Backfiring

[Muswagon]

New 408W, having intermittent issues. My issue sounds like the cam is toast, however, literally the last time I ran the car it was a monster

The crappy carb I bought, a procomp 830 DP, is very touchy and basically junk (replacing with 750 Quick Fuel this week). So keep that in mind.

First off, the ongoing issue I have, even when its running great, is backfiring in the exhaust. From what i have read, that is raw fuel igniting. It runs very rich, again, the carb is not great.

I don't know where the timing is currently set, my cousin built the engine and we had the timing set to just about perfect, so not sure that could be the problem.

The only change from last year is it underwent surgery with the tranny change from a FMX to a TKO 5 spd. The vacuum line that went to the modulator valve I had initially forgotten to plug, I threaded a bolt tightly into the hose as I didnt have a proper vacuum plug around, is it possible this is not sealing very well?

Got the clutch cable where I wanted it and took it for a test drive.

Starts up right away. No choke so had to go through the usual pedal pedal stab. Did not feel right at all, felt like it was fighting itself. Lots of banging in the mufflers.

Then suddenly felt like it did last year, idled, revved up seemingly ok.

Took it for a spin, moved right and sounded right but when I tried to stab it all would get a carb backfire.

Again, I know this sound like my cam self destructed but literally the last time I drove it, it was a champ.

408W, AFR 185 heads, comp cam with 284 int./296 exh duration, 0.541 int./0.544 exh lift. 10.2 ish compression, the aforementioned crappy 830 carb

cam and engine were broken in correctly with zddp break in, and now running oil with zddp in it

Any thoughts?

 

[Don C]

It sounds to me like an ignition problem. A fouled (or weak) spark plug, due to the too rich condition, could be the problem.

I would start by removing all of the plugs and inspect them, check the plug wires for proper resistance, pull the distributor cap and check it for cracks and dirt, check the points and condenser connections. It could also be a bad condenser. CHECK THE TIMING, don't assume that because it was right that it still is, the hold down bolt may be loose.

The modulator hose likely isn't the problem, if it is leaking it would just help to lean out your rich condition.

 

[Muswagon]

Thanks for the input!

 

[72HCODE]

like don said...

additional check the carb, put a flashlight over the carb with the engine running and see if fuel is leaking from somewhere it should not be, like dribbling down the ventries , or secondaries leaking fuel, accelerator shooter dribbling extra fuel or leaking from the transition slot.

if the carb is suspect or damaged it could also be the source of extra fuel going down the intake when it should not be.

---

one question would be did it ever run correctly in the past and if it did then what changed.

the new transmission could of also changed the load requirements on the engine.

additional to the condenser going bad (or if you have electronic ignition) a coil going bad can do the same thing.

so check over things like don suggested and go from there.

 

[coilwire]

Had the backfiring problem with me mach. Changed carbs, readjusted the timing. Still every time I went to leave off the line at the strip it would backfire and die. It started right up and would run good. Last race of the season I noticed the oil pressure very low. When I troe the engine down the cam was trashed and all the cam bearings were mushroomed out. Don't know if this helps.

 

[Muswagon]

No.....that didn't help lol! Praying its not the cam, but seeing how sudden it is I am going to go through the list of things to try that the guys listed.

 

[Muswagon]

Changed over to the new Quick Fuel 750. First turn after bowls were half full was a massive backfire thru carb. Tried again, it tried to start then fireball and after fire on carb so I gave up (after i put out fire). Cannot even check timing now that I cannot get it to run.

I should have said before that I am using a crappy pro comp hei dizzy, which is being replaced this week with a proform hei dizzy. Sounds like it almost has to be an ignition issue.

I caved to pressure from my buddy that the carb and dist would be fine when I was struggling to finance the engine build, I knew it was wrong to cut corners on such important items. When both are installed I will try again.


I also checked to see if the distributor was loose at all, its tight so don't think timing went out

 

[72HCODE]

a backfire like that would be the distributor 180 out or timing is well out of wack.

 

[Muswagon]

Thats the odd part, ran 100% awesome last time I drove it and dist isnt loose at all

 

[c9zx]

If nothing has been changed and it was running fine before, it sounds like ignition, or timing chain "jumped a tooth". Do you have any "friends" that have access to the car and like to play practical jokes? Swapping plug wires around used to be a common, yet cruel, practical joke.

Chuck

 

[turtle5353]

Put a timing light on it and see where the timing is. I have had the distributor not move, but have sheared the roll pin on the gear and had it backfire run shitty etc etc. Gear moving let the timing change enough to put timing out of whack.

 

[Jeff73Mach1]

Not to jump on the band wagon per se, but ignition problems would be the first thing I would look into.

I think the Chinese HEI's are fairly bottom of the barrel parts. I'd rather run a points distributor personally.

I once blew the fusible link in my car. In an effort to start it I used a screw driver across the starter relay (solenoid) to get it to start-but the coil wasn't getting any juice. Trying to just get the car across the street (To an auto parts store) I had such a massive backfire that I completely blew the driver side muffler end cap off.

While your carb may need tuning/tweaking/etc. I doubt it is a lost cause. If you are using a big cam, power valve selection can get really tricky as they tend to open during the lopes of the engine and make it run rich. Plug em and jet up-won't help your mileage, but will allow you to tune the engine to run more consistently.

 

[Muswagon]

My proform dist isnt in yet, so I think I am going to try my mallory I have.

here is the timeline -

Last Sept took for ride trying out new engine, ran great

This March, moved car to other side of garage to start tranny change, seemed to run great but didnt have it running for long and simply moved it over

Completed tranny change, runs like a sack, but still runs. Very easy to start, hard to keep running, aforementioned problems of backfiring exhaust on idle and through carb under load

Changed to my brand new Quick Fuel carb, massive backfire on initial fire up, fireball on second try and lingering fire. Scared crapless so stopped for the night

What is interesting....old carb (still pretty new procrap 830). When taking off the fuel lines, noticed the inlets were very loose, so thinking this could be part of my problem. This carb has a bizarre way of hooking up the lines. The size is very small for the fittings, does not fit either the large or small side that any 4150 style would use. Anyway, these screw in and then a hose is clamped on. I woudl assume that loose fittings would cause issues. They were tight in fall.

 

[Don C]

My first suspicion is also ignition. However, something else to consider is the possibility of a valve sticking, due to infrequently running the engine, and a long cold winter and a brief fire up a couple of months ago.

 

[Muswagon]

It sounds to me like an ignition problem. A fouled (or weak) spark plug, due to the too rich condition, could be the problem.

I would start by removing all of the plugs and inspect them, check the plug wires for proper resistance, pull the distributor cap and check it for cracks and dirt, check the points and condenser connections. It could also be a bad condenser. CHECK THE TIMING, don't assume that because it was right that it still is, the hold down bolt may be loose.

The modulator hose likely isn't the problem, if it is leaking it would just help to lean out your rich condition.

So with the new carb and new dizzy installed, took your advice and checked the plugs. 4 soaked with fuel, 4 black. Could not get 3 of the 4 soaked ones to fire so bought 8 new plugs and started fresh with clean slate. Tested the coil lead to make sure, and its 12V, just wanted to make sure I didnt have a weak signal going to new dist.

Started up right away, set timing to 17 static with vacuum advance taken off. It did not like 18-20.

Leaned out mix screws to about 1.25 turns out.

Scariest car I have ever driven.

Thanks again for all of the help along this learning journey of mine.

I did have some help with the dist and setting timing, however, for the most part I did the entire tranny swap on my own.

I can honestly say to the guys that advise taking on these projects to have fun and learn, you are absolutely right.

Next weekend my buddy is going to teach me how to read timing marks and use a timing light, super excited.