How to tell if carb needs rebuild?

[Green79Scottsdale]

Basic question, yes. But seriously, seems like I hear guys saying their carb needs a rebuild all the time. Why? I realize over time they "degrade", but what are the signs a carb should be rebuilt? Is it the performance? Is there mechanical things easily reached to point to a rebuild?

The main reason I ask is the Qjet on my 79. It's got 100k miles on it, it's 35 years old, and to my knowledge has never been touched. But it still runs great and is giving my no major issues. I know it runs rich as when I park it in my garage the fumes are quite bad. I was/am planning on doing some tuning come spring time to see what kind of possible performance and mpg gains can be had. But I don't really want to go tuning a carb that "needs to be rebuilt". So if a rebuild is truly in order, I would like to get it done before spring time.

I would love to hear your thoughts guys!

 

[firebane]

Its time for a rebuild. The biggest issue is that fuel now contains ethanol which destroys any rubber type product unless its specifically made for it.

When doing the rebuild it allows it to be cleaned as well as re-adjusted and most people find the vehicle runs better afterwards.

 

[chengny]

The Rochester Quadrajet was designed to be a rugged, dependable carburetor that delivered decent gas mileage most of the time (and plenty of power when required).

If your quadrajet can do these 4 basic things:

1. Let the engine idle smoothly

2. Come off idle without stumbling

3. Open the secondaries when you floor the accelerator

4. Get anything over 10 mpg on your 3/4 ton 4WD truck

It is operating pretty much as designed.

You could pay $150 (to have someone go through it and renew the soft parts). It might be a little sloppy inside after 35 years of daily use.

But, be forewarned, when it comes back - don't expect to be blown away by a huge difference in how your engine performs. If your q-jet can do those things (listed above) as it is now, your power plant will probably run pretty much the same as it did before the rebuild.

That's just my opinion but it's based on experience.

 

[Green79Scottsdale]

The Rochester Quadrajet was designed to be a rugged, dependable carburetor that delivered decent gas mileage most of the time (and plenty of power when required).

If your quadrajet can do these 4 basic things:

1. Let the engine idle smoothly

Check. Takes a little coaxing to stay running at first, but it has done that for as long as I can remember. Once it is warmed up it will idle down to 500 rpm and sit there steady all day long. And it sits for months at a time without being driven.

2. Come off idle without stumbling

Check. Throttle application = engine response. No hesitation whatsoever.

3. Open the secondaries when you floor the accelerator

Umm... I never floor it. Easy to check however.

4. Get anything over 10 mpg on your 3/4 ton 4WD truck

Gets 10 mpg. Always have from what Dad has said. 10 mpg empty, 6 or 7 mpg with the slide-in camper. In my eyes not to bad for 400 sbc, TH400, full-time 4wd, and 3.73 gear combination.

It is operating pretty much as designed.

You could pay $150 (to have someone go through it and renew the soft parts). It might be a little sloppy inside after 35 years of daily use.

But, be forewarned, when it comes back don't expect to be blown away by a huge difference in how your engine performs. If your q-jet can do those things (listed above) as it is now, your power plant will probably run pretty much the same as it did before the rebuild.

That's just my opinion but it's based on experience.

Believe me, in no way would I expect this to be a different truck after a rebuild and tune. I am planning on borrowing my mechanic buddies A/F meter for the tuning. If it does perform better and get better mileage, great. If not, oh well, at least I know it's as good as it's going to get! It is after all a completely stock low compression late 70's small block. It did however pass a compression and leakdown test with flying colors recently.

Thanks for your thoughts.

 

[bucket]

If it operates well and your plug readings are not overly rich, I'd say you're better off to just leave it alone.

 

[Quadrajet Power]

Just like mentioned above. Today's fuel can cause some big issues on the soft parts in the Qjet. First issue normally is stumbling on acceleration because of a bad accelerator pump. Other things are more subtle but erratic idle can be caused by trash build up in the circuits and vacuum leaks from shaft wear in base plate.

After building these for years, and seeing the parts after years of age and use, I always put bushings in primary shaft, replace jets and primary rods, since they also wear, and replace the secondary air valve cam and tension spring. Many if the plastic cams are cracked.

So yea, after the years and miles, you could probably see some improvement with a good, correct build, but if it is performing like you need, drive it!

Just don't end up pulling it and bolting on a parts store carb. You'll be throwing money away.

 

[Green79Scottsdale]

I was hoping you would chime in Mark, thanks.

firebane mentioned it earlier too, the current state of our fuel. I hope the lack of fuel I actually put through my truck helps to slow the degradation of the soft parts of not only the carb, but everything else also.

It does what I want it to do in a very good fashion, so maybe I will just keep driving it as is. With my luck I would tinker with it and cause a big problem. To use my buddies meter I will need to put an o2 bung in the exhaust anyway.

With the prices of new carbs, I would rather have someone knowledgeable, like you, work on my original carb. I like the idea of keeping my truck totally original. The only thing I would replace my carb with is one of the new EFI set-ups available. And it sounds like this is a side business for you, so I would rather support your efforts, as I am finding out what its like operating a side business.

 

[firebane]

I was hoping you would chime in Mark, thanks.

firebane mentioned it earlier too, the current state of our fuel. I hope the lack of fuel I actually put through my truck helps to slow the degradation of the soft parts of not only the carb, but everything else also.

It does what I want it to do in a very good fashion, so maybe I will just keep driving it as is. With my luck I would tinker with it and cause a big problem. To use my buddies meter I will need to put an o2 bung in the exhaust anyway.

With the prices of new carbs, I would rather have someone knowledgeable, like you, work on my original carb. I like the idea of keeping my truck totally original. The only thing I would replace my carb with is one of the new EFI set-ups available. And it sounds like this is a side business for you, so I would rather support your efforts, as I am finding out what its like operating a side business.

Unfortunately it doesn't work that way. To give you some idea I had to replace a gasket on my motorbikes gas tank last year and after I replaced it within 1 year due to the alcohol in ethanol it cracked the rubber and I have to replace it again.

Fuel is always sitting in a carb somewhere or in fuel lines and will be degrading the parts over time regardless.

 

[Quadrajet Power]

I was hoping you would chime in Mark, thanks.

firebane mentioned it earlier too, the current state of our fuel. I hope the lack of fuel I actually put through my truck helps to slow the degradation of the soft parts of not only the carb, but everything else also.

It does what I want it to do in a very good fashion, so maybe I will just keep driving it as is. With my luck I would tinker with it and cause a big problem. To use my buddies meter I will need to put an o2 bung in the exhaust anyway.

With the prices of new carbs, I would rather have someone knowledgeable, like you, work on my original carb. I like the idea of keeping my truck totally original. The only thing I would replace my carb with is one of the new EFI set-ups available. And it sounds like this is a side business for you, so I would rather support your efforts, as I am finding out what its like operating a side business.

yes, it is a hobby business for me, which is why I keep my prices low enough to try to help out the average hobbyist driver. I can put the best parts in, and do a thorough build, no "kit" install stuff, and with my low overhead, I stay busy and make enough to keep my race cars and 4x4 running. Well, almost, haha, since only one of the 3 runs currently. But 2015 is the year I decided to have them all back going!!!

I'm really surprised your carb is still doing well at all since the fuel today really does a number on the soft part in the quadrajet. I have pulled many apart this past year when the accelerator pump cup has pulled off of the plunger after somebody put in a parts store kit. The cup swells up and locks itself in the carb, and when the driver pumps the pedal hard to get it free, the cup pulls off. Then there is no accelerator pump use at all causing a big hesitation when first accelerating from a stop.

Let me know if I can help, happy to help any way I can.