Help! Carburetor on 250 I-6

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

JohnTaurus

Full Access Member
Joined
Mar 25, 2020
Posts
152
Reaction score
158
Location
Mississippi
First Name
John
Truck Year
1974
Truck Model
C10 Custom
Engine Size
250 CID Inline 6
Hey guys, first off, let me say that I hate carburetors. It was my only hangup about buying this truck, and sure enough, its proving to be just as troublesome as those I've had on imports and Fords.

This is not the factory carburetor. It came from O'Reilly auto parts and they couldn't give me much info on it. My cousin bought it, and he was there, but they really had nothing for it. Carburetor is not old, maybe 1-2 years.

Issues:
Will not start in the morning without a shot of gas down it's throat. Fuel pump replaced yesterday with Delphi unit, no help. Fuel filter inspected and cleaned as well.

Now, it doesnt like starting warm without pumping the throttle, where as before, it would bust off quickly and idle great after having been shut off for an hour or more, all without touching the throttle. It sat less than 10 minutes and didnt start.

It seems to idle up at 1500-2000 rpms after driving (like coming to a stop at the end of a road). I have to punch the throttle and then it idles down to 1000. Just like my old Mercury, but it was normal operating when doing a cold start. It would idle high for a bit, then you kick the gas pedal and it would idle down to normal and was ready to drive. This carburetor is doing it when the truck is at normal operating temperature, after having been driven 20 miles or more! I have replaced the throttle cable, which was old, cracked and was sticking way up after being driven. Although it doesn't stick at nearly as high RPM, I still have to stab the pedal so I'm not running 2000 RPM waiting at an intersection.

It also "diesel's out" and sometimes it makes this loud SHHHH noise just as it finally does go dead. This afternoon when it did that, there was visible smoke as though it had been blown out.

I am obviously quite the novice when it comes to carburetors. Probably 90% of my car history has been with EFI Systems from the 80s-up, and it seems that every carburetor-eqquipped vehicle I buy, it's a persistent problem. The best was my 1983 Mercury Zephyr, and of the 10s of thousands of miles I put on it, it only had an issue in high elevations and low temperatures. But, still, never had that problem in an old 90s Ford Tempo or Honda Accord with EFI.

All that said, I believe this carburetor is a piece of s#¡t. Any insight would be greatly appreciated, guys. And, if a new carburetor is required (I can not afford an EFI setup right now, and do not wish to park the truck in the meantime), please suggest one for me, within a reasonable price range.

Truck details:
1974 Chevrolet C10
250 c.i./4.1L Inline 6
electronic distributor and alternator conversion done by previous owners
3 speed manual
Aftermarket carburetor, probably the cheapest one O'Reilly's had.

Just in case it matters, truck was sold new and still resides in Mississippi, so nothing to do with California-specific emissions equipment.

I will take pictures of the carburetor if need be. Maybe some vacuum hose is wrong. I'm honestly clueless about this stuff (to do with carburetors and so on).
 

SirRobyn0

Full Access Member
Joined
Dec 6, 2019
Posts
6,755
Reaction score
11,402
Location
In the woods in Western Washington
First Name
Rob
Truck Year
1984
Truck Model
C20
Engine Size
305
I'd definitely like to see pictures that might help. My initial thoughts. #1 look for vacuum leaks, #2 make sure the choke is closing like it should when cold and also opening when it's warm. I'm pretty good with carburetors and have done work on them professionally, so there's a pretty good chance we can figure this out.
 

HotRodPC

Administrator
Staff member
Admin
Joined
Aug 29, 2010
Posts
47,128
Reaction score
9,299
Location
OKC, OK
First Name
HotRod
Truck Year
85 K20 LWB
Truck Model
Silverado
Engine Size
454 - Turbo 400 - 3.73
Sounds to me you problem is the automatic choke is hanging up. Is it electric choke or what? Yep, a pic of it would be helpful to see the linkage and such.
 

HotRodPC

Administrator
Staff member
Admin
Joined
Aug 29, 2010
Posts
47,128
Reaction score
9,299
Location
OKC, OK
First Name
HotRod
Truck Year
85 K20 LWB
Truck Model
Silverado
Engine Size
454 - Turbo 400 - 3.73
Did cuz turn in the OEM carb as a core? I have a feeling he should have kept it and had it built.
 

JohnTaurus

Full Access Member
Joined
Mar 25, 2020
Posts
152
Reaction score
158
Location
Mississippi
First Name
John
Truck Year
1974
Truck Model
C10 Custom
Engine Size
250 CID Inline 6
I'd definitely like to see pictures that might help. My initial thoughts. #1 look for vacuum leaks, #2 make sure the choke is closing like it should when cold and also opening when it's warm. I'm pretty good with carburetors and have done work on them professionally, so there's a pretty good chance we can figure this out.

Thanks for your response. I did check for vacuum leaks and found a couple. It seemed to start okay, but still running at 2000 RPMs until I kick it down with the throttle.

I believe this carburetor is setup for a manual choke, but nothing is hooked up to the linkage.

I noticed when it was running exceptionally poor last week, that when I revved it up with the throttle under the hood, the pin that sticks out of the brass-colored thing moved inwards and then returned to where it was when I let off. The rpms were at there peak when it did this (but I wasnt really going that high). I'm wondering if vacuum being hooked to a non-functioning choke could cause this? Maybe my cousin accidentally put the hose in the wrong place?

I will take pictures and post then this evening. Thanks again.
 

Bextreme04

Full Access Member
Joined
May 13, 2019
Posts
4,439
Reaction score
5,572
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
It sounds like the carb is set up for the factory divorced choke setup and it might not be connected. I had mine come disconnected on my 74 and it had this behavior. You should have a choke spring bolted to the intake manifold with a lever sticking out like the image below. The rod goes through the slot above it and then has a jesus clip that locks it in there. If yours looks like the image, thats your problem. Connect the rod and it should start working correctly.
You must be registered for see images attach
 

SirRobyn0

Full Access Member
Joined
Dec 6, 2019
Posts
6,755
Reaction score
11,402
Location
In the woods in Western Washington
First Name
Rob
Truck Year
1984
Truck Model
C20
Engine Size
305
Thanks for your response. I did check for vacuum leaks and found a couple. It seemed to start okay, but still running at 2000 RPMs until I kick it down with the throttle.

I believe this carburetor is setup for a manual choke, but nothing is hooked up to the linkage.

I noticed when it was running exceptionally poor last week, that when I revved it up with the throttle under the hood, the pin that sticks out of the brass-colored thing moved inwards and then returned to where it was when I let off. The rpms were at there peak when it did this (but I wasnt really going that high). I'm wondering if vacuum being hooked to a non-functioning choke could cause this? Maybe my cousin accidentally put the hose in the wrong place?

I will take pictures and post then this evening. Thanks again.

I'll be home kind of late tonight, but I'll be sure to log on and look for your pictures, I think that would really help.
 

JohnTaurus

Full Access Member
Joined
Mar 25, 2020
Posts
152
Reaction score
158
Location
Mississippi
First Name
John
Truck Year
1974
Truck Model
C10 Custom
Engine Size
250 CID Inline 6
I'm sorry I didnt get in until now, but I took these pictures while stopping along the way for a minute with my cousin.

You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,693
Reaction score
6,865
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
Its so cuteeeeee

Get a monojet for it. Electric choke.
 

JohnTaurus

Full Access Member
Joined
Mar 25, 2020
Posts
152
Reaction score
158
Location
Mississippi
First Name
John
Truck Year
1974
Truck Model
C10 Custom
Engine Size
250 CID Inline 6
I see that is all I got (that turned out okay), but I am off work tomorrow and can provide more if needed.

I plugged two vacuum hoses, one that runs from the carburetor to a metal tube that circles around the valve cover (kinda with the hard fuel line) and ended there. I live a long ways away from town, so I improvised by using some scrap (but new) vacuum line with a bolt in it to serve as a plug.

To my own credit (lol), it did start this morning without gas or starting fluid in the carb. I attempted to start it as I always do, and it sputtered for a but (barely running, reading less than 500 RPMs) and conked out. I tried starting it again, and it ran rough at first, then I feathered the throttle like I do to keep it from choking out when its cold, and it ran. I was thrilled.

However, there is still the problem of wanting to stick at 2000 RPMs anytime throttle is applied. You gotta jab the gas to get it to idle down. It flares when shifting from gear to gear, unless I kick it first before pushing the clutch in. I'm sure it sounds to passersby like I'm a noob at driving a manual transmission, lol!

I don't get why it's doing that. The only thing I can think of, cost wise in mind, is to buy a rebuild kit and let someone who knows carburetors rebuild it for me.

When it "diesel's out", it seems to sorta backflow through the carb. It blew a vacuum hose off yesterday when it did it with the visible smoke "puff". That can't be right.

Thanks again for your help, guys. Love this old truck and I have been driving it A LOT. I put 300+ miles on it last week.
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,693
Reaction score
6,865
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
It sure sounds like your choke and kick down arent working right.

Have you adjusted them? Usuaully the spec is to adjust the choke so that it is partially closed, and I mean like a couple degrees off full open so that way spring tension can pull it closed. For example, if there is no spring tension and its open when cold, it wont run right at all until its warm. Versus if it pulls itself closed and opens with temperature, it will run. The hanging RPM sounds like manual kickdown. Usually you jab the accelerator when starting cold to squirt some gas and set the choke, then it will start at a high RPM to encourage warm up and when you go to drive, jab it to lower the RPMs. At least this is how my ford with a manual kickdown and kickup worked. Some Qjets have a automatic pull off and systems that do that automatically iirc once set initially. You have a 1 barrel of course.
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,693
Reaction score
6,865
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
I'm sorry I didnt get in until now, but I took these pictures while stopping along the way for a minute with my cousin.

You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach
I may be an idjit but that sure looks like your intake has a clockspring and your carburetor has an unused linkage on the choke pull off, maybe for a spring or attaching to that clock spring?
 

JohnTaurus

Full Access Member
Joined
Mar 25, 2020
Posts
152
Reaction score
158
Location
Mississippi
First Name
John
Truck Year
1974
Truck Model
C10 Custom
Engine Size
250 CID Inline 6
It sure sounds like your choke and kick down arent working right.

Have you adjusted them? Usuaully the spec is to adjust the choke so that it is partially closed, and I mean like a couple degrees off full open so that way spring tension can pull it closed. For example, if there is no spring tension and its open when cold, it wont run right at all until its warm. Versus if it pulls itself closed and opens with temperature, it will run. The hanging RPM sounds like manual kickdown. Usually you jab the accelerator when starting cold to squirt some gas and set the choke, then it will start at a high RPM to encourage warm up and when you go to drive, jab it to lower the RPMs. At least this is how my ford with a manual kickdown and kickup worked. Some Qjets have a automatic pull off and systems that do that automatically iirc once set initially. You have a 1 barrel of course.
Yes, its EXACTLY like the "kick down" when warming up a carbureted engine, but does it all the time. Anytime you so much as breathe on the throttle, it back up there until you "kick" it down.
 

JohnTaurus

Full Access Member
Joined
Mar 25, 2020
Posts
152
Reaction score
158
Location
Mississippi
First Name
John
Truck Year
1974
Truck Model
C10 Custom
Engine Size
250 CID Inline 6
I may be an idjit but that sure looks like your intake has a clockspring and your carburetor has an unused linkage on the choke pull off, maybe for a spring or attaching to that clock spring?

Right, that's what I tried to explain earlier: that the choke doesn't seem to be hooked to anything, linkage wise (It does have a vacuum line running to it).

However, the truck was starting fairly normally since I started driving it in February, it only started having this no-start-when-cold and sticky throttle here lately.
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,693
Reaction score
6,865
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
maybe linkage fell off.
You must be registered for see images attach
 

Forum statistics

Threads
44,079
Posts
948,937
Members
36,151
Latest member
rlfdavin
Top