Topic: New bike. Wide open throttle kills motor

[Highwayhigh]

Hey guys. I'm new here and new to the world of motorcycles. Picked up my first bike yesterday. Nice, barn-in-the-middle-of-nowhere find. A 1978 Honda CB550K. It's nice and clean. Mechanically, fairly strong. I am replacing the chain and both sprockets tomorrow. (both are worn and chain slaps on decelleration) There is one more problem with the bike. Granted, it's fairly cold here, but I'm not too savvy on carbs past minor work with my dirt bikes as a kid. Anyways, the bike runs fairly strong, but whenever you quickly pin the throttle, the engine dies. It doesn't gurgle or struggle. It just kills it. Almost as if it's starving the motor out. This is in nuetral with the bike still or when I'm riding. It has the four air filter pods instead of the airbox. Guy put new 4-2 pipes on it. Said he went through the carbs not log ago and worked them over. Not sure what all that entailed. I plan on popping off the filters and trying it then to make sure they aren't starving the motor. Is there anything else I should look at or quick tricks before I have to pull the carbs off and tear them apart? Thank you in advance and I'm glad to finally join the community. Expect many more new guy questions. I've already started tearing through the faqs. Thanks again.

-Logan

[Shenanigans]

Do you have any experience riding anything with a mechanical slide carb? You cant just flail open mechanical slide carbs as it will kill the motor. Does it die when you slowly open the throttle to all the way open?

[bucky katt]

find a stock airbox and put it on. you may get a bit more power with properly tuned pods but it just isnt worth it to me. i run a stock airbox and a k+n filer on my k4 750 and it runs like a bat out of hell and has good throttle response.

[Shenanigans]

find a stock airbox and put it on. you may get a bit more power with properly tuned pods but it just isnt worth it to me. i run a stock airbox and a k+n filer on my k4 750 and it runs like a bat out of hell and has good throttle response.

Worst thing I ever did to my bike was to shave off the airbox mounts!

[TwoTired]

78 CB550K carbs are set lean at the factory which compensates for the stock restrictive muffler and the stock air box and filter.  With this combination the mixture is set just about perfect.

Changing the exhaust, makes the carbs settings too lean for the set up.  Changing the air box and filter makes the carb throat vacuum anemic, and that vacuum is what pulls the fuel through metering jets.

The carbs CAN be adjusted/tuned to compensate for the engine breathing changes.  Unknown if yours have been done correctly.

At idle, the mechanical slide are nearly closed and this makes a nice deep vacuum in the carb throats (this vacuum is what pulls fuel through metering jets in the carb).
Suddenly opening the slides (throttle whack) allows atmospheric pressure at the carb inlets to reach into the carb throat to where the metering jet exits are.  This effectively stops fuel from being mixed with inlet air.  The shorter inlet runners that POD filters bring to the party, make this vacuum loss far worse than the stock set up.

Even in stock form, you cannot expect the engine to respond from idle (particularly under load) when the slides are suddenly opened more than 1/2 of total travel.  If the idle mixture is left sufficiently rich, the engine will respond quite briskly.

The stock carbs will need quite a bit of attention to adapt to the inlet and exhaust changes, and still get decent response to 1/2 throttle travel twist.

[Highwayhigh]

I appreciate all the advice! I don't even know of the stock airbox hardware is still there. The covers are but I'm not sure. And is there any moderately reasonable way to get this bike up to snuff without a tedious carb workover? I haven't riddden a bike this old, but it did seem a TAD underpowered for what I would expect. Not sure if it's tuning or if that's just what this bike can do. Anyways, thank you guys again for your help. All the information given helps me a ton. And for the record, I don't have any experience with mechanical slide carbs. And the bike does accelerate smoothl through the rpms, just it doesn't respond well to sudden throttle inputs

[Kframe]

When resurrecting any old engine there is no excuse for not going through a 'tedious' carb cleaning.  
If you don't, you may have an engine that runs, but it likely won't run well and you'll end up with other issues that will have you second-guessing and adjusting other things trying to compensate for what may be as simple as a clogged circuit or a single out of sync carb.  
Take the time and effort to learn how to completely strip the carbs of the essential parts and clean thoroughly with a nasty dip or something, compressed air helps too.  
If you find rotten o-rings or corroded jets, folks here can help you source parts if you need them.  A great thing about Honda is that they continue to support a lot of their older vehicles.  Meaning readily available and reasonably priced OEM parts.
It may seem like a tedious and/or intimidating task right now, being new to bikes in general as you said, but you'll learn a lot about the inner workings, you'll know you've eliminated several variables if you do end up having poor running issues, and you'll end up with a happy bike and you'll be a happy rider.  
Every expert was a newb once.
-K

[TwoTired]

If the bike seems sluggish, first check the head pipes for even heat.  They aren't too peppy running on three cylinders or less.

Then, look at the spark plug deposits to get an idea of combustion conditions.

Go on from there...

[Highwayhigh]

Alright guys, bit of an update with the bike. I was going to get it registered today but unfortunately, my chain and both sprockets are shot, so she's down until Tuesday for parts. But I wanted to tinker with it more today to figure out what was happening with it. I noticed that it's only unresponsive up to about 3k rpm. Past that you can pin the throttle no problem. I ALSO noticed that at idle, carb three (looking left to right as you're sitting on the bike) is popping and spitting at idle. The header coming out of cylinder three was drastically cooler than the other three pipes as well. I could barely touch the others, but header three I could dang near hold on to it. The throttle response seems sluggish still, and when closing the throttle, the engine is a little slow to decelerate. Because the bike is down for parts, I intend to spend the rest of weekend pulling the carbs apart and cleaning them thoroughly. I also want to check the points and spark plugs. Pretty much just work over the whole bike. But as far as the carb and cylinder three thing, is there anything specific I'm looking for? Maybe it's running too lean? Thanks again for all the help guys. This has already been a tremendous learning experience.

[TwoTired]

For cylinder three, make sure the spark plug is sparking (maybe even swap it to another cylinder and see if the problem follows the plug).

After you are sure that spark is not an issue, and are comfortable assuming the compression in that cylinder is acceptable, then the carbs are the next to check.

Plugged pilot jet is a possibility.  Pretty small orifice in that, 0.016 inch.
Are the carbs getting clean fuel from the tank (rust)?

Have the carbs been vacuum synchronized?  Do a tappet clearance adjust first.

[david 750f]

Swap your 2&3 spark plug leads (they use the same coil) if the cool exhaust pipe problem switches cylinders then it is an ignition problem. If the #3 cylinder is still cold then it is a carb problem.
Good luck

[Highwayhigh]

Checked the plug for spark, which was good. All the plugs were brown but cylinder three was black. Cleaned it off, fired it up, still bad. Well I decided to pull the carbs to just clean them well anyways. Pulled the carbs and the bowl covers. I sprayed everything down with a healthy dose of carb cleaner. The float bowls had some gunk built up. The bike probably sat for a while and the guy didn't drain the carbs. But ya, put the carbs back on and fired it up, bike runs like a champ. So I'm guessing it was just some gunk built up in the jets or whatever. But I still have to wait until Tuesday for the rest of the parts to get this street worthy. This warm weather is going to pass me by! Thanks again for the help, guys. I have a lot of plans for the bike, so hopefully I can see them through. Next step - get rid of the hideous stock turn signals.

[Kframe]

Highwayhigh, if you're gonna stick around these parts for awhile, it'd be helpful for the others if you fill in as much of the profile info as you're comfortable with. 

Nobody cares about your street address, but state or city is helpful if people have parts or tools you may want to buy or borrow. 

-K

[Highwayhigh]

Done and done. Thanks for the advice.