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Post by squishymonster on May 20, 2020 9:59:04 GMT -5
I am working through issues with my carb, pouring over all kinds of information here and there, and I have a couple of questions that I hadn't found answers to yet.
Mainly I am curious if I should be able to idle with the throttle plate 100% closed, and if I should be able to achieve 600rpm that way. I am able to achieve a very low but stable sounding idle with the plate completely closed, but it's a fragile idle in that any movement of the throttle plate kills it (that is, if I disable the metering rod in order to test / isolate the idle circuit).
It seems to me that the amount of air capable of entering the cylinders strictly through the throttle mixture screw port would be far too small, but that's more (false?) intuition than experience.
In that case, it seems very clear that absolutely any vacuum leak in the throttle body would have major consequences since the idle passages are so small, it wouldn't take much to get that small amount of air elsewhere, and then not draw the fuel mixture.
With that low idle, choke 100% off, I made adjustments to the metering rod so that immediately as the throttle starts opening the metering rod starts providing fuel. That means that as I set the idle screw to achieve 600rpm, both the idle mixture screw, and the main jet are providing fuel for any throttle-slightly-opened idle. What I've read so far seems to indicate that is correct operation. Am I on the right track?
With all of that in place, I was idling well with no choke, and my next step is to get acceleration to work properly. My current issue there is after a quick blip on the throttle and quick release I get a fairly reliable backfire up the carb. But before I start tracking that down I wanted to see if the previous things are moving in the right direction.
Second, I have a couple of loose linkages. The pivot between the metering rod and accelerator pump has two plate-like brackets and they separate quite a lot. I don't see any intended mechanical means to stop that separation. I hoped the choke body would hold everything together, but they still separate close to the point where they wouldn't engaging one another anymore.
The two lower brackets on the motor side of the throttle shaft also have a lot of play, resulting in the throttle plate not being able to fully return to the idle position. But I just found another post here regarding that issue. I think i can repair the pin hole with my welder...
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Post by squishymonster on May 20, 2020 10:07:56 GMT -5
I guess while we're at it... I read several posts about new idle jets are smaller than stock. I dug through my spare parts box and found the stock idle jet and it did in deed look to have a wider inner diameter. Unfortunateley, in all of my wisdom, after installing it, and troubleshooting some other things, I decided to pull the idle well jet while the carb was on the jeep, and I did not think about my screwdriver crossing the idle jet... so, it is now good 'n smooshed. I have no choice but to use the aftermarket one.
The question is, do the aftermarket ones actually work alright? I'd assume being smaller, they are even more sensitive to vacuum leaks, and they would also require the main jet to pick up more slack.
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Post by squishymonster on May 20, 2020 11:03:01 GMT -5
Maybe an update in my understanding: the Service Manual says the metering rod / high speed circuit don't kick in until 20mph, and that the idle circuit should be acting alone up to around that point. But it says the throttle opens a little (exposing the idle slot) to provide the extra mixture in this range.
If that's accurate, i should be able to disable the metering rod for idle tests. But the question still remains, how much RPM should I be able to get out of the idle circuit with the throttle 100% closed? It sounds like when I crack the throttle a little bit I should not be pulling anything from the main jet yet.
I think the documentation using MPH is a bit weird since the fuel system doesn't care at all about gearing.
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Post by Scoutpilot on May 20, 2020 11:23:11 GMT -5
I am working through issues with my carb, pouring over all kinds of information here and there, and I have a couple of questions that I hadn't found answers to yet.
OKAY. Mainly I am curious if I should be able to idle with the throttle plate 100% closed, and if I should be able to achieve 600rpm that way. I am able to achieve a very low but stable sounding idle with the plate completely closed, but it's a fragile idle in that any movement of the throttle plate kills it (that is, if I disable the metering rod in order to test / isolate the idle circuit).
The purpose of the choke circuit is to give a cold motor a richer mixture to warm up quickly. The idle RPM with choke will usually be higher than normal. Using the throttle with the choke closed further enriches the mixture and will cause two things to occur. Your plugs will carbon foul and the excess fuel with diminished spark will kill the motor. It seems to me that the amount of air capable of entering the cylinders strictly through the throttle mixture screw port would be far too small, but that's more (false?) intuition than experience.
99.9% of the air enters through the carburetor throat. Some enters through calibrated vents. Its velocity past the vacuum ports creates that vacuum internally and vacuum is what moves fuel. In that case, it seems very clear that absolutely any vacuum leak in the throttle body would have major consequences since the idle passages are so small, it wouldn't take much to get that small amount of air elsewhere, and then not draw the fuel mixture.
Over the years, the throttle shaft and bores will wear into an egg shape due to torsional forces. A gap of 0.005" is a vacuum leak. With that low idle, choke 100% off, I made adjustments to the metering rod so that immediately as the throttle starts opening the metering rod starts providing fuel. That means that as I set the idle screw to achieve 600rpm, both the idle mixture screw, and the main jet are providing fuel for any throttle-slightly-opened idle. What I've read so far seems to indicate that is correct operation. Am I on the right track?
I recommend an idle of 650-700 RPM, but yes, you are on the right track. With all of that in place, I was idling well with no choke, and my next step is to get acceleration to work properly. My current issue there is after a quick blip on the throttle and quick release I get a fairly reliable backfire up the carb. But before I start tracking that down I wanted to see if the previous things are moving in the right direction. When was the last time the valves were adjusted? As well, take a look at this Timing without a timing lightSecond, I have a couple of loose linkages. The pivot between the metering rod and accelerator pump has two plate-like brackets and they separate quite a lot. I don't see any intended mechanical means to stop that separation. I hoped the choke body would hold everything together, but they still separate close to the point where they wouldn't engaging one another anymore.
No longer manufactured, this two-piece lever is can only be held back by one of two repairs.
1. Drill and tap a screw hole in the pin. It must be a VERY small screw.
2. Glue a small block inside the choke housing where you see the wear from the lever's motion. It must be a VERY thin block.
The two lower brackets on the motor side of the throttle shaft also have a lot of play, resulting in the throttle plate not being able to fully return to the idle position. But I just found another post here regarding that issue. I think i can repair the pin hole with my welder...
That is one repair. I typically will drill the hole to accept the next larger screw. That usually tightens it up. The old way to fix it was to braze the two parts together. This is no longer practiced.
Hope this helps. |
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Post by Scoutpilot on May 20, 2020 11:33:33 GMT -5
Maybe an update in my understanding: the Service Manual says the metering rod / high speed circuit don't kick in until 20mph, and that the idle circuit should be acting alone up to around that point. But it says the throttle opens a little (exposing the idle slot) to provide the extra mixture in this range. The service manual is flawed. As soon as you press the gas pedal the metering rod raises. At the same time the throttle plate is opening to expose more of the vacuum port. Both actions increase both air and fuel flow.
If that's accurate, i should be able to disable the metering rod for idle tests. But the question still remains, how much RPM should I be able to get out of the idle circuit with the throttle 100% closed? It sounds like when I crack the throttle a little bit I should not be pulling anything from the main jet yet. You should not disable the metering rod. Rpm is fuel and air flow. Adjusting both the idle stop screw and the idle mixture screw to maximum opens the throttle plate increasing air flow, creating more vacuum which increases the fuel flow. This will result in an idle of over 2000.
I think the documentation using MPH is a bit weird since the fuel system doesn't care at all about gearing. I agree. |
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Post by squishymonster on May 20, 2020 12:40:31 GMT -5
Thank you for all of the answers. This helps me verify that my understanding (so far) is on a reasonable track! I appreciate your suggestions for the sloppy brackets as well, I will definitely try those out soon. That gives me an idea... one of those little drill shank collars might work well on the accelerator pump pivot, and be a non-permanent modification.
About the timing, the only adjustments I've made to the distributor offset so far have been by ear. I have maybe a 15 degree window that I have to be well within. Too clockwise on that 15 degrees and I get a "dieseling" sound on acceleration, where throttle response is decent, but not optimal, and top speed and high end acceleration suffers. Too far counter clockwise and the exhaust starts mild backfiring. I'm somewhere left or right of center.
I'll be more specific after my next update. I'm getting a tach and vacuum gauge after work today so I can get better dialed in. Dwell meters seem difficult to come by these day, but I may be able to borrow one from a mechanic friend.
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Post by Scoutpilot on May 20, 2020 13:28:53 GMT -5
I'll be waiting for your results. Remember...slow and small changes.
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Post by squishymonster on May 20, 2020 23:16:25 GMT -5
Well that didn't go as planned. I'm not sure if I should be happy, or upset. More the latter so far.
My HF tach / dwell meter gives a vastly different reading every half second despite a smooth idle. I also may have cooked it in my efforts to get it working but we will see. I want to try isolating the clamp with aluminum foil and see if it cleans up.
So I decided to go vacuum leak hunting. My new vacuum meter was reading 18 but I don't know the rpm yet. This is with choke off and engine warm.
I decided to torque the carb manifold bolts down to 30+ lbs just to see if I hadn't crushed the copper well enough. Well, lucky me, apparently my torque wrench took a dump the last time I used it so it wasn't indicating torque limit. The forward nut just wanted to keep going. I started getting suspicious too late and I split the forward "ear" of the casting in two.
Now I can still maintain 18lbs vacuum but it requires choke...
Maybe Im Lucky and there was already a crack there I'd have never found. I don't think I will ever use a torque wrench again without starting with a 10lb test...
The real pain is I have a 2nd torque wrench that is still working so I used it to verify torque after the fact. I think I only got up around 35 or maybe a little higher.
On the bright side, I ordered an aftermarket throttle body on Monday, anticipating needing a temp backup in case my throttle shaft needed repair. I was hoping I'd be able to stick with the original.
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Post by Scoutpilot on May 21, 2020 5:24:11 GMT -5
With the ear of the base breaking without having reached 35, IMHO, there was a crack there. As far as the Dwell/Tach goes I have a question. Digital or Analog? Digitals don't do so well on these old ignition systems.
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Post by squishymonster on May 21, 2020 20:19:11 GMT -5
It's a digital tach / dwell. I'll try to find an analog.
I think I'm out of commission until the new throttle body arrives. I may try a cast iron weld if I get brave. I'll follow up after I get going in the right direction again.
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Post by Scoutpilot on May 22, 2020 4:29:59 GMT -5
Sometimes taking a break helps me think...
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Post by squishymonster on May 26, 2020 12:02:07 GMT -5
No throttle body yet, but I did make it to step next. Prepare to cringe: the original throttle body became my first ever attempt at brazing. It's not pretty, but it's holding. When removing it I found that the forward manifold stud was leaning forward a little. I am not sure if it was bent and helped the ear break as it tried to align, or if the ear breaking led to the stud bending out. I put a nut on it and lightly whacked it back straight. It moved a little easier than I expected. Now I'm a little worried the stud is broken down inside the manifold, but for now it is holding a seal. I'm now past the choke issue and like so many others have found, it was a too small aftermarket idle jet... The previous owner had installed it, but threw the original into the parts box. I reinstalled the original (prior to this thread) and noticed some improvement, but then in the process of also cleaning out the filthy idle well jet passage I crushed the original idle jet tip, so I was never able to try both repairs at the same time. On Sunday I realized I could drill out the aftermarket idle jet, and low-and-behold, no more choke was needed. 20 vacuum, but I don't have an RPM yet. I thought "great! now I can tune, and go for a test drive", but it turned out it only unlocked Carburetor Problems Level 2  I suspect this one will be a little more straight forward. My accelerator pump throw is way too short, but since there is some binding / loose linkage, it is not simply an issue of making an adjustment. Today I intend to do some work in there to see if I can remove some restrictions in movement, and try one of your proposals for holding the linkage together. I saw in your video that when your throttle moves, your pump movies immediately. Mine is significantly delayed. As for the soon-to-arrive replacement throttle body, it's going to stay in the parts bin for now. I'm worried about busting a manifold bolt. |
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Post by Scoutpilot on May 26, 2020 13:48:34 GMT -5
Post a picture of the accelerator pump linkage so I can see if the parts are intact and it is properly assembled. And yes, there have been many occasions for me when the Low Speed Jet was too small and I had to drill it out.
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Post by squishymonster on May 26, 2020 21:27:19 GMT -5
I think I'm all fixed up aside from some tuning. I replaced some pump parts and cleaned up the well wall and now the well spring can return the plunger on its own. Even though someone replaced the plunger somewhat recently, I suspect it may have dried out at some point. It was significantly harder than the new leather replacement.
I replaced the check valves. The larger had to be extracted. The new smaller one is a flap and spring style. I picked it because it closes instantly and the ball style has a delay (maybe prefered?)
With the changes, my pump actuated with the throttle in both directions.
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Post by Scoutpilot on May 27, 2020 5:22:18 GMT -5
Let's hear it run now...
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I suspect this one will be a little more straight forward. My accelerator pump throw is way too short, but since there is some binding / loose linkage, it is not simply an issue of making an adjustment. Today I intend to do some work in there to see if I can remove some restrictions in movement, and try one of your proposals for holding the linkage together.