The Stovebolt Site Forums The Shop Area Engine Shop Backfire....causes...stumped!

HI all....took the 54 235 3100 out for a spring fling around town. Beautiful weather and the wife was loving the drive. The rebuilt Rod-B was great and the vacuume numbers were in the 18 19 area at idle. Flat driving 35 to 55 smooth as glass. BUT when we started a climb up a steep grade we got hesiatation, bucking and mother backfireing. I limped up the hill and down the other side flat driving once again and all was fine but as soon as I lifted my foot off the accelerator on a slight down hill slope I got those little backfire farts? Coil new and new spark plugs and timing about 5 btc..18-19 vacuume at 650 idle and engine running cool 160 to 170...I want to install my Langdon HEI system but I thought one thing at a time...what gives with this backfire stuff what is the cause.....? THANKS, Steve

You don't say what type distributor you have. If it is still the stock distributor with points, they could simply be out of adjustment or burnt.

It's also possible you still may be running lean on fuel under a load

Steve,

My guess is that a lot of things might cause the symptoms you describe.

Last year, I had very similar symptoms and figured the "backfiring" through the exhaust pipe was a result of unburnt gas in the exhaust system (perhaps assisted by air leaking into the exhaust system).

My problem was solved by resetting the level of a carburetor float - the float was rising too high and gas was dripping into the intake at all times.

Maybe others will have more helpful suggestions.

Good luck,
Tim

So that was you...........I thought someone was shooting at the easterbunny.
Marty

I think your initial timing is still not advanced enough, which causes the backfire and bucking when you get in the throttle when climbing and you lose vacuum. The initial backfire may have caused a leak in the exhaust system, usually the donut at the headpipe. This causes popping when deccellerating. It is also possible to have and intermittant electical problem that can cause backfiring out the exhaust. Did it backfire through the carb when it was bucking?

hard to do on a comp,,but with what you give for info go thru some steps #1 check dist points then adv,,2 you know the plugs and wires are good???? 3 sounds to me that the problem is in the power system in the carb as going up a hill when engine needs fuel it is getting leaned out and there is your backfiring,,,you should have somewhat the same problem when you go to full throttle anytime if it is in the power system....doc

It could be a number of things, but it does sound like your carb is running rich or the accelerator pump squirts too much fuel.
- Always best to rebuilt your own carb with a special kit, or check every part on a remanufactured one, before use.
It's hard to get a good remanufactured simple fuel pump these days, let alone a carburetor that was done right.

Definetly re-check the float level. - With modern fuel you want to decrease the recommended float adjustment by up to a 1/16" when setting it.

To lean out the mixture, you can try a smaller main jet in the Roch B. if everything else on the carb is good. The shop manual should list the correct size, in the specifications/carb section, if you know your carb is stock.

The leanest setting when timing is usually the best.
Keep advancing the timing in small steps until you hear a slight ping (screeching sound) when you accelerate on an incline in 3rd gear with wide open throat and stock transmission. Then retard back to no ping.

The popping sounds at deceleration comes from unburnt gas in the exhaust, from a too rich idle setting.
With just one carb and the stock exhaust system, you shouldn't get that sound.

I found the best idle setting to be the lowest idle speed that keeps the engine running without stalling after deceleration and shifting back to neutral.
That would be around or a little lower than 500rpm, just as the shop manual suggests.

Yep.....its a stock dizzy.. OK... I will lower the idle a bit...I think you guys are right I'm running too rich as my tail pipes have black soot instead of brown on the insides. It starts right up and did not backfire through the carb just the tail pipes and in quick sucsession upon deceleration and hill climbing at aceleration..... If I advance the timing I should be closer to tdc..agree.. how close can you get less than 5? What about vacuume at idle I thought 17-19 were good numbers at idle? Would it help to have a vacuume gage in cab and see what happens while running around. I can run a hose from wiper port to a hand held gage..what would I look for? THANKS or your advice I still need help... Steve

When you advance the timing (turn the dizzy counter-clockwise) the BB will be below the pointer and moving further away from tdc. Your flywheel is turning clockwise as you look at it through the timing hole. My timing mark is barely visible in the lower part of the window. Don't forget to unhook the vacuum advance line to the dizzy and plug it while you set the timing.

Do not expect to see tan tailpipes with unleaded gas. Even new computer controlled vehicles have black tailpipes.

I haven't seen a tan or white tailpipe since they went to unleaded fuel. :confused: :confused:

let us know what the fix is...doc

To advance timing, just turn the distributor counter-clockwise a tad and drive.
Repeat this until you hear a slight ping at wide open throttle, going up an incline.
Then retard the last advance you made.
Then, if you like, you can use a timing light to see where the marks are and write or draw a note for the future.

Any popping sounds at deceleration should disappear when timing is advanced far enough.
The ping sound let's you know how far you can advance without getting pre-ignition.

Your vacuum reading is good.
Checking the gauge while driving can tell you a lot about the engine. It will not show your carburetor's fuel mix.
I would check the number on the main jet and compare it with the manual.

Also check for vacuum or exhaust leaks.

Check for a air leak at the carb. Make sure the carb base is not warped,same thing with the intake. All bolts tight after warming up?

In my model A (stock) you can leave the timing
advanced and it wont backfire,but let the carb get
a little loose...Bang! Had a 81 f100 6 the
carb came loose a couple of times...Bang!

If you want grey pipes,1 gallon of racing gas
mixed in with a full tank will do it,you will have to reset your idle.

If you want your exhaust to Krackel when you back off the gas,put the muffler at the end of the exhaust with a short tailpipe (couple of inches)

Wick

THANKS...to all of you great people..you have got to love this hobby with people like you! OK...I will print out all the information and get to work this W/E.. and report back....I have another question... the vacuume line to the dizzy which comes from the carb is all metal. What is the best way to plug it off when timing...on my old car the tube was rubber and a golf tee worked. I hate to jamb anything into the tube so what's best.. I was told to cut a portion of the tube out and replace with rubber hose and clamps that way its always ready to go by un-doing the clamp and plugging the hose from the carb? THANKS Steve

Hi Steve,,just unscrew the vac line from the advance mechanism,,and slip a rubber cap over the end of it. (a mini condom)
Theyre usually found in the "HELP" section at the parts store.
Good Luck,Marty

If none of the above works, take a look at the condensor in the distributor. A loose condensor mounting bracket, or a failing condensor will cause backfire under load.

John

Marty...I take umbridge at the use of my name coupled with the words mini-condom...OH well, THANKS!
Condenser..many owners have told me that it has happen to them it would be a first for me but I'm changing it before I advance the timing. In regards to advancing the timing I was told that the engine should be fully warmed and idle at about 500-600 and don't worry about the timing light and disconnecting the vacumme just turn the dizzy CCW while its running and tighten down and go for a drive?? How much is a "just turn" 1/16 1/8 1/4 inch....THANKS, Steve

There you go ... just turn the dizzy small increments at a time.
If you think of a clock - maybe 5 minutes each time and backwards.

You can't hurt anything that way and you WILL hear the pinging (screeching) sound, no doubt, if you have advanced too far.

When you advance timing, engine rpm will go up a little, so in the end adjust your carb settings again to around 500rpm at idle.

I like the way you described the action..never have done this sort of thing....Pinging?? Is that what we called coke bottles rattleing is that the sound I'm listening for under load and then turn just a little CW after hearing the ping/bottles noise??? Should I pull over to the side of the road and turn the dizzy or will that leave me stranded if I turn too much? Or should I take her home and use a vacuume gauge to adj. idle and then back out on the road what would you do??? Would you take the wife with you or is there a chance I will need a tow?? Sorry for the all stupid questions I'm still trying to get it all right! THANKS, Steve

No worries at all!
That sound is not harmful to anything but the ears perhaps.
Your engine will not mind a little glass-bottle-banging sound...never thought of that image before, but will next time....
....I often drive down into lower elevations and always just reset the dizzy by memory and listening, as the timing needs resetting with altitude changes.

If you would advance too much, you couldn't start your engine quiet as easily, as the ignition would fire too soon, before the piston is all the way up.

The sound just tells you how far you should advance. It is not any reason to be worried at all, as long as it's fairly slight and briefly. I tune it out, because I don't like the annoying sound, when I punch the gas pedal, when I need it.

The more advanced, the leaner the engine runs and the less gas you burn in the long run.
- You can advance too far and then the engine heats up or worse, if you went way too far...
But basically, the quieter and leaner the engine runs, the better and longer it will last.
The engine will also feel snappier and more responsive in city/town driving.

Have fun - finding the right timing has been a lot of fun over the years for me, going with different carb setups.

No special tools needed, just your right foot, your ears and driving your bolt all over the place and listening to the engine and how it responds.

OTRA...mountain desert just got back from DV and Pharumph getting hot...I'm printing out your instructions..OK...one more..when will I hear the bottle noise up hill or on the flats with my foot in it and after I turn the dizzy am I elimanating the noise completely or leaving a little and what effect on my vac readings? I can't believe you have fun with this it drives me NUTS. Another owner has suggested to do single injector down the manifold throat like UPS did to their trucks...but I want stock....THANKS for the help! Steve

While I do agree that setting the timing "by the book" doesn't necessarily get you the optimum setting, and I always advance mine as stated above, BUT (you knew that was coming didn't you) I would still use the light to get it "in the ballpark" as they say. It's going to save you a lot of time in the long run and maybe keep some of that $$$ gas in your tank instead of burning it up diddling with your dizzy!

Waldo...YEP.. right now as far as I can make out its at about 5 BTDC...how in the heck did they crane their necks that way...you say use a light but there are no degree marks just...I...O....I... its a 55-57 engine 235...I'll burn a 100 gallons to make it work right as my wife wants a smooth running truck or else..NO truck! THANKS, Steve

A good carb should run for years without problems, once setup properly.

Pinging likes to happen under load and low rpm, as in right when you start getting in high gear and flooring the pedal on the bottom of a hill or incline.

You mentioned before you already got factory timing setting with help of a light.
After that you don't need a light any more, unless you are curious what the final setting looks like.

After you are done advancing, let's hope that solved your issues...
At least you learned something and eliminated one step in trouble-shooting and are one step closer to get it all right.

Never get frustrated. You gotta have fun working and especially fine tuning these trucks and engines.
Nothing like that feeling, when she runs like a dream with the help of your own hands.

OTRA and waldo53....thanks for all of your help...what should my points be set at? I want to re-check them when putting in the new condensor... I will replace the condensor check the points and then warm her up and advance the timing and go on the road...can I call you guys with my cell phone and hold it for you to listen to the engine while hitting the hills...my wife screams quite nicely at me when we are stuck on the side of the road....maybe you guys could calm her down! THANKS, Steve

Hi Steve; If you can use a dwell meter on your points (best method) they should be set at 31-34 degrees. If using feeler guages, set at .018". Be aware though, IF you change the point gap - it does affect your timing, so recheck timing when you're done.

P.S. Leave the wife at home until you get it just right, then take her to A&W or someplace like that - she'll love it.

Waldo..Thanks for the gap info. I will drive up to see the truck this weekend 175 miles from here. I printed out all the information and will advance timing and change condensor and recheck point gap idle and vacuume. Then give her heck and I will report back Monday. As to my wife, I was just kidding.. she is a real GEM the best and that's the truth! I'm a very lucky guy to have her! T/C Steve

Hello to everyone..took the 54/235 out for a Sunday ride after advancing the timing and replacing the condensor and the new re-built carburator did need some more tighting as per your instructions. It all worked out great we had a lovely ride on Sunday about 35 miles round trip! OK...once in awhile while decelerating I will get little BABY backfire farts not many...do I need to advance just a bit more or leave well enough alone as I did not get any pinging and the idle is fine and so is starting up. This was the first ride where heading for the side of the road was not needed and it did very well up a 6 percent grade at 50-55 just like a truck should...what do you think? THANKS, Steve

Hi Steve,,,if its working to your liking,,leave it alone. If you feel you want to adjust it some more,,make sure you mark where the dizzzzy is before you start tweaking it. That way you can put it back to where it was when you were happier with it...........and be kind to the innocent lil bunny wabbits...............
Good luck,Marty

Marty...good stinking.. I mean thinking about marking it out....maybe just a tad more? Kinda of think that I have not gone far enough as I do not have that pinging sound...?????? Where are waldo53 and OTRA when we need them? T/C Steve

Listen and feel for exhaust leaks with the engine running, especially around the exhaust donut between the exhaust manifold and the head pipe. Even if you can't hear anything, try tighteneing that connection. This may clear up your exhaust popping.

On these low compression engines, you may never get pinging on todays fuels, but if you are too far advanced, the engine will surge at cruise and will kick back against the starter.

I'm right here, following along with your progress. You're getting good feedback from everyone and sounds like to me your getting real close to getting it right on. Good job!!

Glad that you pros are lurking around to help us out! OK...the exhaust donut between manifold and pipe is there but the PO did not have it fitting that tight as they welded a second pipe to the manifold for dual exhaust which was done when Fentons were not being made. The secondary pipe is fitting very well the primary has the dognut but you can see that the pipe is a bit out of plumb. The bolts are very rusty and I do not need to bust a manifold flange. SO.. I will feel around the dognut for a leak if there is a leak what would happen if I just leave it alone...those bolts look like they will break off inside the flange they are very rusty! Glad you guys are around! THANKS, Steve

it seems many people are under-estimating the dangers of an engine "pinging".
The optimum timing for ignition is BEFORE pinging occurs.
There should be NO pinging under acceleration or high speed loads such as climbing hills etc.

The noise heard is the combustion mixture EXPLODING from the buid up of heat and pressure as the piston rises within the cylinder bore.
This is NOT a good thing for an engine, as each one of these many little explosions ar hammering the crap out of your main bearings and damage the piston crowns.....enough to burn a hole through them if put under these conditions too often!

Any sound coming from your engine that sounds like a little man in each cylinder belting the crap out of your pistons with a hammer can't be good for anybody!

Engines sre designed for the Air/fuel mixture to be ignited by your spark plugs' at the optimum time to achieve COMBUSTION.
Combustion occurs when the mixture, once fired by the plug, BURNS at a CONTROLLED rate across the piston crown to produce the necessary pressure to operate the crank.

This is why these are called combustion engines!
(NOT Exploding engines!!!!)

Keep your engine from pinging as much as you can, a few times while tuning won't hurt, but don't get into the habit of having an engine running with slight pinging the whole time you are driving it...........it will cost you in the end!

I've been waiting to say that, doc bob. One of the strange books I've read was on how to maintain marine Diesel engines, and it had nearly a full chapter on 'pinging'.

The thing is that you can set your mixture to the point where you can't hear a ping, but the engine is still pinging. (Diesel engines, so the ignition system is not a factor.) Naturally, most of the things that apply to stopping pinging completely in a marine Diesel don't apply to a Chevrolet 235, but the point is that it is hard to detect a 'just barely pinging' engine. Better to be safe than replace a $1200 piston (many marine Diesel pistons cost a lot more than that.), so retard the timing at least five degrees after you stop hearing the ping.

BTW, pinging occurs when combustion starts somewhere else in the combustion chamber than where it should, and before combustion SHOULD occur. The piston may well still be moving towards TDC, rather than away from it. I think you can see that any significant combustion pressure in a cylinder before TDC is going to put a strain on a lot of parts. The crankshaft can't just start going backwards, so you have a case of irresistable force and (kind of) immovable object. Not good for pistons, rods, or crankshafts. Also not good for fuel consumption because the engine is actually fighting itself to provide power.

As to the idea of replacing part of a vacuum advance line with rubber tubing, be careful. My parents '64 Chevelle had a 230 in it and the carburetor end fitting eventually ended up stripped. I replaced it with a nipple and a piece of 'HiTemp' tubing. It started running like doodoo. So I got under the hood, with the engine warmed up, and tried to find the problem. Not too hard to spot. When I revved the engine (increasing the manifold vacuum) I could see the tubing collapse along most of it's length.

Basically, if you do this, be sure that the tubing you use is really suitable for the environment.

After reading my post I think I need to clarify a point. Combustion does start before TDC, but cylinder pressure, in a properly tuned engine, will not become significant until ADC.

It takes a while for the combustion front to reach all of the fuel mixture, so cylinder pressure looks kind of like a geometric curve.

You don't want cylinder pressure higher than atmospheric pressure until after TDC. Otherwise, the cylinder pressure will try to push the cylinder back down instead of letting it pass through TDC. Nothing good comes of that.

That explains the whole process perfectly.
This is what happens when the fuel mixture burns in a controlled fashion.
The time it takes to burn is a relativly slow process that can be measured so that the complete combustion does occur at or just after tdc.

Pinging is just that, the explosive uncontrolled detonation of the mixture as the piston is still trying to move UP the bore.
As you can imagine, the force of such an event on an upward travelling crank, rod and piston is quite extreme.

Rough castings and sharp edges within the combustion chamber can also cause this to happen as these small obtrusions or sharp edges will start to glow under the heat of the combustion process and cause the incoming mixture to ignite before the spark plug can fire........causing pinging again!!!!!!!!

As this can happen without extreme compression ratios or bad timing settings, people get caught out diagnosing the reasons behind an engine pinging ecause it is being formed by "Hot Spots " within the engine and is unseen.

When building a high compression engine, smooth off all rough castings in the heads, round off any machined square edges on pistons and this will prevent this problem from occuring.

Just some extra info for any Stovebolters' having a similar problem with their vehicles.