Ok guys here is what I got. I have a 1972 455 non stage engine 30 over with a T/A Stage One cam ( the one it would have came with if it was a Stage One ) with everything new on the ignition side and using a stock 4-speed 72 carb timing at 10 degrees and the timing gear in the stock position and carring a 30 dwell. The plugs are like they were never installed ( not fouled ) starts up cold and winds up great, But here is the problem; wile not under a load the engine bucks and runs uneven. At idle its not bad, but bring it up to 2000 RPMs the engine shakes and runs uneven. I'm thinking this may be a lean miss and may try to add propain gas to the carb when missfires to rich up the mixture to see if this smooths this out. I have only owned 350s in the past and they have been smooth running at all throttle positions and thought 455 shoould be the same. Can anyone give me some input on this one. Thanks in advance. IDOXLR8 :Comp:
Disconnect your vacuum advance line and block it off. Test drive to see if the problem goes away. Too much vacuum advance while "un-loaded" will feel like surging, bucking, uneven running engine. If this proves to be the issue, install an adjustable unit from, Crane, Accel, etc. and crank it down to just a 2 or 3 degrees.
455 misfire unloaded BadBrad, thanks for your responce on the 455 I will try that. If there is to much advance is it because of the wrong vacuum advance, I'm just woundering, the wong mahcanical advance springs? If that proves to be the problem I would like to get to the root of the problem, Thanks IDOXLR8 :Comp:
Hi, wish I knew your name, There are alot of things that would cause a misfire, most of them basic. I'm going to assume that you have checked the firing order, it is easy to reverse #5&7. You wouldn't be the first guy or the last to do that(I speak from personal experience ) I would check the distributor cap and rotor as well as the plug wires(check the wires for resistance with an ohmeter-1500 ohms/ foot) Check the distributor shaft for worn bushings/wobble. You can perform a cylinder balance test by running the engine at 2000 RPM, and shorting one plug at a time(be careful). Look for a dead cylinder that doesn't drop RPM's. You can use the choke to richen the mixture if you think it is lean. Hope that gives you a few ideas.
Larry, I'm a mechanic by trade and have built the engine. the firing order and wires are fine and as I stated on the other board it runs great and even at idle and take off, just runs uneven criusing and not under a load. AL
Al, That's strange. Hows that Q-jet look? Is the throttle shaft sloppy? You may be developing a lean miss because the shaft is leaking vacuum in certain positions
Larry, I wish my problems were that easy but the engine has a correct N.O.S. 4-Speed carburetor on it. Thanks, AL.uzzled:
I'd ask the ignitionman. what he thinks. He could also rebuild & convert it to HEI. I just have the feeling the distributor is the cause.o No:
I may have missed it, but have you put a timing light on it while it is missing/bucking? I'd be interested in hearing what your timing mark was doing (steady or jumping) and what was your advance when the problem occured. You could do this test both with and without the vacuum advance connected (both ported and manifold vacuum just for kicks). Marc
I was leaning torwards ignition prpblems but everything is new and all the plugs do not show any signs of problems. I'll try all the things you all posted and see what happens. Thanks AL:Comp:
Just remembered something else that caused a similar running condition for me 10 years ago. Idled fine - WOT fine - part throttle cruise surge and stumble. Opened the Qjet and found that one of the air bleed tubes had totally fallen out of its placement in the top of the carb. Carefully tapped it back in with a hammer - problem solved.
Well I checked things out and the timing is steady and I tried all what you all posted and I'm back to square one. This doesn't make sense, I went back to the back of the car and the smell of the exhaust at 2000 RPMs it will burn your eyeballs out. Now maybe there is a rich condition but you would never know it by looking at the plugs. AL
OK, I may be getting somewhere here on the misfire. I run the engine up to 2000 RPMs and placed a rag over the carburetor and the engine smoothed right out. You would think vacuum leak but the engine idles so smooth and even it will hurt your ears. AL
Al, Have you put a vacuum gauge on it to see where it's running? Q-jets are famous for worn throttle bushings which could give you a nasty leak. Try spraying the throttle shaft area with carb spray while it's acting up and see what happens to the vacuum reading and engine operation. Also the stock 72 manual carb should be a 7042243 which is jetted a little lean on the primary side. .073 jets and 45B rods with CX secondary rods(.0567 tips not bad). The Stage1 carb 7042242 has .075 jets and 45B rods with CV secondary rods (.0527 tips)
Well Larry I think we are closing in on the problem, If the throttle shaft proves OK then I need to figure out what I'm going to do about my lean miss, Hello " Carb Doctor " AL .
I just got off the phone with the "The Carb Dotor" and he said the float level is to low according to the information I gave him. I will be buying a new 0.50 top gasket and a new set of #47B rods and #75 jets and a 6019 power piston spring. We will see. AL
It's a lobe sensing pic up module or some older models are magnetic trigger. They replace the points in the distributor. When they work they work great. Problem is they often go bad and do exactly what you described or fail completly.
