Backfire and shaking--timing related?

P

peeeot

Well-known member
I read the sticky articles about the SCV and load-o-matic distributors and thought them both helpful and interesting. One particular point caught my eye. The Ford article shows an extra passage in truck carburetors with SCV that is supposed to allow manifold vacuum to the distrutor at all times in order to prevent backfire. My Mustang always backfires when coasting down in gear with the clutch engaged, and frequently will hiccup as the revs fall off during the 1-2 shift. Is this normal?

I have also noticed that at road speeds of about 45 mph and above a deep pulsing vibration shows up. If I put my hand on the shift lever its shaking matches the vibration perfectly. If I disengage the clutch, the vibration disappears immediately. It only happens in third gear at speeds of 45 mph and up, around 2k + rpm, with my foot on the gas and the clutch engaged. The engine is smooth in that rpm range in other gears.

Could these two symptoms be related to the same problem, namely an incorrect spark advance or advance curve?

Initial advance is set to about 8 degrees BTDC, and I have yet to hear any spark knock. 1100 carb, SCV, load-o-matic.

Thank you!
 
I think you have 2 unrelated issues.

#1, if you are truly backfiring with engine jaking (slowing down in gear) then you have a timing issue. Check to make sure you don't have a vacuum leak and make sure your dizzy is advancing correctly by reving it up to 2000 RPM and checking your timing (you'll need a dial back light if you don't have one). Some sputtering and spitting is normal if you are letting the engine slow you down. It's funny, some people (me included) actually like that sound. That's why I would make sure it's back fire (which you would hear from the front of the car) versus just jaking sputter (which is from the rear).

#2, I think you may have a tire out of balance on the rear end. They can sometimes only show up at certain road speeds, which is why you are only experiencing it in 3rd gear and a certain RPM.
 
Thanks for the input.

#1 The noise I'm hearing is coming out of the tailpipe. I know some sputtering is normal, but frequently this sounds like a pronounced CRACK. The muffler is probably worn out because it's pretty loud, so could that be why it sounds like a backfire? Either way I'm going to check the timing. I figure it makes sense that it would backfire. If the engine is turning 1800 rpm but the throttle plate is shut to idle position, then for one thing the timing's going to be way late at the mere initial advance of 8 degrees and for another the vacuum is going to be pulling a lot harder than the idle circuit of the carb is designed for.

#2 If I had an out of balance tire, wouldn't the vibration continue after I had disengaged the engine, allowing it to coast?
 
"back firing" out the tailpipe is usually referred to as "after-fire". I usually do not consider this to be much of a timing issue unless the exhaust is running very hot and you also have misfires, dumping fuel into the exhaust for ignition.
True back-firing occurs out the carburetor and often is timing related.
Doug
 
Since when does true backfiring only happen out the carb?

I've never heard such a statement, and often refer to popping out the tailpipe as backfiring.

Regardless, if it is popping out the tailpipe it is because fuel is combusting in the exhaust.

One likely cause of this is too rich of idle mixtue. When decelerating in gear, you have major vacuum signal @ the carb. This will draw a lot of fuel through the idle circuit and can/will backfire in the exhaust.

My ranchero used to go off like a 12ga until I figured this out ;)


Back off your idle mixture and see if it gets better.
 
#2, Missed that part about the engagement of the engine. Maybe a worn gear/syncro, or just a weird harmonic with the drivetrain. Could always check the driveshaft. If you have a few extra bucks, may be worth having it rebalanced.

#1, yeah, a worn muffler may make it sound worse. Remember, when you are at 1800 RPM, even if you don't have yout foot on teh gas, your engine is still creating some vacuum. Unless you have your vacuum port plugged on the dizzy, then it will advance the carb some with the vacuum. It's caused usually because the vacuum is pulling in fuel from the carb at the 1800 RPM required rate but the carb is only allowing the idle amount of air in, so you are running rich. The result is fuel is ending up in your hot muffler and burning off with loud pops.
 
Bort62":3mhm7pvs said:
Since when does true backfiring only happen out the carb?

I've never heard such a statement, and often refer to popping out the tailpipe as backfiring.
Click to expand...

Backfiring is only through the carb, else it wouldn't be 'back' firing, which is ignition happening in the wrong direction, ie: through the carb and not through the exhaust...the popping coming from the exhaust is refered to as afterfire. Plain and simple...it's the correct engine terminology.
 
I've never heard of afterfire. I've heard of "backfiring" and "backfiring through the carb." I'll be sure to use the right terms from here on out though!

I have the idle mixture set as lean as I can get it while still maintaining good idle quality. I advanced the inital timing to about 11 degrees BTDC today and test drove it; I couldn't get the car to spark knock (though I didn't try flooring it in third, because that never happens). I think the afterfire was lesser than usual, though that might have been due to other carb tweaks I did to adjust idle speed and mixture with the new initial advance. It sure did rev nicely up to around 3700 rpm though! I've never driven it like that before.

I'm much more interested in the shaking/vibration now. I have some new observations because I took the car on the highway today, which I usually avoid doing. First off, it first shows up closer to 43 mph in a subtle form, but gets more intense the faster I go. It's plainly noticeable at 50 and reaches a whole new level by 60. It doesn't feel like the car itself is really shaking, but the shift knob definitely is, and there is a resonance throughout the car's structure. It is heard more than felt, I think. The strength of the effect diminishes substantially when I put the clutch in, but it doesn't go away altogether as I thought before. I had some moments where it was noticeably present, but still reduced. It still occurs exclusively in 3rd, so it can't be purely engine rpm-related. Mashing harder on the gas at highway speed did affect the sound, but I couldn't really tell whether it got worse or not, just a little different in character. Also, sometimes under otherwise identical conditions, it happens stronger or weaker than others, so there's a randomness element to it.

Also, there was one time where I was coasting and the sound was unusually noticeable for coasting. When I applied the brakes at that time, they were rough, with shaking coming through the pedal, and I think at one point (may have been an altogether different instance) I heard a sound that was almost certainly a brake shoe popping into place against the backnig plate, as if it had somehow gotten misaligned beforehand.

Could brakes be playing a part in this?

I don't recall it ever happening with the old engine.
 
I actually regreased them just today, before my stint on the highway. The boots were crispy; they crackled as the grease pushed through. Sound like a probable culprit?
 
So today I got under the car and disconnected the rear of the driveshaft, rotated it 180 degrees, then reattached it. I believe that has fixed my shaking problems. I feel like there's still a remnant of the effect, but it's dramatically reduced on the highway; before I exited early in order to avoid those speeds but today I just kept on cruising.

I suppose the balance of the driveshaft ad the balance of the differential yoke were being additive rather than cancelling one another out. It's possible that the driveshaft could still use some balancing but I think things are good enough for me (for now at least).

I also checked the drivetrain for slop while I was under there. The front u-joint had no play at all that I could detect; the rear one had a very tiny amount of play. Also, when I rotated the driveshaft by hand, trans in neutral, car on ground, I heard a clanking sound at the limit of rotation in either direction. It sounded like it was coming from the drum, though maybe it was just resonating there. Does that sound normal? Seems like it would have to be play either in the pinion and ring gear or in the axle splines.
 
:D Also,check that there is no crud buildup on the driveshaft and/or yokes.Dried on mud or clay will wreck the balance in a heart beat.
Ask me how I know. :oops: :oops: :oops:
Leo
 
You might also check for slop on the front yoke of the driveshaft/output shaft bushing of the transmission. The bushing on the output shaft can be worn and cause the problems you are having. Depending on the transmission, you may be able to separate the extension housing from the transmission, leaving the tranny in the car. Replace the busing and seal and replace the yoke if it is worn in the seal area.
Doug
 
One more outside thing to check...one I learned the hard way.

Make sure your transmission mount is securely bolted to the crossmember. I once forgot this. Never was an issue until I would put the transmission into 5th...and the shifter would rattle like crazy, car would rattle...basically everything rattled.

I'm not sure why I thought of this...but for some reason it came to mind. This one drove me up the wall until one day when I was having my mufflers swapped the guys told me I was missing those bolts.

Slade
 
I haven't really inspected the driveshaft for gunk buildup. Next time I get under there, I shall.

My brother and I rebuilt that transmission a while ago. I can't remember whether we replaced that bushing or not. I'll inspect that for jiggles too, and the crossmember bolts for tightness. I thought about those being loose but I guess I didn't really check.

I did more driving today and things were still much better at highway speeds, but occasionally, particularly around 50 mph, I got the same effect. Before, the faster I went the more intense it got; now it's intermittent and usually isn't there to speak of. You know how it feels when the windows are open and at just the right speed the air buffets a funny way and makes a kind of helicopter-like sound? This effect is very similar to that but it isn't that. Ugh, I don't know how to describe it! I wish it was more consistent; then it would be easier to diagnose.

I'm tempted to undo the driveshaft and rotate it back 180 degrees again to where it was before and see if things return to being exactly how they were. Then I'd at least know I'm on the right track.
 
okay. Well I think I can handle the $16 investment for a pair of new ujoints.

I'll make that the next item on my part/repair list, then post back with the results.

Thanks everyone!
 
I agree with Bort. The fact you rotated the shaft and it got better makes it sound that way. It's rare that a driveshaft suddenly goes out of balance, but it can happen. I would still check your tranmission mount bolts. Make sure everything under the tunnel is tightened down. Reinspect the spring plates in the rear, etc.
 
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