Bog/rough running at 70mph

[Awann99]

I've been chasing a weird issue on my daily. 350/HEI quadrajet c1500. The truck will drive great but around 65/70 mph on hills especially it bogs down. The vacuum gauge will drop to around 5 from the normal 15-20 and the truck will start to shake/vibrate. Seems like it's coming from the drivetrain. There is a deep droning noise you can hear. It will hold 70 but it doesn't sound like it likes it. If I let off the gas it feels like the motor "catches up" and the vacuum gauge goes back up and the truck stops vibrating and drives better. If I'm going down a hill it will go 70 no problem. Any ideas? I'm thinking the distributor isn't advancing right. Timing is set at 12 btdc. How would I check this? Thanks guys.
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[Grumpy old man]

Sounds like fuel starvation , start with a new fuel filter before tearing into the ignition

[geezer#99]

When you replace the fuel filter, check the volume the fuel pump puts out.
Should fill a quart jar in 30seconds or less.

[tutone]

Rebuild your q jet. Just went through the same crap. It was an Edlebrock though. This cheap junk they call gas eats them up.

[Awann99]

I'll give that a shot. It's a rebuilt qjet from National Carbs in Florida probably about two years old. I'll pick up a new filter for it from napa tomorrow and see if it helps. Thanks guys.
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[dmjlambert]

I bought my Quadrajet from National Carburetor when they were offering lifetime warranty. Did yours have that warranty? I noticed recently they don't mention it on their website any more. When I bought mine there was a choice of whether to buy it with the lifetime warranty for some additional money. If yours has the warranty you can contact them and send it back and they will work on it and send it to you. I sent mine back one time so far.
But fuel filter first thing to try and less trouble to do.

[Lee H]

Also check the "sock" filter on the inlet tube in the gas tank. Another thing you could check is the total advance, should be about 34 to 36 degrees total timing at 2,800-3,000 rpm. Good luck, Lee

[LockDoc]

Originally Posted by Lee H

Also check the "sock" filter on the inlet tube in the gas tank. Another thing you could check is the total advance, should be about 34 to 36 degrees total timing at 2,800-3,000 rpm. Good luck, Lee

I second the sock filter on the end of the pickup tube in the fuel tank.... Usually the inlet filter in the carb will affect it at all speeds.

LockDoc

[Palf70Step]

I agree with the fuel sock issue. Only other thing I have run across is fuel line size. is it 3/8" or 5/16" line. I had a original 6 truck with v* swap that ran fine until the engine really want fuel at high speeds.

[HO455]

Asto the OP's question. To check the timing advance hook up a timing light and while looking at the timing mark on the balancer rev the engine up. The mark should move about an inch. This the very basic test to see if the mechanical advance is working. For more detailed information there are dozens of videos and other articles on the interweb that detail how to set the timing properly.

On the fuel side a quick and easy check would be to remove the gas cap and do a test run. That would tell you if your tank isn't vented properly.

You didnt mention what year or how modified your truck is. That information may help us troubleshoot.

Also any rubber hose between the fuel pump and the tank should be checked to see if it has collapsed. If it looks flat it may be collapsing and restricting flow to the fuel pump.

Good luck and keep us posted on what you discover.

[Awann99]

Originally Posted by HO455

Asto the OP's question. To check the timing advance hook up a timing light and while looking at the timing mark on the balancer rev the engine up. The mark should move about an inch. This the very basic test to see if the mechanical advance is working. For more detailed information there are dozens of videos and other articles on the interweb that detail how to set the timing properly.

On the fuel side a quick and easy check would be to remove the gas cap and do a test run. That would tell you if your tank isn't vented properly.

You didnt mention what year or how modified your truck is. That information may help us troubleshoot.

Also any rubber hose between the fuel pump and the tank should be checked to see if it has collapsed. If it looks flat it may be collapsing and restricting flow to the fuel pump.

Good luck and keep us posted on what you discover.

I'll put in an order for a new sock along with the fuel filter. I didn't even think about that. It's still running it's original tank so it probably could use a good cleaning too.
The truck is a 69 1500 lwd 2wd. Goodwrench 215 hp V8 sitting in the i6 position. I'm running an ancient edelbrock sp2p intake. The truck is as stock as I can keep it. I'm running a 1969 original air cleaner housing as well. The fuel lines have not been swapped over to the v8 size.

Thank you for the advice on the timing. It's good to hear this is probably just an issue with fuel starvation and not timing.
I will keep y'all updated when the parts arrive.
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[dmjlambert]

Under the cab there is probably a short piece of fuel hose that goes between the hard line from the gas tank to the hard line on the frame. And at the engine there is another short piece of fuel hose that goes from the hard line on the frame to the fuel pump. You probably replaced the one going from frame to the fuel pump when you put in the engine. I suggest inspect the one under the cab and if it has not been replaced before you may need to replace it. If it is very old or maybe original be careful because it can crumble in your hand when you give it a squeeze and the gas will siphon out of the tank all over the floor or ground. Since you are going to be removing the sender to put a new sock on it, it means you will be disturbing the hard line that goes from the tank to under the cab and will cause movement at the fuel hose under the cab. The truck original fuel hose was not rated for gas with ethanol, but make sure your new one is. Since ordering a sock you may also need the fuel sender rubber grommet for when you put the sender back in the tank.
I'm not 100% sure on the fuel hose arrangement for 6 cylinder because I have an original V8, so my info may not be completely accurate.

[Awann99]

Originally Posted by dmjlambert

Under the cab there is probably a short piece of fuel hose that goes between the hard line from the gas tank to the hard line on the frame. And at the engine there is another short piece of fuel hose that goes from the hard line on the frame to the fuel pump. You probably replaced the one going from frame to the fuel pump when you put in the engine. I suggest inspect the one under the cab and if it has not been replaced before you may need to replace it. If it is very old or maybe original be careful because it can crumble in your hand when you give it a squeeze and the gas will siphon out of the tank all over the floor or ground. Since you are going to be removing the sender to put a new sock on it, it means you will be disturbing the hard line that goes from the tank to under the cab and will cause movement at the fuel hose under the cab. The truck original fuel hose was not rated for gas with ethanol, but make sure your new one is. Since ordering a sock you may also need the fuel sender rubber grommet for when you put the sender back in the tank.
I'm not 100% sure on the fuel hose arrangement for 6 cylinder because I have an original V8, so my info may not be completely accurate.

I put the new Fuel filter and sock on. The sock looked fine. There was some gunk around the metal part where it slid on the line. The filter had gunk inside of it.
My sending unit was replaced about 4 years ago. I ran rubber fuel line from the sending unit to the metal line under the cab due to the original line breaking. Now that y'all have brought up the collapsing issue I'm going to replace that with a metal line and set it up like it would have been from the factory.
I haven't taken it for a spin yet but progress is progress.
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[Awann99]

I meant to update this a while ago but better late then never. The fuel filter helped the truck. But it is still having that stumble at 60-70. I advanced the timing to around 14-16 before tdc and it ran better at high speeds. I'm thinking maybe the vacuum advance may be not working now. Still a strange situation. New fuel lines are on order as well.

[Grumpy old man]

The Small block Chevy engine that built America using 50+ year old technology in stock form is one of the most basic engines to survive 50+ years and still be a favorite engine in many muscle cars and trucks , They will run with gas that smells like turpentine , 5lbs of oil pressure , No air filter , Vacuum leaks , points ignition , and keep going ! Add an HEI and good maintenance and you've got a runner , Always start with the basics fuel/ compression and spark .

No computers needed !

[tim_mc]

Since you mention timing and vacuum advance, I recall having a similar issue when getting my truck on the road where it seemed to bog/lunge when cruising on the highway at a specific speed.

When building my truck, I installed a new GM HEI P/N 93440806, which came with an aggressive vacuum advance (added 20 degrees at 7.5" per a spec I found on-line). Mechanical advance for that HEI at 2400 RPM is rated at 16 degrees. After messing around and trying out different combinations, I figured I was getting too much total timing at high vacuum/cruising, so I changed the vacuum can to a Standard Motor VC204: 14 total degrees advance, 10-12 max advance, starts at 7-9". That really seemed to do the trick. I'm running my vac can off of the manifold port, and have 12 degrees of static timing (IIRC). I've tried it both ways and my engine prefers the canister on manifold vacuum.

Since then I've had to replace the GM coil and module and went with Davis Unified Ignition (DUI) parts, which really woke up the engine, maybe not related but it runs way better than it ever did on the stock GM HEI.

I know my specific details likely don't apply to your situation, but thought it might give you a few ideas. Maybe block off your vacuum advance and see if there's any difference, give it the ol' suck test or replace it with another?

Here's a good explanation & reference for some common GM vacuum canisters, their specs and typical applications:
http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf

[72SB]

OP

confirm your vacuum advance is working correctly. Ideally use a mighty vac and increase vacuum to what the truck makes and confirm the VA travel is not sticking.