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Onan 4.0 bf cr no vacuum


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  #1  
Old 11-15-2018, 10:34:02 AM
kbelle kbelle is offline
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Default Onan 4.0 bf cr no vacuum

I am in the process of converting this unit to burn propane/natural gas. I drilled the Carb at the upper boss and threaded a 1/8 inch NPT to 3/8 barb so it will provide vacuum to the KN regulator. Put everything back together ran great but no vacuum at the port. I understand the kn needs 1/2 psi but I dont think I am getting any vacuum when the choke is open. the unit runs great on gas. What am I missing?
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Old 11-15-2018, 10:41:13 PM
David C David C is offline
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Default Re: Onan 4.0 bf cr no vacuum

A picture of where you drilled the carb would be worth a thousand words. The hole has to be drilled below the venturi and above the throttle plate. If you drilled above the venture, the KN demand regulator will never see a vacuum.

David C.
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Old 11-16-2018, 05:38:26 AM
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Default Re: Onan 4.0 bf cr no vacuum

Is the throttle wide open?
If not, there will be little to no vacuum at the venturi without closing the choke.
To send the correct vacuum "signal" to the demand regulator, the carburetor must be drilled AT the venturi.
Not above. Not below.
If you drilled the large (upper) boss you're in exactly the right spot.
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Old 11-16-2018, 11:25:03 AM
kbelle kbelle is offline
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Default Re: Onan 4.0 bf cr no vacuum

Thank you for the responses. I have a Marvel Scribner carb and drilled the top large boss with 1/8 NPT with 3/8 hose nipple. Shutting the choke will instantly increase the vacuum but stall the engine. This am I pulled the intake and replaced the carb and intake gaskets. I also pulled the valve covers and set the valves to .005 intake and 13 exhaust --the valves were too tight. pulled the heads cleaned them up as well. from what I can see everything looks good. powered up started and ran great put it under a load 2--1500 watt heaters to get he engine warm. pulled the cap off the 3/8 nipple and the engine began to hunt. the vacuum gauge only trembles at below 0 lbs. the gauge is not so good and I cant feel vacuum on my finger. I am making progress but I doubt this is enough vacuum to pull the KN which is arriving latr today. I thought the engine would produce 18-20lb at idle.
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Old 11-16-2018, 11:52:49 AM
RETCPO RETCPO is online now
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Default Re: Onan 4.0 bf cr no vacuum

18-20 inches of vacuum will only be under the carburetor throttle plate and intake manifold. As asked for before, please provide clear pictures of the work you have done. It will help!
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Old 11-16-2018, 12:18:16 PM
Pete Spaco Pete Spaco is offline
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Default Re: Onan 4.0 bf cr no vacuum

Maybe need to do a little thinking about the vacuum terms and the actual range of the gauge:

"18-20 lb" sounds like psi
"18-20 inches vacuum" is probably inches of mercury.

But the propane input pressure at the final regulator will be, maybe about 10 "inches of water", won't it?


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Old 11-16-2018, 01:19:11 PM
Mac Sine Mac Sine is offline
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Default Re: Onan 4.0 bf cr no vacuum

Further to Pete's post, carburetor venturi vacuum is measured in inches of water column. This is a "weak" vacuum signal, compared to intake manifold vacuum (that is measured in inches of mercury).

The nominal setpoint of a KN demand regulator is to "break-open" (to allow gas to flow to the carburetor) at negative 1/2-inch of water column.
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Old 11-16-2018, 05:16:25 PM
kbelle kbelle is offline
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Default Re: Onan 4.0 bf cr no vacuum

My mistake yes it is inches of mercury not pounds but the fact still remains i still have very little vacuum according to the guage. I suspect I need to grind the valves and seats. I will post photos shortly thanks to all thanks k
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Old 11-16-2018, 05:29:05 PM
Wayne 440 Wayne 440 is offline
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Default Re: Onan 4.0 bf cr no vacuum

The "very little" vacuum at the venturi is probably normal, since it is upstream of the throttle plate.

FYI- .5 inH2O is 0.0367412 inHg - A typical automotive vacuum gauge probably won't budge.
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Old 11-16-2018, 08:17:52 PM
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Default Re: Onan 4.0 bf cr no vacuum

I suggest you do nothing more until you've connected a KN demand regulator, supplied with propane at the correct input pressure, which is about 1/2 pound PSI.
An ordinary BBQ regulator will probably work just fine.
You'll need a valve between the KN and the carburetor to regulate air/fuel mixture.
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Last edited by lokay5; 11-17-2018 at 06:01:31 AM.
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