Rochester 2-jet

[mudman]

I don't know how well this has been discussed, but I have tried to put a Rochester 2-jet on an EA-71. It seems that no matter what I've tried, I have too rich of a mixture. I have 50 size jets in. The smallest that they make are 42. It will run it, but it is very rich.

 

I thought that this would be an excellent carb for a sube due to its simplicity, but I just can't lean it out. Perhaps this carb should stay on older 6 and 8 cyl engines.

 

This link has alot of information about carbs on VW's, I would imagine that the same principals for these engines hold true for the opposed 4 subes.

 

http://books.google.com/books?id=0EaJuPx71_oC&pg=PA34&lpg=PA34&dq=rochester+2+jet+Volkswagen&source=bl&ots=hK3LzqE5iv&sig=cedDLTSp9JVkb6cBWf1oNsWjIKs&hl=en&ei=9PSvSbjGApjAtgfvgtntBQ&sa=X&oi=book_result&resnum=1&ct=result#PPP1,M1

 

Has anyone else tried a Rochester on a sube? I just want to know if it's possible, or should i fork up the 300 bucks for the weber?

[opus]

I wouldnt fork the money for the weber. The Roch is a great carb. Has to be a way to make it work. Wait for someone to chime in and see where you get.

 

How did you get it to bolt up?

[mudman]

I machined the top flange on the existing manifold to fit the two barrels. (I did this so that the barrels sit in line with the manifold and not perpendicular) Then I made an adapter plate on the bridgeport and welded it directly on the top flange of the manifold. I welded all of the vaccum ports on the manifold also, since I don't need them. I also split the manifold with a plate down the center so that one barrel feeds one side and the other barrel feeds the other side. I have absolutely zero leaks with the welding of the adapter plate (did a pressure test), so there is no vaccum leak.

[opus]

Ok, need pics now!

[ivantruckman]

Do ea81 engines have different Vacuum characteristics, i know they have longer intake manifolds, hmmmm. thinking..... a carb should be a carb, airflow across a venturi , sucks fuel out of a jet

[naru]

I don't know how well this has been discussed, but I have tried to put a Rochester 2-jet on an EA-71. It seems that no matter what I've tried, I have too rich of a mixture. I have 50 size jets in. The smallest that they make are 42. It will run it, but it is very rich.

 

I thought that this would be an excellent carb for a sube due to its simplicity, but I just can't lean it out. Perhaps this carb should stay on older 6 and 8 cyl engines.

 

This link has alot of information about carbs on VW's, I would imagine that the same principals for these engines hold true for the opposed 4 subes.

 

http://books.google.com/books?id=0EaJuPx71_oC&pg=PA34&lpg=PA34&dq=rochester+2+jet+Volkswagen&source=bl&ots=hK3LzqE5iv&sig=cedDLTSp9JVkb6cBWf1oNsWjIKs&hl=en&ei=9PSvSbjGApjAtgfvgtntBQ&sa=X&oi=book_result&resnum=1&ct=result#PPP1,M1

 

Has anyone else tried a Rochester on a sube? I just want to know if it's possible, or should i fork up the 300 bucks for the weber?

 

 

Ahh... Your Carter -Weber has shape shifted itself into a Rochester dualjet? No wonder you are having trouble pinning the problem down!

 

That carb was on 5.0 litre engines.You are expecting it to work on a 1.6?

Almost certainly,the vacuum signal to the metering rods and power valves is all wrong.

Your reference says they don`t work on VWs either.

 

Throw that POS away.You are washing oil off the cyl walls and not doing your new rings any favours.An early varijet would have been a better Rochester choice.

 

If you`re determined not to buy a Weber,try a different junkyard carb from an engine no bigger than about 2.5L.

Edited March 5, 2009 by naru

[mudman]

LMAO naru. Yes, I wanted to know the jet sizes of the Carter Weber to apply them to the Rochester. Still LMAO. Shapeshifting carbs.....funny. I figure that if I said I had a rochester, everyone would just tell me to "get a weber and be done with it." I really don't want to spend any more cash, nor do I want to hear what I fear is the inevitable. So I told a little white lie to try and track down the source of my problem. Didn't help b/c the carbs are completely different. Pretty dumb of me, I know. I shall not tell fibs on USMB again.

 

The rochester sounds good, in fact really good. But how can it sound so good while it's throwing that much fuel into there? I suppose that the fuel air mixture is correct if I had a few more cylinders. I just have double the fuel/air that the engine needs, would this be correct? If this were correct, smaller jets would work, right? And if the Webers use 50 size jets, then a 50 is a 50, and a 50 will perform the same in the Rochester? IDK.

 

Perhaps I am under the misunderstanding that any small simple carb should work if adjusted correctly. It mixes the fuel with the air, vaccum sucks it in, put a spark to that, and your piston is firing........Right? Not saying that a quadrajet would work, but I thought these rochester 2G carbs were about 280 cfm.

 

The VW forum does say that "it will blow your mind" trying to get a rochester 2G to work. I want to get this thing running right, but I don't want to spend 300 more bucks to do so. What are the alternatives? Holley 5200?

[naru]

Rochester 2Gs come in several flavours.Smallest is 280cfm.Largest is 435..AFAIK,3.8L is the smallest corresponding engine.

Part of the problem is the synchronous throttles.An oversized progressive carb would be more forgiving.

 

BTW-I do own one,but,it is not on a Sube.

 

You have the same amount of air and excess fuel.

 

Jet size comparisions are not valid w/different sized venturies.

As for idle jets,air bleed size will affect jetting comparisions.

 

Too big a carb won`t be able to meter fuel accurately enough at low airflow rates.Almost certainly,the vacuum signal to the metering rods and power valves is all wrong.

 

Holley 5200 would be a better starting point.There will be several other canidates at the junkyard too.Think smaller engines.

 

Here is a radical thought-How about a stock carb?

[mudman]

I have a hitachi and an extra manifold. I cant get the choke to work right, nor could I get that thing to idle properly at all. I bought it rebuilt, but without emmissions at all hooked up to it, it was a nightmare. Fuel/air mix did almost nothing to it. I am trying to use the simplist carb possible. I thought that they mada a carb with a fuel line to it and thats it. A small simple carb is what I was in search of when I came across the Rochester 2-jet. I was told that it was 280 CFM, but I cant be certain, that only what I was told. Is there a way to tell what the CFM is? The webers are about 300 if I'm not mistaken, but I only need 200 cfm. I figured that if the webers are 300 and work, why wont a 280?

 

I'd rather something not as complex as the hitachi. I have no clue what most of the stuff all over that carb is.

 

I'm looking at other options if the Roch is not 280cfm. Perhaps a single barrel holley or something thike that. I just want to keep it as simple as possible, but it needs to be reliable.

[naru]

2g identification,ripped this from another site:

 

1 1/4" flange, 1 7/16" throttle bore, 1 3/32" venturi - 278 cfm

1 1/2" flange, 1 11/16" throttle bore, 1 3/16" venturi - 352 cfm

1 1/2" flange, 1 11/16" throttle bore, 1 1/4" venturi - 381 cfm

1 1/2" flange, 1 11/16" throttle bore, 1 5/16" venturi - 423 cfm

1 1/2" flange, 1 11/16" throttle bore, 1 3/8" venturi - 435 cfm

 

I sometimes work on a 394 cu. in. Oldsmobile that breathes thru one.

 

Might be worth checking to see if the metering rods are stuck,but I wouldn`t expect miracles.

 

I suspect it doesn`t work because you don`t have enough part throttle vacuum from your 1.6L.

[mudman]

Ok, I have the 278 cfm then. Well, I just checked all plugs. The 1 and 3 are a nice tan color, but the 2 and 4 are black as night. I heard that one side runs dirty on these engines, but come on.

 

Rochesters are not supposed to have each side adjusted independently are they? I thought that if I turn one side a half a turn, I must do the same to the other. Wait, that shouldn;t have anything to do with it. I've got too much gas on one side, not air. Or maybe its not enough air on one side?

 

I rebuilt the engine, and I know it was done correctly, so why would one side run dirty? Could it be carb adjustment, or are we going around and around here and I only have enough vaccum on one side of the carb? It can't be that the carb is too big on only one side. Bad needle, badjet, bad entire side of the carb? This is making me nuts, because if one side is running great with the carb, why wont the other side? The carb is clean, I cleaned it 3 times already. I have no clue.

[naru]

The plot thickens. If one side is kinda OK then something is different between sides.AFAIK, the carb is symmetrical.

 

Adjust mixture screws independently.Should be similar but maybe not

identical.Both should be responsive.

 

I`d compare compression side to side.

[mudman]

Compression was a little low on that side, only by about 10 lbs, BEFORE the rebuild. I thought that this is what was giving me problems to begin with, but I feel confident that nothing is wong with the Engine mechanical system since it was rebuilt.

 

I have no flat spots and throttle is very responsive, so I have to believe that I'm very close. After a half hour run time, the 2 and 4 are so dirty, that they don't want to fire. They are black. 1 and 3 look good.

 

I even plugged the EGR, PCV valve,and I let the valve covers vent to atmosphere. The EGR does come from the side that is running fine, but I think that emmissions have nothing to do with my problem. One would have to guess carb, but I can't understand why. The next option would be weak spark, but I can see the spark jump from the cap to the wire(if I remove the wire). It will jump a gap of about 3/8" or so, so ignition must be good. It again brings me back to the carb or something with the emmissions that I don't know about. Decisions Decisions.

[mudman]

Here is another thought. I do not have a fuel pressure regulator. The Rochesters are supposed to run at 3-5. What does the stock subaru fuel pump deliver?

[mudman]

Well, after alot of research, I've learned that the idle screws are supposed to be adjusted independently on the Roch. One barrel feeds one side, and one barrel feeds the other, so they must be adjusted differently. I feel confident that this carb will work correctly, once adjusted correctly. At 280 CFM, it should work well, but this has been alot of trial and error getting it right. Right now, I'm not sure if I'd recommend using one of these on a sube, but I will see how it performs after some more adjustment and tinkering. Might be a screamer, might be a dud, we shall see. Thanks Naru for you insight, you've helped me out alot.

[DR DLEX]

you may be pulse-ing the carb,by isolating one side of the carb to one side of the engine.does your mounting plate allow intake vacuum from both sides past the throttle plates?without equization the carb won't idle right.also which side does the vac. adv. plug into,check for leaking adv on dist.(spark plugs dirty/clean same side) .

 

 

 

just sharing braincells,have fun...

[scrounge]

That 2GC is probably too much gas for a soob. The smallest engine I know of having one was a Vega 2300cc from GM, the biggest is an olds 455 (7.4 Liter) I have in my garage. They're not the same CFM, but even the Vega carb seems a tad big.

 

Most (If not all) 2GCs have a power valve, which probably opens too soon for a Subaru, though you may find a stiffer spring for it. There's a great book out there called "Rochester Carburetors" by Doug Roe. It has some information on 2bbls as well as the Quadrajets. It's good reading if you're into Rochesters (all my other cars are Oldsmobiles) and has a lot of CFM, jet and rod info.

 

Have you considered an early Rochester dualjet? It's smaller than a 2GC but it's basically a Quadrajet with the secondaries removed and uses the far superior Quadrajet metering system. It's trickier to adjust but worth it IMHO, I get better mileage out of Quadrajets when I keep my foot out of the secondaries.

 

As someone said earlier, you may be better off with a varajet 2bbl. It seems like a staged 2bbl would be better on a Subaru.

 

Good luck.

[7point62fmj]

I realy hope you get it to work but as for me I would just spend the 160 for jerry to mail me out a freshly rebuilt weber that is alredy jetted for my ea81.

Good luck.. interesting:banana:

[mudman]

The carb says rochester 2 jet, which I assume to be a 2G. Nothing fancy about this carb. I don't think that its a dual jet.

 

Idle is a little rough at 800 rpm, but I need to advance timing some still. At 2000 rpm it just purrs. Probably because it not progressive. I have a little flat spot on the throttle response, but it's nothing that I can't live with.

 

I have a plate to isolate each side of the carb inside of the manifold, Vaccum should pull evenly from each side of the engine I would think. The plate does not go all the way up to the carb mounting body, instead it sits about 1/2" below. I put the plate in mainly because this is what I've seen on bigger V8 intakes to regulate fuel flow to each cylinder bank.

 

I am finding out that it is alot of carb, and very primitave. I don;t expect that it will ever be spot on accurate, but I'm gonna play around a little more before I fork up the cash for the tried and true weber.

[DR DLEX]

have you tried the carb w/out the seperater plate.carbs love vacuum~more vacuum=happy carb.seperating the intake into two halves will run good 2000-7000rpm(good vacuum)but will have hard time at 800-1000rpm(or 400-500rpm each side)subi engines fire one side at a time 1-3then 2-4.there is a spot in the valve timing that both intake valves are closed at the same time on each side,thus stopping intake vacuum for that respective side.most "car" carbs cannot funtion w/out constant vacuum,unlike motocycle or lawnmower carbs.one thing to try is to lift the carb off the intake with a 2-3 inch spacer plate .this may even the "air pull" at the base of the throttle plates.

 

 

just sharing braincells......have fun