Cold Starting Issue

[CRM]

I've been pretty busy at work and didn't have time to post this question earlier. When I go to start the truck (IL6 250c.i. 1 Barrel Rochester carb) I depress the pedal a couple of times and start the engine. It idles very rough and if I don't hold the pedal down for a couple of minutes it basically dies. If I hold the pedal down then eventually it holds itself at a fast idle. Once the temp gauge starts going up I can kick the pedal and it goes down to a normal slow, smooth firing, idle. It's been cold here for a little over a month, and this issue started when the temps dropped. I have limited experience with working on carburetors. Is this a choke adjustment problem, or should I be looking somewhere else?

 

[1987 GMC Jimmy]

It's definitely possible. I would also check for vacuum leaks, and I'd make sure you're getting good fuel. You can look down the throat of the carb and actuate the throttle, and you should get a healthy squirt. Even the slightest movement of the throttle should squirt a little fuel. If those results are sloppy, you may need a new accelerator pump diaphragm in the carb. As far as vacuum leaks go, I'd pay the most attention to the base gasket, but anything that has vacuum lines running to it or the lines themselves can cause issues. If you have an EGR valve on that motor, I'd make sure the valve was working. The old style ones you can push up on the underside where the plunger is, cap your finger over the nipple, and let go. If it stays in place, it's good. If it moves, you need a new one. The new style ones you have to take off, depress the pintle, and then you can do the same test. You can test vacuum leaks with an aerosol like starting fluid by spraying it around the areas in question and seeing if the idle changes, or you can use smoke from a cigar or cigarette. I prefer the latter (even though I'm not a smoker), but you can't test the base gasket that way, just everything else. Back to the choke, you should look for it to be nice and closed when the motor's cold, and it should be vertical once the engine is warmed up. Attached is a link for the adjustment procedures for your carb.
http://www.autozone.com/repairinfo/repairguide/repairGuideContent.jsp?pageId=0900c15280051784

 

[74 Shortbed]

Sounds like choke adjustment, but could be other issues along with it too.

 

[CRM]

Thanks for the input. Once it stops snowing around here and it warms up I'm going to try to check it all out. Could be awhile the way this winter's been going. We've had snow on the ground since December which isn't normal for SE Washington state. Normal years we see snow but it doesn't accumulate on the ground. I don't have a garage to work in, just a gravel driveway, which is covered with 4" of snow and more on the way tomorrow.

 

[Georgeb]

Thanks for the input. Once it stops snowing around here and it warms up I'm going to try to check it all out. Could be awhile the way this winter's been going. We've had snow on the ground since December which isn't normal for SE Washington state. Normal years we see snow but it doesn't accumulate on the ground. I don't have a garage to work in, just a gravel driveway, which is covered with 4" of snow and more on the way tomorrow.

If it is a cold weather issue you may want to consider checking it out when its cold out.
Remove the air cleaner and manually open the throttle all the way. Observe what the choke plate does and report. It should drop all the way closed. If it doesn't the linkage may be gummed up or it may be out of adjustment. Some close up pics will help too. There is a vacuum pull off that is meant to pull the choke plate open a bit once the engine starts. Make sure the line to it is ok and that it moves in and out.

 

[CRM]

OK I went out this morning to check it out. It was 36*. I think I figured it out. Usually I get in the truck , kick the gas pedal 2-3 times and start it up. I seem to remember reading somewhere that the gas pedal should be pushed 1 1/4 times before starting the engine. Today I pulled the air cleaner cover and the plate was wide open (first pic). I pushed the gas pedal one time and the plate closed all the way (second pic). I started the engine and the plate opened just a little (pic 3). There was none of the previous problems. The engine rpm's held steady at a reasonably low rpm.Then after a minute or so the rpm's starting increasing a little until it hit a stable high rpm. Once the engine temperature gauge needle started going up I kicked the gas pedal again and it went back down to the low idle. Is it possible that pushing the gas pedal more than once could have been causing the problem?

 

[MikeB]

You should only have to kick the pedal once to set the choke and fast idle, and to squirt a little gas into the carb. Then you can kick it again if the engine won't start. Kicking it several times could flood the engine a little, but the idle should recover in a few seconds after the excessive fuel is burned.

Try the procedure that worked for you, but do it with the air cleaner top installed. If the problem returns, you may have another issue, for example a crushed hot air pipe that runs from the snorkel to the exhaust manifold.

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[CRM]

Thx Mike. The tube is in good shape. Could there be an issue with the air control valve, vacuum diaphragm, or sensor? You say pushing the pedal once sets the fast idle. Mine doesn't idle fast until the engine has run for at least a minute. Should it immediately go to fast idle?

 

[rich weyand]

Push it once and hold it for a couple seconds, then release the gas and hit the key.

When my choke pivots get dirty, the plate doesn't slam shut, it eases shut. You may not be giving it enough time.

Spraying the choke plate pivots and the choke rod pivots with a little penetrating oil (like Silikroil) doesn't hurt either.

 

[CRM]

Thx Rich. It appeared to slam shut pretty fast but a little lube never hurt anyone. I'll try holding the pedal down tomorrow.

 

[MikeB]

I'm more familiar with late 60s and early 70s engines, where it didn't take a half dozen temperature and vacuum switches to control systems like Thermal Vacuum Delay, Throttle Return Control, and Early Fuel Evaporation. Could be your 77 is dependent on more than just choke housing temperature to control the butterfly or heating to the air cleaner.

Never heard of a choke gradually closing unless it was semi-stuck, like Rich said. But what's weird is it worked OK with the air cleaner lid removed.

Someone may have mentioned this, but make sure the engine doesn't have any vacuum leaks (usually at hoses) and that your EGR valve is working properly.

 

[CRM]

I tried Rich's suggestion without not much success. It was colder this morning than yesterday and the engine idled pretty rough again. I guess I'll have to dig a little deeper.

 

[CRM]

I tried something different this evening. I pushed the gas pedal once to set the choke. Once it started it idled rough. I kicked the pedal like I was trying to kick it off of high idle. After kicking it three times the engine went to high idle and stayed. Another thing that might be important is that the mixture screw in the front of the carb has no plastic retainer on it and I have moved that screw in the past. It's possible that could be the real problem. I've been told to turn it all the way in and then back it out three turns. Hopefully this might help someone come up with a reason for the rough idle.