1974 850t no start

[Mark Stevenson]

I'll tell the whole story, I bought a new old bike, MG 850t, then my friend came over and noticed that it wasn't firing on one cylinder. so I tested the spark plug on that side and noticed that it did not spark, I figured I'd buy new resistor cables and resistor for the spark plugs. installed them, no dice during test. so I sanded down the surface of all the ignition points, adjusted them, and I tested the coil and it was fine too.

it was a that point that I realized that I had been testing the plugs incorrectly, not allowing them a ground. as soon as I tested it with a ground it sparked beautifully. So now I don't know if it's been okay this whole time or wether I fixed it, because now the bike will not start whatsoever. I get a few thumps when trying to start after waiting a while, but nothing more.

I check the plugs after trying to start and they're wet with gas (I think), so I test the plugs when wet, and no spark. I'm not sure where the presumed fuel problem came from, but now I'm in a pickle. I've got the idle mixture set at stock, even though the exhaust is straight pipes with baffle, but I've fiddled with the idle screw and can't find what the stock amount of turns outward it was/is. even still the plugs are wet upon start. I could use some help, this is my first time diagnosing a no start/no spark situation.

 

[john zibell]

Straight pipes even with baffles is not good. No telling what was done previously to the carbs to get it to run. Once wet, the plugs may not spark but conduct over the center insulator. Dry them with a butane lighter or matches. Idle mix screws should be around 2.5 turns out from seated if the correct pilot is in use. In you worked on the points, changing the point gap will change the ignition timing as well. You need to verify timing is correct before you go chasing other areas. If you can locate a Haynes manual for the bike they have pretty good coverage for that model.

 

[Mark Stevenson]

Straight pipes even with baffles is not good. No telling what was done previously to the carbs to get it to run. Once wet, the plugs may not spark but conduct over the center insulator. Dry them with a butane lighter or matches. Idle mix screws should be around 2.5 turns out from seated if the correct pilot is in use. In you worked on the points, changing the point gap will change the ignition timing as well. You need to verify timing is correct before you go chasing other areas. If you can locate a Haynes manual for the bike they have pretty good coverage for that model.

thanks a bunch, I had sanded the points but then I reset them poorly, most likely. the manual says .016-.018 inches, and it describes loosely what the feeler should feel like in the point gap. So I must've mistaken a few things. But just as a word of notice, I was not the one to put straight pipes on, that was the owner before the owner before me. any idea what damage this might've done do the engine, maybe a replacement valve of some sort?

 

[john zibell]

thanks a bunch, I had sanded the points but then I reset them poorly, most likely. the manual says .016-.018 inches, and it describes loosely what the feeler should feel like in the point gap. So I must've mistaken a few things. But just as a word of notice, I was not the one to put straight pipes on, that was the owner before the owner before me. any idea what damage this might've done do the engine, maybe a replacement valve of some sort?

I didn't think you put the straight pipes on. Just wanted to let you know they will run like crap that way. These engines run best with a cross over to improve scavenging and mufflers for a little back pressure. T points are difficult to set up. You should feel a slight drag on the feeler gauge. Use the wider specification, they will stay in range longer that way. Also a point file should be used instead of sand paper. You will get a better surface on the points. One cylinder is timed by rotating the distributor body, the other by moving the points mounting plate. Getting both in time is like a balancing act. Sometimes you have to change point gap a little to get the timing correct. Timing marks are on the flywheel and can be seen once you remove the rubber plug on the right side of the gearbox case. So, set the timing. Then adjust the valves and if she doesn't run, start looking at carbs. Also a leak down test will inform you if you have any issues with valves or rings, and which has the issue. A compression test only verifies if you have good compression, but doesn't indicate why it doesn't. BTW, correct exhaust is reasonable at MG Cycle, but get her running before you spend more money.

 

[Mark Stevenson]

I didn't think you put the straight pipes on. Just wanted to let you know they will run like crap that way. These engines run best with a cross over to improve scavenging and mufflers for a little back pressure. T points are difficult to set up. You should feel a slight drag on the feeler gauge. Use the wider specification, they will stay in range longer that way. Also a point file should be used instead of sand paper. You will get a better surface on the points. One cylinder is timed by rotating the distributor body, the other by moving the points mounting plate. Getting both in time is like a balancing act. Sometimes you have to change point gap a little to get the timing correct. Timing marks are on the flywheel and can be seen once you remove the rubber plug on the right side of the gearbox case. So, set the timing. Then adjust the valves and if she doesn't run, start looking at carbs. Also a leak down test will inform you if you have any issues with valves or rings, and which has the issue. A compression test only verifies if you have good compression, but doesn't indicate why it doesn't. BTW, correct exhaust is reasonable at MG Cycle, but get her running before you spend more money.

so what's the consensus on switching to a dyna ignition and coil system? recommended or shunned.

 

[Trout]

Lots have been converted & once set up it's pretty worry free. I'll probably do that to the Mille GT if I ever get the shop built.

 

[Wisconsin v7 Racer]

so what's the consensus on switching to a dyna ignition and coil system? recommended or shunned.

Might want to look into this ignition. http://www.c5ignitions.com/

 

[Trout]

Dang! $500.00 buys a lot of points & plugs

 

[john zibell]

so what's the consensus on switching to a dyna ignition and coil system? recommended or shunned.

Before you go spending money on electronic ignition and such, first get it running on the points system. You may find other issues that will require money.