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Vacuum Guage Test Results
Hello,
If you remember the previous thread "Fuel Starvation, Arg!", the problem was that after running in forward at full throttle for 12 seconds, the engine would bog down to about half thrust.
After checking and eliminating the fuel intake mesh, fuel filter, fuel line leaks, installing a new (and second) fuel pump, the problem persisted. Even tried a seperate portable gas can to further eliminate all of the above.
So Don Moyer suggested that it might have something to do with the intake manifold vacuum pressure. He very kindly sent me a prototype kit of a vacuum test guage.
It consists of a vacuum dial guage that reads up to 30mm Hg. The dial connects to a plastic tube which attaches to a barb fitting that screws into the place where the carb scavenge tube fits into the intake manifold. So, as the engine calls for fuel, you can measure the vacuum demand.
I installed the gauge and ran some tests.
Not knowing what I should expect, here are the data:
Throttle Gear Pressure
Idle Neutral 15mm
Idle Forward 15mm
Idle Reverse 15mm
Mid Neutral 15mm
Mid Forward 10mm
Mid Reverse 15mm
Full Neutral 15mm
Full Forward 3mm
Full Reverse 15mm
As you can see from the results, the intake manifold pressure drops under forward load.
What does THAT mean? Could it be possible that my packing gland is too tight, causing friction and heavy load after 12 seconds of running in forward? Then a rest lets water re-lubricate the flax, then the cycle repeats? Is that possible?
I can sustain high rpm in neutral all day long.
What a puzzle!
__________________________________________________ _________
Here is Don's response:
"Rick,
Your vacuum readings in forward are very consistent with Universal’s test stand values which were 16” at idle, 14” at half throttle, and 3” at full throttle.
My experience (and hence my understanding) of vacuum analysis is very limited. I can only relate to manifold vacuum values as a function of throttle position and RPM; e.g. it is at a maximum when the throttle valve is closed with the engine “sucking” against the almost closed throttle valve at idle (16”), and at a minimum when the throttle valve is fully open (3”) and the engine is sucking air through the open carburetor throat (and whatever dirt may be clogging the flame arrestor screen).
You don’t report your RPM, but Universal’s vacuum data relates to 500 RPM at idle, 1500 RPM at mid throttle, and 3500 at wide open throttle (WOT). I should note that this is test stand data where Universal had complete control over the load they placed on the engine which would have been 12 HP at 1500 RPM and 30 HP at 3500 RPM.
A couple notes on Universal’s RPM data; (1) we find that 500 RPM is a bit low for idle with 800 being more comfortable for in-service conditions and (2), there is no known prop that allows us to duplicate their WOT of 3500 RPM and 30 HP but WOT is WOT and I would think that 3” would be a reasonable value at any reasonable RPM in the range of 2500 and above.
With respect to vacuum values with a partially blocked exhaust, the guide I forwarded to you from our Internet search indicates that if you go to 2500 RPM for 15 seconds and the vacuum drops, and then does not rebound instantly when you retard the throttle to idle; it is a good indication of a restricted exhaust. The guide does not specify if the engine should be under load or not, and I don’t recall whether or not your 10 second running time is in neutral or under load, but I took the liberty of calling a retired Universal engineer with whom I have maintained contact and he speculates that if you don’t get the classic symptoms of a blocked exhaust in neutral that you should try the check under load which might show up as a more dramatic drop in vacuum during the latter part of your 10 second running time.
Don"
__________________________________________________ ____________
Finally, since my vacuum pressure drop seems to be normal, why does my engine still bog down after 12 seconds of full throttle in forward gear?
I'm going to loosen the packing gland and see if that helps.
-Rick
If you remember the previous thread "Fuel Starvation, Arg!", the problem was that after running in forward at full throttle for 12 seconds, the engine would bog down to about half thrust.
After checking and eliminating the fuel intake mesh, fuel filter, fuel line leaks, installing a new (and second) fuel pump, the problem persisted. Even tried a seperate portable gas can to further eliminate all of the above.
So Don Moyer suggested that it might have something to do with the intake manifold vacuum pressure. He very kindly sent me a prototype kit of a vacuum test guage.
It consists of a vacuum dial guage that reads up to 30mm Hg. The dial connects to a plastic tube which attaches to a barb fitting that screws into the place where the carb scavenge tube fits into the intake manifold. So, as the engine calls for fuel, you can measure the vacuum demand.
I installed the gauge and ran some tests.
Not knowing what I should expect, here are the data:
Throttle Gear Pressure
Idle Neutral 15mm
Idle Forward 15mm
Idle Reverse 15mm
Mid Neutral 15mm
Mid Forward 10mm
Mid Reverse 15mm
Full Neutral 15mm
Full Forward 3mm
Full Reverse 15mm
As you can see from the results, the intake manifold pressure drops under forward load.
What does THAT mean? Could it be possible that my packing gland is too tight, causing friction and heavy load after 12 seconds of running in forward? Then a rest lets water re-lubricate the flax, then the cycle repeats? Is that possible?
I can sustain high rpm in neutral all day long.
What a puzzle!
__________________________________________________ _________
Here is Don's response:
"Rick,
Your vacuum readings in forward are very consistent with Universal’s test stand values which were 16” at idle, 14” at half throttle, and 3” at full throttle.
My experience (and hence my understanding) of vacuum analysis is very limited. I can only relate to manifold vacuum values as a function of throttle position and RPM; e.g. it is at a maximum when the throttle valve is closed with the engine “sucking” against the almost closed throttle valve at idle (16”), and at a minimum when the throttle valve is fully open (3”) and the engine is sucking air through the open carburetor throat (and whatever dirt may be clogging the flame arrestor screen).
You don’t report your RPM, but Universal’s vacuum data relates to 500 RPM at idle, 1500 RPM at mid throttle, and 3500 at wide open throttle (WOT). I should note that this is test stand data where Universal had complete control over the load they placed on the engine which would have been 12 HP at 1500 RPM and 30 HP at 3500 RPM.
A couple notes on Universal’s RPM data; (1) we find that 500 RPM is a bit low for idle with 800 being more comfortable for in-service conditions and (2), there is no known prop that allows us to duplicate their WOT of 3500 RPM and 30 HP but WOT is WOT and I would think that 3” would be a reasonable value at any reasonable RPM in the range of 2500 and above.
With respect to vacuum values with a partially blocked exhaust, the guide I forwarded to you from our Internet search indicates that if you go to 2500 RPM for 15 seconds and the vacuum drops, and then does not rebound instantly when you retard the throttle to idle; it is a good indication of a restricted exhaust. The guide does not specify if the engine should be under load or not, and I don’t recall whether or not your 10 second running time is in neutral or under load, but I took the liberty of calling a retired Universal engineer with whom I have maintained contact and he speculates that if you don’t get the classic symptoms of a blocked exhaust in neutral that you should try the check under load which might show up as a more dramatic drop in vacuum during the latter part of your 10 second running time.
Don"
__________________________________________________ ____________
Finally, since my vacuum pressure drop seems to be normal, why does my engine still bog down after 12 seconds of full throttle in forward gear?
I'm going to loosen the packing gland and see if that helps.
-Rick
and then it flops out of the way at low power again.
. 
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