QuestionQUESTION: Working on 1977 MGB (car A), all original I think?? Backfiring & spitting back in carb. & running rich! Thought it was the carb. & rebuilt it... same problem. Check auto. choke and it is working O.K. (Note: Car A carb. worked O.K. after rebuild on another '77 I have (car B) ... just idled high.
Question: Car "A" has electronic ignition with Opus distributor. This distributor has a "thru beam" light looking through a disk with slots to fire (like points?) ... have you ever heard of this? I've tried setting timing ... no luck ... same problem. Could this loading up and backfiring be an ignition problem ... timing or coil?
Also ... Car A - head rebuilt, valves set ... same problem!
Thanks, Mike
ANSWER: Hi Mike,
You need to start from scratch and even though valves were set and head rebuilt you need to do the test to see if the rebuild resulted in a good combustion chamber by just doing a compression test. Give me the results of a compression test. Then set the timing as per the sticker under the hood. After those are completed warm the engine up and set the idle below 1000 RPM and lift the pin under the side of the carb and tell me what the engine did. Don't forget to put oil in the top pot of the carb. (fill to about 1/4 inch below the inner most lip in the top pot) (Be SURE the air filter is in place before lifting the pin on the carb.)
The Opus/Lucas was not an optic system. It used a pickup coil and a plastic reluctor with small bar magnets in it. I have seen an Optic system from the aftermarket used in MGs as most Opus/Lucas systems died one or more times before they even got out of warranty. So most people went back to a pointed distributor or put aftermarket electronic systems in the Opus/Lucas distributor.
Don't try to guess what is wrong, find out by tests only. give me the results of the compression tests and the "lift pin" test and I will tell you the next test.
Howard
---------- FOLLOW-UP ----------
QUESTION: Thanks for the quick response! Here are the compression results: From Radiator back-
#1 = 120-125
#2 = 120-125
#3 = 120-125
#4 = 125-130
Note: Gauge is in 5lbs.psi so this is as close as I could read it.
Checked carb as specified and saw a SLIGHT increase in idle speed ... didn't register on car RPM gauge ... leveled off at top. Do have oil in carb & also new diaphram from rebuild. Air filter was in place during test.
I had already done this test WITHOUT filter previously and saw significant increase in idle speed ... as a result I had adjusted anticlockwise one revolution on needle. Did not change this for the above test.(Previously following recommendation in Moss catalog for Stromberg carb.)
Car has backfired & run rough from day one. Head rebuild was a result of finding push rod loose (bent)when I first started on the car. Have double checked valve settings twice.
Little moe info ... this running rough & backfire (big bang & fire) seems to come & go ... minutes apart ... actually seems to run well in between.
AnswerHi Mike,
That all sounds good except it is responding backwards. A rich conditions usually gives you a increase in idle speed and a lean condition will drop the idle to a point that the engine dies. Just as an experiment, you should note where the needle position is as you adjust the needle down more, which is counter clockwise on the 3mm Allen screw.
If it is rich and you still have a CAT in the exhaust you may be loading up the exhaust with HC that is igniting at times and giving you the back fire.
Run the needle down until you get a definite drop in idle speed when lifting the pin and also note the engine response to removing the air filter element at times. BLM had that carb so lean that if the air filter was out, it would just about not run and it needed the added vacuum behind the filter to get the correct amount of fuel from the jet. The air filter acted like a choke to pull enough fuel in.
Let me know,
Howard
