MGB Back fires on uphill climb

[BobS76]

1979 MBG, stock electronic ignition, Webber carb. Yesterday I had the car out on the interstate, keeping my speed to about 65. Leaving our valley, I started to climb a fairly good hill. To compensate, I started to apply more throttle. Between the hill grade and the "extreme" power of my MG, I eventually put the throttle to the floor. At first I noticed a loss of power. A few seconds later, it started to back fire (sounded like it was coming out from under the hood), more or less once per second, with considerable more loss of power. I played with the throttle, hoped, and prayed and managed to make it to the top of that first hill, which lead into a slight down hill. During the down hill, I eased up on the throttle, and the ignition recovered. I continued to make it out of the valley, at lower speed, with out further problems. I have had no problems since then, but then I have not tried any steep grade at speed either. Does any one have a guess at what happened? <Aside from that, does anyone know why my 'enter' key does not work while typing this post?> Bob

 

[JPSmit]

1. you need oil in the dashpots.
2. you need to richen up the mixture a couple of flats
3. (unlikely) you need to check your timing.
4. you only need to drive downhill (better gas mileage too)

 

[Stewart]

install a supercharger and go up the hill at 90 with much nicer sounds coming from the engine bay.


If it's popping through the carb it's most likely going lean. Do you have access to a air fuel meter or run the hill again with your foot on the floor and safely cut the ignition and then look at the plugs and see what color they are.

 

[PAUL161]

Do what JP said and then, when climbing a hill, shift down to a lower gear and Keep the RPMs up! You can't lug these engines! JMHO. PJ

 

[blueosprey90]

No real authority here. And I can only comment based on my experiences with respect to a '59 mga. You probably have pollution control devices that I don't have and I run SUs, not Weber carbs.

I don't think it is a leaning out problem, unless possibly if you are at a really high altitude. I think leaning out would cause a misfire, not a backfire through the carbs. It would be more like a stumble, maybe even more like a pinging, as if your fuel doesn't have a high enough octane. You might be able to feather the throttle to control.

I would check the timing with one of those dial timing lights. I would check the valve lash. I think under load / full throttle, your timing is advanced and you are firing the plug while the intake valve is still open, thereby causing a backfire through the carb.

I don't know if Weber carbs need to get rebuilt from time to time. I had a high speed misfire that I chased down for three years. This winter I did a carb rebuild. I found that my seals were all buggered and that I had one extra copper washer in one of the carbs. Put new seals and removed that part and the high speed misfire appears to be finally gone. I'll know for sure later today, as I'm headed out to Lime Rock Race Track for a little race action.

 

[dklawson]

Consider this one more vote for "lean under load". However, since the jetting was correct before I would not start by looking at the carb. You reported that the backfire was only under heavy load under conditions demanding a lot of fuel. Take a look at any and all fuel filters and screens. Replace or clean as appropriate.

 

[Stewart]

Another thing to look at which baffled me for quite a while as it was so random was the rubber fuel line from the tank hard line to the fuel pump. The line was cracked and brittle after 5 years (thanks ethanol) and when the planets aligned it would allow the pump to suck a bit air along with fuel which then caused a big loss in power with backfiring at high power. Let off the throttle and it ran fine. It would do it for a bit and then stop for hours days or weeks and then do it again.

 

[LarryK]

Check as above: mix, filter/s, vacuum to dist. and dist. movement. Might be a throttle lean, might be a dist. vacuum stuck, or even just a distributor oiling. Problem does not seem to be real $ problem just sorting of ignition and carb settings. Yes, downshifting on hard acceleration is always recommended. You didn't say if on hard acceleration from stand still?

 

[blueosprey90]

Well Bob, what is the diagnosis?