Runs great with Choke!!!!

[wmbishop]

I have a 1979 johnson 35hp motor. I just purchased the boat and noticed that it seems to be weak; however, when I pull the choke in it really picks up and runs great. I took the carb off and rebuilt it. I checked compression, (110psi), changed plugs, fuel line... Still the same problem. While going down the lake I pump the fuel bulb and it doesn't make any difference so I guess it's not the fuel pump. What is happening. ? Any suggestions would be appreciated.

 

[byacey]

Re: Runs great with Choke!!!!

You must have a substantial vacuum leak if the carb float level is good and the main jet isn't plugged...

 

[phatmanmike]

Re: Runs great with Choke!!!!

you have an air leak.<br /><br />it runs better with the choke on becasue its compensating for the air leak and is getting the air someplace elce

 

[Sherman]

Re: Runs great with Choke!!!!

But the air leak could be at the fuel pump. My 96 6 hp had the same problem until I replaced the fuel pump.

 

[wmbishop]

Re: Runs great with Choke!!!!

Thanks for the help. What a great site. I have considered a vacuum leak but there sure isn't an obvious one. I've visually inspected the motor for leaks, obvious ones, but I didn't consider the a leak at the fuel pump. I know from working on cars you can take a little starter fluid around the intake and listen for a rise in RPM. the boat motor intake is too close to the breather. How do you check for a vacuum leak around the intake.<br /><br />Thanks

 

[byacey]

Re: Runs great with Choke!!!!

You could use a small unlit propane torch with the gas turned on slightly - not too much otherwise you will have a different, more immediate problem on your hands. Just bring the nozzle close to where you suspect air is being drawn into the engine and listen for a change in the motor.<br /><br />I wouldn't use ether, it's too volatile and dangerous to be spraying around ignition leads and magneto coils in close proximity.

 

[Walker]

Re: Runs great with Choke!!!!

If you squeeze the bulb up tight before you start then the motor should run fine for 30 seconds or longer because the carb bowl is full. If it doesn't then I suspect your carb isn't as clean as you think.

 

[byacey]

Re: Runs great with Choke!!!!

Walker, under ordinary circumstaces what you say is true, however if more air is being drawn from another source other than the carburator - ie. a leak, The engine will not be recieving sufficient fuel to run.<br /><br />Does it idle OK in neutral or in gear? Is it just high speed operation that is lacking?

 

[AJ168]

Re: Runs great with Choke!!!!

If it is a leak, then when you pump the ball hard with the engine not running, it will eventually go soft again on its own.

 

[wmbishop]

Re: Runs great with Choke!!!!

The idle is fine (without choke) in gear and out of gear, it is only the high speed. My carb has a high speed orifice. The orifice is or was clean. That was my first thought. It was stuggling for fuel so I changed the float valve and seat, re-used old float and re-adjusted. The carb seemed clean so I thought it might not be the problem. The motor does not miss it just runs much stronger with the choke. How can I determine if it is lack of fuel too much air? <br /><br />Again thanks for the help.

 

[Walker]

Re: Runs great with Choke!!!!

If it idles fine then I doubt you have any kind of air leak in the intake system. I still suspect the high speed circuit in the carb.

 

[wmbishop]

Re: Runs great with Choke!!!!

Walker, I could take another look at it. There is a possibility that some trash may have gotten in the orifice. We'll pull it again and check it out. Is there anything else that should be checked while I'm into it that could cause the problem? It seems pretty straight forward. Gas through the float valve into the bowl, through the high speed orifice up the tube into the throat. If there is anything elese I'm not sure.

 

[byacey]

Re: Runs great with Choke!!!!

Check the float that it isn't binding and it is installed right way up. Make sure the fuel iline to the carb isn't piched or restricted.

 

[krb113]

Re: Runs great with Choke!!!!

Don't forget to check out your plastic fuel tank if you have one. Mine sucked air at the fitting because of the way the line hooked up and it didn't have a "stop" so to speak for the connector to rest on like the metal tanks do. Let's the line move around and suck air past the o-rings. Bad design. Now in dumpster.

 

[Sherman]

Re: Runs great with Choke!!!!

I know on my 79 115 there is a small hose that runs from the botom of the air collector box in front of the carbs to the block. It drains excess oil out of the box and into the block. Yours may have this also, not sure at all but I have had problems with mine twice when this hose has got pinched off under the carbs.

 

[Walker]

Re: Runs great with Choke!!!!

will2112, if you didn't put a carb kit in it the first time, do it this time.<br />Gaskets get old, hard and sometimes don't seal like they should. Floats/needles do the same.

 

[MCM]

Re: Runs great with Choke!!!!

Ditto what Walker said, I made the mistake of using old gaskets once, vacuum leak from hell!!

 

[wmbishop]

Re: Runs great with Choke!!!!

Sorry for the misunderstanding. I did use a carb kit, I just didn't use a new float. Everything else was new. Parts place said I didn't need one because it was a composite float. "Sherman" my motor does have one of those lines but I replaced with new already. Thx, Also the float can only work one way. However, I do have an old metal fuel tank. I replaced the line, bulb, and filter, but not the tank. Is there something I need to look at there? I did not replace the fuel line in the motor but I may need to take a closer look.<br /><br />Thanks,<br />William

 

[MCM]

Re: Runs great with Choke!!!!

What condition is the inside of the old metal tank? if its rusty and the rust is flaking away that may be the source of your problem, pretty sure there's a filter in the tank as well.

 

[KilroyJC]

Re: Runs great with Choke!!!!

Just an FYI - <br /><br />Another (messy) way to check for a vacuum leak is to slowly spray in various suspect areas with WD-40. When you hear an RPM change - you found it!<br /><br />Good Luck -<br />Matt M