Inliners International Forums General Discussion Engines Rochester monojet carb

How are the vaccum lines connected in a rochester monojet with electric choke. There are 4 places where vaccum lines could go. One is on the base of the carb to the right of what i believe is the mixture adjustment screw. There's another one on the base of the carb but on the passenger's side. Then there are two close to the electric choke, one on top of the other. Right now the mechanic has left the vaccum line going to the distributor connected to the top one next to the electrick choke and the other three have a little cap. He said those were for a canister or something like that. Im new to all of this and i cant find any books here in el salvador. Is that how the lines should be coneected???

I can tell you how mine is set up on my '68 C10 with 292 and Powerglide tranny. It's got a manual choke though. My carb has 2 ports, 1 on passenger's side and 1 on front, driver's side corner. Front one is plugged, and the other goes to the vac advance on the distributor.

I can look in the Haynes book on Rochester carbs for more info after I get home.

I used to have it on the front one you mention, but the vaccum line was clogged so the mechanic plugged it on the top one next to where you regulate the idle. Is it the one on the side or the one in front? What difference would it make if it were plugged into a different port and what are the other ports for?

The Monojets came with lots of combinations of hose ports. I've got one that has 4 ports. 3 are small ones (about 4 mm across) and one is large (about 10 mm). If your carb has a different number, then the ones I can describe are going to have the same purpose.

The one at the bottom front next to the mixture screw is a manifold vacuum. It's usually capped off on trucks, and is most often connected to the carbon canister system on cars. If you have any other small ports on the base of the carb (the same height as the mixture screw), they are also manifold vacuum.

The one on the passenger side under the electric choke is a ported vacuum. It normally is connected to the distributor vacuum advance regardless of vehicle.

There is a small one up high on the drivers side, and is a manifold vacuum. When it is used, it is usually routed to the air cleaner cold-air door or to a solenoid mounted on the firewall on some cars (taxis) and larger trucks.

The large one, which is also up high on the drivers side, is a vent that is connected to the carbon canister (if equipped) or capped off if there isn't a canister system.

Manifold vacuum is normally used to operate on-off devices. Ported vacuum is normally used for distributor advance and EGR valves. There are exceptions to this, and you have to know what type of device you are dealing with to get it correct. For example, some cars used a water-heated switch to send either manifold or ported vacuum to the distributor. You can get by with hooking the distributor to only ported vacuum. You should also test the vacuum advance pot to see if it holds vacuum. Hook a (clean!) hose to it and suck on it when the engine is running. The motor should speed up a lot. If it doesn't, there is a problem, and it easily could affect your gas mileage.

Up here in the States, all cars had a sticker under the hood when they were new that showed how the hoses were to be connected. Does your car have a sticker on it, or did it ever come with one? I've seen more than a couple cars that were manufactured in Mexico that did not have any sticker, and used technology that was at least 10 years behind what the US manufacturers were selling at the time.

Keep on trying, and get back with your testing results

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David
newbie #4153

There are two next to the electric choke and one under the electric choke at the base of the carb. The one connected to the distributor is the top one next to the electric choke. The one beneath that one and the one under the electric choke are capped off. What happens if it is connected to a manifold vaccum port?

It sounds as though your distributor is connected to ported vacuum. If you connect it to manifold vacuum, then the ignition system will advance the spark anywhere from 12 to 18 degrees extra at low engine speeds. The motor may not like this and ping (detonate) when you try to accelerate.

Some engines are designed for full vacuum advance at low speeds, others are not. It depends on the camshaft, what kind of emission equipment is installed, the compression ratio, the piston deck shape, the condition and age of the engine, the gasoline octane rating, the transmission, the rear gear ratio, the air temperature, the list goes on and on and on. Without being there, I can't tell you for sure whether it can handle it. But if the motor pings with manifold vacuum, and it doesn't with ported vacuum, then it should only be hooked to ported. If it never pings AND the system is working correctly, then use manifold vacuum and see if the car performs a little better.

Do yourself a favor and test the vacuum pot on the side of the distributor to see if it's working like I described in my last post.

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David
newbie #4153

I took the hose that goes from the distributor to the carburetor and disconnected it from the carb. and only left it connected to the distributor. I sucked on the hose and nothing happened to the engine. It stayed the same, it didnt speed up at all. What does this mean?

It means that you need to replace the vacuum pot or the hose has cracks in it. Double-check the hose before you do anything else.

The pot is designed to move a plate inside the distributor when vacuum is sent to the pot from the carb. If it doesn't hold that vacuum, then it can't do it's job. More than likely it is one of the biggest reasons that you are getting poor performance and bad gas mileage.

In order for you to replace it yourself, you will need to get a stamped number off the side of the distributor and tell the person at your local parts house that you need a vacuum advance diaphragm for your car. They may need that number I just spoke of, they may not. Then you will need a flat-bladed screwdriver to get the distributor cap off. You just push down on each of the 4 "cross-heads" on the outside of the cap, and turn them 1/4 turn counter-clockwise. You don't have to disconnect any wires if you are careful. If you do disconect anything, don't break it, and remember where it came from. Lift the cap off and push it over to the side so you can see whats under it. You can change the vacuum advance now by taking out the 2 screws holding it down and then wiggling the rod out from under the flat arm it's attached to. The holddown screws may be Phillips or 1/4" hex heads. DON'T BEND ANYTHING. Reverse the procedure to reassemble it and be very careful to make sure the cap is seated completely on the distributor and the 4 cross-heads are holding before you start the engine. If you can't get the rod out of the flat arm, then it would be best if you found some help to show you how to do it.

Good luck! Successful completion makes for smiles all around!

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David
newbie #4153

How much does a vacuum advance diaphragm cost? I called up a guy who sells parts for american cars and he said it would cost about $150, is that ok or no? They dont sell that part new here. Nobody has american cars only japanese.

That kind of money would buy you a brand-new distributor up here. Or you could get a new dizzy off the internet for less. Just the advance should cost around $10 to $15 maximum. Ask him if that was for a new or rebuilt distributor or if he is pulling your leg on the price.

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David
newbie #4153

I got one installed for 20 bucks. However i did the same thing, disconnected the line from the carb and sucked on it and the engine didnt speed up. Do i need to apply some throttle to it while sucking or what? Is this vaccum advance not working either?

I took off the distributor cap and sucked on the hose with the engine off of course and something in the vaccum advance I changed moved about 3mm at the most. Does that mean the vaccum advance diaphragm I put in is working and maybe something else is wrong in the distributor or what? beacuse like i said in my last post with the engine running it wont speed up if i suck on the hose.

If you watched the advance move, then it is working. Finding out why the motor is not speeding up is another question. At least you got a reasonable price on the installed part.

You identified the hose connection at the carburetor as a ported vacuum. When running at idle, if you disconnect the hose, the engine speed should not change, because there should not be any vacuum. Connect a timing light, start the motor, and see if the timing marks move when you suck on the hose. If they don't, there is something wrong inside the distributor.

Next question is, ignoring the advance for now, is does the engine have a little more power that before? There are still some other things to check as your diagnosis continues such as mechanical advance, all the vacuum hoses, and whether the base timing is set correctly. Did you talk to your mechanic about his thoughts concerning the possibility of a partially plugged muffler? You should complete your distributor tests before you try to diagnose a bad muffler.

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David
newbie #4153

I felt the car has a little more power after the vaccum lines were connected correctly and the hose was unclogged. Im getting about 11 MPG but hope to get more. I checked the vaccum lines and they are all ok, they are not clogged or anything like that. Ill take it to the mechanic to check the timing and see if it advances when I suck on the hose. People have told me it is possible that it has a bad muffler but ill check the distributor first. What could be wrong if the timing does not advance? What should the ignition timing be?

The base timing should be about 4 to 8 degrees before top dead center. On a stock 250, it's not critical to get it exactly on the factory spec. The vacuum advance should provide about 12 to 18 degrees more advance just by sucking on the hose. There is an assembly inside the distributor called a mechanical advance, and it gives about 12 to 14 degrees more as you rev the motor up. You should check the mechanical advance with the vacuum disconnected, so you can see how much it makes the timing marks move. All you have to do is rev the motor up to about 2500 rpm, maybe 3000, and the timing mark should be done moving.

If the mechanical advance doesn't work, the distributor has to be removed and completely disassembled, cleaned and lubricated. They usually stop working because the grease gets nasty-hard and nothing can slide like it's designed to do. The option is to replace it with a rebuilt unit, but there's usually no reason for that other than money or time. I prefer to take them apart and fix them. Sometimes there are worn-out parts inside; if that's the case, then get a rebuilt.

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David
newbie #4153

Took the car to the mechanic, unfortunately I wasn't there when he checked the timing and the mechanical advance. He said the mechanical advance was fine, and that he couldnt check again because he did not have the timing light with him at the time. He also said that when the car is at idle and i disconnect the vaccum hose from the carb and suck on it the engine should not speed up at all. It should only speed up as throttle is applied because that way throttle and vaccum are working together. He said he set the timing 2 more degrees than what it was before so he said he set it at 12 degrees. However i haven't seen any improvement at all in the car's performance. Im starting not to trust this guy anymore so I'll see if i can get a light somewhere and test it myself. He changed the clutch from my gmc truck and now the exhaust is making a big noise it wasnt making before and he says it might be the exhaust manifold's gasket, but it was just fine before i gave him the car. I'll check it myself and inform you cause im frustrated right now with this guy.

I was looking inside the carb with t he engine running and made an interesting discovery. A little spray of fuel comes out from inside the carb, but not from the jet. It comes out of the "wall" so to speak, apparently there's a fuel leak there. Do you guys think maybe that's what's causing the poor gas mileage and performance????

p.s. Just a six, I took the liberty of sending you an email with pictures to clear up my questions, hope you dont mind.

[This message has been edited by tatao (edited 09-08-2003).]

I had nothing to do one of these days and i went ahead and tried to connect the cannister. I tried to figure out the sticker under the hood, but was unable to do so. I connected one of the hoses to the carb, to the 10mm port Just a six mentioned in his first reply. The other one goes to the fuel tank, but I couldnt figure out where to connect the vaccum hose. After re-reading this post i guess i should connect it to the port at the base of the carb next to the mixture screw right(manifold vaccum)? The large one wasn't connected before and wasn't capped either. I dont remember if it produced any vaccum like the manifold vaccum port at the base of the carb which is producing vaccum at all times. Does this mean that port may be sealed internally? How does it operate, should it be producing vaccum at all times just like the mannifold vaccum port at the base of the carb or does it operate like a ported vaccum like the one connected to the distributor?