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Technical Area => Questions and Answers => MK II B-GTs ('67 to '75) => Topic started by: june2020 on May 10, 2020, 01:09:12 PM

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Title: '69 B-GT Backfiring and Losing Power
Post by: june2020 on May 10, 2020, 01:09:12 PM
I have been dealing with trying to diagnose a problem with my '69 B GT. It was running great up to a few months ago, then coming back from a run it started backfiring and losing power to the point I had to stop and get a tow. I checked the points and they look good, set them again, then went on to the fuel system. I took off the fuel lines before and after the in-line filter to check for flow and can hear the tick of the fuel pump and have good flow. I even replaced the fuel lines and filter under the hood with new but it doesn't seem to matter. The car will start but seems starved for fuel, idles rough, and when stepping on the gas it wants to cut out. Even feathering the accelerator doesn't seem to work very well. Took the K&N air filters off to examine and they look good, no airflow issues here. Thoughts?

John Hale
Title: Re: '69 B-GT Backfiring and Losing Power
Post by: amgba on May 10, 2020, 01:38:23 PM
My two cents is that this is one of those "Back to Basics" kind of problems. Like that old saying "If you hear hoofbeats, think horses, not giraffes"--the same applies to car symptoms. Hit the basics first, not the esoteric possibilities. How good is the spark? Did you use a fuel pressure gauge (you can have a squirt, but not enough pressure)? What is your compression? Did you test for vacuum leaks with carb cleaner or smoke? Did you do a cylinder balance test?

Joseph Troise
Title: Re: '69 B-GT Backfiring and Losing Power
Post by: Art on June 08, 2020, 12:27:48 PM
Hi John,

All good advice from Joseph, but from what you're describing, I'd say to take a look at the vacuum and mechanical advance systems on the distributor.  A 69 B would be equipped with a Lucas 25D4 distributor, which is notorious for problems with the vacuum advance disc failing, often with the advance arm separating from the vacuum disk.  This can happen on the later 45D4 type, but less frequently.

The mechanical advance can't handle any hard acceleration on its own and the engines seems to bog and backfire as it tries because the timing is not advancing, just as you are describing.  I'd check the vacuum pot and the mechanical advance system (springs, weights, etc.) for damage, rust or wear

My old adage is 'if it feels like a fuel problem, it's probably an electrical problem' (and vice-versa).

This may not be the culprit, but it very much resembles gas starvation on acceleration. 

Hope that helps you.

Safety Fast!,


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