# How to implement a rev limiter on a small engine?

#### strantor

Joined Oct 3, 2010
6,338
I have a small gas powered UTV/buggy (think all-terrain golf cart). It has a Chinese v-twin engine and despite countless hours of fiddling with the carburetor I could not get it to run right. Much research indicates the carbs on those engines are just crap. So I replaced the stock intake manifold with a pair of adapters I machined, and put on dual flatside carbs. It runs much better now, but there was no good way to tie in the original mechanical governor. I still want to limit RPMs because it has the original valve springs and flywheel and shouldn't be taken above 5k RPM in this configuration.

So what is the best way to limit RPM? I will have some kind of electronic controller onboard (PLC, Arduino, or other programmable controller) so I will be able to do this electronically if that makes things easier. Can I ground the coils (in the same way the kill switch works) if RPM >4500? Or will that cause other problems? I could override throttle with a servo or something, but that would be more complex. I would prefer a simpler solution like grounding the coils provided that won't cause problems.

#### LowQCab

Joined Nov 6, 2012
2,650
The first plan would be to find a way to incorporate the original Governor-Linkage into
I think that doing everything You can to pursue this route should be the top priority.

The second plan would be to adapt a "Micro-Squirt" Fuel-Injection-Computer
to the Engine to control the Ignition-Timing.
Micro-Squirt has a built-in Rev-Limiter, but I wouldn't use it.
It's very easy to simply severely Retard the Ignition-Advance above the desired maximum RPM.
This gives a very smooth Rev-Limiting action that is barely perceivable.
No poping and banging and misfires.

If your Buggy has a 12-Volt Battery and an Alternator,
You could also up-grade the Ignition-Coils to GM-Truck-Coils,
making for a really reliable and stout Ignition-System.

And, if you're feeling adventurous ..........
It can also be used to implement actual Electronic-Fuel-Injection on your Engine,
regardless of the Engine's configuration.
Which is guaranteed to make it run better than it ever has,
( as long as You have experience in tuning Engines ).

You are seriously pushing your luck by running your Engine to ~5000-RPM,
~4000-RPM should be the Red-Line.
Stresses on the Connecting-Rods go up on an Exponential-Curve, and they WILL BREAK.
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#### shortbus

Joined Sep 30, 2009
9,762
Can I ground the coils (in the same way the kill switch works) if RPM >4500? Or will that cause other problems?
If you do it that way be ready to replace the muffler/s from the backfires when the ignition cuts in again. From the carbs dumping raw gas into the muffler and the ignition starting again. As a young driver did you ever have a stick shift car with points ignition? And shut the ignition off for a second or two and then back on? To here the bang when the ignition came back on.

Did you keep the original governor in place? how about extra bell cranks the connect to the carbs throttles.
While those original carbs are pretty bad, I've had good luck replacing them with the updated carbs they sell on Ebay. Have done 2 motors now with the newer Ebay carbs and no problems so far. Actually 3 motors counting the 1976 Briggs 16HP one.

#### strantor

Joined Oct 3, 2010
6,338
If you do it that way be ready to replace the muffler/s from the backfires when the ignition cuts in again. From the carbs dumping raw gas into the muffler and the ignition starting again.
Thanks. That's along the lines I was thinking, why I asked the question. Wasn't sure exactly what would happen but I suspected there might be a problem since the air and fuel will still be flowing but with no spark.
As a young driver did you ever have a stick shift car with points ignition? And shut the ignition off for a second or two and then back on? To here the bang when the ignition came back on.
Yes I have had vehicles with points ignition but no I never tried the sweet party trick you described.

While those original carbs are pretty bad, I've had good luck replacing them with the updated carbs they sell on Ebay. Have done 2 motors now with the newer Ebay carbs and no problems so far. Actually 3 motors counting the 1976 Briggs 16HP one.
I got these carbs from Amazon. They work way better than the stock carb so far.
Did you keep the original governor in place? how about extra bell cranks the connect to the carbs throttles.
Yes I still have everything. The governor arm is still in place as I wanted to explore options for reusing it. As far as bell cranks, not sure what that is. If it was part of the original setup, I still have it.

The first plan would be to find a way to incorporate the original Governor-Linkage into
I think that doing everything You can to pursue this route should be the top priority.
I know reason when I hear it. I am going to pursue this first. The problem with reconnecting the original governor is that it only has about 0.5" of travel and these carbs need 1-1.5" of throttle cable pull. Making them compatible seemed like more work than an electronic workaround but I now think that is probably wrong. I have been thinking about it and I have some ideas I want to try.

The second plan would be to adapt a "Micro-Squirt" Fuel-Injection-Computer
to the Engine to control the Ignition-Timing.
Micro-Squirt has a built-in Rev-Limiter, but I wouldn't use it.
It's very easy to simply severely Retard the Ignition-Advance above the desired maximum RPM.
This gives a very smooth Rev-Limiting action that is barely perceivable.
No poping and banging and misfires.

If your Buggy has a 12-Volt Battery and an Alternator,
You could also up-grade the Ignition-Coils to GM-Truck-Coils,
making for a really reliable and stout Ignition-System.

And, if you're feeling adventurous ..........
It can also be used to implement actual Electronic-Fuel-Injection on your Engine,
regardless of the Engine's configuration.
Which is guaranteed to make it run better than it ever has,
( as long as You have experience in tuning Engines ).
I have looked into options along these lines in the past and it is very intriguing. It tickles a very specific part of my nerd brain. It is on the to-do list but far enough down that it's probably past the "save for retirement" zone and into "it'll never happen" zone. If I ever do it, it will probably be illegal by then.

You are seriously pushing your luck by running your Engine to ~5000-RPM,
~4000-RPM should be the Red-Line.
Stresses on the Connecting-Rods go up on an Exponential-Curve, and they WILL BREAK.
Again, smells like reason and experience. I officially take heed. I had read in more than 1 place that these engines (Harbor Freight Predator 670cc V-Twin) are safe to take up to 5k in stock configuration but going higher than that requires a different flywheel, better rods, and stiffer springs. But I have no reason to go over 4k so I will avoid doing so.

Thank you both!

#### LowQCab

Joined Nov 6, 2012
2,650
Here's the Owners-Manual in case You don't have it ..........

The Stock-Carb doesn't work well because of Emissions requirements.
You could probably "re-Jet it", ( with Welding-Torch-Tip-Cleaners ),
and it would run much better.
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#### strantor

Joined Oct 3, 2010
6,338
Here's the Owners-Manual in case You don't have it ..........

The Stock-Carb doesn't work well because of Emissions requirements.
You could probably "re-Jet it", ( with Welding-Torch-Tip-Cleaners ),
and it would run much better.
.
.
.
Yes I have the manual, thanks for the link. The carb on it did indeed suffer the impediments of emissions regulation. It had caps soldered over the air/fuel mixture screws which had to be drilled out to make adjustments. The adjustments helped but it still sucked. I have a set of tiny drill bits for drilling jets and I did re-jet it to run its best, and it did run better, but it still sucked. What do I mean by "it sucked" you might wonder?
• Adjustments wouldn't stay put. It would run fine for a few hours and then not. I would have to tune it all over again each time it was ran, or sometimes have to stop and retune it in the middle of use.
• All adjustments were buried under a plastic cover which was covered by the air filter, which was under another plastic cover that couldn't be removed until a 3rd plastic (side) cover was removed. Wouldn't be a huge deal if it didn't require frequent access in the course of "normal" operation.
• The float would get stuck and flood everything, draining a whole tank of gas if you didn't remember to shut off the fuel valve (that I had to add because it didn't come with one). Also it had a solenoid inside the carb meant to prevent this, which didn't work either.
• It was set up for the exhaust that came on the engine. That exhaust didn't fit inside my buggy so I made my own exhaust and the pipes weren't equal length. So one cylinder would run richer than the other. I tried drilling the jets one bigger than the other to compensate and it made a difference at high RPMs but I could never get it to idle correctly.
My new setup has two separate carbs, one for each cylinder, so I can tune them each for their own exhaust. Also these carbs are meant for motorcycles and have separate jets and/or adjustments for idle, mid-range, and high range, where the original carb, if it had any jet or adjustment for mid-range, I never found it. Which isn't surprising considering the intent behind engine was likely lawn tractors, (big) pumps, (huge) log splitters, etc.; relatively-constant-max-speed applications demanding 20+HP and therefore not optimized for the whole throttle range as is needed for a vehicular application. Also these new carbs are quite common so there are plenty of parts for them. For example this 30-piece kit of jets I just ordered. Trying to find jets for the original is hard, and there is no selection; you buy factory replacements and drill them out until you've opened them up too far and then you buy more factory jets and drill them out but not quite as large as you did last time.

#### SamR

Joined Mar 19, 2019
4,492
Yes I have had vehicles with points ignition but no I never tried the sweet party trick you described.
It tends to open muffler cases at their seams and flatten them out... Which kinda overrides the fun of it!

#### crutschow

Joined Mar 14, 2008
31,129
It tends to open muffler cases at their seams and flatten them out... Which kinda overrides the fun of it!
Once my old van was misfiring (don't remember why) so I stepped on the gas to try to clear the problem.
There was a loud bang, and suddenly the sound from the muffler became a lot louder.
Blew a nice hole in the bottom of the muffler.

#### SamR

Joined Mar 19, 2019
4,492
Was on I-65 south of Birmingham AL when my 69 Camaro backfired and blew the Hush Thrush muffler off of the 3 1/2" header collector, no tail pipes (had to add those to get GA certification). I got off the interstate at my turnoff and pulled into a gravel lot and was under the car trying to determine the damage when I heard and then saw this shiny pair of black Riding Boots. Poked my head out to see an Alabama State Trooper standing over me getting ready to write me up for no mufflers. And it WAS loud coming straight out of the header. Told him it just happened and he gave me a week to get it fixed. Better than a ticket any day!

#### strantor

Joined Oct 3, 2010
6,338
Was on I-65 south of Birmingham AL when my 69 Camaro backfired and blew the Hush Thrush muffler off of the 3 1/2" header collector, no tail pipes (had to add those to get GA certification). I got off the interstate at my turnoff and pulled into a gravel lot and was under the car trying to determine the damage when I heard and then saw this shiny pair of black Riding Boots. Poked my head out to see an Alabama State Trooper standing over me getting ready to write me up for no mufflers. And it WAS loud coming straight out of the header. Told him it just happened and he gave me a week to get it fixed. Better than a ticket any day!
My Trans Am had mufflers but it didn't matter, it was loud enough that it might as well have had open headers. Thankfully owned it during the information age so I don't have to just describe it for you. There's video:

I've had over a decade to refine my taste so I don't really prefer that sound any more. Quality over amplitude these days. I would love to have that car again and the first thing I would do is put different exhaust on it. Loud would still be fine, if it was a better kind of loud.

#### shortbus

Joined Sep 30, 2009
9,762
I got these carbs from Amazon. They work way better than the stock carb so far.
Well from the looks of those carbs I think you will have a very hard time making a governor work. I did't know you were talking about Mikuni style carbs. They are for motorcycles or the type of use where there is w twist grip type throttle, none I've ever seen could be governed because they are a slide type, instead of a blade type throttle.

I will tell you one thing to watch for though and that is any air leaks on the rubber "spigot" that connects to the manifold. Mikuni's were a big thing on Harley's in the 1980s. so off came my Bendix carb and a Mikuni took it's place. Going down the freeway and getting to my exit the throttle wouldn't shut the engine down, it was going full throttle. and the exhaust pipes were glowing red. When I got home and started looking the rubber spigot had a crack in it.

Mikuni's can still pull fuel through the jets when the throttle is closed, and the leak in the rubber allowed it to happen and gave enough air to run lean and not slow the engine using the twist grip. The good old Bendix was put back on the bike.

#### strantor

Joined Oct 3, 2010
6,338
Well from the looks of those carbs I think you will have a very hard time making a governor work. I did't know you were talking about Mikuni style carbs. They are for motorcycles or the type of use where there is w twist grip type throttle, none I've ever seen could be governed because they are a slide type, instead of a blade type throttle.

I will tell you one thing to watch for though and that is any air leaks on the rubber "spigot" that connects to the manifold. Mikuni's were a big thing on Harley's in the 1980s. so off came my Bendix carb and a Mikuni took it's place. Going down the freeway and getting to my exit the throttle wouldn't shut the engine down, it was going full throttle. and the exhaust pipes were glowing red. When I got home and started looking the rubber spigot had a crack in it.

Mikuni's can still pull fuel through the jets when the throttle is closed, and the leak in the rubber allowed it to happen and gave enough air to run lean and not slow the engine using the twist grip. The good old Bendix was put back on the bike.
Thanks for the tip on rubber spigot. I didn't like the look of those rubber boot things (my mistrust of accepted practice now seems not so paranoid) so I rolled my own interface between the head and the carbs. Solid aluminum with an O-ring, made from a spare intake manifold that I hacked up on the bandsaw. The carb fits (really) snugly inside, and then is locked in place with 3 set screws around the circumference.

I have an idea for the governor. It would probably be a waste of everyone's time if I tried explaining it in depth (because I won't do a good job explaining, and because I will post pics if it works) but picture an old Volkswagen Beetle's dual carb throttle linkage. I'm just assuming you've seen that, but if not I can post a video. I think I'll take the springs out of the "Mikuni" (Chinese clone = "Maikuni") carbs and replace the pull cable with a rod. Now the slide can be pulled open or pushed closed, no spring required. There will be a rotating bar spanning from one carb to the other, and when that bar turns, it moves both slides in unison. With that setup, compensating for the 1/2" travel of the governor arm vs the >1" travel of the carb slides will be no big deal. It will be set by the ratio of the lengths of the two carb arms vs the "master" arm. And since no more springs, no worries about putting high loads on the governor arm. It should move fairly easily.

#### LowQCab

Joined Nov 6, 2012
2,650
"" It was set up for the exhaust that came on the engine.
That exhaust didn't fit inside my buggy so I made my own exhaust and the pipes weren't equal length.
So one cylinder would run richer than the other.
I tried drilling the jets one bigger than the other to compensate and it made a

difference at high RPMs but I could never get it to idle correctly. ""

It's not the "Exhaust-Length" that's causing Idle issues,
it's the fact that it's a "V-Twin",
it will be a miracle if You ever get the Idle right.
The Factory Carb came with different Jets in each side, with different part-numbers.

Those Carbs are definitely over-kill.
You can probably achieve full output-Power at a ~50% Throttle-Opening,
Full Throttle could actually result in a "hard to Tune-Out" flat-spot or bog.
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#### strantor

Joined Oct 3, 2010
6,338
Those Carbs are definitely over-kill.
You can probably achieve full output-Power at a ~50% Throttle-Opening,
Full Throttle could actually result in a "hard to Tune-Out" flat-spot or bog.
I got (2) 28mm carbs. This is from the Listing:

Size:
21 mm: 50cc to 100cc engine
24 mm: 50cc to 110cc engine
26 mm: 75cc to 125cc engine
28 mm: 75cc to 125cc engine
30 mm: 100cc to 160cc engine
32 mm: 125cc to 200cc engine
34 mm: 125cc to 250cc engine
My reasoning was, these are for dirt bikes which rev 2-3X higher than my engine, so need 2-3X more air. Then consider their guidelines were probably for 2-stroke, so another 2X multiplier. Therefore I figured the provided guidance is 4-6X higher than what I need, let's call it a factor of 5. Since I'm putting on 2 carbs, divide my displacement by 2: 670/2 = 335CC. Then divide by 5, you get 67CC. For 67CC they recommend 26mm, but I opted to go 1 size larger because a company that sell parts for these engines, sells their dual carb kit with 28mm carbs and I figured they tested it and other sizes before deciding 28mm was the right call. Maybe their choice was random but mine wasn't; it was based on theirs. If it turns out you're right, I can go to a smaller carb. They only cost $30 each. It's not the "Exhaust-Length" that's causing Idle issues, it's the fact that it's a "V-Twin", it will be a miracle if You ever get the Idle right. The Factory Carb came with different Jets in each side, with different part-numbers. I will get it to idle right. I bet a case of beer on it. Last edited: Thread Starter #### strantor Joined Oct 3, 2010 6,338 I have an idea for the governor. It would probably be a waste of everyone's time if I tried explaining it in depth (because I won't do a good job explaining, and because I will post pics if it works) but picture an old Volkswagen Beetle's dual carb throttle linkage. I'm just assuming you've seen that, but if not I can post a video. I think I'll take the springs out of the "Mikuni" (Chinese clone = "Maikuni") carbs and replace the pull cable with a rod. Now the slide can be pulled open or pushed closed, no spring required. There will be a rotating bar spanning from one carb to the other, and when that bar turns, it moves both slides in unison. With that setup, compensating for the 1/2" travel of the governor arm vs the >1" travel of the carb slides will be no big deal. It will be set by the ratio of the lengths of the two carb arms vs the "master" arm. And since no more springs, no worries about putting high loads on the governor arm. It should move fairly easily. I did an experiment, removed the throttle cable and spring, replaced the cable with a length of TIG welding rod, and measured the angles at which I can push/pull the slide inside the carb before it rubs or binds on the hole it must slide through. Based on those angles I drew this up. Hopefully it makes more sense than my ramblings: #### LowQCab Joined Nov 6, 2012 2,650 Exactly what I had imagined !!, ( except for using Welding-Rod instead of Cables ). Longer Cables may be "slick-working enough" to allow You to loop each of them together around to a bracket that's more in-line with the Governor-Lever, possibly, greatly simplifying the re-implementation of the Governor. That Governor, and it's associated Springs, are fairly strong, possibly strong enough to negate any "Stiction" problems created by the Cables. I'd still like to see a Micro-Squirt installed, that would give You a 3-D Map for controlling Timing-Advance, making the Idle more stable, and probably adding an extra ~5-or more, ft/lbs of Torque, also making the Engine run cooler, and quieting-down the exhaust-note at cruise, and increasing Fuel-Economy, You could even add an Idle-Control-Valve. But it is a lot of work to install and set up. . . . Thread Starter #### strantor Joined Oct 3, 2010 6,338 Exactly what I had imagined !!, ( except for using Welding-Rod instead of Cables ). Longer Cables may be "slick-working enough" to allow You to loop each of them together around to a bracket that's more in-line with the Governor-Lever, possibly, greatly simplifying the re-implementation of the Governor. That Governor, and it's associated Springs, are fairly strong, possibly strong enough to negate any "Stiction" problems created by the Cables. I already had it running with a two-into-one throttle y-cable and made the judgement call that it would put too much force on the governor. The problem was less to do with stiction and more to do with the force of the springs inside the carburetors. The butterfly valve on the original carb presented almost zero resistance to the governor arm (which is quite long, starting at a great mechanical disadvantage) while the force required to overcome these springs and open the slides on the new carbs is like twisting two motorcycle hand grips simultaneously. Then consider I must put the governor arm at an even greater mechanical disadvantage to step its travel up from 0.5" to 30mm, and it seemed to me like way too much. So I determined the springs had to go, and since "you can't push a rope" that meant rods. I'd still like to see a Micro-Squirt installed, that would give You a 3-D Map for controlling Timing-Advance, making the Idle more stable, and probably adding an extra ~5-or more, ft/lbs of Torque, also making the Engine run cooler, and quieting-down the exhaust-note at cruise, and increasing Fuel-Economy, You could even add an Idle-Control-Valve. But it is a lot of work to install and set up. . . . You just keep on talking mister and you might find yourself embroiled in guiding a moron through a Micro-Squirt installation. Last edited: #### shortbus Joined Sep 30, 2009 9,762 I will get it to idle right. To help with that I have two suggestions from years of working on dual carb bikes. 1. a ballance tube between both manifolds. Most dual carb setups do this in the manifold design its self, Cast as part of the manifold, even multi carb car engines there is a connection in the manifold. This balances things out so the individual cylinders aren't fighting themselves. 2. One really good thing for setting the idle on multi carb bike and even the old Triumph cars is a "uni-syn" Using one of these makes things so much easier. This link is just to show what I'm talking aboutt, shopping can get better prices. https://www.edelbrock.com/uni-syn-a...ment-for-1-and-2-bbl-with-round-top-4025.html You just keep on talking mister and you might find yourself embroiled in guiding a moron through a Micro-Squirt installation. Why would you want to go with a micro squirt? They won't work with your magneto ignition, since the advance/retard isn't even part of a small engine. ; ; ; ; #### LowQCab Joined Nov 6, 2012 2,650 Sounds like you're just as crazy as me. Maybe I can talk You out of it ....... Ignition Control Only ........ Micro-Squirt Computer w/ Harness = ~$344.oo,
Custom Flywheel-mounted "60 minus 2" Optical-Interrupter-Disc, = ~$65.oo, or DIY ( the Flywheel has internal Magnets that will not work with a Hall-Sensor, or VR set-up ) Optical-Sensor, ( any Output-type will work, Micro-Squirt will provide 5-Volts-Ref-Power ) ( If the "60 minus 2" Toothed-Wheel is mounted away from the influence of the Magnets in the Engine-Flywheel, either a VR-Sensor, or Hall-Effect-Sensor can be used without issue, a "36 minus 1" Toothed-Wheel can also be used ) 2- GM-Truck Ignition-Coils = Standard-Motor-Products #UF262T = ~$60.oo-pair new, ( Junk-Yard ),
2-GM Intake-Air-Temperature-Sensors, ( IAT-Sensors ), ( 1 for Intake-Air, 1 for Engine-Temp ), ( Junk-Yard ),
1-GM Manifold-Absolute-Pressure-Sensor, ( MAP-Sensor ), ( Junk-Yard ),
( A PVC-Pipe "Pulse-Damping-Chamber" needs to be constructed for the MAP-Sensor,
Map-Sensors don't work well with less than 4-Cylinders, and a V-Twin is the worst case possible ).
Various "Pig-Tail" Connectors for Sensors, ( Junk-Yard ),

Fuel-Injection ...............
( needs to be larger than ~650cc Internal-Volume, bigger is better, can be any shape )
Small Car Throttle-Body with integrated Throttle-Position-Sensor, ( TPS-Sensor ), ( Junk-Yard ),
( Any ~0 to ~5V Pot attached to the Throttle will work )
1-complete-set, Fuel-Injectors, preferably from a smallish GM-Engine, ( Junk-Yard ), ~$60.oo, ( get more than just 2 in case one or more of them are bad / clogged ), Custom-Machined-Aluminum Fuel-Injector-Mounts, New-Electric-Fuel-Pump, ~$75.oo,
New-Adjustable-Fuel-Pressure-Regulator, ( ~20-psi ) ~$100.oo, New- proper-High-Volume Fuel-Injection Fuel-Filter, ~$25.oo
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#### strantor

Joined Oct 3, 2010
6,338
To help with that I have two suggestions from years of working on dual carb bikes.
1. a ballance tube between both manifolds. Most dual carb setups do this in the manifold design its self, Cast as part of the manifold, even multi carb car engines there is a connection in the manifold. This balances things out so the individual cylinders aren't fighting themselves.
Ok, I never thought of that or heard of it, but it makes sense, and if I have trouble, I'll give it a shot. Thanks for the tip!

2. One really good thing for setting the idle on multi carb bike and even the old Triumph cars is a "uni-syn" Using one of these makes things so much easier. This link is just to show what I'm talking aboutt, shopping can get better prices. https://www.edelbrock.com/uni-syn-a...ment-for-1-and-2-bbl-with-round-top-4025.html
This I have seen, and contemplated purchasing, but I thought it better to try without it first, see if I need it, before spending more. There's a trick with water bottles I want to try too:

Why would you want to go with a micro squirt? They won't work with your magneto ignition, since the advance/retard isn't even part of a small engine.
Yeah it would mean throwing the baby (magneto) out with the bathwater. Basically converting it to a miniature modern ICE. Why would I want to? @LowQCab listed some good reasons but for me they are just icing on the cake. For me, the real reason would be: Just because. I like to tinker, learn new things, etc. and this electronic engine control stuff fascinates me. If you'll think back to my OP, I was trying (stretching) to address a mechanical problem with an electronic solution. Thankfully I was talked out of it, but the itch remains and demands eventual scratching.

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