V-TWIN---MAYBE V-4

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

rick9345

Member
Joined
Mar 28, 2012
Messages
7
Reaction score
7
Things have been crazy/busy around here this past week. My wife organized a "Walk for Parkinsons" charity walk on Saturday. Weather was great, over 100 folks showed up and she raised over $32,000 for Parkinsons research. I've been working on a robotic work station for one of the big 3 automakers and this morning they changed everything that I've done in the past two weeks, but I still get paid for what I've done. Today I made a new piston for the cylinder which was leaking pressure into the crankcase, because I couldn't stop it leaking past the rings. I think the previous piston had it's bottom ring too close to the wrist pin hole. Tomorrow I will pressure test that cylinder again, and hopefully the leak is fixed. I also learned the nasty truth about socket head capscrews today.---I love the look of them, but after you've tightened and loosened them about 5 times, the socket rounds off and then they can't be tightened any more, and even worse, the tight ones can't be loosened. Of course, I'm always an optimist, thinking they will only have to be tightened once.---HAH!!!
over torqued always rounds the small ones .and or inexpensive Allen keys
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
You really need a screwdriver-type torque wrench for the very small torques of small screws - and believe the torque figures from the manufacturer of the screws! I have a lb-in. torque wrench - max 120 lb.-in, and that is too big for sub-1/8" screws!
Then finding adapters to get a small hex end can be problematic!
I have some electricians nut spinners which are OK for those particular screws (8BA, 4BA), but hard to find for many small screws like 2.5mm hex. socket...
K2
 

Brian Rupnow

Design Engineer
Project of the Month Winner
Joined
May 23, 2008
Messages
14,235
Reaction score
7,447
Location
Barrie, Ontario, Canada
After stripping the engine down to it's basic parts, I haven't found anything that would prevent it from running. I replaced the piston on the side that was leaking past the rings a little bit, and I've opened up the cylinder heads a little bit around the sparkplug ends. The Traxxas carb which I had originally ran was a bit questionable, so I have replaced it with a home made carb based on Malcolm Strides plans. The engine is now reassembled, and I'm going over the second ignition coil as I write this. The spark seems to be a lot weaker than I would like, but I have some more testing to do before I replace it. I hope I can sort that out, as a 12 volt coil costs $70 here. I may have to try and return the coil to the automotive parts house where I bought it and hope for a free replacement.
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Hi Brian, 3 checks immediately for what seems like a weak spark...
1: Battery voltage and current when "running"... and
2: dwell angle - I.E. the part of the revolution of the cam when contact breakers are closed. On a Vee-twin, one is more than 360 degrees, the other less than 360 degrees. If the battery cannot handle the current draw of the coil, the current will drop significantly (Very visible on an analogue meter - but not a digital one?). So, in that case you may get a better spark with the longed dwell angle, and a poorer spark on the one with less dwell angle.
But most ignition systems are more than capable of providing good sparks up to 6000rpm or so firing 2 sparks per revolution! But "model" coils are possibly smaller/less robust than "car-type" coils?
3: The 3rd. check is the resistance of the circuit from coil through the contact breakers to earth (battery terminal). If this is higher than "not a lot" (continuity) then the spark will be weak. Clean contacts with alcohol. (Not abrasive, as the platinum is so thin nowadays, you'll rub it off with 2 wipes of emery!).
K2
P.S. Excuse me if you already have done these ...
 

Gordon

Well-Known Member
Joined
Jan 4, 2011
Messages
1,207
Reaction score
265
After stripping the engine down to it's basic parts, I haven't found anything that would prevent it from running. I replaced the piston on the side that was leaking past the rings a little bit, and I've opened up the cylinder heads a little bit around the sparkplug ends. The Traxxas carb which I had originally ran was a bit questionable, so I have replaced it with a home made carb based on Malcolm Strides plans. The engine is now reassembled, and I'm going over the second ignition coil as I write this. The spark seems to be a lot weaker than I would like, but I have some more testing to do before I replace it. I hope I can sort that out, as a 12 volt coil costs $70 here. I may have to try and return the coil to the automotive parts house where I bought it and hope for a free replacement.
Just try switching the two ignition boxes or perhaps just the coil. I have had good luck with COP (coil over plug) coils. You can buy eight of them for less than $70.
 

Brian Rupnow

Design Engineer
Project of the Month Winner
Joined
May 23, 2008
Messages
14,235
Reaction score
7,447
Location
Barrie, Ontario, Canada
After spending a ridiculous amount of time fussing with this engine, I think I have narrowed the reason it won't run to be that wild and crazy intake manifold/fan mount system. The engine has compression, valves are timed correctly, and ignition is timed correctly. The one thing that I am seeing and I haven't seen this before, is that while the engine is being cranked with my variable speed drill it is absolutely not pulling any fuel from the gastank up to the carburetor. I tried this with a purchased Traxxas carburetor and with a home made carburetor. This is a totally new one on me---haven't seen this before. At any rate, I have built a new fan support, which is totally independent of the intake system, and I have built two stubby carburetor mounts that work with Traxxas carbs---I have two new ones on order.
bpE6MM.jpg
 

minh-thanh

Well-Known Member
Joined
Dec 29, 2016
Messages
1,255
Reaction score
803
Location
Viet Nam
Hi Brian !
There's one thing I've been thinking about for a long time that I don't know if I should say......
You should clean - really clean all engine parts before assembling and after assembling the engine
Only one or a few small Swarfs will destroy cylinders, pistons, O rings...and affect intake valves, exhaust valves...
And more , when the engine is clean, you will easily adjust everything and easily find the reasons that the engine does not run or does not run well, ....
 

Nerd1000

Well-Known Member
Joined
Dec 7, 2020
Messages
108
Reaction score
73
Location
Australia
To check whether it's a fuelling issue, why not try running the engine on a squirt of starter fluid? If it fires and runs smoothly that rules out ignition issues.
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Can you make a simple gravity feed to the carbs?
I understand the difficulty of the large volume of intake manifold compared to the displacement of the engine. The manifold depression is not great enough to suck the fuel at cranking and slow running speeds... Maybe a faster cranking motor could help starting, but if you have fuel starvation nothing will resolve that except more fuel.
The twin carb solution was the answer for motorcycle V-twins back before I was born....

Good luck!
K2
 

Brian Rupnow

Design Engineer
Project of the Month Winner
Joined
May 23, 2008
Messages
14,235
Reaction score
7,447
Location
Barrie, Ontario, Canada
You can not gravity feed these carburetors. They do not have a float and float needle to shut off gravity fed fuel. If the tank is higher than the carburetor it floods like crazy and pees all the fuel into the carburetor and then into the cylinder or out onto the floor. This type of carburetor uses venturi produced vacuum to suck the fuel up from the tank which is always located below the carburetor.---As far as keeping everything clean is concerned---I keep everything clean enough. May still be dirty on the outside, but it's clean where it matters.
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Thanks Brian, It was just a suggestion.... being a few thousand miles away means I can't just pop over and stick my nose in.... but I figured you were trying to lift fuel an inch or 2, when carbs usually only lift a half inch or less? And with reduced depression at start-up, maybe only a tiny fraction of an inch? The fuel tank is nearly the same level as the carburettor, so perhaps it just needs filling a little, to just short of flooding?
Ideas are free... but only the right one is more than worthless.
Cheers, you'll crack it soon.
K2
 

johwen

johwen
Joined
Apr 28, 2010
Messages
138
Reaction score
34
Unless your carb has a throttle Brian a smaller choke bore will give better carburation at a slower speed and the engine will run smoother. Give it a try.
Cheers.
John
 

werowance

Well-Known Member
HMEM Supporting Member
Joined
Sep 2, 2011
Messages
1,283
Reaction score
335
so Brian, whats your ideas on why it wouldnt suck fuel with your original intake? im not asking because i have a suggestion or anything, just wondering is all. are you thinking that one cylinder is letting the suction pass thru to the other cylinder during its intake stroke thus not putting enough suction on the carb? or something like leakage around the solder joints or mounting joints? or what are you thinking?
 

Brian Rupnow

Design Engineer
Project of the Month Winner
Joined
May 23, 2008
Messages
14,235
Reaction score
7,447
Location
Barrie, Ontario, Canada
I'm fairly convinced that it had to do with the intake not sealing tightly enough to the cylinder head at the flanges----or---When the one cylinder was on the intake stroke, the other cylinder may have had the piston in a position to let air pass thru it to the cylinder which was on the intake stroke, so that there wasn't enough air passing thru the carb to create the venturi effect, which creates the suction to pull gas up from the tank. Both ignition systems are working okay, giving the right amount of spark at the correct time. The valve opening sequence is correct, as set by using a degree wheel. Both cylinders are well sealed by rings and valves, --when I put 40-50 pounds of air into the cylinder with my adapter that screws into the sparkplug hole, they are not leaking anywhere. The rocker arms are set for 0.010" clearance from the pushrods when the piston is up on compression. I did notice that the flange on the cylinder head which is supposed to be in line with a flat space machined into the cylinder in the same spot wasn't perfectly flat--The flat spot on the cylinder stuck out about 0.010" from the flat spot on the cylinder head. I have since filed the cylinder flat spot to be perfectly flat and in line with the flat spot on the cylinder head where the flanges match up with the intake manifold. I was using cardboard gaskets there, but they may not have totally filled the gap created by the flat spots not being perfectly aligned.
 
Joined
Apr 2, 2020
Messages
2,678
Reaction score
810
Location
Sunderland , UK.
Hi Brian. I am looking forward to your results when your twin carb set-up is fitted. I think that the separation of the 2 intakes - with shorter intake tracts and smaller volumes - will make a big difference.
I read the comments about one cylinder try to suck while the other has the Inlet valve open... surely your valve timing diagram can confirm this? But I would have expected these timings to be well spaced...? Looking at my Moto Guzzi valve timing curves, based on crank angle, inlet 1 opening 18 deg BTDC - call this TDC zero degrees. So we have zero degrees, inlet 1 open, until 230 degrees. Then inlet 2 opens at 432 degrees, and closes at 680 degrees. Then inlet 1 opens at 702 degrees. = 18 degrees BTDC at 720 degrees, or zero degrees. But my engine is a 90 degree V-twin. The gaps where both valves are closed are 202 degrees and 22 degrees. I think your V is closer than 90 degrees, so those closed angles will be closer in value....??
Hope you get it figured out soon....
K2
 

Brian Rupnow

Design Engineer
Project of the Month Winner
Joined
May 23, 2008
Messages
14,235
Reaction score
7,447
Location
Barrie, Ontario, Canada
This morning held a startling revelation!!! I got up early and sneaked down to the bat cave, and as I was pondering why my engine wouldn't start when everything was adjusted correctly, I was fondling the carburetor. When I had been trying to start the engine a few days ago, it had good spark, the valve and ignition timing were set just right, but when I tried to start the engine it simply would not suck up fuel from the gas tank. I've never seen that occur before, and it left me scratching my head and made me decide to move on to two carburetors with individual short inlet pipes and a separate stand for the fan.---And as I fondled the carb, the thought suddenly appeared in my head "I wonder if the carburetor is letting the fuel thru it?"--So, of course, I picked up the carburetor and tried to blow air thru the inlet where the flexible gas line attaches to it. (Another sort of "Blow yer guts out" kind of thing). And it was plugged tighter than a fishes arse. That tiny little tube 0.055" outside diameter x 0.025" inner diameter was plugged absolutely solid. I tried high pressure air to clean it out--no luck. I tried to push a very small diameter wire thru it---no luck. NOW I know why the carb wasn't sucking up any fuel from the tank. Remember, this engine did run briefly, but that was with a different store bought carburetor on it. Big mystery is solved!! As soon as my new Traxxas carburetors arrive, I will try and start this engine again, with dual carburetors on it.
qerhsX.jpg
 

Latest posts

Top