There have been a lot of questions lately about how to best tune the H75 so that everything works properly and prevents abnormal operation.We are honored to invite Engine Man, who will tell us what to pay attention to during debugging.
🔶️ H75 Tuning Tips 🔶️
I got a couple of messages from owners and others who wanted to know the best way to tune the H75 so all cylinders work the same and prevent irregular operation.
I thought I’d do a little write-up here in case others are interested.
Note: Make these changes after your engine has passed the breaking-in period.
1️⃣ Cylinder Compression
The most basic element to start with is ensuring each cylinder has healthy and equal compression. This translates into almost equal intake, along with balanced distribution of power stroke pressure on the crankshaft.
The best way to test, which doesn't need any tool, is to remove the spark plug from the other 3 cylinders and rotate the flywheel by hand (ignition off and no fuel). If you feel a healthy resistance during the combustion stroke, then the basics are good. Repeat this for each cylinder individually.
If you notice a lack or lower compression in a cylinder, most likely your valves require adjusting, which can be done via the nut on the rocker arm.
NOTE: Adjusting all valves on all cylinders equally will not give you the right result.
You need to adjust each cylinder individually to ensure the valves open, close, and seal correctly. If after adjusting the valv, you still have a compression issue, it could either be a valve seating issue on the cylinder head or a potentially defective/damaged piston rubber O-ring.
2️⃣ Ignition and Spark Plugs
It is possible to encounter a defective spark plug or faulty CDI and hall sensors (I only experienced it once in all of my Microcosm engines. With proper grounding, this issue is avoided).
To check, considering safety first, take all spark plugs out, and connect them to ignition leads. Ensure they are securely connected to the metal casing of the engine. Then turn the ignition on and by hand, slowly rotate the flywheel. Now observe the arc in every spark to be equal and in the correct pattern. If you see one with a weak arc or irregular pattern, discard it. In some cases, the problem could be too large of a gap in the spark, with a gentle push against a semi-soft surface, you can potentially reduce the gap safely. I have done it twice with good outcomes and without any observable damage under a magnifying glass.
The H75 setup uses a Wasted Spark system in which all ignite on every round of the hall sensor trigger. Not ideal, but it is sufficient. However, this means your batteries will run out sooner. So keep an eye on them and replace them regularly.
3️⃣ Fuel Delivery
The intake design of the H75 is done through a basic single carburetor with an unequal intake pipe. While the design keeps things simple, it introduces a delivery challenge. Meaning to ensure equal fuel and air intake by each cylinder, there is a need to adjust intake valves very slightly. The two outer cylinders need more opening of the intake valve and the two inner cylinders need less. I will share my method to ensure they are adjusted properly in the next step.
The next item is the carburetor. You need to adjust both the HSN and LSN to get the desired outcome without flooding or starving the engine. This is more of a trial and error step, which will make sense in the next part.
4️⃣ Testing and Fine Tuning
The H75 should idle pretty low and steady and be able to rev high quickly. It should start by one or two flicks of the flywheel by hand from cold or hot (once fuel reaches the carb). I posted videos of mine and there you can see it. There are other great videos on YouTube as well.
The best way to test is by conducting single-cylinder operation at a time. Meaning take out the spark plugs from the 3 other cylinders. Safely secure them to the metal casing. Add fuel and use the drill starter bit to start. What you should see is a steady yet weak operation of the cylinder without stutter. You only need to run each cylinder for 20 seconds or so. If there are issues, refer to the above steps to diagnose. Otherwise, repeat for the other 3.
Now put all the plugs in and run the engine with all 4 cylinders. During the cold start and for the first minute, you might need to let the engine warm up first before revving it. It's crucial to avoid any premature wear. Once it's warmed up, carefully touch the wall of each cylinder and ensure the outer ones are not excessively hotter than the inner ones. If they are, refer to step 3.
From here onwards, anything else to fine-tune is carb and fuel mixture related, so it is easier to resolve any problems. I use a mixture of Coleman (different brand) and gasoline with Lucas Oil upper cylinder lubricant (same as Marvel Mystery Oil). The H75 is a thirsty one! The HSN is slightly above a quarter of a turn and the LSN is slightly above one turn.
Hope this write-up helps. 😎👍
How many parts to assemble the Holt 75 kit?