QuestionQUESTION: Hi Scott. First off, thanks for all of your past answers to other folks' questions. They've been invaluable to me as I've recently purchased a 1973 GT6. This car was running rich when I purchased it (spark plugs completely matte black), but now seems to be running a bit lean, and I can't seem to sort it out. At the risk of rambling on, here's a short history of what I've done.
(1) Rebuilt an extra set of CD150 carbs and installed them. My metering needles are back all the way off (full rich), and they're the stock needles (B5CF).
(2) New air filters and gaskets (not K&N, just standard OEM filters).
(3) New plugs (gapped at .025), new wires, filed points and gapped at .015.
(4) Compression test reads 150, 140, 135, 140, 140, 150; wet test (with 2 eyedroppers full of 20W-50) is 155, 145, 140, 145, 150, 160.
(5) Set rocker valve clearances.
(6) Timing seemed fairly advanced, so I set it back to 4 degrees ATDC. Unfortunately, I believe the vacuum diaphram on the dizzie is shot (when I suck through the hose, I get almost no resistance).
And here are some mods that the previous owner made before I purchased the car.
(1) The rocker cover is vented through a hose to the bottom of the engine compartment -- not the carbon canister. Right now I have the intake tubes on the carbs next to the butterflies capped off, like the previous carbs were.
(2) Not sure if the P.O. did this, but there is no PCV valve or EGR valve.
(3) New Monza exhaust (but still the original manifold).
(4) P.O. had removed the oil pressure sensor, and installed an after market sensor and gauge. I've installed a brass T-fitting, and have both sensors installed as my understanding is that the original sensor also operates the anti-runon valve.
Looking at my plugs, some of them seem to be running lean despite the metering needle adjustment. The side electrodes are gray-ish, but on some of them the center insulator is still very very white. Also, the ones that seem to be a bit lean aren't 1, 2, 3 or 4, 5, 6 -- it's something like 1, 3, and 4. However, I'm running unleaded gas (93 Premium), so my spark plug readings might be useless.
The car starts up fine, runs at a steady 190 degrees (unless I get caught in traffic), seems to have good oil pressure, etc. But before it gets completely warmed up, it also hesitates somewhere between 2000 and 2500 RPMs, usually in 1st or 2nd gear. My assumption is that this is due to the mix being lean (?). However, my coil and distributor are both original so I guess it could be electrical. Otherwise, it idles fairly steady at about 800 RPMs. On occasion, it also diesels ever so slightly when I turn the ignition off.
I suppose I'm wondering a few things. Could bypassing the emissions (tube from the rocker cover) cause something like this? Or could I need a different metering needle? Maybe because of the aftermarket exhaust and/or emissions cheats?
My apologies for the long-windedness of this.
Many thanks,
Nathan
ANSWER: You've done a thorough job of understanding the basic condition of the engine, so that allows us to rule a couple things out.
Reading the plugs to determine mixture is very inconsistent, and has to be done a very certain way (called a plug cut) in order to be able to determine the correct mixture under load. A plug cut is running at WOT driving the car, and then shutting off the ignition, and immediately stopping and pulling the pugs. That will tell you if the carb is set correctly under load.
Trying to read plugs at idle or low speeds wont' tell you anything. Nor will the type of gas affect the read. An EGT or CHT or vacuum gauge will tell you more.
First step, since some of the emissions has been modified, is to clean them up and make sure you don't have a vacuum leak, which will throw off the mixture, no matter what you do with the carb:
1) PCV: since you have some blow by due to the fact the wet reading are higher than the dry readings for compression, you need to make sure the valve cover hose is plumbed to atmosphere with a catchcan & filter, or to the 2 intake tubes on the carb. If you connect to the carbs, you'll suck some oil, but as long as the blow by isn't toooo bad, its OK. If you have access to a blowby gauge set, that will tell you how bad your rings are, and how much you have to worry about PCV. Too much pressure in the crankcase will cause the engine to fight itself, as the pressure that blows by has no-where to go, and will cause rougher running, as well as oil leaks at the weakest points (valve cover gasket, for example) You really don't need an PCV or EGR valve on an older tired motor. It just needs to be able to vent. But make sure that it either ties into the intake vacuum, or has a filter so that nothing can come back "up" the hose.
2) EGT/Colourtune/AF gauge/diagnostic comp Trying to determine the mixture is tougher, and if you have the needles all the way recessed into the air valve, and it isn't running rich at idle, then I would start looking for a vacuum leak. Can of carb start while the engine is idle should allow you to track it down.
3) Spark: if the spark itself is yellow(ish), that's too weak. If the spark is white, it at a better voltage to ignite.
4) Distributor. Since the diaphram is shot, as you indicate; cap it off, it is certainly adding to the leak. I don't know if this distrib is a vac adv or vac retard. Look at which way the timing plate is supposed to move if the diaphram "pulls" toward the side where the vacuum connects; if it moves the plate clockwise, that's advance (I think, don't quote me). You may even have a carb with vac adv and ret setup; I've seen distribs like that from time to time. However, I doubt spark is your issue; its more likely the leak.
5) Carbs. You may have the carbs out of sync, and they may be causing the imbalance in mixture. Float level setting is also touchy on the carbs, and I will assume that when you rebuilt the carbs, you make sure all the odd little passages were clean with air. I've had a plugged passage screw me up more than once. There are also many vacuum ports that may have caps (like the intake tubes) that may be leaking, so making sure they are all capped, even if you think they are blocked, is another precaution.
Let me know if this helps, and what out of the above checks out. If you can't find a vacuum leak, you may have to go farther in diagnostics, putting it on a scope, looking for a crack in the combustion chamber/piston/block or something else that may be causing the fuel mixture problem. At this point you need to do a little more process of elimination so that you know what it isn't. Then we'll go through what is left.
---------- FOLLOW-UP ----------
QUESTION: Apologies for replying a second time before you even had a chance to respond to my first reply. My trip to Autozone was a bust -- the dizzie parts they had marked as Lucas were in fact for Delco. I did get caps to plug off the vacuum retard. And a filter for the breather pipe.
On the drive to the store, I only experienced a tiny amount of hesitation once. In 3rd gear around 3000 RPMs, and not nearly as bad as the hesitation I was experiencing before. Before the car would buck and almost die. This time it was minimal. Also, the tendency to run-on was much less pronounced.
Also, as I was putting the caps in the vacuum, I realized that when the original carbs were installed, a hose ran from the carbon canister to an intake pipe on each carb directly behind the air filter cover. The carbs I rebuilt are the previous model and do not have that intake pipe (only the one close to the manifold). Would the lack of connection to the carbon canister cause a leaner mixture -- since the fuel tank still evaporates to the canister, but those fumes never reach the carbs?
Thanks very much for your time and patience.
AnswerThe carbon canister being unconnected would not cause a leaner condition. It is a storage device for vapors when the car isn't running. When the car is running, it is extracting any vapors that were stored , but that is such a small amount, it has no effect on the overall mixture. A vacuum leak because one of the cannister ports isn't plugged would be a larger issue.
What is the color of the inside of your exhaust tail pipe? If black you are running rich. If the bodywork near the exhaust pipe gets black after running the car, you are running rich. If you have carbon deposits, its telling me you are running rich. If coasting the car doesn't miss, but it does miss under load, you're getting in the range of things difficult to trouble shoot... It could be any of the things we are mentioning, or something completely different. If you never hear the engine ping, I doubt you are running lean. You may be running more into a fuel starvation issue, or vapor lock, than a mixture issue.
