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I just rebuilt my 16.5 HP B & S 28W707-0121E1 motor and it is hard starting (need to take the air filter off to get to start). I'm thinking it might be the rocker arm gap which both are set to .006. When it is running, it has a loss of hp and there is blowback through the carburetor which is most likely the reason it goes through the gas. Any help is appreciated!

If it runs only with the air filter off, or taking the filter off makes it run any different, then you need a new filter. Even if it looks clean, maybe it is oil soaked, and resistant to air flow.

Intake .003-.005
Exhaust .005-.007
Your intake valve lash is wrong.

I did replace the filter as the first step thinking the same thing, but unfortunately it did not make a difference.
I appreciate your reply!

If the intake is set at .006 clearance, why would I get blowback through the carb? One would think the valve would be seating not allowing the blowback??

rpaull said:

If the intake is set at .006 clearance, why would I get blowback through the carb? One would think the valve would be seating not allowing the blowback??

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Also, what reference should I be using while setting the rocker arm clearance?

The valve will be opening and closing at the wrong time. It will also not be opening wide enough.
Set the valve lash at 1/4 turn past TDC on the compression stroke.

I wonder if you don't have a fuel problem, to where it is running way to rich. It's not quite as likely if you have the Walbro carburetor, but if it has the Nikki, those things are kinda picky what with the fancy o-ring setup.

:welcome:

I'm going to move this thread over to our Briggs & Stratton section.

rpaull said:

If the intake is set at .006 clearance, why would I get blowback through the carb? One would think the valve would be seating not allowing the blowback??

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Did you put the caps back on the valves ?? They are very easy to get lost ~!~!

BlazNT said:

The valve will be opening and closing at the wrong time. It will also not be opening wide enough.
Set the valve lash at 1/4 turn past TDC on the compression stroke.

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That makes a little more sense, I've been setting it at top dead center. I'll give to a try in the morning and let you know.

zman111666 said:

I wonder if you don't have a fuel problem, to where it is running way to rich. It's not quite as likely if you have the Walbro carburetor, but if it has the Nikki, those things are kinda picky what with the fancy o-ring setup.

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Thought about that as well but I couldn't find anything wrong. It is a Walbro carb.

Boudreaux In Eunice La. said:

Did you put the caps back on the valves ?? They are very easy to get lost ~!~!

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Caps are on the valve stems

Ok, this one is a little wild, but what the heck:
Did you check the oil again recently, to make sure the level hasn't risen? If suddenly you have more oil than you put in it, that would be gas leaking passed the carb, into the combustion chamber, passed the rings, and into the oil. It would explain the increased consumption, odd running, blow-by (too much pressure in the crankcase gotta come out somewhere) and hard starting. Once it gets to a certain point, the cylinder will become hydrolocked and not want to go at all. Did you try cranking without the plug in? Does it blow out a lot of gas if you do?
Another vague possibility: Is it possible something went wrong when you put on the new piston rings, (missing ring, spaces not staggered, etc) causing too much blow-by there, and increased crankcase pressure? A compression test might help determine either that or a valve not seating.

BlazNT said:

The valve will be opening and closing at the wrong time. It will also not be opening wide enough.
Set the valve lash at 1/4 turn past TDC on the compression stroke.

Click to expand...

Worked in it all day trying different lash positions and gaps - was able to get it to run fair by setting the lash at TDC with .004 intake and .006 exhaust still with excessive fuel consumption . The 1/4 past TDC at the same gaps, the motor would not start.
I just don't get it????

Ok Bert and IL Engine have posted on other threads about this before. Head might not be flat and cylinder block might need a stone on it.

I assume you torqued the head to specs.

It almost makes me think that you might be one tooth off on your cam gear if it's not starting 1/4 past TDC.....

Just a thought....

Boudreaux In Eunice La. said:

Ok Bert and IL Engine have posted on other threads about this before. Head might not be flat and cylinder block might need a stone on it.

I assume you torqued the head to specs.

It almost makes me think that you might be one tooth off on your cam gear if it's not starting 1/4 past TDC.....

Just a thought....

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Yes, I thought about that as well even broke it down again and checked the timing marks and they were spot on - I'm totally lost now!

Double check the valve operations with a pencil down the plug hole while you rotate the engine.
If both the valves are not closed at exactly TDC then the cam is out.
AT TDC there may still be some pressure on the inlet due to the decompressor but the valve must be closed

Boudreaux In Eunice La. said:

Did you put the caps back on the valves ?? They are very easy to get lost ~!~!

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I am not sure what you think this would affect as long as the clearance is correct. Not all of these engines had caps on the valves according to the Service Manual.

Walt Conner

Is it spitting a mist back out the carb, dribbling, or popping and backfiring? Not to be critical, what are the cam timing marks on your engine, and were they clearly marked, and meshed?

Tinkerer200 said:

I am not sure what you think this would affect as long as the clearance is correct. Not all of these engines had caps on the valves according to the Service Manual.

Walt Conner

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Well Tinker this model came with the valve caps on them. That is critical on these mowers. The same as not getting the push rods in just right towards the crank case side.

Getting the right valve clearance is almost or impossible without the caps on and having the rods in right.

You go by the book. I go by experience, in fact over 35 years worth of it. Yes I do make a wrong decision and jump the gun on here every once in a while. Well maybe one wrong one, but I have helped a lot of people on here with the right advice and I try my best to bring other PRO'S in too. I don't try to inflate things or make them harder for the people that don't have the experience like we do.

If we tell people to go buy a service manual that costs them 40 bux with the shipping what good are we being on here to give out advice to help them out. ?? We try to give them advice in easy and step by step lay men's talk. Not techie stuff that we learned in Briggs school....

I am not in any way trying to put you down Mon Ami. I am your friend on here........... Boo

Boudreaux In Eunice La. said:

Well Tinker this model came with the valve caps on them. That is critical on these mowers. The same as not getting the push rods in just right towards the crank case side.

Getting the right valve clearance is almost or impossible without the caps on and having the rods in right.

You go by the book. I go by experience, in fact over 35 years worth of it. Yes I do make a wrong decision and jump the gun on here every once in a while. Well maybe one wrong one, but I have helped a lot of people on here with the right advice and I try my best to bring other PRO'S in too. I don't try to inflate things or make them harder for the people that don't have the experience like we do.

If we tell people to go buy a service manual that costs them 40 bux with the shipping what good are we being on here to give out advice to help them out. ?? We try to give them advice in easy and step by step lay men's talk. Not techie stuff that we learned in Briggs school....

I am not in any way trying to put you down Mon Ami. I am your friend on here........... Boo

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Well while your 35 years of experience is impressive, I have close to 70 which means nothing. I expect the people who designed and built these engines have a pretty good insight into the operation of their engine. One thing I have learned in the ~70 years, sometimes the hard way, is to use the manual. The parts for the engines with and without caps are the same except for the caps. There are plenty on instances on the 6 forums I monitor where people ask if both valves are supposed to have caps as theirs only have one and runs fine. They will adjust.

No need to buy an expensive manual, I have most of them available free for the asking and there are other sites which have them available free also.

Walt Conner

It would be nice to have a sticky thread with links to service and parts manuals.

cpurvis said:

It would be nice to have a sticky thread with links to service and parts manuals.

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What happens when you do that is some A-hole then downloads the lots and whacks them together and sells them on Evil Pay.
Then that come to the attention of copywrite holder and the link gets pulled.
Much better to dole them out as needed.
The K & T site you see on here a lot has most of what people need.
Stihl , Echo & a few others are really anal about chasing down people who publish their material.
You can give a person a copy of something legally.
However you can not broadcast or distribute some one elses material.

Then again you get broken links and orphaned web sites .
Finally there are a lot of "free" manual sites hat will load your computer with malwear or just plain use your details for identity fraud.

First two paragraphs of this post are boring tech talk. Skip if not interested.

An engine with a correctly timed camshaft and good sealing intake valve should not push anything out the intake port except during the "overlap" portion of the camshaft, and I doubt if small engines have much, if any, overlap. Overlap is a condition where, as the exhaust valve is closing and the intake valve is opening, both are open for a brief time. Lots of overlap is what gives that "lumpy" idle to engines with "race" cams.

Overlap is used to improve the high-rpm performance of engines and a 3600 rpm engine doesn't qualify as one. FWIW, overlap is controlled by 'lobe separation angle' and is built into the camshaft on engines with single cams. Today's variable valve timing engines use actuators to advance or retard one or both camshafts to change the lobe separation angle. Widely separated cam lobes (i.e., little or no overlap) are good for low end torque. Narrowly separated cam lobes let the engine produce more power in the upper rpm range. Variable valve timing is a way to get the best of both possible worlds. (They also control basic cam timing itself, i.e., where the cam is set relative to the crank, but that's another subject.)

So much for geek talk.

In this case, the OP says he's re-checked the valve timing and it's correct, that is, the marks are lined up.

My next stop would be a visit to the intake valve to see if it's sealing. Was it replaced during the rebuild? Is it closing all the way? Lapped in? Seat damaged?

bertsmobile1 said:

What happens when you do that is some A-hole then downloads the lots and whacks them together and sells them on Evil Pay.
Then that come to the attention of copywrite holder and the link gets pulled.
Much better to dole them out as needed.
The K & T site you see on here a lot has most of what people need.
Stihl , Echo & a few others are really anal about chasing down people who publish their material.
You can give a person a copy of something legally.
However you can not broadcast or distribute some one elses material.

Then again you get broken links and orphaned web sites .
Finally there are a lot of "free" manual sites hat will load your computer with malwear or just plain use your details for identity fraud.

Click to expand...

Let me re-phrase...It would be nice to have a sticky thread with links to service and parts manuals IF it could be created without any of the issues associated with copywright infringement and computer problems, such as viruses, malware, etc.

Now do you agree? That would be nice, wouldn't it?

cpurvis said:

Let me re-phrase...It would be nice to have a sticky thread with links to service and parts manuals IF it could be created without any of the issues associated with copywright infringement and computer problems, such as viruses, malware, etc.

Now do you agree? That would be nice, wouldn't it?

Click to expand...

There is a popular Forum, which I won't mention here, that has a "sub forum?" which is dedicated to Service Manuals.

Walt Conner

By the way, how the heck do you access PMs on here, I get a message I have one but find no way to see it. Never mind, I finally found it along with a couple of of others I never answered, please folks, don't send me PMs on this forum.

Then how can I ask you the name of the other forum?

cpurvis said:

Then how can I ask you the name of the other forum?

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Address below, put in proper format and remind me what you want.

Walt Conner
wconner5 at frontier dot com

Tinkerer200 said:

Address below, put in proper format and remind me what you want.

Walt Conner
wconner5 at frontier dot com

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Done.