Backfiring!

Had a new experience today. About five miles into my ride home the car backfired several times, but didn't die. I pulled over and checked under the hood while the car idled. At the time of the backfire, I was pulling up a slight grade in 4th gear. I got back in and drove about a mile and a half more and it did it again. Again I pulled over to make sure the car wouldn't die. I then drove the rest of the way home without incident, about 35 miles, mostly highway. The only thing I noticed that was different was that the car did not want to idle down without blipping the throttle.

I'm running HS4's with a Pertronix in the distributor. I typically drive the car about 300+ miles a week. Last I checked my plugs showed no sign of richness or being lean.

Oddly enough, I was going to do a complete tune this weekend: plugs, wires, cap, rotor, valve adjustment, timeing, etc, .

Did you previously fill up the tank.....water maybe. just a guess. Or would that be intermitent ignition issue? Mikek

Recent fill up on the gas, the previous day. It was premium from the same station I always use. From past posts I see it could be a running lean issue (carbs) or ignition (coil?) There are no points or condensor. Jeff redid the distributor a couple of years back.

Sometimes exhaust leak will do that.

Well, maybe it was just feeling old...and you pulled it's finger so it farted.<G>

mysterious...especially the repeats, and then all was fine.

plug wires all seated properly? I've had them loosen, and even throw a wire off a plug.

take it apart and inspect the pieces for wear. if the cam in there has rounded off (should be 5-6-? flats & angles to it) then replacement is in the cards. If it doesn't look worn, clean it and refit.

Hmmm wrong thread. This replay was about brake adjusters.<G>

Robert,

I had a similar incident today. See my earlier post here:

http://www.mgexperience.net/phorum/read.php?1,1374062

Don't go messing with things too much right off the bat. I swear that these cars have souls and sometimes a soul just has to vent or take a fit.

Go on and do your tune stuff and let nature take its course. It is probably not going to let you down without a lot more warning.

It sort of sounds like you have a transitional lean condition as the throttle is being opened.

By "backfire" is am assuming that you meant spitting back through the carbs. If you still have throttle plates that incorporate the poppet valves in them a "backfire" can toast their fine springs and leave the valve a little loose. If that happens the valve bleeds air when it shouldn't and the idle will be elevated to a degree according to the amount of extra air going into the engine when the throttle is supposed to be closed. If you have a doubt, pull the vac chambers off of the carbs and reach down the throat with your finger and press on the spring end to feel the "snap" as the valve closes. A wimpy spring can usually be felt.

Also, use the water squirt bottle routine all over the intake system while the engine is at idle to see if any other vacuum leaks have developed.

Jack

While I am fairly new to MG "lumps" , I've been taking engines apart for 40 years....
Taking the engine apart due to a few backfires would be jumping the gun, big time!
First thing to do, is check your timing...and your vacuum advance...Pull the cap, look at the rotor...If it's one of the Chinese rotors, you might consider the "high-price spread" sold by Moss...You may be getting just enough cross spark , under load , to cause the backfire...Look at the contacts in the cap...are they burned?...
Check the plug wires (in a dark garage), to see if there's any cross sparking, as well as the boots...
Pull the advance tube off the intake (or carb), and suck on the tube....Do you feel pressure, and , with the cap off, can you see the advance arm pulling on the dizzy advance plate?...
Run the engine, and check the timing, both at idle, and a 1500 rpm....Can you see the timing mark move, as you increase the rpms?
You could very well have gotten bad, or dirty gas...(refill truck was there , just before you pulled in to the station, and stirred up the dirt)....Consider changing , your gas filter...better yet, go ahead and change it.
Double check your valve adjustments (I know, it's a PITA!)
Do all of these things, BEFORE your tune up, so you can identify the problem....

Thanks all. I'll take a good look at things tonight - before I replace the cap, rotor, etc. I 've been running about 350 miles a week and almost felt a little guilty not taking the GT to work toady. I guess its a trust issue we'll just have to work through....

Mine would do the same thing but it was drawing in oiI and condensation from the engine cover vents. See you have the engine cover vented to the carbs which is correct.

After prolonged grades on my commute I would get a bit of a backfire through the carb. What led me to the vent was I just changed the oil, it was a bit over-filled and after the second backfire I smelled fresh oil. Once I routed the vent line to the rear of the transmission, no more problems. I put a line from car to carb on the vacuum port, (which is incorrect) but works. I am just going to cap the ports since my air pump is removed.

Robert-
This sounds like a fuel distribution issue. Fuel pump, fuel filter, or internally swelled hoses. Check your float bowl levels, and your floats.

Jim D

if it is a transitional leanout upon opening the throttle, as Jack suggested, changing the dashpot oil to something heavier could solve the problem. A lot of folks like to use ultra light oil in there, ATF or even Marvel Mystery Oil, but truth is you want to keep the piston from lifting quite as fast as it will with light oil in order to allow the fuel flow to "catch up" with the air flow. That's why using 20/50 or even 90 wt gear lube (as I do) can help resolve that transitional leanout.

Any answers yet?

Mine would misfire climbing hills. Changed distributor cap as the old one was carbon tracking and would misfire under load. Distributor caps are cheap.

Use a little spray lube on the advance assembly under the breaker plate, as well a a drop or two of oil under the ignition rotor. It could be starting to stick a bit, which explains the backfire and the high idle.

Robert, any updates? I'd like to see what works before I go diving into mine. It has a similiar issue in that there's just a little 'pffft!' through the carbs on start-up. It'll do it a few tmes before settling down.

Cheers,

Glen