Rough idle at start.

[brown7373]

My 72 C-10 has a GM 350-290 crate engine I installed 3 years ago and has run just a little over 3000 miles. It replaced the original 350. It has run fine until the past 6 months of so.

At start-up after it sits over night or longer, it seems like it is only running on 4 or 6 cylinders. It does this for 10 or 20 seconds, then it quits and smooths out. Once it smooths out, it runs fine and doesn't do it again.

At first I thought maybe sticking lifters, or not pumping up, but that seems unlikely on a new engine. Might it be something electrical, such as a bad coil or cracked distributor cap or rotor. Another thing, which has been since I installed it is HORRIBLE gas mileage. With the original 350 it got 11-13, but this engine gets only about 7!

At install, I rebuilt the Q-jet, installed new plugs and wires and a new distributor with Pertronix.

[RustyPile]

First thing that comes to my mind is the choke pull.. It's a vacuum operated device whose purpose is to crack open the choke blade as soon as the engine starts running. When cold, the choke is fully closed. If the choke pull isn't working, the engine will struggle against the fully closed choke and run extremely rich. Once the choke spring warms up and starts to open the choke blade, the engine runs smoother.

[Steeveedee]

Rusty Pile has given you a first likely place to check. Choke pull-offs have a diaphragm which is perishable because of it being neoprene. Put a vacuum pump on it and see if it holds vacuum.

[brown7373]

I'll check. Thanks

[garyd1961]

I hate to say this but that is one of the worse engines you could put in a truck. It is a low compression motor with an oversized cam. It either needs a smaller cam or better heads to run like it should. The motor's power range is way too high for normal driving.
As for your problem at startup it's probably something to do with the carb. Have you tried patting the gas 1 "one" time before starting. That should set the choke. If it only does it for a few seconds I wouldn't go through a lot of trouble trying th o fix it. I mean I wouldn't buy a new carb or anything.

[brown7373]

I have been told several times about not a great engine for this application, but I am stuck with it. I don't think it is a carb issue. The choke sets up with 1 pump and starts immediately. It has worked fine until a few months ago. Even at that, it starts and runs, except for the usual ru7n on less than 8. A friend and ace mechanic thought sticking or non pumping lifters and suggested an old school trick, a bottle of Risilone. I am going to try that before any more involved procedure, as it is quick and cheap. It does feel like someof the valves are not opening. If electronic, I would think it would do it at other times in addition to cold start.

[brown7373]

My engine continues rough at idle on cold start. Again, I live in south Florida so I am not talking about extreme cold, just sitting overnight 50-60 degrees. I have discovered that if I put it in gear, the engine runs smooth. Shift out of gear and again rough...almost like it has a huge cam. I plan to go through the Q-jet again, but any other ideas? I ease it down the block to the main road and it runs perfectly.

[Grumpy old man]

Pull the spark plugs and give them a look , With only 3000 miles in three years it sounds like your only tooling around town and never putting any highway miles on it and it sounds like it's not been allowed to run long enough to keep the plugs clean . after 3 years it's probably ready for new plugs . I wouldn't jump back into the carb just yet .

[brown7373]

About half those miles are on the interstate at 65-75 mph. I start it (cold) and it is rough. I immediately put in gear and it idles smooth. I can drive away with no issues. So why the rough idle in neutral?

[geezer#99]

Check the weights and springs in the distributor.
Is it an original points distributor with a pertronix conversion or a pertronix distributor?

[brown7373]

Original distributor with Pertronix conversion. By putting in gear it puts a slight load on the engine and the rpms drop.

[fdbdw]

Possibly battery, have it checked maybe it’s not holding a good charge overnight.

[geezer#99]

Originally Posted by brown7373

Original distributor with Pertronix conversion. By putting in gear it puts a slight load on the engine and the rpms drop.

Distributor is likely the culprit.
Found this video. Not sure if it’s a Chevy or not. Same components.
Look at about the 6 minute mark. It shows the points plate and how it’s attached to the fixed plate. Your pertronix module attaches to the plate where your points did. That plate needs to swivel or rotate so it’s attached with that small circlip. If the clip is missing the points plate can move up/down causing your rough running. It changes your timing.
Easy test is try moving the module upward. It shouldn’t move at all. If it does suspect the circlip is missing or fixed plate is loose.
Another possibility is the fixed plate isn’t mounted solidly. It might be held in place by set screws that are installed from the outside on the main body. You need a tiny Allen wrench for those.



https://www.youtube.com/watch?v=l8Xr0hkCyF8

[HO455]

The symptoms you described sound like a mis-adjusted choke. The adjustment settings given in rebuild kits are only going to get the choke in the ballpark. Then it usually takes some fine tuning to get the choke to work correctly on any specific engine combination.
A simple way to check this would be when the truck is cold remove the air cleaner lid. Then have a long thin screwdriver in ready in hand. Have someone start the truck while you watch the choke plate.
When it starts the plate will open part way, if it only opens an 1/8" or so use the screwdriver tip to push the plate open slightly more and the stumble should clear up.
On the other hand if the plate opens more than about 1/4" try closing the plate slightly to see if the stumble clears up.
When the engine is warmed up make sure the choke plate is completely open and that it is being held there firmly. If the plate is able to flop around it will contribute to poor fuel mileage.

Here is a link to how I adjusted mine. It may help you. Scroll to post 618.

http://67-72chevytrucks.com/vboard/s...698377&page=25

If the choke plate doesn't make a difference you can move on to other possible causes without having spent any money.

Back in the day having the choke adjusted was part of the annual maintenance routine.

Good luck and please keep us posted on your findings.

[brown7373]

I think you may be on to something. I will check and readjust the choke settings this weekend.

[rockyrivermark]

Originally Posted by fdbdw

Possibly battery, have it checked maybe it’s not holding a good charge overnight.

How would a low battery give this symptom. Once the engine starts, alternator would supply plenty for the ignition system.
Just curious if there’s something I haven’t heard of before.

[jimijam00]

Choke is too lean

[leftybass209]

Just installing a rebuilt qjet is asking for problems on that crate engine. Is it electric or hot air choke? If the throttle shafts were loose and didn't get bushings, or if the choke linkage isn't bent correctly, it'll give you problems.