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MG MGB Technical - carbon deposits on plugs
| hi 79mgb recently, my mg has no pick up. it runs as i said but no pickup. also, carbon deposits on the plugs, dry, not oily. before i screw things up in my usual manner, how about some hints/ analysis as to my problem, what to attack, etc. i am hoping for suggestions that won't have me rebuild the engine,( or rather, have it done.) thanks john |
| john sutter |
| John. What plugs are you running and what is the gap between the center electrode and the arm? Too cold a plug will carbon up. What is your driving use of the vehicle? Short trips will carbon up a plug because you need a richer mixture when the engine is cold. What carb are you running? What type of choke? Both of these can have an effect on how fast/if you will get a significant carbon build up. Set the engine up with NGK BP6ES plugs gapped to .035" so you are starting with fresh plugs and a known standard. What are your compression readings? When did you last adjust the valves? What ignition system does your car have and when was the last time it was cleaned, inspected, and, if necessary, parts replaced? All of these things can have an effect that would produce the symptoms you describe. Les |
| Les Bengtson |
| Classic symptom of a bad diaphragm in the ZS carb, assuming you still have that. This is an "every few years" replacement item. Remove the 4 Pozidrive screws on the carb top, lift out the piston, and examine the diaphragm for tears. May as well buy one ahead of time, since if it is not bad now it will be soon. 4 more Pozi screws hold the diaphragm to the piston. When reinstalling, look for the little tabs that engage in notches in the piston and carb body - orientation is critical. Strongly suggest you buy a #2 Pozidrive screwdriver, as the screws are frequently very tight and a Phillips will damage them. {ALL "Phillips" screws on MGB are Pozi! Most are #2} Pozi screwdrivers can be hard to find; some of the parts suppliers have them, or SnapOn dealers or industrial tool suppliers. Sometimes you can find Pozi bits in interchangeable bit assortments. All that stuff Les mentioned also, but you are 95% on the diaphragm. Also check that the three screws holding the autochoke (or manual conversion) on the side of the carb are tight and present. They are weird - special, not at all standard - screws, of two different lengths. If they are missing, loose, or a long one is where the short one should be, will cause rich running or fuel leaks. I have gotten them from Moss and TRF, but you will probably have to call and talk to a tech expert. FRM |
| FR Millmore |
| les, why do you say plug gaps at 35 when book says 25. have noticed that a few times on here, mine runs fine at 25. just curious. bob. |
| trebor |
| Bob. The vehicle under discussion is a 79 which has a factory specification of .035" for its spark plug gap. However, I run the .035 gap on my 68 and have found no problems associated with its use. US cars of the period (60's) used the .035" gap with no problems. I suspect, but cannot prove, that the .025" gap was established back after the second world war when the Brit's had to use something described as "pool gas" and the smaller opening helped with this fuel. On my engine analyser, both a .025" and a .035" gap show 11K firing voltage. My recommendations were based on this understanding. Les |
| Les Bengtson |
| Here is a question, What would happen if you 'Jet hot coated' the combustion chamber? I know that exhaust manifold benefit from external and internal Jet hot coating, even the intake manifold benefits. So would there be any benefit in the combustion chamber itself? There was even talk in research circles about ceramic combustion chambers, is this similar then? Mike |
| J.M. Doust |
| Mike- http://www.hotrod.com/techarticles/engine_coatings_review_tech/ http://www.swaintech.com/ FRM |
| FR Millmore |
| The gap changed from 25 to 35 thou with the change from 25D4 distributor to 45D4. This was so long after the war (30 years) which caused a temporary change in fuel available anyway, that I suspect it was more to do with the design of the distributor itself e.g. cam profile, resistance to HT breakdown etc. as much as anything else. The higher plug gap *should* result in a higher peak HT voltage, as the voltage rises rapidly until the plug fires then stabilises. The Leyland Workshop Manuals have examples of engine analyser displays with various ignition problems, including one clearly showing a much higher peak voltage - 7kV to 11kV - at a fast idle when one plug has an excessive gap. Under hard acceleration and correct gaps all plugs show a peak HT of about 11kV as the higher the cylinder pressure the harder it is for the plug to fire. |
| PaulH Solihull |
| hi, again 79mgb my plugs are ngk gapped at 035 put in a new diaphragm, but now it wont start, i did something wrong ? will refit the diaphrgm as soon as it stops raining. more suggestions ?? thanks , john |
| john sutter |
| Was the diaphragm in fact bad? Did you get the little tabs in the appropriate depressions? One on piston, one in body. It will not work if the piston is in the wrong orientation, which is determined by the tabs. How long have you had the car? It is possible that the mixture has been adjusted lean as an attempt to compensate for a failing diaphragm, which means you have to set it back. If you do not have the special tool to adjust, you can use a 1/8" Allen wrench, but you MUST hold the piston while adjusting or you will tear the diaphragm. Possible that the plugs are fouled. Either change or clean. You can burn off carbon fouling by heating the electrodes with a propane torch. Don't pick up the plugs until they cool! FRM |
| FR Millmore |
| hi 79mgb still won't start !! diaphragm looked ok, but i replaced it anyway ! getting gas, so no fuel delivery ruled out. i had cleaned the plugs by gently filing the electrodes. since i can't start the car , how can i test to see if the plugs are firing ? , or to test the coil ? any other suggestions. ? (since it won't start i can't, in desparation, drive it to an expert ??? ) john |
| john sutter |
| Firing plugs? Check almost in darkness. I take a plug out, connect the lead, then crank. You should see the spark. Or, some car accessory outlets have these plug lead testers, you clip them on and crank a little window on the instrument light up when it senses 'High tension'. If you get a spark, then you coil must be ok?Timing could not have gone could it? Mike |
| J.M. Doust |
| It's not the electrodes that need to be cleaned, but the insulators. The carbon makes the insulator conductive, and the spark runs down the side instead of jumping the gap. Either sandblast if you can find an old time plug cleaner, or burn off the carbon. Or try new plugs. All that aside, if the diaphragm was not bad, then the most likely thing is the autochoke, if you still have that. There is a pretty good section in the Moss catalogue on this. Another common fault on these 75-on cars is that the air filter gets clogged up, either from age or an exhaust leak at the manifold. The air flow on the filter is backwards, so the dirty part is on the inside where you can't see it! An exhaust leak at the pipe to manifold can clog a new filter in 500 miles of use. FRM |
| FR Millmore |
| I can't really see all the plugs suddenly failing just after changing the diaphragm. Connect the king lead from the coil to a plug laying on the block, remove the distributor cap, and with the ignition flick the points open and closed. You should see the plug sparking. If that OK refit the cap and king lead, and check each plug lead is firing a plug laying on the block when cranked on the key - with LHD you will probably need an assistant. Not much point in saying more until we know what is missing. |
| PaulH Solihull |
| hi, again 79mgb a review getting gas, ok plugs are firing, but got carbon residue on plugs, cleaned distributor. diphragm o.k..replaced. the top, larger, hole where pin goes is oval BUT the small orifice where the pin actually does it's work is great, nice and circular. mixture right now is too lean, i think, i had been playing with it. nothing else has been touched/ adjusted. try to start and continued operating the starter motor for an extended period of time , got a backfire with a big cloud of black exhaust, but no start. help john |
| john sutter |
| Lets go back a bit - just which diaphragm did you replace, carb or distributor? Have you fitted new plugs? Have you got the firing order correct i.e. 1.3.4.2 counting anti-clockwise on the distributor? And is the rotor point to No.1 when it is at the top of its compression stroke? You determine this by removing the plugs, putting your thumb over No1 plug hole while turning the engine. When you feel compression starting to lift your thumb off, keep winding until the piston is at the top of the bore, then check to see which contact the rotor is pointing at. |
| PaulH Solihull |
| hi 79mgb 175cd carb i upgescrewed. i meant the carb, i haven't touched the distributor. plugs ok firing order 1432 will try your suggestion as soon as the weather clears up a bit. as i think i mentioned, i have no garage. john |
| john sutter |
| John. First thing, read what I posted originally. You have provided an answer to only one of my questions and all are pertinent to the discussion at hand. "firing order 1432" That may be part of your problem. The correct firing order is 1342 in an MGB and most other four cylinder engines. Certainly, having your plug leads in the incorrect order will make the engine difficult to start. Beyond that, with no testing and having acknowledged that the original diaphragm looked to be good, you have decided that the problem is with the carb. Have you done anything to verify this? Pour a little fuel into the throat of the carb and see, with the correct firing order of the distributor leads, whether the engine fires up. If not, you have simply added another variable to your trouble shooting. My website, www.custompistols.com/ has some simple test procedures in the MG section. They might be of use to you. Les |
| Les Bengtson |
| hi again 79mgb 175cd carb man, i can't get anything correct. the firing order is 1342, it has been that for the 12 years i have owned the car. why i wrote 1342, i'll never know. john |
| john sutter |
| Can't even get the correction right either :o) |
| PaulH Solihull |
| hi 79mgb 1342 !! paul with the distributor rotated to tdc at top of compression stroke (pressure felt at #1 plug) the rotor arm points to plug #4, counting anticlockwise from #1,( as best i can describe its position.) when the car was running the distributor was set at about 20 degrees btdc. what do i try now ? another problem is that the distributon has 'slipped' and is no longer set as it was when the car was running. how do you correct this with the engine not running ? first you get the engine running. thanks john |
| john sutter |
| Oh boy, It sounds like a right pickle! John, close the garage door and enjoy the festive season. Come back to the car after a week. Let the Lord work on the car, God bless all, have a Peaceful Christmas. Mike |
| J.M. Doust |
| John, sounds like you have crossed the plug leads when cleaning the plugs... Swap 1 and 4 then 2 and 3. Try starting it and see what happens. You might get a nice Christmas surprise! MGmike |
| M McAndrew |
| When you've got it running, mark your plug leads! Tape, blobs of paint on the caps or nicks in the rubber of the caps, anything to make sure they don't get mixed up. When I looked at my MGB, intending to buy it only just started and ran incredibly rough. I gambled, bought it anyway and was relieved to find that plugs 2 and 3 had been swapped over! Merry Christmas! |
| Mike Standring |
| "with the distributor rotated to tdc at top of compression stroke (pressure felt at #1 plug) the rotor arm points to plug #4, counting anticlockwise from #1,( as best i can describe its position.)" Well that says your plug lead connections are completely wrong. Whichever cap contact the rotor arm is pointing to, when you detect TDC on the compression stroke of No.1, is the one you should be connecting to No.1 plug, not No.4. Your distributor internals, or distributor drive gear, are/is obviously 180 degrees out. Connect wherever the rotor is pointing to to No.1 plug, and count 1 3 4 2 anti-clockwise from there for the others. As far as timing goes start with 10 degrees BTDC static and if everything else is correct it will start and run well enough to fine tune things. Static timing is determined by connecting an test-lamp or meter to the points, ignition on, and turning the distributor until the points just open when the crankshaft pulley mark is about (it's not critical) at the 10 degree mark on the timing cover pointers. |
| PaulH Solihull |
| John. There is air pressure present on both the compression and the exhaust strokes. Thus, simply feeling some air pressure is not, alone, any indication of which stroke the cylinder is in--especially to the untrained individual. And, your knowledge/experience level seems to be rather shallow. The only proper method of examining the timing, especially for a beginner, is to remove the rocker arm cover and turn the engine over while watching the valves open and close. When the intake valve opens (second valve from front of engine), then begins to close, you are moving on the compression stroke. Run the harmonic balancer around to the 10 deg BTDC point and see where the end of the rotor is pointing. This, along with a lot of other useful trouble shooting information is on my website, which I referred you to before and you do not seem smart enough to read and understand. As to the carb you have taken apart, did not find anything which looked bad, so decided to replace pieces of it, I am not a great fan. Neither are most people. While I would never recommend replacing the twin SC carbs with a Weber DGV, I routinely have replaced the Z=S carbs on my cars with the Weber DGV. There is a dramatic improvement noticed when the Weber is fitted and you will keep noticing the improvement, whenever you drive the car, until the Z-S is simply a dim memory of a bad time that happened a long time ago. Again, you need to come up with some form of systematic plan of action, test the various components in a logical manner, and solve the problem. Not a difficult process if done correctly. Difficult and frustrating if done incorrectly. You seem to be determined to do it incorrectly. Consider the fact that your car can be loaded onto a flat tow transport and, for a modest amount of money, be driven to a competent mechanic. Les |
| Les Bengtson |
| "There is air pressure present on both the compression and the exhaust strokes." On a running engine there may well be detectable cylinder pressure on the exhaust stroke. But on one being turned by hand the exhaust valve being open on the exhaust stroke will mean you won't feel any pressure by blocking the plug hole with your thumb, compared to the compression stroke where both valves are closed. |
| PaulH Solihull |
| hi !! 79mgb thank you all,some more than others - saved all the suggestions for later use, the mg now runs, though, not well. i still have to do the timing and mixture. happy hanukkah, christmas, new years, and kwanzaa. till next time, though i hope it will not be soon. john |
| john sutter |
| Paul, I have, several times, demonstrated to the inexperienced mechanic that you will feel some air flow through the spark plug hole on the exhaust stroke. Most especially at the beginning of the stroke when the exhaust valve is just moving off its seat and the air tends to flow more readily out the open spark plug hole than through the almost closed valve. If this happens, the number four cylinder is on its compression stroke and the distributor rotor would be pointing to the number four lead--exactly the situation described. But, since John now posts: "the mg now runs, though, not well.", I guess we will have to see what happens as he performs more tests while dialing in the engine. Les |
| Les Bengtson |
| Les - I don't doubt it. But if you *close* the spark plug hole with a thumb as I suggested then I'd be very surprised if anyone would do it so weakly as to have the piston rising on the exhaust stroke lift their thumb off the hole. It is pressure your thumb is 'measuring', not flow. |
| PaulH Solihull |
This thread was discussed between 05/12/2011 and 03/01/2012
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