Backfiring / bogging down under load ?

tomr · July 31, 2009, 09:16 PM

Hi all:

I left for a weekend of camping this afternoon but it was not to be. I could go along at 60-65 no problem, but as soon as I encountered the slightest hill or incline, the Brave lost power and backfired like crazy. i know this is a FAQ, but I'd love some advice...we managed to limp back home...

Thanks
Tom
'75 Brave D21, 318-3

The_Handier_Man1 · July 31, 2009, 11:02 PM

You might want to check fuel lines, filters and any switches that could block fuel from the tank(s) to the carb. It sounds like a fuel problem to me. Les

mulletman · July 31, 2009, 11:05 PM

yip sounds too lean

tomr · August 02, 2009, 08:04 AM

Thanks guys...Just prior to this happening, I "filled" the gas tank...likely the first time it's been full in years. I suspect I stirred up rust and crud etc ??
I changed a fuel filter that sits between the fuel pump and carb, but it made no difference. I only have one fuel tank, and I don't see a filter anywhere between it and the fuel pump. If that is so, could my fuel pump now be clogged/full of dirt ?

Tom

mulletman · August 04, 2009, 12:14 AM

is it an mechanical or an electric fuel pump? if it is and electric on i would test it with the line off the carb. i have a clear filter after the fuel pump by the carb so i can see if its getting good fuel.

tomr · August 04, 2009, 06:48 AM

Thanks for the reply. It's a mechanical fuel pump. I have a clear filter, and it seems to be getting fuel. Should it be full all the time, or is 2/3 or so ok ?

tomr · August 04, 2009, 08:15 AM

Hi again. At the risk of exposing more of my stupidity/ignorance, I need to mention that on mine, the vacuum line that comes out of the intake manifold just behind and to the left of the carb, is fitted with a brass T fitting. One side of this goes (I think) to the brake booster. Th other side is fitted with nothing...i.e. it's an open "sucking" vacuum. Anyone know what it's supposed to be connected to? Also, could this be the problem? (see how unmechanical I am)

please go easy on me...

DaveVA78Chieftain · August 04, 2009, 02:01 PM

The open vacuum port is a defiently a problem. Results in engine running too lean. Need to at least plug it up.
Here is the top end of a 73 model 318 from GOWINIGO.

Here is the emission control vacuum line distribution from the chassis manual.

Dave

tomr · August 04, 2009, 02:23 PM

thanks Dave...yeah I thought so, but I think something must have fell off because that open end of the brass T looks too clean. Perhaps just a plug of some kind. I'll look closer...

tomr · August 06, 2009, 07:51 PM

Plugging that T made little difference. My vacuum gauge is showing a normal reading (65kpa). There is however, no vacuum reading at idle at the hose that goes to the distributor? Is that normal?

DaveVA78Chieftain · August 09, 2009, 04:19 PM

That brass fitting is not stock so my guess is someone had added something a some point. Could have been simply a better vacuum gauge or aftermarket cruise control.
Most discussions about engine vacuum relate to intake manifold vacuum (like used by the brake booster). Intake vacuum decreases as the throttle is opened up.
Distributor advance vacuum is the opposite and increases with engine load. This allows an increase in timing when the engine is under load (ie. going uphill) above and beyond the mechanical advance. So, yes, low distributor vacuum at idle is normal. Have you pulled the distributor cap off, and pulled a suction on the vacuum advance circuit to see if it is working? (Causes the pick-up coil mounting plate to move in the opposite direction of rotor/reluctor rotation) It is not uncommon for the diaphragm in the vacuum advance can to go bad and will cause poor performance.
Initial timing for a 318 is somewhere around 2.5 +- 2.5 degrees (use emissions label for your year). Set to far advanced will cause the problems you describe.

Dave

tomr · September 02, 2009, 08:35 AM

Sorry for the silence. I decided to vacation without the "Brave", and get back to it later. I ran a temporary fuel line from a Jerry can to the fuel pump, essentially bypassing the tank etc. Same backfiring resulted, leading me to think that it's not a fuel starvation problem. Dave, can you explain the vacuum suggestion in more "simple terms"? Are you saying to remove the vacuum line from the carb that goes to the distributor and apply suction to it? How much suction, and I assume I should see/hear something move/turn ?
Thanks
Tom

DaveVA78Chieftain · September 02, 2009, 08:18 PM

As shown below, vacuum advance is attached to the distributor. All I do is remove the distributor cap so I can see the pick-up coil mounting plate. I then disconnect the vacuum advance hose at the opposite end from the distributor. I then just pull a suction on the hose with my mouth ( N:( ewwww dirty, wipe it off first or use a vacuum tool if you have one). I watch to see if the pick-up coil plate moves. If it doesn't then either the vacuum can is defective, the hose has a hole in it, or the pick-up plate is binding.

Complete ignition description is located at http://www.classicwinnebagos.com/forum/index.php?topic=3424.0

Dave

Distributor
Provides firing control pulse to ECM
Provides Mechanical Advance
Provides Vacuum Advance (Controlled by carburetor vacuum)
Routes high Voltage pulse from Coil to proper spark plug

tomr · September 04, 2009, 07:10 AM

Thanks again. I don't see any movement when I suck on the hose.

Just to be clear, I'm "sucking from" the distributor, and towards the carb.
also, the carb seems to be supplying adequate suction where that hose attaches. The suction gets stronger when I rev the engine...

povertycoach · September 04, 2009, 09:34 AM

tom,
reconnect the hose to the distributor advance can and suck towards the dist. With the dist cap off, watch for rotation of the points plate. It should move 1/4" or more.

Your carburetor needs a good cleaning, and a rebuild to ensure the internal circuits are clean, and there are no vacuum leaks at any gaskets.

p-coach

drev59 · December 14, 2009, 10:01 PM

Had a similar problem on my 73 brave. Intermittent loss of power and backfiring. At first thought it was a fuel starvation problem and checked everything on the fuel system. Still had problem so I went and talked to a older Chrysler mechanic and after explaining the problem his first response was to try a different ignition module. Problem solved. Module would start breaking down especially the hotter the temp got under the doghouse.