Help!!!1968 6cyl w/New exhaust manifold and rebuilt carb. Runs like crap - The 1947 - Present Chevrolet & GMC Truck Message Board Network

RED72blazer · 08-27-2012, 05:12 PM

So I have a 1968 c10 with a 250 straight six and a monojet carb. It has 86,000 on it and is a very original truck. I recently replaced the exhaust manifold and rebuilt the carb. The truck ran VERY well before any of this, except One major problem with it was that the original exhaust manifold was warped and wouldn't seal. It had a horrible exhaust leak. ordered a replacement manifold(very nice quality btw)but it doesn't have the heat riser valve. While I had everything apart I had the carb rebuilt. It wasn't having any problems, but I wanted to prevent anything from going bad and leaving me stranded. I had this done by a shop I have had do at least 10 other carbs for me. I have never had a problem before with their work.

So now with everything back together the truck is whisper quiet, which is awesome. I have the idle screw all the way out, but it won't idle below 800-850 rpm. When you pull out the choke cable the truck starts to stumble and will die. it will also blow a lot of black smoke. If you look down the throat after doing this there is condensation on the throttle blade. If you get the choke cable in fast enough it will stay running but is idling at 1000-1100 rpm and will blow black smoke for a second or two. I can go under the hood and pull back the choke linkage and it will return to idle. If I change the choke linkage around. I can get the truck to idle at 600rpm. But the choke still doesn't work and the linkage looks close to binding. I included a picture of a set up similar to how mine is. I just don't have that big thing at the idle screw hole. If I change the choke to the single hole on the bottom arm is how I get it to idle lower than 800.

Any ideas? I've checked for vacuum leaks and haven't found any. I am going to drive the truck up to the carb shop tomorrow and have them take a look. I am worried that the choke doesn't work because of the lack of heat plate in the manifold. Or maybe they assembled things wrong. or????

thanks for any help.

airdale94 · 08-27-2012, 06:32 PM

Just unhook the choke completely and make sure it stays open. Once it's warmed up a little if it runs fine it's the choke, if it doesn't something in the carb. is messed up. I would say the the float or the needle and seat would be the first check. The shop should have sorted this all out for you though.

68gmsee · 08-27-2012, 07:21 PM

I doubt it's the heat riser. It's primarily for cooler weather to help warm up the carb/intake manifold faster. Most of those would rust open or shut and most people would just cut them out or wire them open if they were able to do so.

As mentioned above, disconnect the choke and see what it does.

GuyO · 08-27-2012, 11:38 PM

Um, sounds like it is a timing issue. Turn the idle air mixture all the way in until it seats, then turn it out 2 full turns. The choke is confusing the issue, disconnect for now then chock the wheels, unplug your vacuum advance but plug off the carb (no vacuum), set the timing to 8 dbtdc, set the idle rpm to 600, turn idle mixture in until the engine RPM drops then back it out 1/2 turn. Recheck timing, reset idle speed to 600, reconnect the choke so that the linkage does not interfere with the throttle assembly.

blakeaz33 · 08-27-2012, 11:45 PM

May not even be related since you had your carb rebuilt but I had a problem similar with the same engine carb combo and the factory fuel filter that is in the carb bowl where the the fuel line bolts on came apart and kept the truck from running at anything over an idle by starving the truck of fuel. Drove me crazy until I figured that one out. Either way got to be spark or fuel. You'll get it figured out.

Wrenchbender Ret · 08-28-2012, 04:05 PM

When the choke is off the fast idle cam should not be contacting the throttle shaft lever. You should be able to adjust the idle speed with the speed screw or the selinoid that is in the photo.
I agree the manifold heat rizor will not cause this problem.
They could have the cam upside down. Sometimes the gasket between the fuel bowl & throttle body is not in correct or they could have usaed the wrong one, as 2 different gaskets come in the kit.

08-27-2012, 05:12 PM	#1
RED72blazer urban fourwheelin Join Date: Aug 2002 Location: Seattle Posts: 1,007	Help!!!1968 6cyl w/New exhaust manifold and rebuilt carb. Runs like crap So I have a 1968 c10 with a 250 straight six and a monojet carb. It has 86,000 on it and is a very original truck. I recently replaced the exhaust manifold and rebuilt the carb. The truck ran VERY well before any of this, except One major problem with it was that the original exhaust manifold was warped and wouldn't seal. It had a horrible exhaust leak. ordered a replacement manifold(very nice quality btw)but it doesn't have the heat riser valve. While I had everything apart I had the carb rebuilt. It wasn't having any problems, but I wanted to prevent anything from going bad and leaving me stranded. I had this done by a shop I have had do at least 10 other carbs for me. I have never had a problem before with their work. So now with everything back together the truck is whisper quiet, which is awesome. I have the idle screw all the way out, but it won't idle below 800-850 rpm. When you pull out the choke cable the truck starts to stumble and will die. it will also blow a lot of black smoke. If you look down the throat after doing this there is condensation on the throttle blade. If you get the choke cable in fast enough it will stay running but is idling at 1000-1100 rpm and will blow black smoke for a second or two. I can go under the hood and pull back the choke linkage and it will return to idle. If I change the choke linkage around. I can get the truck to idle at 600rpm. But the choke still doesn't work and the linkage looks close to binding. I included a picture of a set up similar to how mine is. I just don't have that big thing at the idle screw hole. If I change the choke to the single hole on the bottom arm is how I get it to idle lower than 800. Any ideas? I've checked for vacuum leaks and haven't found any. I am going to drive the truck up to the carb shop tomorrow and have them take a look. I am worried that the choke doesn't work because of the lack of heat plate in the manifold. Or maybe they assembled things wrong. or???? thanks for any help. Attached Images __________________ 1972 K5 Blazer with Cummins 4BT, NV4500 swap in the parts gathering stage.

08-27-2012, 06:32 PM	#2
airdale94 Registered User Join Date: Jan 2007 Location: Topeka,KS Posts: 778	Re: Help!!!1968 6cyl w/New exhaust manifold and rebuilt carb. Runs like crap Just unhook the choke completely and make sure it stays open. Once it's warmed up a little if it runs fine it's the choke, if it doesn't something in the carb. is messed up. I would say the the float or the needle and seat would be the first check. The shop should have sorted this all out for you though.

08-27-2012, 07:21 PM	#3
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Help!!!1968 6cyl w/New exhaust manifold and rebuilt carb. Runs like crap I doubt it's the heat riser. It's primarily for cooler weather to help warm up the carb/intake manifold faster. Most of those would rust open or shut and most people would just cut them out or wire them open if they were able to do so. As mentioned above, disconnect the choke and see what it does. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

08-27-2012, 11:38 PM	#4
GuyO Registered User Join Date: Nov 2011 Location: Kirkland, WA Posts: 343	Re: Help!!!1968 6cyl w/New exhaust manifold and rebuilt carb. Runs like crap Um, sounds like it is a timing issue. Turn the idle air mixture all the way in until it seats, then turn it out 2 full turns. The choke is confusing the issue, disconnect for now then chock the wheels, unplug your vacuum advance but plug off the carb (no vacuum), set the timing to 8 dbtdc, set the idle rpm to 600, turn idle mixture in until the engine RPM drops then back it out 1/2 turn. Recheck timing, reset idle speed to 600, reconnect the choke so that the linkage does not interfere with the throttle assembly.

08-27-2012, 11:45 PM	#5
blakeaz33 Registered User Join Date: Nov 2007 Location: Scottsdale, AZ Posts: 152	Re: Help!!!1968 6cyl w/New exhaust manifold and rebuilt carb. Runs like crap May not even be related since you had your carb rebuilt but I had a problem similar with the same engine carb combo and the factory fuel filter that is in the carb bowl where the the fuel line bolts on came apart and kept the truck from running at anything over an idle by starving the truck of fuel. Drove me crazy until I figured that one out. Either way got to be spark or fuel. You'll get it figured out.

08-28-2012, 04:05 PM	#6
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: Help!!!1968 6cyl w/New exhaust manifold and rebuilt carb. Runs like crap When the choke is off the fast idle cam should not be contacting the throttle shaft lever. You should be able to adjust the idle speed with the speed screw or the selinoid that is in the photo. I agree the manifold heat rizor will not cause this problem. They could have the cam upside down. Sometimes the gasket between the fuel bowl & throttle body is not in correct or they could have usaed the wrong one, as 2 different gaskets come in the kit.



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