Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines
The Regular Stuff: Chat, Buy, Sell, Off Topic, etc. => General Discussion - Scale Model Gas Engines - Hit & Miss - Throttle Governed - Non-Compression – etc => Topic started by: 70Rcode on March 12, 2020, 11:54:20 pm
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Thx Gil, for the helpful valve timing info....on the cold running issue, my experimental brief elevating the cyl head temp >120 deg using heat air gun let me dramatically lean out the A/F needle V setting. Perhaps a simple non-metallic head gasket would cause similar heating & actual
fuel Vaporization phase for cleaner improved combustion ?....thx again,..tom.
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For some reason, my M90 seems to hit harder with more impulse torque with exhaust valve closing early about 25 degs Btdc with 0.012" lash on Coleman + 10% MMO fuel mix.
I am not stating fact here as certain knowledge.... just possible theory.
In the late 1970's, car engines were set up to recirculate some of the exhaust gases back through the intake along with fuel and air. This was supposed to assist with cleaner emissions. I am not certain I followed the logic but I do know that it was supposedly causing the engine to act as if it had somewhat higher compression by having exhaust gas take up some of the combustion chamber space. I wonder if closing the exhaust valve early leaves enough already inert gas in your chamber that there isn't enough room for a full charge of fuel/air mixture. It also might let it run a little hotter and these do run too cool to get the most complete combustion.
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I checked my lash and it was .o25. The engine runs fine and I think I'll leave it alone for now.
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For some reason, my M90 seems to hit harder with more impulse torque with exhaust valve closing early about 25 degs Btdc with 0.012" lash on Coleman + 10% MMO fuel mix.
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Try a valve lash of .031". That will help some. The engine will run good with the exhaust beginning to open 10-20 degrees before BDC and closing at or very slightly before TDC and firing at TDC. (fuel: Coleman or other pure fuel or white gas mixed 15:1 with Marvel Mystery Oil)
Best of luck and have fun!
Gil
Thanks for the info Gil :-)
Ade
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Any predictings on which exh valve Lash (tighter, looser or no change) Works best ?....., tom.
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Contrary to classic H&M theory, adjusting the Cam for corect exh valve losing at Tdc resulted in noticably Slower Weaker firing running !!!. I reset cam back to 38 Btdc .. i will keep it there & play with the valve Lash....mayb didn't this M90 didn't read the "Classic H&M" timing book !!
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Thx Gil for that tuning tip ! I'll open the lash up & see how it responds. It's not likely i would have thought to make a lash adjustment. Big thanks again for the help...tom.
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Hi all, Got around to checking my M90 Exhaust valve timing. It's Opening at 46 deg BBDC & Closing at 38 deg BTDC at the valve (seems advanced). Is this (especially Closing point) off enough to noticably affect running performance ? I thought exhaust closing should be around 5 deg ATDC & does it warrant correcting
as i only have this one.
...thx for any info, ..tom
Try a valve lash of .031". That will help some. The engine will run good with the exhaust beginning to open 10-20 degrees before BDC and closing at or very slightly before TDC and firing at TDC. (fuel: Coleman or other pure fuel or white gas mixed 15:1 with Marvel Mystery Oil)
Best of luck and have fun!
Gil
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Thx Ade for info...i'm running the instruction spec'd lash of 0.5mm (0.20")...Also my 1st H&M engine experience. It usually starts 1st crank hot or cold & hits scary hard (about 45/min) that i'm kinda
relunctant to fiddle unless anyone experienced Knows resetting to Tdc will make noticeable running improvement. Crank & cam was tampered with during prev bearing upgrade so anything non-factory is possible. I need to know if resetting closing from current 38 Btdc to Tdc will make it better running...thx again, ..tom.
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Hi Tom,
I am no expert on engines that use atmospheric pressure to open the inlet valve. I too would be very interested to hear from someone with experience here.
Regarding the exhaust valve staying open after TDC into the inlet stroke, this is a design for higher revving engines where inlet and exhaust valves overlap a little to help start the gas flowing as early as possible. IMHO this is not applicable to these engines for 2 reasons, 1. the engine speed is very low and 2. if the exhaust valve remains open into the inlet stroke then part of the inlet stroke is wasted as reduced pressure cannot form in the cylinder to open the inlet valve if the exhaust valve is still open. However if the exhaust valve closes too early and the piston is still rising to TDC then again positive pressure will be in the cylinder at the begining of the inlet stroke. Therefore i would suggest the exhaust valve closes at TDC.
The exhaust valve duration is strongly effected by the valve clearence, we are taught that long duration is good for power, but again only in high revving engines. The exhaust valve duration on these engines seems quite long with a 'normal' valve clearence setting which results in the exhaust valve starting to open about 1/2 of the way down the power stroke when set to close at TDC before the inlet stroke, so maybe more power could be gained by increasing the valve operating clearence to reduce duration and setting the exhaust valve to open later toward the end of the power stroke but still close at TDC before the inlet stroke?
I'm sure Jin has looked at this and chosen wisely, but these little engines are great to experiment with :-)
Kind regards
Ade
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Hi all, Got around to checking my M90 Exhaust valve timing. It's Opening at 46 deg BBDC & Closing at 38 deg BTDC at the valve (seems advanced). Is this (especially Closing point) off enough to noticably affect running performance ? I thought exhaust closing should be around 5 deg ATDC & does it warrant correcting
as i only have this one.
...thx for any info, ..tom