# Engine keeps going after spark plug cables removed??? **URL:** \ **Category:** Factual Questions\ **Created:** [September 28, 2005, 6:00pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889 "2005-09-28T18:00:57Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [September 28, 2005, 6:00pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/1 "2005-09-28T18:00:57Z")
I was experimenting with my Honda CB500 motorcycle today. I had it pressure-washed the other day and the engine was sounding quite coarse. I guessed that one of the spark plug cables must have gotten wet. So, with the engine still running (inline-2, 500cc) I removed the first spark plug cable and then the other. I expected the engine to stop, but it didn’t :eek: I could even rev up the engine with no apparent problems. How was this possible? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 28, 2005, 6:05pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/2 "2005-09-28T18:05:31Z")
> [@Dog80](#): > > I was experimenting with my Honda CB500 motorcycle today. I had it pressure-washed the other day and the engine was sounding quite coarse. I guessed that one of the spark plug cables must have gotten wet. So, with the engine still running (inline-2, 500cc) I removed the first spark plug cable and then the other. I expected the engine to stop, but it didn’t :eek: I could even rev up the engine with no apparent problems. How was this possible? Dieseling. Motorcycle engines tend to be high-compression engines and once up to operating temperature, this compression creates enough heat to ignite the fuel-air mixture, under the right conditions, just as in a diesel engine. ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [September 28, 2005, 6:06pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/3 "2005-09-28T18:06:08Z")
Offhand, it shouldnt be, but I’m more of a truck person than motorcycles. Its a stupid question, but are you sure you pulled the spark plug wires and not something else? Does it have multiple plugs in each cylinder? ---
**Author:** ![Edward\_The\_Head](https://avatars.discourse-cdn.com/v4/letter/e/ce73a5/32.png) [@Edward\_The\_Head](https://boards.straightdope.com/u/Edward_The_Head)\ **Post date:** [September 28, 2005, 6:23pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/4 "2005-09-28T18:23:55Z")
> [@CynicalGabe](#): > > Offhand, it shouldnt be, but I’m more of a truck person than motorcycles. > > Its a stupid question, but are you sure you pulled the spark plug wires and not something else? Does it have multiple plugs in each cylinder? Most of the motorcycles I’ve seen only have one wire for each plug. Plus wouldn’t it hurt like hell if you removed a live wire from a plug? I did that once on a Mustang while it was running, you can bet I never even though of doing that again. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [September 28, 2005, 6:28pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/5 "2005-09-28T18:28:30Z")
> [@Q.E.D.](#): > > Dieseling. Motorcycle engines tend to be high-compression engines and once up to operating temperature, this compression creates enough heat to ignite the fuel-air mixture, under the right conditions, just as in a diesel engine. Dieseling is the only reasonable explanation, but the compression is not particularly high (10:1), the engine hadn’t reached operating temperature and the spark plugs are one grade “colder” than normal. > [@CynicalGabe](#): > > Its a stupid question, but are you sure you pulled the spark plug wires and not something else? Does it have multiple plugs in each cylinder? There’s only one spark plug per cylinder. I pulled the caps that connect to the top of the spark plug. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 28, 2005, 6:36pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/6 "2005-09-28T18:36:39Z")
You using low-octane fuel in that thing by any chance, **Dog**? Low-octane gas tends to ignite more easily than higher-octane stuff. That’s about the only thing I can think of. ---
**Author:** ![Schuyler](https://avatars.discourse-cdn.com/v4/letter/s/54ee81/32.png) [@Schuyler](https://boards.straightdope.com/u/Schuyler)\ **Post date:** [September 28, 2005, 6:59pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/7 "2005-09-28T18:59:36Z")
Another possibility is that there are enough carbon deposits in the engine to 1) raise the effective compression ratio, increasing your susceptibility to pre-ignition, and 2) create a hot-spot that would provide the trigger. It’s very curious that this happens right after a pressure-wash and (only?) when cold. A bit more investigation and description may provide more information with which to make further WAGs. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [September 28, 2005, 7:23pm UTC](https://boards.straightdope.com/t/engine-keeps-going-after-spark-plug-cables-removed/323889/8 "2005-09-28T19:23:29Z")
I will try again tomorrow. I will let the engine running for longer. Also, I will try removing the spark cables from both ends. > [@Q.E.D.](#): > > You using low-octane fuel in that thing by any chance, **Dog**? Low-octane gas tends to ignite more easily than higher-octane stuff. That’s about the only thing I can think of. I use BP Ultimate 100 octane (RON). That is the best petrol you can get in Europe.