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Fouled Spark Plugs and Sealing Gas Tanks

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Two topics for the price of one this month! What to do with fouled spark plugs and how to clean and reseal gas tanks are two subjects that come up quite regularly on the Stationary Engine Mailing List. Both are problems most engine collectors will have some experience with at some time or another, so some of this information should come in handy. The original query came from one of the young collectors on our list, and the responses were from collectors around the world, young and old! As ever, the following comments reflect a variety of opinions that surfaced during this discussion.

Fouled Spark Plugs

One of my -inch spark plugs is fouled. It won't fire, but I can see it sparking inside the bottom. Can I clean it? These plugs have lasted a year, until I ran them in the Witte. I soaked them in some gas, and got one to fire again .... gonna soak them all night one day and clean them out real well.

Someone told me to soak it in ammonia -said it takes the carbon out. All it did for me was eat at the aluminum screw terminal. Someone also told me to use starting fluid - never did though. I use a needle to dig down inside and scrape it out.

I have been using a long reach Maytag plug in my 3 HP headless Witte and have not had a problem with fouling.

If you are using a 3095 in that 5 HP Witte, THROW IT AWAY and get a take-apart plug. I NEVER could get a 3095 to run my 8 HP Witte for more than two hours. Then had to buy a new one. The take-apart (I paid $20) saved me LOTS of bucks over the years!

Your fouling is probably caused by carbon buildup on the insulator. Carbon is a conductor, so now the electricity has a path that is lower resistance than the air gap between the electrodes. Douche it out with brake cleaner, ether or some other kind of solvent, then with a stiff piece of spring wire (such as comes off a street sweeper), poke a thin rag down there and wipe it off. Take your time and do it well.

DO NOT sandblast the insulator. You'll cause pitting in the porcelain, and carbon will quickly build up again in the pits. DO NOT wire brush the insulator, as a thin layer of steel will be deposited on the porcelain, again causing a lower resistance path for the electricity.

You MIGHT have a cracked insulator and the carbon has filled up the crack, creating a low resistance path. In that case, you're screwed and it's time to buy a new plug. By the way, Autolite 3095 plugs cost less than four bucks each. I'm of the opinion that for four dollars it's not worth it to beat my head against the wall with a problematic plug. I've played with everything from the $22 take-apart plugs to the cheap Autolites, and cannot tell any difference except in the thickness of my wallet.

This is standard operating procedure for spark plugs. A lot of mechanics aren't aware that wire brushing a plug center electrode insulator will mess it up royally with streaks of steel from the brush. I have a final procedure for obstinately dirty plugs which is A LAST RESORT BEFORE THROWING IT IN THE TRASH. Take a pinch of saltpetre (potassium nitrate fertilizer) and sprinkle it down into the recess around the porcelain until the annulus is full. Gently heat the threaded end until the saltpetre melts and coats the porcelain. Let it cool, then boil the plug in a tin can of clean water to dissolve the saltpetre out. This treatment oxidizes the oil and carbon buildup gunk and converts it to plain water and carbon dioxide, leaving things nice and clean. Unfortunately, it is very slightly corrosive to SOME porcelain formulations and will eventually pit it - that is why I recommend it only as a final resort. I used to use this on NGK plugs with impunity, but it was hard on Bosch and AC. I haven't had to use this for about two decades, and the porcelain compositions used by those manufacturers may have changed over the years.

Running slow, my 4 HP headless Witte tends to foul plugs pretty well. I've had excellent success using a take-apart plug. Part of each day's starting routine is to take the plug apart and clean it. It's good for a whole day of running then. I like to soak in MEK for hours, then flush out several times. Install and go!

Every time I visit the dentist I ask for his old discarded (but sanitized) dental tools. The steel in these is VERY tough and works great scraping deposits out from around electrodes.

Sealing Gas Tanks

Gas tanks are often a weak point on old engines. As the metal rusts, the particles get into the fuel system and cause all sorts of problems, as does old gas. But of course, fuel tanks are not designed for ease of cleaning!

What is the best way to get the heavy congealed gas or varnish buildup out of the bottom of a cast iron gas tank?

I prefer Acetone. I know this seems a little volatile and extreme but it works great! It's not the cheapest but it's what I prefer to get the rotten stuff out. I also use alcohol for milder cleaning. It's a lot cheaper and always available.

When I started working on my Ideal 'M' it had about -inch of varnish on the bottom of the tank from stale gas. I found denatured alcohol to be the best for dissolving 'varnished' gas and use it all the time for cleaning supply tubes on small engines.

Denatured alcohol dissolves the gum and doesn't harm the running of the engine should there be some left in the fuel.

On the Ideal, I put in a quart of denatured alcohol in the tank and would rock the engine each day to stir up the sediment. I would dump it out when I saw the mixture getting pretty black. Then repeat the process 'til you're satisfied you got most of it out.

Even now when I start the engine, I can smell the alcohol. But, I've only burned about two gallons of gas running all season.

I've done that with Maytags. I discovered a drawback: The old gum is plastering over piles of rust chips. Dissolve the gum and the rust starts moving around.

Then I assume you want to keep the gum in the tank?

Nope. Just a reminder that the job's not finished when you get the gum out, just getting started. With a Maytag, the only effective solution is to take the top of the tank off, scrape, then blow and rinse it out thoroughly before applying whatever sort of sealer you may wish to use.

I would get as much gum and rust out as I can. I use nuts and bolts as an aggregate. Then, if rust is flaking, you might try coating the tank with Kream, a product you can buy at motorcycle shops to line motorcycle tanks. You have to be able to 'roll' the tank around to coat all sides. Kream has the consistency of melted ice cream.

I used Kream in a pair of leaking '56 Harley gas tanks about 15 years ago. No leaks yet. It really is good stuff. The tank has to be uncommonly clean, though, before you use it, hence the addition of nails, bolts and chain.

Kwic-Poly is FANTASTIC. It's a two-part epoxy that has the consistency of water when first mixed. Pour it in and slosh it around. It's an exothermic curing process, so you really gotta keep sloshing it around 'til it's fully cured, about 10-15 minutes. The stuff seals leaks, traps and binds any loose gunk, and is TOUGH!!!

One usage recommendation: Use a good degreasing dishwashing detergent to remove oil films on the surface to be coated and use the shop vac to dry the tank before coating. It may not be REQUIRED but it sure will give a better result.

If you've got largish holes in your tank, either cover 'em with tape first or JB on some sheet metal, than use the Kwic-Poly. You'll end up with a tank that's armor plated.

I cleaned a 1926 Ford gas tank using heat from a handheld butane torch. I'd flushed with various things to no avail. The asphalt-like stuff was thick - I'd bet it had a pint or more of the crud. I steamed it for an hour. I had to open the top with a three-inch hole saw in four places. Just the right amount of heat turned the gum to ash, which was easy to remove mechanically. I again flushed with MEK, blew it out dry, patched the holes and applied Kwic-Poly. I'm happy with it. I think the heat did the trick. Perhaps a good heat gun would work, also - the heat would not be as local. Be careful!

If you feel the need to agitate (I know I do), use nails, nuts and bolts.

I use a short length of chain small enough to feed through the cap. It's easier to retrieve.

My favorite story is the large full base engine with cast-in tank. My friend jacked up a tractor and bolted the base to a piece of plywood, which was bolted to the wheel. Filled it with acid, nuts, and bolts. Then put the tractor in gear, and just left it on the spin cycle for a little while. No, the tractor didn't fall off the jackstands!

I used a pump jack I rigged up with a variable-speed motor. I strapped the gas tank to the pumpjack arms, put a few assorted nuts in, and filled it about 1/3 full with lacquer thinner. Set the speed to get a nice sloshing motion and let it run overnight. The next morning the gummy varnish was all dissolved and the tank was clean and shiny like new inside.

There has been much said about 'coating' the inside of tanks. I'm NOT in favor of doing this unless there is a problem such as small pin hole leaks. I keep gas in my engines at ALL times and store them in a garage. I wouldn't think of coating the tanks on a Maytag. Once cleaned out, you shouldn't have any problem.

Current list discussions are about the last shows of the U.S. season and the first shows of the Australian season. There are also requests for identification help on newly purchased winter projects. I'm sure that out of this somewhere some topics will appear that will interest the readers of GEM next month.

Engine enthusiast Helen French lives in Leicester, England. Contact her via e-mail at: Join the ATIS mailing list at:

''There has been much said about 'coating' the inside of tanks. I'm not in favor of doing this unless there is a problem such as small pin hole leaks.'

A lot of mechanics aren't aware that wire brushing a plus center electrode insulator will mess it up royally with streaks of steel from the brush.'