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MG MGB Technical - Mgb Head and oil burning
I have a question re: oil burning/usage on my 1968 MGB. I'm trying to decide if I need to go full rebuild because doing compression test, dry & wet, all cylinders stay within 5lbs of each other. They range between 150-155lbs. dry and wet add 5 more to each. I assume this is normal and would not cause an oil consumption problem? When I do valve checks I just do a re-adjustment check of valve lash with engine cold. I usually use three gauge sizes---one above, one below, and one of the actual setting. What else can I do to narrow it down? Others mention leak down test but I don't have equipment for that and have never done one. Sorry for this long message but I hope I covered with sufficient detail. Thanks! |
Don_d |
What is your actual oil consumption and what sort of oil do you use? Blue or black smoke from engine on acc or overrun? Might this be a crankcase ventilation issue? Seems your compression is ok. regards Jan |
Jan Emil Kristoffersen |
Valve stem/guide wear won't be shown up by a compression test, and this usually causes the cloud of blue smoke when revving the engine after idling for a while, or when opening the throttle after a down-hill stretch with it closed. If you don't get that then it could be burning via the crankcase ventilation system that Jan mentions. A 68 model would have been the last year to have the PCV valve which can fail, after that it was done using carb ports which are trouble-free. Disconnect the vent hose between the PCV and the front tappet chest cover and see if they are full of oil. Even if the valve is OK you can get problems with the cover, and that can happen with the carb system as well. There should be a flame trap/oil separator consisting of something like a pan scourer inside the double skin of the cover that may be missing or bunged up and not trapping the oil. But even with that as it should be some people have reported that they have to change to a different design of cover to stop it, see the attached. As a temporary or permanent measure you could fit a catch bottle between the valve (or carbs) and the cover. |
Paul Hunt |
Don, these cars do use some oil especially if driven hard. Add to that leaks. Below are some figures from the mgbmga.com website. BTW this site has good articles on the PCV system etc. ***************************************************** Oil consumption can vary under different driving conditions. I consider the following figures to be a generalization: Under 300 miles per quart - very poor 350-500 miles - poor 500-600 miles - fair 600-750 miles - good 750-1000 miles - very good 1000 miles & over - excellent ********************************************** So a couple of thoughts. 1. What is your oil consumption? See above. 2. Have you inspected your plugs for oil fouling? 3. Do you have a lot of oil leaks? 4. Do you see blue smoke? When? (ex. accelerating from idle, cruising, decelerating) 5. Is your engine a high or low compression? That will put the compression tests in perspective). A high compression is nominally 160 psi and a low compression 130. But, carbon deposits or a skimmed head can increase these nominal readings. That said given the pressure only goes up 5 psi with oil it looks okay. 6. Normally a compression test should be done with a hot engine and the throttle open when cranking. 7. A leak down test is more complex but gives better diagnostics. But, you need added equipment to do this. I've never tried it. I live in Somersworth NH near Dover. My 65 low compression engine (now an 18V) tests around 140 psi. My consumption is probably 750 or so miles/USQ. I have some leaks (nothing major) but I am sure the valve guides are worn. Unless your oil consumption is poor or worse I'd be tempted to drive and enjoy the car. Oil is cheap if you don't run out. Note, certain Audi and BMW standards state 600-700 mi/USQ is reasonable. |
Robert McCoy |
Below is another perspective on oil consumption from mgaguru.com. His MGA figures should also apply to the MGB. ******************************************* 500 miles per quart of oil is border line normal for an MGA, nothing to worry too much about, as oil is still cheaper than internal engine work. 700-800 miles per quart is honorable mention, and 1000 is very good. 1200 miles per quart is very unusual, and maybe only with a new engine just run in. Anyone who claims to go 3000 miles between oil changes with stock MGA engine without adding oil is probably mistaken (or lying). |
Robert McCoy |
Well, gentlemen, it seems I made my message too short! Sorry for that. Should have been even more specific. This car has yet to see the road, so all runs, diagnostics, etc. are being done in my garage. I believe this is the high compression engine, which I checked against the engine numbers. I use Castrol 20w50 and there are no engine leaks, very surprised at that. Weird thing is I don't see a significant loss of oil on the dipstick, which I don't think is the original as it bottoms out in the pan. Of course, the PO did so many poor things to this car that I have been correcting for years when money is available. While idling I don't see much in the way of smoke but it is blue. When kicking in throttle for short time and release, get a cloud of black smoke. Strange though is that I don't get this huge cloud every time! The plugs are taken out after each run in shop and they are coated in black soot, cleaned and replaced for next run. Otherwise they will cause some irregular idling and/or other poor running conditions. I am using the recommended N9YC(?) I believe is what they are. Also, all the smog gear was removed by the PO and the tappet cover with breather hole has just a rubber hose hanging down in the engine bay. I did remove the original mesh and replaced it with copper mesh because it appeared to be almost clogged. The other things I did was replace the HS6 carbs he had on there with a Weber DGV. That's because he had two different jets on them, were causing a vacuum leak, throttle linkage needed overhaul, and that was just the tip of the iceberg. This problem was ongoing even before the Weber was installed. I also did a header system and the tailpipe is coated with black soot. I got desperate last year and got hold of umbrella valve guide seals that fit this car and installed them. No significant changes at all. I hope I've answered most if not all your questions but I am ready to answer more if you have any. Thanks guys. |
Don_d |
Made a mistake on the 4rd line down. Referring to throttle on then off again it should say "blue" smoke not black. |
Don_d |
"The plugs are taken out after each run in shop and they are coated in black soot". Soot and no hint of an oily nature just suggests running rich. As do your other obsevations, i.e., nothing significant on the dipstick. Smoke from exhaust on over-run indicates valve stem/seal leaking. Smoke under power indicateds piston/ring wear. Your compression reading don't indicate the latter Get some decent HS4's and bin the WEBER. Borrow a proper dipstick and calibrate yours. Check the timing. |
Allan Reeling |
18G, GA, and GB dipsticks apparently rested on a dimple in the oil pan. Later versions had stops built in. The block tubes vary by engine model so you need to have the correct pair. Check mgbmga.com technical articles under "oil pan, dipstick, dipstick tube combinations". The oil pan should take 9 US pints. Add about 0.9 pints for filter. Sounds like you are running way way rich. First thing is to get the timing and dwell correct. Make sure the distributor is advancing both mechanically and that the vacuum advance works. Then get the carburetor sorted out. If you keep the DGV check the internet for tuning advice. If it is in good repair and jetted right you should get a pretty clean idle after adjustments. Make sure the choke is operating properly. The biggest complaint with DGV's is stumble off idle. Check the archives here on how to minimize that. I've read that sometimes rich idle will give blue smoke especially at idle. One of the issues with a DGV is what to do for crankcase ventilation. The default option is a draft tube like the DPO tried. You will need a valve cover with a vent tube on it to make this work. I really don't like this approach and if I had a DGV I'd try going with a PCV system. But, get it running right with the draft tube first. If I were king I'd install HS-4's, throttle linkage and original intake and exhaust manifolds for a 1968. These can be found in various states of disrepair on eBay or Craig's List. I'd try to get original air cleaners also. The car just works best this way. That said you should be able to get the Weber working. There wasn't much smog equipment on a 1968. I think a PCV and an air pump with it's various pieces. The latter may be hard to source. I wouldn't worry much about what spark plugs you have for now. All are going to foul running rich. You won't know the right heat range until it is run under load. Just keep cleaning them and if they get too far gone replace in kind for now. |
Robert McCoy |
Your compression readings aren't a major factor if your piston oil control rings are seized in the piston ring lands. This will allow large amounts of oil to get past the control rings and onto the compression rings. The oil volume will exceed the compression rings abilities to prevent the oil from reaching the combustion chamber and being burned. This usually happens due to infrequent oil changes by the previous owner. Overheating an engine can also result in hung oil control rings due to their shape and size. RAY |
rjm RAY |
"Weird thing is I don't see a significant loss of oil on the dipstick" So no oil consumption then. Not really surprised, if it is only run in the garage at the moment, nor the sooty plugs. With the Weber I'd run a modern PCV valve off the inlet manifold to the front tappet chest cover, and if you have the rocker cover with the port on the back then fit a small filter there and use a non-vented filler cap. If you have the early rocker cover with the port at the front that was originally connected to the front air cleaner, then block that off and use a vented oil filler cap. The later port on the back, and the vented cap, have a restriction to control how much air flows through the crankcase, and develop a small reduction in pressure. The early front port is unrestricted and will result in too much flow, which could suck oil in. With SUs if you can get the 69 and later type with the breather ports then that means you don't need the PCV valve. |
Paul Hunt |
I forgot about the later HS4's having the crankcase vent ports. It is a better solution than the earlier version. Thanks Paul. I guess you eat this elephant one bite at a time. I don't think New Hampshire does emissions checks on cars without OBD or of this age. You can check. If so you won't need to worry about the air pump. I still don't like draft tubes. |
Robert McCoy |
Don. Without tearing down the engine, you really do not know what you have. The fact that it had HS-6 carbs is a good indication that there has been engine work in the past. Too much carb for the factory engine. Your 68 came with an 18GF engine which had a "high" (for the Brits) compression ratio, HS-4 carbs, a "mushroom" PCV valve on the top of the intake manifold attached to the front tappet cover, an air pump and air rail on the cylinder head, and vented oil filler and fuel filler caps. If you are running a road draft tube, I suggest you replace it with the proper PCV valve. All of the information you have been provided about MGA engines has no meaning with your engine, assuming that you have the correct 18GF or later B series engine. MGA had a three main bearing engine having a felt crankshaft pulley seal and no rear main seal. They did use a lot more oil than the 18 GF and similar engines. However, my 18V engine, in a new 79B, used no more oil that American cars of that same vintage--less than a quart used between Orlando, Florida and Austin, Texas. Older engines may leak more oil than they did when new. And, as Ray points out, if the oil control rings have not bedded in properly oil consumption will be higher than if they had been bedded in properly. In addition, engines which have not been used for some period of time can have the oil rings gum up and fail to do their job of oil control properly. The Weber DGV series of carbs are not as trouble free as some of their promoters would have you believe. They were a multi-use carb which was designed to be set up for specific applications, preferably by someone with a lot of Weber parts and experience. A "carb in a box", even brand new, is no assurance that the system is set up properly for your car. I would agree that a switch back to the factory original HS-4 carbs would be an excellent idea. Not only is it more original, but my experience (four Weber equipped cars, five SU equipped cars) is that the SU, once you learn to tune them, is an easier to maintain, longer lasting carb, that is far more tunable than the Weber. Most of the "problems" you describe are a result of not being able to adequately drive the car sufficient to develope baselines for it. Once those baselines have been established for your car, you can accurately compare what you are seeing with other vehicles of the same type. But, the first step is to accurately determine exactly what you have in your vehicle. A dipstick that bottoms out in the pan is not from an 18GF engine such as should be in your car. An earlier dipstick, used in an 18GF engine should show a "too high" oil level when the sump is filled to proper capacity as it sits lower in the sump than the 18GF dipstick, hence moving the markings lower in relationship to the actual, intended, level. Find out what you actually have, make a list of it, and get the car on the road before worrying about rate of oil consumption. Les |
Les Bengtson |
Wow, guys! Love all the responses and will check off as many as possible. I'm retired and not in great health but I'd still like to get 'er on the road. Some inspection stations in my area are really hard-case when it comes to any smoke coming from the exhaust. As for switching to HS4s, that's not an option for me on a fixed income. My valve cover does not have any ports in it and I checked the oil filler cap by blowing through both sides and appears to be vented(?). I would think if it were not I'd have even more serious issues than I do. The DGV carb kit came from Brit-Tek some years ago and I haven't changed any parts on it. I called there after it was installed and was told to keep trying to make adjustments. Also, when I checked with Bob, he said not to worry about the PCV as the MG should run fine with the setup I have. Well, it hasn't yet but now, reading all your comments, I have more work to do getting the setup right. Bob (Brit-Tek) felt that it was running on its accelerator jets as well as idle jets. As for the black plugs, to be honest, it's hard to tell if it's from running rich or using oil. It almost appears to me to be a little of both. After reading all your comments though I will lean toward concentrating on getting timing and carb adjustments in synch. It's ironic that you mention driving it to get a better baseline; my brother owns a '69 MGC and suggeted the same thing some time ago. Sorry for the delay in responding, as I get on-line when I can. I can't thank you guys enough for all your help and I will continue to work on this when I can. |
Don_d |
You should be able to get some good advice from Brit-Tek. I think they have some advice on-line regarding set up and tuning. The DGV should have been set up with the correct MGB jetting by Brit-Tek which is a plus. Did Bob give you any advice on how to get onto the idle jet only? Normally when the throttle is nearly closed the only jet activated is the idle jet. DGV's can be leaned out quite well at idle. As the throttle comes off idle then transition transfer ports are opened. Then at a certain opening the main jet comes into action. I'm not certain when the secondary barrel opens. The accelerator pump is only activated when you step on the throttle. There is a power enrichment valve that opens on heavy load when the manifold vacuum drops. At idle it shouldn't come into play. Go through the instructions and you should be be able to get her setup. By the way, it takes a lot of oil consumption to make smoke. Modern 2 stroke engines at 40:1 gas to oil mix don't smoke much at all. |
Robert McCoy |
Yes, Bob at Brit-Tek was helpful but obviously I was still unable, at that time, to get it right. His advice was to keep at the two throttle adjustments until Weber was running only on idle circuit. I've tried that so many times since I swapped it that I can't offer an accurate count! My nephew believes what was mentioned earlier here----get the timing adjustments dialed in perfectly (which I have done quite a few times I thought)then mess with the carb. His reasoning was that we have both tried to get carb adjusted right and it seems to be fine after (no lumpy idle, etc.) although the stumble on accelerating (which lasts just a second or two)never goes away. But when you shut it down, restart the next time, it starts throwing smoke again. I believe secondary barrel opening is determined by the linkage setup. As was mentioned, this Weber kit was setup specifically for the MGB. Understanding that every engine is not exactly the same but you should be able to tune it in with minimal adjustment...at least that is what I was told. Just keep keepin' on! Thanks again to all!!! |
Don_d |
Don, Just one item re; timing. Incorrect timing can have an effect on emissions due to increased combustion chamber pressure. The two part crankshaft pulleys do deteriorate and can give wildly inaccurate timing when the outer moves on the perished rubber. Check the TDC mark is accurate. This can be done roughly by the screwdriver down the spark plug hole. There is a "dwell" at TDC, as the piston changes direction, guess the middle of the dwell by feel. That should get you within a degree or two. When I timed the cam on my last re-build, TDC was about 1/2" out!! Had to buy another pulley. |
Allan Reeling |
"18GF engine which had a "high" (for the Brits) compression ratio" To be fair most markets had high compression originally, only those with known poor quality fuel had the lower, which didn't include the USA. They subsequently got lumbered with low compression as an emissions measure. |
Paul Hunt |
I'm not experienced with DGV's but I have read a bit. The following is from mgbmga.com (British Automotive) discussing carburetor choices. ******************************************************** WEBER 32/36 DGV (all MGB's 1963-1980) While a very reliable carb/manifold set up, a disadvantage with this carburetor is the fact that the primary venturi accounts for two thirds of the throttle open position (no CFM* figures are given for this position) with the secondary venturi becoming active in the final one third of throttle movement. This carburetor requires excessive initial ignition timing advance to avoid stumble on take off (if the ignition timing was retarded, we would have to open the primary throttle plate to obtain the necessary idle; this would expose the progression bleed holes which in turn would require that the idle mixture be leaned to compensate for this richer mixture condition. The end result being very little progressive richening upon further throttle opening, especially at snap throttle and so the stumble). **************************************************** This was written in 2001 but likely not much has changed. I have no idea what "excessive initial ignition timing advance" is. But it sounds like stock timing won't do the job. Maybe someone can help. Bob |
Robert McCoy |
Just an fyi Don...If you are running a non stock fuel pump (say a facet type) you will find that the fuel pressure is about 4 to 5 lbs. The SU Pumps usually run in the 2 lbs range. I find that most after market pumps cause the carb or carbs to flood...because the needle and seat cant stop the pressure. Most times an inline pressure regulator is needed to bring the fuel pressure back to stock. It usually clears up the sooty plug situation. You will have to look to see if your components are stock...if it isnt this may help. Im just guessing.....it might be an issue. Use the info wisely weather it is of use to you. Lots of good info on this thread...The guys are spot on. If by chance you talk to Bob again...Tell him Steve Devine says Hi! Ive known him for 25 years when he was working out of broadway st in somerville massachusetts! |
Steven Devine |
One small thing not mentioned and probably irrelevant because you have no measured oil loss. If too rich, gasoline can wash down the cylinders and cause oil consumption. |
Richard Smith 1 |
This thread was discussed between 05/08/2015 and 30/08/2015
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