Author Topic: CB550F Overheating?  (Read 661 times)

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Offline TheHulk

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CB550F Overheating?
« on: September 02, 2021, 06:12:33 AM »
Hey everybody. I recently rebuilt my 77 CB550F engine, guided by an experienced friend who has rebuilt numerous of the same engine.

When I fire up the bike, everything seems to sound and operate normally. After about 10 minutes of moderate city riding, I start to lose power and have pretty sputtery performance. No backfiring, but I can hear and feel a sluggish change in the engine and definitely have a loss of power. Static timing is spot on. Carbs have been cleaned, rebuilt, and synced. No vacuum leaks.

My first instinct was that the engine was overheating. The bike idles nicely around 900-1000rpm. I've taken some IR readings off of the cylinder head, above each piston, after a 15 minute ride. Cylinder head is unpainted. As you can see, they certainly aren't the same temperature.

But for the sake of this thread... What would be considered too high of a temperature? Should I be concerned with 250º temps? At what temps should I really be concerned?

Cylinder #1: 186º
Cylinder #2: 129º
Cylinder #3: 250º
Cylinder #4: 214º

Pulled #1 and #4 plugs for a quick inspection (no formal plug chop yet) and everything appears normal upon first look. No thick soot, but no ultra white tip.

Thinking I should make an adjustment to the slide needles to richen up the mixture, and try again. Each carb is only about 1/2-3/4 turn open. Thoughts? Thanks!

Offline ekpent

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Re: CB550F Overheating?
« Reply #1 on: September 02, 2021, 06:23:59 AM »
129 sounds too low.
« Last Edit: September 02, 2021, 08:31:28 AM by ekpent »

Offline TwoTired

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Re: CB550F Overheating?
« Reply #2 on: September 02, 2021, 06:25:59 AM »
What exhaust and intake set up do you have?

Do you still have the 069a carbs on it and have you kept the stock setting with original Keihin parts?

My concern is what the heat does to the oil.  And do have proper oil circulation.  And if the fins are at 250, what is happening to the oil inside?  Depending on the oil choice, it can start to break down above 300.

Did you end gap the rings to fit each bore?  And what gap did you use?

What method are you using to obtain those temp numbers?

Cheers,
« Last Edit: September 02, 2021, 06:35:02 AM by TwoTired »
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline TheHulk

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Re: CB550F Overheating?
« Reply #3 on: September 02, 2021, 07:00:28 AM »
What exhaust and intake set up do you have?

Do you still have the 069a carbs on it and have you kept the stock setting with original Keihin parts?

My concern is what the heat does to the oil.  And do have proper oil circulation.  And if the fins are at 250, what is happening to the oil inside?  Depending on the oil choice, it can start to break down above 300.

Did you end gap the rings to fit each bore?  And what gap did you use?

What method are you using to obtain those temp numbers?

Stock Keihin carbs. Some original parts, some from a rebuild kit.

Cylinders were bored and honed to +.5, with a David Silver Spares piston and ring kit, by a reputable engine shop in town.

Readings taken with an IR Gun at the top of the cylinder head (not the head cover) above each piston.

Offline TwoTired

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Re: CB550F Overheating?
« Reply #4 on: September 02, 2021, 04:35:29 PM »
Read about your temp test gun specifics.  The measurement spot size is not the red dot from the laser.  It is often a cone shape that fans with distance.  Mine measures a spot size of 1 inch diameter at 1 foot from gun.  The spot size increases with distance from gun.  So don’t think you are measuring only the spot painted by the dot.  And check if the laser is centered on the IR spot measurement area, as the laser is not centered in the IR detector, but rather in parallel, the cone and the beam will only converge at one point in the distance in front of it.  The owners manual should explain where that point is.

Carb Rebuild kits have pretty loose dimension tolerances, and many have no quality control measures to find out of tolerance specimens.  They leave those checks to the end user.  That’s why many here seek genuine Keihin replacement components in order to keep four separate carbs delivering equal fuel ratios throughout the various throttle settings.

Since you have an engine that runs, nothing is going to scream “I’m obscenely bad”.  The devil is in the details.  As in finding what is different among the cylinders.  It’s tedious.  But without details, finding culprits without being there, is going to be pretty difficult.  Particularly if questions or clarifications aren’t answered.

When I still had the airplane with a Lycoming 4 cylinder air cooled engine, the motor was instrumented at factory locations for cylinder head temperature.  300 to 375 was considered ideal operation @ 75 to 100 % power.  During climbs with lower air speeds, 475 was sometimes seen.  Before adding extra cowl flaps for more cooling air, I saw 500 and 525F for cylinder head temps.  That engine broke off an exhaust valve head to rattle about in the chamber before getting chucked into the exhaust port (saving the piston from being holed).  This engine was made to produce 150 horsepower continuously for 2000 hours.

When you took your temp readings, was there any airflow about the engine?  And what was the air temp surrounding the engine?

Since you recorded uneven temps among your cylinders, you need to refine your measurements for faith in their accuracy, or start looking for differences in the cylinder mechanical operation or the 4 carbs operation differences that feed cylinders that you KNOW are mechanically equal.

Good luck!
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.