whos an older sbc carb motor expert

   #1  
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so, in my long history of wrenching i swear i come across so many "aint never heard of that before" type situations that leave me scratching my head....heres another.

70s sbc with an edelbrock 4bb carb on top. new hei dist, new wires, new plugs. motor absolutely ran fine when parked up on a lift about 14 months ago (when garage got built oddly enough). take it off lift for the first time (doing a bunch of suspension work mostly), starts up just fine and runs like a top in garage. gets up to temp and holds temp fine...revs up..feels snappy...sounds perfect.

shifts great, drives great...up until about 10minutes into a low speed trip (within neighborhood, all below 25mph)...and at this point it feels like someone threw out an anchor and motor loses pretty much all power.

have to bury throttle to the floor just to keep it running (which it barely does). it does have intermittent sputtering while foot is on the floor where it seems like it WANTS to accellerate, but 95% of the trip home (roughly a mile) is just barely running enough to limp into garage again. wont even stay running at idle in neutral at this point without feathering the throttle.

when this happened the first time, wife was home and i could get her to work the throttle to keep it running while I looked at random shit...i could easily tell that 2 cyls on each side of the motor were cold (infrared therm)...so i figure its a distributor or plug issue.

park it, go back out later that night, reconfirm all plugs and wires look good...find a "somewhat" loose connector in the 12v lead to the dizzy and fix that...and it starts right up and runs like a top again in garage like nothign had happened.


fast forward to the next day (today)...go out in garage in afternoon, fires right up...idles for a few min to get up to temp...and i go for the same exact test drive before (just around neighborhood streets). yet again at around the 10min mark...it falls on its face exactly like before! I do the same thing to limp it home into garage where its sitting now (wife wasnt home to do more troubleshooting and i cant keep it running even at idle without goosing the gas).

so im guessing its something to do with something getting heat soaked after a few minutes of run time, as the motor is clearly up to temp before I even leave the driveway (180tstat, works perfect) and holds that temp the entire trip regardless. oil pressure is solid and steady...im at a loss.

my only thoughts at this point are something in the carb going wonky once the intake heats up and thus heats up the carb on top of it and thus causing the bowls to lock up or something? maybe the trans is dying a slow death (th350) although it shifts fine...and i cant even see the trans getting remotely warm at the low speeds im running.

anyone else have any ideas? this is super bizarre for something so old and simple!
 
   #3  

Master-Cylinder

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Used to be able to check the float level/fuel level in old Rochesters through the vent hole in the top of it while it was running. I still have the tool.
 
   #4  
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its a mechanical fuel pump, with an inline see thru filter...its definitely GETTING fuel to the carb...

sadly its a newer edelbrock carb...i dont know of any realistic way to check that without disassembly. thankfully they are about a dime a dozen, so worst case ill just find another one and put it on to see if the problem goes away.

someone else suggested its something with the fancy HEI distributor (or experienced a similar issue with a tiny crack in the rotor that would expand as it got hotter)
 
   #5  

Anal Disco

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I had heat soak problems but it only manifested itself when it sat for 20-40 minutes after being at operating temp, making it hard to start up.

Sounds like a fuel delivery issue. How does your fuel filter look, if you have one of the clear ones?
 
   #7  

Wretch

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Does it pump fuel when you work the throttle?

You might think of the coil as something that can heat up and stop working as intended as well.
 
   #9  
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fuel filter looks fine...or at least nothing that would give me pause.

coil has been a common answer, but its all part of the distributor as well (its one of those fancy schmancy red all in one HEI units).

i mean, im sure its cheap chinese crap...but as many as they sell and get high ratings on...i cant imagine its broken just from sitting still for a year.
 
   #10  
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only reason i didnt think it was a carb "dirty" issue was that it runs so flawlessly at idle and for the first 10min of the drive... response is as snappy as one can expect for a carbed low compression sbc...it makes great vaccum even at idle...it makes solid power (until the 10min mark). choke cable works...as expected...

its just literally like the motor falls on its face after a short drive all at low rpms. i could see if i were out hotdogging it or doing burnouts up and down my driveway...but this couldnt be a more leisurely afternoon drive if I tried and nothing significant happens to signal the power loss...it just comes about almost instantly.
 
   #12  

Wretch

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Usually spark problems are on, or off, no limping home.
I used to be of the same mine but then, within a year saw two failures that produced similar symptoms.

One was in Porsche 911 that only had the problem after warming up and under load.
One of the coils was breaking down.

Then on a friends 67' Ford Pickup.
Get it hot and it would break down stumbling and if you pushed, it would stall out and not run until it cooled off.
Coil was the culprit.
 
   #15  

Wretch

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The ecm in the distributor could be at issue as well.
Did they use the right one?

Needs the dob of dielectric grease under it checked...
...leads to and from it checked too.
 
   #16  
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it all ran fine before the truck went up on the lift (and i had a baby) and sat for a year...

none of that was touched while it sat not running for the past year...only suspension shit underneath. thus why im baffled at how something like that would be the problem.
 
   #18  

Stetson

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Mice chewed some shit?


Seafoam her anyways... :up:
 
   #20  
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will recheck all vaccum hoses, but with the huge lopey cam thats in it...even the smallest vaccum leaks caused significant changes in idle (ie it wouldnt)
 
   #27  

Master-Cylinder

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I remember a coil contact under the coil to the cap that would cause problems. It also had a rubber washer that would get replaced.
 
   #28  
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yea im gonna at least take the cap off to have a looksee...but the entire distributor/cap/rotor/etc are all literally brand new...cant have more than 100 miles on it total even before it sat for a year.
 
   #29  

Wretch

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I remember a coil contact under the coil to the cap that would cause problems. It also had a rubber washer that would get replaced.
Yup, carbon button.
Don't finger it up with your skin oils...
...shoot it with a little brake parts cleaner if it is nasty.
 
   #31  
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probably just replace the whole thing (again)...its only 50 bucks.

thinking back on it...lots of welding got done on the truck and id imagine that could have done some damage to the shit inside the dist. as plausible as any other theory i suppose.
 
   #32  

Master-Cylinder

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Fourteen months is a lot of time for something to get clogged up. Bugs like to build nests in vent hoses, at least on motorcycles. Pop the gas cap and see if that does anything. But then again, if it an older vehicle that cap is prolly a leaker anyway.
 
   #35  
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Fourteen months is a lot of time for something to get clogged up. Bugs like to build nests in vent hoses, at least on motorcycles. Pop the gas cap and see if that does anything. But then again, if it an older vehicle that cap is prolly a leaker anyway.
Yeah, I've been thinking about that possibility too. My Cub Cadet sometimes does the same shit after a bit of run time unless I run the gas cap a little loose.
 
   #36  
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yea tank was nearly empty, id popped it to put a few gallons back in it before i started it back up (not to mention 14 mo old gas probably aint the best...but its a relatively low comp motor anyway).

its also been kept indoors in a garage up on a lift...im sure the garage isnt completely bug free...but its not like its just been sitting in a yard or a driveway etc.
 
   #37  

Wretch

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Ethanol free fuel?

If not, my guess is changing to: there is a pebble of corrosion swirling around blocking the primary circuit.
 
   #40  

Wretch

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It'll have white powdery corrosion in the float bowl/well.

Ethanol carries the water there and then leaves it once it evaporates, sitting and then it slowly eats the carb.
 
   #41  
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yea..ive dealt with that in other motors. this particular eldelbrock always loses the gas in the bowls when sitting more than a day or so...never known why or how to fix it. was kinda hoping with that particular situation, it wasnt all that big of a deal for it to sit this long either.

but the carb isnt something ive ever cleaned, I took it apart to change the springs out when i put in the big cam...but until now its run flawlessly. probably wouldnt hurt to take it all apart even if its not hte problem.
 
   #42  

Wretch

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Ah, so it's a Carter AVS carb and not a Rochester then?

Top comes off nicely on that.
 
   #44  

Wretch

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indeed...only like 40 bolts and cotter pins so small i cant even see on the linkages =D

ill probably wind up doing more harm than good!
Pull the carb, stick it in a box and take it to MC...

...I hear he does a bang up job of carburetors. :biggrin:

I was sure it was only 17 screws holding the top on.
 
   #46  

Wretch

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Squirt the gasket with some WD40 or other oil of that sort before you go to remove the top so the gasket is less likely to tear.
 
   #47  

Master-Cylinder

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Remember when you'd go to the auto parts store and they had walls of carb kits hanging on them?
 
   #48  

ZX11 Man

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It got a catalytic converter? If it does, it sounds like its clogged
 
   #50  
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I changed from points to HEI one time and didn't use a hot enough lead to power the new distributor. It ran like crap, also wouldn't idle.
 
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