My 170 is backfiring out the carb, only on cylinder 3. I had the valves done. It only does it under acceleration. It idles very well and the compression is reading 120-125 on all cylinders including No. 3.
When I pulled the spark plug wire off 3, the backfiring stopped.

When I checked vacuum. It was 17 at idle, and then fluctuated down when it was backfiring.

I pulled the head off again. All of the new plugs look good, except 3. That plug has lotsa carbon build up on the plug (but it was firing).

I have the head back at the machine shop for new springs in 3. It doesn't appear to be sticky.

Any ideas?

Check that you're not adjusting the wrong valve. From the front of the engine E I E I E I I E I E I E. The intakes get lashed hot at .010 of an inch and the Exaust get lashed at .020 of an inch.

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63 Plymouth Valiant Wagon

I've already adjusted and re-adjusted the valves. I set them at .12 and .22 to make sure they werent too tight. Why would I get equal compression all across. . . ..

Check firing order and make sure the pushrods are not bent.

Good luck!

Greg

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http://www.youtube.com/hyperpack

done that. Like I said, it idles perfectly and only backfires when accelerating. thanks.

Accelerating while driving (load applied) or accelerating in Park / Neutral?

I am not sure if that matters a lot, but It could help us figure it out.

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It backfires on 3 either in neutral or when accelerating under load. It seems to do it a little more when cold (but hard to tell when driving). The valve does not appear to be burned and the seat looks ok (we have it out know). The machinist is checking for sticking and putting in a new spring. Pushrods are straight. I dont thin k it is bad rings because compression is full in 3.

thanks

Did you change the plug in #3 cylinder and run it then? Excessive carbon build-up on a spark plug can cause backfiring (dieseling).

Good luck,
Joe

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Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"

Yup, I replace all 6 plugs. Only no. 3 is fouling. Thanks.

Best to check or replace (with known good) distributor cap and rotor, #3 plug wire (can swap from another cyl for test). Maybe distributor shaft is bent or has other defect so try another of those? Not necessarily head/valve issue...

Lou

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Home of Slant6-powered fun machines since 1988

thanks. I already replaced dist. cap, plugs, points, condenser, coil. I hope when I get head back from machine shop with new valve springs it will stop backfiring. The other 5 cylinders are firing well.

Really I think this will be traced to a sluggish № 3 intake valve. No spitback at idle/more spitback with increasing RPM because the faster the engine runs, the less time there is for the valve to close, so a sluggish one will make itself more and more known at higher and higher engine speed. A faulty spring is less likely than something more serious like a too-tight valve stem/guide clearance, a burr on the guide or valve stem, or a slightly bent valve stem.

It's not in the carburetion or ignition. If it were, you might see a misfire on № 3, but the fact that you're getting spitback through the carburetor means the intake valve is open when the spark goes off, letting the fire out of that cylinder and into the intake tract: POP! If this were happening during the time that intake valve is meant to be open, that would indicate a crossfire situation—something the matter with the secondary side of the ignition system, like a bad cap or rotor or a couple of plug wires crossed, or questionable plug wires running parallel to each other and one inducing voltage in another to cause a false spark. But if that were happening, you'd get misfire/spitback at any engine speed; it wouldn't idle right. So I think it's happening when the valve is not meant to be open, and so…

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Too many people who were born on third base actually believe they've hit a triple.

I think you are right. I just got the head back from machine shop. He put on new spring on NO. 3 intake. He said the 3 intake valve was not sticky. So, I should know the answer (I hope) tomorrow.

Thanks.

I found the problem. NOt what I guessed. When adjusting the valves after replacing the head- I found that the lifter on NO. 3 cylinder is not working- The exhaust valve only opens about 1/16 inch. Is there a way to fix the lifter without taking the engine apart?

The pushrod is not bent.

Help!

There are two reasons for this.
1) the lifter is hanging up in the lifter bore, and not dropping down to follow the cam.
2) wiped lobe on the cam not raising the lifter.
Number two is most likely. If that is the case, you will need a new cam and set of lifters.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225