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https://yabb.autonerdz.com/cgi/yabb2//YaBB.pl Picogroup Area (contact administrator for access) >> Picogroup Motorcycle Discussion >> Intake Vacuum Waveform https://yabb.autonerdz.com/cgi/yabb2//YaBB.pl?num=1711932306 Message started by James H on Mar 31^st, 2024 at 4:45pm

Title: Intake Vacuum Waveform
Post by James H on 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%

Cyl_3_Green_compared_Cyl_4_Red.png (103 KB | )

Title: Re: Intake Vacuum Waveform
Post by rayray on 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.

Title: Re: Intake Vacuum Waveform
Post by StitchB on 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?

Title: Re: Intake Vacuum Waveform
Post by James H on 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.

Sec_V__InVac__FLS_Comparison.png (162 KB | )

Title: Re: Intake Vacuum Waveform
Post by rayray on Apr 5^th, 2024 at 9:40am

Can you post your PSD files

Title: Re: Intake Vacuum Waveform
Post by James H on Apr 5^th, 2024 at 10:31am

Thanks for having a look. On drop site. James Hunt GS750E

Title: Re: Intake Vacuum Waveform
Post by rayray on 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.

Title: Re: Intake Vacuum Waveform
Post by James H on 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%

Title: Re: Intake Vacuum Waveform
Post by rayray on Apr 7^th, 2024 at 4:37am

Can you check the valve lift, it may need a valve adjustment.

Title: Re: Intake Vacuum Waveform
Post by James H on Apr 7^th, 2024 at 6:59am

Camshaft lobes look mint, clearance was checked and is correct.

Title: UPDATE Re: Intake Vacuum Waveform
Post by James H on 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.

Head_gasket.jpg (89 KB | )

Head.jpg (3526 KB | )

GS750_Intake_vacuum__FLS.png (242 KB | )

Title: Re: Intake Vacuum Waveform
Post by rayray on Apr 23^rd, 2024 at 3:30am

Good work and thank you for the update!

Title: Re: Intake Vacuum Waveform
Post by JÜJA on 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.