Briggs valve tappet clearance issue

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Mre4
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Briggs valve tappet clearance issue

Hi,  

I have an Hayterette with a 3.5hp Briggs and Stratton engine (code 94908, manufactured in 1980). As an exercise in learning more about small engines I decided to give the engine a bit of an overhaul, and have come across the following problem: When checking the valve tappet clearance, I found it to be greater than the specified figures - intake 0.012" and exhaust 0.020". I haven't ever lapped valves before but I assume that this would be too much clearance to lap away. The manual for these engines says to cut the valve seat to reduce the clearance, but this seems a little wasteful and I don't have the necessary equipment to do so.

What are the possible causes of this excessive clearance, and what avenues of investigation are there for me to try so that I can remedy it? 

 

Thanks,

Mre4. 

hortimech
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There are only a few possible

There are only a few possible reasons for this, the lobes on the camshaft could be worn, but this is unlikely. The valve seats could be loose, again unlikely and easily spotted if the valves are removed. The only other possible reason and the most likely, somebody removed the valves and attempted to grind the valves and rest the clearances, To reset the clearances you need to remove material from the end of the valve and it is all to easy to remove too much, especially if you use a grindstone.

If you cannot recut the seat and/or reface the valve, you will have to attempt grinding the valves in with coarse and then fine grinding paste, though this may not be possible, bearing in mind the large gaps you have.

wristpin
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As Hortimech says the cause

As Hortimech says the cause of your problem is probably a previous bad case of  of an inexperienced attempt to reset the clearances - aka  " finger blight". I would agree that it is probably not a practical proposition to try and recover the situation and if the engine apparently starts and runs OK I'd leave it alone. Only you will know!!

However one downside to be aware of is that the excessive valve clearances may disable the decompression feature that aids starting and result in snatch back on the recoil rope. That said, the inertia of the Hayterette's blade disc may cushion that to some extent. 

One other option is to find a cheap scrapper engine as a donor for hopefully unmolested valves which may give you more acceptable clearances . Over the years when ever I scrapped an engine the valves (together with the ignition coil) got saved and the assorted box of valves is the first port of call in such situations.

Mre4
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New valves are still

New valves are still available from Briggs Bits so I think I'll order some, hopefully that will resolve the issue and if not I'll know I need to keep digging. 

 

Thanks for your help,

Mre4. 

wristpin
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That will solve the problem -

That will solve the problem - at a price but remember that the new valves do not come "ready gapped"  so you will need to do that  - carefully!  Also,to make the most of the new valves the seats should have a light skim so that new is matching new without an excessive amount of lapping.

It is important that the clearances for both valves are set with the piston one quarter of an inch down the bore on the power stroke - that is the official Briggs way and avoids conflict with the decompression feature of the cam profile. When your engine was made the clearances were 5 - 7 thou for the inlet and 7-11 for the exhaust.

Mre4
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Would using old valves from

Would using old valves from another engine as you suggested earlier negate the need to skim the valve seats? I don't know off hand where I'd go to get that done, and from searching online valve seat cutters do not seem cheap! 

wristpin
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No really, perhaps it is an

No really, perhaps it is an unnecessary counsel of perfection ; just trying to make you aware of the options and possible consequences.    Whenever I have fitted new valves it has been necessary to remove a lot from the stem to achieve any clearance at all; give some thought as to how you will achieve this. It's not just a matter of removing metal but also of maintaining a " square" face.

Really, you need to start from a baseline of good valve faces and good seats; what permutation of old and new you use to achieve this is up to you. I don't think that you've answered my question about whether it ran before you pulled it apart. If it did and is just going to be your "play thing" I would be inclined to box it back together as it is. 

 

Mre4
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It did run before, yes,

It did run before, yes, although it was a little smoky. I would still like it if I could make the valve clearance as it should be, so that I can see if it makes any difference to how it runs and for the sense of satisfaction, but I will take your advice into consideration as to the fact that I may well end up worse off than when I started if I do. 

 

Thank you for your help, 

Mre4.