Snap Krackle Pop On Freeway?

[Amigo Arts]

Hi Everyone! I have been gone for a long while but I guess that is what life does to you, house, two dogs, baby, and started a business.Well here is my question. I have a 71 Chevy long bed. I have dubbed it my shop truck. Its kinda hot rodded too.
So here is my problem. It runs fine in the city 25-45mph. When I get on the freeway all is fine too, sometimes. When cruising at around 55, sometimes it starts to krackle and backfire. Totally random though. I can drive 60 miles and only have it happen once or twice, or sometimes it won't happen at all. It seems to be more frequent on long hills but it doesn't always happen.
So here is what I am running:
Original SBC 350 bottom end. I know, ticking time bomb...
Camel hump heads
Edelbrock Performer Intake.
Summit Cam Duration 282/282, Lift .465/.465
Holly 670 Street Avenger.
New Accell Points Dizzy.
New wires.
New Coil.
New Summit 110 gph fuel pump.
Fuel lines to the carb from pump are 3/8" rubber.

I just adjust timing and valves and still no difference.

Thanks for your help!

[Lower]

Have you checked the floats on the carb? Sounds like you might be running out of fuel on longer pulls.

[Amigo Arts]

Thanks for the response, although i did all this stuff to my rig my self i am not really an engine guy. Can you give me a little info of what to look for or a link to another post to learn from?
Dunno if it makes a difference but it also pulses and lurches at the same time as the backfiring.

Thanks

[Lower]

I would say your running out of fuel.
Taken from the holley website
Setting the fuel level should be the first thing you do before attempting to make any further adjustments.The float level should put the fuel level just below the bottom of sight plug hole. You will make the adjustment with the vehicle on a level surface and the engine idling. You will first remove the sight plug, then to make your adjustment you will need to loosen the lock screw on the needle and seat. This will allow you to turn the adjusting nut to raise or lower the float level. Each hex flat on the nut will change the float level approximatly 1/32". When you have the fuel level just below sight plug hole you will then tighten the lock screw and reinstall the sight hole plug. Make sure you have a shop towel handy in case you have any fuel leaks from the Sight plug or needle and seat adjusting nut.

[Amigo Arts]

Sorry for the long time before response. So I checked and saw I have one of those holley carbs that does not have adjustable floats.
This is the carb at summit.
So now what do I do? I plan on calling holley or summit to see what to do.
I do have a carb on the shelf. I think its a 750 4150? i'll have to check. I used to run it a while ago but wanted something more economical. It needs a rebuild badly i think i remember the float sticking on that one and leaking fuel.
Should I cut ties and attempt a rebuild?

Thanks again for the help

[sanford66]

Could you have a plug wire shorting out on the exhaust? If it happens some of the time it sounds like something to do with the ignition system to me. Good luck with it. Joseph

[oldspowered67C10]

I agree that it sounds like its running out of fuel. You need to determine if its a carburetor problem or a fuel delivery problem. Check float level and if thats OK then check for a weak fuel pump or a clog in the fuel line somewhere. If I'm not mistaken all holley carbs have adjustable floats but some aren't externally adjustable.

[cdowns]

rubber fuel lines are a great way to burn the truck to the ground

they can also collapse and cause fuel starvation

[Amigo Arts]

A little bit of success!

I rebuilt the carb, changed power valve and accelerator pump. New air filter, new plugs. Plugs were nasty, some blackened, some whitish, pretty much all that I expected. Im sure I am getting some oil blow back, as far as I can tell there has never been a rebuild on this motor. I bought the truck several years ago and it had all original running gear on it.

So here is my question, it ran great but after getting to the top of a long hill ~ maybe a mile or so on the freeway, it started to pulse. No backfiring through the carb, and if I lifted it caught up and was fine.
I am running about 6-6.5psi at idle and I adjusted my internal floats and changed out the plug or needles what ever they are that are attached to the float. My pops says to run it at 23 degrees advance.... I always learn from more experienced people but I think that might be excessive. Could that be why i am leaning out at the top of a long pull?

Thanks for the help.

[long69]

i had a new summit fuel pump. and also run a holley street avenger 570. i noticed that the pressure was all over the place. sometimes near 0 sometimes over 9 maybe higher. it would just die on me while driving down the street, or really run like crap. most other times ran sweet. when i noticed fuel gushing out the carb, off it came. and i cleaned what i could on the floats, fairly sure they were sticking. and i use a regulator now also.

[frednoah]

23 degrees advance? Is that initial or total? Is it locked?

Sounds like a timing issue to me. If you're starting out at 23 degrees, by the time all of your mechanical and any vacuum advance comes in you may be way high at highway the rpms you are turning on the highway.

[Amigo Arts]

Yup 23 degrees at about 1000 rpm. I'm thinking I should back it down to 18?
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