Rochester Monojet Help

[ST Dog]

Originally Posted by 68gmsee

Actually they're pretty simlar... Your setup is the way my original carb was. I

Well, I think there's a lot of difference. The curb idle adjustment is different, the fast idle cam is different, and the tang on the throttle shaft is different.

I'm even more confused as to the origins of this carb though. Looks to be cobbled together parts from the original '68 carb and the '72/'73 carb indicated by the number stamped on it.

At least it seams to work now. Best I can tell, it has never worked. No way the linkage/tang has worn that much. No signs of wear either.

Now at least I can get a fast idle to warm the engine up. I need to go through the carb though, as it need a few minutes of warm up before it's drivable, eevn in 80* weather.

Thanks for the help. I feel better about the adjustments. Only carb I know well is the Q-Jet from late 60s/early 70s Cadillacs (divorced choke).

[cdowns]

try going to a public library and see in they have any haynes ,motors,or chiltons manuals either of those 3 should have good info on that carb

have you done the basics like sparkplugs and points //those are a good first step b4 jumping on the carb bandwagon// fuel and air filters too

[ST Dog]

Originally Posted by cdowns

have you done the basics like sparkplugs and points //those are a good first step b4 jumping on the carb bandwagon// fuel and air filters too

Yes. This isn't my first "old" vehicle.

As for a manual, did GM make shop manuals for the trucks?
I have one for my Cadillac and it's far more useful than any of the generic books. I have the Motors manual (big hard bound book) for all US cars '69-'76, Chilton's (another hard bound book) for '67-'74 cars as well as the Newer Chiltons and Haynes paper-back books for RWD Cadillacs (covering 70s and early 80s) that the parts stores carry.

The hard bound books are much better than the newer paper backs.
But the GM factor service manual is the one I turn too most of the time.

[68gmsee]

You got me there sarge... I guess if you look at the little things they are different. I was mainly refering to the operation: link, bellcrank, manual choke.

What you have is probably an M/MV series out of a newer vehicle with idle "speed control" where the idle adjustment screw is. My book shows yours to be sort of a combination of different carbs.

Gotta remember that when these carbs were sent back for rebuilding they used whatever parts they could find. Since a lot of the parts were interchangeable, they used newer and older parts to make them work.

The bellcrank "hook" was there on some vehicles so that if you stomped on the throttle it would force the choke open.

But, glad you got it going though. Let me know if you need anymore pics.

[ST Dog]

Originally Posted by 68gmsee

Gotta remember that when these carbs were sent back for rebuilding they used whatever parts they could find. Since a lot of the parts were interchangeable, they used newer and older parts to make them work.

Given the truck/engine history I'll never know.
Maybe I'll find a proper carb some day, or better still, a staged 2bbl.
No point in a Q-Jet on a 250cid engine.

Would I need a different intake manifold for a Rochester 2bbl?

[cdowns]

gm factory service manuals are avaiable from lots of places try the main page here and go to the vendors link most of the guys have them for sale at reasonable prices

[68gmsee]

Originally Posted by ST Dog

Given the truck/engine history I'll never know.
Maybe I'll find a proper carb some day, or better still, a staged 2bbl.
No point in a Q-Jet on a 250cid engine.

Would I need a different intake manifold for a Rochester 2bbl?

Well, actually a quadrajet would be a good one install if you can get a 4 barrel manifold fairly cheap. I've given it some thought and there are others that have done it.

I would much rather go with a 4 barrel than a 2 barrel. The primaries are smaller on a quadrajet and you still have the big rear ones for added boost.

Of course the best way is with a fuel injection conversion, but that's way beyond my means at this time...

[ST Dog]

Originally Posted by 68gmsee

Well, actually a quadrajet would be a good one install if you can get a 4 barrel manifold fairly cheap. I've given it some thought and there are others that have done it.

I would much rather go with a 4 barrel than a 2 barrel. The primaries are smaller on a quadrajet and you still have the big rear ones for added boost.

A Q-jet just seams like over ill for a 250.
While the primaries individually are smaller, I would expect the 2 primaries would still be bigger than the primary on a 2 bbl, leading to more throttling losses.

Not to mention this truck is for my 14yr old, who turns 15 in September. I don't want it to be too much. So I don't anticipate any performance mods. The 2bbl is more for economy. The smaller primary allows for less throttling loss and more effective part throttle fuel atomization (same reason the 2 small primaries on a Q-Jet help with efficiency compared to a square bore carb)

If it was for me I'd be putting a Cadillac engine in it

[68gmsee]

Originally Posted by ST Dog

A Q-jet just seams like over ill for a 250.
While the primaries individually are smaller, I would expect the 2 primaries would still be bigger than the primary on a 2 bbl, leading to more throttling losses.

Not to mention this truck is for my 14yr old, who turns 15 in September. I don't want it to be too much. So I don't anticipate any performance mods. The 2bbl is more for economy. The smaller primary allows for less throttling loss and more effective part throttle fuel atomization (same reason the 2 small primaries on a Q-Jet help with efficiency compared to a square bore carb)

If it was for me I'd be putting a Cadillac engine in it

For economy, I've found out that you won't get much of a change on these trucks unless you get drastic. I've heard estimates of anywhere 10 to 18 mpg. I squeeze about 16 on the highway on my 250 single barrel and I used to get 15 on my 69 with a 350/350 and a quadrajet on the highway. Both with slight tailwind.

The 2 barrel has larger openings than the primaries on a quadrajet. I used to have a 2 barrel and the gas mileage was dismal. If gas mileage is the main concern, about the only thing that may help is rear axle ratio change, overdrive transmission, HEI distributor and maybe larger tires.

[ST Dog]

Originally Posted by 68gmsee

The 2 barrel has larger openings than the primaries on a quadrajet.

While the primary on a 2bbl is lager than either of the Q-Jet primaries, it should be smaller than the combined primaries on the Q-Jet.

Q-jet primaries are 1-3/8" each, or 2.97 in^2 total area.
That's the same as a 1.94" bore. I tought most 2bbs had 1.5-1.75" bores.
I know Holley/Webber made some 2bbls with different sizes for the primairy and secondary.

And I want a vacuum secondary if I can find one, not a mechanical.


Guess I should start a different thread for trhis. It's off the monojet topic.