I bought a 70 340 Barracuda about 3 months ago. The car was originally a 383, 2 barrel. It idles rough and smelled like raw fuel from the exhaust. I went through the carb, Edelbrock 1407, 750cfm. The floats level was way off. I adjusted the floats, the fuel smell is gone. I also adjusted the fuel pressure. My mechanic thinks that may be the engine has a cam, which is causing the thumping idle. The car is an AAR clone. It looks like the previous owner raced the car. Driveshaft loop, upgraded ignition, etc. Is there a way I can check what type of cam I have without taking the cam out? Thanks
Rough Idle
- Thread starter money pit
- Start date
Challenger RTA
Well-Known Member
I always think simple to start. Check plug wires, spark plugs, cap and rotor for corrosion. Worn distributor. Correct voltage to ballast resistor and coil, bad and loose connection. Hot wire it to eliminate the cars wiring. Carburetor dripping fuel inside, choke working, blocked filters, fuel pump good? . Vacuum leak? follow through with compression and leak down test. Don't think Know. Mechanics make money thinking. Make them check and know or do it yourself.
Last edited:
The car came with an MSD ignition box, coil, and distributor. the plugs and fuel pump, and fuel filter are new. I checked many times, it doesn't look like fuel is dripping into the carb. I'll double check, I don't think the MSD is connected to the ballast resistor. The choke is working and there doesn't seem to be a vacuum leak (sprayed carb cleaner). I need to do a the leak down test. I checked the temp on each cylinder (with a laser thermometer). Number 1 cylinder is a liter cooler than the rest of the cylinders. Also, I tried to tune the carb with a vacuum gauge. The RPM goes up but the vacuum stays at 15. I have the timing at 14 BTDC and total at 30. The engine runs fine above idle. Thanks
Challenger RTA
Well-Known Member
I would swap that wire and then the plug to an other cylinder, see if it follows. Not that this is it. Check to be sure the distributor vac adv is working.
Challenger RTA
Well-Known Member
Maybe I missed that you said you had a steady vacuum reading. I had engines run like that. Burnt, bent, sticking valve or other. Use an vacuum gauge to diagnose. Process of elimination before you tear into it.
Xcudame
Well-Known Member
15" of vacuum is good! Vacuum gauge is the Mopar secret tool for tuning and diagnosing!
I haven't checked the vacuum advance. The vacuum is steady at 15, at idle. I'll also try the wire swap. Thanks
I pulled the plugs after it warmed up. The plugs were black. I checked again to see if gas was leaking into the carb from the primaries and secondaries. No leak. I drove the car and checked the plugs. They were a light tan color. Gas must be leaking from somewhere at idle, I just can't find the source. Thanks
Bret Schneider
Well-Known Member
- Joined
- May 6, 2020
- Messages
- 134
- Reaction score
- 80
Does the engine oil have a stronger than normal gas smell? Seem like the oil is overfilled?
If that's the case and you're using a mechanical fuel pump there's a chance the fuel pump diaphragm is leaking fuel into the crankcase. Yeah it should leak out the pump weep hole first but that doesn't always happen. Fuel in the oil can cause rough idling since those fumes are pulled into the intake via the PCV and can drive the mixture rich.
If that's the case and you're using a mechanical fuel pump there's a chance the fuel pump diaphragm is leaking fuel into the crankcase. Yeah it should leak out the pump weep hole first but that doesn't always happen. Fuel in the oil can cause rough idling since those fumes are pulled into the intake via the PCV and can drive the mixture rich.
I don't smell any gas in the oil. I just switched from an electric fuel pump to a mechanical pump. I think there's something going on with the carb. Thanks
540HemiCuda
Member
With 15 inches of steady vacuum at idle, the cam is not a big one. That's good idle vacuum. Try more timing at idle and see what happens. Then, if someone has a carb you can borrow for a test, that would be my next step.
It's running a little better after I replaced the plug wires, cap and rotor. I think I'll try the carb next. I did a compression test. #1 is 95, the rest are 120. I also did a leak down test. It sounds like the rings are leaking. No smoke, yet. I plan on a rebuild after I fix a couple of safety issues. Thanks
Litchkar
Well-Known Member
Compression is low in number 1, should not vary more than 15% between adjoining cylinders. I would test another carb. Engine is tired, but that probably isn’t causing the idle issues.
A few things come to mind.
You mentioned tuning the carb with a vacuum gauge. By that I assume that amongst other things you adjusted the idle mixture screws after resetting the float level? Turn in one side at a time until the rpm/vacuum drops and the engine begins to stumble. Now you're running lean. Slowly back out the adjustment screw until your vacuum peaks. Don't back it out any more for the time being. Do the other mixture screw the same way.
What is your idle speed when it's pulling 15 inches of vacuum? Automatic or manual transmission? I think the autos called for 750 RPM idle speed, the manuals 900, but someone on this forum will know for sure. I have an Edelbrock on my 340 manual tranny and it likes 900.
I know you said you checked for vacuum leaks with carb cleaner. Have you tried pinching off any hoses that carry vacuum? Vacuum advance hose. Brake booster hose if so equipped. Anything else that carries vacuum. If you don't want to pinch it then unplug it and cap off the vacuum source. If you do find any vacuum leak, fix the leak then re-adjust your idle mixture.
I realize you don't know what cam you have in the engine so a "normal" vacuum reading is somewhat unknown at this point. As stated earlier aggressive cams will have lower vacuum than mild cams.
You mentioned tuning the carb with a vacuum gauge. By that I assume that amongst other things you adjusted the idle mixture screws after resetting the float level? Turn in one side at a time until the rpm/vacuum drops and the engine begins to stumble. Now you're running lean. Slowly back out the adjustment screw until your vacuum peaks. Don't back it out any more for the time being. Do the other mixture screw the same way.
What is your idle speed when it's pulling 15 inches of vacuum? Automatic or manual transmission? I think the autos called for 750 RPM idle speed, the manuals 900, but someone on this forum will know for sure. I have an Edelbrock on my 340 manual tranny and it likes 900.
I know you said you checked for vacuum leaks with carb cleaner. Have you tried pinching off any hoses that carry vacuum? Vacuum advance hose. Brake booster hose if so equipped. Anything else that carries vacuum. If you don't want to pinch it then unplug it and cap off the vacuum source. If you do find any vacuum leak, fix the leak then re-adjust your idle mixture.
I realize you don't know what cam you have in the engine so a "normal" vacuum reading is somewhat unknown at this point. As stated earlier aggressive cams will have lower vacuum than mild cams.
I'll try another carb. It runs fine above idle and idles better when it's warmed up, but still a little rough. Thanks
After I adjusted the float level, I adjusted the idle mixture screws. I tried the method you mentioned and with a vacuum gauge. The vacuum doesn't seem to change when I make the adjustment. I'm running the idle at 900 (auto trans). I disconnected the vacuum hoses one at a time and plugged the outlet at the carb, no change. I'm going to try another carb. Thanks
Similar threads
