Topic: Best Cam for Stock Compression? (CB750)

[Plzhalpimlost]

Yes, another cam thread!
Let’s start with my specs:

1975 CB750K
-Complete engine rebuild with balanced crank and all new internal rubbers, chains, bearings, etc.
-.5mm over bore pistons (749cc motor)
-Stock compression!
-APE HD cylinder studs
-Kibblewhite HD Valve springs
-KW valve guides and valves (+1.5 intake, stock size exhaust)
-A full head porting that will be matched to the cam chosen
-Stock carbs
-4into1 exhaust (not that it matters)

I’m looking to get the absolute most out of this setup but want something with a smooth power and and reliable.

I’m pretty settled on Megacycle due to their quality but I’m all over the place for 125-what. I’ve called Megacycle and they recommend the 125-NR. My machinist (doing all the head work) recommends the same.

I greatly respect both of their opinions but you guys are crazy about the 125-65 and 125-75!

I can’t find any reviews on the 125-NR (or the 125-05 for that matter), so I’m looking for some people smarter than me to weight in on what I should go with.

Again, I’m considering ANY of the Megacycle cam options.

Thanks!


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[bryanj]

I have no experience of that firm, or any other as they all seem to be US based but if you have told them all you have done why disregard their recommendation? They are the designers after all

[Terry in Australia]

Don't instal the 125/75 unless you're gonna do more head work, bigger valves, bigger carbs, and ride flat out. My 836cc was a beast with JMR stage 3 or 4 head, bigger valves, CR carbs, electronic ignition etc. The 125/75 is useless below 4K RPM, and didn't really start working until 6 or 7K. If you want an enjoyable rider, take Megacycles advice, the bigger cams are race only.

[PeWe]

Stock pistons (valve pockets almost not existing) means low lifting cams.
Higher lifting cams need pockets in pistons

K7/K8 cam or difficult to find F2 cam.
CycleX has the mildest webcam too CX-1 for stock engines. CX-2 need more space for valves I guess.
Or if you have a cam shop that can regrind a stock cam, grind base circle down a little and increase lift. Maybe 0.5mm is possible?

Megacycle 125-65/75 need higher CR pistons with pockets and a ported head.
More cc is good on a street bike. 836 very common.

Find another cylinder with 836 pistons that will take higher lifting cams when you will have the head for it.
the cylinder you have perfect for a stock build, just find one more bike.

[Plzhalpimlost]

I have no experience of that firm, or any other as they all seem to be US based but if you have told them all you have done why disregard their recommendation? They are the designers after all

Not disregarding their advice, only looking for more opinions.

There is an abundance of posts on here talking up the 125-75 that it makes you think “am I crazy for getting anything less?”.

Most posts about stock cc cam options end up with commenters sitting OP on their lap and telling them to wait until they build a big boy bike before getting anything other than an f cam HAHA. I’m here to have a performance conversation around stock displacement.

End rant.


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[Plzhalpimlost]

Stock pistons (valve pockets almost not existing) means low lifting cams.
Higher lifting cams need pockets in pistons

K7/K8 cam or difficult to find F2 cam.
CycleX has the mildest webcam too CX-1 for stock engines. CX-2 need more space for valves I guess.
Or if you have a cam shop that can regrind a stock cam, grind base circle down a little and increase lift. Maybe 0.5mm is possible?

Megacycle 125-65/75 need higher CR pistons with pockets and a ported head.
More cc is good on a street bike. 836 very common.

Find another cylinder with 836 pistons that will take higher lifting cams when you will have the head for it.
the cylinder you have perfect for a stock build, just find one more bike.

Just to clarify, my head WILL be ported. But I hear you with the pockets in pistons. Can’t escape that.

It’s always another bike isn’t it When I’m ready to build a 750 meant to ruin pants, I’ll probably skip past the 836 all together.
For now, this one is sitting at 748.


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[PeWe]

There are smaller pistons. 760cc
JE 62mm
https://dynoman.net/engine/pistons/custom.html

But flat stock pistons, not much cam to use.  Hotter cam with more duration has more overlap which is when in snd ex valves are open simultaniously letting the  (dynamic) compression out thru the window, more overlap needs more CR  by pistons that often are CR 10.5:1. Next common step is 12.5:1.

I made tests with a rather hot cam DP315 (RC315), its recommended valve lash is 2- 3 times more than other hot cams

I measured compression and saw the lower compression with less lash which increased the overlap but lift too. So it became a hotter cam.
Bike felt weaker on the road with less compression  but higher lift and more duration.
 My pistons are around  CR10.5:1 thicker gaskets lower that number a little.

Cam was hotter but gave less power. Maybe pistons CR >12.5:1 should have matched it. These are normal for racers

Possible to order higher CR or after a mold of your head's chambers.

Thats why a cam with not that much overlap is good for a street bike. 125-65/75 designed for higher CR.  >10 something

The hotter cams  (race) start to give power from 6500-7000 rpm, often very sudden.
More convenient if it happen 2000 rpm earlier.

It is a pity to not use real pistons and cam for your fine head.

Here my cam tests I made soon 2 years ago. Tested 3 different cams within one week.
Ended up with an RC295 cam as my final choice. Still available as DP295 from Dynoman. Cam lift In almost 10mm.
The Megacycle cam might drop into my other engine build, 900cc.


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[Plzhalpimlost]

It is a pity to not use real pistons and cam for your fine head.

If life’s taught me anything, if it seems bigger than stock, they’re usually not real


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[754]

They dont need to be bigger, the tops need to be higher, and valve notches need to be deeper.
 Until you want to do that, stick to mild cams.

[BPellerine]

in short you need more compression to run a big cam,or you will lose bottom end,the right cam gives you more power throughout the rev range.bill

[Plzhalpimlost]

in short you need more compression to run a big cam,or you will lose bottom end,the right cam gives you more power throughout the rev range.bill

Thanks, Bill. What a perfect segway back on track. What is the best cam for my set up? I understand that people are sad I'm not doing an 836, but that isn't what this thread is about. With the way my bike is, here is what I see as my options...

Megacycle 125-05 (125-00 same thing)
Megacycle 125-NR
Cycle X CX-1 (Webcam 41)
Cycle x CX-2 (Webcam 41A)

cycle x cam link: http://www.cyclexchange.net/Tech%20Tips%20Camshafts.htm
megacycle cam link: https://www.megacyclecams.com/catalog/Page34.pdf

I'd like to see solid power across the rpm range, but will be doing high speed runs from time to time, so no need for a torque monster. I'll be swapping a 17 and 18 tooth front sprocket depending on the day/the weather/the wind/my mood.

Thanks again, everyone.

[PeWe]

This bike has "small" pistons. The guy at the shop said 750cc when I talked with him and commented that the head was ported less than mine, by same guy but around +30 years between the jobs.

JE pistons  made for the chambers. Bike for classic racing.

86.8 hp on the wheel is not bad.

[BPellerine]

it will boil down to what cam you and your engine like,with cams being expensive these days it is hard to try diff cams to find the best,my thought would be to stick with a mild cam,as you only can tolerate so much lift with stock pistons,cx1 and similar cams would be my choice.call

[PeWe]

+1 mild cams.
I have an Action Fours cam, SS-1 that was described in their catalog as: "For those who are afraid of hot cams"
It has 9.17mm measured (card 0.360") total lift @ 0 lash.
Said to fit stock engine as is!!

I never saw it as a mild cam until  I read the old catalog only 10 years ago.

Action Fours decribed their cams to be fabricated very well with no need of adjustable sprocket. I used adj sprocket.
Cam has minimum wear after around 50.000km.

That cam should be fabricated again. Lobes have modest shape without any sudden quick lift nor quick closing phase.

This one might be something? I'll never sell mine

Others on this forum might have one as TP holder?

[Medyo Bastos]

I have one under my bed pewe


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[Plzhalpimlost]

This bike has "small" pistons. The guy at the shop said 750cc when I talked with him and commented that the head was ported less than mine, by same guy but around +30 years between the jobs.

JE pistons  made for the chambers. Bike for classic racing.

86.8 hp on the wheel is not bad.

Wow. Yeah if I was in the 80s I’d call that a big win. But I’m assuming his pistons are deeply pocketed..?


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[Plzhalpimlost]

+1 mild cams.
I have an Action Fours cam, SS-1 that was described in their catalog as: "For those who are afraid of hot cams"
It has 9.17mm measured (card 0.360") total lift @ 0 lash.
Said to fit stock engine as is!!

Do you have the full specs on it? I wonder what available cam is closest.


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[bwaller]

Either of Megacycles two cams that were recommended to you will offer a nice bump over a linear powerband. The head work you've mentioned with one of those cams will be immediately obvious to the rider behind the bars. You've done a good job, be happy, no need to overthink further!

[PeWe]

No other cam as I am aware of.   Webcam 41 aka Cyclex CX-1 that most likely is tamer.

The CB750  race bike engine built around 2014. JE pistons with deep pockets and high CR, over 12.5:1.
I saw the head last year at the shop when repaired after a valve had dropped into the cylinder.

I think the Action Fours SS-1 is the cam for your build.

At last install:
Total lift IN/EX: 9,17 mm (.361")/9,17 mm (.361")

Open/close @ 1mm (0.04")lift,  0 lash:
IN:22/55   Dur:257   LC:106,5
EX:56/24   Dur:260  LC:106
Overlap. 46°

SS-1 cam description :
http://www.satanicmechanic.org/a4scam.shtml

Other cams with same numbers but more agressive will have higher risk that pistons hit the valves since they will have the higher of the lift longer time (crank degrees)

A cam like this is probably waiting to work, now hidden under a bed somewhere

Just find yours SS-1!

The alternative moderate Megacycle cams are very expensive new, hardwelded.

[754]

Well you have enough info, pic one.
 At the end of the day , the cam you like...may not be mine or Pewe,s choice , a lot depends on how you ride, and WHAT YOU EXPECT....  there us no magic cam , it's a combination of a few things..

[PeWe]

My cam is magic!

I have so many nice memories with it. 4 tourings to southern  Europe to the Mediterranean sea over the alps with stops there. And via France over Andorra (nice mountains) to Spanish coast at Tarragona.

[bwaller]

It is true what PeWe mentioned about MC's hardweld cams. Always best to pair with hardwelded rockers which costs a bit more, but never need to worry about the combination again.

[Plzhalpimlost]

It is true what PeWe mentioned about MC's hardweld cams. Always best to pair with hardwelded rockers which costs a bit more, but never need to worry about the combination again.

Already have a set of hardwelded rockers from Cycle X


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[Plzhalpimlost]

Well, unless I can find a magical Action Fours Cam I’ll be deciding between the MC 125-05 or the 125-NR.
I’ll be continuing my research into understanding cam vocabulary, but if anyone can translate what the butt-dyno difference would be between the two, let me know!


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[Terry in Australia]

No other cam as I am aware of.   Webcam 41 aka Cyclex CX-1 that most likely is tamer.

The CB750  race bike engine built around 2014. JE pistons with deep pockets and high CR, over 12.5:1.
I saw the head last year at the shop when repaired after a valve had dropped into the cylinder.

I think the Action Fours SS-1 is the cam for your build.

At last install:
Total lift IN/EX: 9,17 mm (.361")/9,17 mm (.361")

Open/close @ 1mm (0.04")lift,  0 lash:
IN:22/55   Dur:257   LC:106,5
EX:56/24   Dur:260  LC:106
Overlap. 46°

SS-1 cam description :
http://www.satanicmechanic.org/a4scam.shtml

Other cams with same numbers but more agressive will have higher risk that pistons hit the valves since they will have the higher of the lift longer time (crank degrees)

A cam like this is probably waiting to work, now hidden under a bed somewhere

Just find yours SS-1!

The alternative moderate Megacycle cams are very expensive new, hardwelded.

When I cooked the top end on my first K1 in 1981, ("Mechanic" used silicon sealer instead of replacing gaskets)  I needed new pistons and rings, rebore (first oversize) new cam towers, rockers and cam. I bought everything from Maurie Quincy (old racer from the 50's and 60's) and he advised me to buy a mild "drop in" cam mainly because the OEM item was too expensive.

I bought an Action fours cam, but it required fly cutting the pistons, so took it back and got an Andrews "drop in" cam, and I remember that bike was much more "peppy" after then. No idea what the model was, but was a great cam, lots of low to mid range power, so I didn't have to wring it's neck to enjoy it. Andrews don't do Honda cams any more, which is a pity, or I'd buy one for my current stock CB750.