Vega V twin Aero engine

[creast]

As I mentioned previously, I am concentrating on getting at least one cylinder running and I have to thank messrs Britnell, Mayhugh and Rupnow for their previous threads elsewhere regarding valve seating which has given me more insight into getting these things running. My Vega is definitely suffering from seating problems. Compression is there and then it isn't. I have boosted the spring force by shimming the springs but it hasn't solved the problem. I am also getting blow back through the carb, sometimes badly.
I intend to move away from the seating in the alloy head and go for a bronze seat guide arrangement which means considerable re-work but I feel I have to get this better.
One encouraging thing is I do know it is capable of running as per the YouTube link, albeit several short runs of around a minute at best.

[ame]https://www.youtube.com/watch?v=n1SHT9w39xc[/ame]

 

[creast]

Well, I thought I was making progress but over the last week or so I have got very close to chucking the towel in. This engine is just is so fickle to get trying to run.
My original heads didn't seem to seal well on the valves and after making some tweaks I managed to wreck one of the heads so made two new ones which had lower volumes since I suspected compression was an issue.Valves seem to seat good in the aluminium.
Just working on one cylinder to run is a challenge and I wonder if I should have picked an easier project first. I have never made a glow engine before never mind a 4 stroke twin!
I have tried a commercial carb to rule out my own but in fact the Vega carb does just as good so that rules that out. I have tried various glow plugs and now on OSF type as they claim to be best for 4 stroke plug.
Fuel is 5%Nitro castor base.
The only way I can get life out of it for short runs of about 30 secs or so is by heating the cylinder with my micro blow torch to get the cylinders warm.
I have altered compression by using a gasket and that does not make any difference.. it will still run after heating on the lower compression.
I thought the cold weather may be the cause but |I remember as a teen running cox baby bees in winter? Today was 6deg C so not too cold.
Does anybody know what might be at fault?

Yours desperately,
Rich

 

[canadianhorsepower]

Hi Rich
Here are my sugestion :hDe:
#1 I would make a small hole anywere in the crankcase (that could be plug latter)
#2 I would build a small plug attachement to replace the glow but allows you to bring the pressure in the cylinder
with both valve close to about 15 lbs to check for any leaks. (the hole in the crank case would indicate piston rings leaking.
#3 when all of the leaks are fixe I would do a mecanical compression test. ( the chamber looks pretty big)
and it will determine your valve timing is good or not.

#4 send it to me Id love to have it as a trophy Rof}Rof}Rof}
good luck

 

[petertha]

The only way I can get life out of it for short runs of about 30 secs or so is by heating the cylinder with my micro blow torch to get the cylinders warm. Rich

Rich can you elaborate. You mean if the engine is cold it wont start/run well at all period? But if pre-you warm the cylinder barrels you get a run but only 30 seconds, or it keeps running thereafter? Sounds like you are doing all the right things, so keep eliminating variables one by one.

Are you keeping power on the glowplug during running? What happens if you pull the power off?

The heat has me mystified depending on your answer. Maybe its affecting compression by the piston/liner fit? Remind me once again, what's the materials configuration?

How about the rocker/valve gap? I've seen a tiny amount descrepancy cause havoc with I/E timing, especially if it floats or similarly effected by heat.

 

[creast]

Hi Rich
Here are my sugestion :hDe:
#1 I would make a small hole anywere in the crankcase (that could be plug latter)
#2 I would build a small plug attachement to replace the glow but allows you to bring the pressure in the cylinder
with both valve close to about 15 lbs to check for any leaks. (the hole in the crank case would indicate piston rings leaking.
#3 when all of the leaks are fixe I would do a mecanical compression test. ( the chamber looks pretty big)
and it will determine your valve timing is good or not.

#4 send it to me Id love to have it as a trophy Rof}Rof}Rof}
good luck

Luc,
Thanks for the input.
The crankcase has a drain hole already and one point is that the bottom end relies on blow by for lubrication (as per design plans) so the ring/rings must leak a bit anyway.
I confess to changing to a single ring but compression does seem fair, but its hard to tell what's right when you have never built one before.
The air test will be good to check the valves so I will make an adaptor.
Sorry but it will stay as an ornament here if it never runs (cheeky Rof})

 

[creast]

Rich can you elaborate. You mean if the engine is cold it wont start/run well at all period? But if pre-you warm the cylinder barrels you get a run but only 30 seconds, or it keeps running thereafter? Sounds like you are doing all the right things, so keep eliminating variables one by one.

Are you keeping power on the glowplug during running? What happens if you pull the power off?

The heat has me mystified depending on your answer. Maybe its affecting compression by the piston/liner fit? Remind me once again, what's the materials configuration?

How about the rocker/valve gap? I've seen a tiny amount descrepancy cause havoc with I/E timing, especially if it floats or similarly effected by heat.

Hi peter,
If the engine is cold then it will do the odd sputter but not enough to run proper. Warming seems to encourage it to fire and once running I try to tweak the mixture which it does respond to but often dies after say 30 seconds. It needs plenty of priming to run and this is often borderline of locking up so I have to be careful.
When running I have found putting the tank level higher than the carb helps but then is prone to flooding when it stops.
I can't help but think a fuel feed issue is part of it.
I need to work on the carb since the mixture screw is too loose and I have to hold it whilst running or it unscrews (not making life easy am I?)
Power is left on the plug and by the time I get to unclip it the engine has stopped anyway.
I think warming is probably helping the fuel to vapourise and making it easier to ignite, but thats just a hunch.
Valve clearances are around 0.002" and I have fiddled with these to see if it changes the character.
Thanks for your input
Rich

 

[James6605]

Rich
Fantastic effort on the engine mate! Well done.

That problem you mention with the mixture screw unscrewing itself rings my alarm bells.
If the screw does not seal properly in the carb spraybar thread, you are losing the vacuum signal generated by the airflow that will draw your fuel into the airstream.
Try slipping a piece of rubber tubing over the needle and spraybar where they screw together to seal it.
This concern is confirmed by the fact that raising the tank helps.

All the best
James

 

[petertha]

I experienced kind of the opposite issue where a (worn out) 4S starts up ok when cool, but fades away & dies after warming up or any kind of load. You then find yourself mixture control tail chasing because one setting cant satisfy transition between 2 very different running conditions.

So your pre-heat thing has me wondering... I've never done this on RC-4S & so not even sure what to expect.
http://www.dynamiterc.com/Products/Default.aspx?ProdID=DYN2514
But do you think you could rig up something like this, apply it to your non-running cylinder, then switch cylinders, then with/without pre-heat. Its almost like the heat is affecting your overall compression, but I'm having difficulty visualizing how (typical ringed aluminum piston in steel liner in aluminum sleeve)? Usually a good running RC engine has a nice compression pop-over bounce over TDC, but that's a feel thing, not really a number. I'm not sure if this gauge preserves the net high pressure on the needle or if it would just bounce around & tell you nothing.

One of those el-cheapo temp gauges might help quantify heat. ie what is pre-heat vs running heat vs (maybe friction induced heat)?
http://www.amazon.com/dp/B00144E8A4/?tag=skimlinks_replacement-20

The elevated fuel tank is essentially making it richer fuel/air ratio all things being equal. But richer on these fiddly things can also have other tangent offshoots with pre-mix oil glow fuel like head temp & to lesser degree compression. But yes, you need the carb needle valve / mixture nailed down first, otherwise everything is out of whack.

 

[petertha]

Another thought (because the Vega carb & induction pipe is a bit different than typical RC) - do you have good seal between carb body & manifold stack interface & stack / cylinder port? Typically a combination of soft durometer o-ring & gasket paper is employed. This can be another temperature induced variable with differential expansion. I've seen cases where the smallest micro-gaps cause grief. You can jerry-rig test with some gasket silicone spooged around the joint as long as the metal is cleaned with brake fluid.

 

[creast]

Rich
Fantastic effort on the engine mate! Well done.

That problem you mention with the mixture screw unscrewing itself rings my alarm bells.
If the screw does not seal properly in the carb spraybar thread, you are losing the vacuum signal generated by the airflow that will draw your fuel into the airstream.
Try slipping a piece of rubber tubing over the needle and spraybar where they screw together to seal it.
This concern is confirmed by the fact that raising the tank helps.

All the best
James

Thanks James, that's a very useful tip. In fact I had seen similar on a second hand engine I bought and thought it just to stop unscrewing but your comment on air leaks makes sense.
Cheers
Rich

 

[creast]

Another thought (because the Vega carb & induction pipe is a bit different than typical RC) - do you have good seal between carb body & manifold stack interface & stack / cylinder port? Typically a combination of soft durometer o-ring & gasket paper is employed. This can be another temperature induced variable with differential expansion. I've seen cases where the smallest micro-gaps cause grief. You can jerry-rig test with some gasket silicone spooged around the joint as long as the metal is cleaned with brake fluid.

Thanks for all your comments Peter.
I am sceptical about the pressure gauge only in how much volume is in the gauge itself could influence the compression reading. A very small strain gauge transducer would ensure minimal extra volume but a lot more complexity.
I take note of your comments and as agreed, the carb needs sorting first. I will also double check the valve timing but I don't really think this the issue.
Yes, the carb track is long and tortuous and I did ponder on trying a carb directly onto the head to see if that works better.
So many things to check! I have stripped this thing down so many times I must confess to losing enthusiasm but when it fires up I get back up. But for how long LOL.
Cheers
Rich

 

[creast]

I experienced kind of the opposite issue where a (worn out) 4S starts up ok when cool, but fades away & dies after warming up or any kind of load. You then find yourself mixture control tail chasing because one setting cant satisfy transition between 2 very different running conditions.

So your pre-heat thing has me wondering... I've never done this on RC-4S & so not even sure what to expect.
http://www.dynamiterc.com/Products/Default.aspx?ProdID=DYN2514
But do you think you could rig up something like this, apply it to your non-running cylinder, then switch cylinders, then with/without pre-heat. Its almost like the heat is affecting your overall compression, but I'm having difficulty visualizing how (typical ringed aluminum piston in steel liner in aluminum sleeve)? Usually a good running RC engine has a nice compression pop-over bounce over TDC, but that's a feel thing, not really a number. I'm not sure if this gauge preserves the net high pressure on the needle or if it would just bounce around & tell you nothing.

One of those el-cheapo temp gauges might help quantify heat. ie what is pre-heat vs running heat vs (maybe friction induced heat)?
http://www.amazon.com/dp/B00144E8A4/?tag=skimlinks_replacement-20

The elevated fuel tank is essentially making it richer fuel/air ratio all things being equal. But richer on these fiddly things can also have other tangent offshoots with pre-mix oil glow fuel like head temp & to lesser degree compression. But yes, you need the carb needle valve / mixture nailed down first, otherwise everything is out of whack.

Peter,
I just found this simple but possible tester by Jan Ridders.
http://modelengineeringwebsite.com/Compression_tester.html
At least it doesn't introduce much additional volume.
Rich

 

[Syd]

Hi Rich. I hesitate to offer advice on small engines when I've never built one in my life, whilst turning skills are such that I live for the day when I'll successfully part off a workpiece without snapping the tool! A Mill? dream on!
However, as part of a misspent youth, I devoted much time and effort on single cylinder motorcycle engines, both for a one kick start and then getting away quicker than the other chaps on the start line.
When you're staring at an uncooperative lump of machinery that won't do what Herr Otto says it should, it's worth going right back to basics:
You absolutely must have a flammable mixture of fuel & air.
Air is sucked into a cylinder by way of a valve, opened to create a partial vacuum created by a downward moving piston. This valve closes around bdc. and the vapour is compressed by the now rising piston and at tdc, a spark ignites the fuel vapour, expansion of which causes the piston to travel down again.
Around bdc a further valve opens and the gas is forced out of the cylinder by the rising piston.
It is very, very easy not to have both valves firmly seated and sealed throughout the compression stroke (and most of expansion stroke of course. Guessing that there will little, if any, valve overlap.)
Air leaks, must be eliminated, the only air entering the inlet manifold, must come from the carburettor inlet. (your loose screw?)
Fuel must be drawn from the jet into the carb. by air passing over it, not by spilling over, otherwise it will not atomise sufficiently. (fuel level at the jet should be same as in the tank, unless the carb. has a float chamber.)
The smaller the engine, the more important good compression, so very small piston and ring clearances.
A reasonable compression ratio (perhaps of around 7-10 ?)
A good spark at the right time. (On my engines it could be anything from 3 to 9mm. before idc, but this was with an 88 - 90mm stroke.)
Hope something here might prove useful, but I thoroughly enjoy all the threads and fascinated by the quality of workmanship.
Cheers, Syd.

 

[canadianhorsepower]

any good news on your vega
cheers

 

[creast]

any good news on your vega
cheers

Hi Luc,
I only just managed to get some time on the engine this week.
I made the Ridder compression tester to compare to other engines I have and the R H cylinder was higher than a rather worn Enya 19 which runs so I thought maybe my compression was too high?
After checking against another engine which gave equally good compression I ruled that theory out and simply tried to run her again. Guess what? She ran for minutes at a time. The only thing different was the ambient temperature, its a good 5 degrees warmer than previously.
I tried the other cylinder tonight and that ran without warming but died after a few minutes and compression was gone. I removed the head and fitted a blank one which is showing its rings/piston.. probably worn out whilst testing the other one! :wall:
So, tomorrow is 'make a new piston/ring combo time' to match the other one which is still feeling good.
Am keeping my runs running rich for now to ease it in.
Cheers,
Rich

http://youtu.be/Xzf2S02-kfk

 

[canadianhorsepower]

well thats good news Thm:Thm:
thanks for posting

 

[creast]

I could do with someone else making one to put me right
All the others that have built this beast either never completed it or do not respond to my pleas for guidance, pioneering stuff eh?

 

[canadianhorsepower]

I could do with someone else making one to put me right
All the others that have built this beast either never completed it or do not respond to my pleas for guidance, pioneering stuff eh?

well well well Thats all I needed:fan:
I'm still waiting on some material to build Brian's engine:wall:
I'll give that one a shot. I have all the material here

so Its a go:hDe:

 

[petertha]

..simply tried to run her again. Guess what? She ran for minutes at a time. The only thing different was the ambient temperature..

That's great news! The other thing I forgot to mention in my 99 ways to cuss at RC engines is old, skunky fuel. Methanol attracts moisture which can be an issue in humid, cool climate even over a single winter storage & maybe further hampered by lights ends vaporizing off. Particularly if the lid is not air tight or a half filled container. Sounds like you are on the right track now, but that's another trick to try. I would also give it a tank test of higher nitro content just to see. Maybe you can find a small qty at hobby shop. If it starts & idles better as is often the case, its a simple fix & cant really hurt anything unless you go to too high for plug/CR(like +30%).

 

[creast]

Thanks Peter,
I have pondered that but I have been working on the 'if the other engines run on it then it should be fine' basis, however the other engines are 2-strokes so probably sustain running easier.
I have also been playing around with various glow plugs but settled on an OS F type for now.
Cheers
Rich