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MG MGB Technical - diagnosis needed for sick 1967 mgb

This one has me baffled...rebuilt engine has 60 miles on it. Running fine while driving around town on its third trip since rebuild, then suddenly begins to sputter, backfire, and eventually died. I pulled over thinking it was the fuel pump, but it checked out ok. I removed the fuel cap and got the 'swoosh' associated with a vacuum. Coincidence or not, I started up the car and it ran ok for about 4 minutes, then the same symptoms happened again. This time removing the gas cap didn't fix anything. I towed it home and let it cool down. Later, turning on the ignition, the fuel pump ticks fine. I pulled each plug and I'm getting a spark on each plug when I turn over the engine. I tried starting the car...it barely starts, then I have to pump the accelerator to keep it barely alive before it dies again. Rechecking the plugs shows #1 and #3 wet with gas. I opened up the carb (Weber DGV) and it all looked ok. My next step is to pull the valve cover and check gaps. This may sound weird but it seems like the timing is all out of whack with out any physical changes in the setup. The distributor is still in its original position. I'm going to start at putting the crank at TDC #1 and begin checking the valve gaps. Any suggestions, or is this deja' vue for any of you and provide any input? Any help would be great as I think it is more of a timing issue than a fuel issue. Thanks. -Rod
Rod in WA

Rod,

My B is a 66 with SU's and I lmow nothing about webers, but you are obviously getting fuel to all cylinders as well as spark ( based on your analysis to date) That then leaves only timing. It is my opinion that if the timing was on, and even if the carbs were not quite right ( too rich on one side ) it would still run, but poorly. So I'm with you, timing. How about a look at the distributor hold down clamp. Mine is a bear to bothe the distributor and the clamp system well seated and if not correct, can move with vibration. If after checking the easy stuff ( distributor ) and plugs that fire on all cylinders, then the only likely answer is timing. A tensioner problem? Chain out of wack?

Good Luck!!
Jim Hanks

What year? If it has the carbon canister i.e. 1970 on North American spec there should be no 'whoosh' as you remove the cap as the tank is vented through the canister and the filler cap is non-vented. Cars without the canister need a vented filler cap, but even then you will still get a slight 'whoosh' as there is a spring-loaded valve in the cap to prevent leaks from the vent in a roll-over. The valve causes a slight vacuum to develop, hence the slight 'whoosh', but it should not be enough to affect the pumping of fuel. If the valve timing has gone out maybe there is a problem with the chain, which will affect distributor timing too. All plugs should show much the same conditions especially with a single carb. Wet plugs usually means a very rich mixture i.e. flooded. Valve timing way out will also affect the relationship between a cylinder trying to suck mixture in and when the inlet valve is actually open.
Paul Hunt

Timing problem seems unlikely since it ran fine you wrote. I once had a similar expierence, It appeared that the condensor failed under load.
You may want to swap it for a new one before you start tearing the engine apart. Goodluck!
Huib Berger

Thanks for the feedback, guys. Paul, the fuel cap is a 'vented' cap for the non-vented 1967 tank, so I was surprised to hear the swoosh as well but maybe a slight swoosh is normal. Then again the car ran great for 60 some miles before it died and won't restart now even with the cap removed.

Is it possible for the timing chain to slip? Maybe the tensioner gave out as mentioned above and the timing chain slipped. It's a single row chain. I guess what I will do is put the engine at #1 TDC and verify that the distributor rotor is pointing the the #1 lead on the distributor cap. My theory that the distributor is generating a spark but not at the proper time. I will also pull the valve cover and check the clearance gaps as well. I won't have a chance to look at the car again until Thursday night, so any additional input is welcome. As always, thanks for the assistance...this is a great BBS! I will report back with my findings. -Rod
Rod in WA

Your problem sounds like a fuel venting problem. Try disconnecting the pipes that go to the overflow part of the carb. Since I don't know what carbs you have they are either at the top of the float bowls or the side of the carb body (HIF) I had a problem like this with a customer car and found that the vent pipe to the rear was plugged.
Jim

Rod - It is doubtful that valve timing "suddenly" went south. On the other hand, it is not uncommon for a coil or condenser to die suddenly. I think that I would try a new condenser first and then check out the coil. You may even be in serious need for a complete tune up - valve adjustment, new points, condenser, rotor, cap, wires and plugs. Good luck - Dave
David DuBois

Dave, thanks for the additional input. I will focus on the distributor components and coil to insure everything is up to par. The distributor (mallory dual point) has a new cap, rotor, points and condenser, but I will open it back up as well to make sure the points are opening/closing properly. I've done all the work myself, so I may be time to call in the calvary and a real mechanic take a look as you mention. The days are beautiful out here and I hate to have the car sitting in the garage missing out on some great 'top-down' weather! -Rod
Rod in WA

The dewll on a dual-point is higher than normal, you need to insulate one pair of points and adjust the other to the standard figure, then insulate those and adjust the other. Spluttering and backfiring isn't associated with fuel starvation, but can be with intermittent ignition. It can also associated be associated with bad valve timing, as unlikely as this may be to suddenly 'slip'.
Paul Hunt

Paul, I have read yours and Les' webpages on checking the condenser/coil and did the test of strapping the main wire from the coil to a piece of wood. Since it is a dual point system, I propped both points open with the ignition on and barely got a spark and no 'crack' sound. I held the same end of the main wire near a bolt on the engine block and got a very small thin spark when I turned the engine over. The parts stores are closed today for the holiday, so tomorrow I'll go get a new condenser and try it out. Thanks again. -Rod
Rod in WA

Turned out to be a bad condenser...amazing how a $5 part can cause so much headache! I bought two of them and will keep one in my spare parts bag in the trunk. Back on the road again. Thanks for all the feedback. -Rod
Rod in WA

Rod, You mentioned you had dual points and a bad condenser. Was this a Mallory and if so can you tell me what parts store carries a condenser for this unit? thanks
John

Baxter's Auto Parts here in the Greater Portland, OR / Vancouver, WA area carries the Mallory replacement parts. I have picked them up at Advance Auto Parts as well. Yes, mine is a Mallory dual points, mechanical advance distributor. When I was at Baxter's, the condenser was $5.50 and the points were $14 each. -Rod
Rod in WA

This thread was discussed between 01/07/2003 and 07/07/2003

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