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Onan 305 CCK Contractor Tri-fuel Gas Conversion Question


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  #1  
Old 09-17-2018, 01:46:38 PM
sbbloom69 sbbloom69 is offline
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Default Onan 305 CCK Contractor Tri-fuel Gas Conversion Question

HI,
I got my new to me 1961 low hour 305 CCK running well (120/240). I've run it up to 3.6kW and I got the governor / vacuum booster set pretty well with about 2.5 HZ drop from no-load to full load. I've run it about 4 hours now with NO problems on gasoline.

I want to add gas (LP and NG). I have the demand regulator, a propane regulator and most of the fittings I need. I'm going to play with LP first, then work on an extension from my natural gas dryer line out to my porch.

I've read as many of the gas conversion articles as I could find here and elsewhere, specifically for the CCK and the Marvel Schebler VD factory tri-fuel carb. I've also considered the Zenith $110 carb on ebay. My 305 CCK already has the short points box, so the drilling out of the VD carb at the "correct" boss location and adding the gas connection there is an option for me. I've also looked at JohnnyC's straight penetration into the carb throat as an option.

In a perfect world, I would look for the factory Marvel Schebler tri-fuel carb. Or as a second attempt, look for the gas adapter block that bolts to the back of the carb after drilling the gas port and drilling and tapping the two screw holes.

Now my question: Onan added the small gas tubing from the gas adaptor block around to the front of the carb, where the extra hole is drilled at the same throat level as the gasoline idle mixture screw (I understand how the gasoline carb works, and that on a generator, it's really a "unloaded" mixture screw).

If I do the factory gas connection port, but with cobbled parts for the adapter block and gas mixture screw, how "important" is running the gas idle tube to the front of the carb (and drilling out the casting into the throat that's there)??

The carb kits that simply add a tube down the throat don't seem to NEED the idle/unloaded gas line, yet Onan specifically included it.

I'll probably start with just making a new flexible air cleaner-to-carb joint and try the tube down the throat (like JohnnyC's on the later engine) just to start getting experience. But I would eventually like to go the permanent dual port / Tri-fuel connections.

Should I worry about the idle tube to the front of the carb?

Anyone have a factory tri-fuel carb to sell? or at least the gas adapter block?

Stu
Los Alamos, NM
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  #2  
Old 09-17-2018, 10:13:12 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: 305CCK Contractor Tri-fuel Gas Conversion Question

I no nothing about your questions however, just to give you a heads up, you will loose about 10% output give or take when using propane or NG.
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Old 09-17-2018, 11:30:46 PM
JohnnyC JohnnyC is online now
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Default Re: 305CCK Contractor Tri-fuel Gas Conversion Question

I'm reading your question, but I don't fully understand what you are writing about to give you an answer. Maybe it's very late and I'm about fall asleep, but watch the following video. In the video I quickly rigged up my propane tank to my little 2LK to get it running since the carburetor was having problems running on gasoline. The 2LK ran full output w/o any problems. The rigged propane system is not a permanent thing in this case. I just wanted to test my 2 LK after restoration any which way I could fire up the motor.
Maybe this might offer you some ideas.


https://www.youtube.com/watch?v=bbHG36N4zHQ

JohnnyC
New Jersey
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Old 09-18-2018, 11:17:12 PM
sbbloom69 sbbloom69 is offline
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Default Re: 305CCK Contractor Tri-fuel Gas Conversion Question

Thanks for the replies. I read your posts specifically, JohnnyC. My detailed question was specifically geared towards the factory Marvel Schebler VD gas / gasoline tri-fuel carbs. They have the bolted-on load block on the back of the carb, near the short points block (late 50's to late 60's CCK engines). I have a spare carb. The locations for the drilled-out ports to convert a gasoline only carb to a tri-fuel carb are cast into the back of the VD carb body. There are several posts on how to drill out the carb on the site. This makes the Marvel Schebler carb basically the same as the propane / NG conversion kits that can be bought for $200. The factory Marvel Schebler gas carb has the cast load block that bolts onto the back of the carb body, after the gas ports are drilled out (and the two threaded holes to hold the block mounting screws). The load block contains the gas adjustment screws, just like the conversion kits. Your setup uses the ball valve to set a good running mixture.

The factory Marvel Schebler gas carb has a secondary, very small copper tube that comes out of the load block, and runs around to the front of the carb, near the main and idle jet needles. It is to supply gas to the down-stream side of the throttle plate (acts as an idle circuit). The factory load block has an additional small idle adjustment screw.

Onan and Marvel Schebler seemed to think it was important to have this extra idle circuit in their design. My question to you, and anyone else who has added to or converted a Marvel Schebler carb to gas, is this extra idle circuit that important. It seems the conversion kits work just fine without it. You seem to have had satisfactory luck just running your gas line straight into the carb throat, using your ball valve as a rudimentary mixture valve (same as the load block on the conversion kits).

I'm getting the rest of the fittings tomorrow to build a simple setup like yours (I already have a HP to LP primary regulator, and the Garretson demand regulator). I'm going to fabricate a modified carb intake hose adapter to accept the gas line. I don't want to ruin my old rubber intake hose while experimenting.

My hole point with my original the question, is that I'd like to mimic the factory setup and add the gas line to the place already made for it in the Marvel Schebler carb. This will keep the gasoline capability, and give me a clean/factory gas installation without hanging a big ball valve off the carb intake.

Like you, I'm still experimenting. Once I get the propane working, I'm going to extend the natural gas line out of my laundry room out to my porch and try that with the CCK.

Thanks for all of your great photos and documentation. It makes it much easier for the rest of us to take the chance and try things. Hell, that's what makes this whole site great. I took two dead almost-free early 60's 305CCK generators and turned them into fine running, reliable machines.

Stuart
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Old 09-19-2018, 12:32:58 AM
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Jim McIntyre Jim McIntyre is offline
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Default Re: 305CCK Contractor Tri-fuel Gas Conversion Question

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Originally Posted by sbbloom69 View Post
...My question to you, and anyone else who has added to or converted a Marvel Schebler carb to gas, is this extra idle circuit that important?
Based on my experience, no.

More here:

https://www.smokstak.com/forum/showthread.php?t=75460

and here:

https://www.smokstak.com/forum/showthread.php?t=113911
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Old 09-20-2018, 09:46:37 AM
sbbloom69 sbbloom69 is offline
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Default Re: 305CCK Contractor Tri-fuel Gas Conversion Question

JohhnyC, I have followed your Vap Induction threads. I have an earlier CCK model that does NOT have the metal guard around the intake tube. In your photos, it looks like your intake fitting just out of the adjustment ball valve, is just stuffed through the 5/8 hole you drilled into the intake. Is that the case? or do you have a spud tube connected to the fitting to get the gas down nearer the carburetor's venturi throat? Based on all of the cobbled together gas conversions I've seen on Youtube, I'm not sure it matters. I just want to get the best gas conversion that I can with minimal modification to the CCK. Please let me know if you jut have the fitting inserted above the carb, or if it has a tube going down the carb throat. I couldn't find any other photo details.

Thanks,
Stuart
Los Alamos, NM
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Old 09-20-2018, 11:07:20 AM
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lokay5 lokay5 is offline
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Default Re: 305CCK Contractor Tri-fuel Gas Conversion Question

Keep in mind that sticking a crude tube down into the venturi will reduce the carburetor's normal air flow. The Marvel-Schebler adapter feeds the gaseous fuel directly to the venturi from the side.
Also, a tube running into the air horn to the venturi will render the choke basically useless.
There are really only two ways to properly convert a gasoline carburetor to also use gaseous fuel while leaving the gasoline portion fully functional and they are
1. The "Snorkle Kit" which attaches a venturi to the airhorn inlet of the gasoline carburetor.
or
2. The Marvel-Schebler tri-fuel VD series carburetor/mixer

Not to say the Mickey Mouse cobble-jobs with their JB Weld, water faucets and garden hoses don't work, they're just not...uh..."kosher".
Since the throttle plate on an unloaded or lightly loaded 1800 RPM Onan engine is only slightly open at what would ordinarily be a fast idle, it is running primarily on the idle circuit.
When a sudden load is applied and the throttle instantly moves to wide open the gaseous idle feed tube eliminates the stumble that is somewhat common with snorkel adapters.
I have an Onan BF and an NH both with Marvel-Schebler tri-fuel carburetors so I am speaking from experience, or perhaps I'm a little biased toward a factory designed system.

My two cents.
Signing off.


(You're welcome go to my SmokStak Gallery for an up- close view of the Marvel Schebler gaseous carburetor and the adapter)
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Old 09-20-2018, 12:25:55 PM
Mac Sine Mac Sine is offline
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Default Re: Onan 305 CCK Contractor Tri-fuel Gas Conversion Question

To build upon Lokay5's explanation, the need for an idle fuel circuit comes about when an engine is operating at no-load and light-load conditions where the throttle plate is closed against the idle speed stop or partially-open from the idle speed stop. At those conditions, the air flow through the venturi throat is low, and the resulting vacuum signal developed in the venturi and communicated to the demand regulator is correspondingly low, therefore, the engine will not receive the air/fuel ratio needed for consistent operation. In the idle circuit, with the fuel port located downstream of the throttle plate, the vacuum signal is strong at closed or nearly-closed throttle positions and fuel will be inducted through the idle circuit.

As engine load increases and the throttle is opened (by the governor) in response to the increasing load, the increase in air flow through the carburetor venturi results in an increase in the venturi vacuum signal and, correspondingly, more fuel is inducted through the "main jet" in the venturi, until, approaching wide-open throttle, all of the fuel required is being inducted through the main jet at the venturi.

As noted previously, if your engine is not run at light-load conditions, you may be able to omit an idle fuel circuit from your conversion. If the engine is unstable / runs rough at part-load conditions, this will be an indication that you will need to incorporate an idle fuel circuit in your conversion.
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Old 09-20-2018, 07:17:22 PM
sbbloom69 sbbloom69 is offline
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Default Re: Onan 305 CCK Contractor Tri-fuel Gas Conversion Question

Thanks for the information, Mac Sine. I understand how the carbs work, and the reasons for the locations of the various orifices and passages up and downstream of the throttle plate, I was just trying to get a feel for the need of the gas fuel idle circuit. It looks like the depends on whether your generator is lightly loaded or not.

I have a spare Marvel Schebler VD carb. I've seen the pictures of how the ports are drilled out in the VD gas carb and how the load block is bolted on, and how the small gas idle tube runs around to the other side of the carb, specifically to provide the idle circuit.

I read where someone asked about the size of the idle gas port, but I never saw the answer. I gather that the brass/copper tube is just press-fit into the carb body next to the main jet needle? I guess it could be rather large, since the factory load block has an idle needle valve itself to set the gas flow.

I would like to mod my VD carb to add the factory port location. I suppose I could machine or modify a fitting for the load block, since I've never seen one come up on ebay or here. I tend to be a purist when resurrecting (or restoring) machines. But, I have to pick and choose my battles. I like to get them running first, then decide on how far to take the cosmetics.

I have quite a bit of carburetor experience. I have two 6-cylinder motorcycles: 1979 Honda CBX (6 individual Keihin carbs), and a 1979 Kawasaki KZ-1300 (3, 2-barrel Mikuni carbs). They are NOT fun to clean/rebuild. But that is the part of the joy of riding and maintaining 39 year old motorcycles.

I've had a lot of fun resurrecting my two 305 CCK Onans. Now I'm going the next step to get at least one of them running on Propane and NG. Thanks for the information.

Stuart
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Old 09-20-2018, 08:04:10 PM
JohnnyC JohnnyC is online now
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Default Re: 305CCK Contractor Tri-fuel Gas Conversion Question

Quote:
Originally Posted by sbbloom69 View Post
JohhnyC, I have followed your Vap Induction threads. I have an earlier CCK model that does NOT have the metal guard around the intake tube. In your photos, it looks like your intake fitting just out of the adjustment ball valve, is just stuffed through the 5/8 hole you drilled into the intake. Is that the case? or do you have a spud tube connected to the fitting to get the gas down nearer the carburetor's venturi throat? Based on all of the cobbled together gas conversions I've seen on Youtube, I'm not sure it matters. I just want to get the best gas conversion that I can with minimal modification to the CCK. Please let me know if you jut have the fitting inserted above the carb, or if it has a tube going down the carb throat. I couldn't find any other photo details.

Thanks,
Stuart
Los Alamos, NM
The tube is short and terminates at the top of the ventui near the choke. It was designed to automatically position the choke in the full open position since when running on propane or natural gas a choke is not needed. To run on gasoline, simply remove the crude homemade device's two hold down screws to remove it and the carburetor has it's full factory gasoline functionality. My Mickey Mouse cobble-job device was fully tested on Walbro and Nikki carburetors on multiple RV gensets and in all cases the gensets produced more than their rated output without any problems. Heat tests using the crude device were performed under various loads (from no load to full load) and in all cases fell within acceptable range. I am happy with the results and for the cost of only pennies . By the way, no JB weld is needed here.

JohnnyC
New Jersey
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