Misfire and backfire

I have a 91 V8 classic on petrol and LPG. It has a bad misfire. Changed plugs, leads, amp, rotor arm, and finally the distributor and checked for leaks. Took it for a ride today and going up a slight hill, it backfired really loud and died. Got out and saw the AFM had been blown off. Put it back on and carried on, it backfired again and blew it off again. Finally managed to get home and changed the disi back to original. Then I checked the head bolts on the left hand side and found one slightly loose. Tightened it back up and started the car. Still runs very lumpy. Has anyone got any ideas please?
.

Long time ago I had a V8 110, the system was really quite old, probably late 80s, and while it worked fine the switch that controlled the fuel of choice didn't always shut the gas off 100% so I got into the habit of manually shutting off the gas with the tank valve - a bit like a plumbing stop cock. If there was just the slightest amount of gas getting through it would backfire quite hard.

I assume it is a single point system with the gas plumbed into the intake pipework.

When they run low on gas, or the gas pressure is too low, they can burp and backfire through the intake pipe, they have been known to blow the MAF apart...so just knocking it off means you got away lightly.

Some resolutions are to keep the tank topped up, meaning the gas pressure is at max or thereabouts, adjust the gas flow slightly, or to fit a 'Fart Flap' - this is a backfire device that when the system does backfire the pressure is released through the flap instead of the back through the MAF.

Fill up and see if it does it again, if so, get the system re-setup by a garage to ensure it is running right.

If it only backfires on gas, try more advance. If you aren't using a timing processor, 8-12 Deg btdc should be a happy compromise as long as your crank pulley is accurate and it doesn't start pinking on petrol. If you have a processor, set it to 16 btdc on gas.
A loose cylinder head bolt is not good and warrants investigation

The tank is half full of gas, but it is the same on petrol. it is a hotwire setup, so I didnt think it would blow AFM off. Might change the leads back later and see what that does.

shauntbh1 said:

The tank is half full of gas, but it is the same on petrol. it is a hotwire setup, so I didnt think it would blow AFM off. Might change the leads back later and see what that does.

Click to expand...

You may like to check the compressions if all else is good (plug leads in correct positions) sounds like you may have a burned or sticking inlet valve.

Started as a very slight misfire, then I started to change things, I am now almost back to original. Notice that 1 and 3 plugs were black. Now it wont go above 10 mph

shauntbh1 said:

Started as a very slight misfire, then I started to change things, I am now almost back to original. Notice that 1 and 3 plugs were black. Now it wont go above 10 mph

Click to expand...

Do you have 1 and 3 leads transposed?

wammers said:

Do you have 1 and 3 leads transposed?

Click to expand...

no, checked loads of times

shauntbh1 said:

no, checked loads of times

Click to expand...

As i said earlier then, check compressions you may well have a blown gasket between those two cylinders.

I meant to say 2 and 4, sorry

shauntbh1 said:

I meant to say 2 and 4, sorry

Click to expand...

Ok you then either have 2 and 4 transposed or if they are not a blown gasket between those two cylinders.

Will check later