DoctorStoopid

I'm going to have carb apart (again) for the third time after doing a rebuild to replace the MCS since its reading low ohms and needs replaced according to the service manual troubleshooting tree. I'm wondering though, during my previous rebuilds, I didn't bother to use the little L shaped ruler included in the rebuild kit to measure MCS travel because I didn't see the point. Am I wrong in assuming any household ruler works to measure total travel of the solenoid. All I'm looking to do is stick a secondary metering rod in there and make sure it travels 1/8 of an inch downward before stopping, right? What is the point of the special ruler? Unless I am really missing something here.

Tuned Performance

iTrader: (92)

Isn’t the ruler used to set float level ?

sofakingdom

iTrader: (1)

Yup, and acoupla other details most easily established that way.

Use it. Follow the instructions. AT LEAST, if nothing about your carb us fornicated in some unrepaired way (*cough* leaking plugs *cough*), you've started out with some kind of known baseline to tune from.

DoctorStoopid

My rebuild instructions don't even have anything related to that ruler specifically mentioned. It just says "use included ruler to measure MCS travel". But how - isn't it just a regular ruler?!

It makes more sense to use to measure the float as well but I didn't bother because I just reused the float from last time.

Am I correct in thinking that what I did initially is right - setting MCS travel to 1/8 up and down is fine?

mikeceli

iTrader: (2)

Having started in Auto Repair in the early 70's, Iv'e rebuilt quite a few carbs. Sometimes I use the paper ruler, sometimes a "Machinist's Rule".

sofakingdom

iTrader: (1)

Right: it's not so important that you use THAT PARTICULAR ruler, as that you use A ruler at all.

Not everybody has one. The paper one is cheeeeeeeep and will keep people out of trouble if they don't.

DoctorStoopid

Excellent, this is very helpful. I was confused why it was being measured in 32nd's of an inch but when setting the float, that makes much more sense. I was just worried that I'd burnt out what WAS a perfectly good MCS by having it adjusted so far out of whack using a household ruler that I was missing something. But as long as the travel is 1/8 of an inch and it's connected up right, I can't see how that happened. Maybe it was just its time to go!

DoctorStoopid

And the issues just don't stop. I decided to actually use the paper ruler when installing my new mixture control solenoid but I actually had to bottom the thing out to get the desired 4/32 in. of travel specified elsewhere on this website. Should I be concerned before I throw this thing back on the car? Four turns out gets me about 2/32 in if that.

sofakingdom

iTrader: (1)

Sounds like the upper stop is set wrong.

"Travel" is the DIFFERENCE between those 2. Since the lower (lean) stop setting, which is the height of the solenoid off the bottom of the bowl, is in the same casting with the adjustment, that one should be set first, to the correct absolute height; then the rich stop should be used to set the travel.

If you try to adjust it like you're describing, you'll have the rods so far down in the jets that the part that isn't even tapered might make it down in there. It'll run TERRIBLE. Way too lean.

DoctorStoopid

I'm about to throw the towel in here.

From what I understand, when rebuilding the carb, all I need to do to set the solenoid is get the screw in about four turns out and then the travel of whatever I stick in the opening should be 4/32 up and down. Then I adjust the IAB about four out to get 30 on the six cyl dwell scale. Am I missing a step?

I put it back on the car this way and it didn't throw any codes. Idled a little high at first and had some backfire after the initial start up and shut down but it seemed to idle and drive alright after it settled in. If anything it smelled RICH - lots of blue smoke at start up.

sofakingdom

iTrader: (1)

Setting the solenoid about 4 turns down is about right... maybe a little low, but reasonable. I use 5 turns, but the threads are REALLY fine, so it's not all that much different. Similarly, the idle mixture screws are VERY fine, and even over their full range of travel have VERY little "authority", so I usually just set those to about 7 turns out and leave them alone from there.

Your description doesn't include setting the rich stop. That's where you're missing it.

Study the shape of the primary metering rods. The active portion of the jets is tapered; when in the full lean position, the large part of the taper is supposed to be down into the jet, and when in the full rich, the tip part, just below the taper, should be the only part remaining in the jet.

The solenoid height is the "lean stop". It controls how far down into the jets the rods go during the energized portion of the MCS duty cycle, which is, if they spent 100% of their time there, how lean the ECM can possibly command the system to go.

The "rich stop" is the thing in the air horn that looks about like a dime. It lets the rods retract upward during the de-energized portion of the solenoid's duty cycle, and determines the max richness the ECM can command. You can adjust it with a pair of needlenose or something when the air horn is off, or when the carb is assembled, it's accessible through a plug on top between the screw that controls the solenoid height and the IAB.

"Travel" should be adjusted first so that the rods make the transition described above: from fully withdrawn, to fully inserted. The ECM will be unable to properly control cruising and moderate-power operation if that is not set right.

The IAB does a somewhat similar kind of thing, except its main contribution occurs ar lower fuel flow and higher vacuum (low load). When the solenoid is in the "lean" position, the IAB opens due to its spring making it follow the MCS. When open it allows more air into the idle circuit. When the solenoid de-energizes toward rich, it pushes the valve back up, closing it, and letting in less air.

When you hook up your swell meter you are watching the ECM try to control the MCS at idle varying the mixture to bring the O2 reading into tolerance; which what it's looking for is, varying back and forth rapidly between richer than stoichiometric (the condition at which the amount of oxygen - literally, the # of oxygen atoms - exactly matches the # of carbon and hydrogen atoms in the fuel), which is roughly a 14.7:1 ratio air:fuel by weight. If the O2 reading that the ECM sees spends too much of its time near 0 mV, which indicates NO oxygen in the exhaust stream, it increases the duty cycle of the MCS to make the mixture leaner; and vice-versa. A stock O2 sensor is very "narrow-band", meaning that it goes to full rich indication (near 0 mV) only slightly leaner than stoich, and full lean (near 900 mV) only slightly leaner than stoich. That is, it's VERY sensitive right around stoich, but can't really "see" anything else. Remember, the O2 sensor doesn't sense mixture, or fuel, or anything else; it senses OXYGEN, and oxygen only.

Correct tuning is achieved when the mixture during light to moderate acceleration using the primaries ONLY (no foot to the floor for this part of tuning) gives good throttle response, no surging or "flat spots", no puff of black smoke, etc. It's not easy to road-tune the MCS travel but it's really the only way you'll ever really get it right, on a modified motor. Then once the MCS travel is established properly to bring the rods into their correct range of motion, the IAB can be set at idle. HOWEVER, and it's a BIG however, the IAB also STRONGLY affects cruising. You can hook up your dwell meter and tape it to your windshield and drive around, and watch what it does; in a perfect world the dwell should be around 50% at BOTH cruise AND idle, or maybe a little higher (indicating that the ECM is needing to SLIGHTLY tweeeeek the mixture toward the lean side, meaning it tends on its own to be just slightly rich), like maybe 60% or so. IOW you want to bias the whole system just slightly rich, to make it run the best; the factory settings tend to be too lean, for emissions. That's also a clue to whether the MCS travel is set right: if idle and cruise CANNOT be made to agree with each other, then what that is telling you is that given the setting of the "travel" and the IAB setting, the IAB by itself (idle) may be about right, but the combination of the IAB and the rods/jets TOGETHER is wrong.

DoctorStoopid

Sofakingdom, thank you SO MUCH. I primarily went by the rebuild guide stickied on the forum but that makes no mention of the rich mixture stop screw because it looks like it didn't need adjusting there. And, like an idiot, the one sheet of paper explaining these adjustments included with my rebuild kit somehow remained in the box when I removed all the other parts and instructions.

As for how to get this adjusted properly, it seems like there are a few gauges I need to do this properly and I don't see ANYWHERE online that sells these. The paper included with the rebuild kit offers the sage advice of "gauge kit available at the dealer" but I have a feeling this is outdated advice.

If anybody can show me where to find this gauge kit or can offer an alternative, I think I'll finally be able to crack this. Thanks in advance!

NoEmissions84TA

iTrader: (1)

I have seen those E4ME adjusting tools pop up on ebay occasionally.

Edit: I just checked - seems like hens teeth now.

This is one of the tools you are looking for:
A little more searching found this:









https://www.thirdgen.org/forums/carb...can-i-get.html

https://www.google.com/imgres?imgurl...gnMCc..i&w=600

And here you go (on ebay right now) and great price: https://www.ebay.com/itm/Thexton-380...-/183544838834
And at Autozone: https://www.autozone.com/test-scan-a...tool/59562_0_0
And for the mixture screws: https://www.toolsource.com/carbureto...l-p-71278.html

If you are handy, you can fabricate most of these tools yourself.

DoctorStoopid

Hmmm well this is certainly a dilemma. How have you all be rebuilding these carbs without these little gauges and measurement scales?! The screw drivers I can improvise but I can't fathom a way to measure the rich stop without the little L shaped gauge my rebuild sheet is describing.

sofakingdom

iTrader: (1)

I've always done it by the tuning method described above. I put them back together the same as they came apart for a starting point, and tune from there, asking the engine what it wants to make it run like I want, rather than setting it up for best emissions compliance, which is what the "spec" settings will give you. Which would you rather have?