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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: engine-man on January 17, 2023, 06:54:02 pm

Title: R90s playing tricks
Post by: engine-man on January 17, 2023, 06:54:02 pm
Hi all,

I just for my first Jin's engine as a R90S. Before getting it I knew it won't be a smooth ride , however I'm facing some issues I can't figure out what's happening.

First thing first, is when I opened the spark plugs, I noticed one of the plug has a gap far more than the other head, almost abnormal  I tried to gently align it , but probably will order set a decent plugs.

I have been using VP Racing 50:1 Fuel in my gasoline engines and used it on this engine since it needs some oil for the engine. I'm not sure if it's the fuel or carb issue but either the engine runs pretty rough and all over the place, or it fails to run at all. I ran it for a short bit on lighter fluid and it seemed okay but I didn't want to run it without adding 2 stroke fuel which I don't have .

What I noticed is mixture of air and fuel coming out of the carb air intake and engine struggles to suck the fuel on its own at times.

I did set the carb screw to 1/4-1/2 turn or even larger but doesn't make much difference. I also noticed there is a screw on the throttle. Which I assume is the LSN. What's the right setting for that?

I know some others had issues like this,  so I was hoping if someone can help.
Title: Re: R90s playing tricks
Post by: rodnoc on January 18, 2023, 04:49:48 am
> …..the spark plugs, I noticed one of the plug has a gap far more than the other

Gap both plugs uniformly or the engine will idle and run rough, and be hard to start.  Ground an adjusted plug against the head to be sure the chosen gap creates a healthy spark, then set the second plug the same.


> …..I have been using VP Racing 50:1 Fuel….

These engines are low compression and therefore do not tend to ping, so they do not need high octane fuel or specialty blends.  Regular 87-octane gasoline mixed 40:1 with Marvel Oil creates a healthy ‘bang’ and keeps the piston’s o-ring wet enough for good sealing.


Wth the spark and fuel sorted out, combustion will be stronger, the head will run hotter, and carburetor adjustments will have more affect.  Then, if you want to get more involved, check valve lash and compression on both sides for uniformity.
Title: Re: R90s playing tricks
Post by: engine-man on January 18, 2023, 02:48:49 pm
Thanks Rod !

Got new set of spark plugs to just eliminate any issues. The tips of the original plug seemed cracked.

Managed to filed the exhaust valve rod on one of the cylinders, so it is sitting flushed and firing okay.

The only other thing, which I also seen in others video too is the right cylinder ( oppose to fuel tank one ) doesn't fire okay and has misses. I see a black wetness out of the exhaust pipe. I swapped the plugs and same happened. The valves seem to be okay from what I can. Haven't added any oll to crankcase and was running the engine on Zippo with tiny bit of 2 stroke oil. Other exhaust is clean as .

What could be the issue ? Almost seems too much fuel in the cylinder and when I take the plug out the gap is covered in fuel but no fouling happened.

Impressive little thing overall
Title: Re: R90s playing tricks
Post by: rodnoc on January 18, 2023, 05:20:37 pm
> ... the right cylinder ( oppose to fuel tank one ) doesn't fire okay and has misses. I see a black wetness out of the exhaust pipe.

Pull the plug from the left side and check compression feel on the right side. Then check the compression on left side, too. Does the right side feel soft? Low compression generates less combustion heat. A wet pipe could mean insufficient heat to burn off the fuel’s oil.  Low compression would also create misfires.

Valve leakage is not uncommon in these engines, so you may need to lap the right side.  


> ….. Haven't added any oil to crankcase….

Both ends of the rods and the crank journals need lubrication.
Title: Re: R90s playing tricks
Post by: engine-man on January 18, 2023, 06:18:22 pm
Thanks again, I think the valves don't seal up properly as I can see fuel spray out of carb intake.  How can I take out the valves on this engine ? I managed to file the valve rockets to sit even which helped.

Compression seems quite good on that cylinder. And spark from coil seems strong.

Whats strange is the black stuff from the right exhaust, I would have expected unburnt fuel to be more clear. It almost seems like oil contamination in the cylinder even though I didn't add any oil to crank case.  Probably need to take it apart and check the piston ring .
Title: Re: R90s playing tricks
Post by: engine-man on January 18, 2023, 06:21:04 pm
I noticed a greyish leak from the base of the cylinder. Does it mean the seal is damaged or an issue with the piston ring ?
Title: Re: R90s playing tricks
Post by: engine-man on January 19, 2023, 06:37:09 am
And the view of exhaust . The left engine is burning clean and works like a clock. The right one is a different story. However in one of the runs the engine worked beautifully and could Rev it high and stable . Not sure if the issue is the valve, carb or ignition system
Title: Re: R90s playing tricks
Post by: engine-man on January 24, 2023, 01:54:05 pm
Update for those who own this engine or interested to know what helped.

Basically I teared down the entire engine and looked at every bits. The biggest issues with these engines in my opinion is poor assembly and quality control, despite being produced in low numbers. But after hours and hours of work I managed to make it work to the level I am happy with and represents the real r90s character.

Things I have done:

× Replaced both piston rings
× Replaced sparks with Iriduim type
× Replaced the valve pushrods with a more accurate lenght to control intake and exhaust valves better
× Adjusted crankshaft position and pushed it towards the rear of engine by .7 mm
× lapped all valves
× adjusted the ignition cable by bringing the head towards the spark to increase ignition
× took carb apart and filed the HSN screw to sit it more tightly
× Added silicon gasket to both parts of each cylinder block to prevent air and fuel leak
× Shortened the fuel line, this helped with faster fuel pickup and less HSN
× Adjusted LSN to bring the idle speed to as low as possible without stalling to represent the low idle character of  real engine
× using mixture of zippo and VP 50:1  fuel with ratio of 3:1. That achieve the best ignition with minimal spark fouling . These engines have low compression and weak ignition. I don't know why the suggest 95 octane fuel but it's defitenly too high to suit. I personally beleive the best option is lighter fluid mixed with 2 stroke oil or marvel mystery oil as suggested by many

TIP ON CARB SETTING: if after priming the engine doesn't start at 1/4-1/2 turn from close, there is an issue somewhere. I start the engine from cold with 1/2 and throttle 1/3 open. I let it warm up with few throttle up moves, then slowly reduce the carb to 1/4. It should pur and run slow on lowest throttle position, might seem stalling but let it settle. As the engineering is not precise you won't have the exact replication of very low smooth idle as it would on the real engine , but close enough

One issue that still exists is the spitting fuel spray from car intake and I think it's as I managed to get the engine to the level I'm happy with, I will leave it as is for now. The casting of cylinder head is different in each cylinder when you look closely,  unless a precise sanding is done ( with no point of return) the valves and suction on both cylinders won't be exactly equal.

That's why I rather have these engined as kits option too, so those of us who are OCD can make it near perfect !
Hope this helps others with the same engine :)
Title: Re: R90s playing tricks
Post by: RedRyder on January 26, 2023, 11:26:26 am
Glad you are getting it dialed in, engine-man.

I use Marvel Mystery Oil at 20/1 or 25/1

I add it to White Gas (Coleman Fuel or Crown camping gas fuel)

or I use yard tool 4 cycle fuel or 50/1 2 cycle fuel with enough Marvel Mystery Oil to bring that mixture to 20-25/1 

I never use automotive gasoline an any of my model engines.
Most of it has ethanol and a lot of poisons designed to be pushed through a cataytic converter at 1400 to 1800 degrees.
Title: Re: R90s playing tricks
Post by: engine-man on February 01, 2023, 09:47:41 pm
Managed to sort the spitting intake on the carb as well. In all honesty Jin should sell these as a kit so we know from ground up to check and recalibrate everything.

Long story short, I found out the intake valve qnd exhaust rods are too long , causing the valve opens too much and too long during the storke. There is an option to loosen the screws that hold the rocker arm to bring it a bit forward but that causes vacuum leak in the cylinder as there is no gasket in either two halves of the cylinders. ( next on my list to make custom gaskets )

The solution I chose was to make my own rods both for intake and exhaust valves. That way we eliminate too much scavenging during the stroke. I used .89 mm pushrod from a hobby shop, they seem to be a bit thicker and provide a better seating inside the rocket arms.

What I noticed was, each cylinder requires a different measurement rod due to massive margin of error when they build these engines. After all it's for display and hobby than a work horse. Many hours spent using trial and error method to find the correct lenght for each. The new rods as shorter by .40 ,.38 , .42 and .36 of a mm. But the difference is big in the small engine.

Now not only there is no dangerous spray and stink of fuel and out of the carb air intake but also much lower fuel consumption and less of oily soot out of the exhaust . Spark plugs( I am using Iridium type) look much cleaner too .

Maybe this can help someone else who experienced the same issue.
Title: Re: R90s playing tricks
Post by: rodnoc on February 01, 2023, 10:13:59 pm
.... Spark plugs( I am using Iridium type)....

Where did you find them?
Title: Re: R90s playing tricks
Post by: engine-man on February 01, 2023, 10:51:19 pm
.... Spark plugs( I am using Iridium type)....

Where did you find them?

Got them from a local hobby shop. They are Rcexl 1/4-32 ME8 Iridium plugs. You can find them online too.
Title: Re: R90s playing tricks
Post by: Fullraceflathead on February 10, 2023, 01:44:47 pm
Glad to see you fix the intake fuel spraying problem anytime you see fuel spray out of the intake you have valve leakage or an extremely aggressive valve timing issue
Title: Re: R90s playing tricks
Post by: Postie Roy on February 11, 2023, 04:13:42 am
These sort of problems should not occur on a new purchased engine which is not cheap to buy
Title: Re: R90s playing tricks
Post by: Fullraceflathead on February 11, 2023, 03:47:25 pm
I agree completely cause the average person cannot fix it. Many of us that are mechanically inclined and or machinist can repair these but this burden should not be placed on the purchasing customer!!!
Title: Re: R90s playing tricks
Post by: engine-man on February 12, 2023, 12:11:58 am
Well after all hard work , somehow either the CDI or Hall sensor gave up. Luckily I used PayPal to claim refund and maybe Microcosm learns to put more effort and quality in their products
Title: Re: R90s playing tricks
Post by: SteamerJ on February 22, 2023, 04:03:23 am
Well after all hard work , somehow either the CDI or Hall sensor gave up. Luckily I used PayPal to claim refund and maybe Microcosm learns to put more effort and quality in their products

Just curious how you return something that you did so many modifications to.
Title: Re: R90s playing tricks
Post by: engine-man on February 22, 2023, 05:24:38 am
I didn't return, I opened a case with PayPal listing the issues it had originally ( I documented them all ) and agreed to partial refund to cover the cost of parts needed.  I certainly discourage everyone to buy this model as it is flawed big time
Title: Re: R90s playing tricks
Post by: engine-man on March 03, 2023, 01:19:17 am
Time for an update, I haven't given up on this yet.

One of the biggest downside of this engine , in my opinion, is the carburetor. The 2 in and the overall design of it, while simple and functional, is not the most adequate in this application.

It's nearly impossible to match both cylinders intake opening identical as I noticed the crank shaft springed push mechanism in each cylinder is set differently. So the pushrod size needs to be different, however due to the nature of the carb, when the intake happens, the mixture takes the most direct path ( right cylinder viewed from the back ) and it leaves the right cylinder with a lean mixture.

You can  see in some videos that the right exhaust pipe is black and has a liquid soot which is unburnt fuel. Upon inspection of spark plugs, the right plug is shows the sign of rich mixture when the carb is opened enough to give enough mixture to the left cylinder.

You might think why not shorten the push rod on the intake valve on the right cylinder ? Well there is a very limited room for change there, otherwise the push rod will slip our of the valve rockers. This is a poor design which does not follow the proper pushrod architecture. Allowing you to adjust using a screw or similar mechanism.

Now, another issue which others have experienced is the CDI and hall sensor failing. 3 reasons based on my experience.
1- electrical "leak" through poor design of the ignition leads, this was also visible inside the CDI unit when running it in pitch black. This has mostly resulted in the fault which causing misfires continously. Tested with a another microcosm CDI ( used ) and confirmed this. I'm waiting to receive RCEXL CDI and put it to test.

 2- Poor ground connection. For whatever reason the way CDI is grounded is not efficient and this has lead to firing the Hall  sensor twice already. Attaching a wire and bring to the bottom of wooden base to touch the ground sorts this out

3- tenperture of hall sensor , the 44E type comes in 2 temperture resistant categories. Entry and Industry. The type used in this engine is the base which withhold upto 85C . The Industry type can stand upto 115C.  It also comes down to the build quality of the sensors. I beleive this has an impact on the longevity of these sensors inside the engine case when it's run for a while. A good cooling/ direct air circulation is necessary if you run these engines more than 5-10 minutes.


Lots of learning here for me, from such a simple engine, yet challenging. Makes me appreciate the marvel of automakers when I took it for granted
Title: Re: R90s playing tricks
Post by: MadeForThat on March 04, 2023, 12:55:26 am
Glad to hear things are moving in a suitable direction, I certainly believe the grounding is a big source of issue on these, on the H75 it goes through a bolt into the base, where the cdi connects to the end of the bolt. I can't say that was all the issue, but improving the ground seemed to help.
Title: Re: R90s playing tricks
Post by: engine-man on March 04, 2023, 06:17:56 am
Glad to hear things are moving in a suitable direction, I certainly believe the grounding is a big source of issue on these, on the H75 it goes through a bolt into the base, where the cdi connects to the end of the bolt. I can't say that was all the issue, but improving the ground seemed to help.

Totally agree with you, since I modified the grounding with the replacement CDI, noticed almost no misfires which can result in arching else where in the CDI or hall sensor.
The extebded round wire is now fixed under one of the foam legs on the base. It's discreet and stays in place with a tiny bit of superglue.
Title: Re: R90s playing tricks
Post by: engine-man on March 17, 2023, 05:26:50 am
A further update on this engine for the interest of other current or potential owners.

One thing I discovered is the vavle rocker's leg ( where it sits the push rod ) can be adjusted from the inside using a plier. That's a big plus and helps with tuning greatly.  As others have mentioned you want to set the rockers with minimal play. So the valves are opened immediately and widely. Exhaust valves need to open more than intake to create a strong scavenging effect. This is due to the nature of 2 in 1 and primitive carb used on this engine and more important to fix the spitting fuel out of carb intake!

Once you adjusted and tested the outcome ( when engine is warm ) you can add some super glue to the base of the rocker to keep it in place.

I can't insist enough on ensuring you have a reliable ground for the CDI. I used a wire from the ground point to under one of the foam legs. Now there is no misfiring or internal spark to kill the hall sensor.

I enjoy running this engine now, satisfying low idle rpm but fast pickup on open throttle. I am using a small desk fan to keep it cool.

Another tip is if you already added oil to the crankcase and using oil mixed fuel, soon or late you will have oil leak, mostly due to the fact that no gaskets are used in these engines. But it's easy enough to add it yourself. One way to get rid of excess oil is to undo the lower screw on the front case ( after warming up the engine and shutting it down), put a tissue under and tilt it 60 degrees over a soft rag. The excess oil will come out from the screw hole. Much easier and cleaner than letting it leak from the cylinders.
Title: Re: R90s playing tricks
Post by: Fullraceflathead on March 18, 2023, 01:27:45 am
Thank you for those great tips!!!
Title: Re: R90s playing tricks
Post by: Junkologist on March 18, 2023, 06:31:36 pm
Can you post some pics of what exactly you did to improve the grounding? Thanks.
Title: Re: R90s playing tricks
Post by: engine-man on March 19, 2023, 05:52:26 am
I connected a cable to the gorund screw where CDI connected and brought it to the bottom of one of the foam legs
Title: Re: R90s playing tricks
Post by: Junkologist on March 19, 2023, 10:23:52 pm
I connected a cable to the gorund screw where CDI connected and brought it to the bottom of one of the foam legs

Thanks.
Title: Re: R90s playing tricks
Post by: engine-man on May 11, 2023, 03:15:06 am
Just wanted to give an update on this engine. Since I made the necessary changes it has been running great. Predictable and very satisfying to run. If you are planning to get this engine be mindful of mechanical knowledge needed to tackle issues and short comings of the factory assembly.

It has gone through 30 tanks of fuel. Nothing broke or worn off so far.