Newly rebuilt 401 - seems to run a bit rough

kaelen

Hello all! I just finished rebuilding a 401 for my CJ7. I have it running and it pulls VERY strong. It made 360HP and somewhere over 420 ftlbs of torque on the dyno. (once I find the stupid dyno sheet I'll post it) My problem is that below about 1500 rpm it seems to run rough. Between idle and around 1500 it shakes everything pretty good. It almost feels like it is pumping a dead hole. Above 1500 it smooths out and runs beautiful. I've run every test I can think of. Compression (153 psi avg), leakage, checked for vacuum leaks, the vacuum gauge is rock solid 15" at idle. All the plugs look good. Checked ignition and wires. I'm trying to figure out if there is actually something wrong or if that's how my combination "feels". I'm wondering if it could be the RPM air gap manifold causing that. I'm using the XE256 cam which should give a decient idle. Here's the specs 401 bored .040 keith black hypereutectic pistons 9.6:1 compression Comp XE256 camshaft Edelbrock RPM air gap manifold Edelbrock shorty headers MSD billet Distributor MSD ignition Holley 670 truck avenger Any suggestions or opinions would be greatly appreciated. PS>.. Anyone else in Montana??

jeepsr4ever

What carb are you running?

kaelen

I'm running a new Holley truck avenger 670 cfm. I jetted it 1 leaner for altitude. I also tried a edelbrock 1406 off my 383 just to see what happens. It changed it a little bit but not much. It smoothed out a little quicker. It ran the worst with a edelbrock thunder AVS. I've had problems with lean spots on those stupid edelbrocks with every AMC i've built. (1 304, 2 360's, 1 401). No lean spot with holley! I've been thinking that my problems are possibly mixture related. But my brand new LC-1 wideband is on the fritz and I'm having to send it in.

ed455

It sounds idle mixture related. Does it improve or die if you pull manifold vacuum hose off? Mixture is usually pretty close if you remove a small vacuum cap or hose off manifold vacuum and the idle gets a little rough. It's been my experience that the idle you describe is often a lean condition caused by mixture or vacuum leak.

kaelen

The idle goes way up if I pull a vacuum hose. I swapped my Lc-1 from my chevy truck. It is running REALLY rich. 11:1. I'm going to go down another jet size or two. The idle mixture screws don't have too much effect. I can screw them all the way in and it keeps running. I double checked the powervalve and gasket and it isn't leaking.

ed455

Jetting has NOTHING to do with idle. Extra fuel is coming from somewhere OR you're already into the transition circuit in the carb if the idle screws do nothing. I know you tested it, but I would replace the power valve anyway. I had one that tested good and was still the problem. I actually washed down a motor with a bad power valve and had to re-ring. You may have to drill the butterflies (google it) to get your mixture right without getting to the transition circuit.

kaelen

Thanks, I do know that jetting doesn't effect the idle mixture. I guess I didn't explain fully in my last post. It is running really rich off idle also. It is running 11:1 at cruising speed. I've dropped 2 jet sizes already and I am going to go one more. Changing the jets has also made a improvement just off of idle as well. I've replaced the power valve to help a lean spot and it is doing the same thing with the new one. I'm thinking it is getting into the transition circuit. The idle screws actually did something when my defective PCV valve was in there. The thing was wide open. No wonder it was sucking oil from the galley.

5150

My 360 did the same thing. It turned out to be a bad power brake booster.It also affected part throttle cruise up to about 50 mph(2000 rpm).

5150

You stated that your idle went up when you pulled a vacuum hose.When I pulled the vacuum hose from my brake booster,my motor died.It seems like you,re getting fuel from somwhere you should,nt be.

1980_Cj7

Check your throttle linkage and your curb idle adjustment and make sure they are not preventing your carb from closing all the way.

ironman_gq

back the idle speed screw off so it will just barely stay running and then turn your idle screws in and see if that helps. It should let your carb get out of the transfer slot enough to adjust the idle mix :t:

kaelen

Thanks, I'll give that a try. The darn thing worked the best when I had forgot to hook back up the brake booster.

kaelen

Is it possible the 670 CFM Truck Avenger is too small for my engine? Should I have gone with the 770?

ed455

No, the size is fine. Maybe a little small for the top end horsepower but for regular street use, it should be fine. Smaller carbs favor bottom end torque which is king on the street and crawling. I personally would run bigger, but I'm a carburetor neanderthal from the drag racing world. I'm my native realm, cams and carbs that are too big, are just right. (Grunt, grunt)

kaelen

Thanks all for your suggestions and advice.

I'm finally getting this thing figured out. :!:

The primary throttle plates were open too far and I was into the transition circuit. I had over .040 of the slot showing and there only should be around .020.

To fix it I closed the primarys down to just cracked and adjusted the secondary plates to bring the idle back up.

There is a small set screw on the right side base of the carb. You get to it from underneath. Depending on your intake you may have to remove the carb to access it. I lucked out and didn't. This adjusts the idle stop of the secondarys.

I was then able to actually adjust the idle screws. It runs much smoother now. Plus my cruising vacuum went way up as well!!

Now as soon as my new brake booster arrives this thing will be idling smoothly!! Everytime I step on the brakes the idle goes up. 🙁:

Ps.. anyone looking for hard to find brake booster for the older CJ7s. These guys have them http://www.priorreman.com/ . $100 + 50 core..

1980_Cj7

1980_Cj7 wrote
Check your throttle linkage and your curb idle adjustment and make sure they are not preventing your carb from closing all the way.

So what do I win??? Glad you got it figured out and it was something so simple.