Carb questions

[srobinsonx2]

I know there are many threads about carb problems. I have read the tuning threads and understand the basic tuning process. My question is about what is the normal carb tuning frequency? Let me explain. I have a Levy Racing stage 4 347. Awesome engine and it runs great. On the engine I have a Quick Fuel 750 SS with an electic choke. I set up the carb like the "Carb 101" tuning thread describes. I have the transition slot on the primaries set so that it is a square. The secondaries are just barely open (just a slight crack). The idle mix screws are effective and end up being about 1 full turn open. My basic approach to tuning idle has been to adjust the idle mix screws to get the max rpms after the engine is warm. It works great and I end up with an very stable idle around 950 rpm. The throttle is very responsive and it drives great. Here are my questions/observations:

1. After a week or so, the carb seems to "drift" and I have to do a small adjustment. An example, ran great for a couple of weeks (cold starts, hot starts, etc). Today I took a drive and the idle was very low (<700) and felt like it was about to die. Had to adjust the carb. Is it normal to have to adjust frequently?

2. When tuned it runs great but seems to be rich at idle. I do not have an exhaust analyzer but smells rich. This is all based on my untrained amateur nose but it sure gases up the garage while it warms up.

Thoughts? Maybe I am just concerning myself with something that is normal. I certainly don't mind tinkering with the carb if needed. It is fun and I am learning something new.

Thanks guys.

FYI. This weekend I drove the car to a "Hot Rod Night" in a small town near me. It was fun and they let me park my car at the front of the show even though I got there a little late. A lot of fun and cool cars. The kids were the best.

http://thefactoryfiveforum.com/attachment.php?attachmentid=73821&d=1505694949

[Avalanche325]

The "carb tuning 101" is written for using a wideband O2 sensor. If you are going to tune your own carb, you really owe it to yourself to get one. Reading plugs will give you an overall, but no detail. For an extreme example, you could be perfect on the primaries, plugs read good because that is where you run a lot of the time. But you could be lean when the secondaries are open and at high RPM. That would be a VERY bad scenario. Tuning a carb without an O2 sensor, or about 25 years experience in a race shop, is guessing at best.

Setting you timing is step 1 of carb tuning. Make sure that is good at idle and full advance.

If your idle is not right, everything could be off. All circuits add to the others. At WOT the idle circuit is still at work. The water up to temp may not be enough warmup. My engine takes about 20 minutes the really be settled in. Drive it around before you set the idle. Your rich smell in the garage while warming up is just the choke doing its job. I could be crazy, but I swear the packing in the FFR pipes holds unspent fuel if you are rich, which will stink up a garage for a while.

Once your carb is tuned, basically that is it, it is tuned, unless you change something in the engine, or move to 10,000 ft above sea level. I do find that my idle changes a little depending on outdoor temp and I am running hard, like doing autocross in August. If I touch the idle stop screw though, I end up putting it right back.

[srobinsonx2]

Thanks Avalanche325. I do not have a wideband O2 analyzer but do plan on getting one. I have the nice polished Gas N Pipes and instead of welding in a bung I thought about trying the type that attaches to the end of the exhaust pipe (i.e. no bung). Have you tried those? Either way I will get something. I know I need to just delaying....not sure why.

I have set timing. 14-15 degrees at idle (950 rpm) and about 35 degrees when I get north of 3000 rpm.

So it sounds like you set your carb and are good except for some minor changes in idle with temp changes. That is the info I am looking for.

I also didn't realize that full warm up might take 20 mins. I have been considering warmed up when the cooling fan comes on (180 F). That usually takes about 5 minutes. I usually get the oil temp closer to 150 F after about 15-20 mins. I will certainly take your advice on waiting a little longer.

I have read a few other posts about rich carbs and it seems I can get the "eye burn" and "garage fumes" with lean conditions as well but that is usually accompanied by hot temps. I know......get an O2 sensor.

Thanks for the help. I probably need some more info before asking these ridiculously vague questions. :o I will regroup and come back with more info.

[srobinsonx2]

Here is a clamp that allows someone to clamp an O2 sensor in an exhaust tail pipe. From the same people that make the LM-2. Thoughts?

https://www.amazon.com/Innovate-Motorsports-3728-Exhaust-Cast-Stainless/dp/B000CO9MF8

[bobl]

With the size of your cam, you will always get fumes at idle no matter how lean it is. That is caused by the overlap in the cam allowing unburned fuel to flow out the exhaust. That's also what gives it the lumpy idle. I wouldn't trust the 02 sensor in the tail pipe, the readings are not real dependable. Just bite the bullet and get a stainless bung welded into your collector. It's easily polished out and you can plug it off when not using it.

Bob

[Jeff Kleiner]

...I have read a few other posts about rich carbs and it seems I can get the "eye burn" and "garage fumes" with lean conditions as well...

I was about to tell you that! So many guys wind up chasing problems the wrong direction. Although I've built a couple of carb cars in the last few years I have been mostly an EFI for the last decade or more but agree with the advice Avalanche offered. Once properly set up you really shouldn't need much more than a seasonal choke adjustment.

Jeff

[CraigS]

Mine doesn't vary much at all. Normal idle is about 850. After a longer drive it will be closer to 950 but that is the extent of variation. To avoid messing up your SS pipes you could put a bung in one of the header tubes. Put it in one of the top tubes just ahead of the foot box. Yes I know some like to sample mixed air and I can't fault the logic, but there are plenty of cars running O2s in the headers for their efi systems, so it does work. Once you work w/ an LM2 for a bit, I think you will realize it is a fantastic tool. BTW, get the rpm sensor too. You can do some of the light throttle stuff by eye while cruising around, but once you get towards full throttle it's best to do a recording and then review it later. Having an rpm line on the chart along w/ the afr line is really helpful.
Edit to add. Go to the other forum and look in classifieds parts for sale. There is an LM@ there for $200

[ThickCobra]

Where can I find this "Carb 101" tuning thread being referred to? I have all the same questions as srobinsonx2.

[boat737]

Where can I find this "Carb 101" tuning thread being referred to? I have all the same questions as srobinsonx2.

http://thefactoryfiveforum.com/showthread.php?2216-Carb-Tuning-101 But the pictures are gone, so... It's not helping me very much.

Calling Wayne Presley: Can you re post with pictures attached?

[srobinsonx2]

Thanks everyone. This feels like another opportunity to document the details of the process. You know..... for those of us that don't know a jet from a jam nut. What do you think?

[Frank Messina]

You don't need a $$$ wideband setup to get the idle mixture right if that's your issue. You can and should use a vacuum gauge plugged into a manifold vacuum source to dial in the idle mixture. You said it runs great otherwise. That's good, but take a look at your plugs and they will tell you if the jetting is correct or otherwise. There's a sequence to follow when performing these steps and it's been well covered so I won't re-hash it again now. Once correctly set though, you shouldn't have to mess with it.

Frank

[Frank Messina]

On your "idle drift" it is possible that when you previously set the idle speed the throttle was hung on the lower step of the choke cam. Once it dropped off for whatever reason the idle went down accordingly and 250 RPMs is about the difference. Chokes can really complicate life and unless one lives at the north pole you're just as well off without in most cases.
Frank

[Avalanche325]

I agree that you can get the idle close with a vacuum gauge. I have found it to still be a tiny bit rich according to the wideband. Yes on reading plugs if you just cruise down the highway. But you really can't tell from the plugs how it is doing across the board if you really drive the car. (old-school Carb experts excepted) You could lean out at WOT and high RPM and hole a piston when your plugs read fine. A wideband will help you get not just the jets correct, but the various bleeds and restrictors and ensure that you are tuned across the whole range. The vacuum gauge will also will let you know if you have the right power valve. I also agree about the choke. Having a Cobra that is snotty until it warms up is all part of the fun.

I did some reading about the clip in sensors when I got my wideband and came up with what everyone else is saying. It won't be accurate on a 4" pipe. It is not that big of a deal to have a bung welded in, and it is fairly hidden. It doesn't put it in the "perfect" spot as it is angled down a little (moisture concern)to clear the body. But, you only need it in the pipe when you are actually tuning.

My biggest problem was at low speed cruise. 35-40 mph. I have an aggressive cam, so I got some "trailer-hitching". I have to run rich in that range to smooth it out. Carb do take some compromises on a high performance engine.

[GFX2043mtu]

Not to say to spend money but in order to decently tune a dual bank motor you need a o2 sensor on both banks as one tends to run leaner then another (weld in type, sniffer types only work well in mid to top end). In fact with a common plenum for all cylinders and a common carb all the cylinders will vary in AFR more then you might think. In the end you are just looking for a good average across all of them. As far as what AFR to look for it depends on the motor. A very mild cam will like less fuel at idle and low speed then a more aggressive cam. How much depends on the summation of parts in the motor and even at that every motor is different. If you have a four corner idle circuit on your quick fuel like mine a vacuume gauge and tach is all you need to get it to run well. Just don't confuse run well for things like cam surge due the nature of certain parts. If you get cam surge at lower speeds but a good idle, mid and upper range then start by pulling 2 degrees of ignition timing only in the area where it surges if you have a programmable box. That tends to settle the motor down a bit and mask some of the intake reversion that is causing the cam surge. If you still can't get it where you want it my suggestion is to take it into a good shop that deals with high performance carb motors and pay them the $150-200 to tune it and when done ask them what was wrong so you know in the future.

[srobinsonx2]

Ok. Bung installation it is. What side of the car do you guys normally install the bung on? Drivers or passenger? I was thinking passenger and just downstream of where the 4 pipes join together in my gas n pipes.

[srobinsonx2]

Just for documentation, here is a picture of one of my spark plugs. New engine with about 480 miles. Sorry the picture is a little orange. I took it as the sun was going down.

http://thefactoryfiveforum.com/attachment.php?attachmentid=73971&d=1506042708

Not sure if this looks good or not but I compared it to some pictures on line and didn't look like the extremes so that made me feel a little better.

[Jeff Kleiner]

Looks pretty normal; definitely not rich.

Jeff

[CraigS]

I have mine in the passenger side. Keeps the wire out of the steering wheel area. Mine is a temporary setup w/ the unit sitting on the console and the O2 wire just draped over the door w/ some painters tape to protect the door and a few pieces to hold the wire in place.

[edwardb]

Ok. Bung installation it is. What side of the car do you guys normally install the bung on? Drivers or passenger? I was thinking passenger and just downstream of where the 4 pipes join together in my gas n pipes.

That's where I installed the bung on my carb'd #7750 build. On the PS downstream of the 4 pipes. I put it on the inside low enough that it didn't interfere with the body. It won't be at the optimal position. Most instructions tell you to mount the O2 sensor between the 9 to 3 o'clock position so it won't collect moisture. That's impossible for the location I used. But since the sensor is only installed for test purposes and not long term, shouldn't be an issue. It wasn't for me. The bung is only slightly visible once installed, so no big deal.

[Avalanche325]

That's where I installed the bung on my carb'd #7750 build. On the PS downstream of the 4 pipes. I put it on the inside low enough that it didn't interfere with the body. It won't be at the optimal position. Most instructions tell you to mount the O2 sensor between the 9 to 3 o'clock position so it won't collect moisture. That's impossible for the location I used. But since the sensor is only installed for test purposes and not long term, shouldn't be an issue. It wasn't for me. The bung is only slightly visible once installed, so no big deal.

Same location for me. My unit is mounted permanently under the dash. I have the O2 and a tach signal going to it. I can disconnect the sensor wires under the hood and take it out of the pipe, which is all the time except when tuning.

One thing I learned the hard ($75) way. Do not leave the sensor in the pipe if you do not have it powered up.

[Frank Messina]

Just for documentation, here is a picture of one of my spark plugs. New engine with about 480 miles. Sorry the picture is a little orange. I took it as the sun was going down.

http://thefactoryfiveforum.com/attachment.php?attachmentid=73971&d=1506042708

Not sure if this looks good or not but I compared it to some pictures on line and didn't look like the extremes so that made me feel a little better.

That's a pretty good looking plug. Here's a chart (http://www.4secondsflat.com/plug_chart.html) that is a good reference.

Frank

[srobinsonx2]

Thank Franks. That chart is very helpful.

Tomorrow I am going to pull my carb and start from scratch with a idle to full throttle tune. I will try and get the idle tune done tomorrow. I have a LM-2 headed my way and I found a guy that will weld in a bung into my exhaust. I had to do some searching to find someone that would tig weld a stainless steel (ss) bung into my nice shiny ss Gas N pipes. I know that is overkill and will most likely never be seen but it will look much better if it every is.

There is a great carb tuning 101 thread linked above. It has given me a good place to start. But like other parts of my build, it assumes I have some basic carb knowledge. Which I of course do not. I will try and document my process in a dedicated carb tuning thread that shares my process in detail. Maybe it will help others. I will post a link here.

Wish me luck

[srobinsonx2]

Here is a link to my carb tuning

Srobinsonx2 Carb Tuning (http://thefactoryfiveforum.com/showthread.php?25905-Srobinsonx2-Carb-Tuning&p=295140#post295140)

[ThickCobra]

My engine, as sourced from Mike Forte, was provided with timing set and engined dyno' d. It runs OK, but I'm hoping to get the idle air/fuel mixture set properly as it won't idle below 1000 rpm even when warmed up. And, some have mentioned my exhaust smells rich with fuel. I have a 351w/427 dart stroker, quick fuel HR 780 cm carb with both primary and secondary vacuum. I have found a local Ford mechanic very familiar with hot rods and this carburetor.

NOW MY QUESTION. Though I do not have an O2 sensor bung (Chris, I know you like that word), do I really need one as both primary and secondary are adjusted via vacuum?

[srobinsonx2]

The experts say it is best. They say unless you have a lot of experience, tuning one properly without an AGE value is tough.