85, C10, 305 start up and backfire

[jtl82md2]

Hello all, I just recently bought my first C10 after waiting for 15 years.  It is a 1985 Chev. C10 with a 305 with the stock quadrajet.  The guy i bought it from said the previous owner did some work to eliminate the EFM system on it.  I can see multiple vacuum hoses plugged and not hooked up.

Well the problem is at start up.  Cold start she revs to who knows, its high, and stays for a long time.  I have to rev it pretty high to get it to come down.  once at idle she sounds great.  Once you start driving, it has decent power but backfires when you let off of the gas quickly.  So WOT let off the gas it backfires a bunch. 

Rebuild the carb? replace it with an edelbrock? or just save the money for a crate 350

[popsthebuilder]

Welcome to the site. I think what you are describing can be caused by an exhaust leak, but I'm not sure, and I'm definitely not a mechanic. Someone will chime in soon enough though.

[74 C-10 Shorty]

Sounds like an exhaust leak to me.. BTW, welcome to the site..

[jaredts]

The high revving until you stomp it sounds like a sticking choke.  A good cleaning with carb. cleaner might fix that.  Welcome.

[bd]

The high revving until you stomp it sounds like a sticking choke.  A good cleaning with carb. cleaner might fix that.  Welcome.

This ^^^^ and adjusting the cold fast idle to spec may resolve your high idle.  You should also verify proper choke function, which includes the choke vacuum pulls.

The backfire I take it is coming from the exhaust?  On deceleration, high engine vacuum enriches the fuel mixture entering the cylinders.  Exhaust backfire occurs when unburned fuel expelled from the cylinders ignites in the exhaust system.  This can result if the air management system or the deceleration control has been defeated or has simply ceased to function.  The air management system (smog pump and diverter valve) controls air injection into the exhaust stream, whereas the deceleration control leans the intake mixture to compensate for the richer fuel mixture during deceleration.  A fault in either system can cause annoying backfire on deceleration.  If you look at the emission sticker on your radiator support, it will tell you whether the engine was originally configured with either system.

[jtl82md2]

Thanks to all for the info.  I will try cleaning the carb and checking the air management system. 

This carb is the original and has over 100K on it.  Should i maybe just replace with a edelbrock with the adapter?

I have heard good and bad things about switching.

[bd]

Hello all, I just recently bought my first C10 after waiting for 15 years.  It is a 1985 Chev. C10 with a 305 with the stock quadrajet.  The guy i bought it from said the previous owner did some work to eliminate the EFM system on it.  I can see multiple vacuum hoses plugged and not hooked up.

Well the problem is at start up.  Cold start she revs to who knows, its high, and stays for a long time.  I have to rev it pretty high to get it to come down.  once at idle she sounds great.  Once you start driving, it has decent power but backfires when you let off of the gas quickly.  So WOT let off the gas it backfires a bunch. 

Rebuild the carb? replace it with an edelbrock? or just save the money for a crate 350

Sounds like your only real complaint with the existing carburetor is the choke, which is easily correctable.  The backfire issues come from a different source, as previously mentioned....

It's difficult to improve on a properly functioning QJ carburetor for decent economy and great drivability.  Most performance modifications incur challenges; few are bolt-n-go.  And, few mods deliver what their hype suggest.  So, unless there's a specific problem with the QJ that defies reasonable correction, or you simply want the "fun" of installing the Edelbrock, I recommend keeping the QJ.  For an in-depth look at QJs, pick up a copy of Cliff Ruggles book, How to Rebuild and Modify Rochester Quadrajet Carburetors.

[rich weyand]

Agree with bd above.  And bear in mind: Any carburetor requires proper adjustment for drivability, performance, and mileage.  Adjusting a carburetor precisely requires instrumentation, in the form of an air/fuel ratio meter, a good one of which will run you about $250, as well as a bunch of time and effort and additional tuning parts.  Your Quadrajet is already tuned to your engine configuration from the factory, in the form of its internal air bleed passage sizes and metering rods and jets and the like.  You can rebuild the Quadrajet if you wish, without upsetting this tuning, to restore any functionality lost to wear of parts such as the throttle plate hinge points, and retain the Quadrajets superior performance.  It's a dang good carburetor.

In any case, this is a moot point, because that is unlikely to be the problem.