MotorcycleRepairFilter: Why does my engine struggle to get above 5500rpm?
May 19, 2010 9:24 AM   Subscribe

MotorcycleRepairFilter: Why does my engine struggle to get above 5500rpm?

This is my fourth season with this bike, and its begun doing some things it hasn't done in the past. Here are my symptoms:

1) The engine struggles to get above 5500rpm. As I approach that magic number, the engine starts to chug-chug as if its not getting enough fuel. I can sometimes get up to 6000rpm by laying on the throttle and 'pushing through it', so to speak. But I fear doing that very often because I don't know what I'm actually doing to the engine.

2) After trying to get above 5500rpm, and then decelerating, the exhaust pops. From what I've googled, this means that the air and fuel mixture is not getting ignited in the engine and is instead being pushed out through the exhaust, and it somehow ignites there.

3) When I park the bike after a ride, I get a strong gasoline smell. Stronger than I remember from the last few seasons.


What's going on? Is there a problem with the air/fuel mixture? Could I have a leak in my fuel line that only evidences itself above certain speeds?

Could this just be related to temperature? I am riding earlier in the season than I did last year, and it's been colder in Chicago for longer. My bike does seem to be temperature sensitive. Maybe the carburetor is too cold to push enough fuel?

Is this something I can investigate myself, or should I just head down to the (very busy) mechanic?

Thanks for all responses. The bike is a 1986 Kawasaki 454 LTD.
posted by lholladay to Travel & Transportation (12 answers total) 1 user marked this as a favorite
Sounds like a bad plug wire or loose connection between the wire and spark plug cap.
posted by jon1270 at 9:35 AM on May 19, 2010

Best answer: I'll put $5 on your carbs. Sticking, clogged, whatever.

Did you leave a gas stabilizer in it over the cold months or run a tank or two through it every month? Sitting gas turns to sticky sludge and can cost a ton to fix (especially on bikes with multiple carbs..sigh)
posted by anti social order at 9:47 AM on May 19, 2010 [1 favorite]

Best answer: Will it spin past 5500 rpm in neutral? IE, is it only a problem under load?

Could be ignition related, but carb issues can definitely cause it to run way too rich at high RPM. The crackling on overrun is also a sign of it being rich. The gas smell is definitely a sign of it running rich.
posted by pjaust at 9:48 AM on May 19, 2010

i came here to guess plugs/wires as well. your fuel is not asploding. plugs/wires is where i'd start troubleshooting.
posted by spikeleemajortomdickandharryconnickjrmints at 9:49 AM on May 19, 2010 [1 favorite]

Response by poster: anti social order: I store the bike with my mechanic over the winter, who takes care of all winterizing stuffs. I'm assuming he did that. But perhaps I'll call and check.

pjaust: I'll have to test it in idle - I haven't done that yet.

jon and spikelee - Spark plug wires, gotcha. I'll take a look at those.

I may look at this stuff during lunch, so I'll report back in a few hours. Thanks for the responses so far!
posted by lholladay at 9:55 AM on May 19, 2010

Response by poster: Well, in neutral, I can get to 7000rpm without an issue. Can I assume that knocks out ignition problems, leaving me with carb related things?
posted by lholladay at 10:13 AM on May 19, 2010

Best answer: Popping on deccel and struggling at high revs indicate "leanness" in the circuit. Open up your carbs and clean them. Something is gummed up somewhere. There should be a guide somewhere on the net for your particular carb set.

Go ahead and slap some new plugs on it too, it'll run like new after you're done, and you might have overheated them running them lean.
posted by 1f2frfbf at 10:21 AM on May 19, 2010

Response by poster: Looks like carbs is the best bet - I'll look into cleaning them myself, but it may be beyond my meager mechanical abilities. Maybe I'll just stay off major interstates this season. :) Thanks all for the help.
posted by lholladay at 11:00 AM on May 19, 2010

but it may be beyond my meager mechanical abilities.

Oh no. It's easy. At the very least you need: a screwdriver, a small adjustable wrench, a can of spray carb cleaner, an ice cube tray and a digital camera.

You may want to buy a new set of gaskets for the carb, it should be cheap and it's one less thing to worry about. If you tear a gasket in removal, no biggie, you've got a new one right there.

Step 1: Find a screw or bolt, remove it.
Step 2: Photograph screw in hole, then place screw in 1st spot in ice cube tray.
Repeat steps 1 and 2 religiously until all screws are out and documented and each is in it's own depression in the tray.

Now look at the weird piece of aluminum and brass you're holding in your hand. Carefully remove all plastic, brass and rubber bits from it, documenting where they go with the digital camera and putting any small pieces in the ice tray. Most things will simply screw out, some things may be press fittings, leave them be. Absolutely no hammering or banging is allowed. If it doesn't come out easily, leave it, and clean it in place.

Next, take your can of carb cleaner and put the red straw on it, and spray into every little hole you can find. Wear eye protection and spray until whatever hole you're spraying in sprays clear. If a particular hole is completely clogged, you can very very gently explore it with a guitar string.

Now let the whole mess dry off in the sun and carefully wipe down all the pieces you've removed with a soft cloth. If they're metal and dirty you can use some cleaner on the cloth, but under no circumstances get it on anything rubber or plastic.

Now, using your pictures on your digital camera and working backwards put everything back together. Extra pieces are bad. If there's something you didn't document (it'll happen, sorry) you can either try to figure it out, or you can disassemble another carb to look, or you can try and find an exploded drawing of the carb online. Shouldn't be too big a problem. Most carbs are very simple inside, you'll have floats, needles and jets (which are tubes with small openings).

When you tighten all the screws, don't lean on them too hard, you'll either strip the head (bad) or strip the threads in the very soft aluminum (worse). Finger tight is usually good enough for interior parts.

Now, put the carb(s) back on the bike and turn the fuel on. Do they leak? No? Good. If they drip a little, tighten the screws up a little more, if the leak persists, you may have torn a gasket, if they stream fuel, you forgot something. Gas needs to be only on the inside. Very important.

Now would be a good time to buy new inline sintered fuel filters so you don't have to do this again for a while. It's $5 well spent.

Finally, ride and be happy! Don't forget to brag about your awesome mechanical skills at keeping a cool old vintage bike running to all your friends, you've earned it.
posted by 1f2frfbf at 11:37 AM on May 19, 2010 [5 favorites]

Oh and here's an exploded image worth at least a thousand words.
posted by 1f2frfbf at 11:52 AM on May 19, 2010

Link fail. Try again.
posted by 1f2frfbf at 11:54 AM on May 19, 2010

Response by poster: With that description and the exploded view, I may just have the confidence to try it myself. I just grabbed a rebuild kit off eBay - we'll see how it goes!
posted by lholladay at 12:10 PM on May 19, 2010

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