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Author Topic: R90s playing tricks  (Read 150 times)

engine-man

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R90s playing tricks
« 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.

rodnoc

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Re: R90s playing tricks
« Reply #1 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.

engine-man

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Re: R90s playing tricks
« Reply #2 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

rodnoc

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Re: R90s playing tricks
« Reply #3 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.

engine-man

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Re: R90s playing tricks
« Reply #4 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 .

engine-man

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Re: R90s playing tricks
« Reply #5 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 ?

engine-man

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Re: R90s playing tricks
« Reply #6 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

engine-man

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Re: R90s playing tricks
« Reply #7 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 :)

RedRyder

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Re: R90s playing tricks
« Reply #8 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.

engine-man

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Re: R90s playing tricks
« Reply #9 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.

rodnoc

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Re: R90s playing tricks
« Reply #10 on: February 01, 2023, 10:13:59 pm »
.... Spark plugs( I am using Iridium type)....

Where did you find them?

engine-man

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Re: R90s playing tricks
« Reply #11 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.

 

Tom Jensen Jr. 1936-2023