Help starting my c20

[Socalrider87]

Long story short i got a new Jet carb to replace my old warped Quadrajet. Once i installed it the truck will fire up and die in about 2 seconds. If i hold the throttle down a decent amount it will stay on but once i go lower or let go it just dies. I replaced the fuel pump as well and still the same. I screwed the adjustment needles in and back out to match the old carbs setting and still nothing. Can anyone give some advice on what to check or adjust? Like i said with the old carb it would run fine just leaked fuel all over lol but with the new one i can’t keep the damn thing running.

‘70 C20 Longhorn
Factory 402

[geezer#99]

Might need to adjust your choke.
Post a pic of the choke.
We might see something amiss.

[RichardJ]

Carburetors are not plug-n-play.

Bench adjustments as delivered or after a rebuild are simply a starting point to get the engine to start, so you can make the proper adjustments that YOUR engine is asking for.

>> I screwed the adjustment needles in and back out to match the old carbs setting and <<

That leaves me speechless.

Put the mixture screws back where you found them or at least so that they are both the same number of turns OUT. If you can do that without turning them in again, then do that. Every time you turn the mixture screws all the way in, you run the risk of damaging the seat.

If it continued to run with your foot on the throttle, were you not able to turn the throttle speed screw IN so that it would continue to run?

[geezer#99]

Talk about speechless, Richard!
Are you trying to help or just ranting.
He’s not gonna solve his problem like that.

He asked for help not criticism.
Let’s get him fixed up!

[GMCPaul]

Originally Posted by Socalrider87

Long story short i got a new Jet carb to replace my old warped Quadrajet. Once i installed it the truck will fire up and die in about 2 seconds. If i hold the throttle down a decent amount it will stay on but once i go lower or let go it just dies. I replaced the fuel pump as well and still the same. I screwed the adjustment needles in and back out to match the old carbs setting and still nothing. Can anyone give some advice on what to check or adjust? Like i said with the old carb it would run fine just leaked fuel all over lol but with the new one i can’t keep the damn thing running.

‘70 C20 Longhorn
Factory 402

Dad always showed me to have both mixture screws backed off lightly seated to 1 1/2 turns. Then I always followed his write up on our web site using a vacuum gauge to get idle mixture set right whichh can be viewed on our Tips page
http://www.gmcpauls.com/TechTips1.htm

Prior to trying to start truck definitely check the choke flap operation, with truck cold remove air cleaner then when trying to start have someone look at the choke flap to make sure it's nearly closed. You may need to step on pedal once to clear dent tabs to allow choke closure.

Here's the portion related to fuel mixture screw adjustment using vacuum gauge once truck is running from off our tips page

Now for fuel mixture adjustment with one.
Disconnect vacuum advance at distributor and plug vacuum line, start motor with timing light and a RPM gauge attached and set idle and timing to correct settings. Connect vacuum gauge to full unported vacuum at base of carburetor, now shut motor off and turn both idle mixture screws all the way in but be sure that you only use light pressure as you can turn the idle mixture screw past being lightly seated by twisting hard and then you are damaging the idle mixture opening with the needle causing it to flare the opening trashing a good carburetor body so please only lightly seat the needles. now back each out 1 1/2 turns, restart motor and check idle and timing and adjust as necessary to correct settings, now back each needle out slowly a 1/4 turn each alternating between the 2 until you reach the highest vacuum reading, check and adjust idle speed and timing, now repeat step one of backing idle mixture screws out alternating between the 2 a 1/4 turn at a time until you have reached highest vacuum reading again. Continue the process adjusting timing/RPM/idle mixture screws until you have no changes occurring and have reached highest vacuum reading possible. You have now optimized your timing idle mixture adjustments. Reconnect your vacuum advance while disconnecting the RPM gauge, timing light, and vacuum gauge.

You'll also find trouble shooting tips using the vacuum gauge on that page and using a vacuum gauge to monitor fuel consumption at higher RPM's.

Hope this helps.........
Paul Jr @ GMCPauls

[beadblaster]

Make sure you have a plug in the rear vacum port at the back of the carb down on the base plate. The port is for vacum for the brakes but if you are taking vacum from the intake this port is left open from jet.

[HO455]

It does sound like a vacuum leak. You might double check to be sure you go have the right carb base gasket installed. There are many different gaskets that were used over the years and some won't work with certain manifolds. It is a common problem to have.

[beadblaster]

I ran into the same problem with my jet performance carb and the 3/8 or 1/4” plug does not come with the carb and you end up with a massive vacum leak. I am pretty sure that is the problem.

[Socalrider87]

Well I’ll be dammed it was that vacuum port on the back of the JET carb, i threw in a 1/2” plug and she fired right up and ran great now. I want to thank you guys very much!

[rockyrivermark]

Originally Posted by HO455

It does sound like a vacuum leak. You might double check to be sure you go have the right carb base gasket installed. There are many different gaskets that were used over the years and some won't work with certain manifolds. It is a common problem to have.

That's my bet. Check for vacuum leaks. Heck just replace all the vacuum lines. Cheap anyway.