Topic: Engine Pulls Hard up to 5.5k RPM Then Not So Much

[Tracksnblades1]

In this old link the OP was able to correct the misfiring and stuttering by replacing the stock coils with 3ohm ones..
However it didn’t address the resistance to freely reving in the higher rpms, as I read..

http://forums.sohc4.net/index.php/topic,176990.msg2057277.html#msg2057277

just a single example which may not represent your problem….?

I personally would reinstall your OEM point plate and adjust the points and timing spot on and verify whether it’s ignition or A/F (carbs)..I’m betting when you get her right she’ll pack the front tire from 5500- red line in 1st..

[HondaMan]

I personally would reinstall your OEM point plate and adjust the points and timing spot on and verify whether it’s ignition or A/F (carbs)..I’m betting when you get her right she’ll pack the front tire from 5500- red line in 1st..

+1
The stock points are good to over 10,000 RPM, and start to bounce (it used to be called 'float') at somewhere near 12000 RPM. I've seen many electronic pointless units that won't do it on these bikes at those kinds of speeds - Dyna S included on that list (it's OFF time is too short, so the coil voltage drooped about 60% or more above 7000 RPM in those tests we did). The OEM coils, while drooping somewhat above 7500 RPM, had the same spark voltage at 12000 that the Dyna 3 ohm "high output" coils had, due to too-short discharge time.

[Tracksnblades1]

I personally would reinstall your OEM point plate and adjust the points and timing spot on and verify whether it’s ignition or A/F (carbs)..I’m betting when you get her right she’ll pack the front tire from 5500- red line in 1st..

+1
The stock points are good to over 10,000 RPM, and start to bounce (it used to be called 'float') at somewhere near 12000 RPM. I've seen many electronic pointless units that won't do it on these bikes at those kinds of speeds - Dyna S included on that list (it's OFF time is too short, so the coil voltage drooped about 60% or more above 7000 RPM in those tests we did. The OEM coils, while drooping somewhat above 7500 RPM, had the same spark voltage at 12000 that the Dyna 3 ohm "high output" coils had, due to too-short discharge time.

+1 at least on the 10,000rpms.. I’ve never measured the rest..
 

[Finnigan]

Put the original points back on and tested them with static 'F' marks and making sure the plugs fire on each side.  Maybe something I didn't notice before, I think I get a bigger spark with points than I did with the electronic ignition, testing tomorrow.

[Tracksnblades1]

Put the original points back on and tested them with static 'F' marks and making sure the plugs fire on each side.  Maybe something I didn't notice before, I think I get a bigger spark with points than I did with the electronic ignition, testing tomorrow.

Don’t let her sneak up on you…😁
She may surprise you when she climbs on the cam…😇

That fat spark may be telling you your condensers are doing their job..
Are you running the Honda condensers or a pair of old Chevy Z28 ac/delco D204….?

[Finnigan]

Well, well!

Switching back to points seemed to solve the issue, after a morning of tinkering and trying to get everything as perfect as I could I took the bike out and didn't want to bring it back, what a machine.  Climbs FAST to 8k (I was almost too shocked to take it any higher not being on a freeway).

I'll pull the plugs when the engine cools and see if they leaned out at all.  After moving the needle clips from 4th to 3rd position I went to stock air cleaner and new filter.  I'd run stacks but an old Yoshi article said they had more success with this setup (seems odd to me but whatever).

I can see how chasing every last bit of power gets addictive.  I'm already looking at my exhaust and wondering how much its restricting

[HondaMan]

My 2 cents: skip the stacks. You'll run longer and with a wider powerband without them - and will save from buying a lot of sparkplugs in your future.

The OEM airbox is not restrictive until a little over 9000 (9200+ was the measure taken in 1971 by Branch Flowmetrics, IIRC) RPM. Then, the K1 airbox met the K0 version's flow numbers and 'stacks' for airflow, both prevent the turbulence at speed that destroys air-fuel mixing with 'stacks'. If you have my book, look up the airbox mods listed therein: they are the best bet for streetable bikes without big headaches and short sparkplug lives. Those listed were/are for the K2-K6 airboxes. My own has had them for many decades of great riding and high-speed touring.

Inside a FULL roadrace fairing that has been carefully crafted around a factory racer, yes, stacks have a place and purpose - that fairing is part of their 'airbox' for quiet air pressures - but on any moving surface beyond 30 MPH the open stacks disturb mixtures, and not a little. Add to that a cam that is trying to soak your boots with spitback until you reach past the intake overlap RPM (like 5500+) and you'll be fighting many 'fires' at once, trying to tune it.

Every time someone tries to tell me how good their idea of 'reversing the head' on a Twin to make it go faster by having more ram-air, I remind them of the [in]famous Bonneville racer most will remember as that guy with only a bathing suit and a helmet and sandals, running the track at 28 MPH less than he had the year before with the same bike - before he spent all that time and $$ to flip the head around. The stacks-forward arrangement stopped the carbs from working at all above 80-something MPH, and he had broken 100 the year before. When I finally talked the guys I worked with (who ran a Lakester-class 1500cc record holder for over 15 years there, using modified KZ1000 engines) into adding an airbox around the carb intake stacks behind the driver, they set their highest gas record ever at 174 MPH. They ran again without that box the next day and only made it to 161 MPH, ending the "argument". Then they added nitrous for the last run (always their MO then) and went 199. The next year they went 202, both records with the baffle-box in place. The last time I saw the car they had made the baffle box permanent.

[Tracksnblades1]

Well, well!

Switching back to points seemed to solve the issue, after a morning of tinkering and trying to get everything as perfect as I could I took the bike out and didn't want to bring it back, what a machine.  Climbs FAST to 8k (I was almost too shocked to take it any higher not being on a freeway).

I'll pull the plugs when the engine cools and see if they leaned out at all.  After moving the needle clips from 4th to 3rd position I went to stock air cleaner and new filter.  I'd run stacks but an old Yoshi article said they had more success with this setup (seems odd to me but whatever).

I can see how chasing every last bit of power gets addictive.  I'm already looking at my exhaust and wondering how much its restricting

Great news..Well done…

Just to verify….🤔. Does your stock air cleaner box still have the factory rubber velocity stacks..?
Some call them adapters, isolators, etc.. They probably won’t be ideal for your bore, cam peak rpm range, and porting but I’d make sure yours are intact. They’re probably too long for a 9,500 peak power camshaft but it’ll be a starting point…Honda had it together on that stock air box…IMO

[Tracksnblades1]

P S…

Your still leaving some on the table at 8k with a 9.5k rev stick…
I’d probably break her in slow too before giving her all of the long stick…😏

[Finnigan]

My 2 cents: skip the stacks. You'll run longer and with a wider powerband without them - and will save from buying a lot of sparkplugs in your future.

The OEM airbox is not restrictive until a little over 9000 (9200+ was the measure taken in 1971 by Branch Flowmetrics, IIRC) RPM. Then, the K1 airbox met the K0 version's flow numbers and 'stacks' for airflow, both prevent the turbulence at speed that destroys air-fuel mixing with 'stacks'. If you have my book, look up the airbox mods listed therein: they are the best bet for streetable bikes without big headaches and short sparkplug lives. Those listed were/are for the K2-K6 airboxes. My own has had them for many decades of great riding and high-speed touring.

Inside a FULL roadrace fairing that has been carefully crafted around a factory racer, yes, stacks have a place and purpose - that fairing is part of their 'airbox' for quiet air pressures - but on any moving surface beyond 30 MPH the open stacks disturb mixtures, and not a little. Add to that a cam that is trying to soak your boots with spitback until you reach past the intake overlap RPM (like 5500+) and you'll be fighting many 'fires' at once, trying to tune it.

Every time someone tries to tell me how good their idea of 'reversing the head' on a Twin to make it go faster by having more ram-air, I remind them of the [in]famous Bonneville racer most will remember as that guy with only a bathing suit and a helmet and sandals, running the track at 28 MPH less than he had the year before with the same bike - before he spent all that time and $$ to flip the head around. The stacks-forward arrangement stopped the carbs from working at all above 80-something MPH, and he had broken 100 the year before. When I finally talked the guys I worked with (who ran a Lakester-class 1500cc record holder for over 15 years there, using modified KZ1000 engines) into adding an airbox around the carb intake stacks behind the driver, they set their highest gas record ever at 174 MPH. They ran again without that box the next day and only made it to 161 MPH, ending the "argument". Then they added nitrous for the last run (always their MO then) and went 199. The next year they went 202, both records with the baffle-box in place. The last time I saw the car they had made the baffle box permanent.

Always good info, yep I have your book and I'll give that section another look.  Thanks

[Finnigan]

Well, well!

Switching back to points seemed to solve the issue, after a morning of tinkering and trying to get everything as perfect as I could I took the bike out and didn't want to bring it back, what a machine.  Climbs FAST to 8k (I was almost too shocked to take it any higher not being on a freeway).

I'll pull the plugs when the engine cools and see if they leaned out at all.  After moving the needle clips from 4th to 3rd position I went to stock air cleaner and new filter.  I'd run stacks but an old Yoshi article said they had more success with this setup (seems odd to me but whatever).

The whole setup is new from Yamiya, from the rubber stacks to the gasket.

I can see how chasing every last bit of power gets addictive.  I'm already looking at my exhaust and wondering how much its restricting

Great news..Well done…

Just to verify….🤔. Does your stock air cleaner box still have the factory rubber velocity stacks..?
Some call them adapters, isolators, etc.. They probably won’t be ideal for your bore, cam peak rpm range, and porting but I’d make sure yours are intact. They’re probably too long for a 9,500 peak power camshaft but it’ll be a starting point…Honda had it together on that stock air box…IMO

[Finnigan]

P S…

Your still leaving some on the table at 8k with a 9.5k rev stick…
I’d probably break her in slow too before giving her all of the long stick…😏

Planning on 100+ mile cruise tomorrow to do just that, nice and easy

[Finnigan]

Went on an all day ride with the bike, everything went very smoothly from the dual disc up front to the engine pulling hard and idling healthy.

I think it could still use some fine tuning, might even book some dyne time if I can convince a certain someone to fire it up.

On my way home, I came close to coming off the bike - I took the twisty roads out and the freeway home, this is the CA freeway that has the long grooves (see picture) and the bike suddenly became so unsteady I felt like I was driving on ice.  Luckily it changed to blacktop as soon as I was an about to pull over but needless to say I had to stop shortly after and wonder what the hell happened.  It felt like the back tire was waving back and forth and the front tire became VERY light.  On regular asphalt the bike feels planted and stable.  WTF?!

Could it be the Shinko tires I'm using?  Cheap shocks?

Tires are Shinko 270 Super Classic Tires both front and back set to 27PSI - checked before every ride
Front suspension is stock with regular fork oil
Rear suspension is cheap 4into1 shocks but are brand new.

[Old Scrambler]

Your getting it done

Tire pressure seems a bit low, but not very many tires like grooves.

Interesting to note the coil info. I may have to change my DYNA black coils on my race bike.

[seanbarney41]

Not familiar with those particular Shinkos, but if they are a classic grooved tread pattern or something similar, the handling problems are typical.  The grooves in the tread and/or the bias of the internal plys are trying to follow the grooves in the road...this makes the bike seem to not be going where you want it to go.  Just relax and let the bike move around underneath you.  It feels sketchy as hell but the bike's stability will continue working.  This is why modern tread designs are no longer grooved...

[PeWe]

Roads with those kinds of grooves can be scary. I do not see that kind of roads here.
Maybe for heavy rains?

I have ridden on similar for some distances on the German Autobahn in the 80's. They had warning signs for motorcycles, max 120kmh.

It depends on the tires, wobble can happen earlier. Worn tire is worse.
I have noticed that Metzeler Roadtec 01 rear tire could handle grooves better than older designs.
Michelin Road Classic ok too as I have seen so far.

[Don R]

 I have a set of Mikuni 29 smoothbores on my 836, I've been toying with the idea of putting an airbox back on it but the Suzuki/Kawasaki spacing is the issue, I wondered if a Z1 900 air box might squeeze in there. It does feed from the top though, I have a feeling that would make the battery interfere.
 Plan B would be to modify the carb rack to the Honda spacing and use the F2 air box. PeWe has done the spacing mod.
 Plan C is just put the Webers on it, they have a breadbox air filter.

[PeWe]

K&N oval pods worked fine on my VM29.
It was a mess to shorten the rack.
A friend working at a aluminium fabrication company helped me with that late 80's.
I continued with it 2013

The welder mentioned high zinc content and welding issues.
It looked good anyway. They cut where I had marked.
I think it was possible to make 1-2 to come 10-12 mm closer to 3-4.

Adjusting, shorten the choke shaft, drill and tap new holes for throttle mechanism and 3-4 throttles in the shaft.
Must be aligned. A little off can be adjusted when synching  the throttles, bench method.

Here more info
http://forums.sohc4.net/index.php/topic,119335.0.html

[newday777]

Went on an all day ride with the bike, everything went very smoothly from the dual disc up front to the engine pulling hard and idling healthy.

I think it could still use some fine tuning, might even book some dyne time if I can convince a certain someone to fire it up.

On my way home, I came close to coming off the bike - I took the twisty roads out and the freeway home, this is the CA freeway that has the long grooves (see picture) and the bike suddenly became so unsteady I felt like I was driving on ice.  Luckily it changed to blacktop as soon as I was an about to pull over but needless to say I had to stop shortly after and wonder what the hell happened.  It felt like the back tire was waving back and forth and the front tire became VERY light.  On regular asphalt the bike feels planted and stable.  WTF?!

Could it be the Shinko tires I'm using?  Cheap shocks?

Tires are Shinko 270 Super Classic Tires both front and back set to 27PSI - checked before every ride
Front suspension is stock with regular fork oil
Rear suspension is cheap 4into1 shocks but are brand new.

1st, are you running original ball bearing steering bearings? If so replace them with tapered bearings properly torqued with a torque wrench.
Second, did you check your air pressure before going out?
3rd, yes California roads suck especially with grooves and ribbed tires. Have you replaced the swing arm bushings to bronze bushings yet? That will help.
Personally I won't use Stinko tires because of the multitude of failed treads I've seen and heard of.

[MRieck]

K&N oval pods worked fine on my VM29.
It was a mess to shorten the rack.
A friend working at a aluminium fabrication company helped me with that late 80's.
I continued with it 2013

The welder mentioned high zinc content and welding issues.
It looked good anyway. They cut where I had marked.
I think it was possible to make 1-2 to come 10-12 mm closer to 3-4.

Adjusting, shorten the choke shaft, drill and tap new holes for throttle mechanism and 3-4 throttles in the shaft.
Must be aligned. A little off can be adjusted when synching  the throttles, bench method.

Here more info
http://forums.sohc4.net/index.php/topic,119335.0.html

I have used them for decades with CR, RS and FCR carbs ....no problem with fueling....u just have to know how the carb circuits work and that the circuits are fluid and effect each other all the time.
K&N's can absolutely suck in the rain.....no doubt about that though on bikes with side panels that cover the carbs it is not a problem.
 I have never attempted to rerack the old VM's but I can believe it would be a bear. The RS carbs rerack nicely....no problems.

[willbird]

Roads with those kinds of grooves can be scary. I do not see that kind of roads here.
Maybe for heavy rains?

I have ridden on similar for some distances on the German Autobahn in the 80's. They had warning signs for motorcycles, max 120kmh.

It depends on the tires, wobble can happen earlier. Worn tire is worse.
I have noticed that Metzeler Roadtec 01 rear tire could handle grooves better than older designs.
Michelin Road Classic ok too as I have seen so far.

Even scarier is metal bridge grating that has a similar pattern and you can see the water below, the feeling is that if you went down what would happen is like a cheese grater and only your helmet with your head in it would escape getting shredded like cabbage for cole slaw or sauerkraut :-).

Bill

[Don R]

 I remember crossing a metal bridge deck on my first day out, new 73 cb500. We crossed the Mississippi river on US34 at Burlington Ia. There was a little tire dance going on and I was trying to not look down. We went into a bar and when we came out it was 40*F, going home it was either break it in fast or ride alone.

[HondaMan]

On my way home, I came close to coming off the bike - I took the twisty roads out and the freeway home, this is the CA freeway that has the long grooves (see picture) and the bike suddenly became so unsteady I felt like I was driving on ice.  Luckily it changed to blacktop as soon as I was an about to pull over but needless to say I had to stop shortly after and wonder what the hell happened.  It felt like the back tire was waving back and forth and the front tire became VERY light.  On regular asphalt the bike feels planted and stable.  WTF?!

Could it be the Shinko tires I'm using?  Cheap shocks?

Tires are Shinko 270 Super Classic Tires both front and back set to 27PSI - checked before every ride
Front suspension is stock with regular fork oil
Rear suspension is cheap 4into1 shocks but are brand new.

First thing: that's not nearly enough tire pressure, given the construction of modern tires. Try at LEAST 35 PSI on both ends. Look for the tiny letters on the sides of the tire that will tell you their recommended PSI: it is likely 32-40 in most cases. For heavier bikes (750 is in this group) use more PSI. I run mine at 40 PSI rear unless riding very light (no loads but me) and no less than 36 PSI front. But (disclaimer) I have Avon RoadRiders, which have only a single semi-groove in the center: this reduces those wiggles to nearly none, even with Colorado's deep mountain-road grooves. When the grooves are linear around the whole perimeter the grooves on the road try to pull them back & forth by gripping the rubber groove sides.

I actually witnessed a guy jump off his bike on a steel-grated river bridge in IL one fine day. We (4 of us) were cruising about 30-40 MPH on a winding 2-lane blacktopper and it went over a sizeable creek with open-grate steel treads: the guy behind me (old Suzy 2-stroker, IIRC it was a Hustler twin) had never ridden over one and when his bike started wiggling he jumped off! The grate really ate up his hide and scratched up his engine cases pretty bad, too. He said he thought the tire was coming off the rim. ;(

[Finnigan]

Hm ok, looking at Shinko website it says max load @ 42 PSI.  I'll pump them up to around 40 the next time I ride and see if I have any brave pills laying around to try the freeway again.

(I really don't want to replace tires but not being able to take the bike on the highway is pointless in my opinion)

[HondaMan]

Hm ok, looking at Shinko website it says max load @ 42 PSI.  I'll pump them up to around 40 the next time I ride and see if I have any brave pills laying around to try the freeway again.

(I really don't want to replace tires but not being able to take the bike on the highway is pointless in my opinion)

That should change it noticeably! Just grip the grips snugly and loose-elbow-it when you get to those spots, let it float along over the ridges. Here in Colorado they started making the grooves almost 1.5" wide (4 years ago) and someone sued the State when their shiny new HD [something]Glide crashed on them (I'm sure it was the rider panicking, from hearing the news reports), so they went 'down' to about 3/4" wide slots now (these are on certain mountain roads where snow/water is heavy). Still spooky, though.

Colorado's roads are the #43 worst in the country, said last year's survey...I think that's optimistic...