1725 backfire (and other fun)

[mferris]

I've had a "new" SV for about 2 months. It has always started, with significant tweaking of the broken automatic choke with a stick - but the backfiring is getting progressively worse - both out of the carb (gas/fireballs flying in the air with the hood open) and out of the tailpipe. We figured that it was the timing/points, or the Autolite 32/36 DFAV carb that a PO had stuck on that needed rebuilding. I've also had to advance the timing to the farthest the distributor will take (in its current rotation). It will run, rough idle, sounds like it's missing - but has always backfired.

However, today I rebuilt the front calipers, managed to get the car started - warmed it up for the 5 minutes that it has required to get to a reasonably steady idle (with tons of backfiring up to that point), and then drove it around for about 10 minutes (again backfiring).

I then let the car idle for about 5 more minutes and it was getting smoother and smoother, great - until I noticed the foam coming out of the radiator overflow. Looked at the fan to see if the belt was loose which it was - but it was turning the fan - plus the temp gauge read 85c (cool day here - normally would read 100+). So I didn't think it was overheating.

We had done a compression test a few weeks ago with disappointing results (130, 120, 100, 95 psi).

My basic question - is this a head gasket (or progressively worse cracked head, or cracked block)? This is my guess as: presuming the normal temp + foaming overflow from the radiator is gasses getting in the coolant system. The backfiring is gasses leaking from cylinder to cylinder and firing out of order, and the compression is a simliar result.

If this is a head gasket, will it be as straightforward as the calipers were? Are there any how-to's that I should follow.

I just want to get it to a reasonable state, for minimal cost.

Thanks in advance
-Mike

 

[Bill Blue]

Mike, while reading your description of all the mal firing, the thing that comes to mind is spark. It sounds like something is routing the spark to the wrong plug. Something like a cracked distributor cap or bad rotor.

Your diagnosis could be correct, but check out the spark before removing the head.

Bill

 

[RootesRacer]

replace the condenser if the points gap is correct and it still backfires.

 

[mferris]

Thanks Bill. Nothing is obvious to my very untrained eye with the disty. Maybe I'll buy new points and see what happens. I'm trying not to buy an electronic ignition until I decide when I can do a V6.

 

[RootesRooter]

Any chance the plug wires are mis-routed? 1-3-4-2.

You mentioned you've turned the dizzy as far as it will go in its 'current rotation.' I suggest moving the wires on the dizzy 90 degrees and start twisting it again.

 

[Bill Blue]

Thanks Bill. Nothing is obvious to my very untrained eye with the disty. Maybe I'll buy new points and see what happens. I'm trying not to buy an electronic ignition until I decide when I can do a V6.

Quite often the problem is not obvious. I really doubt the problem is the points, unless they are misgapped. Many years ago, I blew a muffler apart on a Gremlin, due to points that were barely opening. Hairline cracks in the cap can play havoc with spark, causing it go all over the place. Also the rotor can develop tiny cracks that cause all sorts of mischief.

Bill

 

[Nickodell]

There are many possible causes for backfiring.

Spitting back through the carb is often due to weak mixture; a clue as you mention it taking a long time to warm up. Other causes: incorrect timing, burned/sticking valve, cylinder-to-cylinder crossfiring due to a bad gasket, or tracking inside/outside the dizzy (run it in the dark and you will see external tracking; internal will show as a line in the dizzy cap) or possible shorting in adjacent plug leads.

In the exhaust it comes from unburned fuel passing into the exhaust manifold due to misfiring from an ignition problem or bad plug(s), burned points etc.

This should keep you busy!

 

[mferris]

Ok. So I'll start with this:

New cap, points, condenser
Firing order
Head gasket

And see where it goes from there.

 

[mferris]

Just an update (partially so I keep track), if you have any other thoughts please let me know.

Engine still backfires horribly. However, I thought we had solve it at one point this morning:

When cold (40F) this morning:
- Did a pressure test on the radiator, pumped it up to 7psi. Let it sit for 10 minutes, no appreciable drop - so doubted it was a head gasket that was causing the foaming in the radiator.
- Looked at points, a little brown, gap was (maybe) .012, so we widened it some - thinking ~.15 as spec.

Started engine:
- Engine started on first turn over ! (it had sat for 48 hours)
- No backfires
- Reasonably responsible throttle (little bogging down - but not bad)
- Revved reasonably
- Idle was at around 1400 rpm
- Got engine reasonably warm, did a pressure test - seems to not want to hold 7psi this time - leaks down to 4psi or so after a few minutes - might have been cap.
- Adjusted manual advance until it sounded good at idle and revved
- Probably 20 minutes of idle/revving - worked really really well.

Then I took it for a short drive, and things got bad:
- Ran for about 1 minute before soft backfires started, and got progressively worse.
- Very poor acceleration (wouldn't go beyond idle when pushed to the floor)
- Other times would accelerate fine
- Engine would die on some backfires, and at times would not restart for a minute or so, then would finally restart - run for a few minutes, then backfire poorly again.
- Might be just me - but would run fine, then take a corner and try to get up a hill and it would immediately start backfiring (3-4 backfires out of the carb in rapid succession)
- Took fuel line off, fuel is being pumped
- Measured Vacuum - at idle about 5psi, then revs to 15+psi with throttle
- don't know if vacuum advance is working.

I'm getting *new* points/rotor/cap/wires from VB this week - so going to start there. Then I guess I'll try a carb rebuild.

I'm presuming that given that when cold - there was no pressure loss, that a head gasket is unlikely.

Keep in mind - I'm just trying to get this thing going. the 90psi in the 4th cylinder is indicative that I need a rebuild - so I want to get it going and back on the road (to get my title back from the state) before I start any major rework.

-Mike

 

[anduril3019]

Just a thought added to all the other good advice, my SV (about 30 years ago) had some similar symptoms. You say it's getting fuel, but make sure the volume of fuel is high enough. My transfer tube between fuel tanks was clogged just enough to allow fuel to flow for normal driving but as soon as I put it under load up a hill or around a long accelerating corner, the carbs would run low on fuel and lean out the mixture until I backed off on the throttle allowing the carbs to fill again. Check that the fuel flows easily out of the fuel line connection at the rear of the car. The large flakes of paint peeling off the inside of the tanks can float and shift so that sometimes they clog the outlet, and sometimes not. If you search the forum, I'm sure you'll find similar stories and remedies.

Good luck!

 

[Bill Blue]

Running fine when cold, terrible when hot: Coil or condenser. The condenser can make a fellow swear he has a fuel problem. But consider this, the fuel requirements are higher in a cold engine.

It is very popular for the ballast resister to be removed. The constant 12v. current kills the coil. But coils are spendy, condensers (capacitors) are not. Try a new one, if that does not do it, you probably need a coil and ballast resister.

Bill

 

[Ron67Alpine]

Just a thought, I had all kinds of problems when I got my SV running under load, up hill. Ran fine on flat pavement. It turned out my muffler was absolutly clogged w/rat/squirl debris.
Ron

 

[RootesRacer]

check the valve clearances hot.

 

[mferris]

Thanks all

I'm going to start with a new condenser and ballast (plus points, rotor, wires)

I'm not sure how to check valve clearances but will look into it. Considering how low compression is - I'd bet there is something there.

I have not cleaned out the tanks yet so that is a possibility. The backfiring has gotten much worse as I've worked on it (and put 20 miles on it while doing so).

I'm not sure that there is crap in the exhaust as there is quite a but of exhaust flow (and I've had a few good backfires that would have kicked out any furry carcasses considering how strong they were)

I'll just wait until the ignition comes and see what happens.

-Mike

 

[bohemianway]

CHeck your slide needles

I had a similar problem and it was the slide needle falling out. They fit so snug that sometimes on closing the throttle they would go back in place. A real freaky deal.

A second on Ron's exhaust plugging. After 20 years of sitting I fired the old girl up and revved it in the garage. later I started scolding the kids for throwing dirt and acorns over the entire floor. Oops, I then realized it was all from the exhaust.

Charles

 

[Ron67Alpine]

I thought I'd cleaned it out also, all sorts of crap blew out, until I cut off the muffler/resonator...I didn't have a stock setup. All the difference in the world, all of a sudden I had power. I had it cut open and it was still stuffed. It was also blowing, manifold to downtube, gaskets.
Replacing the dist cap, condenser and rotor is probably a good idea, just on GP's.
Good luck

 

[Cactusmasher]

Backfire etc.

I would recommend not driving the Sunbeam until the backfire problem is solved. It could cause additional problems added to the list of maladies it has now. Backfiring, especially continuous as described, can cause all sorts of problems in the carburetor as well as the possibility of a fire. A sticking exhaust valve is a possibility. Low compression could be a result of sticking or frozen rings. I have found over the years that the #4 cylinder is a common culprit for low compression and other problems. I have also know of oil pans completely blown off the bottom of the engine or severely split open because of backfiring. Even though this is a "new" car with low mileage, sitting dormant for so many years will cause problems as you describe. It's a process of elimination to find the causes and it can be vexing. It's usually something small or not even considered at the beginning. Been there, done that. Good luck in finding and fixing the problem.

 

[BaileyBL]

Interesting thread. My SV suffered bacfiring and it turned out to be retarded timing, enough that it overheated the exhaust manifold until it developed a hair line crack. I too suffered from flaky fuel tanks and plugged filters and carbs. I have an electronic ignition system and an electric fuel pump. Cyl. pressures are 100,120, 120 and 140, I suspect burnt exhaust valves. All of this contibuted to the backfiring but the main culprit was simply ignition timing. It ran not too badly on a flat road but wouldn't climb hills easily without much backfiring. I am now trying to find a replacement exhaust manifold. This winter I will overhaul one of my two extra cylinder headsI have to see how much I can improve cyl. pressures before getting into rebore, pistons and rings etc. I will also replace the coil.

 

[mferris]

Interesting on the retarded timing. While I'm not an expert - I got out the timing light and saw that that it is dead in the middle of the marked range at idle and advances some (so I presume the vacuum advance is working). Whether or not at this stage of such low compression (high 90's on the 4th cylinder) the timing marks are relevant we'll see.

I'm hopeful that new points/cap/wires... will at least change a few things. We will see this weekend.

-Mike

 

[1980e12mpl]

The fact that it is both backfiring and exhibiting problems under load, would lead me to believe that those two problems are ignition related. So, I think it sounds like you are on the right track to eliminate that side first.