Misfiring after installing new carburetor.

[E.D.75GMC]

Today I received my "new" Quadrajet from National Carburetors. Installation went without a hitch and it idled just fine afterwards.
Then I decided to take a drive around the block.
I had to give it a bit of hard throttle as I live at the top of a rather steep hill and as soon as I let off the gas it started to misfire. I checked for leaks, there were none. Checked the fuel filter, it was clean. Let the truck sit for a couple hours incase it was just flooded and went to start it up and still misfiring. All the hoses to the carb are okay. Plugs and wires all seem okay too. Really not sure what to check next, hopefully someone can provide insight.

I recorded a short video of it running but it won't allow me to upload it (something about a security token)

[Gregski]

when is the last time you did a tune up, checked timing?

[K5owner]

Cliff Ruggles one of the masters of these carbs does not mince word when he describes National Carburetors. Many of the large companies that rebuild quadrajets don't do a great job and it is kind of a crapshoot. What year, engine and style quadrajet is it?

[E.D.75GMC]

It hasn't been tuned up in about a year. Oh yeah it's a 1975 350 with just the stock divorced choke Quadrajet

[E.D.75GMC]

Here are some pics of the new carburetor

[tucsonjwt]

I hope you get it fixed because I just order a Qjet from National.

Cliff has a 12 month waiting list, or I would have sent my carb to him.

[tucsonjwt]

Just a thought, chokes have been the big problem with all of the carburetors on my old squares.

I would stop when it backfires and pull off the air cleaner to make sure that the choke is all of the way open. If the choke is not functioning correctly, you are supposed to bend the connecting link to adjust the choke - I was never able to do this successfully. In fact, on all automatic chokes, I could either adjust them to close all of the way when the engine was cold, or open all the way when the engine was hot - but I could not get both conditions - so it would either give good cold starts or hot runs but not both. So, I now use manual chokes only.

Backfires are sometimes vacuum related (so I read) - you might double check the vacuum hose routing - on both the carburetor and the other end of the hoses.

[E.D.75GMC]

Okay thanks, I'll check the choke out when I get home later. I pulled the choke off and cleaned it up before reinstalling the carb maybe I didn't hook it back up correctly or something came loose.

[E.D.75GMC]

It's just odd it fired right up as soon as gas got to the carb, I tapped the gas and choke came off and it went to idle speed and I let it warm up to operating temp then gave it a few good revs to make absolutely sure there were no leaks and it still ran perfectly. Then I drove it and when I got home it just didn't want to run smooth.

[K5owner]

What kind and how old are the plugs? It could be running rich and fouled one out. I would also double check your idle mix settings with a vacuum gauge and make sure they are set the same.

[E.D.75GMC]

Champion plugs, about a year and a half old.. just pulled these from cylinders 1 & 3

[tucsonjwt]

They look like the champion plugs I just replaced - same thing, all carboned up.
Mine was running very rough with some black smoke on start up. I changed the plugs and all is well. I think the carbon is from running too rich. I used Champions because they were free with a rebate. I will go with AC 44RT when I run out of Champions. Rich running would be consistent with a stuck choke. Unless you just replaced plug wires also, I would install a new set of Belden plug wires from NAPA also.

In my case, I only run my 454 in town on short trips, so that contributes to carbon build up. I usually replace my plugs every year, and that is only 2-3,000 miles or so. I am going to try some BG 44K and see if that helps.

[K5owner]

That is what I thought. It is way rich and fouled the plugs. I heard autolites are more resistant to fouling then the AC Delco's. I would get a set of the appropriate heat range Autolites. I would also look into see if your ignition system is giving enough spark. Is it an HEI or standard distributor? If you haven't recently I would just do a tune up with new plugs, cap and rotor, check timing and adjust the carb to the highest vacuum setting at idle. That should get you up and running pretty good.

[E.D.75GMC]

I replaced all the plugs to no avail, so I'm thinking either the carb needs a bit of adjusting or it's in the distributor like you said. It's HEI, but all of it was all replaced about a year and a half ago.

[K5owner]

Carb it is then. They come adjusted close but final tunings are required

[tucsonjwt]

Did you check to see that the choke was still open all the way after backfire? And that all of the vacuum hoses are hooked up properly?

[E.D.75GMC]

Vaccum hoses are all good, but I'll take a look at the choke when I get home from work!

[geezer#99]

Did you check your vac pot on the dizzy?
If it's blown you'll have a vac leak.
What tranny?

[Dead Parrot]

It is possible that the previous bad carb was masking one or more other problems. Looking at the old spark plugs, it is likely the old carb was running way rich. If you also have a vacuum leak, the way rich carb can mask the leaning effect of the leak. If your new carb is perfect, and you didn't fix the vacuum leak, you now have a too lean condition, which often can cause backfire and misfires.

Now is a good time to check everything for leaks. Unless you know they are new, now is a good time to replace all of those vacuum hoses. It doesn't cost much to replace them and a bad one can cause a lot of headaches. Make sure your vacuum advance and Thermac pot are working. If you have power brakes, make sure that doesn't have a vacuum leak. Also make sure you didn't miss a vacuum connection on the carb. Qjets can have them in weird places.

Check things near the carb. It is possible you pulled, pushed or nudged something loose in the process of swapping carbs.

Any chance the suction line from the gas tank chose this point in time to fail? That can cause misfires on acceleration.

[tucsonjwt]

Did you fix the problem yet? I just got my National Carburetor today and am curious if you can point me to any problems I might encounter that you have fixed. Thanks.

[E.D.75GMC]

Oh yeah it's running great now it just needed some adjustment to the mixture screws

[tucsonjwt]

Thanks for the update. I hope my BB likes that NC carb.