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2019/03/19 07:55:50

backporch

Briggs and Stratton Intek

Before I tear this motor apart to verify that my camshaft compression release is broken, can anyone help me understand what I see. When I turn manually through the compression stroke, the intake valve doesn't really close until about 1/2 way up. Not sure if that is normal and when the compression release takes place. I still have significant resistance at that point and the starter stops about there. Also, what I find strange is that manually turning the motor through the power stroke is difficult. Am I drawing a vacuum??

With the spark plug out, the starter will spin the motor at what I guess is a normal speed. I have not verified that the starter is in good shape or sanded/cleaned every contact yet.

The motor is on a 2004 sears with an 18.5. I purchased it non-running so don't know what is really normal

38 comments Leave a comment

AVB

Unless someone has been in the engine you may not be the middle of the compression stroke as the valve closes but just after BDC when the intake valve closes. The both valves stays closed until just before TDC of the compression then ACR bumps the intake valve to bleed off some of the compression. Use a dowel rod to verify the piston position.

As for the resistance when hand turning this the compression that you are feeling. If compression release (ACR) was working properly there would still compression to fight but at a lesser amount. This high compression is what causes starting problems with the starter as they are not strong enough to spin through the high compression of 130+psi but when ACR is working this is as little as 70 psi.

Also before tearing down adjust your valve clearances to .003-.005 IN .005-.007 EX. These clearance on OHV open up over time which leads to ACR not being able to do its job. Plus make sure the valve stem button(s) depending the setup are in place. Once the clearances are set then you watch for the ACR bump as the piston nears TDC compression stroke. If not present after valve adjustment then the ACR on the camshaft has failed.

2019/03/19 08:48:03

backporch

AVB,

I did have the spark plug out to determine and mark top and bottom dead center. I adjusted the values to spec. Found a few videos that said the adjustment should be just beyond TDC. One video said let the piston travel down 8mm... probably to avoid the bump.

I also tried to put a compression gauge on the motor. The starter didn't turn to TDC on compression. Maybe 25-30 degrees before and stopped? just a guess. gauge read 30PSI. Could be leaky seal, dry cylinder or crappy gauge.

Thanks for the info on the decompression bump. I did not know when the bump happens. I see nothing just before TDC on the compression stroke, so my cam must be crap.

Any warnings about picking up a used cam? looks like the part specified was used on lots of motors of different years and HP ratings. Some are solid gear, others spoked design.

The resistance on the power stroke has me puzzled. I took that to mean that my decompression bump had worked, I had been passing through it too slowly and was drawing a vacuum after that point. I guess in the event that the cam was taken out and put in incorrectly I can resolve that when the motor is open.

2019/03/19 11:54:47

AVB

It will be a spoke version. You double me on this that the engine has the 793880 camshaft by looking the model and type number of your engine. The 793880 ACR do fail more than they should which why I keep 1 or 2 793880 camshaft kits in stock at my my shop. If the ACR did fail make sure you get all the pieces of the engine as it can damage things like governor/oil slinger gear.

As for used just make sure it is in good shape before buying it. Most are probably from an engine that has been ran without oil that had broken the rod and put a hole in the side of the block.

2019/03/19 12:14:49

backporch

owner's manual says 698492. Looking on ebay there a lots of hits, many look like smaller motors with a solid gear. Are the cam lobes any different on those? Thanks for the help

2019/03/19 12:53:11

AVB

I haven't seen any that are solid but the number you posted is currently superseded to 793880. Since the 698492 supersedes to the 793880 the lobes should in the correct position. Apparently Briggs went from a casted camshaft to one they can assemble.

2019/03/19 14:19:43

kshansen

"When I turn manually through the compression stroke, the intake valve doesn't really close until about 1/2 way up"

If this is true then something sure sounds wrong inside.

1. Intake should open close to top center and stay open as the piston moves down on the "Intake Stroke".

2. Then it should remain closed on the compression stroke until piston is close to "Top Center" and then "bump open" for a short time to vent off a little compression to take load off starter.

3. On the power stroke both valve should be closed till piston is at, or very close to, bottom center on "Power Stroke"

4. Then exhaust valve should open and stay open till piston is at, or very close to, top center on exhaust stroke. Just as the exhaust closes the intake should start to open. Actually both might be slightly open at the same time, this is referred to as "Valve Over Lap".

Now go back to #1 and repeat.

Unless I'm completely misunderstanding that sentence at the top of this page there is a cam timing problem in this engine.

2019/03/20 09:08:51

backporch

The exhaust valve seems to follow an expected path. I am probably confusing intake being open with there being a gap until closer to the top of the piston's travel.

I have definitely determined there is no bump in the intake valve. The work I have to do on the power stroke to turn this engine by hand has me very puzzled, but my next step is to take the engine off and check the camshaft.

Thanks for the help. will follow up with what i find

2019/03/20 14:58:18

kshansen

Hope you understand that many times a question or remark is made in an effort to understand what is happening. Things that one could see and know in a matter of a few seconds if there in person with hands on the engine may take several minutes of thought and dozens of words to express in text!

Case in point, you seem to indicate, if I understand correctly, that the engine takes what you feel is more that the expected effort to turn when on the power stroke. What I would want to feel would be does the engine almost try to turn on it's own if you stop part way through the power stroke and begin turning it in the opposite direction.

If having both valves closed and piston going down is creating a vacuum you maybe able to feel that to some extent. Can't say I ever noticed it but who knows?

2019/03/20 15:23:42

backporch

Unfortunately I didn't see your last post and just got to work dismantling.

I pulled the motor and opened up the case. I found that the camshaft looks like it is intact. The bump mechanism is there and there is some very faint spring action. Probably just the way it is supposed to be. Oil is pretty dark and thick. Maybe a thicker oil than I am used to SAE 30?

Tomorrow I am going to visually check the cam lobes, lifters and pushrods to look for anything weird. Any advice of what to look for would be great.

I probably should have spent more time examining the wiring to the starter. but I was sure that something was wrong because of the lack of a bump in the valves and I was pretty sure I adjusted the valves to spec... on the tight side of the range.

2019/03/21 07:25:53

backporch

Here is a picture of the camshaft. Orange flakes are a dusting of pan bolt's loctite

2019/03/21 09:12:20

kshansen

At this point I guess one has to start with the basics.

If cam has not been removed make sure the timing marks on the crank gear and cam gear line up.
If this is one of the engines with the plastic gear and lobes inspect real close to confirm they are not loose or somehow out of position.

I think it's possible to post pictures to this forum, if so how about a picture or two inside the engine showing cam, gear and lobes?

2019/03/21 14:17:25

backporch

I posted a picture 6 hrs ago. pending approval still. If that gets approved, let me know what you think. All metal gear.

2019/03/21 15:14:15

backporch

Looks like its up now.

2019/03/21 17:15:09

backporch

I took a measurement with my digital caliper. Measured the cam diameter with and without the bump. The readings I got in inches was 1.016 on the bump. and .9895 where the bump isn't. thats a difference of .0265 in. Given the gap at the intake was set to .003, seems like there should have been some movement.

If I am thinking straight, the crank should rotate at half the speed of the camshaft. At one rotation of the crank both exhaust and then intake should do there thing. That's 1/2 rotation of the camshaft. I was surprised to see the lobes were more than 90 degrees offset. I think that lends some support to my finding that the exhaust was right on time and the intake started after TDC and closed after BDC. The bump would have extended the "open time" or at least the tightness in the valve clearance till almost TDC on compression. Its kind of what I saw. I didn't notice the "bump" that I had seen on youtube videos was pretty pronounced. Does any of this make sense?

2019/03/21 20:08:12

backporch

Hello Mikel1,

The valves were adjusted to the tight range of the clearances. I dont have a good mower battery but I did jump start from my truck battery as well as two jump packs. They basically all had the same result. Stuck before TDC on the compression stroke.

The wire connections to the starter were tight and looked pretty clean. I didn't follow grounds or look to the other connections on the solenoid.

Looks like the acr on this camshaft is 90 degrees off the max cam lift for the intake.

2019/03/21 22:18:05

Mikel1

I see so can you manually move past that spot?

2019/03/21 22:26:12

backporch

Sure. I was able to rotate the motor by hand. There was a good amount of resistance, but I was able to do it without too much effort with just two hands on the flywheel screen. As I said before I found it weird that there was resistance on the power stroke. I wish my compression gauge could have registered if there was any vacuum

2019/03/21 23:11:18

backporch

I realize this post is getting long, so I will summarize where I am

1. Couldn't crank past part way into compression stroke
2. Set valve clearance to tight
3. Didn't see the noticeable bump from the ACR when manually turning the motor
4. Resistance during compression and power stroke
5. Removed cam, ACR was intact
6. Measured with micrometer and see difference of about .025 due to ACR

issue 3 could have been me comparing my motor to what I saw on youtube
issue 4 is odd about power stroke, but maybe a vacuum??

Not sure where to go next. I am going to reassemble when my gasket comes in. probably also will clean up all electrical connections on starter/solenoid connections and grounds

Maybe get a different starter or motor

2019/03/22 08:30:41

kshansen

First off the cam rotates at Half Crankshaft speed. The gear on the crank has half the number of teeth as the cam!
And the compression relief is before not after top center. The idea of compression relief is to make the crank easier to turn for the starter. If it was after top center the compression pressure would be pushing the piston down and helping the starter turn the crank!

If you do in fact have 0.0265 lift at the cam from the compression relief and the valve is set to 0.003 you should see obvious movement of the valve when it comes up on the compression relief. Not sure what the rocker arm ration of an Intek engine is but should not be enough to account for the 0.0235 difference between no compression relief section of cam and compression relief point!

Something is either wrong in your description of what you are seeing or something is wrong with the compression relief on the cam. I'm still trying to wrap my mind around the statement in you first post " the intake valve doesn't really close until about 1/2 way up". This makes no sense to me.

Not trying to be a jerk with any of my comments but just having a problem understanding what is really going on!

2019/03/22 09:18:38

AVB

Correct the Crankshaft to Camshaft turns ratio is 2:1 with the crankshaft making two turns per one turn of the camshaft.

The spec is correct on the ACR lift (as the used camshaft here has a .0235 lift on the ACR bump) so it should be working. The Exhaust lobe is 3 pass the timing mark, Intake is 18.5 teeth from the timing mark. Timing gear is 66 teeth and crank gear is 33 teeth if camshaft gear teeth count is correct. I got a crank gear here but I didn't record which bin I put it in so it currently considered lost in the inventory.

Just a thought here. Are you turning the engine in the correct direction? CW Flywheel side, CCW PTO end. If turning backwards by hand that would explain why the ACR is not working.

2019/03/22 10:09:05

backporch

Kshansen,

No offense taken at any comments. I really appreciate everyone who sticks with me on this.

I don't think I was the one that said the ACR was after TDC, but if I did, it was a mistake.

I made a comment about crank and cam rotation as I was concerned about the cam lobe orientation. I noticed that the shaft is splined but I am overthinking things. Just to verify, the exhaust and intake are more than 90 degrees apart at their maximum opening?

Before I took the engine apart this is what I noticed.
1. i took out the spark plug and marked the flywheel orientations for TDC and BDC
2. valve clearances were way too wide. I corrected that by adjusting slightly after TDC on compression/power stroke
3. I looked at the action of the exhaust and intake valve. The exhaust opened pretty close to BDC and closed at TDC
4. The intake seemed to start open movement after TDC ( should have noted degrees)
5. It finished closing I beieve after BDC, maybe not too much after
Now here is what I think I saw that you find confusing...
6. The gap that was set for the intake didn't seem to appear until much later in the compression stroke. No rattle, couldn't get a feeler gauge in...
7. The starter couldn't make it to TDC on compression. maybe got hung up 20 degrees or less before that point.

I tried to help the starter with one hand... other was on the key and I couldn't do it. with two hands not too difficult to put palms on the flywheel and turn it.

2019/03/22 10:13:40

backporch

Looking down I was turning the engine CW. I believe that is what the starter was also doing.

2019/03/22 10:15:05

kshansen

I'll try to back off here a bit as my thoughts are only based on basic engine functions. AVB has a much better incite into the specifics of this engine. To be totally honest the only time I ever touched a wrench to an overhead valve Briggs other than to install a couple new ones in my snow blowers was several years ago. That was when i adjusted valves on a rider mower at work that we originally thought had a bad starter because it would lock up while trying to start. Set valves and it would crank like crazy again!

Now give me a 16 cylinder Diesel and I'd feel right at home!

2019/03/22 10:23:28

backporch

The Exhaust lobe is 3 pass the timing mark, Intake is 18.5 teeth from the timing mark

Trying to understand how I can verify mine. If I put the camshaft on a table, gear down, would the high point at the intake be 3 teeth in the CCW direction from the timing mark? intake 18.5 teeth same CCW direction from the timing mark?

If the difference between max open on both lobes is 14.5 teeth and there are 66 teeth, that is less than 1/4 or < 90 degrees, which mine is not. Its not in front of me, but I would guess mine is more like 110 or 120 degrees.

2019/03/22 10:23:57

Roy

Stop overthinking this. When the piston is at tdc, the upper limit of it's travel. both lifters should be riding on the cam lobes. That is, the exhaust is closing as the intake is opening.

2019/03/22 12:15:55

backporch

Fair enough. My cam looks like cam's for sale online, so I am done thinking about that.

When I get this motor back together and if I still have an issue with starting, assuming:
- the valves are correctly set
- the wires are good and have clean connections
- it doesn't work with a car battery or any jump pack...

How do I go about evaluating the starter itself?

2019/03/22 12:51:46

backporch

took the starter off and checked its internals. Pinion gear and shaft are now clean and sliding freely, no broken magnets, commutator is clean and brushes look solid. Now signs of burning or discoloration. Not sure how else I can evaluate a starter. Going to give it a key turn after I get some oil in the motor.

Previous owner was jumping the starter directly on the post. He must not have noticed that the 20 amp fuse between the igntion and battery was blown.

2019/03/27 22:31:55

Mikel1

So did u get another cam?
On the fuse make sure no bare wire(hot) is touching frame

2019/03/27 23:36:02

backporch

I removed the cam and it was fine. the ACR was complete and working fine.

I put the motor back together, and installed the starter.

Same situation. The motor spins until it sees compression and stops. I marked the flywheel and it stops about 45 degrees before TDC. That looks to be about the same place the intake valve stops moving from the ACR.

I jumpered right to the starter lug. no better.

2019/03/28 23:06:32

kshansen

backporch
That looks to be about the same place the intake valve stops moving from the ACR.

Just trying to be clear on this. Are you saying that the intake valve does open from the "bump" of the ACR?

2019/03/29 07:54:14

backporch

ordered a new starter from Amazon last night. will arrive tomorrow

2019/03/29 08:53:33

backporch

I do see movement on the intake valve. The intake opens around TDC on the intake stroke and finishes closing after BDC but does move again and closes once more in the compression stroke, closing by about 45 degrees before TDC. The starter can't get it much past that point.

2019/03/29 08:54:59

backporch

I was just at the big box store and I looked at the mowers outside. similar HP briggs and stratton. Starter looks the same. I tried to turn the motor with my palms on the flywheel screen like I did for my motor. Just as much if not more resistance on this one. I think the starter will be the fix. If it does, the old starter will meet my sledge hammer so nobody ever thinks it is worthy of another crank.

All this is based on the starter really being the same. The battery was under the seat on this model and might have been a little beefier... hard to say. I didn't check CCA.

Also, before I ordered the starter, I read a review on amazon where the reviewer said that even though his starter engaged and would spin, it didn't have the strength to crank the motor. the new starter fixed that. Good enough for me.

2019/03/29 14:10:25

backporch

Starter didnt help much still can't get past the start of compression.

2019/03/31 11:21:39

Mikel1

Any idea of the compression(tester) when it hits that spot?
Did you use just the battery or the jump pack?

2019/04/02 20:30:57

backporch

I used a HF compression tester that jumped to 30PSI.

I also took off the screen on the flywheel and was able to rotate the motor with a torque wrench at 15 ftlb or higher. it did click below that. Not very scientific, I know, and that was the bottom of that wrench's settings.

As far as power source, I tried two different jump packs ( the dead lawnmower battery was removed) and a truck battery connected to the mower's battery cable wires. I also tried to go directly to the mower starter from a truck battery with jumper cables clamping hot to the post on the starter and ground on the exhaust. Motor reacted the same way

Thanks for checking back and staying with this thread !!

2019/04/03 15:58:02

backporch

A final update on this long thread.

The mower started to crank normally and almost without any fix. The only thing I did was put the jumper cables and a jump pack on the cables at the same time to try and maximize the contact for current to flow. Starter cranked quickly and compression rose to 90-100 psi. Amazing.

After that I connected only a jump pack and it still worked.

Since then I bought a new battery, got it to fire up, adjusted the PTO and verified that it could move in forward and reverse. I will take it for a longer run later today, but it appears to be working pretty well.

Thanks to everyone for their help. I wish I could share a good reason why I had so much trouble.

2019/04/16 09:49:27

Mikel1

Thanks for the update

2019/04/16 17:14:33

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Briggs and Stratton Intek

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