MGB Update: Timing Is Everything!

MGB 18G Engine

Project updates on my MGB have been slim because it has provided reliable service for the most part. It has started to develop a rough idle lately though. The problem was worse on hot days, so I figured a new condenser and perhaps a coil would be a good idea. Replacements are cheap and it’s good to have a couple of spares on hand so I went ahead and ordered them. While I was in there I also did a quick valve job. The engine was also getting a little noisy, but I figured a loose valve could be the culprit. The compression was good so I knew there weren’t any major internal problems. The work helped, but the rough idle and ticking were still present, so I decided to check the timing.

Old VS New Distributor

After making a line on the pulley with some white out, I hooked up my timing gun and pointed it at the mark. The strobe effect of the light makes it possible to see how far advanced or retarded the engine is running and you can adjust it by turning the distributor. Well, getting a reading was going to be difficult because the mark on my pulley was jumping all over the place. That could explain why my otherwise well-tuned engine was having a hard time and making more noise than it should have. This symptom points to a worn timing chain, but I was hopeful that it was something more simple. The vacuum advance on the distributor can also cause problems and since the idle would sometimes drop at random, I went ahead and ordered a new distributor.

Timing Chain Test

The new distributor definitely helped. The engine maintains a much more steady idle, but it clearly wasn’t the fix-all I was hoping for. To be certain that the chain was the problem, I performed a little test. After removing all the spark plugs and the distributor cap, I put a 1 5/16″ socket on the pulley bolt and turned the engine one direction until I saw the rotor move a little. Then I stuck a piece of masking tape on the side of the pulley and made a mark with a pen. Then I pulled the wrench in the opposite direction until I saw rotor movement again. Another mark went on and with almost 1/2″ of space between the two, I think it’s safe to say that the timing chain was our culprit. Replacing it isn’t a complicated job, but the coolant does have to be drained and the steering rack unbolted so it’s a little more involved. So, any suggestions for me as I continue to sort out our little MGB?


  1. kenzo

    Quick question.
    Why would you change condenser, coil and do a valve job prior to checking the timing? If it is running a bit rough timing is usually #1 to check. Quick and easy.
    Just curious.

    • Jesse Mortensen Jesse Staff

      Good question. I should have have done that first off. I was having more symptoms when the outside temperatures were higher though. I tested the coil with a multimeter and the vacuum advance in the distributor wasn’t always working, so replacing those parts wasn’t a total waste.

      Like 1
  2. Randy Forbes

    I have one (1) pertinent tip. Although it’s been almost fifteen (>14) years since the last time I worked on an MGB, but starting in 1973 when I bought one new, I ultimately had sixteen (16) of them! The last three (’68 Tourer, ’71 BGT/Buick 3.8 V6 & 74-1/2 BGT__1 of 1178) were left behind in my ex-garage, with my ex-wife, but that;s another story!

    Anyway, where was I? The Tip!

    Avoid the temptation to use any type of gasket sealer when replacing the timing-chain tensioner (and definitely replace it, as I guarantee that the pad will be well worn). There’s a pin-hole that supplies oil to lubricate the tensioner, and it doesn’t take very long__running without the oil there__to get two (2) nice troughs across the pad!

    While you’re at it, I’d suggest breaking out your Wire-Gauge drill-bit index and (by hand, or in a pin-vise) make sure that pin-hole is open to the galley behind it. If the front-plate gasket was installed with sealant, it’s almost assured to be blocked off. Remember, you’re not trying to drill out the cast-iron block, just making sure there’s no sealant blocking the passageway.

    Okay, an overly long diatribe, just to say “clean the fricking hole!”

    • Jesse Mortensen Jesse Staff

      Great tip Randy! I’m hesitant to go to the trouble of replacing the chain though because, from what I have read, they usually last the life of the engine. The compression is good, but I don’t know the history of the car beyond the last owner so it’s possible that the engine underwent a partial rebuild at some point. When I do dig in there I will be sure to open those holes and I never liked gasket sealer much anyway. Thanks!

  3. jim s

    the car has some new parts and still needs some more, it is not a problem since he is not a line mechanic in a shop. and he has a backup car. been there/done that.. thanks for sharing the tip.

    • Jesse Mortensen Jesse Staff

      Yep, I’m no expert. Still learning new things all the time and that is part of the fun. Sometimes it’s fun to throw new parts at a car and hope they fix the undiagnosed problem too. Heck, even a few mechanics prefer that method!

  4. viking

    Hi, I think the ticking is a rocker clerance problem. The valve stem has worn a grove in the rocker arm, so if you use a feeler gauge, you can not get the proper valve lash . Instead set the lash by feel, bring the piston to TDC on the compression stroke, then feel the lash by moving the rocker arm up and down. Compare the lash ply to other valves, so don’t use a gauge. You get the hang of it by practicing. I have done it for over 40 years without feeler gauges,remember the exhaust valves have a little more play than the intake valves.

    • Jesse Mortensen Jesse Staff

      The valves very well could be the cause of the noise. I can see why your technique makes sense too, but I don’t think they are the source of the sporadic timing mark or the very slight pulsation I see on the tach and feel on the seat of my pants. I have heard that the timing chains usually last the life of the engines in these things, so I could be wrong about my theory though.

    • Pete

      That was my thoughts also. I would check the clearance of the valves. If the valves are too loose the motor will run badly. If the valves are to tight the motor will run badly also. If I remember correctly to tight will also cause a starting problem.

  5. Mark

    Have you checked the SU carbs? The bushings for the throttle shafts wear and cause poor running conditions due to the air leaks.

    Just a thot.

    • Jesse Mortensen Jesse Staff

      I was trying to resist the urge Mark! When things start to run rough, it’s only natural to start fiddling with the carbs. Unless they are worn, there is usually something else that has changed and caused the engine to run rough. That’s why I focused on the ignition and timing first. I think you could be right though. Leaks could very well be causing my problems. Im not sure if it would cause the timing mark to bounce around like that though. Any thoughts?

  6. Jesse Mortensen Jesse Staff

    I probably should have just presented this and let everyone here diagnose it instead of pretending like I knew what was doing. It could be the timing chain, but I’m sure it could be something else. Any other ideas?

  7. viking

    Get rid of the su’s and manifold, and install a pair of weber dual port side draft carbs, problems solved…also check the fuel pressure. I have also found that induction timing lights are unreliable, jumping all over, my trick is to hook it up to the coil wire, it will blink very fast but is more reliable.

    • Jesse Mortensen Jesse Staff

      The SUs are going to stay, but I will try your timing light trick. Thanks.

  8. rogerowen

    New condensers can often be a problem, they are generally poorly manufactured. I have had several instances of condensers failing within a few 100 miles of installation. I serviced an MGB-GT that was running ok, but a little while after the service it began to run rough – getting worse and then not starting at all. Then there was an MG Midget 1500, same problem, replaced the condenser again and the problem re-occurred. As an emergency measure to get the customer mobile again I found an old condenser unit from a 1966 Triumph Spitfire MkII kicking around the workshop, fitted it, and off it went – and has been running on this condenser for several years without problem.

    There’s an old adage – If it ain’t broke, don’t fix it!

    2 VW T2 camper vans in recently – same problem, put old condensers back in – problem solved.

    Generally speaking the poorly made new condensers work OK at first but then start deteriorating as the electrolyte gets hot and starts to break down.

    Poor quality modern replacement parts are a serious issue to classic car maintenance.

    • Bobsmyuncle

      Funny I’ve heard this quite a few times just in the past few months.

    • Jesse Mortensen Jesse Staff

      Yep, I thought it might be the condensor too, but after replacing it once and installing a new distributor, we are now on #3 so I think we can eliminate that as cause.

    • Jamie Palmer Jamie Staff

      We lost both a replacement condenser AND rotor button (no, not one of the good red ones) during our first race with the Marina (MGB-powered).

  9. Dan10

    I could be the timing chain tensioner is not operating properly and the chain itself is fine.

    • Jesse Mortensen Jesse Staff

      Could be Dan. If I’m going to dig in there, I might as well replace the chain while I’m in there though.

      • Jesse Mortensen Jesse Staff

        Looks like the tensioner has a spring release so if it was rebuilt, perhaps someone forgot to release it?

  10. Claus Graf

    Check the SU carburetors diaphragm.

    • Jesse Mortensen Jesse Staff

      Will do Claus!

  11. RIC Parrish

    I had a little British car repair business in Iowa City while I was in Engineering school. A fellow working on his Phd in the electronics of the human brain came to work with me. He taught me the FIRST thing you do with any old British car is clean the SUs with spray carb cleaner (while still on the car) free them up, clean them out. Works miracles on B’s, 3’s anything with SUs.

    • Jesse Mortensen Jesse Staff

      As previously mentioned, the carbs very well could be the problem. I will get the old can of carb cleaner out and let you know how it goes.

  12. Jesse Mortensen Jesse Staff

    Checkout this article on Hemmings:

    After reading it, I realized that I’m still a “parts changer” at times. I also just proved my “lack of knowledge of internal combustion mechanics” here. Oh well, we are all learning and unless you have formal training in automobile mechanics, you will have to follow my trial and error technique. I’m lucky though because I have a whole crowd of experts waiting to lend some advice. Thanks guys!

  13. Rufus

    Quick valve job??? Did you remove the head and grind the valves, or set the valves? If you simply set the valves, be aware that valve clatter is pretty common in B’s. If you want them quieter, there is a bit of leeway in the adjustment. You can adjust them to .013 and they should not be so loud.
    It is fairly common to start blaming the SU’s for everything from running rough to the National trade deficit. The accepted diagnosing process is 1. ignition 2. valve train 3. carburation. Finish the ignition sequence before you move on. You didn’t say where you got the replacement distributor, and that may still be a problem for you as a large number of the available units are a poor substitution. I have solved the scatter problem by having my original 25D rebuilt by Advanced Distributors in Minn. Probably a similar amount of money to your new aftermarket distributor, but better than new OEM quality.
    Now, after double checking the advance, you can re-adjust the valves.
    To trouble shoot the SU carbs for vacuum leaks, the old carb cleaner on the running engine trick works fine. Pay attention to the area around the throttle shafts. They don’t go bad often, but when they do, this is where. More likely than not you will have a vacuum leak at the manifold or one of the fittings. @Viking,,, with all due respect, a twin side draft Weber is probably far too expensive for Jesse, far too much carb for a tired ole beater with no performance mods, and way too difficult on initial set up to get right. A single 45 while a common replacement shows little improvement over a well maintained set of SU’s, and is, as I said, very tedious to set up.
    I’ve been fooling with British cars since the 70’s, and it is important to remember to only deal with one fix at a time. The ole Scientific Method. Make one change, chart the effect, then go on to the next .
    I’ve been following your exploits with this B, and enjoy hearing you spread the word of reliable, inexpensive and fun collectible car ownership. These are easy, fun cars to drive and maintain.
    Have fun

    • Jamie Palmer Jamie Staff

      @ Rufus — excellent advice. +1 for Advanced Distributors!

    • Jesse Mortensen Jesse Staff

      I should have said valve adjustment. Thanks for the clarification and advice. It would seem that I was doing things in the right order and even after reading everyone’s suggestions, I’m still thinking the timing chain needs replacing.

  14. Rufus

    I would be surprised if the timing chain is the culprit. It’s not that they can’t go bad, they just usually don’t.
    Is the timing scatter still as bad as it was? Try static setting the timing at around 10 degrees (a good ballpark figure). Then, be certain the vacuum advance is working properly with no leaks in the hose or fittings (take the dist. cap off and suck on the tube and watch to see if it is advancing smoothly).
    If the ignition system is all fresh and checking out right, I agree you should move on to the carbs. Sometimes just tightening the nuts on the manifold can help. Also, if they are HIF’s check the poppet valves.
    Good luck with it. I’m pretty sure its going to be something simple.
    Have fun

    ps, you can pm me on MGE as Rufus, if I can help more, I’m happy to.

  15. Mikee S

    Having worked on MGBs for many years, even back to when they were new, I always get suspicious when I hear that the engine’s tappets are getting noisy.
    Cam and lifter wear was a problem on the “B” series engine right from the start and modern oils can make the problem worse.
    Might be time to do a cam lift check. If cam and lifter wear get really bad the pieces of hardened steel floating around in the engine can do untold damage.

  16. John

    One other thing that is often ignored,check the fuel pressure. The B’s fuel pump is made by Lucas. ‘Nuf said.

  17. Rufus

    In reality, if indeed he still has the stock pump, it was made by Skinners Union (SU).
    I believe we can get around to the fuel system tomorrow, if he hasn’t found the problem in his spark delivery system.

    Have fun

    • Jesse Mortensen Jesse Staff

      Looks like a previous owner dumped the SU pump and installed an aftermarket solid state unit. I have the receipt and it looks like it is only a couple of years old.

  18. Monsieur le Baton

    Keep the SU’s Jesse, i switched mine to electronic ignition too, saves so much hassle, and join best mg forum around! :)

  19. Jesse Mortensen Jesse Staff

    Here is a challenge for you all. Can you diagnose the problem just by listening to the tailpipe?

  20. rogerowen

    Interesting theme. Condenser.

  21. Rufus

    Is the rattle in the clip the one you have been talking about?
    That isn’t valve clatter. Is it coming from the front of the engine? Is that noise the reason you think the timing chain is bad? I’m reeal curious about the rattle.
    I have heard a noise like that on a B before. I think, if the rattle in the clip is coming from the engine, I would spend my effort diagnosing it. Does it sound like metal on metal?
    I feel like I’m playing 20 questions. Is it Groucho Marx???
    While I hear the occasional miss from the tail pipe, that rattle should move to your first priority. Then again, it could be my computer connection.
    Have fun

  22. viking

    Hi, Does the car run ok. Sounds as you have crossed a couple spark plug wires.

    • Jesse Mortensen Jesse Staff

      Actually it runs good. Give me a little credit.

  23. mike young

    Cam may be flat…chain may be fine! Use a high Zinc oil (after cam,tappet, chain, sprocket,tensioners relaced)

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