18.5hp Briggs OHV Intek...Cranks after Valves Set when COLD ONLY!

[Alphadog]

Hi Guys,

New guy here. I'm a decent mechanic and a licensed electrician but, I've been fighting this problem on and off for days. I was determined to figure this out without help but I just can't get it. I wish I knew more about small engines. I've looked over this forum and still can't get the answer I'm looking for. Maybe someone can explain a few things to me so here goes.......

I have a 42" Craftsman Model 917.274811 with an 18.5 Intek OHV Briggs Model 310000. If you need more info just ask. From what I have found on that model, I need to set the Intake valve between .003"-.005" and the Exhaust valve between .005"-.007" at TDC plus 1/4" more on the piston. Does this sound correct to anyone?? I've done this procedure in every kind of way imaginable. I've done this at TDC and 1/4" past TDC on the piston. I've gone with the lowest settings and the highest settings. I've set them with one valve open and the other closed and visa versa. I can always get it to run afterwards, like a top, but after 30 seconds or so if I shut it down and try to crank it, well, then it's the same old problem again.....too much compression to turn over.

I've read about the exhaust valve opening slightly at the end of the compression stroke but I've never seen this on mine. I DO see the Intake Valve do a double dipper just before it closes but not the exhaust. Do I have them backwards??? I mean, the exhaust valve (top) is directly in line with the exhaust port, is it not??? It is only opening and closing. The intake valve (bottom) opens and closes and double dips slightly at the end. Just in case for some reason I'm wrong, I set them the opposite to see what would happen. No crankie. Way off.

I've also read about this, "The compression release is an arm on the cam gear that keeps the INTAKE valve from closing completely until late into the compression stroke at cranking speeds to relieve some compression. When the engine starts, this arm swings out of the way. In some instances, there is a bur somewhere in the cam arm ***'y that keeps the arm from working, sometimes the spring breaks, sometimes the arm comes off."

Was that a TRUE statement???? Does the cam really have an arm that does this?? I've read so many different "opinions" that I'm all mixed up. Of course, I don't have a clue so I'm asking for help, real help. Can someone tell me what I'm doing wrong? Or right? I'm very very frustrated!!! I can send pictures, more info, or anything else you may need. Much thanks to anyone who can set me on the right course.

 

[ILENGINE]

The camshaft has a weight that has a spring return that bumps the intake off it seat at cranking speed but turns out of the way when running. I have seen the weight break and will be laying in the engine sump, I have seen the spring break and not allow the weight to return, and I have seen the weight stick out and not return. All three will can kickback and hard starting.

If I remove the engine to inspect the cam issues, I just install the new cam while I am at it because it takes too long to get to the bottom of it sometimes.

 

[Alphadog]

The camshaft has a weight that has a spring return that bumps the intake off it seat at cranking speed but turns out of the way when running. I have seen the weight break and will be laying in the engine sump, I have seen the spring break and not allow the weight to return, and I have seen the weight stick out and not return. All three will can kickback and hard starting.

If I remove the engine to inspect the cam issues, I just install the new cam while I am at it because it takes too long to get to the bottom of it sometimes.

Well, that's good to know. I only found that information in one place and wasn't sure. You mention that the spring return bumps the "intake". Is that the little intake bump I mentioned in my post??? I thought there was supposed to be a bump on the exhaust (according to others). So, that's why it's so easy to crank cold and not hot?? I hate to sound stupid but, here goes.... I can understand the need for "less" compression at start-up (cold or hot). What I don't understand is "why" I have the problem only when it's hot, unless the weight is in the sump and it's just plain easier to crank when it's cold. Does that sound like a perfectly good scenario??? Also, to be clear, were my descriptions correct in my post?? You know, such as gap, exhaust on top, bump in intake, etc.? Did I miss anything? If I decide to break it down and check out the cam, are there any words of caution or direction you might like to share since I've never attempted it before (but always willing to try)? I appreciate all your help Lawn Pro.

 

[ILENGINE]

Your clearance sound correct. the bottom valve on the single cylinder ohv engines is the intake. Crossflow head. The bump is the compression release working, on in this case the intake valve. They moved from exhaust valve release to intake release mainly due to emission standards.

what I suspect is happening is you have the sticky flyweight, that when it sets the spring returns the weight to the compression release position, but gets stuck there after starting. Sticky weight for lack of better term. If you pull the engine to check on it, just go ahead and install the new camshaft. And make sure you get the timing marks lined back up when you install the new cam. You will also need to readjust the valves after everything is reassembled.

 

[Alphadog]

For the first time I actually understand what is going on inside. Best answer I've seen on the problem. I'll dig deeper inside. If I have any problems, I know where I can get some answers. Thanks again Lawn Pro

 

[Modify Man]

It seems I may be having this same issue.
I set the valves and it starts fine a few times. But then, uhhg.
For a while I couldn't figure out why the exhaust valve was so loose until I noticed the rocker arm stud was coming loose.
Finally I thought that maybe this would end my constant fiddling with the valves.
But NO. Again, I'm out there this morning messing with it because I want to give my other mower a break. Figure out its
hiccup.
Needles to say this just makes things sound worse and yet some solace knowing that I wasn't doing my adjustments wrong. And why don't they have the valve clearance numbers in the owners manual?

 

[slomo]

Intake valve between .003"-.005" and the Exhaust valve between .005"-.007" at TDC plus 1/4" more on the piston

Into the bore you mean. It's TDC on the COMPRESSION stroke plus 1/4" past TDC. Remember there are 2 TDC's on a 4 smoke engine.

As you guys were talking about, the larger valve in the intake. Ex valve is smaller. Make sure you are on the IN valve and EX when setting the gap.

slomo

 

[Tinkerer200]

"I thought there was supposed to be a bump on the exhaust (according to others)."

Be careful where you get your info from. "Others" don't always know. Most B&S engines relieve compression thru the Intake Valve. Vanguard twins are the exception.

I can send you a Service Manual for the engine IF you like. Address below, put in proper format and remind me engine model and what you want.

Walt Conner
wconner5 at frontier dot com