Ok, I had the HB on the road now for close to 2 months.

im getting an ave 27mpg city/hwy (60/40).

Here is the problem. after sitting overnight, starting, a quick warm up, i get going and i get a single backfire while shifting. after it gets to operating temp, no more backfires.

would it be a sticky valve causing the problm?

i using 10w-30 oil, do i need to do an internal cleaning(motor flush)?

thanks,

More likely a exhaust leak that as the motor heats up it goes away enough to not allow any air in the exhaust. What your hearing is unburned gas being combusted in the exhaust. You need air for combustion and therefore i bet you have a blow out manifold gasket or two...or at least one starting to go.

More likely a exhaust leak that as the motor heats up it goes away enough to not allow any air in the exhaust. What your hearing is unburned gas being combusted in the exhaust. You need air for combustion and therefore i bet you have a blow out manifold gasket or two...or at least one starting to go.

 

I checked, bolts are tight, no gaps and no exhaust leak. But it seems that its occurring more and more now even after it has gone to operating temps. more specifically during the gear shifting phase.

 

I rechecked the firing sequence, and plugs and wires, aside a bit of wear on the wires, nothing appears out of ordinary.

I have thought it may be a timing issue, but adjusting it all the way to 0* BTDC isnt helping. still bf'ing on a 10 mile test drive.

 

After more reading and research,

Would this be a weak exhaust valve spring floating and allowing the resulting spark/explosion out the tailpipe?

Did you feel and listen for an exhaust leak. Just cause the bolts are tight does not mean you have a blown out gasket. I never asked..are u running a cat still or straight pipe to muffler? If its the second your most likely going to just have to live with that noise.

Does your year/model have an anti-backfire valve? Maybe it's toast. They seem to expire at a rather young age, judging by when mine croaked.

 

And they are very expensive to buy new (and I can't think why anyone would want a used one, given their short lifespan).

Block the air suction valves (ASV's) with quarters.

 

GD

check if your intake manifold gasket is not blown. I had the same problem, The car would die if I didn't ramp up the fuel, causing the backfire.