Backfiring while driving, not while sitting still?
 

Hope someone here can help me out. I built a 351w, and after finally getting everything put together, it ran great for the break-in. Sitting still, it runs and sounds great. Problem I'm having is when I'm driving it's backfiring out the carb, but only while accelerating. Timing is set right, all vacuum lines are new, new fuel tank, lines, filter, complete new motor, trans, carb, etc. I'm bangin' my head on the wall now trying to figure out why it won't do it while sitting still in my shop, but as soon as I get on the road, Pop.


Anyone have any ideas to help a youngster out?

Thanks!

John T.

timing advance module bad?
oil pressure constant? keeping a intake v open at high rev?

Check your firing order sounds like spark plug wires crossed. If that doesn't fix it pull the valve covers and look for broken parts.
good luck

:D :D ....Am i right this only happens under load in drive.....might want to ck the adjusters on the intake valves....either too tight or broken and not holding setting

Well...not the adjusters, checked them earlier in the evening. Also put a different distributer in it just to make sure it wasn't the pick-up coil. Rechecked the firing order, and all the plugs/wires. No broken parts under the valve covers - everything looks great and new.

Hmmm...bangin my head against the wall one more time. It did start doing it in my shop while in park, but only while revving the motor.
Any other ideas would be greatly appreciated.

Thanks again!

John T.

IF IT IS UNDER LOAD THE ONLY THING HAS TO BE SOME SORT OF ELECTRICAL MALFUNCTION :shock: :shock:

I HAVE SEEN A WEAK BATTERY CAUSE THIS SAME THING TO HAPPEN IN A GRANPRIX I WORKED ON :evil: :evil:
JUST A THOUGHT BUT MIGHT BE WORTH CHECKING!!

LIVELY

Lively is on the right track I think, low voltage could cause a problem.


I had one setup that ran fine sitting there but when you got into it, it'd miss ended up being plugs. Assuming you have checked them all, and ohm'd all the plug wires ? What ign setup are you using ?

flat tappet or roller camshaft ?

flat tappets were bad about knocking lobes off camshaft resulting in the backfire your speaking of.

Next i would check Timing chain, also did you degree camshaft ?

i would rule out electrical 1st..but would look here in the end if the rest doesn't clear it up

food for thought.

ANOTHER THING :shock: :shock: --IS IT A MISS OR A DEAD STUMBLE?? ARE YOU RUNNING A CAM OUT OF A 351W OR FOR A 302? DIFFERENT FIRING ORDERS!!!

LIVELY

i had that problem one time with the spit fire spark plugs,
went and bought the reliable a/c and that fixed my problem,
sounded like the timing was off, bad spark plug, might
check that, i zapped my hand checking plugs, man what
a charge

Using flat tappet. THe cam is a 351, and I already switched the wires to a 302 order just to make sure that wasn't it. After doing that, I replaced the plugs just to be sure it wasn't something like that. I'm still using a stock ignition for now - yeah, I know...young racer on a tight budget :cry:

It did start doing it while in park and revving the motor.

It's only flaming out the pass. side of the front of the carb. I pulled the cover off that side, and everything seems to be working fine.

...was it a new cam and lifters ?

Yep, new cam and lifters. What I don't get is if I just gradually rev it, it doesn't do it. Only when revving it quickly, or when taking off in drive. Guess I'm gonna have a beer and call it a night. I'll try to remember my camera and maybe put a video link up.

So confused.

Thanks for all your help so far! I Really appreciate it.

A lean mixture can do it, so can a bad distributor cap. Vacuum leak also can. If you have a vacuum gauge, and can hook it to manifold vacuum, put it on and see what it reads at an idle. Valve lash set too tight?

could be any of afore mentioned, but i am thinking lean condition, float level too low, air/fuel mix, etc... what type carb are you useing?

backfire's under a load
 

Do you have a msd box??? If so, does the timing mark jump around when you check it w/ a light? How much lift do you have on the flattappet cam & did you break it in w/ just the outer spring's first. Check the plug's first to see if if they are burning lean. example: do they look the same as they did when you first installed them(white)?

Low voltage defiantly could be the problem. If you have a 6al or better make sure you dont have a 3000 chip installed. Some of my buddies use to think that was funny to put a 3000 chip in when someone just built a new engine.


If you really want to know what the true firing order is,

Start with number one. Make sure both valves are closed.

make the dampener t.d.c. mark align up with the pointer.
then
just rotate crank with a wrench one quarter of a turn,
whatever the cylinder is that both valves are closed,
it the next cylinder that is supposed to fire....
it is probably #5,
or #3 and so on........

I'm using a holley 650 double pumper carb that I bought after it was rebuilt (after giving the guy my engine specs, it was supposed to be able to bolt it on and go HA HA) So far, I had to reset the floats - fuel level just at the bottom of the sight glass. Haven't checked the jets or anything on the carb yet.

No msd ignition, and it's still running single springs on the heads. All the lifters are still moving like they should, and I double checked the firing order one more time.

Ever feel like one of Dr. House's patients? :lol:

How did you adjust the idle mixture, did you use a tach, or a vacuum gauge? Did you adjust the Idle mix screws? It sounds like it could be lean, Usually a low voltage can cause a miss, but I'm in no way saying it couldn't cause a backfire, If it's already lean at idle, and you crack the throttle, especially under a load, that could cause a lean backfire through the carb.

Have you pulled the plugs and checked the plug wires yet ?

Double checked timing again ?

John start your engine and let it warm up a second and then take spray bottle with water in it and spray each header tube close to the head, to see if you have a cylinder not firing or weak. The water should sizzle and evaporate immediately.

Used a vacuum gauge to set the carb. Also tried the water on the headers, all are hot. Still does it.

John do you have another carb to try? It would be nice if we could eliminate the carb as the problem. There is a lot of things in the carb that might cause this--bad power valve, bad base gasket, debris in jet, ect..

how would it be a lean issue, when its not getting enough fuel ?
and its coming back through the intake valve! so why is it firing when the valve is open
it still sounds electrical/advance some how?
wrong distributor gear?

I don't have another carb to try, unfortunately (unless I snatch the one off my father's 64 Fairlane, but I like being alive :D) so tore mine apart last night. It's got 63 jets in front, wiht 80's in the back, put in a new power valve (6.5), and everthing in it was still Really clean.

Not sure if I posted it, but I did put a different distributor in it night before last with no luck on that either. Also tried a different ignition box (motorcraft).

My father told me that I may have the wrong size pushrods in it. The cam Grind # 270-2h 204/214@50, 270/280, 448/472 from Crane. It's got stock size pushrods in it now. I called Crane and found out how to measure and get the correct size, just to rule that out too.

Have you tried beating the shit out of it with a BFH?

Sooooo close to trying that!!

Double check your valve adjustments, its got to be either in intake valve too tight or exaust loose.

They're non-adjustable.

if all else checks out and it seems you have ruled out eletrical and you got to go into the engine..i would start with cam and timing chain....I PERSONALLY think your gonna find your problem somehwhere between either one of these and i'm gonna say wiped out lobe on camshaft...

( Not that you did something wrong but that it just happens )

everything you have stated leads me to this conclusion...i mentioned this earlier in the post with a few reservations but i am almost convinced at this point.


i am sure you will find it either way

If it wiped out a lobe, wouldn't it be acting up even when slowly revving from low-high rpm's, not just when quickly revving it?

Is the cam a WG1107 or a WG1108? And is there any chance one or two of your lifters aren't pumped up.

Cam is the 1108. Not sure about the lifters?

Also, if it is a lobe on the camshaft, is there a way to tell w/o tearing the motor apart again? Not sure if it would work, but it seems like I could pull the plug wires off one at a time and see if it quits doing it with one of them??? Would this be possible, or is my brain fried pie from all the thinking it's done today?

Thank ya'll so much for the help so far!

back firing
 

Sounds like a valve train issue to me. If it is back firing through the carb under load. I have had the same thing happen with a fresh set of rebuilt heads, the valves were not cut right and it would run fine at idle but when it was put under a load it would start popping. Just a thought ?

John, when you put the timing chain on is there any chance you got the timing marks off?

I'm starting to agree with bjuice.

Just my opinion but my next four steps would be.
1. borrow your dads carb for a second to eliminate your carb.
2. pull your valve covers to see if all the rocker arms look like their
moving the same amount.
3. check your timing chain to make sure your dots are lined up at tdc.
4. do a compression check on all cylinders to see if they are the same.
The compression check will tell you if you have a valve leaking.

Yep, I ws hoping that it was a carb or ignition problem, but it aint looking too good for that. When you pull your rocker covers, put a dial indicator on the rockers. You can check lobe lift at the pushrod end, or valve lift at the valve end. Compare in. to in., and ex. to ex. as I believe you said it was a duel pattern cam. this will tell "FOR SHORE" if you have a flat lobe or one that is going flat.

that won't work if it is a cam lobe....exhaust valve probably not opening (worn lobe) so when the intake valve opens it goes that way...just my thoughts ...not as experienced as most here are.

If it was a totally flat lobe it would be hissing back thru the carb at idle or low rpm something you could definately hear, not to say it's not, a compression check would definately tell that story, what no one has mentioned if a week or broken valve spring, just a thought, because i too have no idea what the problem is without looking at it and investigating it myself.

JMO

Zip.

GOOGLE CRANE CAMS WG1108K Then click on the top choice (Jeg's) and look at *** NOTE. I'm with Gimmemud , something is wrong in the the adjustment of an intake valve. Too much preload on an intake lifter ???

Very good advice DADMOD and your first post. I got the noted part DADMOD had referred to.

***Note: To provide the most accurate valve adjustment on hydraulic lifter camshafts, the heads must be machined to accept screw-in studs (on engines not originally equipped). On engines equipped with bottleneck type studs, using 270-99768-16 positive locking nuts will permit valve adjustment. For engines equipped with pedestal mounted rocker arms and hydraulic lifters, excessive lifter preload can be easily remedied by using Crane's rocker arm pedestal shim kit 270-99170-1. Crane offers a pushrod guideplate and rocker arm stud conversion kit 270-36655-16 for street applications, enabling the 1977-00 302ci and 351W engines with pedestal mounted rockers to have adjustable rocker arms without cylinder head removal or machining.