I've been finding this site to be so helpful, that I thought that I would try to find some advice on my rich running carb. It's really gassy. I can smell it throughout the rpm range, even going down the highway with the windows closed. It's getting 6 - 8 mpg. It's a freshly rebuilt 1972 455, with 9.5 - 1 compression. Performer intake. Crower cam 230@ 50. It's an 800 cfm Q-jet that's been rebuilt: drilled idle passages, new float set at .375, 75 jets, primary rods are .026 tip to .050 (it's a 1979 carb), .041 secondaries, choke removed, new vacuum pull off, no leaking plugs, clean passages etc. Enlarging the idle passages helped more than anything. Some of richness did clear up, and it idles and runs much better (no surging at cruise), but it continues to run rich like before. I tried some drilled throttle plates, and it may have helped gas mileage a little, but the idle lope came back, and it ran a lot like it did before I enlarged the idle tubes. I feel it ran tighter with undrilled plates. I am still running the stock ignition, which is points, for now. I was wondering if adding a super hot coil with hotter plugs would work. The car was not gassy at all before the rebuild, and fuel efficiency not noticeably that bad. Thanks, Gabe
Gabe, were any modifications done to the carb's primary power piston spring? Since the engine will have less manifold vacuum with the aftermarket cam, it's not uncommon to have a rich condition if the power piston spring is too strong, allowing the primary metering rods to withdraw from the jets at low rpm cruising. Often a couple of coils (or so) may have to be cut from the original spring to accomodate less manifold vacuum. Also, are you funning a vacuum advance diaphragm on the distributor, and is it plumbed to manifold vacuum also? If you haven't already, check out Larry's thread on setting the ignition advance curve: http://www.v8buick.com/showthread.php?t=63475 Devon
That was one fast reply! I was going to add the power piston spring looks like it had been already cut, but I don't have another to swap in. I don't think with this later style Q-jet I can push down the rod hanger when the engine is running to see it clears up the richness. May try cutting he spring more? Vacuum advance is going to a small bottom front driver's side tube on the carb.
I thought you could check the position of the power piston on all q-jets, but I admit it's been awhile. I wouldn't do more cutting without some backup springs just in case, unless you know for sure you need to. Sounds like the vac can is getting manifold vacuum...but is the vac advance canister working properly to begin with (holding vacuum)? Devon
Yes, Devon, the vacuum advance diaphragm is working. It's going to take me a little while to understand the thread you've linked to, but I have the engine set at 9 deg initial timing. I believe that's supposed to be 10 - 12 initial (minus the 4 degrees that the cam is set back) for 30 - 32 total advance all in at 2500 rpm. Can my timing and/or lack of softer springs on the mechanical advance really affect fuel efficiency and cause overly rich exhaust to this extent? Thanks, Gabe
Perhaps not, but with the big cam I'm running I've found that running as much initial (and vac) advance as possible really helped clean things up at the low end. The four-corner idling adjustment has helped as well. I'm out of ideas for the moment! Devon
Just went out and timed the car. I'm getting 31 deg. all in at 1800 rpm. I figure this is safe as my cam has a 3 way key and it's set to 4 deg. back, giving me a total timing of 35. I hope this is right and not backwards giving me a total of 27 instead. The car has a good amount of power, and it would be a different story if it was getting 6 mpg and not blowing out gassy exhaust. I feel like the excessive richness might be something really simple that I am missing. -Gabe
Cam timing effects the timing of valves opening in relation to piston. Ignition timing (31* at 1800) is the timing of the spark to piston position. Cam and distributor timing are not related to each other. By that I mean that advancing the cam does not effect spark timing. I hope I don't offend you I am just trying to help.
Thanks. That's good news then. My timing should be set. When timing, I thought that one had to simply keep in mind if the cam had been advanced or retarded. In my my case, it is the latter. All kidding aside, I forgot to add that I checked the power piston, and there is sufficient vacuum to hold it down at idle. Nothing I've done so far really seems to have anything to do with it's running rich (and me going poor).
Do you know which jets & metering rods you're running? Maybe the carb was once calibrated for a different application. Devon
Gabe, Unless someone has added light springs to your distributor, it is highly unlikely that your timing is maxxed out at 1800 RPM. If it is, you must also restrict the amount of vacuum advance degrees in the canister or it will surge while cruising.
Thanks for the questions. The carb runs rich no matter what float level is set. I played around and got everything from surging at low float level to flooding out the carb at high level, with no affect on the richness. I've switched Q-jets with no effect. I thought that the idle circuit might be the solution, and while it did make the car drive better, it did not help the richness or economy a noticable amount. The distributor springs were a little bit of a suprise, but I did get this car from a car guy who loved the '72 Riv-v's. I put a 30 deg mark on the balancer, revved it with a light on it, and it would rise till it stopped right about zero and 1800 rpm. I adjusted the timing 'til it stopped between 0 and 4 (giving me about 32 deg total). I would not suprise me if the previous owner put some springs in there to give a 1800 - 2000 rpm total. I've had no adverse effects using the vacuum advance, even though it has not been altered in any way. It seems to give a little kick off idle, but the car can run fine without it too. As for jets, I have .075's in there now, but had .079's in there the other day. The difference is subtle with no real change in exhaust richness. The rods are .026 at the tip and .050 at the widest part. This is a 1979 carb and has the shorter rods, but I chucked them up in the lathe and turned the .036 tips down .010. I had ealier Q-jet on with .075 jets and .045 rods, but no better condition. Maybe I'm blaming the carb for another problem elsewhere. Ignition coil? Thanks, Gabe
I was just going to tell you that I had the same problem with mine and it went away when i put a new coil cap and rotor. I assumed it was due to weak spark.
Thanks, James. I put a new cap and rotor on it. It also has new condenser and points. For a few bucks, I might as well change 'em all out again. I put a new coil on it, but it turned out to be defective so back to Napa it goes. I'm going to try getting a better one this time. Any recommendations? Everybody is being so helpful. The exhaust richness is really excessive. I really don't know a whole lot, but I'm learning. Thanks, Gabe
Gabe, The stock canisters usually have 14-18* in them. Combined with a quick mechanical advance, that is usually way too much. If you're at 32* at 1800, and you add the vacuum advance in, you are looking at 46-50* at a steady cruise. That will make the engine surge all by itself. I usually limit them to no more than 10*. Around 40-42* will give you the best fuel economy.
Thanks, Larry. I'm gonna put a limiting nut and bolt on the advance to limit it's travel to 8 or 10 deg. I've got a MSD Blaster 2 that I'm gonna pickup today, hopefully I'll star seeing results! -Gabe
Plugs look normal. Oil is thin and gassy. Put a pressure gauge on the fuel line. It's reading between 4 and 6. Put on a regulator and turned it down to 4.5 and then all the way down, no change. I've got a new fuel pump coming tomorrow. Thanks, Gabe
Heres another update. New fuel pump and oil change (oil was really gassy), but so far the exhaust is still too rich. Maybe after driving it a while, it'll improve. My friend suggested a vacuum leak where the heads meet the intake. He said that he did not like the way the machine shop milled the heads. That's all! -Gabe
If you think there is an intake leak, before you R&R the intake, try and re-torque the intake bolts (in proper pattern) and see if they are loose. If you think they are really mismatched, you may need your intake milled, and/or port matched. There are tests to check for an intake manifold leak. Keep in mind, none of this may be why you are running rich. Do you set your dwell with a meter, and do you have a vacuum gauge? good luck, ElectraJim
