The Stovebolt Site Forums The Shop Area Engine Shop Tell me it isn't the carb (1958 283 2GC)

Edit: Jon always says it is never the carb, so someone tell me it isn't that carb.

We were getting the truck running today and ran into an odd issue.

The engine was cold (35°F air temp), we pulled the choke, turned the key, and it cranked and started fine.

After letting it sit and idle for a bit (90 sec?) it started running rough, so I pushed the choke in and it smoothed out. We let it sit and idle for a couple minutes while checking other things and it idled fine.

When we tried to rev it up in neutral, the throttle gets to about 1/2 and it starts hissing loudly (from the throat) and stumbles and tries to die. Letting off the throttle, it smooths out and idles just fine. The throttle position that it stumbles and dies is very consistent.

If I pull the choke out about 1/3, it still idles fine and will rev as high as you want with no noise or stumbling.

My first thought is a small vacuum leak causing a lean condition at rpm and a bit of choke is enough to richen it up. I would not expect that to get worse at higher rpm though.

I didn't have time to get the propane torch out today, so I figured I would ask if I was totally off base on this. I would order a kit from Carbking, but I can't find any tag or numbers on it.

could be a small piece of something stuck in a jet, but yes, it does sound like carb to me.

You think it is worth pursuing a vacuum leak? Or just pull the carb and go from there?

I did try pinching off the vacuum advance with no effect, but there are plenty of other places for vacuum to leak.

To me it sounds like the engine isn't warmed up yet. Mine is the same, 283 with a 2G, takes a couple of miles to get the engine warm and smooth out the carb and RPM. I'm running a new 180 degree stat but thinking of trying a 190, my AutoMeter temp gauge will hit 180 then the stat opens and runs at 170 degrees the rest of the drive. Does your engine have a heat riser on the right manifold? in the 2 ton application such as mine, the system did not use a heat riser, it just relies on the exhaust cross-over in the intake manifold. Could be loaded up with carbon and not heating the plenum floor.

Ok, I wanted to make sure it was going to be safe to run before actually taking it out on the road. Maybe I will give it another shot tomorrow before I tinker with it any more.

Turn the idle mixture screws out another 1/4 - 1/2 turn and try it.

If you order a kit, he won't need any numbers, just knowing it is a Rochester 2G will get you there.

It's not the carb

Change the fuel filter.

Jon

I did already replace the fuel filter and the fuel lines. I left the originall mechanical pump in place for now. It will probably get a rebuild, but I have not seen any indication that it is having any issues.

If you've got a piece of crud in a carburetor passage, there's an old shade tree trick that works often enough to be worth a try. With the engine revved up pretty well, clap your hand over the air horn and let the manifold vacuum suck liquid gas through all the passages in the carb. Just before the engine stalls out, let it rev up and do it again- - - -several times. You're using manifold vacuum to power flush all the places you can't reach with a spray can of carb cleaner. If the problem is a clogged passageway, the liquid gasoline acts like a field-expedient carb cleaning. If that doesn't help, pull the carb and do a more thorough cleaning job by boiling all the bits and pieces in a solution of dish washing soap and water, followed by a HOT water rinse and LOTS of compressed air through all the holes in the castings. The air blows away all the crud the solvent softened.
Jerry

The fact that partial choke allows the engine to run normalish is indicative of a vacuum leak in my experience.
Partially closing the choke as the throttle plate is opened, causes an increase in vacuum applied to the carburetor compared to what the vacuum would be with the choke open. This increase in vacuum allows the carburetor to act closer to normal when there is a vacuum leak.
Have you done anything to the engine lately which could cause a vacuum leak?

Another possible cause could be a nonfunctioning heat riser. This problem really raises its ugly head during cold weather. Lack of heat applied to the intake manifold during cold weather will cause the fuel to fall out of suspension and puddle up on the floor of the intake runner. Puddled fuel will not burn. Incidentally, partially closing the choke can actually increase the speed of the volume of air/fuel through the manifold, forcing the puddled up fuel to move into the cylinders, clearing out the puddles.
It will be easiest to check the function of the heat riser. Do that first.

Use an UNLIT propane torch to check for vacuum leaks. It works better that anything I have ever tried.

BTW, Its never the carburetor.
Even if it is, its much easier to do an inspection of the heat riser and do a vacuum leak test than to open up the carburetor and stare at the guts and see nothing wrong even if there is something wrong. The other problem with tearing into the carburetor is that if you don't put it back together correctly, and it was never the problem to begin with , you will then have created the dreadful two-problems-at-the-same-time scenario without the ability to check for a vacuum leak because it won't run because now the carb is jacked up.

I use acetylene from a torch tip- - - -it's more controllable flow-wise and location-wise than Propane.
Jerry

That will never work for me. I can't ever seem to remember to turn the valve off from 3 weeks ago and never have any acetylene.
That may well be a personal problem that others may not suffer from.

If the OP has to tear into the 2G, it's not the end of the world. It is not a complicated carb and pretty easy to clean and re-assemble.

To answer the question about recent work: this truck has only moved on/off a trailer and around the yard in the time I have owned it. This is the first time it is to the point of actually testing it out real driving.

Jon's site says the tag number is mandatory for getting a kit for these, but I will give him a call tomorrow and see if he can help me out. Based on the tables he has posted, all non-governed 283 2GCs use the same kit.

I can see signs of previous weepage between the bowl and the airhorn and have no reliable history.

I meant to check the heat riser yesterday but got distracted. I will do that too.

It could be a low float level. You are running to long with the choke on. That creates a super rich mixture & can carbon up the plugs & pollute the oil.
Its possible the sock in the tank is plugged up if it has one.

I don't still use acetylene enough to justify the rent.

For decades, I have used propane as the vacuum leak finding tool. Have a base set on the workbench with the torch firmly fixed in the base. Neck down the tip to a small (3/16 works well) neoprene hose of 10~12 feet (whatever one's shop requires), cut a 6 inch piece of 1/8 steel tubing and insert one end into the free end of the hose from the torch. Now one can really pinpoint a vacuum leak.

The "tag is required" is for newbies to the old vehicle hobby, that are used to driving newer vehicles which are virtually impossible to modify. We have found AT LEAST 75 percent of vehicles before1965 have had their carburetor replaced at least once. Our kits are custom, and include the proper orifice fuel valve, the correct length accelerator pump, main jets, idle mixture screws, the power (economizer) valve, etc. If the prospective customer has been around older vehicles for awhile, and is confident the carburetor in question is correct for the vehicle, and willing to accept the responsibility for this decision, we have no problems with selling the kit.

Jon

Sounds good Jon, I will call you in the morning to order the kit. Work exploded on me today and I didn't get a chance while you were open.

I've got four sets of oxy-acetylene tanks that belong to me, and my suppliers swap them out for full ones without having to pay rent. That's the only way to go.
Jerry

The heat riser moves smoothly and opens with a heat gun.

Ballpark how long would you expect it to take the engine to warm up enough to open the riser itself and run with the choke completely pushed in? A mile down the road to the gas station?

With my old 350/Quadrajet and an electric choke, it would take about 2 miles (almost all the way to work) before the choke shut off and it would stay running. That had stock cast iron manifolds and no heat riser.

Some comments above make me think maybe this should heat up faster?

I tried calling Jon a handful of times on Tuesday but didn't get through. I am assuming he is closed for Christmas and maybe New Years, so I will try again next year.

The only time a heat riser activated by a bimetal spring will release fully will be at highway cruise speed with a pretty good flow of exhaust gas coming through the manifold. Within a couple of minutes after startup there will be enough heat in the manifold to unwind most of the spring tension, so the flapper valve will open partway. You should be able to fully open a manual choke within the first mile or two of driving and the engine will idle at its normal speed without stumbling or stalling.
Jerry

I missed this message initially. Should the idle mixture affect it at higher throttle/rpm?

My list for today:
Check for vacuum leaks
Adjust idle mixture as above
Hand block it to suck out any debris
Drive it to get it hot

I will give it a try this afternoon. I will plan on checking/adjusting the float when I rebuild it.

Idle mixture adjustments stop having any effect somewhere around 1K RPM. There's a "part throttle" port slightly above the one with the idle mixture needle on most carburetors that's drilled according to engine size to aid in transitioning from idle to part throttle. it isn't adjustable.
Jerry

Ok, more info and Jerry gets a cookie.

I fired it up waited about 15 seconds, then took the chole down to about 30% on to test for vacuum leaks.

I got an RPM increase when I put the propane nozzle at the carb inlet, but nowhere else. I pulled and blocked the two vacuum lines with no effect. Based on that, I am reasonably confident I don't have a vacuum leak.

At this point (2 min later?) I finished taking the choke off and left it all the way off for the rest of the tests.

I let it idle for a couple minutes here while I tinkered with the charging system and warm up a bit more. At this point the heat riser started to open.

I revved it up as high as it would go before starting to stumble (maybe 1500RPM) and slapped my hand on it. When I tried revving it again, it went higher before stumbling so I tried that 2 more times before it stopped making a difference.

It pulled hard enough that the air cleaner stud hole left decent marks on my hand. Of potential note: my hand was wet with fuel after each attempt.

Now if I ease into the throttle, it revs to about 2k, then hesitates for a moment before jumping up and continuing to rev.

If I snap the throttle, it gets to about 2k rpm, stumbles just enough to hear (with that same hissing sound I mentioned before), then continues revving fine.

I adjusted the idle mixture screw at that point to see if it would smooth out the transition, but it did not seem to have any effect.

Watching the heat riser when I was doing the throttle snaps and such, it was open maybe 1/4 at idle and would swing open most of the way as I revved the engine.

I did try holding it all the way open when doing a throttle snap to see if the riser spring was just too stiff, but that also did not do anything.

At this point I have a couple theories:

1. The bracket shown below holds a rubber stopper in the top of the carb that looks like it should open when I get on the throttle, but both the linkage side and the carb side are bent. Based on the position, I am wondering if that is part of the power piston circuit and it should be opening more/faster with the throttle?

2. Based on the lack of a tag, the carb has almost certainly been tinkered with and might have incorrect pieces in it. If so, maybe that is contributing the to dead-ish spot in the middle.

3. There is still some amount of yuck in the carb that need cleaned out as part of a rebuild. The fuel filter I took off was the gravel sized glass ones that were popular in the 90s. It had a decent amount of debris in it, but I have no concept how long it had been there. The new paper element type I put in does not have any visible gunk in it yet, so the tank might or might not be filthy. Cleaning that is a project for another day.

4. There is evidence of seepage most of the way around the airhorn. That gasket could be messed up/leaking inside as well. That would be a rebuild fix as well.

Any other suggestions, (on-topic) comments, or thoughts? At this point I am inclined to move on to other projects until I can get a kit for it.

Check the centrifugal advance for free movement. That one's easy- - - - -it's right under the distributor rotor- - - - -no distributor disassembly required to inspect/lube the weights and springs. 50 years ago those distributors were notorious for the weight pivots to get worn or rusty, and since there's no oil in that part of the distributor, the weights and springs were SUPPOSED to get a dab of high temperature grease (same stuff that's used on the point rubbing block) at every tuneup. It virtually never happened, so the weights are usually rusty and ratchety. A momentary hangup of the advance weights will create a stutter or stumble.
Jerry

I ran out of light and warm today, but the pump rod and atmospheric vent (the "stopper" I mentioned above) are both out of spec according to the Shop Manual.

I will check the centrifugal advance, but probably wait on the rest of the adjustments until I rebuild it and just make everything right at the same time.

That vent has only one purpose, and it has nothing whatsoever to do with how the engine runs. it's there to provide a way for vapors to escape from the float chamber during hot idle to prevent the engine from loading up at stoplights. Normally, the vapors would be vented into the air horn through that big angled port under the air cleaner. Go ahead and tinker with it, as it's also connected to the accelerator pump linkage, but you're spitting into the wind concerning the problem you're chasing. Ask Grace Slick what happens when you start going down rabbit holes!
Jerry

Give Jerry another cookie!

On manual choke carbs, the idle vent valve was introduced in mid-year 1958, and discontinued in mid-year 1960; so that really narrows the range of your carburetor. Based on that information, we can do a kit (CS-3460).

The linkage should be adjusted such that the vent is open at idle, and closes when the throttle is slightly opened. If you complete remove the "stopper", nothing changes except for a source of "dirt" entering the carburetor. The excessive idle vapors would be vented, as designed.

I agree with Jerry that this valve is not your problem.

When I suggested to change the fuel filter, I should have posted to test the fuel delivery at the carburetor. A weak fuel pump, a cracked fitting in the fuel line from the tank, or a partially clogged "sock", or even chunks of crud in the tank that might be sucked into the tube at higher RPM (higher fuel requirement), and then drop out (gravity) when the engine was turned off.

Please understand I am certainly not adverse to selling a rebuilding kit, but I am a long way from certain that it will help.

Is there any way you could rig up a temporary fuel tank (maybe a outboard marine tank) with an electric pump to totally bypass the current fuel delivery system?

Jon

Thank you for the feedback. As far as the vent, I was just going through my manuals looking for things that were incorrect with an eye to fix any obvious problems before attempting to replace parts. I will not worry about the vent for now.

We drove it a couple miles to the gas station the other day after tinkering with it and I did not feel the stumble there, so it *could* have been something that got flushed through. I will verify that the issue still exists, then try a separate fuel system.

I didn't notice a fuel pressure spec in the manual. Based on what I have read, is roughly 3psi correct?

As much as anything, I am inclined to rebuild it just because the overalll maintenance on this truck seems to have been sporadic at best and I don't necessarily trust that the carb has the right parts in it. I guess if it is functioning, then there is no real reason to mess with it.