I am still trying to get the 429 running in my hiboy, and I am out of ideas. I am getting BAD backfires out of the exhaust and the intake at anything below 1200 rpm. I already split one set of mufflers it backfires so bad. Above 13-1500 rpm it seems to run fine and sound good. Here is what I have and what I have done so far.

1971 429, D0ve heads, d1ve block, flat top pistons with two valve reliefs. Heads were refurbished by a machine shop, milled .006, new valve guides and umbrella seals. All valves checked good for sealing and wear. Engine partialy assembled (short block and cam, timing cover and new 68-71 roller timing chain and water pump together) in truck when I bought it. Heads were installed with fel pro head gaskets and torqued to 140 ft/lbf. Compression was 170psi with no spark plugs installed, and no carb, repeated test wet gave the same readings. Intake manifold is an edelbrock performer 460, assembled with new hardware using fel pro gaskets and torqued to spec. Distributor is a d8ve duraspark distributor, brand new reman. Coil is a blaster 2. I have three duraspark modules that came with the truck, all tested good. Carb is a 1850-5 list holley 4160 600cfm vac secondary with a fresh rebuild. The only vacuum accessories on the motor are the trans, power brakes, distributor, and vacuum gauge. Timing is set to 6' btdc, sticker on valve covers says 4 deg, but it diesels badly if dialed that low. Advancing timing any more than 10 deg just makes it extremely hard to start and backfires more. Plugs are Autolite 45, new at the beginning of all this, gapped to .044. Plugs are now black and sooty. Idle mixture screws started at 1 3/4 turns out, tuned to highest vac. at only 3/4 of a turn out. Vac is fairly steady, needle doesn't seem to vibrate rapidly with fluctuations less than half an inch. Highest reading at as close to idle as I can get is right around 12, with light throttle vac climbs to 20 at around 2000 rpm in park. The engine has a severe stumble then backfires through the carb if the throttle is applied with anything but a feather touch.

I am lost. I don't know what else to try. Any and all suggestions are very welcome. Thanks in advance for all your help, and if this would belong in a better forum than this, then please feel free to move it.

 

I suspect a blown power valve.

 

If it is carb related, i"ll just save up for a edelbrock 750, holleys and I have never agreed...

 

Holley carbs have a "power valve" that enrichens the fuel mixture at or near WOT. Unless your carb has a power valve protector installed any little backfire can split the dyaphram in the power valve causing it to leak extra fuel into the intake thru the vacuum port, screwing up idle and low speed operation. Easy check, turn the idle mixture screws into 1/4 turn or less and if the engine runs better the power valve is blown. Simple and cheap cure, buy an after market power valve protector and install it when you R&R the power valve. Since I installed one 10 years ago, have never blown a power valve in that carb again and it has very expensive dual stage power valves in it ($25+ each). A Holley is a simpler carb than the Edelbrock carb. The problems with the meterig rod carbs was why the Holley 4150/4160 series was developed. If you can tune a Edelbraock carb (in reality a Carter AFB) you can tune a Holley in less than half the time and with half the smarts; as the Holleys are "KISS" peices of equipment.

 

Double check your plug wires and make sure you don't have one out of place. It will cause the problems you are describing.

 

When the motor was essembled did the timeing gears get lined up
correctly (btwn the cam and crank) Cuz if its off a tooth you'll never get it in time. or maybe the distrubitor is off 90 deg or
less.

 

I am not totally sure on the timing gears, the PO already had that installed before I got the project. Plug wires have been triple checked using the haynes manual by me and three other friends who were as stumped as I was. I will order the power valve protector and the new rebuild kit again and try that. I know it is a new timing chain, there is almost zero play between moving the crank and watching the rotor move on the distrbutor in both directions. Guess I will have to pull the front of the motor down to check the marks on the cam and crank.

 

You don't need a rebuilt kit, just get a power valve. They sell them seperate and the standard valves are cheap.

 

Yeah, just looked, $12 for blowout kit, $8 for power valve. I'll start pulling the front of the motor down tonight, I'll post later to let you all know the results of the timing chain. Thanks for all your help, it is really appreciated.

 

sounds like a timing set to me, maybe it wasnt installed right

 

im with bear, this sounds like a carb issue to me.....if it where the timing chain being a tooth off it would run like crap at high rpms more than low rpms, a good running engine will usually idle fine with the timing being + - 10 degrees off, but once you hit the gas you hear the backfires........ since you say it runs fine at high rpms, im inclined to say that the carb is overloading by means of the powervalve or possibly the bowl float in the carb not closing the needle properly...if i were you i would do the powervalve thing and then check the float level before tearing in to the motor......

 

Update: timing chain is dead on, brand new, no slack, and lined up properly. I just spent the last 5 hours tearing it all the way down and reassembling it., so it looks like it is probably the carb. I'll go get the parts to try and fix it tomorrow and will update again tomorrow night. In the meantime, by all means, keep spitting out ideas, I really don't mind checking everything just to eliminate the possible causes.

Could too much backpressure cause these symptoms? I know the mufflers (one is now split wide open from the backfire) are pretty old, could they have collapsed inside and make it backfire?

 

Did you check push rod length ? Sounds like it may need shorter push rods, considering the head work that was done.

 

You need to do a compression test to determine is the motor is truely healthy or if there is a mechnical problem (ie: the push rods to long, etc). I doubt that dual exhaust would ever cause a back pressure problem.

 

Compression test was done after heads were installed, lowest cylinder was 155, highest was 180, the other six were all between 165-170. Repeated test with a little oil in the cylinder to see if that changed anything and it did not. I will repeat test this afternoon when I get back from work and post each cyl. pressure.

Is there any way to check pushrod length with hyd. lifters? After installation, all were snug between lifter and rocker, with just enough play to be able to spin them with fingertips. Motor uses early style non adjustable rockers.