Low speed misfire.

Technical MGB discussion
Ian Fozzard
Posts: 207
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Low speed misfire.

Post by Ian Fozzard » Sat Jul 06, 2019 9:13 pm

Thanks Vic. You are missing some good "MG weather"!!
Best of luck with getting it running right,

Ian F
Vic Butler
Posts: 348
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Low speed misfire.

Post by Vic Butler » Sun Jul 21, 2019 11:14 am

Fitted the distributor to Lumenition box plug then fitted the distributor. With the plug connected I started the engine. It was kicking back so I backed the distributor off and then it fired. It's running rough but it was cold. However the low speed misfire seems to have gone. I'll still change the plugs and then give it a road test.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Ian Fozzard
Posts: 207
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Low speed misfire.

Post by Ian Fozzard » Tue Jul 23, 2019 9:39 am

Hi Vic, thanks for the update, maybe all OK by now?

Not sure which distributor you are currently running, but if you are using one with no vacuum advance, then it might be worth trying a standard distributor with the standard static advance setting, and leaving everything else the same?
If that cures the issues you are having then you will have identified the source of the problem?

Kicking back and hot running sound very much like a timing setting problem to me.

What does Peter C think?

Ian F
Vic Butler
Posts: 348
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Low speed misfire.

Post by Vic Butler » Tue Jul 23, 2019 9:56 am

Morning Ian.
I swapped the distributor for one with no vacuum advance but it didn't work at all well so I refitted the original with a vacuum.
The problem was the connector from the Lumenition optical unit to the control box. I had to swap it between distributors and it wasn't making a proper connection. I contacted Lumenition who told me to return the optional unit and they would check it. They returned it 3 days later with new connecting pins all for free. It was that causing the misfire. I'll change the plugs next and give it a road test.
With the non vacuum distributor fitted there seemed to be very little advance which contributed to the hot running. Once I've done a road test I'll be able to sort the timing.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Ian Fozzard
Posts: 207
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Low speed misfire.

Post by Ian Fozzard » Tue Jul 23, 2019 1:27 pm

Thanks Vic - that all sounds good. Still plenty of time to get some Summer runs in!

Ian F
Vic Butler
Posts: 348
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Low speed misfire.

Post by Vic Butler » Wed Jul 31, 2019 3:22 pm

IT'S GONE. I changed the plugs today and I can now definitely state the misfire is no more.
The old plugs were in a dreadful state and I'm ashamed to admit I can't remember when they were fitted. I ran the engine in the garage (with the door open) and checked the coolant was circulating as some years ago it overheated and for some reason I removed the water pump and found the drive to the impeller had sheared and I did wonder if the same had happened again but that seems to be OK.
I still have to do a road test (very short one initially) but that will have to come later.
The tick over is still a bit rough but much improved on the last time.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Vic Butler
Posts: 348
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Low speed misfire.

Post by Vic Butler » Tue Aug 20, 2019 3:09 pm

Other things have taken priority but today I had another go at the. MG. I weakened the mixture off by one flat and speeded the tick over up so I could leave it running to check for overheating which occurred when I last took it out. I left the engine running until the electric fan cut it and left it running until the fan cut out so all seems OK there.
It was puzzling though that it was still slightly advanced as it kicked against the starter but did start. That is probably the cause of the rough tick over. Then I remembered why. I aligned the mark on the distributor body with the gap in the previous clamp but the current one is a replacement as the previous one was distorted and the gap was likely in a slightly different place so I'll back the distributor off a fraction. If I can get a smooth tick over I'll time it by ear. I timed the Land Rover that way about 9 years ago and have never needed to alter anything since.
Bank Holiday Monday I'll give it a test run.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Post Reply