I've been messing around with my Rochester Monojet MV on my 250 in my 69 Buick and I don't know what is wrong but I cannot for the life of me get my mixture right. Now I usually just set the idle to where it peaks the idle RPM and I recently rebuilt the carb. Before the rebuild, it took about 2 and a half turns out for it to idle and run well but now... I can screw it all the way in, and then only a quarter of a turn out, it runs the best. I'm not sure why and I don't know what's up but that doesn't seem right.

There's not much that can be wrong. All the passages in the carb were blown out good, after you gave it a swim in carb cleaner. The setting for the power needle was checked. IIRC you have to push down on the piston, then check the distance below the rim. The only problem I had rebuilding any of mine was I didn't leave the screw on the air horn loose, and would die coming to a stop. I don't have the carb sheet here, so I'll have to check tomorrow.

It runs OK otherwise.

Larry

How do you do you check the power needle? The power needle is the thing that is connected directly through the throttle and it drags a needle out of the carb jet as you increase throttle right?

And it doesn't completely close off the jet when the throttle is at idle. Should it?

Actually, what do the power piston and metering rods do?

I'm probably going to re-rebuild the carb tomorrow or sometime soon.

Thanks

Darn forgot to bring the carb sheet home.

Let's start with the power piston and metering rod. There is NO direct connection between the throttle and the power piston. The only connection is the vacuum at the throttle plate. At maximum vac the metering rod is held down in the main jet limiting. It should not be closing the jet off. The carb sheet in the kit for rebuilding the carb will give the metering rod setting for your engine. You can then play with that setting for best performance or mpg, but start with correct setting first. When you open the throttle the vac comes off the power piston and the spring under the piston raises the metering rod allowing more gas thru the main jet.

Do a search on the internet for "rochester monojet tuning" there is a lot more information out there than I could possibly type in the next year.

Larry

alright I re-rebuilt the carb and did everything perfectly this time but it still runs rediculously rich. I've done everything perfect this rebuild... blown it out, cleaned it, wiped it down, blown it out more, reassembled... but it still runs best with the mix screw all the way in. What is wrong?

Pg. 16 is my carb http://www.walkerproducts.com/_pdf/rochester1barrel.pdf
Now 37 (power piston rod) is connected to the throttle plate and pulls down 36 (power piston) which is attached to 28 (The metering rod) which goes down into 42 (the carb jet). That is how I understand it. Is the spring supposed to be pushing the metering rod into the jet or pulling it out?

Things like this can be really frustrating, sometimes it is best to look for the simple things first.

If it runs real rich at idle and the idle screw adjustments don't work as they should, perhaps you should look at the float level.

Some of the early plastic floats can get 'gas-logged' so even with the proper float height settings they will sink a bit causing a high fuel level in the float bowl and the gas will overflow internally causing a rich condition.

You really can't tell if this has happened by looking at the float so I would suggest replacing the float and checking the float height settings before changing any adjustments or parts that came 'stock'.

If you should happen to have a brass float, check it for gas inside, they will leak too sometimes, it doesn't take much change in buoyancy to affect the float level.

Did you get your idle stop solenoid issue resolved?

Good luck,

Tim

The spring pulls the metering rod OUT of the jet so the mixture can richen up under load/full throttle.

Vacuum works against the spring so vacuum at idle/cruise sucks the metering rod down INTO the jet, leaning the mix out for cruise.

You could try leaving the spring #38 out of the carb, put it together and try it. Now, the metering rod will fall in to the fully lean condition. Don't go out and hammer on it as it will be too lean under load - but at least you could see if it leans things out at idle. If so, it would tell me that the vacuum piston is leaking or something and cannot pull down and overcome the spring.

Otherwise, over-rich usually means a blocked air bleed somewhere (check all air bleed passages with a wire) or high float level.

alright. I cannot for the life of me find out how to set the float level correctly. You just bend the metal tab so that it pushes the needle closed all the way when the float gets up enough right? I just don't get it because it should leave plenty of room for it to open up right? I have it set to where it stops filling right when it gets to the top of the fuel bowl. If I set it any lower, the float doesn't all the needle to rise up hardly at all. Should it only let the needle raise up a very small amount or what?

Why would I remove #38 if the throttle is going to pull #37 which will pull down #36 anyway??

EDIT: and my float is brand new

The instruction sheet will have the float height measurements, and your rebuild kit will have a float gauge in it. Basically, take the top off the carb, turn it over so the float is on top, and set the measurement as directed. If the fuel reaches the top of the fuel bowl, it is waaaay to high.

Tim

Alrighty guys. I might be retarded. There is a plug hole in the intake manifold and when I was reassembling everything, I didn't tighten it up. After I tightened it up, the idle changed so loose threads must've been hemorrhaging air to richen out the mix like crazy. Now the best mix is 1 full turn out I think so that sounds pretty good.

After I got that fixed, it slowly lowered rpms and died so I think that means that the float wasn't letting in enough gas. I'll open up the float a bit when it's not 95 degrees outside. I'll let you guys know how it goes.

Thanks for your help guys.

"When adjusting the low idle mixture adjust the screw(s) to until maximum vacuum is achieved" 1967 Chevrolet service manual - A vaccum gauge would've identified this problem quickly - they're cheap (less than $25on Amazon or Ebay)!

Well never mind... tightening that manifold screw thing helped but when I cover up the vacuum port, it changes the mix so now the best thing is about an 1/8 turn out. I guess there's still a vacuum like somewhere. But it runs well right now so I hope I'll eventually find the leak hopefully. Where can I find a rubber cap for vacuum ports and for the hole in the intake manifold on the middle runner?

If you use a can of starting fluid and gently spray around the base of the carb, along the intake flanges and on the vacum ports a little at a time, you will find a vacum leak if there is one, you will hear the engine speed up. I'm not talking about spraying like a can of paint , but just small bursts. I've had vacum leaks at the power brake booster and found them this way.

Both Autozone and Advanced Auto carry a Spectre kit that has 3 or 4 of each of the different sizes. I think for less than 10 or 15 bucks.

alright guys. I might be retarded. I just remembered that there is a gasket in between the manifolds that allows exhaust gases to pass into the intake manifold. I thought that was stupid and had to degrade engine performance so I put in a flat metal plate in between them so that air couldn't get through. That has to be making everything run really rich since the exhaust gases aren't there to balance it out. Is there a way to keep all of the air fresh instead of having old exhaust gases?

Now i'm going to have to take off the manifolds AGAIN and take em apart and replace that gasket. :-/ aw well. Now I can fix my exhaust leak.

What's the point of recirculating exhaust gases?

Thanks guys.

Alright now I'm more confused. I replaced that exhaust gasket between the manifold so now it's recirculating some exhaust gases but the best mix is still all the way in. I even re-rebuilt the carb and it's perfect this time. What is wrong? could it be having a loose idle tube (the tube that is that picks up gas that sits in the fuel bowl)? It doesn't seal very well at the bottom so could excess gas get in by the idle tube at the base?

Exhaust gas doesn't enter the intake manifold.

Can you post some pictures of your carb and manifold so we can see exactly what you are referring to?

Tim

Here is some information that may help,

First off the gasket between the intake and exhaust seals exhaust gases from leaking out, the exhaust circulates around the intake to heat the manifold so the gas vapors stay vaporised. The exhaust does not enter into the fuel mixture, unless your car is equipped with EGR valve, which in '69 its not!

So for now, get the right gasket in place and sealed up. Make sure you snug the intake/exhaust bolts going into the head before you tighten the three bolts holding the exhaust manifold to the intake manifold. Its very important that the intake seals to the head completely.

Now, make sure you have the correct gasket under the carburetor, it should have four small cutouts on the inside evenly spaced around the center bore. These cutouts let vacuum up into the carburetor.

Now, pull the top off the carb and check a few things,

Remove the float and needle and set them aside. Remove the metering rod from the power piston. Back the idle speed screw off till the throttle blades are closed tight up against the throttle bore.

Hold down on the power piston and with the top gasket removed, check the clearance from top of carb body to power piston arm. You should be in the .080" range, bend the power piston arm up or down to get this.

Reinstall the metering rod (check to see that its straight and in the jet). Does your needle (needle and seat) have the small clip on it that hooks to the float? Install one if it doesn't, there should be one or two in the kit. Hook the needle to the float from the inside, not the long slot facing away from the plastic part of the float. The float will not go through its full swing if you hook the needle in the wrong place. The needle should hook between the plastic part of the float and the metal legs of the float. Install the float hinge pin and drop it into the bowl area. Set the float level to .250" from the top of the housing to the tip of the float with no gasket in place.

Install the top gasket and lid and tighten the six screws.

Idle screw should be in the 1 1/2 to 2 turns out range.

A vacuum leak will cause the mixture to go lean, never rich. Check all vacuum hoses for cracks and splits, replace them if you are not sure. Vacuum plugs can be bought at any parts store along with new hose.

If the engine rev's higher then normal, you have a vacuum leak. If it chugs and smokes, you are running rich.

Joe

I recently rebuilt my carb and it ran worse that it did before.

Turned out I had the wrong base gasket. Dont know if the 250s use the same type of gasket, but on my 235 the vacuum port for the advance and I would guess the power circuit go through here. I disassembled the carb twice before i figured that one out. Cesar