1972 c10 350 surging under heavy acceleration

[Mike_The_Grad]

Hey everyone, so my truck will start surging heavily when I mash the pedal. It does not always do it. I first noticed it about 2 months ago on a real hot day. Like high 90s. I could not go more than slight throttle or else the truck would start bucking and surging. Never stalled, never died, did not over heat, had no issue with hot starting or vapor lock. Yesterday I drove it around and gave it a good flogging. The first hour ran perfectly no issues. After I parked it for about 2 hours and the outside temps went up, me and a buddy made a hardware store run and the truck did not like anything over half throttle. Even if i got into the surging and tried more throttle or less throttle it still surged. Basically forcing me to do the posted speed limit. Not a bad thing at all, but i wanted to show my buddy what she's capable of. Oh well no problem. But the surging thing shows itself at weird times and sometimes when I need it to perform.
What would cause this intermittent surging issue? Here's some info on the truck: 350 4 bolt main, bored 0.040 over, flat top pistons, 041 cylinder heads, CR at around 10.2:1. Obviously running only premium gas forever. Roller tip rockers, edelbrock performer EPS intake, edelbrock 1406, rebuilt about a month ago, fully calibrated to run as rich as I could make it with their calibration kit, yes I know this is not good but I have already blown a head gasket 2 times in the last 5 years, cam is elgin 1875= 1/4 race cam, davis Unified iginiton distributor custom tuned for my truck, headman elite ultra duty headers ceramic coated, 3" super 40 flowmasters dual exhaust, truck has been fully rewired, trans rebuilt less than a year ago, th350 with shift kit, 3.08 posi rear end with new ring and pinion, caltracs traction bars, long bed. Truck is fun as hell to drive and my only vehicle daily driver. Also has dual electric fans and brand new copper brass radiator.
Im going to check timing but I think it's around 12° btdc which should give me around 36°@ 3000. No pinging, not that I could tell. Spark plugs are autolite AR73 race plugs straight from the box into the motor, moroso super 40 wires.
Im gonna pull plugs and check them and reset timing and brake out vacuum gauge to tune the carb. Also clean up the carb and distributor some.
The last time I did this little maintenance routine truck ran b****'n so maybe thatll fix what ever it is I'm experiencing.

[notsolo]

How about your fuel pump? Something else to consider...Got a pressure gauge on it....too much pressure and it blows past the float and throws in too much fuel.

[Mike_The_Grad]

Fuel pump is less than a year old. Airtex 3 fitting pump with the return line back to the tank. I have a holley fuel pressure regulator in line set at 5.5 psi with a metal fuel filter after that and a liquid filled fuel pressure gauge right at the base of the carb.

[Grumpy old man]

It sounds like your distributor fix didn't work .

http://67-72chevytrucks.com/vboard/s...d.php?t=823362

[Mike_The_Grad]

My distributor fix worked to ensure my timing was rock solid. I reset my timing to 11° btdc. I'll include a picture of the 4 spark plugs from the driver side cylinder head. I dont have a picture of the passenger side cylinder head because I had already cleaned them and put them back in before I thought about maybe taking a picture to help provide some insight for this thread.

I spent this weekend working on my truck. It was awesome. I hardly get anytime to work on my own stuff. Between my dads 64 c10, my girlfriends XJ, my buddies xj, or another buddies 73 duster, or his buddies 68 firebird. Let's just say I stay busy. So I took time to clean up the carb, clean up the distributor, clean the spark plugs, hook up my vacuum gauge, and hook up my timing light. All in all it went well, no surprises. No having to run to the auto parts store. It was nice. I will say it did not fix my surging issue. As i was telling my other buddy today about my weekend maintenance and the surging I'm experiencing. He cut me short and said it's a fuel delivery issue. He told me that he had past experience in his dads 73 ford work truck with a 460 big block. While getting on to the freeway basically flooring it, the truck would start hesitating and surging. Because the engine would suck the float bowls and the fuel pump couldn't keep up with the demand. Sounds legit to me. I am running a stock mechanical fuel pump still. I know my timing and igniton side is 100% working as it should. And my carb wasnt that dirty, considering i rebuilt it about a month ago. So process of elimination starts to narrow down the possibilities. It's either a fuel line restriction, dirty fuel filter, weak pump, clogged pick up tube, bad gas, or fuel leak. The fuel pump isnt showing signs it's bad because the weep holes are dry and clean, the fuel line connections are tight and dry, and the oil dipstick doesnt smell like gasoline. I guess I'll start with replacing the filter and all of the rubber lines from the tank to the carb. It's been a couple of years now that I think about it for the rubber fuel lines, the fuel filter was replaced about 5 months ago. That's gonna have to wait til next weekend though. Granted I'm not working on something else. I'll keep it posted though. Thanks.

[Mike_The_Grad]

Forgot the picture of the spark plugs.

[tim_mc]

May not be your issue since your surging is under acceleration, but thought I'd mention it just in case or for other ideas. When I was experiencing surging while cruising, I found that the vacuum canister I was using was adding in too much advance. It was the vac can that came with the GM HEI 93440806, which I believe the spec is 20 degrees @ 7.5" Hg of vacuum.
I replaced it with a Standard Motor VC-204, which has 14 total degrees advance, 10-12 max advance, starts at 7-9" Hg.).
My latest adventure with the truck running poorly turned out to be the coil and module - now replaced with DUI components and running great. (https://67-72chevytrucks.com/vboard/...d.php?t=822473)

I saw that you just fixed the distributor, but there may still be something going on with it now that the bushings were replaced and the mechanical advance is working as it should. Maybe disconnect the vacuum advance to see how it runs?

As for fuel-related issues, are you sure it's a lean surge and not a bog since you have the 1406 set (too) rich per Edelbrock's chart? I had a 1405 and now a 1905 AVS2, and found that after I fixed my intake and ignition issues my roller cam 350 runs like a top with Edelbrock's out of the box jets and rods. I'm running a step leaner on cruise and power, with .101 mains instead of the original .100 jets, but stock rods and secondary jets.

[67 twins]

Maybe disconnect the vacuum advance to see how it runs?[/QUOTE]

Vacuum advance does nothing at WOT which is where OP says his problem is. Disconnecting the vacuum advance will not affect his problem

[BigBird05]

Could be the sock inside the fuel tank. How old is it?

[tim_mc]

Originally Posted by 67 twins

Vacuum advance does nothing at WOT which is where OP says his problem is. Disconnecting the vacuum advance will not affect his problem

You're right...it shouldn't affect it at all at WOT and probably not related as I stated, but would be one less item and vacuum hose while troubleshooting. I wonder if the mechanical advance curve has changed a bit with the recent work on the weights. Mike's other thread here: http://67-72chevytrucks.com/vboard/s...d.php?t=823362

[Ol Blue K20]

I had the surging problem in my truck. It had a new fuel pump filters etc..I finally replaced the new fuel pump with a new pump. POOF! Problem solved, the first pump had good pressure but couldn't keep up with demand....

[68 P.O.S.]

Originally Posted by tim_mc

You're right...it shouldn't affect it at all at WOT and probably not related as I stated, but would be one less item and vacuum hose while troubleshooting. I wonder if the mechanical advance curve has changed a bit with the recent work on the weights. Mike's other thread here: http://67-72chevytrucks.com/vboard/s...d.php?t=823362

It depends on which vacuum source you're using. If using manifold vac, as you should, the vac advance cuts out at WOT because rich A/F mixtures burn faster and need less timing. If using ported/timed vac, the vac advance is all in at WOT, which is completely wrong.

[j_cst_10]

Most of the time surging is from a lean condition. This could be a number of things from float height, fuel pump, air/fuel ratio being too lean at or near WOT from jetting, fuel sock blockage like someone else mentioned, etc. The plugs won't tell you much since they are mostly run at cruising speeds unless you install a new set, run down the quarter mile, and shut down at WOT.

[kwmech]

Surprised that not one person mentioned a dirty fuel filter. All it takes is one trip to the gas station to get this issue

[HO455]

You could have a piece of rubber fuel line that has failed. (Yay! Modern fuels)
A soft hose on the suction side may collapse and shut off fuel to the pump.
Or on the pressure side of the pump the inside of a hose may have developed a flap or other deformity that limits flow. These can be difficult to find and truly annoying if you find one after you've replaced the fuel pump. (Don't ask!)
Good luck and keep us posted!

Having read your first post again you mentioned the carb had been rebuilt a month ago. If you don't find a fuel delivery problem I recommend giving the carb a good look over for loose screws or a gasket failure.

[tdangle]

Originally Posted by 68 P.O.S.

It depends on which vacuum source you're using. If using manifold vac, as you should, the vac advance cuts out at WOT because rich A/F mixtures burn faster and need less timing. If using ported/timed vac, the vac advance is all in at WOT, which is completely wrong.

Actually the only difference between full time and ported vac is where the hole is located in reference to the throttle blades. Full time vac is below the throttle blade, ported is just above so there is no vac at idle. Both have very little or no vacuum at WOT. If you don't believe me hook up a vac gauge inside the cab and try both ways at WOT.

[70StepsideC10]

I had a surging problem a few years ago. Truck would cruise fine but would surge under hard acceleration, going up big hills etc when I was putting a load on the engine. Pulled the fuel filter out (Qjet with internal filter) and it was full of junk, replaced it and corrected the problem.

[Mike_The_Grad]

I have disconnected the vacuum advance recently to reset my initial timing. It was around 13° btdc. The distributor manufacturer suggests 12° vtec. I find that my truck runs best around 10-11°vtec. This is checked with vacuum hose disconnected and plugged off and idle speed lower than 700 rpms. I run my vacuum advance at full manifold vacuum. Truck likes it that way. The sending unit in the tank is maybe only 2 years old. My old one had the resistor part of it wear out. Then a replacement units float filled with gas 2x's! The brass ones they get over seas use the minimalist amount of solder to seal them up. So I switched to a plastic float from a cj5. They are almost exactly the same dimensions as the brass ones, no solder to leak. Built well and it was a simple swap and works correctly. Plus it was about $5. The fuel lines have all been replaced in the last 2 years. Including the metal ones and all rubber hoses from sending unit to pump to carb. The fuel pump was replaced in the last 2 years because the old one was leaking out of the weep hole and it had a weak spring. The carb is good and leak free. I just did some clean up on it last weekend and it was still dry in and around the parts you can get to without taking it apart. Floats were set at factory specs. I have holley regulator plumbed in with a hard line from the pump to the regulator which is mounted on the front of the passenger side cylinder head using the accessory bolt holes. From there it has rubber fuel line to inline stainless canister fuel filter then onto the edelbrock drop down fuel line with a liquid filled fuel pressure gauge. Gauge is set at 5 psi. I should double check it though to make sure. It's been a while since I looked at it. I might be due for a new rotor and cap. They've been on there for about a year. I usually replace them with a MSD cap and rotor kit they sell at oreillys. It costs about $50 but they give you 1 year warranty and in the kit the give you extra rubber disks and carbon rotor buttons that go under the coil. I have about 8 of those now. Cool thing is I just return the rotor and cap in the package before the one year warranty and I get a brand new one for free.
I reset my idle mixture screws to get the steadiest and highest needle reading on my vacuum gauge. Truck fires right up with the flick of the key. And idle sounds great. I've actually gotten a couple of compliments this week on how good it sounds.
Im leaning towards clogged fuel filter or weak pump. Im running stock 3 line mechanical fuel pump that has return line to tank. I guess these metal fuel filters are supposed to be replaced every 5,000 miles. I think I'm just going to make it part of my oil change regimen. Which I plan on changing oil today. So I will just change the fuel filter also for s&g. And maybe put a bottle of seafoam in the tank.

[Mike_The_Grad]

I also forgot to mention I have my vacuum advance canister restricted to only provide about 10° of timing. I did this with a homemade restrictor plate that i made from some sheet aluminum. Thin enough not to interfere with the pick up coil but strong enough not to fold or bend under the spring tension of the vacuum advace can spring. With the vacuum line disconnected i have my initial timing set at 10-11° btdc. When i connect the vacuum advance back up to full manifold vacuum the timing jumps up to 22° btdc. And the specs on my distributor from davis unified ignition say that the mechanical advance built into it is 22° @3000 rpms. So as the vacuum advance falls away at wot the mechanical is all in with my initial timing at about 32-33° nrdc. I think my truck likes 34° so maybe I will reset my initial at 12° btdc. My CR is 10.2:1 and I only run premium 91 octane gas.

[tim_mc]

You've covered a lot! Sounds like the fuel pump and fuel filter may be the way to go. Speaking of fuel, one thing you might consider with a 1406 is a fuel pressure gauge. I installed one as I wasn't sure how my Mr Gasket fuel regulator was calibrated (or working), and the mechanical fuel pump was pushing gas past the needle & seat in my old 1405 - bad gas smell in the garage & carbon soot in the exhaust - but no more now. This is a VDO P/N 153002 - made in the USA! ($21.99 at Summit). You can see in the pic that the regulator is set to "4", but the gauge is reading slightly over 5 psi...probably still OK for the Edelbrock AVS2 I'm using. Awesome carb BTW. Best wishes on getting that surge figured out...I know how frustrating a nagging problem can be!

[Mike_The_Grad]

My buddy owns a gauge calibration company. He can get or has in stock any size or type of gauge. He hooked me up with a really nice McDaniels liquid filled fuel pressure gauge that reads from 0-15 psi. I have it set at 5 psi.
I think I may have found and fixed the issue, well at least most of it. I replaced the inline fuel filter. When I disconnected the outlet side of the filter I had a small glass jar under it to catch any fuel in the line. Well no fuel came out of the line. I didnt think much of it although it's kind of weird because that never happened before. I even tipped the filter to make sure no fuel was gonna splash out. When I disconnected the inlet side of the line, gas came out of the filter. It was practically full. So i had a major restriction inside the fuel filter. Since i replaced the filter the surging only happened one time and it was less intense than it used to be. I think it might also have something to do with the quality of gas I been getting lately. I do live in southern California and gas prices are ridiculously high. So I have been hunting for the "cheapest gas" I can find. Which at $4.19 is about as good as it gets around here. And that's for premium 91 octane. I usually stick to one or two stations. But those stations prices are over $4.75/gal. Now! Thanks uncle sam. when I can afford it I get the "good gas" and one station in particular has the "good gas". I instantly notice a difference in how the truck fires up, idles, and performs. Plus I better mileage. It's not all in my head either. I drive my truck everywhere everyday so I find myself in different towns at different times and I get gas when I need to not when I want to. When gas was cheaper I would go out of my way to get the "good gas" now that has changed according to budget.

[tim_mc]

Good deal! File this one under "do the cheap and easy fix first". Sorry to hear about the Left Coast gas prices...here in Missouri 93 octane is $2.99 at Costco.

[Sparky dave]

Mine just started surging in hot weather after installing 195 thermo switch for electric fans. It used to run at 180. If out of down to extra heat booking the fuel lines that are running across top of engine. It only does it after sitting in traffic on idle then taking off ( when fuel flow is slow)
Well done good fix by the way

[D.PASSMORE]

Down in Florida, talk about fuel mixtures, I run a Air / Fuel mixture gauge on my Blazer. Noticed while dialing in the correct A/F numbers, by finding the correct jet/rod set up for my OE quadrajet carburetor. There was a fine line between fuel companies, summer gas and winter gas. I set my numbers to run summer shell 89 octane and run shell 92 octane in the winter months. The winter mixtures always run leaner especially BP gasoline (Old Amoco). I run around 9.5 compression in rebuilt SBC 350 / mild CAM.

Lean fuel mixture will be noticed by crappy acceleration, backfiring and/or surging at WOT.

[Mike_The_Grad]

So I think I have this one figured out. I was greasing the chassis the other day and I looked up at the fuel supply line from the frame to the fuel pump. The connection at the pump was damp. So I tightened the band clamp. I noticed it was just spinning and not tightening up so I figured I may as well replace it. Then I looked closer at the hose and it was all cracked and the ends were soft like spongy. I replaced them about 3 years ago. I pulled that Section of fuel line and it needed to be replaced. The band clamp was actually ripped at one of the slits for the worm screw so that was bad also. New fuel line and a new fuel line at the sending unit to the hard line going down through the floor with brand new made in usa stainless steel band clamps seemed to do the trick. No more surging. I guess the broken clamp and soft hose would start to leak at higher rpms and pressure. Thank you all for your input and putting some valuable info in this post for anybody who might experience similar symptoms.