Any ideas as to why my qjet will not idle . It works great when the choke is on but when warm it will not idle . Professionaly rebuilt at Carbco in St Louis. These guys have an impeccable reputation on these carbs and they are scratching their heads . The soft plugs were epoxied and the carb was cleaned and all new gaskets installed . maybe I need a new core . John
Don't forget the most obvious. Do you have a performance cam? If so they require more tuning than stock to get them to run right! What does it read on the vacuum gauge at idle? Less than 12 psi and it will run rough. Been there!
Vacuum leaks are often a source of a poor idle with any engine. Throttle bushings are also a possibility, but since the carb was professionally rebuilt, will give the rebuilder the benefit of the doubt, and hope that the rebuilder addressed this issue, as it is a very common problem with the Q-Jet (until fixed). Very easy to check the bushing thingy; as the design tolerances were 0.004~0.006 inch. Wear to 0.009 inch would be acceptable if the owner were building the carb, but not to a professional shop. Another possible area of concern on the Q-Jet, particularly if the carb has been sitting any length of time after running on modern fuel is the cavity beneath the idle tubes. When modern fuel evaporates it forms "gum" or "varnish". If a piece of this varnish broke loose in the cavity, virtually impossible to remove without removing the idle tubes, and this could definately cause the rough idle. This is not normally an issue, but it can be, especially if the carburetor is/had been in an ethanol environment. I would suggest checking with the rebuilder to see if the idle tubes had been removed and replaced. Jon.
Rebuilder states that he ran a drill the correct size thru the tubes by hand and then blew thru them . I am guessing this is not adequate ? I just bought a Carb off ebay a 7042240 . I am hopeing this rebuilt carb will run correctly . I will inform soon. John
Qjet John, Running a correct size drill bit through the bore obviously won't correct a wear/oversize condition. I would buy a bushing kit or send to John Osborn and have the bores reconditioned.
Q-Jets are wonderful carbs, but O.E. units are very finely tuned to the specific application. Not sure the exact year Buick dropped the compression ratio, but if you have a 1970 (user name), and the 1972 is lower compression; then it is unreasonable to believe the 1972 carb in stock configuration would be perfect on the high compression 1970 without some modification. As another poster mentioned, Mr. Osbourne has an excellent reputation on these forums. Another possibility is Cliff Ruggles of Cliff's High Performance, whose recently released Q-Jet book is rapidly becoming the talk of the carburetor sections of many forums. Generally, blowing through the idle tubes may be sufficient, but if a flake of foreign material from modern fuel exists in the cavity; it may be larger than the orifice sizes on both sides, and would thus NOT blow out. This is when the more thorough procedure of removing the tubes and cleaning the cavity then installing new tubes will eliminate a problem. Jon.
Jon , thanks for the info . I have a 1972 Stage 1 engine in the wifes car . The 1970 name is for my 70 GS thats had the stage 1 size valves installed with a KB cam . Anyway the first carb I had was a 7043240 which ran great for 6 or 7 years . The car sat for the better part of 18 months and then all the problems started . I now have a 7042240 at the carb shop and I am hoping that it will work . I tried a 7044240 that I had here and its too had internal problems . So I should know something soon . John
