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TinyTach setting for 4-stroke single cylinder?

Old 09-11-2019, 06:22 AM
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Question TinyTach setting for 4-stroke single cylinder?

Acting on the advice of v12mac, I bought an el cheapo tachometer for my 7hp engine. I see it is a ripoff of a TinyTach, it looks just like it and has the instructions to wrap the wire around the spark plug wire. However it instructs you to enter the spark plug firing revolution, which I do not know. The choices are twice per revolution, once per revolution, or every other revolution. What do 4 stroke single cylinder engines typically do?
Old 09-11-2019, 08:51 AM
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Normally a 4 stroke fires every other rotation.
2 stroke fires every rotation.

Is your motor 1 or two cylinders? There are settings for that.

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Old 09-11-2019, 09:44 AM
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Just one cylinder. The engine is unbalanced and vibrates the boat a lot.

This is a knockoff meant for the American market (the English manual is a dead giveaway), so there's no setting for one or two cylinders. Just twice per revolution, once per revolution, or every other revolution.
Old 09-11-2019, 09:50 AM
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Some small 4-strokes fire every revolution, wasting the spark on the exhaust stroke. Probably just because it's easier to time off the crankshaft instead of the cam.

No idea if this applies to your engine but I'd think it would be pretty easy to just pick a tach setting and run it. It should be obvious if it's reading double speed or half speed.
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Old 09-11-2019, 09:59 AM
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Most one cylinder engines spark every revolution since the magnet for the coil is mounted on the flywheel. See if the coil is next to the flywheel. If so, it sparks every revolution like a 2 stroke.
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Old 09-11-2019, 10:06 AM
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Well, one reason I'm getting a tachometer is because I think my engine is running way too slowly and maybe the throttle isn't set correctly. So part of putting the tachometer on is I have no idea what the actual RPMs are. What should it read at idle and at full speed? Sorry if I'm too dumb, I hardly know anything about gasoline engines. I'm a computer guy. But I'm trying to learn.
Old 09-11-2019, 12:40 PM
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Hook it up. Video it on each setting and by using state of the art software (ears and brain) we can tell you which one is correctly displaying your rpm.

now that I think about it I had trouble setting mine up. I just assumed the manual was wrong.

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Old 09-12-2019, 03:06 AM
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The video is uploading. While we wait, can anyone tell me if this looks like a "long" or a "short" lower unit? The manufacturer offered two styles, and I chose "short" but maybe they sent the wrong one?

Old 09-12-2019, 03:26 AM
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I wouldn't be able to guess. Can you take a photo showing the height of the propeller in relation to the bottom of the boat?
Old 09-12-2019, 06:00 AM
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Here's the video.

I started at idle and put it on full throttle. It's set on once per revolution. I tried to get some of the environment passing by so you can see how fast it's going.
Old 09-15-2019, 06:39 AM
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I can't see the tach, I'm going to have to look at the video on a pc.

One thought is that sounds more like an air cooled motor that may throttle down at rpm. Like a lawn mower.
Old 09-15-2019, 05:59 PM
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That's a Hangkai, right? Are you sure it's a four stroke?

If you measure the transom height, is it around 42cm? Assuming the ventilation plate (cavitation plate) falls at just about the bottom of the transom, the distance from the bracket (where it sits on the transom) to the cavitation plate would be around 42cm for a short shaft.

Last edited by jobowker; 09-15-2019 at 06:05 PM.
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Old 09-15-2019, 06:11 PM
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Found it - it is a four stroke!
https://www.aliexpress.com/item/3298...7b6f77b3FMiC0Q

Looks like max rpm is 4500-6000
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Old 09-15-2019, 07:54 PM
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yours is a short shaft. I own a long shaft Tohatsu/Mercury/etc.
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Old 09-15-2019, 09:05 PM
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Yes it is air cooled, 4 stroke, 7hp.

The tach displays 1260/1380 at idle and 3300 at full throttle. The number didn't decrease after it had been at full throttle for a while. According to the engine sound, do you think it's it set on the right setting of once per revolution?

When it says max RPM 4500-6000, that sounds like a very large range. Does this mean I can run it at 6000? Or 4500? Or...how's that work exactly? I'm not really that worried about the longevity of this engine. It was an el cheapo in the first place. But I don't want it to burn out before I'm done with it. It's just that I don't expect it to last for 20 years like some of the used engines that I see people selling. I may well end up replacing it with a 12-18hp next spring when the boating season starts up again.

If it maxes out at 3300, it seems the throttle is not set correctly. I'm going go the marina and get them to set it to max out at 6000.

Ah, I see in the tech specifications that jobowker set out, the "minimum load steady speed" is "1300 r/min". This sounds like Chinglish for idle speed is 1300 RPM. OK I didn't think of looking on that site. I didn't realize this is an export model as well, so therefore there should be documentation in English. It seems the tachometer is set correctly.

I'll bring a tape measure and do those measurements when I'm on the boat later today. Thanks to everyone here for the kind assistance!
Old 09-15-2019, 10:26 PM
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Had to tell in your picture but it looks like you have it sitting too low on the transom based on the waterline stains on the lower. That will create a lot of drag and slow you down. Ideally the flat plate above the prop should be about level with the bottom of the boat.
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Old 09-15-2019, 10:30 PM
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Well, it's clipped on where it clips on. I can't make it go any higher or lower. There's actually a little dip in the back of the boat that makes the engine go lower in the water. Will post pics later.
Old 09-16-2019, 05:53 AM
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Originally Posted by bbnuser View Post
Yes it is air cooled, 4 stroke, 7hp.

The tach displays 1260/1380 at idle and 3300 at full throttle. The number didn't decrease after it had been at full throttle for a while. According to the engine sound, do you think it's it set on the right setting of once per revolution?

When it says max RPM 4500-6000, that sounds like a very large range. Does this mean I can run it at 6000? Or 4500? Or...how's that work exactly? I'm not really that worried about the longevity of this engine. It was an el cheapo in the first place. But I don't want it to burn out before I'm done with it. It's just that I don't expect it to last for 20 years like some of the used engines that I see people selling. I may well end up replacing it with a 12-18hp next spring when the boating season starts up again.

If it maxes out at 3300, it seems the throttle is not set correctly. I'm going go the marina and get them to set it to max out at 6000.

Ah, I see in the tech specifications that jobowker set out, the "minimum load steady speed" is "1300 r/min". This sounds like Chinglish for idle speed is 1300 RPM. OK I didn't think of looking on that site. I didn't realize this is an export model as well, so therefore there should be documentation in English. It seems the tachometer is set correctly.

I'll bring a tape measure and do those measurements when I'm on the boat later today. Thanks to everyone here for the kind assistance!
I have an old 6hp 2 stroke. The max RPM should be around 5000-5500. It should be able to reach this RPM regardless of whether it's pushing an eight foot dinghy or being used as a trolling motor on a 16 footer. The key difference is the prop, which acts like changing the gearing. That's why people want to know what RPM's you reach at wide open throttle.

Applying that idea to your situation, it may be that the prop has too much pitch for what you are using it for, it which case it cannot rev above 3300. This would be fixed by getting a lower pitch prop.
It might be that there's something wrong with the outboard, so it's hard to definitiely say it's the prop.

In terms of the wide operating range, it means that at a minimum, your rpms at wide open throttle need to be at least 4500 rpm to avoid lugging the engine and enabling it to reach something near 7hp. The 6000 means that when you are running it wide open, the rpms cannot exceed that number. It has no bearing on the rpms you operate at, but rather where the rpms are when you are operating at full throttle. This is used as a way to ensure it's dialed in properly.

Hope that makes sense.
Old 09-16-2019, 05:57 AM
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If there's no separate distributor running 1/2 engine speed, it fires every revolution.
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Old 09-16-2019, 07:16 AM
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Originally Posted by jobowker View Post
I have an old 6hp 2 stroke. The max RPM should be around 5000-5500. It should be able to reach this RPM regardless of whether it's pushing an eight foot dinghy or being used as a trolling motor on a 16 footer. The key difference is the prop, which acts like changing the gearing. That's why people want to know what RPM's you reach at wide open throttle.

Applying that idea to your situation, it may be that the prop has too much pitch for what you are using it for, it which case it cannot rev above 3300. This would be fixed by getting a lower pitch prop.
It might be that there's something wrong with the outboard, so it's hard to definitiely say it's the prop.

In terms of the wide operating range, it means that at a minimum, your rpms at wide open throttle need to be at least 4500 rpm to avoid lugging the engine and enabling it to reach something near 7hp. The 6000 means that when you are running it wide open, the rpms cannot exceed that number. It has no bearing on the rpms you operate at, but rather where the rpms are when you are operating at full throttle. This is used as a way to ensure it's dialed in properly.

Hope that makes sense.
Respectfully, this is incorrect. The RPMs the motor will reach at WOT is absolutely effected by the load. The WOT RPMs on my HenryO will change significantly just from having my 42gallon fuel tank full or nearly empty. It is propped appropriately. In order to determine RPMs at WOT, the boat needs to be loaded as the user intends to use it. Put the boat in the water and get up to WOT. At that point, check RPMs.


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