Alright guys, I need some troubleshooting advice. I've been trying to figure out why my '71 Mav's freshly rebuilt 302 is fouling cyl. #8 and the rebuilder says he thinks it a vacuum leak, possibly around the vac. tree on the back of the intake, which he said is driven by cyl. #8. Stock Autolite 2100 carb was rebuilt recently (but possibly done poorly by an inexperienced shop), PCV is new, new plugs/wires/dist cap and rotor/Pertronix Ignitor III/Flamethrower Coil. Under no load (Neutral/Park), I'm pulling a very steady 18" according to my vacuum gauge. I've sprayed carb cleaner around and can't find any leaks. If I quickly blip the throttle, she seems very smooth and responsive. If I creep up the RPMs, anything between around 1200 to 3000 or so is very rough. In drive though, the motor SEEMS smooth even though I get the random misfire when pulled up to a light. Plug #8 has to be removed and cleaned weekly due to the amount of sooty buildup on it. The vac. advance is fairly old judging the looks of it and seems pretty stiff but I can get it to move a little by applying vacuum to the hose. I don't have a compression check gauge yet, but if you all think it might be a stuck valve or bad rings, I'll just get one. I'm on a tight budget (aren't we all?) and looking to troubleshoot for free as much as possible. Throw some ideas my way so I can get this thing figured out. Jory
Where do you have the vacuum line to the trans connected. You might be drawing up trans fluid from a bad mod. valve. Check that hose and see if it has oil in it. This is assuming its an auto trans of course.
Yes, it's an auto tranny. The guys that put the motor back a section of vac tubing from the vac tree at the rear of the intake manifold, which plugs into a metal tube that runs rearward and then plugs into another rubber vacuum line toward the bottom of the tranny on the passenger side. I hope that makes sense. I tried unplugging that line and then plugging the tree but got the same result. I don't think I plugged the hose that ran to the transmission though because I didn't think it would matter being that the link to the motor's vacuum system was broken.
Also, the vacuum line going to my vac advance is from the port just below the automatic choke, which runs to the port on the front of the vac advance. The rear port on the advance has been plugged and the tree that was used before, on the thermostat housing is open/unplugged. Is anyting wrong with this setup?
Where does your PCV valve vacuum line go into the intake? If its on the vacuum tree on the #8 runner, change it to the back of the carb or the spacer under the carb. Power brakes? Try temporarily removing and plugging the vacuum line to the booster and see if that helps. (Obviously, don't drive the truck that way.) Booster may be leaking. Swap the #8 plug and wire with another cylinder and see if the problem moves.
On the PCV thought...what valve covers are you running. If not factory make sure they have a baffle under the PCV valve in the valve cover.
I'm currenly still using stock valve covers. The PCV line goes to the back of the carb. The only lines coming off that tree is to the vacuum canister and to the tranny. The elbow on the passenger side is only for the vacuum of the carb to clear out the gasses, right? That shouldn't be my problem though, right? I tried disconnecting and plugging that side with no noticeable results. I've found some articles stating that for a auto tranny (like my C4), the vac advance needs to be hooked to manifold vacuum, not the port on my carb near the auto choke where I have it. Thoughts? My car doesn't have power brakes. It's a b*tch to stop, but hey...it's a great leg workout. I'll try swapping the plug and wire for a week and recheck. This is the 3rd plug I've used on that cylinder though, the 1st was old, 2nd was fouled and now this one is constantly needing to be cleaned.
Not sure what you mean buy "elbow on the passenger side". Picture would help. Did you check the compression on #8 cyl? Hope you don't have a problem with the rebuild. You could check for a baffle under the PCV valve on your stock valve covers. Someone might have removed it for aftermarket rocker arms. If you cannot find the problem you might get around it by installing a hotter plug. Shouldn't have to do that but, if all else fails ...
No, I don't have access to a compression gauge at this point. I'm really hoping it's not the rings/valves on this rebuild. I've attached what the elbow looks like...just a standard PCV elbow. I didn't think this had any function (not like the PCV itself) other than providing a connection to the passenger valve cover for a hose to the air-filter to recirculate gasses. What does the baffle look like? Which valve cover will it be on? I think the rocker arms are all stock, like everything else with my motor (other than ignition). So could it improve things if I go to manifold vacuum for the vac advance instead of where I have it now?
I would check the compresson on all cylinders well before your warrantee period runs out. Especially when one plug is dirty. One valve cover gets the PCV hose like you have it. The other gets a hose connected to the air cleaner. The connection to the air cleaner allow clean air to be drawn into the crankcase when the flow thru the PCV valve is more than enough to suck up blowby. When blowby exceeds what the PCV is passing, the connection to the air cleaner allow the excess to go into the intake and get burned rather than venting to the atmosphere. Shouldn't matter which connection goes to which valve cover. I don't have the air cleaner connection but have a breather cap instead. So, I'm not positive but I think you're right, that's just an elbow. If you pull the PCV valve out and look down into the valve cover, you should see a piece of sheet metal. If the baffle is missing you'll see the rocker arms. Would expect baffles on both valve covers. If your engine idles and runs better with the vacuum advance on full manifold vacuum, that's how I'd run it. Knowledgeable people give conflicting advice on manifold vacuum vs ported vacuum. Don't know if changing to manifold vacuum will affect your total timing but wouldn't hurt to recheck it.
I sure hope not, considering it's new. Think I could get lucky and just check the bolts for the intake?
Nice info! Thanks for taking the time to explain all that! I'll take a look to see if those baffles are in there. About the ported/manifold vacuum...yeah, I've always heard different preferences from different people. This has worked for me well but just currently I'm having a problem and wondering if I need to switch it up for my current situation.
I checked over the weekend and there is a baffle under the PCV. I tried hooking up the vacuum advance to the manifold vac but didn't really gain anything...it still has that intermitten miss that makes it idle a little rough like it loses power for a split-second every few seconds. It does seem that the RPMs advance and the motor runs a little more smoothly when I disconnect the vac advance and plug the carb port. Odd. I checked the intake bolts which were all tight and so was the vac tree at the back of the intake. Oh, and I swapped the plug/wire but the problems persist. Any other ideas?
