Topic: Result from Dyno run: Too rich at 4500-6500 rpm 4-2-1 exhaust better than 4-1?

[PeWe]

Tested my bike on the Dyno today.
The Mikuni TMR32 carbs were OK with pilot 25, main 132.5.
There is a strange very rich area @4500-6500 rpms.

The cam is a Megacycle 125-20 that has rather special data, the exhaust especially.
Measured at installation @0.04" lift (1.0mm)
IN -  Lift 0,377"   30,5/58,5°     Duration 269°     Lobe center 104
EX - Lift 0,329"   76/22°     Duration 278°    Lobe center 117
Overlap 52.5°

I'll try another cam and make another Dyno and see if it change. I must remove the cam anyway when I have a raising cam stud in need of a Big-Sert in the head. I'm very curious to know what the cam does.
After that needles 1 step lower (leaner) if A/F is the same.

Exhaust, cam or carbs or a mix of all? Higher CR will not change,or?
Maybe someone here has had same "problem" and could make it better...?

Next cam to be tested: Action Fours SS-1 I used it most of the 80's.
Measured at last installation  @0.04" lift (1.0mm)
IN -  Lift 0.361"      21/53°     Duration 254°     Lobe center 106
EX - Lift 0.361"       54/21°       Duration 256°     Lobe center 106 
Overlap 42°

[calj737]

It would be my inclination that ignition timing is contributing to this studder and drop. It's almost like you're not getting into advanced timing quickly enough for the fuel rate and thus combustion is suffering.

Then again, I could be way off...

[turboguzzi]

It would be my inclination that ignition timing is contributing to this studder and drop. It's almost like you're not getting into advanced timing quickly enough for the fuel rate and thus combustion is suffering.

Then again, I could be way off...

note likely, most systems go into full advance by 5K.... if runs well beyond 6.5K, then sounds carb related to me. just form my experience, are you running stacks? if yes, try without, if not, try to mount them.... seen stacks doing strange stuff in the past.

BTW, what rwhp numbers where you getting?

And yes, 4-2-1 are regarded as superior overall to 4-1, maybe not on peak hp, but surely on torque range. last time i walked around a WSBK/WSS paddock, havent seen even a single 4-1 system mounted

[madmtnmotors]

Did you raise the needle (by moving the clip to a lower notch) or is it in the stock position? The needle position affects fuel delivery from 1/4 throttle up to about 3/4 throttle as indicted here:

http://www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm

Sounds like the fuel is coming in too early in the mid range.

[Old Scrambler]

According to the spec-sheet from Mega-Cycle...........your lobe centers should be 105 and 111.5.

You may need to rotate your cam a degree or two for improved valve-timing.

[PeWe]

The carbs have KN pod filters. The adaptors on the carbs seems to be good, almost like stacks.
Ignition is OK at idle 'F', full advance at around 2700 rpm's. Tricky to see the tacho and ignition at the same time. Sure after 2600 and before 3000 rpms.

I talked to the guys running the Dyno. Needles are not involved when this was full throttle. I feel same when driving the bike. It runs very smooth when increasing the throttle slowly. Full throttle from 4000rpms cause way too rich. The acc pump was not involved either.

The tuner was sure about the exhaust design. 4-1 with a large collector, no good. 4-2-1 should work much better. They have seen many examples of 4-1 on dyno during the years. I got recommendation about an old RC 4-1 and weld job inside the collector, a plate dividing the all 4  to 2x2 something.... This would be better....

I'll change cam and see if the dyno will look different. My tuner has special weekend next week, dyno run for less than $25US.

[PeWe]

According to the spec-sheet from Mega-Cycle...........your lobe centers should be 105 and 111.5.

You may need to rotate your cam a degree or two for improved valve-timing.

Yes, the megacycle spec of the 125-20 cam differs.
Here is the cam card shipped with the cam.

I could have timed it different, retarded it to get closer lobe centers, but the low rpm might suffer with the late closal of IN, right?   @0.004" lift
IN 27/62     dur 269    LC 107,5   
EX 72.5/25,5 dur 278    LC 113,5

[turboguzzi]

peewee, if you showed us the run graph maybe it could help, any hot cam is bound to have a dip, depends really how deep it is....

would try without filters too, they could be another source for strange inlet resonations @ certain rpms.

[PeWe]

peewee, if you showed us the run graph maybe it could help, any hot cam is bound to have a dip, depends really how deep it is....

would try without filters too, they could be another source for strange inlet resonations @ certain rpms.

Yes it is attached in first post. PDF file almost invisble

[turboguzzi]

for me thats a very typical hot cam dip in the midrange, my dyno curves look pretty much the same. longer stacks could help, but will take some jetting changes. it goes "rich" essentially because around that rev range resonation spits back mixture and carbs "carburates" it twice... when i blip the revs in my racers without filters you can see how they make a spit back cloud as the motor goes through that rev range. thats the nature of these cams.

83 is a nice number, but what's the difference between the light and dark blue curves?

[Tim2005]

My 460 has a similar mid-range rich spot too, but nowhere near as pronounced. I wonder if different taper needles would improve things significantly?

turboguz.. I'd assumed the light blue is torque, or am I missing something?

[PeWe]

Yes, hp (83.4) hp and torque in Nm (70.59). CB750
Turboguzzi: Your thoughts seems to match what my tuner talked about. I'll change cam this weekend and I'll see if the too rich range will change. I had plans for a K0 replica 4-4... After what my tuner said I must try a 4-2-1. Some more dyno runs..
Cam in combination with a bad 4-1 (flow wise) will cause the bouncing back scenario in the carbs. I advanced the cam to make IN close earlier to minimize eventual spit back in carbs on lower rpms.

Better exhaust ,ight make it possible to use tha cam that is really fun. I often notice too late that I have passed 9000-9500rpms. OEM rods. I guess I must pull engine next winter and correct my mistake with std CB750 rods.

[Jim F]

you might have to change out the needles to a leaner needle or a leaner needle jet
on all of my double over head bikes with the various cams that I run, I have found the a 105 In and a 107Ex '
is the sweet spot on all of my bikes
but with a single cam your stuck but the 105/112 numbers might get you in the ball park
I would really take a look at your needles and check your ignition (What type of ignition)
good luck
Jim

[scottly]

for me thats a very typical hot cam dip in the midrange, my dyno curves look pretty much the same. longer stacks could help, but will take some jetting changes. it goes "rich" essentially because around that rev range resonation spits back mixture and carbs "carburates" it twice...

I agree that the issue is most likely the cam timing, but the increased duration on the exhaust side makes me wonder if this cam was optimized for the flow of a stock head, rather than a ported one? My theory is that the mixture at midrange is "following" the exhaust out the exhaust port, rather than "re-carburating" it.

[PeWe]

you might have to change out the needles to a leaner needle or a leaner needle jet
on all of my double over head bikes with the various cams that I run, I have found the a 105 In and a 107Ex '
is the sweet spot on all of my bikes
but with a single cam your stuck but the 105/112 numbers might get you in the ball park
I would really take a look at your needles and check your ignition (What type of ignition)
good luck
Jim

The guy that ran my bike on dyno had full throttle, needles not involved. When increasing throttle slowly up to full, no problems at these rpms or needle area throttle lift.
I'm searching for a good 4-2-1. CycleX has one that look good.
http://www.ebay.com/itm/Cycle-X-Honda-CB750-4-2-1-Race-Exhaust-Chrome-Header-Only-Chopper-Bobber-Cafe-/151526109837

I found another 4-2-1 http://www.ripplerockracers.com/product/cb750-hindle-vintage-racing-exhaust-system-fits-1969-78/

Which design is better for a CB750 SOHC? CycleX design of 1-4 + 2-3 seems to be more common when searching on 4-2-1 headers. 1-2 + 2-3 might not work as well as  1-4 + 2-3?
Firing order 1,2,4,3 and need of not getting exhaust gases backwards into a cylinder from another cylinder earlier in the firing order.. (this might be the case here when the cam exhaust has very early opening, 76 degrees BBDC wich is around 20 degrees earlier than most other cams.

Improved midrange fits my engine well when the max power at red line is not that important with weak rods. I had planned 4-4 pipes, I think I have to find another bike for them

I have to test other cam on dyno with existing 4-1 and new 4-2-1. Later on install the megacycle 125-20 cam again and see how a 4-2-1 and measure the interaction of them both.
The Megacycle 125-20 cam might need higher CR? I have longer rods (CB900 DOHC) that can fix that in next engine build.

[Old Scrambler]

Great tuning thread ..........Lots of sound advice ..........and PeWe is willing to experiment

I would add this for consideration...........your carbs are a little extended from the head which usually is best for low and mid-range......but your stacks are short which is best for high-end velocity. For your application, I think Turbo-G makes a great suggestion about longer stacks. The critical area is the edge of the bell.........they would improve with a 180-degree or better edge.

[turboguzzi]

and would try to run without filters, they might worsen that flat spot when the engine is not on the cam cause the spit back fuel cloud remains trapped. 5 mins to try with / without

[bwaller]

This is an 836cc? I see this kind of dip on mine I associate with bigcarbitis. If you solve the issue, good investigative work by you.

Hindle exhaust for the SOHC is interesting. Mike Rieck has an original Hindle pipe on one of his, it should be a quality exhaust.

[PeWe]

836cc.I think old NOS RC mold pistons.
Hindle has strange 4-2-1. 1-2 + 3-4. All header threads make me think that 1-4 + 2-3 should work better. But I do not know.

I'll change cam this weekend to see if the cam will change. The other cam has more "normal" exhaust opening.
New Dyno run next Saturday.
But I have to know if my 4-1 exhaust is the restrictor as my tuner say it is, probably together with cam. I think that CycleX 4-2-1 with an RC megaphone design should be good.

For the winter: I hope it is possible to pull engine, open the lower case, unbolt the rods with pistons still in bore and replace CB750 OEM rods with stronger rods allowing me to enjoy the green area beyond the red area? >9300rpm.

[MRieck]

RS carbs have the same dipin that RPM range when opening the throttle quickly. If you think about vacuum you are going from a high vacuum situation (throttle held steady at a "cruising" speed in 4th) to suddenly no vacuum when the slide is whacked opened. During that time you see the initial leanness (because no vacuum to pick up fuel). Then the accelerator pump throws more fuel into the engine only there is no vacuum to pick it up efficiently. You also have for a split second (when it transitions from the high vacuum state to the low vacuum state) the needle is in the greatest part of its taper. The needles are always involved even WFO as it sits in the needle jet.....the needle is never completely out of the needle jet (at least it shouldn't be). It's only a second put it pulls a good amount of fuel in there. Once the vacuum picks up and the engine can clear its throat your AF looks good. That differential in vacuum is even worse with the flat slide design.
 On the RS carbs shortening the pump duration and playing with needles and needle jets really helped alot....at least for me.
 The reason it can handle the more deliberate (slow) throttle opening is because vacuum stays pretty constant.
 I'd also think your mileage would not be good with that cam as the exhaust is open a long time and the extended overlap will pull fuel out of the cylinder and into the exhaust.
 The power and torque you are making is good...I'd like to point that out. i do not think changing the pipe will make a dramatic difference. Ignition timing won't make a big difference either as you advance is all or almost all in depending on your ignition and the curve you are running.

[MRieck]

 I'd also add the Harmon "F" and "D" grinds use similar exhaust #'s and I have seen them work very well. Those Harmon grinds are copies of some old Ducati grinds.

[scottly]

Yes, and the Harmon grinds you mentioned are for sustained high RPM applications; not meant to work well in the 4500-6500 RPM range.
Brent, dips in the power curve due to "Bigcarbitis" are generally reflected in lean mixtures from the lack of velocity at the lower speeds. Attempts to cure this by increasing jet size usually results in over-rich mixtures at higher speeds. I think this is a case of "Bigcamitis", at least on the exhaust side.  

[PeWe]

Thanks Mike for your comments. I'll try to adjust the pump and lower the needles. I have to find out how to do it..

[PeWe]

Update: I have done what you guys recommended me to do. I changed the needles to leaner. The needle setting surprised me when it was to the richer. Clip in notch 4 of 6. I thought the needle should be in middle. I changed 2 step up to 2 of 6.

Now not too rich when twisting the throttle as before. I have installed my old cam that is more rod friendly, very boring just before redline and after. Not like the other cam that made the tacho to spin, quickly passing the redline to the green area after.
I'll see what the dyno will say. A/F and whp. I'll update when it can be interesting for other CB750 guys.
I do not know how to adjsut the acc pump, I hope I do not need to.

I attach photos of the needle attachment on the Mikuni TMR32 carbs. Inyternet is not full of TMR info so I took photos. There is an allen head nut to remove. Under it, the needle to grab and lift with a thin plier. There is a plastic washer under the clip to keep eyes on so it will not fall off.

[scottly]

You re-jetted the carbs AND swapped the cam? Best to make one change at a time, lest you start chasing your tail.