Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Written by Engine Man

⚠️👉 Comprehensive Guide on OKMO V-Twin Engines
Welcome, engine enthusiasts! 😎

This guide is focused on the following models :
R28, R29, R30, R31, R33, R34 , V35 and H08
🟠  Inspection and Preparation
We all know the excitement of unboxing our new engine and getting it going as soon as we can.  However, when it comes to model engines, it is strongly advised to take the time to fully familiarize ourselves with the structure and assembly of the engine.
Start by a visual inspection of the engine thoroughly , ensuring nothing has been bent , damaged, or loosened during transport. Whilst these models are secured well in their boxes, the postal services are not always kind to them during shipping . If you notice any damage or unusual appearance of parts, take photos and advise the seller .
The next step is to rotate the flywheel  (check the correct direction in the manual as some of these V twins rotate clockwise , and some anti-clockwise). What you want to feel is compression in both cylinders, almost equally.  Note that these engines use O rings instead of metal rings, so the compression is less strong.
If the compression is not equal, don't panic. This might be solved once the fuel-oil mixture enters the cylinders or refers to the Compression section of this guide.
🟠 Spark Plugs
Now, gently take the spark plugs out and ensure the gap on both is equally set. There is no specific measurement to use, but the majority of them are set to 0.2 -0.4 mm gap.
One issue that is commonly seen is the undesired arcing from the ignition lead tip to the lower part of the spark plug via the ceramic shell of the plug. To eliminate this potential issues I strongly recommend to use a wire heat shrink tube on the ceramic part on all plugs.
Make sure your 3 AA Batteries are healthy and have at least 1.2v each. I found some rechargeable AAs don't work as well as others due to lower mAh rating. So keep that in mind.
🟠 Ignition wires and grounding
For a healthy and consistent combustion, a solid ignition is a must. That leads us  into the next point.
Sometimes with these ignitions the expose part of the wire is quite short, and lacks full contact with the top of spark plug.  I suggest to take the ignition boots off and inspect each wire. If required, cut more of the wire casing.
In addition, I recommend to add some solder to the expose wires to ensure a strong electrical transfer is guaranteed .
You can put back the boots and test the contact with the spark plugs.
Grounding/earthing is crucial saving point for the hall effect sensor in the CDIs used in these engines . My suggestion is to attach a wire from the ground connection of the CDI to the bottom of the base, in a way that it touches any surface you put your engine on.  This will save the hall sensor !
🟠 Lubrication
As these models do not have oil lines and pump for full lubrication, it is crucial to use the correct type of lubricants in various areas, to ensure longevity and prevent any unwanted damage.
There are 3 areas you need to lubricate before starting the engine:
■ Crankcase :
these models only have crankcase vent hole right above the flywheel. So adding oil is not a very easy job, however can be done with patience and using syringe and a needle.
My choice of oil is SAE 20 non-detergent and I add 3-5 ML. You can use any oil you have as these are not too sensitive. But pay attention to not overfill. 
The so called oil change is also not too difficult. After running the engine to the temperture and once fuel tank is empty. Tip the engine to its flywheel side. Put some tissue over the vent hole and let it sit horizontally ( use a stand or cloth under cylinders to ensure oil is not reaching the pistons. Once the old oil is out, repeat the above step.
■ Cam gears:
In some of the models I received there was a small amount of grease added to cam gears. Although the amount and type needed my attentions.
To access the cam gears, you need to remove the from cam cover.  It is absolutely crucial to be patient and ensure removing the cover does not dislocated the timing gear both for ignition and valves. I put the engine on its side facing up  and gently take the screws off, then with very light touch of fingers lift the cover.
My choice of lubricant for can gears is Yamaha Moly Grease. You can use any other grease, however Moly grease proven outstanding in terms of protection and longevity in all my engines.
■ Valve lifters and rockers arms:
The final touch of lubrication is under valve covers . Take the cover out gently, and you will see the beautiful rocker arms and lifters. I use the Lucas oil upper cylinder lubricant on all moving parts there. The reason I use that is to avoid any oil buildup and burning on top of the valves.
🟠 Fuel
Due to low compression of these models, you can run them on low octane fuel including ethanol free petrol, Camp Fuel and lighter fluid . You must add either 2 stroke oil, Marvel Mystery oil or Lucas Oil upper cylinder lubricant to your fuel to ensure the correct lubrication of upper cylinder parts. Some of the oil will also passed the rubber o ring into the crankcase.
I personally user 20% gasoline , 60% Camp Fuel and 20% Lucas Oil upper cylinder. Marvel Mystery and Lucas Oil provide low to zero smoke when burning.
🟠 Break-in Process
The break-in process on any engine is the crucial time to warrant the correct bedding, therefore the longevity of the engine. With models engines, the story back 60 years in time, when breaking in was very different to modern engines.
With these V twins, it is the part that you must be patient and not rush things through.
Make sure you pay attention to above points and prepare the engine adequately.
Follow these steps:
1- In majority of cases the carburetor is configured for break-in when it leaves the factory, so don't change it. Unless the engine is not able to start. Make a full note of needles position before changing it.
2-  Use the drill starter bar that comes with the engine. This will help to diagnose if carburetor changes are required while the flywheel is Rotating at high speed .
3-  The First run should be 30 seconds without any throttle. Followed by 20 seconds of cooling cycle.
4- The Second run should be 2 minutes with throttle not open more than 30%. Allow 30 seconds of cooling cycle.
5- Any further run should be between 2-5 minutes  with gradual increase of the throttle for at least 3 full tanks of fuel. Don't forget to allow cooling cycle for correct metal coupling.
Something to note is these engines run hot, and to prevent any damage or pre-ignition, I recommend to either have a small desk fan running behind the engine, or operate them somewhere with a good air flow to keep the temperature in the safe limit.
During rhe break in period, it is common to experience mis-fire or lack of compression in one or both cylinders momentarily.  I suggest to continue with the break-in guide for at least 2 -3 full tanks of fuel.
🟠 Troubleshooting Tips
■ Carburetor
The ones on these engines are very sensitive to adjustments and temperature. Any adjustment needs to be done in fractions of turn for both High Speed Needle ( main screw) and Low Speed Needle ( throttle screw).
In most cases, the HSN is set to 1 and half turn and LSN to 2 turns from the factory. This is adequate for break in period, however once the engine has settled and broken in, I reduce each to 70% of turn for HSN and 120% for LSN.
Depending on how you like your engine to run, you can modify each to suit.
The idle screw is usually set correctly from the factory and doesn't require any adjustment in most cases. I changed it on my H08 Hoglet as it is converter to power a propeller for a Blériot RC plane project.
Every engine is different,  so you might need to adjust your engine's carburetor according to the best operation for it.
As these carburetors are small, they can  get blocked easily if there is any contamination in your fuel or the fuel tank.
Any form of alcohol,  brake cleaner or Nitro cleaner will do a great job in cleaning the carburetor. Just remember to take it off the engine before hand.
■ Spark Plug fouling
Beside from R29 and V35 which use 1/4-32 spark plug type, the plug is the smaller 3/16-40 type.
While the smaller plug allows lower profile and smaller engines to be produced, they can foul easily. The biggest factor is the oil used in fuel.
I recommend to take them out after each full tank of fuel, and gently clean them with alcohol or similar solvents . Be careful not to damage the tip.  You will notice a lot of build up on them. Cleaning will help with better performance and less of the build up inside the combustion chamber and valves.
■ Valve clearance
This part is for more experienced users, because any change can impact the operation of your engine and you need to be confident in knowing the effect each change males.
Once the break-in period Is over, like any other engine, the valve clearance might be required to ensure the optimum level of operation.  On these models, you are able yo easily take the rocker arms off and adjust the valve guide screws. This will help with change the intake and exhaust pressure, which can make a big difference in how these engines work.
Easiest way to observe the movement and valve opening is to take the spark plugs out, and using your hand rotate the flywheel and through the spark plug hole see the movement of the position in accordance to the valves.
Note , there is no specific valve clearance value. In reality, I found myself adjusting the clearance on each cylinder different to other one to ensure an equal combustion and fuel consumption by each cylinder. Trial and error as well as knowledge of engine operation are the keys here.
Biggest give away with this issue is the carburetor air intake spitting fuel. Keep a close eye on the part to identify if this is happening and find out which cylinder is causing it.
■ Compression
Let's see what are the common causes of compression loss on these engines :
1- Spark Plugs ring
The engines either come with rubber o ring or metal ring for spark plugs. If they are slightly loose, you will feel the Compression loss or intermittent combustion in that cylinder. Make sure they are tight but don't over tighten them. Hand tightening is the safest way
2- Piston Ring
These engines use rubber o ring instead of piston  ring. This means by nature you have less compression, but with good lubrication they hold up well. The rubber o rings can deteriorate fast if there is ethanol in the fuel , not enough lubricant or overheating occurs. They will shrink and harden up, meaning loss of compression.
In some cases you might need to replace the rubber o ring, which comes with the engine. Do check other items before attempting to replace the o ring.
3- Valve clearance and seating
In some rare cases, the machining done on the cylinder head doesn't pride a flushed surface for the valve to fully seal. And could result in loss of compression.  This can happen a lot during break in period , so it is important to let the process takes place so the metal surfaces mate correctly.
However, in some cases, you might need to remove the cylinder head, use a very fine sandpaper (2000 grit) or use a coarse car polish cream and gently run it over the top of the valves and the seats for them.
In most other cases, the incorrect valve clearance can cause this issue, then you need to identify which valve is the problem.
Easiest method is to take the other spark plug out, and rotate the flywheel by hand. Hear carefully for any noise of air leak while listening to the valves with the cover off.  And make slight adjustments as required.
■ Valve and/or ignition timing
This is another advanced topic for diagnosis but a good one to focus on. In very rare cases you might face the incorrect valve timing issue. Mostly the valve timing gear could be off by only one tooth, but that is enough to cause a problem with compression or operation.
The valve timing on all of these engines are the same . To set it correctly follow these steps.
Move the flywheel screw hole to fully vertical and down position. Now align the second lobe from the rear of cam gear with the flywheel screw hole ( both fully vertical and downwards) and secure the cam gear. You might need to try it few time to get it right.
The ignition timing works based on 2 magenta crossing the hall sensor, which triggers sequential ignition. Depending on your engine you can change the timing from 10 degree advance to 5 degree retard. But only change this if you must.
■ Hall sensor failure
One of the common issues owners experience is the sudden shutdown or inability to start the engine. Mostly due to no spark.
In some cases bad battery or loose ignition wires are the issue.
In most cases you might experience failed hall sensor.
Hall effect sensors are extremely sensitive or any voltage and current changes. This is a direct result from lack of adequate grounding, especially if any arcing takes places within  the CDI ignition path.
The best way to avoid this is to read and implement the grounding solution I described in preparation section of this guide.
Replacing the hall sensor is easy, however you must be sure you are using the correct one. Every hall sensor has a code and number. For instance a common one used is 43F 920 type. Direction of hall sensor, depending on the type, also makes a difference. So it is important to pay attention to the way it is installed on your engine, before taking it off.
This guide covers majority of tips and trick for Retrol/OKMO V-Twins, however there is always something that might have not been covered , perhaps part 2 might be released soon !
Good luck and enjoy your engines 😁👍

[ Guests cannot view attachments ]