L18 backfiring under load

[leonardo_milan]

Hey guys having a problem with my 720 and was hoping someone on here would be able to help or atleast point me in the right direction.

 

When under load the engine backfires constantly to the point it is undriveable and no matter what adjustments i make on the carb i cannot fix this issue. the engine has new plugs, coil and timing has been set to factory standard. it will idle (although not very happily and far too fast) and rev without hesitation but as soon as any load is on the engine it splutters and backfires constantly. it is an L18 with a hitachi dch340 carb. this problem seemingly came out of nowhere as i was driving the car the day before with no issue 

 

any help is greatly appreciated. cheers 

[Bleach]

Could be timing advance is suddenly not working.

 

You might try advancing the ignition more at idle and see if it drives under load better and then retarding timing at idle if the advance does not improve drivability.

 

In the end, if that works, your distributor may need a new vacuum module or the rotating plate inside may be seized up.

 

Also, make sure that hose on the distributor is in good condition and connected. If the idle is too high, you might have a vacuum leak somewhere. Double check the carb. Is it seated solid or is there any wiggle to it.

[datzenmike]

From most to least likely...

 

Check the valve lash. A tight intake won't close tight enough and the firing of that cylinder under load may be able to squeeze past between valve and seat and light up the fuel in the intake.

 

Distributor cap or rotor may be carbon tracked or even cracked allowing spark from one cylinder to fire another.

 

See if one of the exhaust rocker arms has popped off. The cylinder is basically deactivated but some gas and air gets in on the intake stroke but without much volume it burns so slowly it's still alight when the intake opens again and back fires. 

 

Likewise an intake rocker may be off and jamming the valve from closing properly.

 

Intake valve seat is worn out and not capable of sealing in and containing the cylinder firing.

 

 

 

 

 

 

 

[leonardo_milan]

thanks guys i’ll have a look and let you know how i get on with it 

 

cheers 

[Stoffregen Motorsports]

Sounds ignition related to me. Chewed plug wires maybe?

[datzenmike]

Had a friend who had a '79 Monte Carlo with the 265? The ones that the cam lobes always wore off, like the 305s. You could drive it but the slightest load it would constantly backfire. The worst cylinder was the exhaust valve not opening and what little gas and air was exchanged would light up the intake as soon as the intake valve opened.