Diagnostic chart

[Hotrodrobert]

Hey Cliff,
How about a chart of ailments and cures??  Even a book would do!!!!

[makomark]

You can try this; it appears to be a re-post of one of Lars' postings from a while ago:

http://digitalcorvettes.com/forums/showthread.php?t=88376

[Cliff Ruggles]

We pretty much covered the causes for carburetor troubles in the rebuilding chapter of our book.

I did not include a list or chart for specific troubleshooting. 

One thing to consider these days is that any q-jet in service with few exceptions has been out there for quite a while.  The castings of the q-jet are excellent, but they still give troubles in some areas simply  due to the age of the material, and many heating up/cooling down cycles, and drying up filling up cycles.

The biggest enemy of any carburetor is dirt, debris and build up from bad fuel and repeated drying out situations.

Most modern fuel also contains ethanol in some percentage.  Ethanol hates rubber parts, and many parts used in the q-jet were rubber.  Some kits will have ethanol resistant parts in them, most do not, because they are still using up old stock.

We get hundreds of calls each month from folks who have installed over the counter rebuild kits in their carburetors, only to have them give troubles soon after they were put in service.  It is almost always the accl pump, accl pump seal, or fuel inlet needle.  On occassion we will hear of a float sinking, but this is quite rare, as the modern floats don't appear to be effected by ethanol.  We actually see more brass floats fail than nitrophyl, as they will have or develope tiny leaks at the soldered joints and sink to the bottom of the bowl.

If you decide to install a brass float, it is highly recomended to submerg it in a glass of warm water and look for any tiny streams of bubbles that would indicate a leak.

I would highly recomend to anyone who is considering rebuilding a q-jet for any performance level to get a copy of our book.  It not only covers complete/correct rebuilding, but also how to effectively seal up bottom plugs, install shaft bushings, and other procedures needed to get a good end result.

Good parts are also the key to success, check us out for the best parts currently available.  We also support folks who are having troubles with their rebuild, here on the website and via emails.

We don't always have the time to help out with phone calls, so try the email or post your troubles on the website, it's the best way to get in touch with us......Cliff

[Hotrodrobert]

I am more interested in modifications and their reasons for doing them and just what they affect.
Like: increase the upper air bleed size-what do you do that for??  Things like that.

[Cliff Ruggles]

That information doesn't exist far as I know.  Setting up a carburetor, q-jet, Holley, AFB, Theroquad, etc, involves modifications to several areas of the carburetor to get a good end result. 

Although I've seen some published information on jetting, and jet/metering rod combinations to determine the delivery area, such as those listed in some of Lars writings, there isn't any specific information on airbleeds, idle or main, that would indicate the exact effects across the entire airflow range as to what the exact effect(s), or results would be?

Also keep in mind that other items effect these results, such as fuel pressure, float level, size of the inlet seat, size of the float, airbleed size at the nozzle, which nozzle is being used (there were at least 4 different nozzles, probably more) in any particular carburetor, etc.

The idle system also compliments the main system up through transition, as fuel is delivered past the idle transfer slots and thru the lower idle airbleed at increasing throttle angles.

For these reasons, tuning becomes a matter of using a "recipe" to many areas/parts of the carburetor, other than simply one particular item, such as an main airbleed.

I could easly write an entire book on custom modifications to airbleeds and custom tuning the other areas of the carburetor associated with airbleed and jet/metering rod tuning.  I'm sure there would be some who would benefit from this.

If/when we are asked to give our Q-jet book a "face-lift", I've planned on going deeper into specific tuning and more high performance modifications, and go into greater detail with some of the areas we already covered.......Cliff