erractic idle and loss of RPM

[thistle3585]

My AMC 304 is idling erratically and I'm having to screw in the main idle screw to bring it up to about 900 rpm for it to idle smoothly. High but smoothly. I rev it up and hold it and the RPM's drop off while the accelerator pedal is still pushed in and it starts to gasp and back fire. If I rev it up high then let off the pedal it will die unless I pump the accelerator. I recently rebuilt the carburetor and it was running fine but then I started having this issue. Initially, I thought it was the fuel pump because the fuel pressure gauge was jumping around from 0 to 10 psi and then finally went to 0 and I couldn't get it started. I just replaced the fuel pump today and it is running at a steady 6 psi but with the earlier mentioned problem. Any thoughts?

 

[Torxhead]

I think you should first check the fuel mixture by examining the spark plugs, there will be a chart in your repair manual. Some guys do not set the floats correctly when they do an overhaul.

 

[Lumpy Grits]

First, replace fuel filter.
Does your f'filter have the 2 outlets?
If so-Make sure the smaller one(return line)is at the 12 o'clock position.
Make sure BOTH your idle mix screws are turned out the EXACT same amount.
Ck all manifold bolts and carb base bolts/nuts to be sure they are tight.
Look for a vacuum leak--
LG

 

[gpdno]

Check you carb float level. I had a similar problem and once I reset my float level everything else fell into place.


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[thistle3585]

Float level is fine and it is getting enough fuel. It seems like it is starting to falter as the engine gets hotter. Here are spark plugs from cylinders 1 and 3. The plugs are Champion RN14YC.

According to the manual, Symptons: Blistered, white insulator, eroded electrode and absence of deposits. Recommendations: Check correct heat plug range, over advanced timing, lean fuel, intake manifold leaks, sticking valves and insufficient engine cooling.

 

[Lumpy Grits]

Plugs look fine.
When was the last time you replaced the dizzy cap, rotor and wires??
My SWAG and knowing the issues with the 2100's. Vacuum leak at the base or in the manifold itself.
AGAIN, I ask-WHAT ABOUT THE IDLE MIX SCREWS? Did you ck'em??
LG

 

[gpdno]

Have you checked your mixture with a vacuum gauge? You can get a gauge for under $10 at most auto parts stores. Very simple and easy to set the proper carb settings using the gauge.

 

[thistle3585]

Also forgot to mention that the engine is backfiring.

New distributor, wires and rotor about 6 months ago. Idle mixture screws are 3 full turns out.

 

[gpdno]

Not sure what carb you are running but 3 turns seems quite rich. I've got a Holley and my screws are set at just over 1-3/4 turns. I would check your tune-up with a vacuum gauge.

 

[Lumpy Grits]

3 out sounds 'rich'.
I'm sure you have a vacuum leak somedangplace now.
Could be an internal issue with the carb also. I don't know 2100's at all.
Get the engine run'n-Use VERY soapy water and spray the carbs base plate area. IF there is a leak you will hear a change in engine sound/pitch and see the bubbles suck'd inside.
Have you put a wrench on ALL manifold bolts to be sure they are TIGHT?
LG

 

[mtnwhlr]

Not sure what carb you are running but 3 turns seems quite rich. I've got a Holley and my screws are set at just over 1-3/4 turns. I would check your tune-up with a vacuum gauge.

and backfiring is usually a timing problem.

 

[thistle3585]

Got the idle screws set at 1.5 turns. It idled erratically for awhile. If I put the manual choke on it jumped about 500 RPM. I sprayed starting fluid around the base of the carb without any increase in RPMs. It idled for about 30 minutes then died. Now it wont start. Kind of frustrated at this point, so I'm going to go back to it tomorrow. Thanks for the help.

 

[Lumpy Grits]

Forget the starting fluid-It's made to burn.
Use the soapy water-There's a reason for this--
When it dies-Can you put and hold your hand on the coil?
LG

 

[thistle3585]

I replaced the PCV and the vacuum hoses and checked the connections and didn't find any leaks. I think I hear a hissing, so I'm guessing its somewhere around the base. Is this the correct "assembly" for the carb. Manifold, thin fiber gasket, aluminum mounting flange, thin fiber gasket, spacer, thick fiber insulator then carb.

 

[thistle3585]

I spent some more time on it. If I put my hand over the carb opening and cut off all air it idles well and seems to run better when accelerating. Doesn't this mean it has to be a vacuum leak? I've checked the hoses and replaced the gaskets on the carburetor, so where else can there be a vacuum leak? Also, nothing happens when I screw the idle mixture screws in.

 

[Lumpy Grits]

YES-You have a vacuum leak !
LG

 

[mtnwhlr]

Take some carb cleaner and spray it around the base of the carb if you have a leak when the cleaner hits it the idle will change.

 

[thistle3585]

While looking around for info on vacuum leaks, I came upon this photo saying I should seal off the port in the carb. Any thoughts on this?

 

[Foulplay]

I read somewhere the other day about a faulty or exhausted charcoal vapor canister can cause the symptoms you describe. Not sure if it is the solution for you, but it's something to look at.

 

[thistle3585]

I do not have a vapor canister on it. I sprayed some carb cleaner on the front side and it changed idle speed. I replaced all the gaskets and hoses with no effect. My only guess is that its the idle mixture screws themselves. I'm not sure where else a vacuum can be drawing on it from the front. I'm just tired of messing with it and decided I'd be better off time and money wise by getting a new carburetor.