Why do engines shake and stutter at low rpm/higher load?

[SmithSwede]

Lately I’ve been putting a lot of miles on an old Ducati 900 SS.  Two valve heads.  Carbed.   It is geared to the moon tall.

The engine will stutter and shake if I give it too much load from a stationary position at low rpms.  Like 2000 rpm or lower.  If I
let the clutch out at 2,500 rpm it’s ok.  I’ve learned to deal with this.  Some people just add more teeth on the rear sprocket and lower the gearing.

Anyway, here’s my question.  Why exactly does the engine stutter and shake like this?  It doesn’t do this at idle.  And yeah, I’m releasing the clutch, but the engine surely can’t be under that much load.   

So what is the physical explanation?   Incomplete combustion given that load condition and throttle opening?  Why?  Why doesn’t it just lack power—as opposed to violently bucking and lurching?

It almost seems like a negative feedback loop.  Once it starts to stutter, it only gets worse until you slip the clutch.  Why?

[Antietam Classic Cycle]

Light flywheel? My old Loops with their 13 lb. flywheels don't do it, nor does the Convert.  I really wish the V7 Stone had a bit more flywheel weight.

[insignia100]

Maybe more in depth than you want but here ya go: https://www.youtube.com/watch?v=soJea7xEt-8

Try revving the engine more when releasing the clutch. ;)

[oldbike54]

 Modern short stroke engines with relatively large valves suffer from slow intake and exhaust gas speeds at low RPM's , leading to incomplete combustion . Combined with the slight rocking forces you feel the engine shuddering . All modern twin cylinder engines do this to some extent . With BMW airheads the R 50 was smoother at low RPM's than the R 60 , which was smoother than the R 75 , so on and so forth .

 Dusty

[pete roper]

If you're riding a 900SS around at sub-3,500/4,000 RPM I hope you're happy with rebuilding three part cranks and replacing major bearings on a regular basis.

That engine is basically a sodding great gearbox with a couple of pistons inside. It was also the direct descendent and based on a true 'Race' motor. At lower RPM oil delivery is marginal at best and you will end up galling up big end bearings in short order and, as suggested, that means rebuilding the crank as it's a pressed up assembly. Not a job for a novice.

The way to use bevelies is very simple. Run them on straight 50 oil and change it at very short intervals, warm it up carefully and once it's warm beat on it like a red headed step child and never lug it around at sub 3,000 RPM. It'll last for ages.

Pete

[Tusayan]

Based on prior posts, the engine is a belt drive SS with one piece plain bearing crankshaft, not a bevel drive engine.

Get the pilot jetting right to eliminate misfire on small throttle openings under light load - given your description that might be a big part of the problem.  The stock pilots were very lean, and leaner in the US than for world market bikes with both being otherwise the same.

Otherwise, the basic reason the engines are a bit intractable at low rpm is speed variation due to the intermittent, irregular torque pulses that are intrinsic to a V-twin.   The function of the flywheel is to absorb kinetic energy and then release it to reduce the speed variation, also with a penalty in acceleration when you want to increase rpm.  The heavier the flywheel the bigger effect in both positive and negative ways.  At idle there’s no external load to slow the engine between torque pulsations, so you notice the effect less there, especially given the tall first gear that is in play at low speed with stock gearing.

Ducati installed a light flywheel on the SS.  If you’d like a bit more inertia it might be possible to fit an ST-2 flywheel, you’d have to investigate the details.  Otherwise do as many before you have done and lower the final drive gearing with new sprocket (s).  The gearing on a belt drive 900SS was very tall from the factory to aid passing the EPA noise test.

[Penderic]

[wirespokes]

Yes, to all of the above! But to answer your question - it's called lugging. The heavy flywheels store the energy from the pistons/crank and deliver it to the rear wheel. When there isn't a heavy flywheel, the power from each piston's power stroke goes directly to the wheel.

If the wheel doesn't want to move, then something else will.

Consider this: to generate power, a solid base is needed, something that's often overlooked. To use a vise, it must be mounted to something solid, obviously. In this case, the power unit is mounted in a motorcycle. The engine turns a shaft, and if the shaft won't turn, the engine will try to rotate around it.

Makes sense?

[Rough Edge racing]

 I have a 96 900 Monster. I spent some time tuning the stock Mikuni CV downdraft carbs, most run the low speed far to rich giving lumpy low speed performance..It's got the V2 heads with big valves and hotter cams like an SS. I'm using no airbox with large pod filters.The engine turns 3500 rpm at 60 mph in 6th gear....It takes off easily from a stop at just above idle...It does shake a bit if you use throttle at low rpm's, but so do Guzzi's?  The Monster is quite happy to troll through town smoothly at 2500 rpm in third gear on level ground and it's second to none in acceleration....It's all in the tuning....

[chuck peterson]

energy cant be created or destroyed...control led explosions can be tricky to move a static object.

You use a lot of muscle, at first, to slide an object across the floor...easier once its moving

Bring it to max torque, then dump the clutch, everything should smooth tight out.....!!!!

 

[rocker59]

Back in the '90s, everyone was putting light flywheels on their 900SS to make them quicker.  I would check to make sure what flywheel is on the bike.  It's entirely possible that a previous owner put a really light flywheel on the bike.

Almost everyone with those bikes also went down one tooth on the front sprocket immediately.  Some also went up a couple teeth on the back sprocket. 

I don't know which carbs you have, but Keihin 39mm FCR was the hot setup, at the time.  They were better in just about every way than the stockers.

Like mentioned above, there is quite a distance between power pulses on a V-twin.  At low RPM with a light flywheel, you're going to feel that as "bucking" if you put it under too much load.

Those 1990s Supersports were happiest running at 4,000 rpm and higher.  I wouldn't recommend running around in 5th or 6th gear below that RPM. 

More info on the deteails of your particular bike would be helpful.

[Rough Edge racing]

 What's the actual difference, if any, between my 96 900 Monster with stock V2 heads and a SS? My bike is very happy to cruise at 3500@60 mph..Is it just the gearing?

[SmithSwede]

Thanks for all the info.   I should have been clearer.  I’m not deliberately lugging the engine—it obviously doesn’t like that.  And yes, it’s happy at 4,000 rpm and above, which is how I run the gears.   I often don’t even get into 6th gear since it is geared so tall.  I may well change to different sprockets when these wear out. 

I’m simply asking for the physical explanation for why it bucks and shudders on those rare occasions when I release a bit too much clutch for the low engine rpms.  Or why any engine does that.

It’s a 1996 SS/CR.  Pretty sure it has the stock Mikuni CV carbs.  Don’t think it has a lightened flywheel but don’t know for sure.

[LowRyter]

You're lugging the engine.

The question it's like asking why have a transmission at all?

[oldbike54]

 It seems some of us are not understanding the question , which is why a twin cylinder engine shudders at low RPM when the throttle is opened , which goes back to low intake and exhaust gas speed . Light flywheels do have an effect , but that isn't what is causing this .

 Think about an older design like a 500 cc Brit single which were long stroke designs with relatively small valves and ports . They would pull smoothly from low RPM's because the small valve sizes kept gas flow moving faster , imagine a water nozzle on the end of a hose , at small openings the water is escaping at a higher velocity than at wide open . The carbs are dumping more fuel into the combustion chamber at a slow speed than the engine can handle , it's why really modern engine control systems don't suffer quite as much from this, but even they can shudder at low RPM's .

 Dusty

[Wayne Orwig]

Beyond the obvious physical properties of needing more flywheel and such to smooth out the lower RPM torque pulses. You are likely dealing with carb issues.

A lot of effort is put into carbs so they fuel correctly at idle, and they fuel correctly at higher RPM. But the transition is a nightmare. Many carbs even have a passage that flows fuel at idle, then flows air off idle. The transition, is a mess. So loading the motor, just off idle, is going to make for some really poor fueling.

I vote that the majority of the problem you are seeing is carbs.

[John A]

There are too many variables for me to have an answer that I’m comfortable with.  Carbs for sure but so is ignition timing, valve timing and on and on.  I’m thinking mixture speed through the intake port may be the single most contributing factor,  as well as the inertia of the gas if we can get the mixture to reliably fire.but a close second would be flywheel weight.  I’m trying to think in terms of stuff that affects the engines behavior just off idle where it would have the most noticeable shuddering.  I’ll read over it later,  there are lots of papers on it,  probly under “drivability “

[BrotherJim]

I had a '95 900SSCR with an FBF lightened flywheel, which made it hungry for higher RPM.  I miss that bike from time to time.

[Rough Edge racing]

 Do any of you remeber how an old manual trans car/truck  engine would sound if you lugged the engine? With 6 or 8 cyclinders it would sort of resonate or rattle the exhaust ...and maybe ping....
  There's a lot of nonsense about Ducati's much of it internet gossip...If you're rolling along at let's say 2500-3000 rpm with the throttle barely cracked open in a lower gear and the engine running smoothly and it responds to very slight changes in throttle opening, you are not lugging the engine....Anyone who wants to ride a proper running stock carburetor Ducati or ride my giant killer 79 Triumph 750 do stop by my house...Lunch and beer is on me... 

[John A]

Cool

[Sheepdog]

I agree with Tusayan on the throttle mixture circuit. My '07 Vintage, my '03 Triumph, my '99 KLR, my '82 Suzuki, and my '81 Honda all needed their low speed throttle circuits recalibrated to correct a bad lean condition. My Vintage got a FI re-flash, but all the rest got (at a minimum) the aluminum plugs drilled out of the carbs, the air/fuel mixture enrichened, and the needle raised a mm or two to ease the transition from low to mid-range. The Triumph needed a richer pilot jet, but the other bikes just needed tuning. The carbureted bikes suffer bad low range performance under the newer emissions standards. The test protocols concentrate on idle and low speed emissions, so top end and mid-range tuning are generally spot-on, but the idle mixture is very lean (even worse with 10% ethanol fuel which burns 3.5% leaner still). An overly lean engine bucks and pops and even stalls if alowed to get hot (which is more likely to happen with lean tuning). Get your carburetors synched and try adjusting your idle mixture for a more optimal setting and raise the needles the thickness of a small washer. This will likely make it run cooler, smoother, and with more linear throttle control.

On second thought, check out the discussion below. Seems like a bunch of folks are going up one or two sizes on the pilot jet and switch to a needle with a different taper...

https://www.ducati.ms/forums/57-supersport/409529-900ss-jetting-advice-what-your-setting.html

https://www.6sigmajetkit.com/ducati-900ss-jet-kit.html

[Rough Edge racing]

 I believe the first thing to do with the Mikuni CV's on a Ducati is the check the float level..And you have to secure the press in float assembly to be accurate...14mm to the carb bowl flange to the highest point of the "square" float.. #40 pilots, Factory Pro needles and jet with the clip in the middle groove..150 main jet...Carburetion is perfect from idle to redline, no surging at lower rpms..I have pods and Marving mufflers..

[SmithSwede]

Thanks.  Lots of good info.  Much appreciated. 

But I’m stubborn and ignorant.   I still don’t think I understand the phenomenon here. 

For example, a lot of you are saying the buck and stutter at low revs has to do with idle mixture.  Well, if it’s fuel mixture, then we are holding constant stuff like speed of gas flow/large valves, or weight of flywheel or 270/450 degree timing, etc.  So why would the fuel mixture be the driving factor in causing the bucking?

I see how suboptimal mixture or gas flow/large valves could cause **less power.**  But then, why don’t I just get less power, yet what power I get is smooth with no buck/stutter?  After all, a little 125cc machine can smoothly pull from a stopped condition, so we are probably only talking about a power requirement of 1 or 2 horsepower to do this.   

I’m not asking why can’t I accelerate really fast with lots of power from stationary position and low revs.   I’m asking why I can’t even creep up to 5 mph without the shakes and shudders. 

Or is the theory that at low revs, the correct mixture actually ignites each power stroke, giving a relatively smooth power delivery with, say, 15 power strokes per cylinder per second.   But suboptimal fueling causes a high percentage of non-ignition events on the power stroke, maybe 7 power strokes per second per cylinder, leading to a hammering and stuttering effect for the few strokes that actually do ignite.  ?

The latter explanation at least makes some sense to me.

[oldbike54]

 At low RPM's the gas flow is slow leaving some of the old partially burned mixture in the combustion chamber meaning the fresh charge is diluted .

 Dusty

[beetle]

Physics.

[oldbike54]

Physics.

 No such thing as a psychic ... oh , wait ...never mind   

 Dusty

[Tusayan]

Or is the theory that at low revs, the correct mixture actually ignites each power stroke, giving a relatively smooth power delivery with, say, 15 power strokes per cylinder per second.   But suboptimal fueling causes a high percentage of non-ignition events on the power stroke, maybe 7 power strokes per second per cylinder, leading to a hammering and stuttering effect for the few strokes that actually do ignite.  ?

That is a pretty accurate description of how misfire with lean mixture works, although it doesn’t take misfire on as many as half the power strokes to create a problem. Misfire is regardless a statistically occurring event, as you’ve described.  Basically the fuel is not uniformly distributed through the air and as the flame front travels from the plug across the charge, if it hits a spot that is too lean to burn, the fire goes out.  With a little bit more fuel, the leanest spots aren’t that lean so it doesn't occur as often.  The best mixture for maximum power is therefore a little richer than the theoretically correct ratio of fuel to air.

[Dukedesmo]

My 3 bikes are all 90 degree V-twins of similar displacement. The order of smoothness is exactly the same as the order of flywheel weight with the Guzzi being the heaviest and smoothest, it will pull from idle without 'lugging'.


The 916 will gets lumpy under load at less than 2.5k and the Monster (900ss type engine) at anything under 3k.


There is also the state of tune, length of stroke, drivetrain backlash etc. which must have some effect but heavier engine internals certainly smooth things out. That said throttle response on the Ducatis is so much quicker especially the 916, on the which the throttle is pretty much an on/off switch...

[Rough Edge racing]

 The older Ducati 900's with the V2 heads have 40 degrees of valve overlap measured at one millimeter valve lift...This is a fair amount and causes the wonderful choppy idle sound of a Ducati or a hot rodded OHV V8....It also can create low speed tuning issues like misfiring especially if the tuning is slightly off or the rider forces the issue....In constrast a stock Harley has less than 10 degrees overlap...And heavy flywheels ..and you can take off literally from idle fast with no clutch slipping and no engine shuddering...Later Harleys also have a vacuum advance like the hot rodded V8 should have..This throws extra timing during light throttle high intake vacuum situations when the mixture is leaner..leaner mixtures need more time to burn completely....Ducat i doesn't have this but leaner idle mixtures with fire with proper tuning and a considerate rider.
  I have found opening up the Ducati spark gap to .035" improves low speed running. Yes, a stock ignition in good shape will fire the larger gap..Use solid core wires with 5000 ohm plug caps or non resistor caps with resistor plugs..

[SmithSwede]

Maybe more in depth than you want but here ya go: https://www.youtube.com/watch?v=soJea7xEt-8

Try revving the engine more when releasing the clutch. ;)

Ok, finally watched your suggested video.  Thank you! 

Low Speed Pre-Ignition.

That at least makes sense to me as a potential source of what otherwise seems like inexplicable shuddering.