Chevrolet 305cu engine problems

Hi, I have a '72 Elky with a 305 in it.
When I start it, it runs great. When I take it for a drive, it starts good and pretty soon it sounds like a few cilinders stop working. I figured it was a bad ignition (stock point ignition) or maybe faulty plugs or wires. So ordered and installed a new Streetfire HEI ignition, with cables and MSD plugs. First few minutes it sounded like the problem was solved but during the test drive same thing happened, not running on all 8. Weird thing to me is, when I let it idle for a bit it'll start firing on all 8 again.
I'm not losing coolant or oil so don't think I've got a blown gasket or cracked head/cylinderwall.
But I'm running out of ideas, was wondering if anyone here might have any

I also have a 305 from my '88 silverado laying around which I know works perfectly, but it's a TBI. Can I just swap my intake with carb and ignition from the '72 onto that one or are these 2 different engines all together?

Thanks in advance.

Check to see if the choke is closing on you.
Have you tried changing the plugs?

fuel filter ? if its got valve problems or cam you can put a piece of paper at the tailpipe when its running if it sucks it in and out . you cave a valve issue . if your not loosing water I wouldnt think you have anything cracked ... can you hear a vacuum leak ? .. can you rev, it up when it skips? if not may be starving for fuel ( filter) or sucking air (hole in line) or hole in line going inside of tank .. does it run good on full tank of gas ? Hey Im just throwing stuff out there ,, ive had all these problems Ive suggested

Maybe

My daughter's 350 70 camaro would idle on 8 but only run on 4, turned out to be the carb. it had a split plenum manifold, each half of the carb fed half the eng. plugged main jet.
some thing to look at [S
good luck

Old Iron said:

Check to see if the choke is closing on you.
Have you tried changing the plugs?

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As far as I can see and tell, my choke remains open at all times (unless I close it manually),
It's got brand new iridium plugs all around.

Yankee Transplant said:

fuel filter ? if its got valve problems or cam you can put a piece of paper at the tailpipe when its running if it sucks it in and out . you cave a valve issue . if your not loosing water I wouldnt think you have anything cracked ... can you hear a vacuum leak ? .. can you rev, it up when it skips? if not may be starving for fuel ( filter) or sucking air (hole in line) or hole in line going inside of tank .. does it run good on full tank of gas ? Hey Im just throwing stuff out there ,, ive had all these problems Ive suggested

Click to expand...

Had to replace the fuel tank because it had junk in it, so also placed a new filter and sending unit.
Might have to try the piece of paper thing. And everything that needs vacuum gets vacuum (atleast that's what I feel when I disconnect the lines and put my finger on it).
I can rev it, but it sounds horrible. Haven't tried it with a full tank yet.

soltz said:

My daughter's 350 70 camaro would idle on 8 but only run on 4, turned out to be the carb. it had a split plenum manifold, each half of the carb fed half the eng. plugged main jet.
some thing to look at [S
good luck

Click to expand...

Maybe some of the dirt and junk from the old tank made it's way inside the carb. Gonna take if off and pull it apart to clean it, see if that might be it.

Thanks for the input guys, I know what to do upcoming weekend.

you don't say if you have solid or hydraulic lifters.

you might have the valves adjusted too tight and when it warms up the valves grow in length and won't close all the way.

just a thought.

I've had a similar issue and it was debris inside the carb. Hauling ass and then it fell on its face. A lil gunk can wreak havoc. A gasket kit is like $20. Pull the fuel bowls, metering plates/blocks and spray with some cleaner. Inexpensive, and you can cross it off your check list. Best of luck, I hate issues like this.

I think junk in the carb but, you may want to check the EGR valve, if still attached. You can buy or make a plate to cover the holes if you want to remove the valve. If not working correctly, or hooked up wrong, it can cause all sorts of crazy things to happen. Good luck.

most likely some trash got by your filter .. My 65 would run good just easing down the road but you try and running it hard and it would, spit, fart , backfire , and if you held it there it would quit , after taking the carb down and cleaning, then got a new coil thinking (braking down of fire) looked at the wires and all of this for about 2 days I finally took sending unit out of truck tank (thinking something was sucking up to the bottom of the line ) which found nothing there , I seen pin holes rusted threw my suction tube ... It would suck enough threw to run easy but when needed more fuel it wouldnt suck enough because of the holes ..