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1984 GMC Sierra runs bad when warmed up
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grayta
User
Jun 29, 2011, 12:03 PM
Post #1 of 6
(2826 views)
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1984 GMC Sierra runs bad when warmed up
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I have a 1984 GMC Sierra 4.1 Liter(250 CID) with 106K miles. When I bought the truck, it had a cracked head. I replaced the head and can't seem to get it right. When it is cold, it will fire right up and runs good. Once it is warmed up-185 to 190 on the temp gauge, it starts running bad. It will "cough" through the carb, and sometimes has a timing ping to it. I have replaced the wires, plugs, fuel pump, distributor, cap and rotor, rebuilt the carb-twice, thermostat, put new pushrods in with the head, and adjusted the valves/rocker arms several times. I am not sure what else it could be. I talked to several different people and every one said float on the carb is sticking, but I had a gut who builds racing engines rebuild it the last time, and it is still thew same. It runs good with plenty of power, until it gets warmed up. Any help or suggestions will be appreciated. Thanks.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jun 29, 2011, 12:40 PM
Post #2 of 6
(2820 views)
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Re: 1984 GMC Sierra runs bad when warmed up
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Open to several things but seems it tolerates the "rich" period for starts and initial driving and probably too rich for when warmed up.
OK: You've been at carb a few times and may be setting it dry and kits I ever bought didn't come with a new float. They can get heavy such that the setting would be too rich but unseen during a rebuild which is mostly cleaning and settings as you know.
No more info for specs for this but there is a known weight of the float or it sinks and fuel level is higher than you wanted. I doubt it even for a 1984 but probably a plastic float vs the old hollow brass ones you could just shake in hand and feel fuel inside.
If clever and good, you can run the engine with top of most carbs and with a finger in the right spot watch the level but go easy and NOT drop anything!
Other if all else seems to check out is vacuum leaks even any diaphragms anywhere but more so (I think) on carb must be right. Choke must completely shut off too.
You can buy just a float (I think, even at the age) and unlikely for it to be a stock item to compare weights at the counter.
Also, watch real, actual intake manifold vacuum on a "T" so device controlled still has it's vacuum. Should hold steady at idle and read the same at like 2,000 RPMs with no load. At sea level should read about 18Hg and the same at 2,000 and smooth or there are things to chase down..............
T
PS: Don't have tons of info in this specifally but head bolts might be one time use only and new ones needed if head was off for any reason...........
(This post was edited by Tom Greenleaf on Jun 29, 2011, 12:45 PM)
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grayta
User
Jun 29, 2011, 12:55 PM
Post #3 of 6
(2813 views)
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Re: 1984 GMC Sierra runs bad when warmed up
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The float is plastic, and when I first took it apart I did shake it to see if it had any gas init-it didn't. I will check the manifold vacuum and then try to watch the float and see what there is to see. I removed most all the vacuum lines and plugged the ends earlier because I thought that might be the problem. I will double check and see what I have actually hooked up and let you know what happens. Thanks.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jun 29, 2011, 1:55 PM
Post #4 of 6
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Re: 1984 GMC Sierra runs bad when warmed up
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Did a lot of carbs back when. The composite plastic floats should not be hollow so you wouldn't hear gas in them. Never did cut one open but pretty sure they are solid but can absorb gas and weight is wrong so they sit too low in the bowl making intermediate and WOT throttle too rich.
The idle mixture screw was probably made tamper proof but can be defeated and need to to clean it out. Trucks may not have done so much of that but cars did. Remember that screw is only for idle to intermediate throttle then does nothing for full power then jets with exact size hole and fuel level in bowl dictates.
Is choke shutting down completely? That can screw up the show for sure,
T
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grayta
User
Jun 29, 2011, 5:33 PM
Post #5 of 6
(2796 views)
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Re: 1984 GMC Sierra runs bad when warmed up
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I am going to get a new float and see if that takes care of it. It does run way too rich when warmed up and I didn't think about it absorbing gas and getting too heavy. This makes sense for what it is doing. I will let you know after I have replaced it. Thanks.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jun 30, 2011, 2:41 AM
Post #6 of 6
(2783 views)
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Re: 1984 GMC Sierra runs bad when warmed up
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Deleted a post for an escort service. Go for it if you want but not part of this forum,
Tom
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