289 idle

I am having a problem with a rough idle on a 1966 Mustang with a 289, hydraulic lifters and a 4-barrel carburetor. First the background. I replaced the engine about forty thousand miles ago with a rebuilt short block. I have never been able to get the engine to idle smooth since I installed it. It doesn’t consistently idle rough like it would with a high performance cam (the cam is only slightly hotter than stock). I thought at one time it might be a warped carburetor casting periodically dripping raw gas into the manifold. That is probably the best way to describe it. I am a pretty good back yard mechanic but when unsuccessful I have taken it to three different mechanics for adjustment/analysis. I have replaced the carburetor, distributor, plugs, wires, points, condenser, distributor cap, fuel filter and still the rough idle persists. I have adjusted everything twice and then had professional mechanics adjust them again. About a month ago a valve spring broke so I pulled the heads and had a valve job done. Still the problem persists. I first adjusted the valves cold (zero lash and then ¾ turn). With this adjustment the intake valve was not closing on one cylinder. A compression test indicated about 140 lbs for all cylinders except one cylinder that was zero. By backing off the adjustment to about ¼ turn I could get compression. I readjusted the valves hot and still couldn’t use more than ¼ turn on the one problem cylinder. Now for the question, I think I am having a problem with an intake valve hanging open, is there any way I can test to verify this or should I just replace them all?



Charlie Stephens

Charlie.... A vacuum gauge should show a valve hanging up. Good, steady needle shows all is OK and a jumpy needle shows a valve not sealing well. Could be sticking, burnt, or bent.

Charlie,



Run accurate comp test. Any variation might be your culprit. Vac will show wiggly with a slight cam too, so don't jump to conclusions. Valves hanging sounds not good at all, doesn't sound like good machine work, or something odd about oil uptop? If retarding timing a shade and readjusting carb doesn't smooth it out, and compression is even, thoughts of an inaccurate cam come up. Suspicious that it came at get go with rebuilt!

Check the intake valve that has the weird adjustment, and see if the rocker arm stud has pulled out of the head somewhat. The rocker arm studs are just pressed in on 289's. There are aftermarket screw-in studs available, or if you are careful, you can drill a small hole and pin the problem stud. Either way, this may settle the valve adjustment problem, but as to the bad idle???

Fred/Sam/JD,



Thanks for the help so far. I had run a compression test using a mechanical gauge that screws into the plug holes and found them all to be 140 psi to 145 psi so that is not the problem. Vacuum test shows 15 inches of mercury and “wiggle“ is plus or minus one inch. Does this sound reasonable? When I said I thought a valve might be hanging open I should have said a lifter might be sticking and holding the valve open. This is just a guess but I am running out of guesses. There is plenty of oil getting to the rockers. Changing the timing and adjusting the carb doesn’t seem to smooth it out. I assume it is not an inaccurate cam since the problem is not always there, only about half of the time when I pull up to a stop. On the plus side it runs great under power. I will keep posting as it evolves.



Charlie Stephens

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OK, you said the magic words! Time to do some carb work: Check fuel pump pressure, I like to see under 4, but just so it's not 9 or somthing. Fuel filter: paper element type, be sure it has not started passing dirt (assuming there is one on fuel pump still?) NEW if in doubt. Float needle and seat: get viton or some resilent needle seat combo, and setting float level needs wet needle and top rotated 90 deg so there is no force on needle (as is, upside down) NOW try readjusting, and during warm up not when motor is red hot, and see if idle isn't smooth (Grin) Complications: Smog equipment? Some had a thermostatic air valve on manifold, opens when it's hot, screws up idle mixture no end! (one carb had it inside) . All above was often the customer who came in with "I need new plugs" (and didn't)

JD,



Thanks for the suggestions. I will have to put the project on the back burner for a couple of weeks. I will post again when I get more data.



Charlie Stephens

Just a follow-up to say thanks for the input and tell you the final outcome. I went to another mechanic who told me the valves needed to be adjusted. I had adjusted them three times in the last month all to slightly different criteria without improving the problem so I kept looking. Finally I went to a local mechanic with electronic diagnostic equipment and asked him to analyze the problem. Mechanics had unsuccessfully tried to solve the problem using diagnostic computers three times over the last couple of years. This person concluded that the spark duration was too short, replaced the coil and the problem was soled. This solution surprised me since there were no problems under load or at high RPM, which is where I assumed a bad coil, would show up.



Charlie Stephens





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