carb exhaust

Some years ago I built the Centaur engine but have have not got it to run.I have good compresion and I think that the valves are good.I think the carbureter is at fault with my mistakes in construction.I have started to make a new one from bar stock and as I can't form a 90 Degree bend as the original casting I wondered if ther are any problems with drilling two holes at 90Degrees to form an exaust in the carbureter,my concern is the sharp change of direction for the exhaust gasses.

Frank

I can't imagine it restricting the exhaust to the point of not running. Usually it is not as critical as the inlet port, due to much higher pressure of exhaust gasses, and have seen model engines with intake ports made with two right-angle drillings. It's not unusual in OHV model cylinder heads.

Did you see the thread some months back about someone having trouble getting a Centaur to run? There was some confusion over the cam timing diagram and massive valve overlap in ETW's drawings apparently. Dont remember the issue being resolved though.

That might have been a thread started by me. I eventually made a pair of cams with 5 deg. more opening angle than the drawings and set the timings in about mid-postion to that specified, but opening periods are still less than the design values. This cam modification was suggested in an ME article from Nov. '76. My Centaur runs, but not very well. Starting isn't easy and it will only run at wide throttle openings, misfiring and with a rich mixture. Clearly work to do, there are examples out there that appear to run well. In my case I suspect the carburettor but I'm not sure how to make improvements.

However, I've put the Centaur to one side for now and have turned my attention back to my loco. I'll be interested in Bricky's progress.

No idea of the actual part except from your description. You could certainly use a ball ended end mill for the drilling to obtain a nice swept curve on the outside of the bore. If you can get in there with riffler files the sharp edge on the inside can then be softened.

regards Martin

When I built my Centaur I used the Carb casting but cut the exhaust passage off as I didn't want the "hot spot" exhaust passage.

I used a separate stainless steel pipe bend, you could maybe do the same with your new carb, don't ask me why I did away with the hot spot it was just fad on my part, and it didn't affect the running of the engine.

Thank all of you for your replies and after seeing Michael's solution I am going with his idea.I did think of doing it like this but didn't think the fuel would reach the cylinder but it oviously does thanks michael for showing such a superb example of this engine.

Frank

Here's a link for my Centaur to show the running that I referred to in my post above.

https://youtu.be/Pka5BRnpSLM

Edit; why doesn't the video embed in the post?

Edited By Clive Brown 1 on 29/10/2022 11:34:11

Michael, what cams and timing did you use on your Centaur, just out of interest?

It certainly seems to make sense to keep the hot exhaust right away from the carb and intake port. Looks the goods too.

You have to click on the little YouTube icon at the top of your post when editing it, then follow the instructions to copy and paste the share/embed link from YouTube. No idea why. Here tis. Looks good.

Too much valve overlap could make it run rough at low revs like that.

Edited By Hopper on 29/10/2022 11:39:11

Thanks Hopper, I thought I'd followed the instructions but obviously not.

You probably followed most of them, but the crucial but is on Youtube itself.
Rather than copying the URL of the video, go to Share, as you may have done, but then then Embed, and you will get something like this to copy, rather than the shorter version you saw

<iframe width="560" height="315" src="https://www.youtube.com/embed/Pka5BRnpSLM" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

Post that lot (control V) into the Youtube radio button window on this forum, change the Width to 450, and the height to 253 and hit return.
Initially on your post you just see a blank white space with a red surround, but the video appears when you add your post to the forum


Bill

Hi Hopper, I made the cams as the drawing, the valves are timed with no lap (Video of engine)

Ah! further thanks. I think I've now got it. I was looking on my Youtube Channel Content page where I could only find "shareable link". I'll stop drifting this thread now.

On the face of it, a gas fueled engine should not need a hot spot, since the fuel is already a gas, and the fuel/air mixture is easily induced into the cylinder..

A petrol fueled engine might well need some heat input to the mixture to provide the latent heat of vapourisation of the petrol.

In the case of a paraffin, (kerosene ) fuel, heat will, be needed to vaourise the fuel. (Which is why TVO engines are started on petrol, to generate the heat needed to vapourise the TVO (Tractor Vapourising Oil, aka paraffin / kerosene.

Excessive heat input into the induction side should be avoided, to minimise charge heating, and loss of charge mass. the engine produces power based on the mass of fuel burned, not volume.

To me it sounds as if the problem with starting and running stems from valve timing. Excessive overlap will waste fuel by allowing mixture to be carried over into the exhaust, with the need for an apparently rich mixture.

The overlap is there to compensate for the inertia of the fuel/air column, (Which is why the Inlet opens just before tdc ) and the exhaust valve closing just after tdc, using the departing exhaust gas to help "suck" the mixture into the cylinder..Once the fuel/air column is in motion, it's momentum will continue to propel it into the cylinder, so that the inlet valve can remain open until slightly after bdc.

At a wild guess, try Inlet open 5 before tdc, close 5 after bdc; Exhaust open at tdc,(Opening too far before tdc will cause residuals to be injected into, and oppose, the incoming charge ) close 5 after tdc.

Those figures might get the engine to start and run, hopefully

Howard.