My first 4 stroke engine runs, but blows trough the carb !

[Gedeon Spilett]

Hi
This is my first 4 stroke gas engine, a 10cm3 (0.65ci), that I have build in the last 4 months. In the picture, my engine wears his first carb, ready for a glow run, but now, he is fitted with a true sparkplug, HT coil and breaker points, and burns car gas.
I started with cheap alloy castings and a kit sold by a young buddy here in France, a true fond of model engine, his site (in french) is worth a visit, hence a link
http://fabrication-moteur.over-blog.com/pages/kits_moteur_proposes-1726109.html
Not a long and difficult built I think, but the development took a large part of it and is far from over, even if my engine is running for good now, short burst between long failure are gone (crossed fingers).
Without prior experience of a 4 stoke, it was really dificult to realize how sharp were the settings of the carb for instance.
I understand that a lot of tuning and tweaking is required to get the nice regular purring of some engines shown here!

I still have misfiring followed by unwanted explosions, visible (audible in fact) in the video, which are not so mysterious, but I also have spitting and coughing into the intake, yes, I feel some blow coming from the carb, and I'm scratching my head actually, how come this happens ?
Cheers

Here is the link for the video in my Picasa ME albums.
https://picasaweb.google.com/lh/photo/2CPZqZ0mkLXq5slfdEs6RNMTjNZETYmyPJy0liipFm0?feat=directlink
And for some pictures of the built.
https://picasaweb.google.com/113249...rAEssence4T10ccMK3?authuser=0&feat=directlink

 

[gus]

Nice looking engine.No worry.During my attachment in the British Army Engine Rebuild Factory,I was told not to look over direct at the Carb as it may backfire. The Engine Tester rev up and rev down the engine speed and true enough it spewed flames. His advice was bad carbs have a tendency to spit fire when revved down rapidly to low speed.Old fashion carbs are very hard to tune and fine tune.
May I suggest try putting in a motorcycle carb. While working on an antique Villiers Engine,I put a Honda carb and all my trouble went away.

 

[jasonh]

Gedeon-

Nice looking engine - I like the crankcase work and the cast carb. Your buddy has got some nice engine designs. It warms the heart to see those all those castings ready to be cut from the sprue.

I had a similar problem with a carb the other day. My issue was caused by a too long intake cam. ie- The intake stroke would pull in the fuel, but the intake valve would not be closed when the piston began it's compression stroke. So - fuel/air was being pushed in the wrong direction, and I was losing a significant part of the compression ratio. I'm not sure if this is your problem, but you may want to check that the intake valve closes at (or maybe slightly before) bottom dead center.

Regards - jasonh

 

[vcutajar]

Gedeon

Congratulations on getting the engine to run. Now it just requires fine tuning.

I thought I saw that engine somewhere else. I saw it on your friend's website some time ago. It's the Kiwi Mark III.

Vince

 

[Gedeon Spilett]

Thanks for comments !
looking at the actual valve opening diagram on my engine, it is rather close to the angle value generally acepted, and indeed admission closes after BDC, as is the case for a hudge number of 4 stoke engine, which do not backfire ! there is something here that I dont get !

Vince : you are right it is a kiwiMK3 !
I like the way you manage the built of your ETW KIWImKII a great thead and a lot of work, thanks

 

[Henry]

It looks great!
Could be that the valves are floating?( I am not sure about this translation to English from Spanish. Maybe the springs in the valves are not strong enough and them have not time to close well.

 

[robcas631]

Awesome! You say you are getting exaust in the intake? Have you tried to advance the timing?

 

[Gedeon Spilett]

hi,
the stiffness of the valve springs is something that is difficult to assess, I've tried several springs, and each time I change for a harder one, I will continue in this direction...simple
to change timing is now not so simple since I have pinned the gear to the camshaft, I can change by one tooth steps, 10°, it seems to me a lot, but doable.
Thanks

 

[Henry]

How is going this engine?
Are you been able to pin point the problem finally?

 

[Gedeon Spilett]

a careful examination of the opening and closing of the valves showed me that the timing were correct, so no change here.
But the thing that really changed something was a large increase in the stiffness of the valve springs, (to give an idea about 1.7 kg on the valve head are required to open the valve now). The effect was really huge on speed, about 6500 rpm now, and also on the noise. The idle adjustment and acceleration were also improved. It is now a fine runner.
So the problem was valve floating.
Unfortunately, these improvements were short-lived as the flywheel lock screw was unscrewed (I should have put a left-handed thread !), the flywheel has expanded the pin hole and went through the workshop, and gears, my own home made gears are in pieces, but no other damage.
I finished with 2 gears to be cut again, and for fixing the flywheel (probably oversized) I do not have too many ideas ... apart to do a new crankshaft.

 

[deverett]

That's bad luck with your fine engine.

One way to secure the flywheel, if you have sufficient metal to modify the crankshaft is to use a taper collet. These have been described several times on this forum, but basically you machine a taper on the crankshaft, make a collet with a matching taper. Split the collet lengthways. Bore the hole in the flywheel to be a close fit on the outside of the collet. As you tighten the screw on the end of the crankshaft, the collet expands and grips the crank and flywheel.

This system grips really tightly and the engine can run in either direction without any problem.

Dave
The Emerald Isle

 

[Gedeon Spilett]

Thanks for suggestions !
I must confess that I should have fixed that flywheel mounting before, as on 2 occasions the flywheel nut was unscrewed, and the locking pin on crankshaft was sheared off cleanly, so I put a hardened steel pin. This time the pin stays intact but its hole on the crankshaft is enlarged, there is room for 2 pin now.
There is a cone on the crankshaft and the flywheel, and the flywheel is clamped on the cone by a nut, a pin in the crankshaft and the flywheel prevents its rotation, a fairly standard mounting, I think.
Perhaps the execution (the matching cones) is not up to the standard required to support the forces involved in this engine. The flywheel weighs 320 g, mostly peripheral, running between 2500-5000 rpm, vroom vroom, quite funny to run.

I'm ready to change to a tapered collet as you suggest, I looked on the forum several examples, it is clear that there is something to change...

 

[Art K]

Hi Gedeon,

I used the collet type of mount that Dave mentioned. You bore the flywheel at the same taper setting as you turn the taper portion of the collet. then drill six holes in the flange of the collet. tap three as extractor bolts and drill the other three over sized to match drilled and tapped holes in the flywheel, these tighten it to the flywheel. I used this on my engine, the thread is "my first engine". If you do this my advise is to make the collet last to fit the flywheel.

Very nice looking engine, I look forward to seeing it running again.

Art