35hp crankcase pressure

[mphelle8vld]

I've been troubleshooting low fuel pressure from the fuel pump and after changing it for a known good pump, I'm now suspecting the crankcase pressure. Anyone out there know how much pressure to expect from the crankcase hole which supplies the pump? Right now I'm only getting a slight puff of air at cranking speed, doesn't seem to have any pressure. Are there any reasonable ways to check if the crankcase will hold pressure?

 

[Paul Moir]

Re: 35hp crankcase pressure

At cranking speed, it's exceedingly low. <br /><br />Here's what I do when I suspect a crankcase chamber leak. It's kind of stupid but it works well. Take off the fuel pump and rotate the engine so it's piston is at the top of it's travel. Then blow in to the fuel pump port. I'll use a piece of tubing here if I expect to be interrupted. <br />It'll give you a real good idea if there's a serious leak as you'll be able to blow in quite easily and you can hear where the air is rushing out - through a blown crank seal, through a bad reed, etc. There will be some normal leakage out the exhaust as there's only the piston sitting there to block it.

 

[JB]

Re: 35hp crankcase pressure

Well, let's see. <br /><br />Is it a Johnny or a 'Rude?<br /><br />Is that a 1957 RD-19, a 2005 35, or something in between? <br /><br />Two cylinders or three? Cross or loop scavenged?<br /><br />In each case the fuel pump is operated by alternating between vacuum and pressure in one of the crankcases. Average pressure is zero.<br /><br />Paul's method sounds like a really creative one, and I bet it works.<br /><br />Let us know what engine you have.

 

[mphelle8vld]

Re: 35hp crankcase pressure

Good to have a couple of the best reply, it's a 83 Johnson 35hp model# J35ECTS. I'll try injecting 5psi through the fuel pump hole and see if I can determine where the leak(s) are, thanks for the quick response.

 

[mphelle8vld]

Re: 35hp crankcase pressure

Well, with 5psi I'm hearing some leakage through the exhaust as Paul noted, and just a slight hiss through the carb which I assume would indicate the reeds. I can't tell if the crank seals are leaking without pulling the flywheel and/or powerhead, should that be my next step? <br /><br />My main problem is the motor will die at idle and fast idle unless I keep pumping the primer bulb, it runs OK at faster speeds.

 

[Paul Moir]

Re: 35hp crankcase pressure

Sounds about right. You'll get a little leakage through the reeds if there's a *slight* bit of dirt, etc.<br />But if you were able to put 5psi on the crank case, you don't have a leaking oil seal. It would have leaked air perfusely.<br /><br />If compression on that cylinder is good, and it seems that the crankcase is well sealed, then I would try looking for other things - restrictions up stream of the fuel pump, and air leaks in the fuel line. A really good test for air leaks is to put a peice of transparent line (temporarily) between the carb and the fuel pump and watch for bubbles in it.

 

[mphelle8vld]

Re: 35hp crankcase pressure

Thanks Paul, you were spot on. I was in the process of using your clear tubing idea when I noticed the fuel line from the pump was pinched between the crankcase and the heavy battery cables to the starter. I guess it took time and heat to collapse the tubing making the pump work extra hard to deliver fuel, because the motor worked fine until last week. Thanks again