damn thing won't run

[AllThumbs]

I built the twin in my profile pic 2 years ago. At that time I tried to run it and it wouldn't go. There were several issues. No compression being the biggest. I put it away. I finally got it back out and replaced the head gasket, redid the valves, put a bigger flywheel.

It now has great compression, but it still won't run. If I put a drill on it I have heard it pop twice but it's not really even trying. It's a single coil spark (both plugs fire at the same time (ie both plugs in series). The crank is offset 90 degrees so it sparks twice on each cylinder, only one at the right time for each cylinder if that makes sense. With the plugs out I can see them spark but it's acting like no spark under compression? There is lots of fuel and the timing is right so that's not it. I am running a 12 V coil on about 9V. I am currently charging a 12V battery so I will try it on that soon. If I get it running you will be the fist to see the vid.

Any other suggestions?

Eric

 

[Maryak]

It's a single coil spark (both plugs fire at the same time (ie both plugs in series).
Eric

IMHO if both plugs fire at the same time then I think they are in parallel and maybe there is not enough voltage to generate an adequate spark under compression for both plugs at once. Not great on electrics but it just doesn't feel right to me.

Hope this helps

Best Regards
Bob

 

[rleete]

Firing twice per cycle is not uncommon (it's refered to as "wasted spark"), but that's on full size engines with plenty of juice.

Between the 9V on a 12V system and the dual spark, you aren't throwing enough spark to ignite. Bump the voltage up, and try again.

 

[Maryak]

Another thought

Could you remove one plug and see if it will run on one cylinder. ???

Best Regards
Bob

 

[AllThumbs]

Ok, it's firing but on only one cylinder now. I get that familiar colman fuel burning smell and I can hear it fining, plus that exhaust pipe is getting hot. Still won't run. The two cylinders share the same carb. I wonder if I need to run dual carbs now. I am not sure how it's liking sharing the mixture. Only the number one cylinder is firing. I know the spark is working for sure. The problem with the spark was actually the condenser wasn't connected. :.

Eric

 

[stevehuckss396]

You say your crankshaft is 90 degrees out of phase. It seems to me that both pistons should be going up and down together. They should be firing alternately. This way the wasted spark scheme would work perfectly.

Am I thinking correctly?

Maybe you could drive a distributor off the cam some how and then you could get the spark to the cylinders when they need it.

Was this build done over at CNCzone? I think I remember it.

 

[AllThumbs]

The spark is working now. There is a cam on the cam shaft that drives the spark 90 degrees apart and then nothing for 270 degrees. Cylinder 2 doesn't seem to get fuel. They both have a common carb. It should work (only one intake valve open at a time). Not sure why it doesn't. I think a new manifold and dual carbs might be next. It could be that the two cylinders need their own fuel metering setting. I am using an OS Max carb and I think the bore is too large as well. Yes, the build was on CNCZone.

 

[AllThumbs]

Here is a better image:

 

[Ed T]

Try blocking off the intake arm going to the cylinder that's working and see if the other will fire. Not familiar with the engine, what's the crank arrangement? Do the pistons travel up and down together but fire alternately or is one going up while the other is going down?

 

[AllThumbs]

My own design engine.

Correction, the crank is 180 degrees out of phase, not 90.

I will try blocking one cylinder and see if the other one will fire.

Eric

 

[Rustkolector]

Allthumbs,
I can't see how your intake manifold is sealed against the head, but a 'slight' leak in the intake manifold can make starting difficult.

You haven't mentioned the fuel system. Try closing the throttle to nearly closed when starting. That provides good fuel draw and higher air velocity for better atomization. Make sure fuel level is 1/2" or so below the carb. I would also consider using a degree wheel and confirming the cam and spark timing, just to be sure.
Jeff

 

[stevehuckss396]

Cylinder 2 doesn't seem to get fuel.
I am using an OS Max carb and I think the bore is too large as well.

A single carb should work fine. Try a carb that has a bore of about 1/8 inch or so. You would be surpised how small the bore needs to be. Also be sure the cam is opening the valves at the correct time. Check the rings and valves for a good seal. Other than that, I got nothing.

 

[kuhncw]

Hello Eric,

You said the spark plugs were connected in series, so both fire together (waste fire). You also said the crank throws are 180 out. At least that is what I understand.

If you fire #1 at TDC, #2 will be at BDC on the inlet stroke. Could you be igniting the mixture in #2 at this point which would mean there is nothing to burn as #2 reaches TDC compression and gets a spark. I believe you said #2 spark follows #1 by 90 cam degrees (180 crank degrees).


Hopefully I have not misunderstood something here. Like Steve said, waste fire works when the pistons travel together, but not when they are 180 out.

Regards,

Chuck Kuhn

 

[Lakc]

. Cylinder 2 doesn't seem to get fuel. They both have a common carb. It should work (only one intake valve open at a time). Not sure why it doesn't.

If you have a lot of valve overlap, one cylinder blowing out when the other is supposed to suck in can kill the fuel out of the carb, especially in tiny intake runners.

For difficult to start engines, we used to overfuel them and then use a blast of compressed air. For model engines, an aquarium pump or a stiff wind from a fan may just do the job. Propane feeds a lot easier then liquid fuel and wont flood a plug out either. Simply waving an unlit torch near the carb ought to give both cylinders a big dose of combustable hydrocarbons.

Spark can be a funny thing at times. It can spark all day out in the open, and still jump down the insulator when faced with actual compression. At least swap the plugs between cylinders and see if you move your misfire.

 

[AllThumbs]

All good advice. The wasted spark is indeed at BDC of the intake stroke on cylinder 2. I didn't think it would cause a burn due to no compression at this point. A crank with no offsets would work but then I would also need a new cam, and definitely 2 carbs. The 2 stroke snowmobile engines use a wasted spark with a 180 degree offset crank and it works. so I thought this should work too. When I get time I will block off the intake to cylinder 1 and see if two will fire on its own.

Eric

 

[gbritnell]

Eric, I have been following this thread, reading what everyone has said and all have made some good points.

Here's what I would do.

I know you have done some of these things but just bear with me. First go through the basics of the engine. Check the compression, the timing, the rocker arm to valve clearance, you know, the basics.

With those things being done there are certain issues that shouldn't be causing a problem, the waste spark being one of them. As long as your ignition will handle 2 sparks at the same time it shouldn't be a problem. As far as the waste spark occurring on a cylinder without any compression it won't (shouldn't) ignite the mixture. I have a similar situation on my V-twin engine. Although my waste spark occurs when the one cylinder is half charged it still doesn't ignite the mixture.

The siamesed intake also should not cause any problems. I have the same thing on my OHV 4 cylinder engine and it works fine. As to the size of your carb, it looks quite small so that problem should be minimal.

At this point let me ask a question, how are you trying to start it? By that I mean what are you spinning it over with? It's possible that with the cam configuration you have it will require a fairly good spin to get it to fire. I use a model airplane starter for my OHV 4 cylinder engine.

I would set your air bleed screw to half to three quarter open and the needle valve just cracked, maybe 1/2 a turn. Now start turning it over. Open and close the throttle lever in small amounts. If it fails to start open the needle valve a little farther. Repeat until it seems like it wants to start. From there the carb will need to be adjusted a very tiny amount. Don't even worry about the air bleed screw until you get it running at 1/2 to 3/4 throttle. Once it will run continually at higher speed adjust your needle until it runs 'clean'. With yours being uncooled it won't take long to get hot so I would just concentrate on getting it running first.

Once you have a good setting on the needle let the engine cool and restart it. Now you can adjust the air bleed screw. As you shut the throttle down you will have one of two conditions. The mixture will be rich or lean. If it's rich the engine will get lazy and slow down or die. If this is the case, open the screw a little. If it gets lean if will quit all most immediately. For this situation close the air bleed screw a little. If you find that no matter how much you open this screw, to the point of totally uncovering the air bleed port, that the engine is still rich then the air bleed port and screw will need to be enlarged.

Above all be patient. One some of my engines the fine tuning takes weeks at times, changing the timing, modifying the carb etc. If all the basics are good the engine will run.

gbritnell

 

[AllThumbs]

Thanks.

I am using a drill to try and start it. I know from experience that this is a bad way to do it but if I can get the motor banging on both cylinders I will switch to rope start.

I don't have time right now but soon I will try some of the suggestions.

BTW the carb has a bore of about 3/16". I would prefer 1/8". I have built a single cylinder engine (webster) with a home made carb that does run, so I can build a simple carb if I need to.

It's true the engine is uncooled as is altho it does have water jackets built in for liquid cooling if it ever runs.

Here is a pic of the parts.

 

[kuhncw]

George, thanks for the comment on the waste fire. I had not realized you were firing your Vee Twin that way. I'd been thinking a distributor was required on a vertical twin with crank pins 180 out.

Regards,

Chuck

 

[Ken I]

I just picked up on this thread.

I hope I am not misreading this but in a "waste fire" configuration a duplex coil is used - a single primary with two H.T. outputs.

You cannot connect two cylinders off a single H.T. output in paralell as the non compressing cylinder will offer way less resistance and all the spark goes there.

I read a comment about plugs in series - I suppose that will work as long as there in no odd current path to the block.
I presume you have the H.T. lead on one plug and the "ground" end of the ignition coil on the other - but then how are you driving the coil - if you are using points its going to strike to earth here as well with this set up.

As Chuck pointed out this only works (as on a VW Golf) for paired cylinders travelling together (one coming up on compression to fire and one exhausting - waste fire on the exhaust does nothing).

Hope you get it running.

Ken

 

[AllThumbs]

The plugs are in series. One coil. "Ht" on one plug, "ground" on the other. The points are insulated from the block of the engine. Both plugs are indeed firing at the correct time and I do not believe that spark is the issue at this time.

I really appreciate the interest in the thread. I have several new ideas to try and isolate the problem.

Eric