Rochester Monojet Help

[Sgt.Booya]

Hi Guys, I just got my truck and I'll be the first to admit I'm a bit of a "tool" when it comes to mechanical expertise, however, I'm usually brave enough to try

I'm planning on replacing my inline six at some point but for now I want to get it running smooth. Hoping you guys could give me a hand?

The Problem:
It's REAL hard to get going.
If I just feather on the gas I can get it to catch after about 15 starting attempts. If I let it idle for about 5 mins I can gradually apply more gas very slowly and raise the RPMs. Too much gas and it stalls out. After its up to tempature I can drive it.

The Equipment:
1970 Chev Rochester MonoJet for an inline 6. 250cc. Casting number# 7040021

What I think is wrong:
I think it's the choke. I've never seen a carb like this before it doesn't have a choke on it? I think it's detached? (See pic)

.


The Solution: (and here's where I need your help)
* I can't seem to find a replacement choke (rebuild kits yes, chokes, no). Anybody point me in the right direct here?
* Is there another carb I could buy that would work on the inline 6 with the choke incorporated? Remember the engine is going likely in the next 12 months [It would have to be low $$$ solution] and there looks to be a fairly elaborate mechanism to operate the specific carb on there now. Looks like anything else (newer) would require from creative fabrication.
* Has anybody ever hooked up a manuel choke under the dash for this carb?

I'd appreciate your input and advice of what you feel might be the easiest way to fix this.....Thanks

Ps: New points and wires....I think its definately an air/fuel thing.

[texnician01]

What your arrow is pointing at is not the choke that is an idle solenoid. The choke will be right under the threaded rod for the air cleaner. It looks like it is there in the pics but I can't tell for sure. the shaft closes to the top will be the choke shaft. the small linkage rod on the back of that shaft should go to the throttle butterfly and the one on the front of that shaft should go to the Intake manifold and have a bi-metal spring under a small cover. The spring is all that controls the choke. There are a few other carbs that will fit that engine without too much trouble. My old 292 had a Model 1940 Holley on it. worked well. About any single barrel rochester will fit but the linkage is the hard part to adapt. You may be able to put a cable choke on the one you have but it may involve fabricating a bracket for the cable housing and getting one of the little brass endpieces to activate the choke. Sorry if that wasn't much help. I don't have any of these carbs around anymore or I could maybe help with some parts.

Nathan

[shelby987]

Pic. isn't loading for me, but my truck had that same carb, and it had a cable choke mounted under the dash. It appeared to be one of those off the shelf k-mart jobs. The only bracket you will need will be one to mount it to one of the bolts that holds the valve cover down.....I would bet you could get a piece of angle iron from Home depot and just use some creative bending.....I had the exact same problem though, with or without the choke....I think the carb might need a complete rebuild.......

If however, you are just looking for an easy way to start it....i found that if I tapped the gas when turning the key (continued tapping, about 1/3 of the way down), I could get it to start....once it kicks DON'T stop tapping the pedal, begin to increase the depth of the tapping until you are a llittle less than half way to the floor....then stop tapping, holding your foot half way down, this will allow you to get to an idle of about 2k or so (never had a tach when I had this problem), after about 2 or 3 mins. of high idle, you can let your foot off and it will idle fine.....if you had a stick shift like mine, you could just slide it into gear, but with an automatic it might just stall you out again.

Hope this helps......if you just want it to run and start, this should work, but this is by no means a final solution.

[Sgt.Booya]

Shelby987, I'm sitting here laughing my a$$ off. The way you described the sequence of events you have to go through to get it running is perfect. That is EXACTLY my problem. I just never thought anybody could desribe it so well.

Still at a bit of a loss though. Since the idle solenoid definately isn't the choke. (I warned you, I'm a mechanically challenged)

Guess I'm down to 1) Rebuild 2) Bolt on something different and figure out the linkage 3) Try a manual choke cable

Interested to see how others would handle it? Opinions?

[PanelDeland]

I had pretty much the same problem with mine.It would start on the manual choke but would stutter and balk when trying to take off(manual tranny).It was also sometimes hard to start.I found the problem when I changed the points and plugs.When I timed it,the timing wasn't changing when the throttle was pushed.The vacuum advance was bad.I replaced it and the engine starts better and there's very little to no stutter when taking off.To check just remove the vacuumn hose and plug it.If the advance is bad there will probably be little change.Vacuum is high at idle so it will affect your idle if the vacuum advance is bad.

[texnician01]

I think I would go find a good book with exploded views for that carb or look on the net, rebuild it, and put on a manual choke, shouldn't be too bad. Wish I still had mine, you could bot it on, hook up the choke, throttle rod and one hose, throw out the rest of that crap you have on that poor motor. lol! Of course you always have the option of the offy 4 bbl intake and header...

Good luck
Nathan

[texnician01]

try a book like this

http://cgi.ebay.com/ebaymotors/Book-...spagenameZWDVW

Nathan

[shelby987]

As stated, I had the same problem and I had a working manual choke, the choke would help to keep it running once I got it past the sputter, sputter, stall point, but it took at LEAST a min or two of warm up before the choke made a difference, and there was still only a small "sweet spot"
I have actually since swapped my carb for the 4bbl and headers....and the engine just sounds MEAN! My points and vaccum advance are fine, totally a carb problem. IIRC, the motor craft 350 2bbl carb is a bolt on???

[Sgt.Booya]

Humm, OK so even with the manuel choke shelby987 had the same problem, so it's probably not entirely a choke problem...and PanelDeland sharpened his up by correcting the vaccum advance. Based on your advice here's what I plan to do:
1) Check and test the Vaccum advance. If that doesn't work, I'll........
2) Take Texnician01's advice and pull the whole Carb off, clean and rebuild it. If that's not it, I'll......
3) Bolt on a whole different Carb and figure out some new linkage.

Ps: Hey shelby987 aren't MotorCraft parts made for.......(Dare I say the "F" word on this website???) Gulp........FORDS?

Thanks guys, appreciate the insight.

[Canadian72GMC]

the arrows in the picture clockwise from top left are ; choke rod, choke plate and choke thermostatic coil. Make sure that on a cold engine, the choke plate is lightly closed. The choke thermostatic coil is actually a temperature sensitive bi-metal spring that puts tension on the choke rod as it cools off. You can buy these, and it is likely your problem.

[ST Dog]

Resurrecting an old post.

Anyone got pictures of the monojet linkages?
I think have a problem with the fast idle cam.

'68 GMC, manual choke. Story is the engine was replaced in '72 with a new engine from a Nova (dealer warranty replacement best I can tell).

The number on the carb points to a '72 carb.
I think it was modified some for the manual choke though.
The coil is still on the intake with the linkage rod, but it's not attached to the cab, and no place to attach it either. There is a rod from the rear of the choke shaft to a cam at the base which looks like a fast idle cam.

Anyway, when I close the choke it doesn't move the fast idle cam much (if at all). If I move the cam by hand, noting in the throttle linkage touches it. Looks like I need to bend a tab on the throttle linkage to set the fast idle, but until the choke moves the cam it won't help much.

[68gmsee]

Here's a bump for you...

I've got a Rochester monojet with manual choke on my 68 GMC. I'm not sure how much of a difference there was in the linkages between the early and later years, but I can post a pic later tonight.

Is the choke linkage and fast idle cam the only pic you need?

[ST Dog]

Originally Posted by 68gmsee

Here's a bump for you...

I've got a Rochester monojet with manual choke on my 68 GMC. I'm not sure how much of a difference there was in the linkages between the early and later years, but I can post a pic later tonight.

Is the choke linkage and fast idle cam the only pic you need?

I think so, the rest seams OK. I just want to verify that everything is there, or figure out what's missing.

[68gmsee]

Well, the pics I took looked like crap so I'll get some with a different camera tomorrow morning.

[68gmsee]

Here's a pic. Hope it helps.

[ST Dog]

Quite different. Here's what I have.


(Hard to fit my camera in there for a good picture)

I guess it's all there. I adjusted the linkage from the choke to the fast idle cam so that it pulls the cam all the way at full choke.
Then adjusted the tang closed to that cam to touch and set the fast idle. About 2000 rpm when warmed up at the position the cam goes to for full choke.

[68gmsee]

Actually they're pretty simlar... Your setup is the way my original carb was. I did some swapping out last year but still have all the original stuff except for the oil breather- must have gotten left behind in one of my moves.

The tang is adjusted so that it has a slight clearance when the choke is fully open. As you engage the choke, the lever pulls up on the bellcrank to increase the throttle opening.

When you push the choke in, the bellcrank should "free fall" to it's normal rest position.. Very simple mechanism, but works great!

forgot to add: on mine, the link pushes the bellcrank down as opposed to free falling.

[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

[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 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.

[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