Hot Rods Does this look like an exhaust leak that could cause a backfire?

Definitely hurt the ceramic coating. Such is life.

 

Still working on tracking down this backfire. Smoked the intake and exhaust today with a smoke machine. Smoke into the exhaust yielded zero leaks. When we hooked it to the manifold vacuum port on the center carb I got smoke from the center air cleaner area which makes obvious sense. But also smoke from the vent holes at the rear of the rear carb. The other 2 carbs have these holes filled in. There was no noticeable smoke from the rear air cleaner...I wish I had thought to pull that air cleaner off during the test to see if it was escaping past the throttle plates....I would think it would have to be because those vent holes are above them

 

Every now an again you read 4 pages and think to yourself, what hasn't been covered? The only thing I can think to offer is, are you sure the distributor isn't a cog off? I'm thinking you have a leaking exhaust and that's the rabbit trail your following. But it's the wrong trail.

3. BAD IGNITION TIMING
Once both sets of valves are closed, that's when the spark will fire inside the cylinder -- or at least that's when the spark is supposed to fire. However, if the spark timing isn't just right, the spark could fire too early before the intake valves are closed, or too late when the exhaust valves have already opened. When this happens, the air/fuel mixture in the exhaust or the intake can combust, leading to a backfire.

Now you tried the carb adjusting from what I remember. And it's not indictive of burnt or bent valves because all the testing you did where it didn't show a problem or did it? Cylinders #1 and 7?
Besides... It's a crate engine. It hasn't been run enough to do that damage from what you've told us. I've seen small blocks run that were a tooth out and the distributer twisted enough to sound good in the shop, they'd back fire. Did you mention if it was a points or HEI? Other then that, I'm all out of answers.

 

If the distributor was a tooth off the timing wouldn't be right. As long as the initial is set right it shouldn't matter right?

Leakdown and Compression tests were good so no valve issues.

Ignition is HEI

 

After reading through the pages of this post and each of the approaches that were suggested then subsequently tried. The obvious (at least to me) thing that has not really been spoken about is pulling the manifold off and replacing it with a conventional 4 barrel carb. Yes the end carbs were capped off
(as I remember) but multiple carb intakes are notorious for causing headaches. Sometimes the gorilla in the room really is a gorilla

What would it cost? A set of gaskets and some time. If you don’t have the intake and carbs on hand it can’t be that difficult to tap a friend or buddy to borrow one. It’s not like this is a Jaguar or a straight 8…

 

If the distributor was a tooth out, you could still time it. It would retard or advance from the out a tooth position to set initial timing. After that it will matter.
I'm not saying that is the problem...just that it warrants being sure. I don't even care what your timing is set at. Just that if you are at top dead center on the timing mark, where does the rotor point in relation to a terminal position?

Without going back and re reading your testing procedures...

This might be your best solution?

But if it was my headache to deal with, start at the beginning. Rotate to top dead center, check your timing mark, scratch a mark from the #1 terminal of the dist. cap to the body, remove the cap and check where the rotor is pointed. And if that isn't to the #1 pin... there's a problem.

At the same time, how's the cap/and rotor. Did you service the distributor or just plunk it in? Call it a dumb question but are those HEI wires? Any cross firing from the wires? Must be frustrating, it's been over a month now.

 

Exhaust backfires can be caused by electrical issues. Have you looked for pinched or electric wires on exhaust or choke crossover. Try tugging on your wiring with engine idling see if there are any changes. Look at all your wires under the dist. Cap for chafing. Shorted wires going through the firewall and to ign. Switch.

 

Are the #5 and #7 spark plug wires separated in such a way that they can't crossfire?
Can't remember if this is the thread where we talked about that.

 

All ignition components are brand new. I dropped the dizzy in with the rotor pointing at #1 and then adjusted my base timing from there. Plug wires have separators most of the way to the plugs and being new I don't think they're crossfiring. One thing I have noticed is the cap to distributor clearance allows for some movement of the cap. I don't think it's enough to effect anything and an internal inspection after about 300 miles didn't show anything weird inside the cap.

I'd hate to pull the intake again but may consider trying to borrow a 4bbl setup this winter.

One other odd thing I noted weeks ago is that with the engine idling I could pull each of the plug wires on the driver side individually and not hear a change in the engine. I didn't try the p***enger side. Attributed it to the lopey idle making it hard to hear a miss then.

 

Well... Sounds like hearing loss to me but hey, all good, and most of us have some.

You pulled 4, heard nothing and stopped there. Follow thru is the key. But why were you pulling plugs? Must have hoped to discover something. For the record, we've all done something hoping to learn from it. Ok...I have done some things. But this isn't about me.

What's the story on the dizzy? It brand new? Name brand or off shore? Good used and by who's accounting?

I'm going to leave you with a suggestion to recheck the simple. And while we're at it, what kind of plug wires are you running? I haven't a clue if that matters but it might. Maybe impress us with a few more pictures as well.

Another post revealed an improperly installed condenser wire because someone caught a small detail from a picture. Also, you know what they say about solving crimes, well yours is seemingly becoming a cold case. We need some fresh lead to follow.

 

Distributor is a brand new Pertronix unit with their cap, rotor, and wires. Plugs are Delco set to .040.

I've about hit the point where I'm going to leave it until I hopefully run a full exhaust this winter. Like I said above smoke test this week showed no exhaust leaks. Only my tear carb doesn't have the bowl vents closed up....had smoke from there when I smoked the intake but not really the air cleaner on that side which makes no sense. I'm going to just put a little dumdum over those holes tomorrow before my local cruise night but I expect no change.

The reason I only pulled 4 plug wires with it running is that my backfire is only on the driver side so that was my focus.

 

From left to right, the wires are 5,1,3,7.
This has been the suggested wire routing since the 1950's.
As I suggested weeks ago, I agree that a proper exhaust system should muffle this problem.

 

I'll have to check this. Pretty sure I have the 5 and 7 wires routed next to each other in the wire looms.

 

Pertronix Ignition "Problems & Failures" (It's Actually YOU) - YouTube

Troubleshooting for Pertronix Ignitor and Coil Installation

I'm getting schooled up on Pertronix. Did you mention the model by chance? Seems searching the H.A.M.B. yields some results about the various systems.
pertronix igniter 3 | The H.A.M.B. (jalopyjournal.com)
Technical - Pertronix Ignition Problem Has Me Stumped | The H.A.M.B. (jalopyjournal.com)
Technical - Pertronix III problems solved | The H.A.M.B. (jalopyjournal.com)

 

It's their Bundle 001....billet distributor with Ignitor II and the Flame Thrower 2 coil. I'll have to read through those links thanks

 

If you have one of those old hand held tachs hooked up, you should see a change when you pull a plug wire.

 

Is that a plug wire above the dipstick? it looks like it`s smashed between a boss on the block and spark plug? crossfire there?...

 

No sir.....all my plug wires are under the header

 

I'm starting to wonder more and more about the distributor. The truck starts and idles well and has good low end power. As soon as I open it up more though into the end carbs I definitely don't get the kick in the pants power I would expect in such a light vehicle. Maybe the distributor isn't keeping up with it?

 

How quickly (what rpm) does full mechanical advance occur?

 

10* mechanical at 1500 and all in (20*) by 2500. GM calls for total of 32* and my base is 12* confirmed with a piston stop.

 

How Ignition Timing Works: Vacuum and Mechanical Advance Explained! - YouTube

4 pages... still back firing. I don't know man...? It's not looking good is it.

I reviewed the (3) pictures of your motor. You could have stepped back a bit and shown more.
Did you ever confirm where things were for top dead center and where the rotor points? Proof would be a picture or two.
But I'm going to ask, is that a vacuum advance distributor? And the piston stop, can you enlighten me on that. Thanks.

 

I'll snap a few pics later this morning but it's a very simple setup. The distributor does have vac advance. GM calls for mechanical only and I started with it as such but mileage was terrible so now I have ported vacuum to the distributor and picked up 2 or 3 mpg cruising.

Rotor was pointing to #1 when I dropped it in at TDC....I'll get a pic of that later. As for piston stop.....that's the tool you use to confirm that the 0 mark on your balancer is correct. Install it into a plug hole and rotate the engine by hand clockwise and counterclockwise marking where it stops both times.....center between the marks should be TDC used for timing purposes.

 

Then the horse is dead, beating it further will not make it run any faster.



Seriously though...us sitting in our arm chairs can only guess, and maybe wonder a bit on what you got and what your doing with it. I'm no mechanic, don't claim to be either. But I manage to keep my stuff running. Something is off with your numbers, and the picture being painted, at least from my seating position and limited knowledge.
With a grain of salt, due respect, keep it simple stupid.
As I see it, you go back to the beginning and start rechecking, retesting and confirming.

Regarding the piston stop. I get ya and got one. I was confirming in the interest of clarity that was what you meant. Also what you might have done to get where you are now. The process if you will.
Mines made from an old spark plug. That's what happens when you read car magazines. You get smarter or think so anyways. Old age and experience confirms and denies it.

You have any idea how many would have taken that crate motor and not complicated it up by thinking they needed to use a piston stop? Me for sure.

You said you didn't disturb the cam, so... here's where I'm getting confused, where did things go wrong that we are page 4 and your not fixed and running. Because you should be.

By the way, who am I to second guess your dropping in of the distributor to not be spot on?
I'm no one. But... if I was looking for conformation it was, from what you've mentioned to date, I couldn't do that if I was an expert. I'd say sketchy close maybe. For the record, if you asked me for my opinion...I think the distributor position is off. Your convinced it correct so not much more for me to offer. But good luck.

That said, if it wasn't the process then that means it's the unit. As in one unit for the distributor, or units, as in multi carbs.

 

HELP me find Top Dead Center PLEASE! - Third Generation F-Body Message Boards

 

I agree this feels like beating a dead horse haha. For sake of my sanity let me ask this regarding distributor installation. Lining the rotor up with the #1 cylinder on the cap is only a general installation technique to get you in the ballpark right? You rotate the distributor body after that to actually adjust to the correct base timing. I've always ***umed the idea of being a tooth off meant you just didn't have enough room to rotate the distributor housing enough to get your timing right then without moving it back said tooth. Spark firing at the correct initial advance as per a light is the same regardless of tooth position isn't it?

I didn't get to touch the truck today and get pics.....worked on a customer vehicle in my shop all morning and now have a 12 hour night shift ahead of me.....gotta love those 24 hour days of work

 

You are correct as long as you can rotate the dist. it does not matter . You can put number one any where as long as you wire the cap accordingly.

 

Actually managed to run out and snap a handful more engine pics before I left for work....tomorrow I will take more with the cap off showing rotor position.

Also I've seen other posts about guys having issues with Pertronix modules if their tac pulls power from the coil or something like that. I'll have to read more on that.

 

So you have number one around on the drivers side ? and your sure the firing order is correct ? you can take a temp gun and check each header pipe and make sure all the cylinders are working like they should, or you can just use some spit on your finger and touch each tube . Seems like you said it was just the left bank back firing.

 

Yes 1 is on the driver side towards the front center on the cap. Firing order on cap has been confirmed several times now.

Heat gun shows cylinders 1 and 7 are the 2 coolest on the engine....on my backfire side no less. But they're not dead. About 70*-90* cooler at idle than the rest.