Engine runs at a higher RPM but doesn't idle - possible causes?

EurekaSevven

EurekaSevven

Active Member
Joined
Jan 14, 2021
Messages
245
Reaction score
95
Location
North Carolina
So, I decided to put a completely new carburetor on my 383 instead of trying to repair my junk original Carter 2 barrel. I got the same model, albeit from a 1969 model, but it's still pretty much the exact same type of carburetor and bolted right on. It was an ACDelco rebuild, I adjusted the idle screws to 1 and a 1/2 turns and tried to adjust it: it doesn't want to run at all at the idle RPM, but if I rev it it fairs a bit better.

I've heard several suggestions from elsewhere and on this forum (many thanks to CBODY67), ranging from a vacuum leak from the carburetor gasket, to a timing problem, to an incorrectly adjusted or clogged carburetor.

On this carburetor there's two vacuum lines, one for the distributor advance and another on the very back. I didn't cap these although I read later that you should, so I'll take that into consideration for next time.

It is worth noting that my original carburetor, while having an issue with acceleration, actually handled idle well, which is why I'm not sure if this is a timing related problem. I have a thick style gasket for the carb which I could put on instead if that would make a difference, I just want to go from the least intensive task to most intensive. Any thoughts on this situation?
 
Leaving the vacuum fittings open can cause a sizeable vacuum leak which will prevent the engine from properly idling. Fix that and if necessary, open the idle screws to 2.5 turns and see if it will then idle.

Dave
 
Well, that's noted. I will come back with the results tomorrow - good thing I mentioned that, dummy mistake, but better than me trying to try to rebuild a distributor or crack open a carburetor again, was hoping not to (although that would not be an unwise thing to do in the future)...
 
Update on the situation:
I plugged the back vacuum port on my carb and and the PCV stem, she fired right up - she doesn't run too good but she does idle and fire decently enough. I noticed there was a loud hissing noise throughout (air leak it seems), and she still doesn't like throttle input - but maybe I wasn't waiting loud enough for it to warm up. Maybe 30 minutes is enough for it to warm up?

I might end up switching the current gasket on there with the thicker, OEM gasket.

I am wondering if this has to do with the fact that my current (rusted) vacuum advance on the distributor doesn't work - I have the other vacuum port routed to that currently. I guess I will have to get rid of the rusted bolts and replace it with my new one and go from there, although I don't think idle is affected by that.
 
Last edited:
A loud hissing noise could be a vacuum leak. Has the intake manifold been off recently? Check to see if the base of the carb. is ok. Is it possible the noise you are hearing is the air being sucked into the carb? If it idles OK then I would check the timing.
 
As far as I know the motor's never been apart, but I haven't taken it off - I currently have one of the really thin carb gaskets on the intake, which is why I might end up replacing it with the thicker one to see if that would fix it.
Another area I suspect is somewhere down at the base of the carb since I have a weird fuel hemorrhage situation going on - but that seems to be coming from my fuel line more than the bottom of the carb bowl itself (never happened on the other one, and probably a non-issue in this case, but still in my mind).

Timing is a definite possibility and I will check that next time I get it running - she starts up nicely enough, at least. Although maybe the vacuum leak is possible because I went to adjust the screws once warmed up and it didn't seem to make a noticeable difference.
 
Let me help with some pictures of a ‘68 383 2bbl. The proper vacuum lines must be hooked to the correct function

1- vacuum advance this must go to the top side port. It basically picks up the pressure differential when you start cracking your throttle to advance your timing. If you think your vacuum advance is bad. Pinch the hose or cap it off to get the carb fine tuned. For initial tuning it is recommended to cap this off any until you get your carb and initial timing set.
image.jpg


2- the small bottom one on the base is for your choke-as your engine revs the vacuum opens the choke for more air so your cold engine doesn’t flood out. You can cap this if you have a manual choke or your choke propped open when the engine is warm. (10 minutes should be more than enough time to warm up your engine.
image.jpg

3- Large port back lower carb to PVC valve recycles engine blow by for emissions. There is a whole section in the service manual on internal air flow.
image.jpg


4- don’t forget about your manifold vacuum that goes to the brake booster. Does not hook up to carb, but a bad hose or a T-that is not capped will give poor idle.
image.jpg

Start with those things before you start monkeying with your timing that seemed to be reasonable before the carb swap.

Oh and I would use the thick carb gasket as that was meant to be from the factory on a 2bbl. 1/4” - 3/8” thick.
 
I have all of those hooked up as they should be - PCV on the back angled slot, distributor advance is capped (but was hooked up to the right side of the carb, engine facing forward), intake manifold port to the booster I didn't touch, and the choke is hooked up as well - Although, thinking on it, the hissing sound seemed to be louder on the passenger side than the driver side, so maybe the choke line was bad, somehow.
 
Choke line can get fried from the exhaust crossover and choke well heat. It may have been fine on your original carb, but once you started moving it to hook to the new it could have been brittle enough to crack.

With a New carb you should replace the vacuum lines
 
My carb came with a choke line already on it - and I sucked on the line itself to see if it functioned and it seemed to work fine when I did it. I still have the original line, though. It was an ACDelco rebuilt Carter carburetor, same model as my previous one (your 2bbl), but from 1969 instead of 1968, so there's a few little differences in the way some things are set up, but it seems to be the exact same carburetor.
 
If it helps, I found my vacuum leak with some careful sprays of carb cleaner with engine running. Whenever idle smooths out, you have a winner.
 
I seem to remember learning in HVAC training that soap and water can do the same function - I think I will try that next time I go out to work on it, in this case.

And I think the thick gasket I had was actually too thick - it made the choke thermostat ride too high (almost to WOT) and barely bolted onto the intake manifold itself. Although, looking at HWYCRZR's choke linkage, his is completely straight while mine is slightly bent: still an issue with the nuts though.
0SSH565.jpg

vaJHLcD.jpg

u32Nvza.jpg
 
I could take a look at my P/N to reference yours, as a curiosity.
But it's looking like I might have to go back to the thinner gaskets in this situation, unless there's one that's more middle of the road in terms of thickness - my other one was paper thin. Although I never had an issue with mounting my other carburetor onto it - maybe one of the carburetor gaskets has a leak in it? About the only thing that makes sense to me at this point.
 
EurekaSevven said:
I seem to remember learning in HVAC training that soap and water can do the same function - I think I will try that next time I go out to work on it, in this case.

And I think the thick gasket I had was actually too thick - it made the choke thermostat ride too high (almost to WOT) and barely bolted onto the intake manifold itself. Although, looking at HWYCRZR's choke linkage, his is completely straight while mine is slightly bent: still an issue with the nuts though.
View attachment 464094
View attachment 464095
View attachment 464096
Click to expand...

Mine is an early ‘68 (scheduled date of August 15th ‘67). It works well up to 3/8” thick gasket and the studs are long enough to engage all the nut threads.
Also sometimes a quick mist of carb cleaner or starting fluid around the base of the carb can pick up a vacuum leak by increasing the rpm if there is a leak.
 
Some awesome advice and support already, man I wish this site had existed when I was first banging knuckles!

You might try using a short piece of hose (garden hose works great) as sort of a stethoscope to help find the leak. One end near your ear, the other poking around looking for hissing sounds. The tubing not only directs sound to your ear and localizes the source, it also acts as a bandpass filter to cut out a lot of lower frequency noise and let you hear the high pitched hiss better! (Also great for figuring out which lifter needs cleaning out!)

Definitely a trick to add along with the cleaner spray and soap bubbles, just remember that anything you put on a vacuum leak likely ends up in the engine so plan accordingly!
 
You must log in or register to reply here.

Similar threads

J
1963 Dodge 880 Hesitation on Acceleration
Replies
6
Views
1K
jhermis1861
J
6
The old stalling when put in gear syndrome
2
Replies
31
Views
799
CBODY67
C
SwissMonaco
Ignition timing adjustment
Replies
7
Views
2K
CBODY67
C
68-NewYorker
Stalls when put into gear
2
Replies
21
Views
2K
halifaxhops
halifaxhops
MoPar~Man
318 rough idle (20+ years in storage)
2
Replies
20
Views
3K
CBODY67
C
Back
Top