Some carbs have a low and high jet. I'm not talking about moving the setting, but clearing the orifice that the needle valve is poised above. Keep your original setting. But clearing any blockage in the orifice can smooth idle and add power at higher rpm. It's a simple thing to do and all you need is a screw driver and 5 minutes.

 

It was mentioned above that the issue could be a sticky float valve. But while on the road pulling a carb might not be my first choice. Could be a fuel limiting issue as well as a plugged exhaust. I don't suspect the accelerator pump unless there is a flat spot when pressing the gas pedal down. Testing for that and checking the higher end performance can be done easily enough. Put it in neutral and from idle if a quick stab on the pedal gives you a flat spot or hesitation then the accelerator pump is faulty. Bring up the rpm slowly and check the higher rpm towards the red line. Should be smooth and strong. If not you are likely looking at a fuel limiting issue. But could be a plugged exhaust too. It's been my experience that a plugged exhaust affects performance at higher rpms, at idle you might not see any issue.

 

Thanks for the suggestions so far. We’ve been camping about an hour from home the past two nights. Not quite what we intended when we first set out, but still very nice. We tend to make minimal plans and go where the wind blows us, so this still fits the bill. Coffee is brewing now and eventually we’ll hit the road for the short leg home.

As long as the engine runs reasonably well, I’m not going to touch anything until we are safely home. I am still leaning towards a problem with the power valve. Haven’t seen a good method to diagnose one other than remove it for a physical inspection. Carb has to be removed for that, so not opening that can of worms just yet.

Accel pump seems okay. Under nearly all circumstances, there’s no problem when I step on the gas. No stumbling or hesitation. It’s just that weird band around 50-60 where power drops off. If I can get through that, she’ll run like a champ at 70. I don’t have to hold the pedal to the floor at 70, so there’s still reserve power. There’s just no reserve power in that dead band, so climbing a slight grade at that speed has been the primary issue.

I’m also skeptical of a fuel delivery issue. I can drop into 3rd and floor the gas on steeper grades, places I wouldn’t expect to stay in 4th. Pulls just fine like that. Wouldn’t that be higher fuel flow? Thinking my logic is okay, showing the pump and lines are delivering an adequate volume and pressure when consumption is highest.

Will keep you updated on what I find. For the moment, breakfast calls.

 

Between 50-60 mph. Where are the rpm's at that point, try and match that rpm, in a lower gear and see what happens.

 

Totally fine at the same RPM in 3rd. Only thing is a mild backfire when shifting to 4th, which is new.

The primary fault seems to occur at the combination of 4th gear and 50-60 MPH. Must be just the right mix for it to act up. Had a little trouble getting on the freeway but all was okay after that once I got the speed up.

Home safe and sound now. Gonna let things cool down before I start digging. Might have to take a nap, too.

 

Mild backfire, so that's pointing towards the carb ? Maybe some kind of internal leak ?

 

Pulled the carb today and found the dry side of the power valve full of fuel. Turns out I found A problem, but not THE problem. I replaced the power valve and the symptoms were improved, but still not good.

The existing power valve itself tested okay. I hooked up a vacuum tester to it and the diaphragm was not leaking. This power valve came installed on the professionally rebuilt carb I installed about 5 years ago, and had not been touched since then. I did find 2 gaskets stacked together. That can't be right, is it? I installed a replacement power valve, being very careful to properly seat the new (single) gasket. The new gasket was about as thick as the two old ones together, so maybe it wasn't assembled incorrectly. Whatever, it was no joy on a test drive. Only a slight improvement.

What of power valve ratings? How critical are they? I still have the original carb stashed away. That power valve (factory original?) was a 7.5 in-hg. The rebuilt carb had a 9.5, which kinda surprised me. This 9.5 has worked well for the last few years. It's the same carb I was running before replacing the engine. When I first installed it years ago on the old engine, I didn't have to adjust anything, including idle speed. It worked fine right out of the box. It handled the same heavy load just fine on the old engine. When I transferred the rebuilt carb to the new engine, once again I didn't have to adjust anything and all seemed good until the break-in was complete and I worked the engine hard.

My suspicions led me to the power valve, so I thought I was hot on the trail to find the dry side full of fuel. Even though the power valve itself tested okay, I wanted to replace it to be sure. The only power valve I could find locally in stock was a 6.5, so that's what I tried today. Did I shoot myself in the foot? Should I have waited for another 9.5, since that's what the rebuilder installed?

I've got a lot more things I can try, including reinstalling the original carb with its 7.5 valve. But I also wanted to follow the general troubleshooting rule to only do one thing at a time in case you introduce new problems. For example, I did NOT pull the top off the carb to check the float level, because I do not have a spare gasket on hand and didn't want to risk it. I was hoping to make everything good by fixing the one obvious fault, but that didn't work this time.

 

You have to remember that old motor most likely had different vacuum readings at different RPM and load levels so the power valve value would change with the new motor.

That's my story and I am sticking to it
Dave ----

 

Makes sense. Been reading up a little on the subject. The power valve should generally be rated at half the idle vacuum, and then round down to the next available size. For example, if you’ve got 18 in-hg, half of that is 9. Power valves all end in .5. So you’d select 8.5.

Might very well be I made things worse trying a 6.5. I may also be way off in the weeds, too. Going to review my work from yesterday and make sure the dry side is not wet with fuel. Also going to take a vacuum reading and select the correct power valve.

Got to wondering if a vacuum leak could be part of the problem. Generally more problematic at lower throttle settings, but it can’t hurt to check.

Also going to check the float level and for the presence of any crud in the float chamber. BTW, I cut open both fuel filters and they were clean as a whistle.

May also have to admit defeat and try the original carb again. Not quite a simple swap as I converted to a full electric choke, so all that stuff needs to be transferred over.

 

I would try the 6.5 valve. If you have a part throttle hesitation when you start to demand power from the engine and it was lean, the 6.5 should make it worse. The 9.5 will come in earlier. But none of this explains why it was running good originally and then started running bad. But experimentation is the only way sometimes a engine can "talk" to you, possibly leading you to the real problem.

 

Reinstalled the original carb. Gave it a new float valve and accell pump. Set the float to factory specs. Made sure the little screen at the float valve was clean. No joy. No change in symptoms. Anybody got any ideas? I'm sure I'll figure it out eventually, but was hoping to get her fixed and back on our camping trip in short order.

Hemmed and hawed about which power valve to use. Decided to keep the original 7.5 because it had been okay previously. It checked fine with a vacuum tester.

After removing the existing rebuilt carb, I revisited my previous work. That power valve was wet on the dry side. Did I mess up the installation? I was very careful to keep the gasket centered.

Let's say I've been totally screwing up the installation of the power valve, and fuel was leaking into the side where vacuum is aensed. Would it cause the weird symptoms I've been having? Kinda seems it would make things too rich, which generally doesn't reduce power unless very severe. Please correct me if my line of thought is out in left field. Don't want to keep going down the wrong rabbit trail.

Any other ideas? Sure seems like I've ruled out a lot of things by trying my spare carb. That is, unless I keep screwing up the power valve installation somehow.

BTW, I hooked up a vacuum gauge for the teat drive. Had about 19 in-hg at idle, down as far as 4 while accelerating. When I got the engine to act up, the needle stayed steady around 4. That should have been in the range for the power valve to open. No way of knowing if it actually did.

 

So you tried your spare carb and it runs just the same? That suggests it not the carb. I’m still trying to figure out how it could be timing or ignition. Did you change the distributor? Are you running the same mechanical advance springs? How’s the vacuum advance? Are the distributor and cam gears compatible?

Is there a leak before the fuel pump, could you be sucking air? Any difference between a full tank of gas or a low one?

I liked the clogged exhaust theory. Is there a butterfly valve between your exhaust manifold and exhaust pipe? Could it be closing under certain conditions? Is it installed upside down?

 

I agree also, it doesn't seem to be a carb problem now. You may want to put a timing light on it, and slowly rev the engine and see if the timing advances smooth and steady. Or does it seem to stick in certain places.

 

One other important thing to check is the base timing as set by the mouthbreather who installed the distributor. All I can figure is I did a quick tune by ear when I first fired up the new engine, and forgot to come back with a timing light. Could have sworn I did but apparently not. The base timing was at 10 degrees AFTER TDC, not 8 BTDC as required.

It ran like that for nearly 1000 miles and seemed fine, but the truck was empty until now.

Suddenly having second thoughts, wondering if I should check the roll pin at the distributor drive gear. Would a sheared pin advance or retard the timing? Or does the engine just quit completely?

BTW, I did give the distributor a good going over. With the vacuum line disconnected, I was able to test the centrifugal advance by itself. The vacuum advance wasn't leaking and responded nicely with vacuum applied. Also checked the pickup wires weren't rubbing inside the housing.

Checked a lot of other things, too. Came to the wonky timing as part of a methodical diagnostic routine. Just wish I had started there... Sure had a lot of red herrings, especially the leaky power valves. Thanks again to everybody for the help.

 

Wow, the timing was way off. You did mention backfiring and that is almost always caused by ignition timing being off or a swapped plug wire. I wrote that on another thread recently, bang bang out of the carb. I figured you had the basics covered as you are a holder of the golden wrench award.