Topic: Solved: Why do they sound "sick" at low rpm?

[Kaze]

Why do (some) drag and other race bikes sound so "sick"? Is it because they aren't meant to idle but go fast?

Listening to a well-tuned CB750 drag bike, or other race bike, when they are coming to the starting line, the sound is sort of similar to my stock CB750 when it's running poorly. Sputtering and "coughing".

I realize that my bike and a race bike aren't the same, but what's the cause of this sputtery sound? It's almost like the rider has to fight to keep the thing running at lower rpm.

[Don R]

 If you are seeing it on video, it may be the microphone being unable to capture a race engine sound.
 Often when staging, the rider hits full throttle while the engines go on an electronic rev limiter that alternately drops cylinders to maintain a pre-set launch rpm. Sometimes called a two-step which is usually an MSD product, it has one pre-set rpm limit while staging and another pre-set rpm limit while going down track.
 For gear changes, an air cylinder pulls on the shift lever and when the shift light comes on the rider hits a button to trigger an ignition switch that kills power for a milli-second so the trans can shift.

 More than what you asked for but what I think I know about it. An actual bike racer may correct some details.

[PeWe]

Forum member Dragracer have some videos.
I was impressed how nice his bike idled despite RC327 cam.

Here a link to one search
http://forums.sohc4.net/index.php?topic=143269.msg2103397.msg#2103397

[Tracksnblades1]

The type of fuel can affect what the idle sounds like.

Listen to this engine and see if you can hear and see when the good fuel makes it to the engine after it starts…

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjj0YKc2_OBAxXdrokEHY6aAPgQtwJ6BAgWEAI&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DV56St37tYas&usg=AOvVaw3_HtuwyexAxVBC2Xigib85&opi=89978449

[Don R]

 A sort of related fun fact, when you go to the NHRA races there won't ever be a nitro team pitted with the pit side of their rig on the open end of a row. They want every pit located between trailers so if a nitro motor explodes, the trailers shield as much of the crowd as possible.
  That's also why a team member always turns the motor backwards every time it is shut off, to get the Nitro out of the cylinders so it can't hydraulic and blow later.

[Kaze]

Sorry I didn't have a better word than "sick". It's more like a popcorn machine. When a normal road bike pops and backfires, something is really wrong. These elite machines make all kinds of wild and crazy sounds.

It's not like I never watch any races, just never had anybody around to ask, y'know? And fact is, when I was younger I was too busy riding my bikes and chasing my wife to care about sport. ;P

DonR. Don't hold back on my account. Good stuff. Seems like these bikes are just nothing like our stockers. (I kinda know, but I don't really... y'know? lol)
Pewe: Yes, Dragracer has seriously cool vids. Love that POV stuff.
TracksNBlades: When the plumes of smoke and the "popcorn" starts?! Holy CRAP! That thing is 1/2 fighter jet.

Here is an example, at about 2min.20sec. of what I hear and think "Is this okay?" It would be bad on my bike, but my bike isn't a racer.

[PeWe]

Carbs can not be properly jetted.
The sputter when relasing the throttle a little from not that high rev might be too rich needles.

I've read that the old Keihin CR tickler carbs had bad or not existing pilot circuit. So that might be the case here.

If that engine should get Yoshimura MJN32 carbs (Improved Mikuni TMR32 carbs) or TMR32 no sputter at all.
Even old Mikuni VM29 should make it smoother if bother to jet the pilot circuit correct.

Forum member dragracer's much hotter F runs silky smooth on those videos I've heard.

Spidermans nitro bike another reason of a harder pounding engine.
Russ Collins jr nitro triple soon to be started. That will sound wonderful!! 3xCB750, around 1000cc each.

Search for Mrieck's video on dyno.  1005cc 1975F.
He ran a rather hot race cam, Megacycle 125-30 if I remember correctly. Hotter cam than that on a racer is probably not actual. Mikuni RS34 carbs.

My K6 in same power range.  Really smooth idle. It has harder pounding engine than stock where compression is relatively low.
If riding it harder on a curvy road, let it rev out on 3:rd gear and slow down from 8000-9000 rpm by throttle almost off it will sound very nice with hard bangs when mixture it will get at higher revs than the idle circuit is jetted for.  Then just give full throttle and it will sound very nice making the horns in head to grow way too much.
I've done that in my town on 2nd gear in city traffic. Checked mirrors direct for blue flashing lights!

Different and harder more explosive bangs when jetted too lean.
Pilot circuit must be correct for idle and slow throttle increase to around 4000rpm on 5th gear. Slow decrease in that area without sputter too.  Easier to see that instead of throttle lift wich is the real thing. Needles on my TMR32 affect the entire range from idle to at least 3/4 throttle.
I have tried numerous combinations until carbs got correct needles with right taper.

[Tracksnblades1]

Sorry I didn't have a better word than "sick". It's more like a popcorn machine. When a normal road bike pops and backfires, something is really wrong. These elite machines make all kinds of wild and crazy sounds.

It's not like I never watch any races, just never had anybody around to ask, y'know? And fact is, when I was younger I was too busy riding my bikes and chasing my wife to care about sport. ;P

DonR. Don't hold back on my account. Good stuff. Seems like these bikes are just nothing like our stockers. (I kinda know, but I don't really... y'know? lol)
Pewe: Yes, Dragracer has seriously cool vids. Love that POV stuff.
TracksNBlades: When the plumes of smoke and the "popcorn" starts?! Holy CRAP! That thing is 1/2 fighter jet.

Here is an example, at about 2min.20sec. of what I hear and think "Is this okay?" It would be bad on my bike, but my bike isn't a racer.

What PEWE said…..plus big long duration cam with smaller displacement engine…music to my ears…

I’m betting that bike still has the long duration intake reversion (loping/burbling) till 3500+rpm  before it finally cleans up.

Like PEWE, DonR, and BryanJ may have inferred in their other posts, each high performance engine is unique and requires tuning outside of the OEM manuals box. But that said they’re all the same too, most will require similar tuning with similar camshaft durations and engine modifications. Special attention will generally always be needed.  Especially to achieve satisfactory results below their intended rpm range when the intake and exhaust pulses are not complementing each other.

A camshaft like the one in your video is only designed to run in a specific RPM range. Probably really high rpm with all the burbling going on at lower rpms. Carburation will be a compromise outside its intended rpm range especially the lower rpm range and idle circuit when the intake and exhaust pulses aren’t complimenting each other..

I think Frank and Bill Benton has some YouTube videos of JMR big blocks still slobbering like that, as the lights are dropping down to green 😁

[Kaze]

I'm way out past my depth here, but it's still interesting.
"A camshaft like the one in your video is only designed to run in a specific RPM range."
I guess that makes total sense, I just never thought of it. They aren't meant to go to the market after all. Amazing.

J. Frank Lonely's bike sound just rips through ya!

[PeWe]

More videos when at it
Here a dyno run at my local shop that have ported my heads.
First one 1983, same guy ported the last head 2021 my K6 has now.

Video, CB750 vintage racer,  I think 62mm pistons. Rather high compression, JE pistons shaped together with chambers.
Norris RX2 cam.

[PeWe]

This is the race idle, right?
https://youtube.com/shorts/o-hp3I0Bz_E?si=c8BICFJPGaRwo7gb

[Kaze]

Your same bike was ported in 1983 and again in 2021?
Which is your bike? The one on the mobile phone video or the Dyna one?

The one on the Dyna sounds more like a track racer than a drag bike to me (but what do I know?)

[PeWe]

3 heads ...
The long story a little bit shorter.

The bike on earlier dyno is a vintage track racer.  Not dragracing. 
Those guys at Motospeed has long history of dragracing since the owner raced Probike in the 80's.
Plus their customers racing bikes, engines to be modified.

The first of my CB750 heads ported was my K6 stock head, then for 836cc winter 1983.

Same head restored spring 2014 after long storage. Once again 2016 when it got 5mm valve kit.

2
Another ported K2 I had bought cheap. Improved and balanced the ports, increased In to 34mm.
Done 2019 as a spare head.
Came into use late 2020 when the 5mm valve guides gave up. Fast wearing bronze guides.

3
At that time I bought a K7 head from UK since it looked good for a final port job before the mechanic retire.

I had plans for this head I discussed with the mechanic.
- Yoshi style port job in front of guides that has been discussed on this forum.
-Chamber sizes to match my 1005cc Billet block with pistons.
-This time stock iron guides, Kibblewhite 33.5/28.5mm valves.
Plus the message to reach   110whp I mentioned Mike Rieck in USA with similar 1005cc setup that has a dyno with 106.5whp.

That head got correct combination with a 970cc cylinder I fixed to avoid torturing the fine Billet block 1005cc with wrong jetted carbs. Correct needles hard to find back then.

The higher compression with thinner gaskets helped to reach another 5-6whp reaching 107.5 whp din.


The old head with 5mm valve kit got CycleX 6.5mm guides, CycleX 34/28 mm ss valves.  This head has bigger chambers and will be used in another ongoing build with 890cc.
Cr 12.5:1 JE pistons I quickly tested between the other head swaps.
Too high compression so crowns shaved 2mm that will avoid doubble spark plug gaskets to not hitting the pistons. 2mm off felt right
I tested with 3 different cams,  DP315, Megacycle 125-20 and Megacycle 125-75.

These mods, quick cylinder, heads swaps with different cams thanks to forum member 754 (Frank) frame kit.

I learned that the RC295 cam give more power than RC315 profile, from low to high.

The cam profiles with more duration has often rather low lift. (Except for Dragracing profiles)

Probably to avoid changing guides, new guides with new cut grooves for stopper ring to let it sit deeper in head. Guide has to be reshaped and cut in port area too.
This to avoid the K valves crazy high waist (groove) for keepers.

That slimmer part of valve will otherwise dive inside guide seal and make oil to be sucked inside the in port.

In the beginning higher oil consumption at inspired riding. After a few rides oil will just disappear during a slow ride.

The first head had no problem with 9.14 mm lifting cam at 0 lash (0.360"). It had F2 34mm In valves with a much shorter and higher sitting groove for keepers.

Stock sitting guides with K valves must have cams lifting under 9mm, not much higher. This can explain the old Web Cam profiles with relatively low lift but longer duration

CycleX has shorter guides for higher lifting cams.

Another video for sound check of midified CB750

My today blue K6 when it had 836 with first head.
80whp. A year after its comeback 2014.

I liked that screaming Alfa 4-1 that reminds me about the 80's.

I have an old K1 head in need of new guides and valves mounted.
Need to contact the mechanic that loves his job and passed retirement age 7-8 years ago.

[Kaze]

That 836 doesn't sound sick at all! I think that black one might be my dream bike...

Interesting the sorts of challenges you face with modifying the engine. "High hips" and such.

[dragracer]

I'm not certain what you're hearing at the starting line for a dragbike but if its after the bike prestages, the rider will typically roll on the throttle to  electrically activate a low side rev limiter incorporated into some type ignition controller.  The controller intermittently cuts fire to the coils or inside the controller at the riders preset launch rpm. This allows the rider to fully lock the throttle until releasing either a button or a clutch lever. The sparadic ignition interrupt then drops out allowing the engine to go full throttle. This process allows for consistent launches and keeps from overrevving the engine at the starting line.

If you don't hear a dragbike running with a crisp idle/lope, there's a  carb tuning issue.  We normally run Lectron carbs on bigger engines that are only designed to do 2 things; idle or operate at full throttle.  Being a venturi type carb, it has no jets but mixture is tuned by adjusting a metering rod attached to the slide. Bigger cams obviously create a rougher idle.

[Kaze]

My original question was a bit ignorant, but that's how life is. "Race bike" isn't a very good term, considering how many different types of racing there are. A circuit bike sounds nothing like a drag bike. (Oops)

All my confusion started when I began looking at CB750 racing. I never had questions when the bikes were obviously completely different than what I'd have. All I cared about was "Damn that's fast". But there is a familiar CB750 sound in there with the bikes I've been watching lately, and they sort of look like mine... I think I just got mixed up by that. ...you'd think I was 12... lol  Drag bikes especially seem like a whole different animal. Like... human and the Hulk. Loney Motorsports videos are a ton of fun.

I appreciate you guys trying to explain these things. Half of what was said here I had to Google. But this one line really summed it up well, "designed to do 2 things; idle or operate at full throttle". Wow. That's like... I start my bike... and POW I'm at the store. Gimme my groceries, quick! POW! I'm home. (Pow, Sheriff is at my door...)

[PeWe]

A wide open exhaust system help to produce the sound. Less back pressure that calm down and help an even idle.

If made of thin metal adds some too.

[Leino]

[willbird]

My original question was a bit ignorant, but that's how life is. "Race bike" isn't a very good term, considering how many different types of racing there are. A circuit bike sounds nothing like a drag bike. (Oops)

All my confusion started when I began looking at CB750 racing. I never had questions when the bikes were obviously completely different than what I'd have. All I cared about was "Damn that's fast". But there is a familiar CB750 sound in there with the bikes I've been watching lately, and they sort of look like mine... I think I just got mixed up by that. ...you'd think I was 12... lol  Drag bikes especially seem like a whole different animal. Like... human and the Hulk. Loney Motorsports videos are a ton of fun.

I appreciate you guys trying to explain these things. Half of what was said here I had to Google. But this one line really summed it up well, "designed to do 2 things; idle or operate at full throttle". Wow. That's like... I start my bike... and POW I'm at the store. Gimme my groceries, quick! POW! I'm home. (Pow, Sheriff is at my door...)

I follow a lot of the honda car turbo stuff on youtube, many of those engines come stock with variable cam timing called "Vtech".....when they start to heads towards "full race" setups they do away with that. Those engines have a distinctive sound at lower rpm. Not "sick" but different for sure. The cam timing is optimized for 7k-10k rpm and vtech is no longer present to provide timing more suited to lower rpm's.

[Kaze]

All your guys' examples are VERY different. I was primarily going off that RSC video above (the one with the guy with the checkered jacket). That one didn't sound right to me, like it was sputtering or hesitating at the low range. But this has sort of been cleared up by the notion that the bike isn't set up to go slow. TracksNBlades1 said:

I’m betting that bike still has the long duration intake reversion (loping/burbling) till 3500+rpm  before it finally cleans up.

I understand most of that, but had to go read about "long duration intake reversion". I don't know anything about engineering... I'm reading this now: https://www.motortrend.com/how-to/ctrp-1008-intake-reversion/ This says that the effects of inversion are not necessarily beneficial to efficient combustion... hmm. That seems related to what I was thinking, the bike sounds "sick" like the gas isn't exploding well. The rest of the article is over my head.

I need a remedial course on motorcycle performance. 👍

[PeWe]

It carburate twice running rich on low rpm.
The intake valve is open that long so it will suck in fuel-air mix and spit back thru carbs and in again, right? Overlap a factor here.

Cam must open rather much before 30* BTDC, close after 60*ATDC at 1mm lift (0.004") and 0 lash. More overlap  too.

My K6 cam open around 27.5/54* at 0 lash. Idle OK.
But I have reduced duration and overlap with more valve lash on ex.
In/Ex 0.004/0.006"

I have had cam with slightly longer duration idling fine too.
A wide open exhaust without any kind of baffle help it idle worse with hotter cam.
DP315 (RC315) cam with more normal lash will have longer duration. I noticed lower cylinder pressure when tested different lash with compression tester.
 (It's specified lash tripple than many other cams)
Bike ran best with its specified lash of 0.012" despite a relatively low lift that gives. Less overlap helped.

[newday777]

Per
"DP315 (RC315) cam with more normal lash will have longer duration. I noticed lower cylinder pressure when tested different lash with compression tester.
 (It's specified lash tripple than many other cams)
Bike ran best with its specified lash of 0.012" despite a relatively low lift that gives. Less overlap helped."

Did the DP315(RC315) come specified to have the 0.012" lash?

[paul_cb836]

I previously had 295, currently 315 advanced 4 degrees (12 thou lash). Has heaps of mid range and very nice top end. 315 no noisier than 295, even with that lash. Overall I much prefer 315. 295 had nice low end power and mid range but boring to ride compared to 315. This is in 836.

[Tracksnblades1]

All your guys' examples are VERY different. I was primarily going off that RSC video above (the one with the guy with the checkered jacket). That one didn't sound right to me, like it was sputtering or hesitating at the low range. But this has sort of been cleared up by the notion that the bike isn't set up to go slow. TracksNBlades1 said:

I’m betting that bike still has the long duration intake reversion (loping/burbling) till 3500+rpm  before it finally cleans up.

I understand most of that, but had to go read about "long duration intake reversion". I don't know anything about engineering... I'm reading this now: https://www.motortrend.com/how-to/ctrp-1008-intake-reversion/ This says that the effects of inversion are not necessarily beneficial to efficient combustion... hmm. That seems related to what I was thinking, the bike sounds "sick" like the gas isn't exploding well. The rest of the article is over my head.

I need a remedial course on motorcycle performance. 👍

Read what PeWe posted about Carburetation. Visualize an intake mixture of 12:1 already in or near the cylinder. Now that same mixture is pushed backwards through the carburetor but doesn’t leave the air box. The same reversed 12:1 then is sucked back through the carburetor again to be mixed again with the same amount of fuel to create the original 12:1 mixture that was reversed due to low rpm operation with high rpm cam and intake/ exhaust inertia and pulse timing.

The link explains a little of the theory. It references two strokes but it applies to four strokes too.
Remember the top fuel spitting the good fuel out the exhaust at an idle in the above video. Once at it’s designed rpm and boost pressure you’ll only see twelve foot flames…😇.       https://www.google.com/search?q=top+fuel+flames&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari#fpstate=ive&vld=cid:691747e6,vid:UZuUpMzWwZc,st:0

https://en.wikipedia.org/wiki/Inertial_supercharging_effect

[Kaze]

Okay! Got it. Reversion is crazy. I always thought carburetion was a one way path. Come to think of it, some gas must be going backwards, because my air filter stinks like petrol (unless that's something else).

I hate to state the obvious (but I'm going to)... if I better understood basic principles of carburetion, I'll bet my bike would run better.

Pewe: This video? I wish my bike ran that smooth!