71 Rochester Monojet MMV Carburetor tuning

[trevarthan]

I've got a 71 Rochester Monojet MMV Carburetor on a C10 with a 250 L6. Just recently rebuilt it. When I bought it from the original owner, he said he thought something was wrong with it. After rebuilding and tuning, I think I see what he means.

If I tune the idle mixture correctly, the truck runs and doesn't stall, but it hesitates badly when cold and noticeably (but not terribly) when warmed up. Pulling the plugs, they're dark with black deposits, so it's clearly running rich.

If I tune the idle mixture a half turn lean, the truck runs fine. No hesitation when cold or warm. Unfortunately, it stalls randomly at idle, when pulling up to stop lights and such.

From what I've read, the mixture screw mostly controls the idle mixture. It affects the mixture under throttle, but not a whole lot.

This has me thinking my main jet is too large. Am I correct?

If I change the main jet, do I also need to change the metering rod?

I'm in Tennessee, so pretty low elevation, but sorry, I didn't know I needed to take a picture of the main jet when I was rebuilding the carb, so I'm not sure what the number is on it. I'll have to remove it to find out.

Alternatively, could I also lean the mixture screw and raise the idle speed?

Thanks.

[rpmerf]

If the Monojet is like a Quadrajet, the jet mostly effects WOT mixture, the rod controls cruise mixture.

What is your idle RPM? In drive for an automatic.

Have you tired setting idle for maximum vacuum or idle RPM, or have you just been setting it where it feels good?

Are you sure there are no vacuum leaks?

Here is a document for tuning the quadrajet. A lot of the theory should be the same: http://www.capitalcorvetteclub.ca/at...ng%20Paper.pdf

As it mentions in the paper, It is usually best to go to the stock jet and rod and work from there.

[Wrenchbender Ret]

You can do minor adjustments on the mixture by adjusting the metering rod height. The instruction sheet tells how to measure & adjust it. The measurement is usually about 1/16 in. If you bend the lever down a bit it will lean it out. Make sure the float level is adjusted correctly & the float not soaked up with gas. Check your ign. timing also. You usually can set the timing a little higher then specs to run better.
George

[Ironangel]

I'd take a close look at the choke...Make sure it's set correctly cold, and make sure once the motors warmed up, (at least 15-20 minutes on cold days) that the choke is opening completely...Motor should idle @ 600 rpm with no stall once warmed up...Check the choke and make sure the timings right before you go to fiddling with the metering rod and jet...A huge mistake and often misunderstood fact about these older first generation motors is, they must have time to warm up to run properly. There are no computers to adjust the Mass Air Flow and Air Fuel Mixtures to ported or direct injection systems...You are the computer, let the old girl warm up then let her tell you what she likes...

[trevarthan]

Originally Posted by Ironangel

I'd take a close look at the choke...Make sure it's set correctly cold, and make sure once the motors warmed up, (at least 15-20 minutes on cold days) that the choke is opening completely...Motor should idle @ 600 rpm with no stall once warmed up...Check the choke and make sure the timings right before you go to fiddling with the metering rod and jet...A huge mistake and often misunderstood fact about these older first generation motors is, they must have time to warm up to run properly. There are no computers to adjust the Mass Air Flow and Air Fuel Mixtures to ported or direct injection systems...You are the computer, let the old girl warm up then let her tell you what she likes...

choke automatic actuation as the vehicle warms up from 40 degrees F BEFORE carb rebuild (8 and a half minutes long - skip to the end if you want - turn your sound down - loud): https://youtu.be/94MVneCeIDE

[trevarthan]

Originally Posted by Wrenchbender Ret

You can do minor adjustments on the mixture by adjusting the metering rod height. The instruction sheet tells how to measure & adjust it. The measurement is usually about 1/16 in. If you bend the lever down a bit it will lean it out. Make sure the float level is adjusted correctly & the float not soaked up with gas. Check your ign. timing also. You usually can set the timing a little higher then specs to run better.
George

We double checked that it was set properly according the spec sheet at 2mm. We didn't change it.

1/16 inch is more like 1.5mm. I wonder if that would help.

We also double checked the float height per the spec sheet and I replaced the nitrofill float with a brass float, so I'm pretty sure it's not taking on gas.

Timing is set at 14 degrees before TDC.

[trevarthan]

Originally Posted by rpmerf

If the Monojet is like a Quadrajet, the jet mostly effects WOT mixture, the rod controls cruise mixture.

What is your idle RPM? In drive for an automatic.

Have you tired setting idle for maximum vacuum or idle RPM, or have you just been setting it where it feels good?

Are you sure there are no vacuum leaks?

I set the idle as best I could to 550 RPM, per either the stick on the hood or my haynes manual, I don't remember. I used a laser tach on the harmonic balancer to set it and I had to get uncomfortably close to the fan for the laser to read the rpm properly.

This is a manual transmission.

Now that I think about it, I changed the mixture after setting the idle speed last time, so I should probably double check it's set properly still.

[Ironangel]

I commented on the video, heres what I said and asked... .What is that unplugged barb at the base of the carb? Is that the vacuum advance barb? That should be plugged when setting idle speed and timing...Motor sounds good, the rpm sounds a bit high but that may be due to that open barb and the choke opening up...Choke looks good as far as opening enough...

[trevarthan]

Originally Posted by Ironangel

I commented on the video, heres what I said and asked... .What is that unplugged barb at the base of the carb? Is that the vacuum advance barb? That should be plugged when setting idle speed and timing...Motor sounds good, the rpm sounds a bit high but that may be due to that open barb and the choke opening up...Choke looks good as far as opening enough...

that's the manifold vacuum barb. I put the vacuum gauge on that when I set the mixture.

I think when I set the idle speed I had the vacuum gauge attached to it.

When I checked the timing the air cleaner was installed. That barb attaches to the vacuum actuated flap on the front of the air cleaner snoot when the air cleaner is installed.

I don't think I have a vacuum leak. I spray carb cleaner all around the engine bay and I never notice a difference in idle speed or engine sound.

That video was created before we rebuilt the carb, so I knew a lot less about carbs when I shot it.

[trevarthan]

Originally Posted by Wrenchbender Ret

You can do minor adjustments on the mixture by adjusting the metering rod height. The instruction sheet tells how to measure & adjust it. The measurement is usually about 1/16 in. If you bend the lever down a bit it will lean it out. Make sure the float level is adjusted correctly & the float not soaked up with gas. Check your ign. timing also. You usually can set the timing a little higher then specs to run better.
George

You are absolutely correct. In this case, even though I had the metering rod height set to spec, it needed to be a little higher. I don't know why spec isn't accurate here. Maybe it has something to do with the gasoline mixtures we run these days?

I ended up buying a remanufactured carb from Autozone and tossing that on just to see if it was tuned better. It wasn't. That carb ran way lean. I cracked both carbs open and compared the metering rod height. The carb we had rebuilt was set to spec. The Canadian rebuilt carb's metering rod was about 3mm higher. We split the difference and lowered the Canadian carb's metering rod about 1.5mm and installed it (I liked the Canadian carb better because it came with a nipple on the float vent and mine didn't have one).

After running this for a few days and pulling the number one spark plug it looks nearly perfect, so it seems we nailed it.

Thanks for the advice!