Inliners International Forums General Discussion Engines Rochester Monojet at Altitude

Greetings, I'd like to re-jet my rochester monojet for altitude but I can not find here on anywhere the size of the factory jet. I live in Denver and as far as I can tell folks suggest buying 2-3 jets a few sizes smaller. I know where to access the size inside the carb, but the car is running and driving and I'd rather not take her down and out of commission for that long. She runs pretty well, but is very obviously not jetted for Denver, and I intend to take it up over the passes around Colorado. The engine is in a '69 GMC with a 250 and as far as I can tell it's original. The car/engine was originally sent to Montana, if that helps. I'm sorry for the noob question, but the information I've found has been all over the place and I either suck at navigating the search or haven't found a bench march size in the threads that discuss this. Thank you.
-Nate

Monojets are a maligned carb around here, they are a metering rod type carb, and sometimes need both jet and rod resizing to get the desired results. Just changing the jet will affect both the power and the cruise circuits. Changing the rod alone will only affect the cruise stage only as all the rods have a common diameter on the power portion of their shank.

Doug Roe has an excellant Rochester Carb book ( an HP publication) that still shows up at Amazon-get it. Roe was a Rochester engineer and he has a few pages on modifing the monojet which is of the same design family as the quadrajet. Roe has tips on modifing the power piston spring tension, the adjustible part throttle jetting as well as simply changing the main jet/metering rod. I had a mono on a '69 230 motor and was able to turn it into a really nice mannered standard shift carb (meaning you can monitor carb performance more easily with a stick than with an automatic with its masking torque converter action.

My gut sort of thinks that just a smaller jet will do the trick because I want less fuel all the time, due to having less oxygen all the time. I was just wondering if anyone had the stock info, I guess I'll just tear the guy apart and see. I'll post up since I haven't been able to find this anywhere on the internet. I had a real hard time with my previous monojet. we'll see how this one treats me. Thank you!
-Nate

In case a search comes up for this. I pulled the jet out last night from a carb running fairly rich and it was a 107. I ordered a 105 and 104 size for testing/replacement. The engine was running REALLY rich, so I went for the 2-3 size down, not just the 1-2 that was sort of originally suggested. I have an o2 sensor and will try to dial it in. I also will not be messing with the metering rod.
-Nate

Nate,
I missed the part about Denver the 1st time.
Typically for your altitude I would have said about 4 sizes leaner ie 103, but since today's gasoline is "cut" with ethanol, I typically jet up 1-2 sizes to compensate for that. So if 107 was good for real gasoline at sea level, I'd say 103 in Denver, and 105 in Denver on today's 10% "gasohol" blend. You should be close with 104 and 105.

That said I have a Monojet or two, I will see what jets are in them. Shop manual should say also

Nate,
I just pulled apart a "Medium" MV:
Throttle 1-11/16 (or a tad more)
Venturi 1-5/16
Main Jet, .104" hole
Metering Rod, .080" cruise, .048" power

So, it matters what size your carb is too, there are at least 3 sizes of the MV carb!

Ahh, nuts. Well, we'll see when I get it all put back together. I can't do much adjusting, but I can't imagine it'll be much worse as it should be "correct".

I'll drive around with my O2 sensor and see what the air/fuel is this weekend to get a baseline and see where I am when I get the new jet in. My metering rod was .081, I didn't check the power. The carb is the 7043014 model... whatever the hell that means. I should probably get the book, but honestly, I'm trying to just put a LITTLE money and work into this and get it to last for a bit till I really figure out what I want to do.

I just went through all of the ignition from coil to plugs and adjusted the valves so I figure now I have a good base to work on the carb stuff. The engine runs well just rich. I do have a weak cylinder though.
-Nate

Okay so here's another update. I had originally pulled the 107 and .081 jet/rod from my old carb, and I bought the 104/105 sizes based on that. Oddly, when I opened up the NEW carb, the jet was unlabeled. I looked over it with a magnifying glass and couldn't find any markings at all. But I could tell that it was smaller than my 107, and apparently larger than my 104. The metering rod was .074. I put the 104 in and the engine seems to be running well, so we'll see as I put more miles on it. I also advanced the timing and tried to dial in the idle a bit since I was having an off-idle stumble if the engine wasn't HOT. It seems to be runnign well, but I haven't had time to futz around the last few days. I think I'll run it how it is, if it's still rich I'll throw my .081 rod in and see what that does.

With the O2 sensor (a/f mixture), I should be able to see if it's rich/lean at WOT (where both rods have the same diameter), and medium throttle, where they differ, and probably dial it in a little more. I didn't realize how easy it was to change the jet/rod. It was all of 10-15 minutes with just a screw driver. Anyway, I realize this is just me replying to my thread, but troubleshooting this carb has been fun so far.

Also if anyone things I'm doing something terribly wrong or dumb, just let me know!

Thanx for the details .

I'm keen to hear how it all turns out for you .

The long # you posted , is the GM part # and if it's never been changed during periodic overhaul , can be decoded to tell you what engine and tranny combo it was designed for ~ GM always went a little bit nuts with the fine tuning, a good thing as it turns out as you can search for a specific venturi and jet size combo once you learn what you want .

I hope you're not near the fires .

Might check Motors Manuals at the library or oldcarmanuals on line. The 7043014 carb (thats the number on the little tag, right?) looks like 1973 vintage, probably car since I dont see it in the truck sheets. So my guess is a 1973 car 250cid, you'd have to hunt for the rest.

Both of my "MV" parts carbs have the same jet and rod as above.

Thanks, okay. So put about 50 miles on the truck this weekend running errands and buzzing around town. Once warm she runs like a sewing machine, smooth and quiet with good power, no luls, nothing funny.

Starting/cold it is a nightmare. I have to crank for a solid 10+ seconds to get it to start and it won't idle for like 2 minutes. The timing is fairly advanced, should I retard the timing for starting? Any other tips? Thanks guys, once I get this figured out she should be a good runner. When warm, it starts with just a bump of the starter. Weird.
-Nate

Is your choke working?

MBHD

For shits and giggles, next time you go to start it cold, BEFORE you touch the key, pop the top of the carb open and check the fuel level in the bowl. When I lived in FL, and parked the car outside for a couple of days in the summer, the carb fuel bowl was empty and the weak mechanical pump took forever to prime the line and carb. Problems went away 100% after switching to an electric fuel pump. Key on, wait 3 seconds for the pump to prime the carb, and start.

x2 what MBHD and 70nova said but I'd add:
1. Some of those later carbs like the MV like the choke ALL the way shut when you go to start em, IIRC the MV likes this especially if jetted lean. Pop the hood and check it, after U pump the pedal but before cranking.

2. Easier than pulling the top off the carb is
2a. Make sure its getting pump shot, give it a couple squirts (key off of course).
2b. Pull the fuel line off, use a squeeze bottle to "fill" the carb. It will always take some in the morning, but if it takes a whole bottle it means the carb was empty. It should just take a little bit, an ounce or two maybe. If more, its either leaking or boiling off so theres no gas in the bowl after that.

It had always started perfectly before I started futzing around with timing and jets and stuff, so it doesn't quite make sense that it's losing the fuel in the float. I've never had to use the choke, but it's manual, so I'll just pull on the lever next time and see how she does. Electric fuel pump is probably a good idea anyway. I've been swamped the last few weekends and just unable to really do anything, but thanks for the tips folks.
-Nate

Choke seemed to do the trick. I guess I just figured it was 90 degrees outside and the choke seemed unnecessary. I remembered that when I went to change the jet the float was full and I hadn't driven the truck in a few days. It's still cranking a little harder than before, but it's running really well, so I'm happy. Thanks guys.
-Nate

My daily driver has a 250/monojet and I have to choke it every first start of the day. I'm not real sure which mono it is, I never have looked at it very close. As long as it keeps working good, I'm cool with pulling out the choke once a day. I'm just a little bit higher than Denver at 5500' and it runs great, but it sure does run better down at 4200' in the Salt Lake Valley. I'd love to see what it would do at sea level. She gets real winded at 7000'+, though, but I'm not up there too much.