2.0L CU air cooled - intermittent backfire

[neil3965]

Hi all - hoping for some insight.

Problem is random, loud backfire (anything from 2 to 150 miles in between each event) - just a single very loud bang out the exhaust with momentary hesitation. Usually when cruising at 35 to 55mph in fourth, but occasionally in lower gear. On two occasions the engine cut out after the backfire, but restarted immediately.

Not related to deceleration, and can't be induced. Runs downhill on overrun with only mild popping.

Symptom appeared a month or two before trouble with #4 exhaust valve, loss of power, loss of compression, turned out the seat had receded. Swapped for new AMC cylinder head, and new RHS heat exchanger (as there was an obvious leak in the downpipe). So, bad valve, exhaust leak - thought I had found what was causing the backfire (as well as the loss of power).

Passed MOT, CO 1.2, HC 48ppm.

Power is good now, but random backfire persists. Compression test this morning was fine ( 9.2 / 9.5 / 9.2 / 9.8 ), but vac gauge on idle was flicking rapidly back and forth between 10" and 14"Hg (smooths a little to 15-17 on higher revs, but still flicking).

Starts OK, idles fine, if a little rough at times, no stalling. Cruises and climbs hills OK except for the random backfire.

New coil, HT leads, dizzy cap and rotor, spark plugs. No vac leaks found. Mechanical fuel pump, good spurt from pipes on engine turnover, filter clean, pulled line off tank and strong flow of clean fuel pouring out.

So - ignition problem, carb problem, dying fuel pump? Any thoughts?

Heading out to check for exhaust leaks, good earth from transmission to chassis, fuel pressure, float chambers in carbs, recheck for vac leaks. Maybe try new ICU.

[Paul Weeding]

Sounds like a missed ignition cycle, so the unburnt fuel is the igniting in the exhaust.... As to why, it could be time for a new coil and TCI unit...

[neil3965]

Thanks Paul - the randomness made me think ignition too. But wanted to go through my tick list before swapping out the ICU.

Couldn't find any exhaust leaks, or vac leaks. Fuel pressure was 2.7 to 3.0 psi, which is low (spec is 2.9 to 4.5). Transmission earth OK.

But when I looked down the throat of the left hand carb I noticed that fuel continued to drip from the pump nozzle at med revs, even when the throttle was held still.

Carb out, top off - and behold, bits of muck swilling around at bottom of float chamber. Some varnishy looking stuff encrusting the chamber, and a dozen or so lumps of soft, black matter, that smeared to paste between the fingers (perished rubber?). All fuel hose is new, could be from deteriorating pump gasket or diaphragm? (See low fuel pressure).

Anyway, cleaned out the gunk, blew all ports and jets through with carb cleaner and air, new needle valve and gasket, checked float level (14mm, spec 12-14) and reassembled. No more dripping from accelerator pump nozzle.

Could be that random backfires were caused by piece of gunk getting sucked into main jet, left side cylinder getting leaned out to point of backfire, gunk not solid enough to permanently block jet?

Not sure how the gunk would cause the dribbly acc pump though... Muck trapped under check ball allowing siphon effect with open throttle?

Whatever, drove 130 miles plus today with no backfires so hoping that's it! Will check and clean the other carb ASAP, and check out a replacement fuel pump.

[Paul Weeding]

That does sound like your problem!!

Electric fuel pump might be a better option... I've heard of the new mechanical ones failing prematurely

[neil3965]

As usual, spoke too soon!

After 200 miles or so the backfiring returned, along with hesitation, lack of power. Cut out a few times, but would restart immediately. When examining the engine (while idling), I noticed that there were air bubbles visible in the fuel line at the semi-transparent t-piece that splits the feed to the two carbs. Suspected a failing fuel pump.

I fitted an electric fuel pump and new fuel line, and removed and cleaned both carbs, but the problem persisted, and got worse - the engine would occasionally misfire, and randomly cut out and not restart for a while.

Finally cut out on a busy road in Ireland, friendly passer by towed me to a layby where I could safely work on van. Now convinced the problem was electrical. Went through Bentley's ignition system troubleshooting procedure, and got no response from voltmeter on Step 7 (voltage across terminals 1 and 15 on coil should rise to 2V when ignition turned on, then fall back to zero after a second or two). Suggests faulty hall sender or open circuit in wiring.

Tried starting engine, and it ran. Went back, fiddled with various bits of wiring, and was able to stop engine by wiggling plug on side of dizzy. So either the contacts in the plug are dodgy, or a wire or joint in the Hall sender unit has broken down inside the insulation.

Limped to a campsite near Bantry and ordered a new Hall sender unit from Brickwerks in case I couldn't repair the old one, just waiting for it to arrive now. Dismantled the dizzy, no obvious break in wires, might just have been the contacts in the plug, but will replace it just to be on safe side. Will keep old sensor and plug to test and repair if possible.

Will let you know what happens, just in case anyone searching has similar random problems.

[Paul Weeding]

Sounds like some good deductive work!!

[neil3965]

Don't you hate it when someone says "I'll let you know what happens" and then doesn't? So, many apologies for not closing this thread earlier.

Long story short, I replaced the Hall sender and, hey presto, problem solved!

All the niggling hesitations, random backfires and occasional cuttings out of the engine have gone.

Moral of the story, if you have similar random symptoms, check the Hall sender. There are three tiny wires inside that flex and bend every time you touch the accelerator (as the advance/retard plate rotates). After 30 years of this, those tiny wires begin to perish.

If wiggling the plug on the side of the dizzy makes the engine die, then suspect the Hall sender. But of course the only way to be certain is to replace it with a unit that is known to be good.

Only aftermarket parts now available (Brickwerks), and require widening the slot in the side of the dizzy body to accept the wider plastic socket (quick and easy with junior hacksaw and triangular file).

[Sir Brixalot]

Thanks Neil for coming back to finish it off, so many don't

[Sir Brixalot]

What a concidence. I'm having the odd backfire and random conking out. Changed the pump twice (and rod) and beginning to wonder if it could be the hall sender. Anyone know if the Brickwerks one is easy to fit bearing in mind a different plug has to be added.

[tobydog]

Could also be the ignition amplifier

[sarran1955]

Hello,

This may be a simple solution..

Sufficiently cheap to even carry a spare..

https://www.youtube.com/watch?v=KkyBAMMHpa4" onclick="window.open(this.href);return false;

Cordialement,

[Sir Brixalot]

I've done the amplifier. Will have a look at the Powerspark.