1986 K10 Hard to start when warm

[wheezer]

Have an 86 K10 305CI. Bone stock, original. Thought I’d pick yalls brains on this. I am by no means an expert wrencher, can do the basics. So here we go...

She fires right up every time when it is cold after pressing the gas once. If you drive it around town and temp gets to normal range of 185* and come back to fire her up, it turns over. Sometimes it will catch after 1-2 turns, but if you miss that window, she turns over and doesn’t catch. I noticed that when she catches, small white puff smells like gas that hasn’t burned comes out of the exhaust

Here’s what I’ve done so far, but little else since she runs good as is:

Battery is new, starter is strong. Choke appears to be vertical when I pull the filter out. Also, she tends to fire up better when the filter is removed. If you let her sit for about 1 hour, she fires right up. Lol.

At this point, she runs extremely well considering it’s 30+ year old original motor.

Ideas on how to get her to fire up better when it is warm? Don’t want to be stranded at a parts store waiting for her to cool down and end up buying a basketful of parts and gadgets while waiting for her to cool down.


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[Dutch Rutter]

Just because I know someone will ask this but since you said its an original motor we assume it has a Quadrajet carb. Is that correct?

 

[1987 GMC Jimmy]

Really dumb question here, but you’re not pumping the gas when it’s warm and flooding it, are you? On my Quadrajet, I’ll do two pumps when it’s cold, and I won’t pump it again until the next day. Sometimes if I’m driving it the same day, but it’s been like ten or twelve hours, I’ll give it half a shot, and that works fine. If I remember correctly, the 1986 Chevy Pickup owner’s manual says to pump and hold the throttle at about a quarter. I’ve never done all that, but I’ve never found that to be reprehensible.

The choke is vertical when it’s warm? And it looks fine cold?

Is the timing in a good spot? Mine won’t do well without premium gas over 5* BTDC. I think 0-5 is the range you should shoot for in a 305.

If you have a Quadrajet, it’d be a good idea to check the acc. pump and overall carb cleanliness by looking down in the carb and moving the throttle. You should see two good streams of fuel start the instant you move the throttle lever. If you haven’t changed the fuel filter recently or even ever, it’d probably be worth doing. Quadrajets have it behind the flare fitting, and just remove the big bolt. Be careful because the metal’s soft, and there’s a spring in the filter housing. If you don’t have factory fuel lines, make sure your lines aren’t draped over or near anything really hot like exhaust.

Finally, make sure it’s running at the correct temperature and not too hot. Should be 195* unless someone put a cooler stat in there. The gauge is good as a monitoring/warning mechanism, but you can’t get more accurate than an infrared thermometer pointed at the side of the cylinder head where the sender goes.

 

[Snoots]

If you never done anything to the carb:

#1-The well plugs in the fuel bowl are leaking

#2-The carb pump gasket is shot by todays' ethanol fuel mixes

 

[wheezer]

Just because I know someone will ask this but since you said its an original motor we assume it has a Quadrajet carb. Is that correct?

I will double check when it’s daylight tomorrow. I can take some pics of the carb to help y’all confirm.


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[wheezer]

Really dumb question here, but you’re not pumping the gas when it’s warm and flooding it, are you? On my Quadrajet, I’ll do two pumps when it’s cold, and I won’t pump it again until the next day. Sometimes if I’m driving it the same day, but it’s been like ten or twelve hours, I’ll give it half a shot, and that works fine. If I remember correctly, the 1986 Chevy Pickup owner’s manual says to pump and hold the throttle at about a quarter. I’ve never done all that, but I’ve never found that to be reprehensible.

The choke is vertical when it’s warm? And it looks fine cold?

Is the timing in a good spot? Mine won’t do well without premium gas over 5* BTDC. I think 0-5 is the range you should shoot for in a 305.

If you have a Quadrajet, it’d be a good idea to check the acc. pump and overall carb cleanliness by looking down in the carb and moving the throttle. You should see two good streams of fuel start the instant you move the throttle lever. If you haven’t changed the fuel filter recently or even ever, it’d probably be worth doing. Quadrajets have it behind the flare fitting, and just remove the big bolt. Be careful because the metal’s soft, and there’s a spring in the filter housing. If you don’t have factory fuel lines, make sure your lines aren’t draped over or near anything really hot like exhaust.

Finally, make sure it’s running at the correct temperature and not too hot. Should be 195* unless someone put a cooler stat in there. The gauge is good as a monitoring/warning mechanism, but you can’t get more accurate than an infrared thermometer pointed at the side of the cylinder head where the sender goes.

Good point & advice. I think I might be flooding the carb accidentally by pumping the gas if it doesn’t catch. I will try your advice first - pump 1x then don’t pump again until the next day when it is same as air temp.

Choke is vertical when warm. I have not checked when cold - I’ll do that in the morning.

For timing, I will have to check that. I don’t have a timing light.

As mentioned in previous post, I’ll take some pics of carb when I take off the filter cover.


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[wheezer]

If you never done anything to the carb:

#1-The well plugs in the fuel bowl are leaking

#2-The carb pump gasket is shot by todays' ethanol fuel mixes

I avoid the ethanol based fuels whenever possible. I run ethanol free 90 octane or better. It doesn’t run well when I use 87, it tends to diesel after ignition is cut off - maybe that may be related to why she’s hard to start when warm...

I use GasBuddy to find ethanol free gas - if that helps y’all any.


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[1987 GMC Jimmy]

I avoid the ethanol based fuels whenever possible. I run ethanol free 90 octane or better. It doesn’t run well when I use 87, it tends to diesel after ignition is cut off - maybe that may be related to why she’s hard to start when warm...

I use GasBuddy to find ethanol free gas - if that helps y’all any.


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Yep, this is what my 305 does above 5* BTDC without premium gasoline. Less than that, I can run 87 and be fine.

 

[86suburbanNJ609]

After my truck is warmed up it also doesn't like to start right up I ease the throttle 1/4 down while cranking starts right up.
Edlebrock 1406 though I don't have experience with qjet.

 

[1987 GMC Jimmy]

Good point & advice. I think I might be flooding the carb accidentally by pumping the gas if it doesn’t catch. I will try your advice first - pump 1x then don’t pump again until the next day when it is same as air temp.

Choke is vertical when warm. I have not checked when cold - I’ll do that in the morning.

For timing, I will have to check that. I don’t have a timing light.

As mentioned in previous post, I’ll take some pics of carb when I take off the filter cover.


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By the way, when I said I don’t pump it again, that’s when I’m only running errands in the afternoon or going back and forth throughout the day, and the car doesn’t sit for more than 8 hours at a time. More than that, I’ll give it half a pump like I mentioned.

 

[HotRodPC]

All my suggestions have been made. #1 being timing. Then the well plugs and choke.

 

[Rusty Nail]

Hi!

If it was ME, I would buy a new MOTORAD gas cap and install a new fuel filter.

Then I would pull the codes from the computer's ALDL.
All you need is a paperclip!

If I was gonna bet, I blame the vacuum thermo-switch dealy on the thermostat housing. I wager the computer believes the operating temperature is cold and it is attempting to start under an imposed choked condition - that retards ignition timing.

 

[1987 GMC Jimmy]

This isn’t a California CCC truck, is it?

 

[Rusty Nail]

And being that close to the ocean his MAP sensor input is very important and it's connected by vacuum line to that thermo switch on the t-stat housing I think.
He didn't say anything about a new thermostat but it's EASY to lose that ground there messin with that stuff.
The output voltage of that switch is probably LOW.

I don't know much about it..only a guess on the internet! .

 

[wheezer]

Update. Tried not pumping gas, fires right up after being warm. Will have to keep eye out.

Below are pics of carb and label in glovebox. I have the high altitude emission, code NA6.

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