Welding Near a Carb....?

Yes, this seems like a bad idea. I have some structural strengthening to do in the strut tower area and where the firewall meets the front frame. If I had been thinking, and it wasn't 100F plus outside, I would have done this with the engine out.

So, I have a Weber 38/38 with the typical K&N filter and yes, occasionally I can smell gas. This never happens after the car has set. Just run and been shut off.

The strut tower is about eight inches from the Weber filter. How bad of an idea is this? It'll be MIG, so plenty of sparks. Would soaking several towels in water and covering the engine and especially carb do the trick? Should I just pull the carb?

I ask what seems like a silly question because I know and have welded extremely close to fuel lines...but as we know, liquid fuel does nothing, it's the gas form that goes boom.

Thanks JP

Throw a wet rag over it and any rubber hoses and go.

That simple, huh? What about a certain line that runs from the tank. I believe it's part of the return or charcoal canister--which I no longer--or a tank vent. Car has zero emissions.

Fireproof blanket? I have a small one for situations just like this. Or just soak a bath towel in water and cover the engine.

Wet denim sheet and off you go.

I keep an old towel around for this. Soak it and cover what you don't want vaporized.

And remember a nice welding blanket over the wet towel on anything you don't want to get all deteriorated from the sparks..

100F in February?!

STM317 wrote: 100F in February?!

yeah..no kidding. Must be in the southern hemisphere.

Aussie? New Zealand?

Don't forget to disconnect the battery before you weld.

^ I read that as "If I had been thinking a few months ago when I did this, and it wasn't 100F plus outside then, I would have done this with the engine out."

I am a weenie. I would park the car outside, disconnect the fuel line from the inlet nipple, block the line with a proper diameter bolt wrapped in Teflon, and dry-crank the car for a bit. Then come back a day later with the welder.

Did you mention a return line? Was the car converted from split-reg fuel injection? I had one of those and we capped the return line down at the firewall nipple with a piece of fuel line and two clamps. I don't think welding sparks are going to cook through a pinched-off piece of fuel line, but I would put a piece of sheet metal on top to deflect 'em just in case.

In reply to WonkoTheSane: Yeah, it was August that was 100F outside. I'm in Oklahoma not far from the drag strip or Hallett. It's currently 65F with Saturday supposed to hit 70F plus. Good time to get things done.

The car was always carbureted. I just pulled off all of the emissions and put on a Weber...with a few other goodies. So, currently, there is no return line. There was one as part of the emissions.

So, good large wet towel and welding blanket and I'm good? Anything I should look out for? Strong gas smells?

Thanks for the help fellas.

Best John

You're way overthinking this, there aren't enough fumes coming out of a cold carb with an air cleaner on it to support combustion. You'd be lucky to get a little lighter sized flame shoving a lit match right in the bowl vent. I guess if it really bothers you, you could pull the fuel pump relay (or disconnect the suction line to the mech pump) and let it idle to death. The towel is more to keep a chunk of slag from burning a hole in the air cleaner than starting an actual fire.

Speaking of wet towels and welding: I watched an old guy mig weld a body tab onto the underside of a painted stock car hood without damaging the paint once. He just held a dropping wet washcloth on the top of the hood and carefully spot welded. I was amazed (and I realized that I'd never be able to do that).

BrokenYugo wrote: You're way overthinking this, there aren't enough fumes coming out of a cold carb with an air cleaner on it to support combustion. You'd be lucky to get a little lighter sized flame shoving a lit match right in the bowl vent. I guess if it really bothers you, you could pull the fuel pump relay (or disconnect the suction line to the mech pump) and let it idle to death. The towel is more to keep a chunk of slag from burning a hole in the air cleaner than starting an actual fire.

That's the first time I have been a accused of over thinking anything. Wet towels, maybe a welding blanket. This is all I needed to know.

So, getting off but sort of staying on topic--what volume of fumes does it take to get a good fireball? I still see guys around here smoking as they fill up at gas stations. I know it's pretty hard to under most circumstances...

Funny side story--my friend had just bought a house that had this nasty overgrown brown garden in the middle of the backyard. He decides he can't stand it, fills up one of those backpack pesticide sprayers with gas and starts spraying the garden. Then he tries to light it--nothing. He sprays more gas and VOOMF! It suddenly decided to ignite and he did the universal "Oh E36 M3, my legs are on fire" dance. He still doesn't have hair on his legs to this day.

It's perfectly ok to overthink not blowing yourself up. This can be calculated to some extent. Per Wikipedia the explosive limits for gasoline fumes are 1.4-7.6% (by volume in air), I'm struggling to find how much vapor you get from a given volume of liquid gasoline, but I did find that the CDC says you can smell it starting at 0.25PPM, that's 0.000025%. So (if I did my math right) 56,000 times a faint odor's concentration to get a fireball.

Like I said, the concern here isn't sparks, it's a chunk of slag burning a hole in a fuel line or lighting the air cleaner element.

I had to modify the throttle arm on a TBI unit once. I tack welded it in place shortly after the engine was running.

Plastic bag over the inlet, rag over the plastic bag to keep any potential spatter from going through the bag, didn't catch anything on fire.

I'm good at catching things on fire when welding. Usually rags, but the fun one was when I was welding my rearend housing up after it had split open again and I cleaned the weld area with brake cleaner. It was too humid for it to evaporate so there was a puddle of brake cleaner on the floor, that I was standing in...

pull filters, duct tape over openings and bowl vents

wet towel over anything ya don't want spattered, foil cover could work for hoses n such

I have set fire to enough carbs, just with backfires, that I would not have given this a second though.

BrokenYugo wrote: It's perfectly ok to overthink not blowing yourself up. This can be calculated to some extent. Per Wikipedia the explosive limits for gasoline fumes are 1.4-7.6% (by volume in air), I'm struggling to find how much vapor you get from a given volume of liquid gasoline, but I did find that the CDC says you can smell it starting at 0.25PPM, that's 0.000025%.

Yeah but it's hard to smell a concentration that low, what with the smoke from the lit cigarette.