305 surging at Highway speed only

[tutone]

Hey all. I chuckle at some of the threads that have obvious answers to, but I think I am troubleshooting one for myself that has me scratching my head.
Truck-- org 6 cyl
Org tank and 5/16 gas line to pump
(I have a new sender-3/8) not installed
Installed a 305/700r4
I have driven it 500-600 miles, mostly around the house, 40-50 mph and it funs like a champ. Trans is adj properly, I unscrewed the gas cap while it was acting up, no change.
At 55 plus it acts like the thing is starving for fuel but only at that speed or faster. It will run then fall flat on its face, then pick back up and do it again.
Pull off the road and let it idle a minute, your good for a mile or two then all over again.
Plan is to change the sender, fuel line, and fuel filter.
Any more ideas or anyone have a similar problem that they solved.?

[CC69Rat]

My thoughts would be:

Vacuum leak
Vacuum advance
Fuel pump

Do you hear any hissing sounds that sound like they aren't normal? When it surges, does it crack, pop and backfire any at all?

[tutone]

Fuel pump is new. Of course that doesn't mean much.
With the r4 it has by the time it reaches 55, it has ran through the higher rpm ranges. Timing is set at 4-5 degrees btdc. Ive adjusted it slightly to see if the vacuum advance is working properly. It seems to be. The dist is brand new as well. As I stated before, up to 45-50, it's the best running truck Ive ever built. No backfire, popping, noise of any kind. It just feels like a fuel starvation problem.Edelbrock 600 carb that feels good until the surging starts. Idles great as well.
One thing that may or may not matter. The fuel line from pump to carb is 3/8.

[geezer#99]

What rpm are you running at when going 55?

[tutone]

1650-1700 with lockup engaged.
Slightly higher without it but it makes no difference in the surge.

[tdangle]

I agree it is a fuel issue. Only focus on that. you didn't mention what carb is on the 305, but I'm guessing the stock 2 bbl or 4 bbl. Most of these have fuel filters in the inlet., Double check or remove it for testing. I agree upgrade the tank pickup and lines to 3/8 as supplied with our V8 trucks.

Another test is does it fall flat on it's face on a full throttle run? that would confirm a fuel issue. the fact that if you leave it set and then it's OK for a bit tells me the carb bowl fuel level is on the low side. By the way has the carb been gone thru or has it been sitting for a while. Might be time to refresh.

[geezer#99]

Originally Posted by tutone

1650-1700 with lockup engaged.
Slightly higher without it but it makes no difference in the surge.

What does it do when not in o/d and locked up?
Have you tried running it in drive only?

[tutone]

The carb is an edlebrock 600 in good shape. I'm not getting into the secondaries at 1700 rpms, I dont believe. I also haven't tried the 3rd gear or full throttle test. But I will do that for sure next time I drive it. I hope it turns out to be a filter issue or something simple. Thanks for the input.

[geezer#99]

Hook a vacuum gauge up with a long hose in to the cab so you can see how much vacuum you have during different tests.

[Wrenchbender Ret]

The sock on the end of the fuel intake tube in the tank is plugged up. When you replace the sending unit you will solve the problem.

George

[tdangle]

Originally Posted by Wrenchbender Ret

The sock on the end of the fuel intake tube in the tank is plugged up. When you replace the sending unit you will solve the problem.

George

That is a great thought!

[tutone]

Originally Posted by Wrenchbender Ret

The sock on the end of the fuel intake tube in the tank is plugged up. When you replace the sending unit you will solve the problem.

George

The truck was off the road for many years. Even though I have flushed it and ran several gallons through it, it very well could be. I will know this weekend when I pull the seat out and check it. It is the exact same feeling as running out of gas, only you never really run completely out.

[CC69Rat]

Reading the rest of the comments and your confirmations I would agree, it's a fuel issue. If the truck sat that long i have seen the sock on the end of the pickup get so decomposed(?) With time and sitting in gas for that long.. it will get brittle and dried up to the point it clogs the pickup. As someone else said this will likely take care of most of it.

I also want to share my experience with the Edelbrock carbs. The factory fuel pumps produce too much fuel pressure for the carb. A stock GM pump will put out about 9psi. The 1406/1407 likes about 4.5 psi. So if you are having hard start issues, drive for a while and stop to get a drink or whatever, jump back in the truck and it won't start.. it's likely the fuel pump. Just saying if you replace it take a look at the Edelbrock specific fuel pump. It is made for that series of carbs and pushes the correct fuel pressure for them. As an alternate you can put a fuel pressure regulator on the truck. I'd also suggest one of the spacers to put under the carb to keep it from percolating the fuel in the bowls. If the carb is sitting close to the intake it's pretty common to do this. Ask me how I know.. but just saying if you think about another fuel pump consider the Edelbrock pump to fix two issues. Good luck with it man.

[tutone]

I really appreciate all the input and comments. I will work on this issue and post the improvements I make and the solution that actually fixes it. I run the same carb on my 55 and it is exactly as you describe. Cold start fine, hot start semi flooded. I will definately put a regulator on that engine and if it fixes that problem, this truck will get one as well. So far it has started good but it may develop troubles as time passes. I am not a big fan of Holley carbs as I never could keep one tuned. May drag out one of the old Q jets and see if I can find a rebuild kit for it. No telling how many 2 bbls and intakes I gave away over the years. It would serve me as well as anything to put one on this old truck.

[MARKDTN]

I had a similar issue with a '71 Eldorado. It would start and idle fine. Slow speeds, no problem. But 35-40 mph or above it would stumble. Built the carb-no change. New fuel pump-no change. Turns out it was junk from the tank getting caught in the elbow on the fuel sending unit. That was 20 years ago and sending units were not available. I just blew it out from time to time.

I think you may be starving for fuel. If I was you, I would change the sending unit and sock since you have it and blow out the fuel line into a bucket. See what comes out. If it's clean you are probably OK on the lines. I don't think you absolutely need 3/8 on a stock 305 to go 60-70mph. Nice, but not required. If the fuel in the bucket comes out nasty and rusty then change the fuel line. And may as well go to 3/8 if replacing. You might try running it off of a can directly to the fuel pump and see if that helps.

[68 P.O.S.]

I had a similar issue a few years ago. Mine ended up being a rubber line at the fuel pump. PO replaced the steel line with rubber and it would get kinked only on the freeway at speed. Probably not your issue, but worth a look.

And yes, get a regulator on that carb (Edelbrock recommends 5.5 psi). Also get a heat insulating gasket like the 9266.

[tutone]

Yep. Fuel sender, fuel filter, new 3/8 hard line and rubber line where needed are going in. That should get me on the highway.

[cwcarpenter98]

My fuel pump died a slow death a couple months ago. I could cruise around just fine at the beginning as long as I didn't floor it heavily or go highway speeds. Anything usually below 55 was good. As it got worse, I started to not be able to just cruise around without struggling to pull a hill. New pump brought it back around real quick

[Sheepdip]

Originally Posted by cwcarpenter98

My fuel pump died a slow death a couple months ago. I could cruise around just fine at the beginning as long as I didn't floor it heavily or go highway speeds. Anything usually below 55 was good. As it got worse, I started to not be able to just cruise around without struggling to pull a hill. New pump brought it back around real quick

Me Three! Same symptom's and I knew I had to replace it, but it gave up the ghost before I got to it. The pump came out in pieces

[tim_mc]

I also strongly suggest a fuel regulator for Edelbrock carbs. If the fuel system checks out, it might be a lean surge caused when running on the cruise circuit of the Eddy carb. I'm running an AVS2 now (chart attached) but the tuning concept is the same. You might need to fatten up the cruise circuit if your carb has a factory electric choke (Model 1406). Edelbrock leaned out the 1406 (600CFM with electric choke) as compared to the 1405 manual choke model. They claim it is calibrated for economy. Here are the factory settings - note smaller mains and fatter rods when vacuum is high:
1405: #1428 (.100") main jets, #1451 (.070" x .047") metering rods.
1406: #1427 (.098") main jets, #1459 (.075" x .047") metering rods.
Both have 0.098 secondary jets.
I've read that many have had to change the metering rods and/or springs to improve performance of the 1406. Here's a link to the tuning guide - 1405/1406 starting on page 14.
https://edelbrock-instructions-v1.s3...ning-guide.pdf

[tutone]

I ordered 2 regulators from Oreilly's this morning.
The 55 is getting a regulator as well. I have always had hot start issues with that carb, not knowing why.
While I am re-plumbing the truck, the other one will land there. Unaware all these years the Edelbrocks had this issue. I guess I have just lucked out on several.
Also, as stated in my 1st post, this surge developed after 4-500 miles of trouble free driving. So I'm not discounting the carb tuning, because it could probably benefit from it, but my belief is, with it surging at highway speeds (when the truck demands the most fuel to keep the speed maintained), then catches up so easily at a complete stop and runs fine again, I was trying to find out what to look for 1st. Input says, sender, filter, fuel reg, check fuel pump, and possible carb tuning. I have pretty well ruled out timing, transmission problems, linkage issues or anything like that.
Order of changes
1. Sender/ hard line increase/reg/ new filter
Test drive
2. If those work---done
3. If those don't work/ new fuel pump
Test drive
4. If that works --done
5. If not/ carb tuning/rebuild/ or change
I'm hoping with this attack plan, I will get to the bottom of it

[geezer#99]

After 500 miles there’s a good chance your timing chain has stretched a bit. That can cause surging due to timing fluctuations.

[68 P.O.S.]

Your timing sounds low, not enough advance. If it's still set at 4-5 btdc, I'd increase it 12 btdc. That alone will make it run much better, but still need to find your current issue.

[tim_mc]

I agree with the 12 BTDC initial timing suggestion. Just another thought - You mention that you have a new distributor and you think the vacuum advance is working properly. An easy test is to unhook the vacuum can, plug off the line and just run on mechanical advance to see if it behaves differently.

[tutone]

Originally Posted by tim_mc

I agree with the 12 BTDC initial timing suggestion. Just another thought - You mention that you have a new distributor and you think the vacuum advance is working properly. An easy test is to unhook the vacuum can, plug off the line and just run on mechanical advance to see if it behaves differently.

Did that and yes the advance seems to be doing it's job. If you have ever been cruising down the highway and just ran out of gas, this is how it feels, only you never quite run out. I think it would ping , backfire, hesitate, do something, if it were strictly a timing issue. I would say without hesitation, that it runs and drives as good as any stocker I have ever had. That is except for this issue. I still plan to attack all the fuel issues first, then move onto timing issues if this plan doesn't work. I have a couple of long road trips planned, so I will need to call on it. I definately will report back to you guys as I devide and conquer.