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Author Topic: High rpm sputter, compression seems bad  (Read 853 times)

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Offline valleycb

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High rpm sputter, compression seems bad
« on: June 29, 2018, 01:09:41 PM »
Hey, I recently rebuilt a 78 CB550.  Full engine tear down, new rings, cylinders honed, valves dressed, new o-rings and gaskets, carbs synced.  The bike runs well.  Very well actually, until high rpms under load.  It's fine in neutral up to god know what rpm, but on the highway it just seems to run out at 6000rpm.  The last bit of throttle doesn't yield any response.  Some days I can only go 100km/h.  Some days 120km/h.  I've tried a number of things to troubleshoot.  Finally got around to a compression test today.  I got 95, 95, 95, 105 dry.  Didn't seem any different wet.  No so good eh?  Is this my problem?  Is this how the bike would run with low compression?  It starts well.  Any input would be appreciated.  Thanks!

I'm thinking I screwed up my valve job.
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Offline valleycb

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Re: High rpm sputter, compression seems bad
« Reply #1 on: June 29, 2018, 01:18:57 PM »
Hang on...a buddy told me I was doing my compression test wrong.  Didn't open the throttle.  Now I get 135, 130, 120, 135.  Thoughts?
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Offline Bankerdanny

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Re: High rpm sputter, compression seems bad
« Reply #2 on: June 29, 2018, 02:06:46 PM »
Those readings look fine. I would not be concerned about the 120 on the one.

Your bike should comfortably cruise at 120kph all day long with power in reserve. It sounds to me like you are leaning out, which is why you have no top end.

I would look very hard at float heights (too low means not enough fuel at high RPM's) and make sure that the main jet and emulsifier tube on your carbs are perfectly clean.

Ignition advance could be an issue. I would also make sure that your ignition advance is working properly.
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Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki K10 80, '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline TwoTired

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Re: High rpm sputter, compression seems bad
« Reply #3 on: June 29, 2018, 02:09:02 PM »
Stock exhaust and induction components?

Honda used a small engine compression tester for their numbers, which adds very little volume to engine cylinders.  See the engine FAQ.

If you suspect a valve issue, try a leak down tester.  You can then hear them leak.

What do spark plug deposits look like?

Cheers,
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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

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Offline robvangulik

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Re: High rpm sputter, compression seems bad
« Reply #4 on: June 29, 2018, 03:51:54 PM »
With low compression, the bike will run bad at low revs, and only make (some) power ar high revs.
This reads as wrong mainjets!
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Offline uksparky

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Re: High rpm sputter, compression seems bad
« Reply #5 on: June 29, 2018, 04:53:30 PM »
I had the same problem with my 550, took me ages to figure it out..... when i bought the Honda it had the foam air filter, went back to the paper filter problem solved....
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Present bike 1982 900C Custom
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