What is causing high failure rate of Carb rebuilds?

[Grover]

Sent my carb off "to a guy" who had an excellent reputation for rebuilds. I won't go into the particulars, but I got it back, bolted it on, and took it for a shakedown cruise in which it failed badly. I monkeyed with the adjustments a little (Holley 4160) and let the guy know my results. He said, "send it back". So I did.

So here comes my question: A couple of local PCM shops won't even do rebuilds any longer. They say they have so many problems with them that they simply don't offer the service. Why is this? I mean, they could likely make as much doing the rebuild, and keep their guys and gals busy doing them in the off-season, so I'm not sure it's all profit motive.

I've done engine work, plenty of it, but I just didn't want to dive into a carb rebuild, no matter how simple it seems. Anyway, why is it so hard to get one right? Was I handed a line of fiction by dealer? Is ethanol the villain? What gives?

[Wylietunes]

A carb rebuild is nothing more than tearing it down, cleaning, and reassembling the carb with the new parts and gaskets. Basically, if its rubber, plastic or paper, its gets replaced while doing a rebuild. In my experience, here's what goes wrong:
1) misdiagnosing the root cause of the poor performance = carb overhaul doesnt fit the problem.
2) poor cleaning of the small passages. Doesnt run any better afterwards.
3) poor quality kit didnt have all the necessary parts, or all the parts that needed to be replaced, didnt get replaced.

IMO, I would never build a carb and send it to someone to put on and tune themselves. Too many things can go wrong. Find someone that will remove, rebuild, reinstall and tune for you. As long as a carb is not physically damaged and a quality, complete kit is available, there is no reason a successful overhaul cannot be done.

Ethanol blended fuels can easily lead to the cause of poor performance, which could result in the need for a carb overhaul, but ethanol blended fuel itself, does not cause performance issues. Its the side effects of ethanol, really.

[92SupraComp]

I have never had a problem with a rebuild. However we send them to my uncle in Florida who knows everything there is to know... He knows all the little tricks you have to do while tearing down and reassembling...

[CJD]

It's the 22nd century now, and carbs have gone the way of the dinosaur. A carb has not been put on a car since before most of the latest generation of techs were born. It's not that they "can't" rebuild them...I bet the shops just don't want to deal with them anymore, or be bothered training their people how to.

I used to be into performance cars and Holleys. I mean really into Holleys. In their day, they were incredible in their ability to adjust the fuel curve over a very wide range of loads and RPM's. I could tell you what to change, what to drill out, and what to fill in to adjust the mixture curves. Once I got where I understood them completely, I realized that they only give perfect mixture under 2 conditions...those are wide open throttle at the top of the power curve, and idle (sometimes).

At all other throttle and loads, they are at best a compromise. The worst fuel injection system is better than the finest tuned Holley. The FI uses atmospheric pressure, maps, O2 feedbacks, and temperature inputs to immediately change fuel mixture. The Holleys require jet and power valve changes, and sometimes re-drilling passages, to do that. Even in their prime, there were only a handful of decent Holley rebuilders that understood them. They are still great pieces of equipment...but it will continue to get harder and harder to find decent rebuilders.

Look at it this way... The model T's were delivered with wood wheels. Every repair shop knew how to repair and rebuild wooden wheels. Wood wheels evolved into metal wheels, which were better in every way. As the industry changed to the new fangled metal wheels, there became fewer and fewer shops that would rebuild the old wooden ones. Today there are only a couple rebuilders of wood wheels in the whole country.

Carburetors are on their way to being wooden wheels...

[Grover]

Great responses. Makes perfect sense. Thank you.

[Britter]

For me, the problem was little "tiny" pieces of debris. An inline "glass" filter with a replaceable element just before the fuel pump corrected all my problems...after cleaning and rebuilding the carb. I had the same problem, and finally had enough. Each time I located debris inside the float valve hanging the valve open...which would lead to the eventual flooding. The debris looks like fuel line delamination/degradation. (little pieces of rubber). Problem is, I have replaced all my lines with high quality stuff and have cleaned the fuel tank. I have taken extreme care to prevent this debris with no luck. I finally spend 10 bucks on a cheap filter, and haven't had a problem since.

Good luck.
Brian
1986 Sun Sport

[Grover]

Sounds like chicken soup, it might not help me, but it can't hurt. Certainly one less thing to worry/wonder about.

[michael hunter]

CJD is spot on with his post ethanol is the enemy of any internal combustion engine.Its even worse in a marine application. I would also add as the carbs get older the throttle shafts start to wear causing air leaks and sticking so they don't go back to min idle rpm. The housings can warp beyond what the gaskets can handle. I have rebuilt hundreds of carbs mostly Quadrajets and would say they were mostly very successful however a few were just not rebuildable and had to be replaced.