If it ain't broke - backfiring issue

Q-Roadster · Post by **Q-Roadster** » Wed May 17, 2006 10:17 pm

So against better judgement, decided to start replacing things on my perfectly fine running Roadster.

Basic tuneup, replaced cap, rotor, plugs, wires, points etc. Got to the points and ran into problems. Prior to the tuneup, had very minor backpressure when going down a mild slope @3500rpm 40-60mph, figured a tuneup would fix it. Couldnt get the replacement points to work so went back to original set, car fired up ok, but now I've got what sounds like backfiring from the manifold/carbs and loss of power after she's warmed up. Didnt mess with the carbs before or after, is this a matter of adjusting the points till the backpressure stops, or is there something else I should be looking at?

Thanks in advance.

Q

datsunrides · Post by **datsunrides** » Wed May 17, 2006 10:31 pm

Is the firing order correct? You may have crossed some wires. When you did the points, did the ground wire get hooked up? Don't know if this would cause a problem, but it's something to check. Another thing to look into is the possibility the cam is going "flat" on an exhaust lobe. Easy enough to check by pulling the valve cover and seeing if all the rockers move the same approximate amount. Not very likely this would be the cause, but timing chains are not know to break either.

Mark

Post by **dbrick** » Wed May 17, 2006 10:43 pm

Did you remove the dist from the engine? I'm with Mark on the firing order, followed by timing and point gap. Silly easy thing, do you have a picture you took of your motor? Look at the plug wires in the photo, see if they match. Info here got me through it.http://www.311s.org/tech/ignition/dizzyfaq.html

spyder · Post by **spyder** » Thu May 18, 2006 1:56 am

What does "had very minor backpressure when going down a mild slope @3500rpm 40-60mph" mean? Are you describing a popping noise in the exhaust. A rich running vehicle with a slight exhaust leak, which introduces oxygen into hot unburned fuel vapors, will do this. Newer computer controlled cars turn off the fuel in this situation, carburated cars don't. Mine is very pronounced but it doesn't annoy me.

project_timemachine · Thu May 18, 2006 10:04 am

Spyder, my roadster also has that popping sound in the exhaust when it's in idle...is this something to worry about? I've had quite a number of tune-ups already and it seems that I can't get rid of it. I've already replaced the points, condenser, spark plugs, ignition coil, cleaned out the fuel filter and gas tank. I also use premium gasoline and recently had the valve clearance adjusted. Is this a carb problem??

spyder · Post by **spyder** » Thu May 18, 2006 11:18 am

"Sound in the exhaust when it's in idle" ... At idle the engine had minimum aitflow into the intake. It stands to reason this is the condition the carbs would have the hardest time accurately metring fuel. Throw into the mix 30+ years of wear. My thoughts are if it gets good mileage, does not overheat, has reasonable power, no smoke or ping/knocking don't worry!

Post by **dbrick** » Thu May 18, 2006 5:08 pm

A burned exhaust valve will cause a miss at idle and the popping also. Worn carb shafts will cause a vacuum leak, similar simptoms and an idle that won't settle down.

As Spyder said, if it's minor and just at idle, don't worry too much

From my Fathers diagnostic procedures, circa 1940:

Simple check, with the car idling, hold a business card or crisp dollar bill (use a $100 for 67.5 2000's) about 1/2 inch away from the tailpipe and vertically, so ir would cover the opening. Move it closer or farther till it starts to flutter If the exhaust pulses just "rattle "it and it waves back and forth a bit, all is good. If it gets sucked back onto the tail pipe whenever the engine misses or pops, usually means a dead miss or a burned exhaust valve.

Dad was usually right

project_timemachine · Post by **project_timemachine** » Thu May 18, 2006 6:40 pm

My thoughts are if it gets good mileage, does not overheat, has reasonable power, no smoke or ping/knocking don't worry!

Spyder, yup so far the engine has no smoke/pinging, doesn't overheat, etc...I thought the popping sound was something serious. Now I can sleep better at night.

Dave, thanks for posting your father's diagnostic procedures...that sure is a simple way of checking for dead miss. I'll check it out later.

Thanks for the help guys!

Q-Roadster · Post by **Q-Roadster** » Thu May 18, 2006 9:59 pm

Thanks guys, I think I've narrowed it down, the minor popping I experienced prior to the tuneup was probably just that, minor, and maybe a slightly rich running car. I just need to fiddle with the points a bit more to get it to run smoother. I didnt pull the dizzy, and the firing order is correct (learned that lesson earlier). What I'm experiencing now is at running temperature, the backfiring is at the carbs/manifold (usually when coasting/decelerating), probably too rich.

Thanks again for the info.

Q

Post by **dbrick** » Thu May 18, 2006 10:18 pm

Double check the ignition timing when you get a chance, moving the points around can alter it.

spyder · Post by **spyder** » Fri May 19, 2006 1:55 am

To add a little more to this thread, I had a head which was cut so far the lower head studs on the intake were almost touching the block. It had insane compression and performance but I overreved it and dinked an exhaust valve. It idled a bit funky and occasionally would pop on a throttle hammer. I did a compression test and decided to replace the head with one which I had spent a bunch of $ rebuilding. It idles fine but does not have the same git up and go.

Moral of the story? Old cars are temperamental, at least mine is!