Plug the brake power booster vacuum line to see if it makes a difference. That is often overlooked as a source of vacuum leak, and is a huge vacuum hog.
The distributor and both advances are fine, I checked this morning with a dial timing light. Timing light flashes the same at each wire. When you say voltage at coil do you mean the red wire coming from the ignition switch? If so, it checks out fine. Booster is brand new, I just changed it last month. I never thought of or heard to take the screws out while running. Now that I think about it, it should be obvious! I'm going to try spraying in there tomorrow morning (dark and rainy, no garage :ball: ). If I do have a piece of gunk in there it'd be nice to not have to gut the whole carb to get at it. I appreciate everyone's help so far! I haven't posted much but I've learned a lot by reading things on this forum over the past few years. Resources like this are invaluable.
Never rule out new parts...it's an easy test. I just replaced the distributor in my Yukon that I replaced a month ago because it crapped out on me.
I'd defnitely look for another coil to try. Easy swap. Cheap and you can return if it doesn't prove to solve your problem. Check voltage at the coil too as already suggested. Distributor isn't loose, right? Sounds silly but it's been done. Running an aftermarket points eliminator kit like Petronix? (Rotor, magnet wheel, cap etc tight and clean) Fuel pump? Usually just crap out all together but worth checking. Borrow a fuel pressure gauge if you don't have one.
Ok I'm back, sorry for disappearing for a bit. The weather's been poor and I've been sidetracked. I went out this morning to tinker with her some more. Tried another coil, no difference. I'm running a stock HEI ignition that's in pretty good shape. The distributor is tight, and both the centrifugal and vacuum advances are working well. The fuel pump should be fine, as I can feel the line throbbing and opening it a little while running yields a nice squirt of gas everywhere. I get about 14V at the coil. I took the carb off and flipped it over to inspect the passages on the bottom and make sure the screws hadn't come loose. Everything was clean and tight, so I fiddled with it a little and put it back on. Now the thing won't stay running at all any more unless I keep the high idle cam kicked up. Perhaps I've knocked a bit of crud around inside. I'm going to head on down to the parts store for a rebuild kit later today. Do you gentlemen have any suggestions for me when doing the carb or anything else I could try? Thanks a million.
Hi Noah, sorry to hear you are still having trouble. I would remove the well plugs from the carb and add Marine Tex to them and put them back in. The plugs leak and cause idle issues and wasted fuel unless fixed. Also inspect the throttle shaft for play, this can be a source of a vacuum leak.
I'd like to know what your vacuum reading is, at idle ,in Park, fully warm, when it is running at 1000 RPM (the conditions you describe as running right). I'd also like to know what cam is in the engine.
When it's running well the vacuum in idle is normally around 20 in neutral and perhaps 16-18 or so in gear with my foot on the brake. The cam installed is a Comp Cams 252H. Now that it's running poorly I'm not getting much more than 13 or 14 at idle unless I rev it up a little, in which case it returns to normal levels. The needle is steady and not bouncing around or anything funny. I have the vacuum gauge installed inside the car normally so I can keep an eye on things like that. I'll try the Marine Tex. The throttle shaft is pretty tight as far as I can tell.
Ok, I have the carb kit and pulled and disassembled the carb. The thick gasket between the throttle plate and body was wet in front of the primary bores and the opening in the underside of the carb body above this area is black and crusty. I guess this indicates a leak? Is cleaning it and reassembling with new gaskets enough or is there something more I can check? Thanks.
Is there a chance the primary well plugs are leaking badly? http://www.hotrodders.com/forum/leaking-quadrajet-138013.html I'd toss that EGR valve. You've already recurved the dist. for HP usage and with a little richening of the carb there won't be any problem. In fact I'd block off all exhaust to the intake except for a 1/4 hole to warm the choke up good. Exhaust to intake leak? You intake is cast iron but you never know: http://www.v8buick.com/showthread.php?t=237500&highlight=intake+leak+300 That cam should idle nice at 600 rpm. How many miles have you put on it? I know I've just recently(last few years) run into exhaust valve seat recession on my 455's. On two different sets of heads after about 10K miles the valves start loosing their seal and the idle develops a slight miss. Maybe try a leakdown test. Compression test doesn't really pick it up but in you case maybe it would. I think it is the alcohol in the gas because I never had the problem before. Your later heads may have hardened seats but who knows?
Plugging the brake booster makes no difference, unfortunately. I've got the carb all cleaned up and reassembled. I filled the bowl with water and let it sit for a while over a pail and it wasn't leaking. I also noticed that neither of the pull-off diaphragms are working, although the choke one in the back used to work but always was a bit slow. I should have new ones in tomorrow or the next day, then I'll put it all back on the car and see what happens. The engine had about 60,000 miles when I got it, and I've put about 20,000 or so on it myself. I completely went through it when I got it, and the cam has only seen about 10,000 miles, maybe less. I find the EGR valve helps a lot with mid-range ping, so I'll probably keep it. It's inactive when idle anyway and that's my main concern at the moment. I wouldn't doubt there being a little bit of valve leakage, as there is a *little* chatter when it gets good and hot after extended highway driving. The heads haven't had any work done to them other than being disassembled and cleaned by hand and new stem seals installed. How sloppy should the rockers be on the shaft? It may be time to replace some things. The lifters and push-rods are new, put in with the cam. Intake leak, eh? That was an interesting read. I may have to look into that if this doesn't help. What really eats me up about it is that it's an intermittent problem, and I wouldn't think that a crack in the intake would alternate back and forth, seeing as I can find no connection between temperature and idle quality. But, who knows? If this doesn't solve it then I'll have to go somewhere next! Now to wait for the new parts.... Thanks!
I've just run into the same problem on a 51 mercury I was working on it turned out to be a cracked fuel line on the tank. The car had a manual pump and for some reason it was pulling just enough air to keep the fuel level in the bowl low enough to lean it out but when you got on it the pump pulled enough vacuum to supply good fuel pressure. Not sure if this is your problem but its something else to look at. If you have a clear filter install it up stream and look for air are if you've pulled the plug for easy primary tuning you can check your bowl level when it acts up. If thats not it the only other thing I can think of is to tear the q-jet down and pull the tubes then check for trash .hope this helps and good luck.
Last week I rebuilt the carb, filled it with gas, and it fired right up and ran beautifully. I wound up with about 5 turns out to get a smooth idle on both sides, which is a bit above spec but if it runs well then I don't guess it really matters. I drove with my girlfriend an hour up the highway to visit family doing 75 and 80 with no problems, drove around town up there for a couple days, again with no problems, and then on the drive back idle just disappeared about 1/2 mile from home. I've since been out there trying all sorts of things. I can put the fuel line from the pump into a gas can and it slurps it down pretty quickly and doesn't make a difference, and I can fill the bowl by hand and it still doesn't make a difference, which rules out any tank or pickup problems, I think. I did notice that I was missing the power piston spring, but at idle it should be in the down position anyway, right? Therefore that can't be the problem. I found a small spring and stuck it in there just for now until I get my hands on the proper one. It stays down when idle so it should do for the time being. I'm at my wits end! I don't really have the dough to shell out for another carb (poor college student here ) but I don't know what else to check. I very thoroughly cleaned out the carb (put it in the dishwasher!) and blew out all the passages so I'm positive that there's no gunk in there anywhere, and I have an inline fuel filter on the suction side of the pump and am still using the old steel line on the pressure side up to the carb, with the carb's filter removed. What else can I try? Thanks!!
Well sorry that wasn't any help. The idle properties on a q-jet are not an issue per spec unless thats the motor the carb came on I often have to drill out idle runners or air bleeds to get one in specs but without looking at it I still think it's trash. Did you pull the idle tubes out of the carb when you cleaned it I've seen trash hide there alot
I think u are on the right track. Smal fuel trash could have lodged itself in one of your main jets. Happened to my holley 600 i have on our boat. Started runing rough. No accel. Poor idle stumbling etc. Rebuilt it and found a very tiny grain of dirt in the main jet. But all the advice on this site is true and correct.the trash in fuel these days is high. Try to also find a fuel source that is alcahol free. Good luck and let us know what u find
I pulled it all apart again this afternoon and went through it with a fine-toothed comb. I didn't remove the idle tubes because I'm not 100% sure I could do so without damaging them, but I did poke around with a fine wire and made sure there were no obstructions. I removed the main jets, and poked and sprayed every tiny little passageway I could find, and then put it back together setting the idle screws at 4 turns out. Previously when it would run well I had to have them at around 6 turns out. I put the carb back on the car and it started almost immediately and ran beautifully. I let it warm up and ensured that the choke was fully open and that it was on low idle, and it was smoothly idling at around 900 in park and 600 in gear, with no adjustments being made. I drove it a few miles without any problems and then parked it for a few hours. I went back out about an hour ago (it's rather chilly out so the engine cooled quickly) and drove some more. About 8 miles down the road (doing 50 - 55) after several successful stop light stops, all of a sudden the idle in gear dropped to around 400 and the car was shaking a bit more than normal. I turned around and came back home, the whole time driving great until I needed to stop. It's cold and dark and windy out now so I'm going to wait until tomorrow to mess with it anymore. What could it be? Maybe the base plate is slightly warped and the manifold heat is enough to break a gasket seal? Tomorrow I'm going to tighten the screws on the bottom and see if that makes any difference.
It sounds like you may have a rusty fuel supply. I suspect you keep getting scum into your carb after each rebuild... I always start with a clean fuel tank, lines, then carb rebuild. There are a lot of vehicles that NEVER run right due to a rusty fuel tank.
A rusty tank, eh? That's actually the best thing I've heard yet. I don't know why I hadn't thought of it sooner, because it makes perfect sense. I guess the inline filter I'm using isn't quite enough to collect all of the junk. I'm eyeballing new tanks online and trying to decide if I should just bite the bullet (I remember the filth my dad dumped out of the tank in his '71 Ford pickup a few years ago...) and get a new one or try and clean my old one out. On new tanks, can the old filler neck be attached easily or is it better to spend the extra and get one that comes with it already attached? It's so close I can almost taste it.... ou: Thanks!
