The Stovebolt Site Forums The Shop Area Engine Shop 261 only runs good with choke on.... need carb help

I dont know what carb it is, but the engine runs a little rough or even dies with the choke in (butterfly vertical). Fully warmed up.
Drives underpowered also. If you hit the gas, theres a dead delay, then take off.

If you pull the choke out to varying degrees, she runs like a top. Ive adjusted the idle jet 7 ways from sunday, as well as the idle screw, but no improvements compared to just running with the choke engaged.

1951 chevy 3100

I'm assuming you have a Rochester carburetor, yes? No matter what you have it sounds like you're running lean. If that is the case, you should first get a vacuum gauge and see what it tells you. You may have a vacuum leak somewhere (carburetor to manifold gasket or somewhere in the intake manifold...possibly manifold to head gasket but not generally that). You're getting too much air relative to the amount of fuel you're getting. If vacuum is good (at least 15 pounds at idle and relatively steady) then I'd take the carburetor apart and clean it, check the float level (it might be set too low...which will cause it to run lean) and see what else is happening in there generally speaking. I'm certain you know this already, but the idle mixture screw will only affect the idle and not much once you get past that (or that plus off-idle).

I deleted my post because I had my brain on backwards today. I confused lean and rich. duh...

Post a pic of your carb so we know what you’re working with.

Thank you. I will order a vacuum gauge.

You have the infamous Rochester B, which was only intended to survive for the usual 2 year warranty period 70 years ago. Now it would be a good paperweight, but since it's been exposed to gasoline, it will be a stinky one. "The ABC's of Carburetion are Always Buy Carter!" That was Carter's sales pitch in the 1950's, and those words are still true today, if not more so!
Jerry

That’s a Rochester B. You could try a couple things before you get your vac gauge. Turn the idle screw in and gently bottom it out. Turn back out 1.5 turns. That just gives a baseline to work from.

If the engine will run and idle, even if it’s rough, you can hunt for vacuum leaks by spraying something flammable at various places. Carb base gasket, intake manifold gasket, etc. if the idle smooths out or picks up, that place is leaking vacuum and needs addressed. In other words, it’s pulling the spray in through a leak in that area.

Some folks do the same thing with an unlit propane torch.

What specific carb might i hunt for, to replace this one eventually?

Good question but the answer is sort of a challenge. Rochester made a carburetor for the 261, but we don't know if what is on there is that carburetor or some other Rochester B and we don't know what problems it may have because of what is called the rebuilder's mixmaster. If you want to know if you have the one made for the 261, remove it and turn it upside down. Measure across the throttle body opening (the round hole). If it measures 1.75" you at least have the right throttle body for the 261 and you might also have the right carburetor in total. If it measures 1.5" you have probably have a 235 carburetor. At least as I recall. Don't quote me on the measurements but the 261 Rochester had a larger opening than 1.5".

Carter never made a YF model for the 261, so many folks have used YF models made for other similar engines. Jon (carbking) would be the best person to ask about this as he knows the specifics about the various Carter carburetors and how they might work for the 261. And there are other (more expensive) options like Zenith. AMC used a Carter YF on a 258 engine, but that was as I recall in the early 70s. There may or may not have been smog get-up on it. If so, I would expect some adjustment might be needed. Offhand I can't remember the model number but the size of the engine is close to 261.

Great info! Thanks. I'll have a look. Ive got obvious exhaust manifold leaks, so im guessing maybe some intake leaks aswell. Im gonna order some gaskets and get at it.

The YF for the AMC/Jeep 258 engine has slotted bolt holes in the throttle body flange, so it will fit both of the common stud spacing dimensions without any modification to the carburetor. The same carb is also specified for the 232 AMC six, so it appears to be sort of a "universal fit" application that might require a little jet or metering rod tuning for best performance. The vacuum ports in the throttle base DO NOT have the same spacing as a Rochester B, so be sure you get the correct gasket for the carb, or you can block off one or more vacuum passages.
Jerry

There is also a Carter YF4145S that was used on the Chevrolet 194, 230 and 250 engine as well as the YF3821S or YF3821SA. Both of those have a manual choke and I tried a 4145S on a 235 once upon a time. It worked ok but the throttle rod connection was on the wrong side (I had to make an adapter which wasn't difficult at all) and it seemed to run slightly rich on the 235 so that one might be ok on the 261. Those were much cheaper than some of the YF carburetors out there. I have seen some of them on the big auction site for $50 or less. They'd need rebuilding but almost anything you get will need that.

There are lots of good suggestions here about potential replacement carbs. Your Rochester B is probably a turd, as are most. I am a YF convert and will never go back.

However, I’d put carb shopping off until you shore up nearly everything else. Find and address vac leaks, get ignition and timing dialed in, get the valves properly adjusted, etc.

If you get those things right, your 261 will most likely run pretty respectably….even with a B. Then when you do upgrade carbs, you will appreciate the sweetness that a well-designed and well-tuned carb brings.

One of the "261" engines I'm building is going to end up very close to 300 cubic inches. A Carter YF for a Ford 300 six is probably going to be included in that one.
Jerry

Rochester B works just fine for me and many others.

Is there a tag on the carb? Or a number stamped into the base? I would see if it is rebuild-able. The white crap on the outside makes me wonder what the inside is like. At one point I had to use a welding tip cleaner to get what looked like calcium out. (suspect ethanol was the cause)

Looks like a gasket between the carb and the spacer; it probably shouldn't be there.

IF you decide to replace the carburetor, then:

A.M.C. starting using the YF on the 258 in 1971.

Federal smog emissions started in 1968, so ALL of the A.M.C. carbs would have some smog calibration. The 1971~1974 carbs will have less smog than the 1975 and newer (the smog requirements REALLY accelerated in 1975).

When smog emissions came about, Carter used HUGE main jets and much thicker than pre-smog metering rods. WHY? Because the wear on the thicker rods is less of a change than with the thinner rods, thus the carbs stayed in specification longer.

The carbs were actually calibrated using the rods.

So changing to a larger jet (the 1971 versions used 0.104 jets) is quite difficult.

So folks have told me they have "sanded" rods to make then thinner. This sounds like a Dr. Goodpliers trick, but cheaper than buying custom rods (which are available).

I do NOT know how the throttle arm would adapt to the Chevrolet linkage.

I like the Zenith carbs used on a G.M.C. 270 for the 261, but they are somewhat difficult to find, and significantly more expensive than the A.M.C. YF's.

IF you decide to keep the Rochester, then the first order of the day is to positively identify the carburetor (some will have a 4-digit number STAMPED, not raised) on the throttle body). For years, the commercial rebuilders grouped the 235 Rochesters and the 261 Rochesters as being the same.

If the carb is correct for a 261, read Jon G.'s blog on straightening castings. And if you wish to rebuild, major overhaul kits are available.

Jon

Jon, you’ve mentioned the Carter WA-1 as a potential Stovebolt carb as YF supplies dwindle.

Would a WA-1 calibrated for the 257 Olds inline 8 be a candidate for a 261 carb? Several different tag numbers of those available at the big auction site, using your website to cross to the application.

A guy would have to live with an auto choke, of course.

Carter made a WA-1 with manual choke (414s).

The WA-1 is an excellent carburetor. It does have a leather accelerator pump. Technologically, somewhere between the W-1 and the YF (so in a perfect world, out of 100 points, one might give the W-1 88, the WA-1 92, and the YF 94). Since today's world is slightly less than perfect (ethanol), the WA-1 might be more reliable than a YF. Today's figures might be W-1 88, WA-1 92, and YF 85.

The "but" here is that the carburetor is older than the YF and had a much shorter production run; thus parts (other than that grumpy Missouri hillbilly, if its Carter, he probably has it) would be less common than for the YF.

The 414s is a universal design, so the throttle arm is adjustable. Other than the 413s (auto choke), and the 414s (manual choke), I think all of the other WA-1's have a fixed throttle arm.

The WA-1 does share metering jets with the W-1, other than than, not a lot of interchange. And the auto choke units are larger in diameter than the YF. I am unaware of any current manual choke conversions that would fit the WA-1.

As far as calibrations go, the metering rod WILL PROBABLY REQUIRE CHANGING TO A DIFFERENT PROFILE which is going to add to the cost. A passenger engine has a significantly different fuel requirement curve than a truck engine. The calibration MUST be done using the rod, not by simply changing the jet. I did not check all the WA-1's, but the ones I checked have a rod profile totally unsuited for the truck engine.

I don't often mention these because of the parts issue. I try to answer these questions without grinding my own axe.

Are the rods available? Yes, I can cut ANY rod profile for any Carter rod pre-1975. But these are hand-cut on a manual lathe, time consuming, and tedious (remember, grumpy Missouri hillbilly ), therefore they are not cheap.

Jon

Don’t the universal WA-1’s have an oddball accelerator pump that’s really expensive? I have some fuzzy memory of you commenting on that.

When you say the rod profiles are wrong for a truck…are they wrong for a truck that does actual work? Or wrong for a 1/2 ton truck that mostly hauls a couple passengers to the drive in and maybe grabs a couple 2x4’s at the lumberyard once in a while?

The universal WA-1's do have an oddball accelerator pump, and it is expensive.

The fuel curve is way different from passenger engine to truck engine.

Jon

I learned something. Thanks for all the replies.

Jon's comment about basically similar-appearing carburetors having widely different calibrations is something I learned while going to GM training schools on Quadrajets in the late 60's, 70's, and 80's- - - - -There were something over 200 different QJ part numbers over a 20+ year production run, calibrated for everything from a 230 cubic inch Pontiac OHC 6 cylinder to a 500 Cadillac engine. They all look very similar to the untrained eye of a salvage yard scavenger!
Jerry

Im looking strongly at Holleys 1 and 2 barrel EFI systems. Just to simplify things.

Well i got a vac gauge. Heres a vid of it at idle. (i dont have a tach)
https://www.youtube.com/shorts/ujJn-i8XXyY

I tried the ether spray method, but got no discernible reaction by spraying it at intake ports, under carb, on carb gasket, on carb, all around the carb. And when i would spray a quick squirt into the air filter, the engine would die or nearly die.

Try it with an unlit propane torch. First thing I like to do is get the unlit propane torch near the carburetor throat just to get a feel for what the increase in RPM will sound like when I get the propane near a vacuum leak. Since propane is a gas, it is more easily sucked into a vacuum leak than a spray.
I know that ether is quite volatile, but not as volatile as propane.

Is that idle video with the engine warm and no choke all the way open?

Engine is warm, choke halfway out. Otherwise it will sputter and eventually die.

Might need back up and remind us what you’re working with here.

1) Is this an engine that sat for long-ish time that you are trying to revive or an engine you have seen run well that is now running poorly?

2) Have you verified the rotor is pointing at #1 at TDC compression stroke, and that firing order is 1-5-3-6-2-4 from there? Points and condenser new or unknown?

3) Has a valve adjustment been performed?

Not trying to speak condescendingly or anything. When I first tried running my engine, it acted a lot like yours. I wasted some time chasing hunches rather than getting back to the basics. Had the timing 180 degrees out at one point. Bad condenser. Manifold heat riser locked up. A number of things that contributed to poor running, even though I had rebuilt the carb.

Believe it or not, a Stovebolt can run with the firing order all screwed up. I “know a guy” who has seen it happen.

I recently had a similar problem on my fresh 261 after removing a 235 carb and installing a 261 carb , turned out the .55 main jet was to lean so I replaced it with a if I remember correctly a .72 main jet I had hanging around . It has worked fine since .

John

Thanks all good points.
She was an annual cruiser but sat for last 3 yrs. Gonna crack open carb and look for partial blockages.
Ill check all other thing u mentioned. I just set the valves before that vid. They were quite far out! Intake was probably .020+
Fuel filter looks great still. Theres a filter under cab. Ill check it too.

There are only 4 screws plus the fuel line holding that carb together. Pull the top off & see what you have. Its probably got a bunch of white stuff or water in the fuel bowl.
If it is not too bad clean it all up & put a kit in it. It might be like Panel Fan says, it might be a 235 carb & too lean for a 261. Although a .072 jet is too big for either one. Otherwise get another carb.
Those model B carbs are not all that bad. There are thousands still running around out there every day & at car shows.

George

Good info George, what size jet would you recommend, I'll give one a try.

Thanks.
John

Mine recently wouldn't run unless I was choking it. I changed the fuel pump and that fixed it!

If it sat for 3 years with Ethanol fuel in the carburetor that is likely where your problem lies.

The 235 carbs usually come with a 58 or so jet. I can't remember what was in a 261 carb. Sometimes I had to drill them out to 1/16". (0623) to get the mixture good. If you had to use a 72 jet to get it to run right your power valve is not working & you are running on the main system only.

George

The 261 carb has a 64 jet. At some time in the past, somebody put a 57 jet in my carb - maybe it came with a carb kit. I followed George's suggestion and reamed it out.

contact Jon at www.TheCarburetorShop.com to get the correct rebuild kit for your carburetor and engine.

Thanks for the info George , will drill out a smaller jet as well and try that out .

John

I was having the same problem with my 235 in my truck that I’m trying to bring back to life. I adjusted the screw on the engine side of the carb until it sped up a little. Then I’d push the choke in until it struggled a little and repeated that process until it idled with the choke all the way in.

Doug

Heres the carb. Says "C" if that means anything. Opened it up. Its spotless. But i suspect the vacuum advance brass pipe fitting may have been sucking air a little. So i doped it.

I will measure jets tomorrow.