Back Firing!!

My '68 Commando, Buick 225 w/ HEI conversion began sputtering, lightly backfiring and losing power when I reached 55 mph. If I push the clutch in and rev the engine it seems to be fine. Only seems to have the problem when I am "under power". Any ideas as to where/what I should be looking for?

[RRich]

Could be any number of things, ranging from fuel, to distributor, to even the timing chain.
I'd start wuith fuel first.
Filters dirty? Does it do it going downhill as well as up? - filter(s.)
Could be a piece of dirt or water in the carb jet. Rev it up to about 3000, put your hand over the carb
top choking it off fully. Keep it there till it dies. Restart and try it, maybe fixed now. The extreme
vacuum from choking it with your hand can pull out the trash.
If you have a manual choke, run it till it sputters, pull it out - if it helps it's lean.
Check dwell, then timing.
Tell us what happened. Rich

RRICH, Thanks for the ideas. I started with the filters, replaced them & had same problem. Monkeyed around with the timing but have misplaced most of my info on HEI timing. Can you help? I think it should be set @ 5 degrees @ 800 RPM with the vacuum hose plugged. Is this right? Also, what's a good way of knowing if timing chain needs to be replaced?

Thanks, Craner

[RRich]

The timing spec for the 225 is 5 degrees BTC. It makes no difference whether the type of ignition is HEI, Duraspark, standard [points, or any other aftermarket, even if you just manually touch wires together by hand, the spark should still happen at 5 degrees!
You can probably get away with 10 initial. The vac should advance it about another 15 or so - but only at high speed 2000+ .
The centrifugal advances another 15 degrees. Total would be about 5+15+15 ==== or 35-40.
Always set dwell first, it affects timing.

Unfortunatly there is no easy way to determine if the timing chain is loose. The method of rolling the engine forward by hand a little, then rolling it back to see how far you can turn it back before the dist moves does not apply here. Normally the rule of thumb is 10 degrees, but the 225 has a chain tensioner. It will always show as good, right up till it jumps!
There is only one way, take it apart and look. They came with a plastic coated cam gear, if you find one in yours, replace it. They are only reliable for about 70,000 miles +\-, then can disintegrate.
I still think it's fuel if your timing is close.
Did you try enrichening it?
Trick: Take the end off a propane torch. Make sure you can get a good volume of propane out, you may have to take out the tiny orfice. Put a hose on the tube. Put the other end of the hose in the carb - wire it to the stud. Drive it, make it buck. Add propane (it burns like gas.
If it helps, it was too lean. (Called propane enrichment.)
Rich

[Paul Calman]

The most common cause of backfiring/missing under load with Delco HEI ignition is bad wires or the dist cap.
Wires are easily checked with an Ohm meter, they should be within 1000 Ohms of each other.
The caps tend to allow spark to jump internally to ground on the coil. You can usually see the "dust pattern" where the arc goes. If it looks dusty, get a quality replacement. Napa ,and Delco are the only 2 I would use.
Make sure that the coil's ground strap is inside, connected to a coil screw, and use the supplied dielectric grease on both sides of the rubber cookie, and on the connector plugs to keep them clean and dry.

To Email, leave my Mom out of it.

RRICH, Got it working fine again! Under power the carb was not feeding half the cylinders. Pulled top off the carb and cleaned the main metering jets, replaced plugs and rotor. Everything is fine. Thanks for your help.

Craner