1906 Carb issues/tuning

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essenz
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1906 Carb issues/tuning

Post by essenz »

Sorry if this is beating an old horse, but I cant seem to tune the 1906 per Edelbrock guide. This is on a 66, so 462 MEL

To eliminate vacuum leaks - the manifold port (for booster) is plugged tight. The booster port on the carb is plugged tight, correct vacuum advance port is connected to dist, the other is connected to a vacuum gauge. My PCV (new valve) is running to the PCV port on the front of the carb.

I start at 1.5 turns out on the IMS with warm engine around 600rpm - car is in PARK. Turning the IMS out (leaner) does NOT increase RPM, only turning it in increases RPM. The strange part is RPM keeps increasing all the way till I have both IMS screws fully turned in/seated (richest). But that doesn't make any sense? Where is the air coming from yielding such a lean mixture/high RPM if the idle mixture screws and turn in all the way?

Does anybody else have this issue on their 1906? What should I look for/troubleshoot next? Does it sound like a vacuum leak? At this odd setting, the cars idle is smooth around 580RPM, vacuum is right around 18 - just feels incorrect like this. Other people who use the 1906 I've heard before say 1.5 turns out and their good. My car such an oddball....
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Re: 1906 Carb issues/tuning

Post by 1Bad55Chevy »

essenz wrote: Fri Nov 18, 2022 1:59 pm

I start at 1.5 turns out on the IMS with warm engine around 600rpm - car is in PARK. Turning the IMS out (leaner) does NOT increase RPM, only turning it in increases RPM. The strange part is RPM keeps increasing all the way till I have both IMS screws fully turned in/seated (richest). But that doesn't make any sense? Where is the air coming from yielding such a lean mixture/high RPM if the idle mixture screws and turn in all the way?

This had me messed up since it was opposite of everything I had ever done on a carb...

From your instruction manual..
Screenshot_20221118_135600_Samsung Notes.jpg
You are turning your screws the wrong way and when the idle picks up you need to lower it with the idle screw.
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Re: 1906 Carb issues/tuning

Post by essenz »

Sorry mispoke. Turning the screw in is lean, out is rich. When the screw was 1.5 turns out, it was clearly very rich (my eyes were burning). The screw almost all the way in leans it out (it should stall, but it doesn't).

This is an fresh out of the box 1906. From what I'm reading online, if the car/carb idles with the screws turned all the way in, it means the car is getting gas/fuel some other way. Not sure how though, the idle is generally low (under 700) so I should still be running on the idle circuit alone. The floats could be off causing excess fuel to pour in - but this is a brand new carb - the floats should be correct from factory.

I'll tinker some more tomorrow.
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Re: 1906 Carb issues/tuning

Post by 1Bad55Chevy »

Last year I had an edelbrock carb right out the box that I installed on a SBC on my neighbors 84 chevy 1500 that had similar issues. Real long story short of me chasing my tail and just about throwing away my tools my dad tells me to check the actual float and sure enough there was a very tiny hole in it causing it to hang low in the bowl. I blame the fact that everything has gone to complete shit since covid....
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Re: 1906 Carb issues/tuning

Post by TonyC »

Sounds similar to an issue I've been having with the Carter I have on my 468 now. I have the fuel-mixture screws turned out only about half a turn, and the engine still runs rich; I can tell by the smell. I haven't really tried making idle adjustments as I should have after discovering the float problem, I admit; but I'm not sure whether any adjustments would make a difference on this carburetor. That's why I'm rebuilding another stock, 1966-specific Carter, hoping all my fuel issues go away.

Sorry to go off on a tangent; I just meant to say I'm having similar issues with the carburetor I have, though admittedly it has differences to it from the original Carter designed for '66.

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Re: 1906 Carb issues/tuning

Post by frasern »

I'm sure you have already checked for vacuum leaks, Perhaps verify that both step up rods are moving freely, and one didn't get jammed during shipping.
Sinking floats are something we discussed a while ago when Dave was still here. I have seen a brass float ingest fuel, Dave confirmed the plastic ones can as well, but they can also dissolve. I know Tony has brass floats, but I am almost certain a new carb would be the foamy plastic type.
A simple shake test will tell you if a brass float has fuel inside, but a better option may to weigh both, perhaps with a re-loading scale, to compare.

Another thing, off topic for a new carb, but years ago I heard of someone soldering a leaky float. He tested it in a bowl of water and it was fine, but when installed it would sink. Turned out, the solder had added weight, and gas has a different specific gravity than water. The reason I bring this up is; I have always wondered if corn oil gas makes a float ride higher?
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Re: 1906 Carb issues/tuning

Post by essenz »

Ugh, I really dont feel like tearing this carb apart to check floats... But I will....

What about the accelerator pump, if thats sitting too low - could that cause excess fuel to leak in?

The accel pump linkage has three holes - default is to use the middle one. I've heard other people to use the top hole (Edlebrock used to call this the "winter/cold" setting). I might try that first today.
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Re: 1906 Carb issues/tuning

Post by frasern »

Edelbrock is something I don't have any experience with, but anything that's binding could hold the butterflies just a tad off idle, so if that linkage is tricky, it's a good place to look. I expect it's the same as a Carter. Also look for clearance around the mounting.
I responded to the float issue because it had been raised, but I think a float problem is unlikely on a new carb, unless it's defective as mentioned.
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Re: 1906 Carb issues/tuning

Post by frasern »

SO now, I can't stop thinking about the base plate, if you're using the 462 unit, it's very tight to the butterflies inside the bores, and could be stopping them from seating?
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Re: 1906 Carb issues/tuning

Post by TonyC »

Yes, if you're using a spacer that doesn't match the dimensions of the throttle plates, that would be a problem. Verify that. If the plates do not bind against the spacer, then move to the carburetor itself. I will admit that the shop manual gives conflicting information on the hole settings for the accelerator pump, but I have done research to clear that up. The top hole is for summer, when the outside temps are at their hottest. The bottom hole is for winter, when it's cold. Of course, the middle is for average temp conditions. My numerical references for using those holes are as follows:

1. If the lowest temps are above 60°F and the highest above 90°F, use the top hole.

2. If the temps range between 40°F and 90°F and don't deviate out of that range, use the middle hole.

3. If the highest temps are below 60°F and drop below freezing for the low, use the bottom hole.

There is no written reference for those numbers; those are just reference points I've adopted to define hot, average, and cold conditions. They work for me.

If you suspect the accelerator pump, measure it. First, set the link to the top hole, then use a 1/64" ruler (you can find those precision rulers in part stores) to measure from the casing to the top of the pump with the throttle at rest. It should be at 17/32" (or 34/64"). If it isn't, then bend the link at its elbow to adjust its height. If you have a dashpot plunger on your carburetor, measure that as well; it should be at 1/8" with throttle fully closed and 5/16" with throttle wide-open. If it isn't, the plunger arm needs adjusting, which is a bit trickier and will likely involve removing and reinstalling the arm each time you make an adjustment. The shop manual gives instructions on how to do both. The only thing about that guidance is the conflicting info on the accelerator-pump holes, which is cleared up above.

Of course, that guidance applies only to the stock Carter AFB. Edelbrocks may have different settings, so if you have adjustment references for your specific model of Edelbrock, use them instead. If you don't have such references and cannot acquire them, then try the specs set in the shop manual, see what effect that has.

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Re: 1906 Carb issues/tuning

Post by LithiumCobalt »

Installation instructions for the 1906 say to check the float levels before you install. Mine were slightly off new out of the box. No need to tear the whole carb apart. A few screws off the top and the horn comes right off.
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Re: 1906 Carb issues/tuning

Post by essenz »

Thanks Dan.

So its interesting. I have 93 octane, and with a timing light I'm right at 10 BTDC. And to be honest, if go further to say 12 or 14 BTDC vacuum/rpm increases. But I've always stopped at 10. In my mind, I dont understand how this car (designed for leaded fuel) could even run at 10 BTDC.

I also called Edelbrock and they confirmed that this carb is right on the edge for this size engine, so even with everything set right, it may be OK to have the Idle Mixture screws almost all the way in.

So I'll check the float levels, check the timing via vacuum, then tune the carb again too lowest RPM and see where I'm at.
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Re: 1906 Carb issues/tuning

Post by 1Bad55Chevy »

This is going to start some serious controversy...

Initial timing does not matter... only thing that matters is overall timing at full advance. For example if you want 38* at full throttle and your distributor has 24* of mechanical advance you would set your distributor at 14* Initial.

Proof that Initial does not matter is your vacuum advance. Hook your timing light up with the vacuum advance connected, depending on the motor and how things are set up you could see values as high as 60*. The reason this works is because at part throttle the mixture is extremely lean meaning less fuel particles across the combustion chamber. With a lean mixture at part throttle the burn time across the chamber is much slower so the ignition event has to start much sooner.

I never understood why people get so hung up on initial timing. Just set it between 8* and 12* and roll out!

Again I have never worked on a MEL engine but I would assume it follows the same basic laws of physics like any other internal combustion engine..
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Re: 1906 Carb issues/tuning

Post by TonyC »

I have to agree there, for the most part. The only reason to adjust initial timing would be to reduce the chances of knocking...but, thinking more into that, knock would happen more at higher revs, meaning that the advances would warrant retarding more than initial. Where the actual number is concerned, different engines run and handle loads differently, which is why one engine needs 10 ahead and another needs only 4 ahead for their respective sweet-spots. One thing I have to point out, though, where initial timing is concerned is that the engine's hot idle needs to be smooth with vacuum advance taken out of the picture. Vacuum diaphragms fail; and when this one does, you don't want to end up stranded with a stalled engine that relies heavily on its vacuum advance to keep the at-idle running within spec. That's a good reason to adjust initial timing with vacuum disconnected, though that may not be the historical reason to do that adjustment that way.

Now, where performance engines like the MELs (or their Chevrolet "twins," the 348/409 series) are concerned, timing is critical, so it has to be done precisely; not merely initial, but the two advance settings as well. A long time ago, the original guru of vintage-Lincoln restoration, Ron Baker, published in a newsletter exactly what the advance numbers should be at what revs. Unfortunately, that guidance is long-since lost, so I've had to guess based on what I could remember. What I have now works, though it may not be exactly on point with Ron's numbers.

---Tony
Last edited by TonyC on Mon Nov 21, 2022 3:52 pm, edited 1 time in total.
"Don't believe everything you read on the Internet, just because there is a picture with a quote next to it." (Abraham Lincoln, 1866)
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