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James H Senior Member Picogroup Offline Bike Nerd Posts: 165 Waterloo Ontario Canada Gender:	Intake Vacuum Waveform Mar 31^st, 2024 at 4:45pm This is from a motorcycle engine so intake is separate with its own carb and no airbox. Red is off of a good known running cylinder. The other green waveform runs but runs weak at idle due to low vacuum and picks up as revs climb. Thoughts on what is causing this? Bit confused as the bad cyl has good compression and leak down of well under 10%
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rayray LSD Guide Picogroup Offline Posts: 2,207 Toronto Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #1 - Apr 1^st, 2024 at 5:13am To compare intake stroke vacuum pulls, the engine should be cranking. Put a pulse sensor in the tailpipe with the engine cranking, to compare the exhaust stroke pressure pulses. Put an amp clamp on one of the battery cables to do a relative compression test.
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StitchB Super Nerd Picogroup Offline Posts: 1,314 lake stevens Washington USA Gender:	Re: Intake Vacuum Waveform Reply #2 - Apr 1^st, 2024 at 5:52am James, could we get a file on this. Was wondering what is the gasket between the head and the carb face? Seems to straighten out when reves come up is why I ask? Ray Rays comment are 4 sure relevant?
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James H Senior Member Picogroup Offline Bike Nerd Posts: 165 Waterloo Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #3 - Apr 4^th, 2024 at 6:26pm Here it is under cranking with ign syc and fls in exhaust. From the top. Good cyl ign sync, Bad cyl ign sync, Good cyl intake vacuum, Bad cyl intake vac, Good cyl exhaust fls, bad cyl exhhaust fls The intakes on both cyls are not connected (do not share a common manifold, the exhaust is however.
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rayray LSD Guide Picogroup Offline Posts: 2,207 Toronto Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #4 - Apr 5^th, 2024 at 9:40am Can you post your PSD files
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James H Senior Member Picogroup Offline Bike Nerd Posts: 165 Waterloo Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #5 - Apr 5^th, 2024 at 10:31am Thanks for having a look. On drop site. James Hunt GS750E
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rayray LSD Guide Picogroup Offline Posts: 2,207 Toronto Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #6 - Apr 6^th, 2024 at 1:35pm Excellent captures. Cylinder 3 cranking vacuum at 3.9 psi and max compression at 103 psi Cylinder 4 cranking vacuum at 6.8 psi and max compression at 116 psi I would try to compare the compression blow-by pressures in the crankcase, engine cranking and or running.
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James H Senior Member Picogroup Offline Bike Nerd Posts: 165 Waterloo Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #7 - Apr 6^th, 2024 at 4:56pm Thanks, I will check blow-by however weird thing is leakdown test Cyl 3 is 6%, Cyl 4 is 11%
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rayray LSD Guide Picogroup Offline Posts: 2,207 Toronto Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #8 - Apr 7^th, 2024 at 4:37am Can you check the valve lift, it may need a valve adjustment.
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James H Senior Member Picogroup Offline Bike Nerd Posts: 165 Waterloo Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #9 - Apr 7^th, 2024 at 6:59am Camshaft lobes look mint, clearance was checked and is correct.
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James H Senior Member Picogroup Offline Bike Nerd Posts: 165 Waterloo Ontario Canada Gender:	UPDATE Re: Intake Vacuum Waveform Reply #10 - Apr 22^nd, 2024 at 5:35pm For anyone who might care It appears the issue causing the problem is a leaking head gasket between Cyl 3 and 4. The motor is air cooled. I am confused as to why leakdown was decent on both cyl however. If I am looking at things correctly Cyl 4 was causing a small positive pressure in Cyl 3 intake.
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rayray LSD Guide Picogroup Offline Posts: 2,207 Toronto Ontario Canada Gender:	Re: Intake Vacuum Waveform Reply #11 - Apr 23^rd, 2024 at 3:30am Good work and thank you for the update!
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JÜJA

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Re: Intake Vacuum Waveform
Reply #12 - Apr 24^th, 2024 at 4:58am

Thank you for the feedback James.

Just looking at your last picture with the overlay I can see an increase of around 1 psi in #3 intake waveform. Also #3 intake vacuum not only is significantly lower during the intake stroke but in addition it stays low for a shorter period time/rises earlier compared to #4.

The achieved vacuum levels while cranking, in particular of #4, look surprisingly high to me. The high cranking speed of well above 300 rpm may, partially, cause this.

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