Small Engines (Lawn Mowers, etc.): Small Propane-fueled engines, shaft position, screw out

Question
QUESTION: I have a custom engine, manufactured by B&S for Generac. 25Hp, V-twin, mounted vertically, and drives a 6.2kW alternator via belt drive. Has at best 14 hours on it. Mfd in 2001, last run in 2004. Stored in climate-controlled environment during this time. It runs on natural gas or propane. Starts & runs fine, but at ~3/4 power, it starts surging/hunting, Linkages nice & smooth, excellent fuel delivery via two regulators, one pressure, the other vacuum-demand. Any ideas what might be causing this?

ANSWER: Hello Richard:

If the Fuel Pressure is Correct, then the Main Jet has a Restriction. If this Carburetor has the High and Idle Air Mixture Screws, then Turn the High Mixture Screw Out 1/4 to 1/2 Turn and See if the Engine Responds. If it Does, then you May be Able to Operate the Engine foe a Short Time with the Carburetor Adjusted this Way and Do Not Damage. However, you will Eventually need to Clean the Carburetor. Even Propane and Natural gas have Corrosive Properties. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 9pm EST (US). My New Day Starts then. Thanks.

Respectfully

John

---------- FOLLOW-UP ----------

QUESTION: John:
That was fast! Thanks. Carb has no adjustment at all for fuel flow, thus, the demand-type regulator. The only adjustment there is is an overspeed limit screw, and limits the butterfly shaft position. Machine is clean enough to eat off of; have carefully removed carb and it is literally shiny
ANSWER: Hello Again Richard:

Are you Absolutely Sure About the Pressure Being Correct? On Propane and Natural Gas Carburetors, they Require a minimum of 0.2 Mpa of propane pressure. Translated, that means that you must supply a minimum of 30 psi to the generator, or it won't run Correctly. A Slight Pressure Loss Usually is the Cause the Problem you Described. Send me the Model Numbers Off the Engine, Please. With these I can Locate the Proper Manual and Better Assist you. I Require the OEM (Walbro, Zama, ect) and the Model Numbers Off the Carburetor. With these I can Locate a Breakdown of the Carburetor and Better Assist you. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John

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QUESTION: John:
You are one dedicated individual! I apologize for being a pain, and I recognize the value of your time. If this gets to be too much of a load, let me know and I'll go away. Your time can be better spent helping folks that have difficulties with which you are more familiar. Mine is not a device you will probably ever see.

As requested, here are data from the engine cover:

From the opposite side of the engine from the full-pressure oil pump:
"C1198"
"00260"

From the engine cover on the side with the oil pump:
Family:    YGNXS.7242NV:EM
Model:    04268-0
Serial:    3206992
Volts:    120 - 240
Amps:    51.6 - 25.8
Watts:    6,200
RPM:    3,600 (note: this is the max speed of the engine; the manual says it operates at 2,900)

I will not be surprised in the least if you find no information on this engine. It was a collaborative effort between Generac
ANSWER: Hello Again Richard:

I Do Work on the Engines, Just Not the Generator. You can Possibly View a Breakdown of the Engine at this Site Addy; http://www.generator-parts.com/ and Select the Manual for the V Twin Horizontal and Vertical Engines Under the Service, Diagnostic and Parts Manuals Section. You can Purchase a Manual for this Engine at this Site Addy; http://shop.briggsandstratton.com/BShopProductListingPage.asp?MecID=100&CatalogI. The Manual Shown is the One for this Engine. I have Found that a Small Amount of Dust can Restrict the Gas Flow througn the Carburetor and Simply Blowing Out the Internal Passages will Usually Correct this Problem. I am Assuming the Demand Regulator and the Pressur Regulator are Operating Correctly. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John

---------- FOLLOW-UP ----------

QUESTION: John:
Tnx for the reply. Looked at variety of manuals; while very informative, did not find anything on troubleshooting LPG-powered engines, particularly the surging. Found info on carb and 2ndary regulator systems. Will have to hunt some kind of pressure gauge I can connect to find out what's really there. Done for now. Will get back to you when I find the solution. If you find anything, please advise.

Answer
Hello Again John:

The Surging is Caused by Inproper Fuel FLow at Higher RPMs. I will Get Back to you if I Locate Anything. Till Next Time. May All Your Mechanical Problems be SMALL ONES and May the All Mighty Bless You and Yours. Thanks Again.

Best Wishes and Good Luck

Respectfully

John