What limits RPM on clone engine?

thedude306

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Hi all

We are having great fun on our snowmobiles this year but I was having trouble getting the speed I thought I should and reached out to a facebook group and they got me chasing RPM and it turns out my tach was junk. Found a better one and I am now in test mode. I am getting 6000rpm on the engine and I thought I was getting 6800rpm.

My question is what limits RPM on these engines and is it possible to get more rpms out of this engine?

Engine is a Pro Point 208 from Princess auto in Canada. It apear to be a Ducar motor.
NR Racing cam 280-0611: Rated as a "Good Torque Cam w/ excellent power. 7500 RPM"
NR Racing bored .625 Honda carb
cromo push rods
reinforced rockers
26lbs springs
billet connecting rod
thinner head gasket Thickness: .009"

Some mild head work. Took off a few thousands, ported and polished, lapped the valves.
 
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thedude306

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I do have a bigger carb and AL fly wheel. Added it to original post.

I wonder if coil is an issue?

Maybe I'm expecting too much out of this engine?
 

JTSpeedDemon

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There's actually far more to RPM than it might sound, yes the raw components make most of the difference, but if you research professional engine building, there's so many intricacies to engine performance that range from the mathematical proportions of the bottom end to airflow swirl in the intake port. But that's a bit out of range for what we're talking about here.
Some clone engines do have rev limiter coils, make sure yours doesn't have that.
Also, are the 26 lb springs the recommended weight for the cam? Mismatched spring weights to the cam can kill power.
No 7K RPM is not too much to ask of that engine, but you do have to start getting very specific and professional (and sometimes expensive) to get to that level, usually involving more extensive head work for 7K+ RPM ranges. Building an engine involves much more than simply popping in "high performance" parts like most Youtubers do.
 

thedude306

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Yup, no professional here! LOL

I am just trying to push these engines to the max that I can to my abilities. I did spend countless hours polishing and tweaking the head. Well not tweaking, just getting all the imperfections out. I by no means thought I was building a champ engine. Just getting it better. (flat mating surfaces, casting imperfections etc)

Springs are matched to cam. I've messed with timing and valve clearance and it all seems really good. The engine really pulls nice and seems to be a good performer. It's just the RPMs, and on a sled (and I am sure karts) that's king. Actually the more I think of it the more I am thinking coil. It just kinda blahs at that 6000 rpm range.

oops. How do I tell if the stock coil is limited and what should I get for replacement? Preferably an amazon option as I am in no mans land Saskatchewan
 

OPmini

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take a look into changing the gearing you can change it so you have less bottom end, but pull hard around 6k, and its usually cheaper to buy a sprocket than an engine part lol

almost all stock coils are limited, I would order a racing coil from Pauls Karts he has the best stuff for these little engines.


Pauls Karts
 

thedude306

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take a look into changing the gearing you can change it so you have less bottom end, but pull hard around 6k, and its usually cheaper to buy a sprocket than an engine part lol
I've gone as low as 1.66:1 gearing..... (18 tooth clutch with 30 tooth drive) with a 2.594 pitch 4 tooth driver for the track. I'm back up to 2:1 but need to mess with clutching. Want engagement at 3000rpms + sleds are different then karts but yes I have tried all gearing combos. I do have a 5 tooth driver coming. But I would like to get a little more out of the engine too.
 

karl

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The regular Ol #30500-Z0T-003 Genuine honda coil has no limiter.

The newer UT2 honda engines do have said limiter coil. Just don't order one of them.
I think it limits it around 5k, I may be wrong, so that is probably not the issue.
 

thedude306

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The regular Ol #30500-Z0T-003 Genuine honda coil has no limiter.

The newer UT2 honda engines do have said limiter coil. Just don't order one of them.
I think it limits it around 5k, I may be wrong, so that is probably not the issue.
Thanks I'll start there. For $18 it's worth a shot. Even if I can just start to tickle at the 7000rpm doorstep i'll be happy.
 

karl

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Thanks I'll start there. For $18 it's worth a shot. Even if I can just start to tickle at the 7000rpm doorstep i'll be happy.
It can't hurt. For the price I put one on my bike just for peace of mind, extra reliability.

The rubber dry rots on the clone coils in 3-5 years. I have worked on 30+ year old honda engines, and the lead and rubber is fine.
 

thedude306

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But how does it know to cut spark? Is it heat or a resistor or what?

I'm as low as I can go on the gear ratios. My hub won't take a smaller gear. The sled will still spin it's track in the snow at 2:1 At 1.8 it won't spin but still has good power. And a 1.6 it is a slug off the line but is better on the top end. Original gearing was 4:1 with a 3.5hp engine. I would think my modded engine is in that 10-12 hp range. At 2:1 with a 2.594 pitch 4 tooth driver using the following formula I'm losing 5 mph per 1000 rpm. To run 35mph at 7000rpm would be the sweet spot at 2:1 More then fast enough on the snow to keep up with a bigger sled (well not keep up but not slow down considerably)

RPM/Gear Ratio X (Sprocket pitchxteeth)/12 X 60/5280 = MPH

 

JTSpeedDemon

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From what I know, Honda rev limiter coils have some kind of transistorized system that cuts spark at a certain RPM, don't know much beyond that since I mostly deal with Briggs flatheads.
 

65ShelbyClone

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But how does it know to cut spark? Is it heat or a resistor or what?

Most information I find about the coils says they're "digital" (which is vague). They probably just have some simple transistor logic circuitry that compares time between pulses and then grounds the primary when the time is too short. Just an educated guess on my part.

I'm as low as I can go on the gear ratios. My hub won't take a smaller gear. The sled will still spin it's track in the snow at 2:1 At 1.8 it won't spin but still has good power. And a 1.6 it is a slug off the line but is better on the top end. Original gearing was 4:1 with a 3.5hp engine. I would think my modded engine is in that 10-12 hp range. At 2:1 with a 2.594 pitch 4 tooth driver using the following formula I'm losing 5 mph per 1000 rpm. To run 35mph at 7000rpm would be the sweet spot at 2:1 More then fast enough on the snow to keep up with a bigger sled (well not keep up but not slow down considerably)

RPM/Gear Ratio X (Sprocket pitchxteeth)/12 X 60/5280 = MPH

This is assuming the engine makes enough power at those high revs to pull the speed you're after. I suspect it would be beyond the power peak by 7000. You said you have a larger carb, but not what it is or how big.
 

thedude306

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The transistor logic makes sense.

My carb is a bored out honda. I cant remember the jetting on it but it has been flawless as far as stating and performance (as far as i can tell). It was the recommended carb from NR racing. It runs rich when warm out (above 0C) They recommended the jetting as well for in the cold. I think you are right I will start to run out of power. But a guy can try! Even another 5-800 RPM would be noticable and welcomed.
 

itsid

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The newer UT2 honda engines do have said limiter coil. Just don't order one of them.
I think it limits it around 5k, I may be wrong, so that is probably not the issue.
IIRC those UT2 coils are limiting at around 6k (5.8-6.2)
the problem is.. it's a CDI coil and as far as I understand it's main benefit is it advances timing with increasing rpms..
so some might have issues with this coil reaching the 6k IF they installed an advancing key or flywheel that - together with the coil-
cause a too early ignition slowing the engine down again...
but yeah it cuts off at around 6'ish k revolutions either way.

it's easy to test however!
if you own an oscilloscope it's as easy to wrap a bit of insulated single strand wire around the spark plug wire and
with the help of a small capacitor deed that to the input.
next best thing :
just get a timing light.. rev the engine up and if you see the timing light flickering at 6krpms.. it's the coil refusing to spark.
(easy to see the difference if you compare that to slightly under max rpms ;))
since it fires once every revolution it appears to be "always on" at 100 Hz and a limiting coil would cut that that to less than half
so you'd end up with a less than 50Hz signal instead (and we all have a decent idea of how that looks and sounds, no?)

With the old non CDI non advancing and most of all non limiting coils,
you might loose another hair of power either in low to midrange or on top depending on what your mechanical timing is set to.
but that can (mostly) be dealt with by adjusting the flywheel again.

And while you did some testing already..
I still am inclined to keep one finger still pointing at the gear ratio...
it's easy to overestimate the top end torque of a modified engine,
You cannot judge that by it's low end torque at all I'm afraid,

So if you see the no-load rpms rising to 7k easily and you can only put 5.8k onto the road (*cough* snow in this case)
you might want to consider gearing lower

'sid
 

thedude306

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Great info on the coils. I never thought about the possibility of the CDI coil and controlling timing but that makes total sense. That's what our big 2 strokes on our 800 sleds are doing, (well with external CDI box)

Timing light.... oi I think I have one of those in a drawer somewhere! It's been a while since I had to set timing and points on a truck but I think I can find it. That is an easy test.

gearing is still an option. As is clutching. No load RPMS are 6040 right now and 5980 with a load. That's pretty good I think. My assumption is that I won't get to the higher RPMs without a long straight away. That's OK. We have lots of those and that's the only place the extra speed is really needed. And by long I mean a mile or better. Lots of places have 3 or 4 mile long straight aways. Our last ride out we had 48 miles on the odometer.

I went and looked at some torque and HP dynos of these style of engines and I was surprised that the torque came on so early. and peaked in that 2500 rpm range. Im not used to the sound of the RPMs either. Hopefully with time I'll get some of that figured out, It's also extremely difficult to "test and tune" with a 6 year old! It's hard enough as an adult! Stickers and loud exhausts have gotten us "super fast" according to me son...

Our playground


 

thedude306

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if you have a stock coil it prob has a built in governer

just get a timing light.. rev the engine up and if you see the timing light flickering at 6krpms.. it's the coil refusing to spark

WINNER WINNER CHICKEN DINNER!

Thank you gentleman, found my old timing light and it is definitely the coil. Starts to flicker right around 6000 rpm (dropped a little after warm up)

In some of my reading over the last couple days it seems there is some evidence that coil gap really effects engine performance. How do you quantify this outside of a dyno? Just set it and forget it? I am at .40"

New coil shipped, I should see it monday!
 
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