Backfiring: Fuel, spark, or valvetrain?

[mountainexplorer]

I have a 402 BB in a '72 Motorhome. The motor starts, idles and runs fine, but backfires through the carb when under a load or when given full throttle w/out a load.

It had a points distributor and Q-jet carb; both with remanufactured stickers. I figured I'd put an HEI in it and see if that cured anything. But still does the same thing. I haven't swapped out carbs yet. I have advanced and retarded the timing, and it seems to have no effect. There are no vacuum leaks, and all plug wires are in proper order.

At idle it's fine. But I can notice a slight ticking noise. It's more noticable standing along either side of the motorhome than from up on top of the motor. Initially I had considered a cam lobe or lifter as the problem. But I pulled both valve covers and watched all the rockers as it was running, and nothing seemed out of the ordinary. There was a very slight lesser movement on #2 intake, but barely noticable. Could have just been my eyes playing tricks on me. Wasn't enough to make it backfire like it does.

If it's not a spark or fuel issue, and not a cam lobe or lifter, only other thing I can think of is a valve or bad seat. Am I missing any other possibilities?

Other facts:
The motor does not smoke, and has approximately 97,000 miles according to the odometer. All the date codes on the motor indicate it is the original motor.

It backfires regardless of being cold or warmed up, in gear or not, but is more noticable when a load is on the motor.

When I got it, the motor had a new cap, rotor and plugs. The Carb was rebuilt recently as well as the point distributor, and it all looks new and in good shape. This indicates to me that possibly someone was trying to fix the same problem in the past.

 

[deadtoyou]

prolly a sucked valve. rip the cylinders off and get em machined and get new valves in em, just doing the same thing in my buds 454. doesnt cost too much

 

[deadtoyou]

prolly a sucked valve. rip the cylinders off and get em machined and get new valves in em, just doing the same thing in my buds 454. doesnt cost too much goe to the cylinder shop on francis, pretty cheap!!

 

[diesel4me]

more things to look for...

Have you tried different plug wires??--or plugs??(hope they aren't those Bosch Platinum plugs!)...sounds like you tried swapping carbs and distributors,so I guess I cant blame a failing condenser--how about the ignition coil??--got a full 12V to your HEI if you put it in??..if it has less than 12V,or an intermittent short in the hot wire to the coil,it can cause a backfire..

It could be a burnt valve,or a bent pushrod,or sticking rocker arm(make sure the rockers havent cut slots in the rocker arm studs!)..weak or broken valve SPRINGS are often overlooked on big blocks,and are a leading cause of backfiring...if the cam lobes seem to all have sufficient lift,its not likely to be the problem--if an exhaust vale loses enough lift,it will cause a backfire thru the intake next time that cylinder's intake valve opens!...I wonder if a restricted muffler would have any effect??..good luck!

 

[mountainexplorer]

I haven't changed carbs yet. Just swapped from points to an HEI. I used an HEI distributor I knew was good, and changed the plug wires when I did it (not new, but different used ones). I ran a heavy guage wire direct from the battery to the distributor to ensure a true 12 volts, and taped off the power wire for the old coil. I haven't physically checked the plugs, since I can see the new empty boxes of Autolite plugs that were just put in.

The previous owner also claimed that it was running fine when he last drove it.

All rocker arms seen to be moving properly. There seems to be no binding, no visually bad springs or pushrods. The motorhome has dual glasspacks, and there seems to be no restriction. It would pretty much have to be a valve then, or bad valve seat. I have a pair of newly rebuilt open chamber truck 427 heads, and a new cam I can throw in it. I just want to try and drive it from town to the other place where I can permanently park it and rip it apart.

I guess I'll just be backfiring all the way. I should probably have a fire extinguisher. It throws some pretty big flames.

 

[diesel4me]

Wont hurt to look...

I'd make sure the wires are good,and pull the plugs anyway and look at them--one might be fouled or have a cracked insulator or whatever...Previous owners ALWAYS say "it was running great!"... ---I bought a truck with a 454 that SEEMED to run great--until I got it home!--then after 2 days,I had to walk home due to a junk fuel pump--after I got that fixed,the Holley carb backfired and caught fire,melted it into a puddle,and scorched the cap and plug wires...after I got a"new" carb and wires,I found out it would not rev over 4 grand without backfiring!( Iassumed it was the carb causing the backfiring)..

I tried EVERYTHING I could think of!--but nothing helped..I finally pulled the plugs out,one had about 1/2 of the center electrode missing,it was way up inside the porcelain!..but it ran only slightly better after I replaced the plug..I later found one rocker with a pushrod nearly all the way thru!--fixed that,and it ran much better,but would still "break up" at high rpm...I looked more closely,and saw one rocker arm was hardly moving compared to the others...time for a new camshaft... I drove it 3 weeks,and it was backfiring so badly I set the air filter on fire!..almost lost my "new" carb!

After I put the cam in,it ran great for a month or so--but a wrist pin or something started rapping slightly,and I got disgusted...sold the motor to a friend who wanted it for a spare for his ramp truck!--and I bought a 78 Burb with a 400 SB to put in its place...He rebuilt the motor,said 2 pistons had broken skirts,they were in the oilpan!...that was one 454 I didn't miss much!.. ....

Hope its something simple and cheap to fix!..good luck,check everything before pulling the heads as a last resort!..that 454 I told of also had 3 toasted exhaust valves--but a compression test showed 125-150 lbs in every cylinder!

 

[Mad-Dog]

A compression test would be a good idea........when you mention #2 intake valve movement and backfiring thru the carb in the same sentence it always leads me in the same direction............burnt or bent valve, just my $.02

 

[73k5blazer]

A compression test would be a good idea.....

What he said. Simple to do. Leakdown test on any low reading cylinders as well

 

[mountainexplorer]

It must have been a lifter sticking a bit from sitting. I drove it 85 miles last Thursday night and it backfired the first few hills then never backfired again. Now that I can get on the throttle without it acting up, that thing has loads of power, for a motorhome.

 

[theperfectgarage]

It must have been a lifter sticking a bit from sitting. I drove it 85 miles last Thursday night and it backfired the first few hills then never backfired again. Now that I can get on the throttle without it acting up, that thing has loads of power, for a motorhome.

Well thats good news, I was gonna offer the thought of perhaps a well worn timing set ? That's before I read it was " fixed ". I would like to hear what diesel4me thinks of that, I always respect his posts.

 

[diesel4me]

Valves and lifters...was hoping it wasn't that "deep!"

So it must have been a stuck lifter ??..I guess that could do it,I've had lifters "pump up" in my 68 GTO's 400 when I did a long burnout one night,and it would backfire and break up anytime I got it over 3800 rpm's after that!..

I changed everything I could think of to get rid of that high speed misfire!..new points,condensor,plugs,wires,cap,rotor,even another distributor in case it was worn bushings..still "broke up"!..rebuilt the carb for nothing too..

I then assumed the cam must have lost an exhaust lobe..So one weekend I brought home a cam,set of lifters,and a intake gasket set from the parts store I worked at--also some extra pushrods,in case I bent one(I was hoping it would be a simple fix like that!)..

Took off the valve covers,and eyeballing all 16 rockers,they all were moving equally,so no cam lobes were worn I assumed! ..no bent pushrods either,and I'd just put new timing gears and chain in it recently,so I doubted that would be the cause..I ran the motor with the covers off another minute or so,and noticed the last valve on the drivers side was "lower" than the rest!.I shut it off,and checked the valve spring..it looked perfect,but the valve wasnt closing all the way!--my heart sank,assuming it was a bent valve head or stem!..

I reluctantly pulled the intake off,and the valley pan..almost started taking ff the head bolts,but this was my ride to work monday AM,so I decided not too at the last moment..

I then took out a few lifters..and when I looked at the lifter that was on that valve I suspected of being bent,I noticed the little "lock ring" dewhickey was GONE,and the plunger was sticking out of the lifter a good 1/4" .. --

I found my reason for the valve hanging open just enough to cause it to break up at high speed!..put all 16 of the lifters in,got it back together the next morning,and took it for a test beating..smoked the tires 300 ft ,and no more high speed misfire! ..

A few years later after I sold it to my friend,it had started to guzzle oil, and had tons of blowby--we ended up putting rings on the #7 piston,after discovering it had only 30 lbs compression!..all other cylinders had 160-175 lbs..it took us another whole weekend,but it ran perfectly again!..the compression rings were broken into 1" peices on that one piston,but no damage to the cylinder at all..still had hone marks showing with 110,000 miles on it!..

By the way..we found more than one of those "lock rings" for the lifters,AND a plunger from one stuck in the sludge at the bottom of the oil pan!..guess I wasn't the first guy to pump the lifters up too much!. .one lock ring was in the oil pump screen,so we put a new pump and screen on too..

Thanks for the kind words theperfectgarage!!...

 

[mountainexplorer]

Oh yeah, just a sticking lifter that free'd up. Just needed to be driven. Last August after I drove it up to the place, I was burning out with it all over the place, raced a motorcycle up the road and won, and nosed up to a '75 Chevy truck sitting on the ground and pushed it forward till I ran out of room.

It really made me wish I was able to take it out to the racetrack once before dismantling it. It will plain get up and move... my 440 powered motorhome and a 454 parts motorhome I also have are no match for the 402 motorhome in the acceleration department.

 

[Zeus33rd]

I voted cam lobe. Had the same thing happen to my old 402. The backfiring sounded like a little midget under the air cleaner lid bangin on it with a hammer. Tore it down and one of the cam lobes looked more like a journal. The lifter had a 1/8" deep dish in it.