Thread: Sbc bogging at around 40 mph and above

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I bought a 98 jeep wrangler with a 350 in it and it ran good for a while. I replaced the carb for a brand new edelbrock thunder series off-road carb. It ran fine for a month and now I'm having problems. That's the basic back story but here's the problem I'm having now. It starts and runs fine first thing in the morning until it gets up to normal running temp which is about 190 degrees. Even at that temp it runs good upp to about 40 mph the. It starts to bog down. I have replaced all plugs and wires distributor cap rotor and coil. New fuel pump and for what it's worth a pcv valve and all the vacuum lines that I know of. Oh and the fuel filter. I have know idea what to do now. Please help. Anything is appreciated. I'm not much of a mechanic just learning as I go.

Is your new carb jetted correctly for your motor and setup right out of the box? To me it sort of sounds like the air/fuel is too far off above 40. I'm no carb expert so hold on for more experienced input.

I honestly don't know. According to edelbrock it is ready to go but that's probably what they all say. How would I go about finding out?

Originally Posted by Treygan

I replaced the carb for a brand new edelbrock thunder series off-road carb. It ran fine for a month and now I'm having problems.

Contact Edelbrock.com, they will tell you how to fix the carb, most likely debris has gotten into the carb and plugging a passage.

Ok I'll see what I can figure out. It is an offroad vehicle so that's very likely. Thanks.

Treygan,
First, welcome to CHR. Glad you're here, and hope you'll stick around, post some pictures and keep us in the loop with your jeep!

You say that the 350 ran fine for a month with the new carb, but then you started losing power under load above 40mph. To me that says that your carb cannot be too far off on the jet sizes unless your plugs gave you some clue on mixture when you pulled them. Also you mention changing the fuel filter, so hopefully it's keeping junk out of the carb.

When you say that "...It starts and runs fine first thing in the morning until it gets up to normal running temp which is about 190 degrees." do you mean that it will accelerate up above 40mph while it's warming up, but once warm it starts bogging down? If it's only bogging down after it gets fully warm I'd be looking at something in the ignition/electronics breaking down with heat. What type of distributor/ignition are you running?

Unfortunately, like most of us, I believe he changed the fuel filter after he started having problems instead of when he installed the new carb.

Also, check for a vacuum leak, a vacuum leak can cause this type of problem too (vac gauge will tell you). The choke will compensate for the increased air until the choke opens, at which point the metering rods won’t operate correctly because of the low vacuum.

And, if you have an automatic transmission check the vacuum line connection at the transmission to see if it has come off or burned against the exhaust (often overlooked).

Originally Posted by 36 sedan

Unfortunately, like most of us, I believe he changed the fuel filter after he started having problems instead of when he installed the new carb.

Also, check for a vacuum leak, a vacuum leak can cause this type of problem too (vac gauge will tell you). The choke will compensate for the increased air until the choke opens, at which point the metering rods won’t operate correctly because of the low vacuum.

And, if you have an automatic transmission check the vacuum line connection at the transmission to see if it has come off or burned against the exhaust (often overlooked).

All good thoughts. At this point the most important thing to me is for Treygan to reply about his symptoms, and especially if it's accelerating OK on initial start. Without that I'd just be guessing.

On the fuel filter, if it had a decent in-line filter in place and he changed it out and still has the symptoms I don't see how he would have got crap in the carb unless the filter ruptured, and I've never seen that happen in all my years. Your idea about vacuum affecting metering rod action is a great point, and I had never really considered that possibility. Hopefully Treygan will get on-line and give us a few more clues!

Sorry guys. I worked last night so I just woke up. So to try to answer your questions. Up to this point it has ran fine until I get up to 190 degrees. I can drive 75 mph no problem. And I changed the fuel filter the day I changed the carb which was about a month ago. I was looking at it at a little bit though and found that the spacer for the air cleaner, which had been painted orange at some point, had some paint peeling and bubbling on the inside of it. So I know it could have easily come off and gone right into the carb. As for the ignition it's just the old style distributor. And the plugs when I changed them didn't look bad to me. They had a tan color to them, some worse than others. I hope I did ok answering. I'm not a great mechanic and probably not even a good one. I am taking the carb with me tonight to work and my boss who has do e it before is going to help me open up the carb and take a look. Thanks guys for the help and the welcome. And I'll post pics as soon as I get some extra time. Thanks again and keep it comin.

Originally Posted by Treygan

Sorry guys. I worked last night so I just woke up. So to try to answer your questions. Up to this point it has ran fine until I get up to 190 degrees. I can drive 75 mph no problem. And I changed the fuel filter the day I changed the carb which was about a month ago. I was looking at it at a little bit though and found that the spacer for the air cleaner, which had been painted orange at some point, had some paint peeling and bubbling on the inside of it. So I know it could have easily come off and gone right into the carb. As for the ignition it's just the old style distributor. And the plugs when I changed them didn't look bad to me. They had a tan color to them, some worse than others. I hope I did ok answering. I'm not a great mechanic and probably not even a good one. I am taking the carb with me tonight to work and my boss who has do e it before is going to help me open up the carb and take a look. Thanks guys for the help and the welcome. And I'll post pics as soon as I get some extra time. Thanks again and keep it comin.

The passages that you'd plug on the carb are on the fuel (liquid) side, not the air going through the venturi, IMO. You say "...old style distributor." Is it an HEI type like this with the coil on top?

HEI.jpg

Your plugs being "tan color" is ideal. Says your mixture is good. I'm thinking you've got an ignition module breaking down when it heats up. Just need to know exactly what kind of distributor/ignition you have.

Oh and it is a five speed ax15 jeep tranny right now. I'm hoping to slip in a th350 when I get the motor figured out. But for now I'll stick with what I have. I also don't know about vacuum leaks but I replaced all the vacuum hoses that I know of. I replaced the one to the break booster and to the pcv valve as well as changing the pcv valve. I changed the vacuum line going to the distributor. I don't know of anymore but let me know if I missed anything.

Edelbrock has a lot of good tune up and calibration information on their website, you might want to visit the site, print out the information pertinent to your carb, and take that with the carb for your boss to look at.

Rspears,

Yes it looks like that and I replaced everything on top of it 2 days ago.

Dave, I'll have him get on their website with me tonight at work before we tear into it.

Originally Posted by Treygan

Rspears,

Yes it looks like that and I replaced everything on top of it 2 days ago.

I'm betting you need a new ignition module, like this or similar in appearance:

Module.jpg

Think about what you've told us - it starts great, runs 75mph until it gets up to temp, and then starts falling on it's face. That's typical of the module breaking down and giving you erratic firing under load.