Backfiring

[Sea Ranch]

Greetings,

So I'm driving home from school/work and get within a block of my house when I've run out of power and, it feels like, fuel (am definitely not out of fuel, though).

It's now a struggle to get the motor started; it turns over without firing, then eventually starts up. Does not accept throttle well; you have to nurse it up to 2 or 3 thousand rpm and that's about all it will do. Further, it putts a bit and will occasionally backfire like I'm going to blow out/up the muffler.

There is fuel in the stock glass-bowl fuel pump. I pulled the filters out from under the large brass nuts on the top of the Weber DCOE40s but there was precious little foreign matter there at all. Turned over the motor to see if fuel was coming in - it is.

Have not checked condition of spark at plug leads; have been working to get my dead computer up and running (and waiting for the "third shoe" to drop). Electronic ignition has always been faithful.

My theory is that it ingested something from the fuel tank into one or both carbs (though how it got past the fine mesh screen on the inputs just past the banjo fittings I don't know).

Thoughts? Any insights gratefully received . . .

Randy

[nebogipfel]

Randy, Just a quick thought, possibly an induction vacuum leak on the carbs, possibly an "O" ring. Perhaps a fractured headlamp vacuum pipe?

[Sea Ranch]

Whoa. Never thought of that!

I've had the headlamp vacuum leak thing in the past and what I experienced was just mild surging in the lower revs.

On the other hand, what I have currently is making the car positively undrive-able.

Coincidentally, I have had "plugs" in the ends of the two fuel tank breather tubes (where they exit under the car) for the last few days, in order to cut down on fuel evaporation and the accompanying smell (works really well). I pulled the plug out of one side and drove the 20 minutes to work today with no problem (one tube venting, the other still plugged). When I drove home and developed this problem, it seemed to me like the fuel was not arriving at the carbs and so I got out and unplugged the other breather tube as well (thinking that perhaps the one breather I un-stopped was in fact plugged further back in the tube for some otherwise unknown reason) and that I had left a plug in the only real (breathing) breather.

Alas, unplugging them both has made no difference to the car's operation, after many miles of driving, inspecting, etc.

I always make a point of not discounting "coincidences" like this. But so far I can't see any connection.

And if the engine is indeed fuel-starved, why does it occasionally backfire, as though fuel is loading up in one cylinder and then being lit by an irregular spark?

[hatman]

I suffered similar symptoms with mine some years ago. Turned out to be as a result of intermittent failure of the anti-theft switch in the glovebox (common problem, I gather). Is yours still fitted? If so, it's gotta be worth a check.

[ardee_selby]

<snip> Further, it putts a bit and will occasionally backfire like I'm going to blow out/up the muffler. Randy

Randy,

As above, plus...

Elsewhere you wrote "I really feel one of my ports is leaking based on the telltale single cylinder quiet "putting", but haven't figured out which one yet. I could save myself the trouble and re-gasket them all..."

Is this the same "putting" as before?

Ref: "Unburnt air/fuel mixture passing through the exhaust and igniting also results in backfiring. This is usually caused by a temporary loss of ignition or rich idle mixtures in combination with exhaust air leaks"

In addition, may be wise to check ignition timing to eliminate it from your enquiries

[Sea Ranch]

Thanks, Hatman and Richard. Really appreciated. I'm off to the car now to have a look 'round, test some things, including spark, etc.

Thanks for noting the similarity to previous post. My exhaust does seem to have a minor leak on one of the exhaust port to manifold joints. This has not changed; the farting/putting I'm describing now seems definitely to be unburnt fuel igniting at the wrong time, as in your reference quote, Richard.

Thanks. I'll report back soon . . .

Randy

[Sea Ranch]

A running summary:

- this morning pulled plugs, all produced nice spark, visible in daylight, despite #2 being soot-fouled
- pulled tops off Webers, pulled out accelerator pumps; all seemed clean and clear
- part of the carb top's casting that holds the pivot point for one side of the float (on front carb) was broken off and laying in the bottom of the carb. I retrieved it, inspected float pivot, it seems to still be functional despite being held in place on only one end. Could this cause a problem with float function?
- re-assembled. Car would not start; seemed to have no firing, as though no fuel or no spark in combustion chambers. Turned it over lots, then pulled plugs again and they were all wet, so presume there is fuel.
- cleaned plugs, re-tested for spark. All looked good.
- re-assembled, car started and ran for a while. Was able to rev it, but it would "fall on face" easily off idle with even a little too much throttle. Set the idle to about 1200 to keep it turning over to see if it would warm up. After about 2 minutes of running, it slowed down and then died. This now does not sound like ignition but rather carburetor or fuel supply.
- pulled screen filters under the big brass nuts, there was fuel in the inlet areas.
- after the car sat for 10 minutes, it started again quite readily, running the same as above. Eventually died again.
- the fuel pump's glass bowl seems always to have lots of fuel in it, even when the car dies (fuel to the carbs but not in the float bowls??)

Thoughts???

[richardcox_lotus]

Very similar symptoms to when my sprint started misbehaving.
Turned out to be the coil failing when hot. Let the car cool off & it seems to be fine again - infuriating I know !!

I also think the fact that it's come on somewhat suddenly points to a breakdown in the electrical side - in my experience fuel faults are slow deterioration rather than sudden failure.

Rgds
Richard

[nebogipfel]

Randy,

I would suggest you go back to basics and check throught the car methodically.



I always start with sparks.

Check the LT do you have a good supply to the coil? Is the wiring to and within the distributer OK? Check all connections.

You need to go through the distributer and check that you do not have problems with any of the HT side. Is the cap OK or is it tracking? likewise the rotor. You have electronics but has it developed an intermitent fault? perhaps you could check by temporarily refitting points.

Is the coil failing, test by substitution.

Hang a timing light on, is it correct? Has the advance stuck fully advanced?

Then to fuel,

Check for the aforementioned air/vacuum leaks. Has a vacuum hose split? Has it eaten an "O" ring?

Is the pump OK? remove the supply pipe to the carbs and pump fuel into a bottle when cranking on the starter (take care with stray sparks)

IMO If the car is on four cylinders when running it's unlikely to be a carburettor problem

BUT You need to correct the float issue you have mentioned.

I know you have checked the tank venting but have you popped the filler cap when the engine is running to see if a vacuum is developing.

It's worth doing cylinder compression tests to check for more serious issues which may explain why the car doesn't want to idle.

That would be a start

[Sea Ranch]

Another "whoa"! Thanks, Richard and John. Lots of great thoughts. I will look into them.

- Have kept the fuel filler cap popped to make sure there's no venting problem.
- Will call around to see if I can find Weber parts in town (don't think there's any way of repairing the broken float pivot point without replacing the entire top piece)
- Can coils be intermittant? Spark present on test but not strong enough to run properly?
- Last night driving the car around, it Definitely was running on three cylinders some of the time, but clearly on all four at other times. Seemed to switch back and forth.

At least I've got my main computer fixed now (was having a "Job-like" week here!)

I'll check back later . . . thanks so much . . .

Randy

[nebogipfel]

Another "whoa"! Thanks, Richard and John. Lots of great thoughts. I will look into them.


- Can coils be intermittant? Spark present on test but not strong enough to run properly?

Your're welcome Randy, hope you get it sorted.

Yes, coils can suffer from tracking problems with the HT randomly running away to earth, but more commonly they tend to start breaking down when hot but will function OK again as they cool down. This was also a fairly common problem with condensers when we all used them

Also if you can determine which cylinder is dropping out (assuming it's just one) I would also substitute the plug and lead to that one to see if it helps.

[Sea Ranch]

Thanks again for reply.

I confess I'm confused. Not sure whether to focus on ignition or fuel!!

Finally a couple pix, now that computer is online . . .

Here are the plugs, in order, cylinders 1-4, top to bottom, when I took them out this morning after a number of miles of intermittant running on three, backfiring, not idling, etc. last night:


And here are a couple pix to show the broken pivot point on the front carb (yes, that means that #2 cylinder's sooty plug is being fed by the carb with the broken float pivot; connection?)




The first pic shows the rear carb and the slot that is cut into the casting on the one pivot, and the second pic shows how on the front carb that slot has allowed a piece of the casting to break off. Really, though, I can't say that the pivot and float are that much affected, unless the extra bit of slop translates to shaking that goofs up the float level? Could this cause:
- backfiring
- inability to idle (for more than 10 seconds)
- low power
- hard starting
- not revving easily
- as though it's sometimes operating on 3 cylinders

Thanks . . .

Randy

[Sea Ranch]

Update:

Just got off the phone with a Weber specialist in Vancouver. Told him I was looking for a top cover for my DCOE40 and explained why. He said, "Sure, we sell used parts: $150." I said, "For the whole carb?!?" He said, "No, the whole carb is $400+"

Anyway, gave me opportunity to speak to a knowledgeable person about whether this broken float pivot could upset my carburetion enough to cause serious driving problems and he said, "Sure". Something about maintaining a small movement in the float needle, etc. and flooding. I told him I didn't think the broken mount amounted to much change in the float's movement but he thought any little change would be too much.

So, what do you all say? Could this be "the problem"? Could it add up to the range of symptoms I have experienced?

I will be looking at an epoxy repair of the broken pivot, to see if I can salvage the part before paying out the big dough on a used top cover.

(Apparently older DCOEs have this split pivot point but newer Webers do not, precisely for this reason.)

[bill308]

Yes Randy, the broken pivot will likely cause the float to not shut off fuel at the required level, resulting in an overly rich mixture on the 2 affected cylinders. This has to be fixed before you look at anything else.

Bill

[Sea Ranch]

Okay, then. Thanks, Bill. I'm on it. Here's hoping "JB Weld" is as good as they say it is. (Actually it's holding up quite well - 2+ years - as a patch over a puncture in my old Suzuki's oil pan, after my daughter ran the car up onto a median and flattened a sign - the sign that's supposed to tell you not to run up on the median )

I'll check back later . . .

Randy