Need help fine tuning new carb

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44bz
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Need help fine tuning new carb

Post by 44bz »

I don't have much experience with this. It's a new Holley 4160. I've got the idle set to 550 and timing is at 5 degrees, per the tsm. I'm still fiddling with the choke but got the high idle set to 1550 which is what the manual recommends. The choke staying on a little long so I'm adjusting that still.

The issue is that there's a stumble or something. I can hear it in the exhaust note, kinda like an occasional puff sound. It runs and drives great but the puff bothers me.

I put the timing light back on this morning and watched for a bit. It seems steady then there's an occasional increase, I think. My eyes start to play with me if I stare at the marks too long.

So what's going on here? I haven't made any other adjustments to the carb other than choke and idle.


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Tatsadasayago
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Re: Need help fine tuning new carb

Post by Tatsadasayago »

When is this puff or 'hiccup' happening? Idle, tip-in, transfer stage?
Since you're in Oregon I assume you are at lower altitude. The 600 CFM 4160 comes jetted nicely for sea level to 1000' so I doubt it's a mixture problem.
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44bz
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Re: Need help fine tuning new carb

Post by 44bz »

It's only at idle. It accelerates great and without hesitation. I'm at about 200ft.


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Tatsadasayago
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Re: Need help fine tuning new carb

Post by Tatsadasayago »

I would suggest you perform the Dollar Bill Test®
While the engine is idling, place a dollar bill across the exhaust pipe and watch it flutter with the exhaust pulses. If the engine hiccups the bill will get sucked into the pipe briefly. If this is the case you have either an intermittent ignition misfire, lean misfire OR an exhaust valve that isn't closing/sealing before the piston starts it's way down creating a momentary suction. A slightly burnt exhaust valve will intermittently leak at idle but won't be noticeable above idle.
You can confirm if it's an ignition misfire by placing a timing light on each plug wire and watch for the light failing to flash at the same time you hear the hiccup. If the light doesn't show anything then you may want to do some curb idle mixture tweaking.
I would lean one side out at a time and see if the huccuping gets worse. If it does, turn the screw out until it goes away. A half turn max should fix it if one cylinder is running lean, yet not making the others run too rich.

Putting the timing light on the coil wire is another way to visualize a problem with the ignition system.
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440sixpack
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Re: Need help fine tuning new carb

Post by 440sixpack »

Is your vacuum hooked to ported or manifold ? set your idle mixture to best lean idle.

I wouldn't mess with the carb before you get your timing right. this has been discussed much here lately but just to keep it short check your mechanical all in at 2500 RPM, it should be 34-36 as a rule. and then again at 2500 with your vac advance hooked up. it should be 50-60.

see if it idles better . if it still starts good try it
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44bz
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Re: Need help fine tuning new carb

Post by 44bz »

Tatsadasayago wrote:I would suggest you perform the Dollar Bill TestImage
While the engine is idling, place a dollar bill across the exhaust pipe and watch it flutter with the exhaust pulses. If the engine hiccups the bill will get sucked into the pipe briefly. If this is the case you have either an intermittent ignition misfire, lean misfire OR an exhaust valve that isn't closing/sealing before the piston starts it's way down creating a momentary suction. A slightly burnt exhaust valve will intermittently leak at idle but won't be noticeable above idle.
You can confirm if it's an ignition misfire by placing a timing light on each plug wire and watch for the light failing to flash at the same time you hear the hiccup. If the light doesn't show anything then you may want to do some curb idle mixture tweaking.
I would lean one side out at a time and see if the huccuping gets worse. If it does, turn the screw out until it goes away. A half turn max should fix it if one cylinder is running lean, yet not making the others run too rich.

Putting the timing light on the coil wire is another way to visualize a problem with the ignition system.
Thanks for the suggestions, I'll try some of them this week. I spent the day building forms for concrete steps and I couldn't get to the jeep.

Everything ignition related is new as of a month or so ago, tuneup time. I didn't have this issue before bolting the new carb on so I feel like it must be something with the carb that isn't right yet.


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Re: Need help fine tuning new carb

Post by 44bz »

440sixpack wrote:Is your vacuum hooked to ported or manifold ? set your idle mixture to best lean idle.

I wouldn't mess with the carb before you get your timing right. this has been discussed much here lately but just to keep it short check your mechanical all in at 2500 RPM, it should be 34-36 as a rule. and then again at 2500 with your vac advance hooked up. it should be 50-60.

see if it idles better . if it still starts good try it
The vacuum advance is connected to the timed spark port on the carb.

I'll retry with the timing. When you say 34-36 and 50-60 are you referring to vacuum?


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Tatsadasayago
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Re: Need help fine tuning new carb

Post by Tatsadasayago »

44, there are two different schools of thought regarding vacuum advance. Some go with ported vacuum advance and the smart ones utilize manifold vacuum. Setting the base timing is dependent on which vacuum source you choose and the effects are vastly different.
When using manifold vacuum to control the advance, 50-60 degrees total advance at 2500-3000 RPM under a light load is normal.
Carb tuning is very dependent on which method is used.
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440sixpack
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Re: Need help fine tuning new carb

Post by 440sixpack »

Normally I use manifold vacuum because you get the advantage of the advance at idle . but my HEI distributor didn't like it so I went to ported.

As far as the 34-36 and 50-60 I'm talking about degrees of advance it 2500 RPM. if you don't have a dial back light you'll need one to check this. initial timing is meaningless as long as your engine starts good. these are just general numbers what you need could be a bit more or less but it's a starting point.

Ignition timing is by far the most important factor in tuning an engine. how well it runs , how much power and how long it lasts all start with timing. once that's correct then you can fine tune your fuel induction.
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44bz
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Need help fine tuning new carb

Post by 44bz »

I need an ignition timing 101 course...

Really all I've done for timing is let it idle and check the timing marks with my harbor freight timing gun and confirm the correct hash mark lines up with marker on the timing cover.

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440sixpack
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Re: Need help fine tuning new carb

Post by 440sixpack »

Funny you should say that, the best article ever written on the subject is titled just that. it's worth the read.

http://www.corvette-restoration.com/res ... ing101.pdf

You're going to need a dial back light to do it right, you're only checking your initial timing now and that's the least important of the 3 checks made.

Read the article and I bet it answers most of your questions and dispels many myths. if you still have questions they can probably be answered here.
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44bz
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Re: Need help fine tuning new carb

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440sixpack wrote:Funny you should say that, the best article ever written on the subject is titled just that. it's worth the read.

http://www.corvette-restoration.com/res ... ing101.pdf

You're going to need a dial back light to do it right, you're only checking your initial timing now and that's the least important of the 3 checks made.

Read the article and I bet it answers most of your questions and dispels many myths. if you still have questions they can probably be answered here.
Thanks for that! I'll give it a read and report back when I have some news.


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berettajeep
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Re: Need help fine tuning new carb

Post by berettajeep »

44bz wrote:
440sixpack wrote:Funny you should say that, the best article ever written on the subject is titled just that. it's worth the read.

http://www.corvette-restoration.com/res ... ing101.pdf

You're going to need a dial back light to do it right, you're only checking your initial timing now and that's the least important of the 3 checks made.

Read the article and I bet it answers most of your questions and dispels many myths. if you still have questions they can probably be answered here.
Thanks for that! I'll give it a read and report back when I have some news.


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Re: Need help fine tuning new carb

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That article changed my mind a few years ago and I can tell you it made a Tire-Burner out of my Chief.
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Re: Need help fine tuning new carb

Post by coolhand75 »

Thanks for the info mine is doing the same thing


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Re: Need help fine tuning new carb

Post by 44bz »

I think I finally figured this out and it was pretty simple. I adjusted idle mixture screws out about 1/4 turn and the intermittent puffing sound immediately went away. My vacuum gauge also settled on a steady 22" from manifold vacuum. So I guess I was a little lean?

I had to reset my timing because it was way off but I'm at 5 BTC again per the TSM and it's idling smooth as butter!


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Re: Need help fine tuning new carb

Post by 44bz »

Well I probably spoke too soon but I think I've got it fixed. I took it for a drive after I posted the update this morning and the J fell flat on its face - no power, could hardly accelerate from stop...so I watched some videos on timing because I have no experience and no one to show me how.

I have initial timing set to 5* BTC as the tsm calls for. I had my neighbor rev the motor to 2500rpm so I could check the advance but I couldn't really get more than 25* with the timing light. I know you guys suggested closer to 35* but when I advanced the distributor it seemed to have more problems.

So I left it there, reset my idle mixture screws and cleared up the exhaust puff, and got my hot idle back to 550.

Took it for a drive and it felt good - lots of acceleration, steady at idle, no popping or anything. So I dunno, but it must be close...


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letank
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Re: Need help fine tuning new carb

Post by letank »

If you want a little more pep... increase the timing to 7 or even 10... and go for a test drive. My 74 is a slug at 5, but gets me grinning at 12... of course dufferent engine and carb, but it is all about gas and controlled ignition
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