1/4 scale V8, first project.

[keith5700]

George, thanks for your drawing. I'm going to re-design my plug and see if I can get some sort of bush soldered to the end. High temp adhesive isn't going to work very well in ptfe I suspect.
You are more organised than me looking at your drawing!

Ian, the valve heads are 8mm dia, and the stem big dia is 1/4".
I happened to have the springs already, but they originally came from Entex Springs in Nottingham, UK. Whereabouts are you?

I would be wary of giving you the exact spec at the moment as I have no idea if they will be strong enough for the job.
Valves take 1.3kg (2.9lbs) to just start to open. I suspect the valves are going to bounce at high rpm but I'd rather start off
too soft than too hard.
Cheers.

 

[keith5700]

Just taken some full view pics as a lot has change since the last ones.

It doesn't look much different to last time really, but nearly all the front is new, as well as bits and bobs elsewhere.

 

[rythmnbls]

Those full view pics remind me of publicity shots of F1 engines published by the likes of Ferrari or BMW, simply exquisite.

Regards,


Steve.

 

[camnefdt]

hi keith

would love to see/hear this beauty running!! hows the build coming?

 

[keith5700]

Hello strangers!
For the last 6 weeks I've been hoping to post some news on how the engine sounds when it's running, but I'm still struggling to get it to start.
Note this is with a simple Walbro carb and not the injection.
I figured that if it won't run anyway then there's no point spending 6 months on the injection system.

So, I wired up the ignition system, or 'bodged' is a better word and fitted the surface discharge plugs.
Then spent 2 weeks over Xmas trying to get it to run.

I can honestly say I never got a single bang out of it the whole time. This is on petrol, propane and even Easy Start.

Eventually I decided it had to be the spark plugs which were the problem.

The Powerspark ignition system is quite handy in that if the plugs aren't firing then it shows up on it's display.

With the plugs on the bench they sparked fine, but I already knew that it is harder to fire them once they are under pressure.
So I made a small pressure tester from some clear acrylic bar.
The upshot was that the plugs wouldn't fire over about 20psi. During testing I knew the cylinders were generating about 60psi during cranking.

Then I made a 'normal' type spark plug, like George Britnell described in an earlier reply.
This managed about 35psi, whatever the gap was set to, or wherever the spark energy was set to on the Powerspark.

I'm guessing that when fuel is added then the spark can jump easier than in dry air? otherwisw there's no point carrying on.

I'm just making 8 more plugs at the moment.

Took a week out and made a big wing sump tho, just to keep my interest up!

 

[trumpy81]

Keith, that is disappointing for sure. But keep at it, you'll crack it sooner or later.

Any chance of running glow plugs and methanol/nitro? Or maybe ditching the Powerspark for something else? You can't beat a transistor assisted Kettering System

What version of Powerspark are you running?

What's that noise I hear in the background Keith, it sounds like a real motor .... lol

 

[t.l.a.r. eng]

Sounds more like a weak ignition coil.

 

[vcutajar]

Keep at it Keith. I am sure that eventually you will manage to get it to run. Looking forward to hear it rumble.

Vince

 

[alihureiby]

amazing :bow:

 

[keith5700]

The ignition system is Powerspark Rci-200. This is their twin cylinder version. I reckoned if it will run a twin cylinder 2-stroke at 30000rpm then it would be fine for mine. I think now that I should have gone a bit more basic just to get the engine running, as this is another unknown really, but then again it should be more than up to the job.

Another unknown is the crappy carb I've fitted. The donor assured me it came off a running 50cc engine. Even if it's not 100% suitable I would have thought I'd have got a few pops and bangs at least by now.

Poor old engine looking a bit bedraggled:-

Ignition set up with 2 x cheapo coils

New spark plugs, gapped at 20 thou'

If anyone knows for sure that any part of this set up is rubbish then I'm quite prepared to junk any of it and seek alternatives. Apart from the engine!

 

[keith5700]

Luc, I've had a vacuum gauge on it. This was in place of the carb, not from a manifold tap with the carb installed. It pulled just over 2"hg, which is 3-4psi, at around 1000rpm.
I realise a 50cc V8 won't pull anything like the vacuum a single cylinder 50cc would, but I don't know if that vacuum will pull any fuel through that carb.
Or if that's a normal vacuum for this size of engine.
Then again, it won't even bang once with an ether based fuel, which leads me to think the problem is ignition based.

 

[trumpy81]

Keith, for starters that carb looks way too big for this engine. A rule of thumb is that the venturi size should be roughly equal to the size of a single intake valve.The reason for that is because there is only one intake valve open at any given point in time.

Luc's suggestion of a wet plug test is also a good indicator, but I suggest you flood the tip of a plug with a drop of motor oil, enough that it covers both electrodes. If the plug still sparks whilst it is flooded with oil then it is definitely not ignition problems, except for timing perhaps.

What are the coils you are using?

They look to be a good small size for model engines. I might try one of those myself.

 

[Lakc]

Sometimes life was easier back in the days of points.
If you loose spark at that little pressure, Your either short of ignition energy (coil size, coil primary voltage) or short of insulation and the spark is finding an easier way home. Swap the coils out if you can, use a big old fashioned oil filled automotive type.

 

[trumpy81]

Keith, looking at the first photograph again, I think your cam timing may be a little off. It could be just the angle or parallax error on my part, but cylinder 1 looks OK, but cylinders 3,5 & 7 look to be off a little, particularly cylinder 7. Cylinder 7 appears to have almost 180 degrees of separation but this could just be my interpretation of the photo. Might be worth checking though?

 

[trumpy81]

Luc, I don't quite understand what you mean?

 

[keith5700]

Ok. 3 or 4 psi is vacuum not positive pressure.
Cam timing is spot on, I must have checked that 20 times, each time hoping to find something amiss, but have not.
Lobe separation is 110, inlet full open 106 after tdc.
I swapped the plug centre electrodes tonite to copper. They were steel, but no change.
The power spark keeps going into sulk mode, even with the plugs on the bench.
With full sized plugs it fires them fine. I put some 6k resistors in series with my plugs to copy the full sized ones, but no difference.
I,m ready to junk the power spark and get some points set up, and a big fat coil.
Meanwhile I'll look for a smaller bore carb too. Maybe a small rc plane one?
Thanks, Keith.

 

[trumpy81]

Ok. 3 or 4 psi is vacuum not positive pressure.
Cam timing is spot on, I must have checked that 20 times, each time hoping to find something amiss, but have not.
Lobe separation is 110, inlet full open 106 after tdc.
I swapped the plug centre electrodes tonite to copper. They were steel, but no change.
The power spark keeps going into sulk mode, even with the plugs on the bench.
With full sized plugs it fires them fine. I put some 6k resistors in series with my plugs to copy the full sized ones, but no difference.
I,m ready to junk the power spark and get some points set up, and a big fat coil.
Meanwhile I'll look for a smaller bore carb too. Maybe a small rc plane one?
Thanks, Keith.

Thanks Keith, I guess it's just the way I am looking at the cams in the photo, which is what I suspected.

There's no reason you can't make a temporary carb. Use a sewing needle for the main jet and a simple barrel as the throttle. Don't forget to include a proper venturi in the barrel though. It doesn't need to be fancy as long as it meters fuel. I can do a plan for a simple carb if you like.

If the Powerspark is indicating problems then maybe points is the way to go, or at least another ignition module. The coils should be OK though, a simple test would be to wire them straight to a battery and simply touch the - negative lead to the terminal quickly and watch for corresponding sparks from the plug. You may need to reduce the spark gap a little when in use though.

Using copper for the electrode should have indicated whether or not it's the insulation breaking down, or it could indicate that there is too much resistance in the electrode, given that it is a very long electrode. I would think that it would be unlikely, but anything is possible ... lol

 

[trumpy81]

the firing order if like a GM is
1-8-4-3-6-5-7-2
knowing that the cam is 2;1 ratio with the crank
so it fires the fisrt 4 ( 1-8-4-3) ten start all over
with ( 6-5-7-2) in other word take a cam shaft data for the firts
four cylinder and off set it for the next4 by 180 degree and your in business.
how can I explain this in a simple way:idea:
if you would use a piece sign (two finger) and your cam lobe would point
at 10:00 and 2:00 for number 1 cylinder then the 5TH cylinder cam
would point at 8;00 and 4:00 180 degree apart
this goes for a 4 bagner also
hope I wrote it properly:fan:

Luc, if I understand you correctly, then what you say is true.

However, I was referring to the lobe separation between the intake and exhaust cam lobes of cylinder 7 (the last cylinder on the right of the photo), I thought the photo showed them to be at 180degrees apart and not 110 degrees as they should be.

It's OK though as Keith has checked this and all appears to be correct. It is just the way that I am viewing and interpreting the photo.

 

[trumpy81]

Luc, that is completely untrue in regards to the capacitor. The purpose of a capacitor is too suppress the arcing across a set of points. It has nothing to do with the actual operation of a basic ignition system.

BTW I was not offended by your statements.

Luc, parlez-vous français?

 

[Lakc]

Luc, that is completely untrue in regards to the capacitor. The purpose of a capacitor is too suppress the arcing across a set of points. It has nothing to do with the actual operation of a basic ignition system.

BTW I was not offended by your statements.

Luc, parlez-vous français?

Yes, but any primary energy used to sustain the arc at the points does not contribute to inducing the energy you need in the secondary (spark plug) side.

With full sized plugs it fires them fine. I put some 6k resistors in series with my plugs to copy the full sized ones, but no difference.

The extra distance between the electrode and shell in the full size plug adds a ton of dielectric strength. Its possible, or even probable, that your spark is finding an easier path somewhere along the plug.
What material are you using for an insulator in the plug?