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ChickenLips

AC 720 will not start.

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ChickenLips
As I was using my 720 this afternoon, the engine started to miss as if it was running out of gas. I stopped and checked and it looked like it needed gas, so I filled it up. When I restarted, it ran for about 30 seconds and quit. It will not restart. It has spark at the plugs and is exhausting a white vapor which I found to be gas. I lit a torch and put it near the exhaust and it lights off on each stroke. Any ideas or fixes?

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Brent_Baumer
Not to be a smartACE, but here's one suggestion, don't be torching the white vapor anymore. Sounds kinda dangerous to me. Wouldn't want the muffler becoming shrapnel(sp?). Brent

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HubbardRA
Sounds like maybe your float stuck and is flooding the engine. Had a B/S vertical shaft engine that liked to do that once in a while. Never knew when it was going to happen. Needed a carb rebuild, but traded the engine off instead. Rod H.

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powermax_paul
Mark, Is there spark at the tip of the plug or between the wire and the plug? Check the plug itself, might be fouled, happens quite often on these onans. What plugs are you using? I gave up on Champion H10C's. Usually dont foul both plugs at the same time, but it's possible. Try some Splitfire SM21F or NGK B6L plugs. Also, check for water in the gas tank, it can suddenly affect your engine when you change the slope you're working on. Also check you crankcase breather. This needs to be cleaned periodically and the check balls need to be free to operate properly. If this gets clogged the motor won't run worth a darn. Paul

Paul Kjorlie, The Norwegian

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ChickenLips
Thanks for the suggestions. There is spark at the tip of the plug. I checked both plugs. I am using the plug recommended in the onan engine manual. I think H8's. I cleaned the crankcase breather about 2 weeks ago. I will check the carb float today to see if it is stuck. Could a valve be stuck causing the raw gas to come out the exhaust? Mark

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ChickenLips
I checked the float and it was stuck. I reasembled it and it started right up, but now it will only run for a couple of minutes and then starts to die out. Would you suspect the float is not adjusted correctly? I am also not sure how the spring is supposed to be installed on the float. I have two carbs and they each had the spring installed differently. Does anyone know of a source for a parts (pictue) of how the spring should be installed? Thanks, Mark

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jackcobb
The raw gas, is throwing me? But I have had the same symptom happen, runs for 30 seconds or so then die, when the fuel pump goes bad. Also had a 620 with rust in the tank, plug the filter and do the same thing. Make sure your pump is pumping gas.

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powermax_paul
Mark, I agree with Jack. It seems odd that that you have raw gas coming out the exhaust. I had a cracked diaphram in my fuel pump once and it filled my crankcase with gas and it spit black smoke and gas out the exhaust, but that doesnt sound like your problem cuz it would be black vapor coming out the exhaust with the oil and gas mixed. (If you ever have a fuel pump problem, replace it with an off-shelf electric. It'll save a lot of headaches.) As for the float spring, I looked at the manual and it's difficult to see how it should be mounted. I had mine apart a couple of months ago. I think the pin goes thru the spring and the long perpendicular end goes against the fuel bowl while the other end wraps around the float adjustment tab. Sure your choke is ok and no mice have built a nest in your air intake? Seems every other time I start one of my onans a mouse goes running! By the way, where do you find the H8 plugs? I thought Champion discontinued them. Paul

Paul Kjorlie, The Norwegian

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Floydster
As far as I know Champion has not made the H-8 plugs for many years, the H10C is the plug that is closest to the H-8. By the way, the best thing I ever did was to install electric fuel pumps on my 620's. Thanks to PowerMax Paul giving me the info a long time ago. The CCKB's love to eat up points, about every 50 hrs. I replace mine, that and a shot of seafoam in the fuel and they just keep on running. Floydster

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nighteye
Hey Powermax Paul, The more I read your responces to these powermax questions, the more I am convinced that you are a true genious, or, part powermax. I am enjoying the education. Ken. Splitfires Rule Nighteye

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ChickenLips
Paul, Champion does make a H8C plug. I get them at Fleet Farm. The box says 587, and H8C on the end tab and the plug has H8C on it. It is a Copper Plus. As for my AC720. I found the float was originally stuck open and the fuel pump was filling the carb with tons of gas. I think you were right about the spring going against the bowl. It now seems starved for gas. It will idle but will not run with the throttle turned up. I unhooked the fuel line and turned the engine over to see how much gas is being pumped. It wasn't exactly high pressure but it put about a 1/4" in a 3" dia. jar in about 20 seconds, does that sound like enough? If I install an electric pump, what should I hook the power lead to? I took the cover off the points and checked the gap. It was .018, it should be .020. Is that enough to cause a problem? I also watched the points while it was running. There is a lot of blue spark there. I that normal? What is the purpose of seafoam?

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jackcobb
Chicken Your description of the pump worries me. Actually 20 seconds is quite a long time. I never tried mine as you did, but my original manual pumps would really squirt a stream when they shot. It's a pulse as you know. If you go the electric route, you want a low pressure. I think the spec calls for about 4 PSI. Some auto pumps put out way to much pressure, like 15 PSI or so and I understand thats a problem. The electric mounts nicely on the large black heat shield between the fuel tank and engine after you drill a mounting hole. Right by where the two fuel lines go through. You can pull power off the + terminal of the coil, which is real close to the baffel holes/location. Maybe Paul has a better read on your fuel pressure? The Seafoam is an additive that has been praised on this site. I'm not much for remidies in a can, but the stuff seems to make a difference. You add it as directed to your gas. Plug filing will definitly go away. Splitfire plugs are praised here as well, I'm going to try them next season. Manual fuel pump kits are available. The ONAN kit is really pricy. Kohler has one that the Milwaukee Onan engine dealer uses. The parts, gaskets etc are the same and all there. It is a fraction of the Onan cost. Good luck

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MPH
Sounds like your short on gas to me. Have you replaced your fuel filter??Even if its new, try by-pass it to see if your fuel flow increases. Had a similar problem with my 4040 last spring, put on a new filter and alls fine. Puraltor makes a pump that puts out 1.5 to 3 lbs.. Paul was kind enough to post a pic for me last spring how he mounts them, sorry but I haven't a clue how to link to that. MPH

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HubbardRA
If you go for an electric pump, they make them about the size of a fuel filter. Get one for a foreign car. They only run about 2 to 4 psi. American cars usually run 6 to 8psi. If you use a higher pressure pump you will also need to purchase a fuel pressure regulator. Rod H.

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powermax_paul
Rob, Good point, but it really doesn't matter what pressure the pump runs at because of the return line. The pump is really only delivering fuel to the tee at the carb. What the carb doesn't take, returns to the tank. Leave the tee in place, run the pump into the end of the tee so its flow creates a little pressure (due to velocity) into the carb, and run the return line from the 90 of the tee fitting. Cap off the inlet and outlet of the mechanical fuel pump and leave it in place. Connect the electrical supply of the electric fuel pump to the hot side of the ignition coil. Pump will run only when the ignition is on, and the coil is right next door. Fasten the new pump to the firewall above the transmission cooler. Floydster got me going with seafoam. That seafoam is great stuff. Haven't fouled a plug since I started using it. Thanks for the tip! Can't really give testimonial for the NGK plugs, but they gotta be far better than Champions. I've only been using Splitfires and the Champions I have left over. Paul

Paul Kjorlie, The Norwegian

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jackcobb
Paul, what do you think?? My Onan dealer was very emphatic not to go above 4 PSI with a pump. Granted there is a return line with a T. But presure of a system with 4 PSI at the carb, is different than a system trying to maintain 15 PSI. He explained that the flow from the T is not big enough to reduce the pressure significantly of the 8 or 15 PSI auto pumps. It will too be 15 PSI at the return line into the tank. That much presure into the carb, has a detremental effect on its performance and operation he claims. He likes the electric pump route, but goes the low PSI route. Do you think he's right?

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MPH
Not sure what the OEM pumps produced for presure, but the 'T' was on there with them, so it makes sence to me not to exceed that presure by much. I would think a large increase would affect neddle valve/foat part of the carb. Bewarned, I'm only half done with my second cup of morning coffee, so my thinking may not be worth typing yet..MPH

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thedaddycat
If you're going with the electric pump but are concerned about excessive pressure, just use a larger T fitting and return line. Common brass plumbing fittings should work, and the larger return side should provide enough flow capacity to keep pressure down. Keep the supply side the same size. (Ever try using a 3/4" garden hose suplied a from 1/4" pipe?) Another thing, how it is piped may make a difference. Route the supply and returns as a straight path with the carb off the side. This way there will be minimal head loss to the return line due to the T.

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HubbardRA
Paul, I was not aware that Onan used the "T" type return line. I have never owned and Onan. Most of my experience is with B/S and Kohler. We used the "T" technique on some of the motorcycle powered pulling tractors because the seals under the float seats would not take high pressure. Many folks thought it was float needle leakage, but it was really the seat seals. We had one guy who wanted a little more pressure, so he ran the return line up around his roll bar behind his seat and back to the tank. Rod H.

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powermax_paul
Jack, You're right. If the flow of the electric pump is high enough, the static head produced at the carb could maybe reach 4 psi. I'd have to calculate it based on the specific gravity of gasoline and the velocity in the tee. Don't have my books right now cuz I'm up at the lake (Phillips, WI) this weekend. I could be wrong, but I don't think the head losses in the return line would be enough to create that much pressure with a low viscosity fluid like gasoline. Based on the viscosity of gas and the relatevely rough inner surface of the gas line hose, there should be turbulent flow in the line. That's where it gets complicated. If the velocity in the hose is low enough, you could have laminar flow which gives a greater head loss. If the velocity is high enough you should have turbulent flow which gives a lower head loss. Also, if the hose wall surface is rough, you will get turbulent flow and thus lower friction/head loss. (that's why they put little dents in golf balls). As you can see there are a lot of factors. I'd recommend getting the lowest flow electric (automotive) pump you can buy. Daddycat is also correct in that if you have too much pressure due to flow, change the tee so that the flow goes straight thru to the tank. IE with the perpendicular part of the tee toward the carb. Final note, I used an electric pump made by Purolator off the pegs at a chain auto supply store and it works fine. Don't necessarily endorse this brand, but it works for mine. Paul

Paul Kjorlie, The Norwegian

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ChickenLips
I replaced the points, condensor and plugs. The tractor started right away. It ran for about 20 minutes and then began to smoke(black). The enginge slowed, started to missfire and then quit. When I cranked it over I had gas vapor coming out the exhaust. I pulled the rubber connector between the carb and the air filter off and gas was running out of the carb. I took the bowl off the carb again to see if the float was stuck open, it wasn't. I checked the float to see if it is sinking but it is not. I looked for debris in the needle valve seat but it was clean. With the needle valve closed, could the pump push fuel up through the bowl vent and back into the front of the carb? I check the return line to the fuel tank thinking if it was plugged it might pump it up through the vent but it was clear. Help Mark

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gretsch
Have you tried blowing into the fuel inlet of the carb (when it is off and apart of course) and testing the fuel needle valve (sits above the float near the hinge)? That is a good test to see if the valve is shutting off fuel supply when float is up. I am not sure on the Onan (I've only had one Onan carb off to rebuild, very similar setup as a Kohler I might add), but most carb floats have an adjustable (by bending) tong that the needle valve rides on. Adjust it until you cannot blow air through the fuel inlet when float is up.

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ChickenLips
I took the carb off to check the float one more time. I did try to blow through with the bowl with the seat closed and it was working. The Onan manual says the float should be adjusted to 1/8" to 3/16", this one was set to 1/8". Since gas was pouring out of the carb I figured it must not be closing so I adjusted it to the 3/16"+. I put it back in and I just finnished an hour of Mowing with no (finnally) problems. Thank you very much, for all the help and sugestions everyone gave me. This is a great (very usable and usefull) website. Sincerely, Mark Forster

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