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MG MGB Technical - Rough Running?

Hello all,
I hope you can help!!! Here is the lowdown!! Sorry for the length but I wanted to make sure I covered all the angles!!!

68B with all smog equipment still attached. A few weeks ago she died on the way back from Britfest in DOVER, NJ...Lost spark... so I replaced the pertronix ignition module, rotor, cap wires and a new shiny 40K volt Flame Thrower coil to match the electronic ignition. She started right up but ran a bit rough. I brought it to my mechanic and he readjusted the valves, synced the SU's, and reset the timing. I picked it up last Saturday and he is about 15 miles from my home. As it was cool out and a bit wet from the remnants of the hurricane it took a little while to warm her up. So as I left the garage I had the choke all the way out.

I assume this richens the mixture? about 2 -3 miles out on the highway I slowly released the choke in small increments as the engine began to get warmer. The temp usually sits between "C" AND "N" well as it finally came up to operating temp and I pushed the choke back it started to run rough? If I pulled the choke out a 1/4 of the way it began to smooth out but as soon as I released it, it began to "sputter". If I'm just running around town it runs fine, but when I'm on the highway and I put some load on it it "bogs" down. Now I noticed when I got home the belt that runs the air pump was disconnected. I asked the mechanic why and he said the air pump was shot and it's innards sounded like they were about to go. I do admit the engine compartment does sound quieter!

Anyway!!! I'm going to make an appointment to go back to the mechanic and have him keep it for a few days and run it both on the highway and around town. Once again it tends to run rougher on the HIGHWAY! I've tested the Vac advance on the distributor 25D by sucking on the hose and it seems to move the plates and do what it's suppose to. There doesn't seem to be and leaks. I've advanced the distributor a little bit but nothing to speak of in difference running on the highway. Since the air pump is no longer working and the belt has been disconnected, would that have an effect on it's running or is that strictly an emissions issue? Everything is still attached...air rail and such. I'm bringing it back this week, so any advise you have that I can let the mechanic look for would be appreciated. I'd like to take her out a few more times before it gets to cold!!

I thinks is a carburetor issue as it tends to run smoother as I pull the choke to richen it up or it could be a timing issue as the more load you put on the engine and as the revs increase shouldn't there be a faster spark? Perhaps the springs in the distributor need adjusting or replacing?


Thanks all,
Dan

DS SUKEL

Sounds lean if when you richen the mixture the car runs better. Give the adjusting nuts a 1/4 turn clockwise to richen it up a bit and see how it runs.
Mike MaGee

Make sure you have oil in the carb dashpots. A bad ignition condenser can also cause rough running at higher speeds.
John H

A bad condenser gives a weak spark as well as noticeably retarding the ignition. You should be able to make a spark jump at least 1/4" from the coil, and as much as 1/2", with a bad condenser it will barely jump a plug gap. You can test this by removing the HT lead from the coil and replacing it with a ground wire. Flick the points open and closed as you gradually move the ground wire out of the coil HT socket, and you will see how far you can get. Unless your atmosphere is very dry you may even hear it cracking with the wire completely removed, although this can cause tracking. Then try the same but disconnecting and reconnecting the coil to distributor wire, which simulates a failed condenser.

I'd also recheck the carb synchronisation and mixture balance. As far as mixture balance goes with the car fully warmed simply turn each nut in turn until you get the highest idle speed - it will drop if either too weak or too rich. That is a coarse adjustment. For the fine adjustment use the lifting pins to raise each piston in turn by about 1/32" (you will have to lift the pins more than this before they touch the bottom of the piston). If the engine stumbles the mixture is too weak. If the idle increases and stays increased while you hold the piston in position it is too rich. When the mixture is correct the idle should momentarily rise then settle back down while the piston is still raised. The difference can be quite hard to detect, but the difference between rich and weak *is* noticeable, and that will help you train your ear to judge the mid-point between the two.
Paul Hunt 2

Thanks to all for your suggestions! Hope to have it all sorted out by the weekend. Fall foliage looks to be in full force and the weather is suppose to be in the high 60's for the drive!!

Dan :)
DS SUKEL

This thread was discussed between 31/10/2005 and 03/11/2005

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