After warming engine up, I would run it a high idle, then turn the main jet adjusting screw in until the engine starts to falter. Note the position of the screw. Then back it out until the engine starts to falter again. Note the screw's position again. Turn the screw in half the amount between the two points. That should do it. I like to leave it just a bit rich, since I'd rather burn a bit more fuel and reduce the exhaust temperature for longer engine life.

Example - turn the screw in 'til falter. Turn screw out 'til falter - say one turn. Turn screw back in ½ turn. My preference would be back it out an additional ¼ turn to run a bit rich.

My Vanguard has a non-adjustable main jet, which I don't like. It runs lean in the cold winter air, and to get it running properly with the snow blower, I have to run it with the choke on a bit. It's done that since new, and the idle adjustments don't help. Thanks, EPA.

 

I usually warm it up, put throttle at WOT an crank in on the main screw a little at a time till it faulters like it's running out of gas, then back it out a little at a time till it runs smooth. Each machine is different. If the plugs are black and it smells rich, it definately is rich.

 

Here is the manual info

 

Thanks again Guys for the help. I don't think any of you know what a HUGE help you are with some of the questions that I have asked in the past. A GREAT bunch of Guys. Thanks, Litz

 

Hello Guys, I'm having some problems with getting the carb adjusted properly. With the engine warmed up and at idle I adjusted the idle mixture (screw closest to the front of the tractor) turning in until the engine misses, and then backing it out past the point of running smooth and then adjusting it mid-point and the engine falls on it's face when I go to wide open throttle. The only way the engine will accellerate is if I adjust the idle mixture very rich. The engine doesn't respond to any adjustment to the main screw. What the heck am I missing here? I know the carb is spotless. I checked the float level again and it's at exactly 1/8" (New brass float) I removed the main screw and sprayed carb cleaner through the circut which is clear. Why will it not respond to any main screw adjustment? When I run the idle mix VERY rich the engine seems to run fine, and accelerates ok. When I re-built the carb I had everything apart and soaked it in carb cleaner, and also sprayed all the circuts with carb cleaner followed by blowing it all out with air, so I'm sure there isnt any dirt in the carb. The fuel tank was also cleaned, and fuel line, and filter replaced. I'm certain that dirt isnt the problem. The plugs are as black as any spark plugs that I've ever seen. any suggestions? Thanks Litz

 

I think I had the adjusting screws a$$ backwards!!! I thought the front screw was the idle mixture, and the rear one the high speed mixture!!! I'm going to go out and attack it again! VERY embarassed here!! I'll let you know how I come out. Later, Litz

 

Back again, I went out and adjusted the carb again and it runs better. Still VERY little change in rpm while adjusting the(correct) idle screw. If the screw is turned in until it seats the engine will continue torun. That doesnt seem to be normal? Shouldnt that stop the fuel flow to the idle circut? The engine definately responds to adjustment to the main screw. I worked the tractor pretty hard for about 1/2 hr.moving dirt with my gannon and at about 1/2 throttle under load it quit. No sputtering, or backfiring. Just stopped. It started right up but didn't want to run much above idle. I re-adjusted the carb and it ran ok for a while and then the same thing. It makes me think that maybe there is some moisture in the gas. I don't know how that could possibly happen since it's been parked inside and dry since I cleaned the tank and replaced the entire fuel system. Any suggestions?

 

Hi Litz,
I just spent an hour or so on my carb. When i first started, I got no response from the idle mixture screw either. One thing to look for, the idle screw should be the one with the longer tip. Also make sure the tip of the needles are spotless and have no pits or damage.

Once I cleaned mine up, I am now able to idle it down to a put..put.

The dying and then not running above idle still sounds like a fuel delivery problem or vapor lock possibly. Where is your fuel line routed?

 

A1CNC, thanks for the reply. The fuel line is routed in the original location. I'm going out now to double check the adjustment screws to make sure they are in the proper location. I really feel like a dumb A$$ thinking the idle screw was the high speed screw. I think SmallFry sent me an exploded diagram of the carb a while back and I cant find it. I'm too damned embarrassed to ask again. Anyway I'll let you know how I come out. Thanks again, Litz

 

Well I think I finally have the carb. adjusted properly. It idles smooth, has good throttle response, and has pleanty of power. But after working it for 15-20 minutes it just quits. No backfiring, etc. After I let it set for 10 min. it starts and runs fine for another 15-20 min. The engine isnt getting hot, and the temp here is only about 65 degrees. The carb, and manifold arent hot? Could a condenser possibly be doing this? Just when I thought we had it whipped something else pops up! I'm beginning to not like this onan too much. I'm sure open to more suggestions. Thanks Guys, Litz

 

Never be afraid to ask :thThumbsU

This one was with a separate fuel pump-

this is with the built in fuel pump

 

GT 80 thank You for the diagram! It puts my mind at ease knowing that I have the mixture screws in the correct places. And it's running good with the exception of the problem of the engine quiting after working it for 15-20 min. After it sets for a few minutes it starts and runs perfect for a while and then the same thing. The only thing that I can think of is maybe the condenser? The weather was really cool here today and the intake, and carb weren't even close to being hot so I don't think it is vapor lock. I'll keep workin on it. Thanks again, Litz

 

Make sure the gas cap vent isn't plugged. It sounds like a fuel delivery problem, perhaps a partially blocked filter or hose. If heat isn't the problem, I'd suspect the fuel delivery system.

 

Yeah Fcubman, I was stewing on those thoughts last night. I'll check the fuel cap this morning. About a month ago I replaced all the fuel line and filter, but I'm also going to replace the fuel filter also. If that doesn't fix it I'm going to run it and when it quits I think I'll pull the top of the carb off and see if the float bowl is dry. I don't know of anything else to do with it. It runs great until it quits. Then after it sets for a while it starts and runs fine. Thanks for the info, Litz

 

You might try it without the gas filter for a trial. Some gas filters restrict the flow for gravity feed systems just enough to cause these symptoms. There are specific filters for gravity feed systems. I don't think the vacuum operated fuel pumps are very strong, either, so the restrictive filters may affect them, too. Just another thought.

 

Found the problem. I went out today and ran the tractor until it quit. Then I loosened the two fuel pump housing screws and there was no fuel to the pump. Then I pulled the fuel line off the carb and lowered it below tank level and there was no gas coming through the filter. I had just replaced the filter and cleand the tank a month or so ago. What had happened is this black residue had formed on the cork float that works the gas guage and you could barely see it but it plugged the new filter almost solid! I REMOVED the cork float, replaced the filter and it runs perfect. I'm using the little tiny white filter that was recommended on a website for the Onan engines. Why do they want to use a filter so small? I've been using automotive 5/16 filters on my Briggs 18 hp twin tractors for years with no problems. Anyway I want to thank all you guys for your suggestions, thoughts, and advice. I just wish I would have checked the filter earlier but I was so damned sure that the fuel system was spotless that I didn't bother to check the simplest thing first. Thanks again, Litz

 

The small fuel filter actually flow really well and the Onan pumps don't really have a lot of suction.

 

I was flushing the tank on that 16/6 I bought last week. Tip the tank up side down to dump it and out comes this black rubber band looking thing. It was the gasket to the gap I think. It was all soft and falling apart for the ethanol I'm guessing ? Also plugged the filter.

Some guys that sell Onan parts use a bigger filter, I like the screen type (Big or small) for reason James stated.

 

Thanks again guys. I'm going to a bigger filter. I'm a little embarrassed for making such a foolish mistake. I was just sooo sure that it COULDN'T be the fuel filter. But the engine is running like new. Thanks again, Litz

 

That's how we learn best - from our mistakes. No shame in that! I'll bet that doesn't happen to you again! :trink39::fing32:

 

In the engine maintenance manual it says to adjust the main power needle as to throttle response. Lean the mixture (turn the needle in) until the application of throttle causes the engine to hesitate on acceleration, now en-rich the mixture (turn the needle out) until the engine accepts throttle without hesitation. Check that the o-ring on the needle is in good shape, replace it if in doubt.

 

You're right about that!! Every now, and then I make things way more complicated than they are! And You're right I won't make that mistake again!!! Thx Guys, Litz