Sbc bogging at around 40 mph and above

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?

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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.

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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).

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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).

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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.

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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.

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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?

Attachment 57713

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.

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Originally Posted by Treygan

Rspears,

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

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

Attachment 57714

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.

Ok. Is that inside the cap. If so I didn't change that. So I guess I stand corrected. I didn't change everything on top. Thanks.

I've not messed with the HEI personally, but your symptoms point to a component break down to me. Here's a pretty comprehensive article on the units - http://www.crankshaftcoalition.com/w...ributor#Module Lots of guys opt for buying a new distributor vs tearing into it and replacing parts. Maybe Dave Severson or one of the other guys can chime in here?

Thanks rspears. I will change that as well. I will see if I can do it before I go to work and go for a test drive. I'll let you guys know the outcome.

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Originally Posted by rspears

I've not messed with the HEI personally, but your symptoms point to a component break down to me. Here's a pretty comprehensive article on the units - http://www.crankshaftcoalition.com/w...ributor#Module Lots of guys opt for buying a new distributor vs tearing into it and replacing parts. Maybe Dave Severson or one of the other guys can chime in here?

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Well I didn't have time to replace the module but I did get it out. I will get one tonight and change it when I wake up in the morning. I am going to go ahead and take the carb apart. It may not need it but it will be a good learning experience and who knows if any paint got in there from the air cleaner spacer. I will get it all put back together tomorrow and give it a test run. Wish me luck. And again thanks to everyone on here that has commented. I am a member on a couple other forums and nobody has given this kind of response on either of them.

Be sure to use the heat sink paste it dose make a differance

should also check wires going to pick up coil for broken wires inside insulation,have you checked fuel pressure,does jeep have catalytic? if so check to make sure its not pluged.

:HMMM: Hmmmmm, wonder what Treygan found on his Jeep? Been a couple of days, and no word. :HMMM:

Sorry guys. It's been a busy couple of days. I just really got to drive it today. I cleaned the carb wasn't much in it but I did find some of that orange paint I was talking about and I also replaced the ICM. it seems to be doing MUCH better. No the only problem I have is when I mash the gas it hesitates and almost dies and then it takes off. Where should I start with that. But as far as the initial problem, I think it's fixed. Thanks to everyone for your input.

Treygan

sounds like the accelerator pump in the carb..

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Originally Posted by cffisher

sounds like the accelerator pump in the carb..

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Would it make a difference which hole I put the linkage into. There's 3 to choose from and I put it in the top. It was in the middle from factory.

Yes, moving it closer to the carb body will increase pump's action. But first, did you make sure the accelerators pump’s nozzles are squirting into the front barrels? There may be debris in the nozzles or the check under the nozzles could be stuck.

When you disconnect it from where you have it press down on the pump and make sure you have a good squirt of gas from the discharge tubes and then out it back in the middle. Make sure you don't have a vacuum leak as it will bog on hard acceleration because of low vacuum to the secondaries.

Ok guys I will check that stuff when I get up tomorrow. I have another question as well. I'm new to all this so I probably will have a lot more before I will be able to help any of you. But anyways, I can go out first thing in the morning, turn the key on push the gas down once and then turn the key over and it fires right up. Once I drive it though and shut it off, unless it completely cools down, it has to crank over FOREVER before it will start. What would be the cause of this and how can I fix it? Thanks in advance.

there are lots of things that cause that..running rich flooding the plugs.advanced spark.choke not working properly. not being there its hard to guess.

I'd question the float level, and maybe also the need for an insulating spacer (phenolic) beneath the carb to keep it from soaking heat from the intake. If your float level is too high the residual heat in the engine can heat up the fuel in the bowl, expand it and cause it to dribble into the intake giving you a flooded condition. If you let it sit long enough to cool down that fuel has time to evaporate, and it starts better. Another thought, you've not mentioned what type of fuel pump you're running, but if you're running an electric pump without a regulator you might be on the ragged edge of too much pressure, and when you shut down the fuel line pressure goes up with residual heat, pushing past the needle & seat, flooding your bowls and overflowing into the bores. Just a couple of options to consider.

have you checked your exhaust sys especially for a plugged up catalytic converter....ted

I don't have a catalytic converter. But I checked the accelerator pump and it seemed to be working right. I moved the linkage back to the middle and it helped a bunch. It will still hesitate al little but it doesn't seem like its about to die.

Good thoughts so far.....check fuel filter, fuel pressure, and the choke to make sure its coming off and not pulling back in choking the carb off.......