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backfireing - HELP

R

Rob Z

Member
The car is a 69 with 302. Engine is internaly stock. Headers, C4, 4brl. original dist converted to Ignitor pointless.

About 2 months ago was having flooding issues with original 2brl. Changed to Holley 4brl (600, vac second, elec choke) and a dual plane intake. The car ran GREAT except for developing an oil leak. This was eventualy traced to a blown rear seal. During the oil leak investigation I repulled the intake in the event the gaskets were leaking there. While I had the distrib out I cleaned it with break cleaner, being careful to only clean the outside(hopefuly).

The next day going into work the thing began backfireing and could just keep 50mph. No power and backfireing like crazy. I checked the dist when I got it home and it looked fine. The vacume and mechanical advance were working.

I pulled the engine to replace the rear main and discovered the timing gear on the cam was SHOT. Guessed it had jumped. Reassembled the engine and it started and idled great. Took it around the block, very easy to insure all was well. Then tried a litle harder accelleration from a stop and BACKFIRE. So I checked the dist, timing, wires, etc. Can not see anything out of place.

The dist is original, but converted to igniter pointless. New cap and rotor, new wires, new plugs. Idles great. Prior to the backfireing it ran very strong and getting 17+mpg.

I'm stuck. It sounds like an ignition issue. I made sure the timming marks on new timming chain set were correct. Top one at 6 and bottom at 12.
 
Was the 4 bbl Holley new? Older (several years) ones had issues with a valve being blown out with a backfire. New ones are ok.

So, the car developed the backfire after the intake and dist. was pulled, You could have a vac. leak along the intake gasket. You could be off the timing a bit. I would check that first.
 
For an engine to eat a distributor gear.... something is going on. So it was backfiring "like crazy" and you found the dizzy gear wiped out. You rebuilt the engine.... and now it's backfiring again?

I hate to suggest this, but are you sure you're using the correct distributor? What do the teeth of this second distributor look like?
 
Dave, I read it as eating the timing gear on the cam....not the dist. gear. I see no rebuild either.....
 
AzPete - The Holley is a 4160 rebuilt by National Carb in Fl. The intake was pulled and reinstalled during this last work. So hopefully I did not screwup install twice. But I did use a different spacer. I'll put the original back in and see if that resolves the issue.

dave - AzPete is correct. It was the cam gear which was toast. It had worn away and showed slipping. THe dist is the sameone that has been on the car. The rebuild consisted of rear main seal and timming set. All new gaskets. The intake was pulled and reinstalled during this last work. So hopefully I did not screwup install twice.
 
I once did an intake install three times so don't write it off as being good until you double check everything.
 
I just put the original 1" spacer back under the carb. - No Change

I checked the timming = 8deg BTDC with vacume disconnected.

Full advance looks to be about 50deg BTDC. Vacume and Mechanical. Do not have a tach but appears to be full on around 1500 - 2000rpm

Still farting.
 
Take some carb cleaner and spay along the intake gaskets and around the base of the carb. Any increase in RPM or stalling out indicates a leak.
 
Great Idea :ecit Dang. out of carb cleaner. Will swing by and get some on way home Thurs and let you know. Will also pickup an extra set of Carb gaskets, just in case.
 
Ooops. Distributor gear or Camshaft distributor gear.... what caused it to fail? When you pulled the engine back apart did you inspect your oil pump? I'm not focusing on the back-firing issue, but more concerned about the cam/distributor gear failing.
 
Thanks to everyone for you help. Here is the rundown so far

Sprayed carb with cleaner - Nothing
Sprayed carb with quick start for better indication - Nothing
Checked PCV - Good
Checked for cross fire using inductive tach - Nothing
Checked for loose dist connections - All OK
Dismantled dist to check for sticking mechanical advance - OK
Used tach to check for mis-fire, ran to 2k RPM watched for change - Found 5 plugs were mis-fireing. RPM droping with throtel constent.
Pulled plugs - No visable issue, but found 3 set to .045 :( - Replaced each plug

Now runs much better. Only backfired twice when hard throttel at 65MPH. Still something there.

I'm using Autolite 45 plugs. Are these OK or are ther better?

Thanks, I'm getting closer
 
The Dist gear was fine, as was the oil pump and shaft.

Here is a pic of the cam timing chain :sad

4931077018_f137d0e5a8.jpg
 
I would put in a set of points and condenser. I bet the electronic unit is having a breakdown. New plugs are easier to fire for w weak system. That or replace the electronic unit.
 
"Rob Z" said:
Thanks to everyone for you help. Here is the rundown so far

Sprayed carb with cleaner - Nothing
Sprayed carb with quick start for better indication - Nothing
Checked PCV - Good
Checked for cross fire using inductive tach - Nothing
Checked for loose dist connections - All OK
Dismantled dist to check for sticking mechanical advance - OK
Used tach to check for mis-fire, ran to 2k RPM watched for change - Found 5 plugs were mis-fireing. RPM droping with throtel constent.
Pulled plugs - No visable issue, but found 3 set to .045 :( - Replaced each plug

Now runs much better. Only backfired twice when hard throttel at 65MPH. Still something there.

I'm using Autolite 45 plugs. Are these OK or are ther better?

Thanks, I'm getting closer
Click to expand...


You sprayed carb cleaner around the base of the carb at the carb to intake seal, but did you spray all of the way around the intake to head/block seal?
 
Well another day another backfire :(

Sprayed Carb cleanre and Starter fluid anywhere I could reach that might have a vacume leak. Found the PCV connection into the carb was loose. Tightened and no change to running.

Replaced Pertronix back to points and condensor. No change in running.

Found the fuel line was sucking air at the out of the fuel pump. Replaced fuel line. No change to running.

Did compression check. one at 145 all otheres at 150-155psi.

So to me there could only be one of two things. PLEAST tell me ther is something else.
1. Mis adjusted valves. THis seems a bit of a strech, but never know. I had adjusted them befor the engine was put back in. Placing to TDC on compression, thghten rocker till no play on pushrod then1/4 turn.

2. Timming chain not on correctly. At most it could be out one tooth on the cam gear. I'm not sure how much of an issue this would be, but I'll put it out there.

Any more thoughts???
 
STOP the PRESS'

Looks like I located the problem. Just remember KISS - Keep It Simple Stupid - Well the Stupid part comes into play here :craz

When I went to the Holley 4brl, it had an elect choke. I grabed 12v from the feed to my pertronix. Sometime between then and now, not exactly sure where in the process, I moved the elect choke feed to the coil. THought was it was feed from a resistor wire and the lower voltage wold allow the choke to remain on a little longer this winter. Well I did not think about the coil and choke being in searies with the wire. THis would increase the load resulting in less voltage to the coil, which could cause weaker and/or missing spark. I just removed the choke and am reading 10.4v on the coil. Also noticed the coil is a 12v coil so I changed the feed to a solid 12v ignition feed. The car purrs. I went on a test drive and try as I might could not get it to fart out the exhaust.

One of those small changes that NEVER entered into my mind that I had done or that it would make any difference. THe fact that several issues appeared at the same time, Oil Leak, bad timming gears and all along it was something I did :craz

I want to THANK EVERYONE for their aid and ideas in isolating this problem. Y'all are the best.
 
Sounds great. Aren't these old cars just great. Just think how many things you learned here.......
 
Glad to hear you figured it out.

I'm sure I've said it here as I tell the same thing to EVERYONE I speak with when there is a problem being diagnosed. It bears repeating.

If a machine of any sort is working prior to someone "fixing it", or "tuning it up" or even just "cleaning it" and afterword it is not working properly, go back and figure out what that someone did to it. This sounds so simple but hardly anyone ever uses this logic. When you're the repair guy they call it's great but as the owner/user of the machine not so much.

I would get calls all the time to fix a broken machine and upon inspection I would find things comparable to someone having installed a couple plug wires backward at the distributor. I'd always start a repair job by questioning the people around the machine as to if anyone had "touched" it. After I'd find the problem (like the switched plug wires) someone would eventually say something to the effect, "oh yeah, we did have to unplug a few of those when we were...".

So from a seasoned technician listen to a few basic commandments of diagnosis and repair;

1. It is very often something somebody just did to it causing/having caused the problem. (This is especially relevant to automotive tinkering/upgrades)
2. Make one adjustment at a time. (Sure sometimes it takes more than one different adjustment to make it right but doing multiples makes affect harder to evaluate. Being able to independently assess the affect of each adjustment makes it easier to determine the full course of action.)
3. When an adjustment made does not give the desired affect...put it back where it was before trying the next. (This is why people get "lost" and usually end up making things much worse. Mark or somehow otherwise give yourself a way to know exactly how something was prior to touching it.)

The other thing that this thread reminded me of is the use of the correct part names. By calling the timing sprocket (sprockets have chains around them, gears mesh with other gears), Dave (who happens to be one of the very best here at diagnosing things from afar...except for Craig's little ruse :)) was completely thrown off track thinking the problem was a damaged distributor drive gear. That took Dave and his advice out of the help giving pool.

Believe me, everyone sometimes forgets these basic things when a problem arises. If you take a second and think about the problem before you dive in to fix it, you have a much better chance of a faster positive outcome.
 
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