doubt they're leaking air through. I tried the starter fluid trick and thought I heard a slight uptick in RPM but it wasn't dramatic.

If I go through the carbs again, I'd replace the gaskets. Can anyone recommend a kit that includes the basics? And I'm open to replacing the rubbers on either side of the carb and I heard about the Speed Triple trick with bigger jets. Is that a thing and is there a well-known formula for the jets?

Thanks for any input.
May be tight valves have you checked them
 

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Discussion Starter #15
OK, so a bit of an update here. I came home with an hour of daylight to work with and moved both the coil and wires (separately). The afterfire (ok, now I understand) still occurred on #1. So I think I can safely rule out electronics.

But what I discovered is that it purrs at slow idle, say less than 1400RPM. The pops start occurring at over 1500RPM and I'd say they get progressively worse. I know I've run across posts that list which jets or sub-systems kick in when (idle vs. 1/4 to 3/4 throttle vs. full throttle), so I assume I'll be able to narrow it down once I google-fu that. I think it's running rich at higher RPM. Progress.

It's actually pretty constant. The backfire (I'm sticking with that term until I understand better what "afterfire" is) is actually pretty sharp/loud and I wanted to make sure I wasn't doing any damage. So I'm not sure I'll be able to tell a lot from the plug.

I'm going to try to drop by the local shop today and pick up a new set of rubbers. My money is on an air leak somewhere. the vacuum cap on that carb doesn't match and looks a little sketch, but it keeps suction on my tongue so I assume it's OK. But I'll move it over to another cylinder because it's easy. And I'll move the coil and/or wire over to eliminate that. Again, because it ran well 25 miles ago and then sat for a while, I doubt it's something electro-mechanical but if it's easy to rule out, I'll do that first.

I did put the o-ring around the vacuum port back, but used the original gaskets for everything so it's conceivable that lets in air. I did the same with the petcock and now that leaks pretty good so it's definitely possible.

I'll shoot out an update with new info when I get it but it'll probably be the weekend. Thanks for all the input guys, I absolutely appreciate it.
 

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Have you tried the started fluid check for air leaks again? At higher rpms when it’s popping?
 

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Yes, NOW I would check valve clearance and compression. Might also check for a valve sticking open. Also might make sure the slide on that carb is not sticking. Let us know what the outcome is. ...J.D.
 

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Discussion Starter #19
OK. Lean or rich can cause the afterfire though, right? I guess I was thinking there's too much fuel, some of it escapes unburnt, and then pops in the exhaust.

I created another video. Same popping, but I got better audio at the pipe. Doused the carb with starting fluid and I don't think there's much of a difference in RPM, but there IS if I spray it at the junction between the two airboxes. Not sure if it would make a difference if air came in there though, as it's still upstream of the filter and literally across from the snorkel. https://youtu.be/bE6id2wO23Q

Apparently the needle jet and emulsion tube are factors once it moves off idle and the pilot jet. At least that's what I'm gleaning from my search. I tried but did not get the plastic insert and emulsion tube out when I went through the carbs. I've since found a video covering that. Does that seem like a reasonable next step? I don't see how the valve clearance could have changed significantly over the last 25 miles and I'd prefer not to dive in that deep right now.
 

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There are four tiny holes in the throat of the carb near the idle jet. They control fuel at small throttle openings. If they or the passage feeding them are blocked it will stumble off idle. They are fed by the pilot jet and it's emulsion tube (all one piece.) There is an associated air bleed jet in the carb inlet. the needle and needle jet are fed by the main jet and its (separate) emulsion tube. (Both emulsion tubes have 8 or 10 air bleed holes in the sides that are easily blocked.) the main circuit has a separate air bleed jet.

A week or broken spring on the slide can cause the slide to lift too rapidly making an extremely lean mixture. Also, confirm the float level is correct and the needle valve is feeding adequate fuel.

If you go to full throttle does the afterfire subside?
 
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