Just wondering if anyone has used an inductive tach to measure rpm on things like leaf blowers and line trimmers
I just bought one like the attached picture and am having some problems working out speeds.
I managed to find that setting 2 is for 2 strokes, and setting 1 for 4 strokes. I worked this out as setting 1 gave idle at 1740 rpm for a Briggs Classic 35, and setting 2 gave idle of a Ryobi trimmer at 2100rpm (about right)
My issue is measuring full speed rpm on 2 strokes. If I go full throttle on the Ryobi, the tach goes up to 7000 odd rpm. In the Ryobi manual (see photo), it says 12000 rpm maximum. Without the trimmer head (split shaft) it will do about 10,500 rpm, I can get it out to 11800 by leaning the mix, but it runs awful with the head back on.
It said full throttle (governed) on Briggs was 2800 rpm.
I assume the max rpm numbers Ryobi supply are off - max possible speed maybe. I bought the tach so i could simply dial in the high speed jet precisely to the required rpm, but the numbers don't add up.
I have the exact same unit that I've been using on my briggs 4 stroke mower for some time now. I pretty much use it for keeping a record of hours for my oil changes. That being said, I have used it a number of times to check/set governers and carbs on old mowers that have come my way for fixing. I read the ENGRISH manual that came with it (I bought from china on ebay) and it made very little sense to what was acutally happening. When pressing and holding the S1 button for more than 3 sec, you cycle through the list of modes from 01 to 05. In my experience, I've found that you use 02 setting for both single cylinder 4 strokes AND single cylinder 2 strokes. 02 Measures every time the plug fires (or rather every time the current shoots down the lead) 03 doubles that figure and would be suitable for 2 cylinder engines bearing in mind that you are measuring only 1 cylinder that is doing half the work despite having 2 cylinders firing for overall revs. I have no idea what 01 would be suitable for as this halves the revs of 02.
Example: My briggs single cyl 4 stroke mower measures around 2940 at run (full) speed (should be governed to 3000) on setting 02 and my 2 stroke echo trimmer measures around 3400 idle (should be 3000) and 7700 flat chat (should be 7000 max). I know my trimmer idles too high as the head spins at idle and it probably runs a little lean as the carb has never been serviced since new. As a side note, I run 98 RON BP Fuel and use with STABIL additive l so I'm probably getting a little more HP from it also.
I'm assuming you bought it cheap from China like me - about $8 so there may be some inconsistancy with the workings between yours and mine BUT there should be a dramatic difference between 01-02 and 03. Maybe you have old fuel or worn rings or an oil mix ratio that is too high or a spark plug that isn't firing well or even a high ohms reading on your ignition coil. All this asside, 12,000 RPM seems a little high to me. Maybe this is the RYOBI Redline of the engine itself with nothing attached to drive shaft. You might need a RYOBI expert to weigh in on this. If it runs, accelerates and cuts well, I wouldn't get too hung up on the numbers.
I normally have about 3 wraps around the plug on mine and keep the lead away from heat sources. Hope this helps mate.
I bought mine off ebay from an Aussie seller - $13. Its just I hate giving China money directly, at least with an Aussie seller, some money goes to the govt by taxes. Although in thinking how much interest they probably pay to China in loans, I should have just saved the $6 haha. Anyway, took 1 week v 1 month.
ENGRISH manual is right - it lists as follows : 01 - 2 stroke 1 cyl, 02 1 stroke Per cyl, etc.
As soon as I saw it say the Ryobi was idling at 1050 rpm (mentally calculated 2100 rpm as the correct idle and changed the setting) Thinking about it, I may have had it set on 02 for the Briggs as well, not sure. Got the trimmer up to 20 something thousand on setting 3, so that was ruled out.
I tried another trimmer (homelite with different engine) and got about the same, bit faster idle (no clutch) and same top speed (roughly about 200 rpm more) The engines on both are perfect inside, and good plugs on both.
Checked my generator 1/2 hour ago - 3000 rpm on setting 2 - exactly what it should be without load.
I think you are right - 12,000 does seem pretty high. It also says the head spins at 9000 rpm - that is definitely faster than I think they run.
By the way, I use Caltex 98 RON and the machines seem to have better throttle response and starting. I reckon running high octane doesnt have any downsides (unless you use engines in 50 degree heat). Many trimmer manuals actually specify 98 octane (disguised as 91 octane in the AKI scale).
I haven't seen anything with a flywheel magneto system in lawn care equipment that spark more or less than 1 spark per rpm. The Briggs 4 cycle engine with magneto ignition are wasted spark setup otherwords they fire the spark plug on both TDC compression (actually about 15-20 degrees before) and on the exhaust stroke making them firing once per rev. Of course they don't produce power on the exhaust stroke but can produce flame out if there is a miss fire on compression. I had one Kawasaki that nearly burned my leg on miss-fire by shooting a one foot flame plus made me jump from the report. Even the v-twins with two coils will still be the same setup.
I think the only time you are going to need the other settings is if things are driven off the camshaft or in the automobile that using a single coil and a distributor where you are sensing the coil wire before the distributor cap.
On that generator I am assuming you are using 50 hz so the off load speed should be only slightly higher than 3000 to produce about 51hz (3060 rpm) as generator have slight governor droop from off load to full load.
As 12K being high it might be right as the chainsaws I work on can be as high 14K depending on the make and model.
Thanks AVB, I was wondering if Briggs had a wasted spark
The generator is a GMC (what you would know as Harbour Freight Tailgator 2 stroke) - I set it at 3040 as I remembered reading something about setting just over. There is a bit of no load irregular hunting - between 3100 and 2980, but being a cheap genset, it won't be that precise.
I might try adjusting again tomorrow (all the screws were played with by the previous owner - who put the governor arm on wrong)
Even multi cylinder motor cycles engines run wasted spark, 4 cylinder 2 coils 2 trigger points on the crank. I have not had anything to do with the Honda CBX 6 cylinder ones but I would think they must have 3 trigger points on the crank and 3 coils?
At the mower shop I worked at we never used a tach on trimmers, there are too many variables. Ultimately what will govern the speed is the length of the line. To tune, you want maximum line the guard will allow, flat out, wind the screw in until it leans and revs go way up, screw it out until it 4 strokes (labours -rich) and screw it back in until you find the sweet spot where the note is just on the smooth side of rich.
That might be fine for string trimmers but when it comes to hedgers and chainsaws that there are limits that if exceeded will shake things apart on hedger or exceed maximum chain speed allowed. On top of that many trimmers, hedgers, and chainsaws are now coming with limited coils where if you exceed the max design speed things just get crappy; unless, you across an unit that is dieseling like I did back in my Spring here.
The coil was limited to 9300 but the hedger was exceeding 12000 without a single spark. I figured it out when the tach was showing zero rpms at wide open.
There a lot times that I got to set the idle rpms below the clutch engagement speed even though it can much run higher at the sweet spot.
Hi nath, that is what I normally do. low speed is idling, screw in until it is about to stall (lean) then out until about to flood stall then half way between the two with a little bias to rich, then adjust for fastest and smoothest rev pick up - just trying to check with tach.
AVB, was it the timing causing the dieseling, or carbon deposits causing it?
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