Newbie Question - Intake Valve Stem Clearance

[JPar]

Hi everyone, I've been lurking here for awhile but this is my first post. I have a model of a Fairbanks-Morse hit & miss engine. This engine was machined by someone else from a casting set sold by Tom Stuart. The engine came into my possession a couple of years ago. I've been fiddling with it off and on, but have never gotten it to run. It will fire weakly, but not strong enough to run. I don't think the builder ever got it to run either.

The engine has compression; not great, but should be enough to run. When I got the engine, it had a mechanical ignitor but I couldn't get it to work reliably. So I've converted the engine to spark plug ignition with a Hall sensor. For fuel, I've tried both gasoline and propane (via demand regulator) with about the same results.

One thing I've noticed is that the fit of the intake valve stem seems to be very sloppy; I can wiggle it from side to side quite easily. The drawing calls for a 0.124 diameter stem in a 0.125 guide, giving a clearance of 0.001 (or 0.0005 radially). I measured the actual diameter of the stem with my calipers at 0.0120. Assuming (for now) that the guide was correctly drilled/reamed to 0.0125, I've got an actual clearance of 0.005 (0.0025 radially). This seems excessive to me.

So my question is, with this amount of clearance could I be getting enough air leaking around the stem to affect the mixture to the point where I'm not getting good combustion?

Any and all thoughts/ideas would be appreciated.
(I was hoping to insert a photo here, but it looks like I can only insert photos hosted from a 3rd-party site.)

Thanks,
John

 

[JPar]

I figured out how to upload a photo:

 

[retailer]

I have no experience with this type of engine but going by my experiences with small stationary engines if the valves are leaking you can usually hear the air leaking when you put your ear close to the inlet or exhaust port while slowly turning the engine over.

 

[Cogsy]

If you can wobble the valve around then the valve will not be returning to its seat consistently and will not be making a good seal. When I make valves and cages/guides I use a reamed cage as a go/no-go for the valve stem as I turn it. I aim for the tightest fit I can get that doesn't bind. I think you need to remake the valve (luckily they're not too difficult to make).

 

[JPar]

Thank you for the suggestions. It looks like I'll have to make a new valve. I've got a lathe, but have never made a valve. One of the model suppliers sells valves with a 1/8" stem, would it be best to start with something like that and then turn the seat to the correct size? Or should I just start with a piece of round stock? The drawing doesn't specify the material. Would it be OK to use a leaded steel like 12L14?
John

 

[Brian Rupnow]

John--I make my valves out of cold rolled steel. I made two this afternoon, you can see them on my Sideshaft Horizontal thread. There are two schools of thought about how tight or loose valve stems should be in the guides. I, like Cogsy, go for a close sliding fit in the guide. However, I have heard other model engine mechanics who say that the fit shouldn't be too tight in the valve stem area, because if there is any misalignment between the guide and the seat a zero clearance guide will prevent the valve from aligning itself in the seat to provide a good valve seal. For me that second solution is not really a factor because my guide and seat are all machined in one set-up, so there is never an issue of concentricity.
Here is a link to a tutorial I wrote on making i.c. valves a few years ago.
http://bbs.homeshopmachinist.net/th...el-I-C-engines?highlight=machining+i.c.+valve

 

[Rustkolector]

John,
The intake valve could very likely be the problem, or not. It could be the exhaust valve, the piston ring side clearance, or the roundness and fit of the rings in the cylinder. The fit quality on the valve guide you mentioned could extend to any, or all of the above possibilities. The guide clearance should be corrected since an air leak there will upset the fuel/air ratio. Start with a vacuum check of both valve seats from the intake and exhaust ports on the head. A hand vacuum pump (brake bleeder from HF) is the best valve seal checker. A good sealing valve seat will allow pumping down to 25" of mercury with a very slow, or no leak down. Yes, you actually can get an absolute valve seat seal, but it isn't necessary.

If the valves seal OK then you have to look at the cylinder roundness and ring fit to piston and cylinder.

Jeff

 

[JPar]

Thanks for the responses. Jeff, you have raised a good point - there are other possibilities in addition to the intake valve. The stem clearance bothers me, so I'm going to go ahead and address that first. And then tackle other issues one by one, until I can get it to run in a satisfactory manner.
John

 

[JPar]

I'm happy to report that after 4 months of fiddling and diddling, I finally got the Fairbanks running! I got a new intake valve from Debolt machine, and lapped both the intake and exhaust valves. That helped with compression and the engine would fire weakly, but not strong enough to run. I was trying to run on propane, and had drilled out the needle valve body a bit thinking that might help. I decided to re-try with gasoline and thought it would probably be a good idea to make a new needle valve body with the original size hole (#70) as called for on the print. While making the new valve body, I discovered that the original part was not to print, it projected too far into the venturi! I made the new part to print, and put everything back together. That did the trick!

Here are the old (top) and new needle valve bodies. The outside shape is different, but it doesn't affect the function.

Here's a photo of the engine. The blurry flywheel proves it is running!

Is it possible to upload a video here, or does it have to be hosted on a 3rd party site like YouTube?

Thanks again to everyone for your help.
John

 

[DanLins]

I enjoyed reading this thread, since I purchased one of the last casting sets from Tom Stuarts widow this last weekend. I think your kit or plans may have been an earlier version, since mine shows clearly on drawing FM-10 that the intake valve is to be fabricated from stainless steel. That sheet was last revised 5/3/2006, Revision A, relating apparently to an update on the valve port dimension.

This thread has certainly given me additional food for thought as I start down this path over the next few months.

Thanks,

Dan

 

[JPar]

I enjoyed reading this thread, since I purchased one of the last casting sets from Tom Stuarts widow this last weekend. I think your kit or plans may have been an earlier version, since mine shows clearly on drawing FM-10 that the intake valve is to be fabricated from stainless steel. That sheet was last revised 5/3/2006, Revision A, relating apparently to an update on the valve port dimension.

This thread has certainly given me additional food for thought as I start down this path over the next few months.

Thanks,

Dan

That's really interesting; thanks for posting! I just looked at my drawings and they are all dated 11June1988, and only sheet 12 (Ignitor) is marked with a revision (A). The intake valve material is not specified on my drawing.

Does your drawing set include plans for a cooling tank? Mine does not. I heard that Tom added that to the set at some point.

I wonder if it's possible to obtain a set of updated drawings? I would be willing to pay a fair price for them if they're still available.
John

 

[DanLins]

John,

Sure, I'll be happy to work with you on the plans. Since I received them this last Sunday, I've gone through and highlighted a few items and made a couple of additional notes. Let me see if I can scan them and just send them to you via email or messaging on here. Save you a little money.

Mine were all prepared by Wayne Austin, who worked with Tom on them, dated 1/21/2006, but many have revisions. PM me with your email address?

Thanks,

Dan

 

[JPar]

Let me see if I can scan them and just send them to you via email or messaging on here. Save you a little money.

Thank you for the kind offer to scan the plans, but I think that might infringe on the copyright and I would rather not go there. Have you been in recent contact with Mrs. Stuart? If so, can you please send her contact info (email if possible) in a PM. I can contact her to see if she can provide updated drawings, and if not, I can seek her permission to get a scan from you.
John

 

[DanLins]

No, sorry, I'm sure she does not have an email. She will turn 90 soon. Let me check with her daughter and son-in-law to see if it will be ok to scan and send you a copy.

Dan

 

[Longboy]

You can isolate and determine compression leakage as well as porting leakage with a blow off nozzle for your air compressor. For your intake valve stem, at the carb mount to the manifold give it a blow through at a few pounds PSI and see what flies up that valve stem. A running engine with excessive clearance, stem to guide, may have an oil mess around the cylinder head coming up the stem. Over on the exhaust side a burnt carbon oil mess. You will certainly hear the hiss on a dry engine too.