Author Topic: Building a New Engine - Advice Needed  (Read 2506 times)

0 Members and 1 Guest are viewing this topic.

Offline stikman

  • Bit of an
  • Enthusiast
  • **
  • Posts: 189
Building a New Engine - Advice Needed
« on: December 09, 2017, 11:17:17 AM »
Hey guys, I've had this build in the works for a while now and my plan is to get it done for spring.  It'll be my main all-round ride, city jammer and highway cruiser.  It's another chopper, old hardtail frame with a +12 springer.  I'm looking for a bit of advice on a few choices for engine performance pieces.  I've currently got a NOS set of Wiseco 836 pistons I got from Bill P, my jugs are already bored.  I'll be porting the head myself.  Got multiple sets of roundtop carbs to choose from so will probably stick with those (w/ breadbox or v-stacks) and spend the rest of my "budget" elsewhere.

So one intake valve guide is chipped from a PO having bent a valve in the past.  I've been going back and forth on what to do about it, though I've decided there's no good reason to leave it be if I'm putting time and money into this head.  All the valves will be replaced anyway (PO didn't maintain valve lash and they got pretty hammered), so what are your thoughts on going with bigger intake valves or just replacing the one guide and using fresh stock valves?  As it's a chopper I'm looking for more low/mid range than top end.  So I'm a bit unsure just how far to take my porting and intake valves, as I don't want to lose my low end torque gains.

Also trying to decide on a cam.  I've got a new Webcam 41 on the shelf, but since I decided on the Wisecos over the cast cruzinimage 836 kit I'd like to take advantage of the higher CR and valve reliefs.  Leaning toward the DP295 based on everything I've heard about it over the years.  The power range seems right for my application.


So to sum up:  What advice do you fellahs have on specific porting characteristics, intake valve size, and camshaft choice for my application of a hardtail chopper looking for mainly (but not exclusively) low and mid range torquey power on a 10.25:1 836 engine?  Thanks in advance!


Current Specs:

Dyna Ignition and Coils
Roundtop Carbs with Velocity Stacks or Breadbox
MAC Drag Pipes 4-4  (I know.. I know  8))
Wiseco 836 Pistons
HD Head Studs
Titanium Valve Retainers
Porting specifics?
Intake valve size?
Camshaft?


« Last Edit: December 09, 2017, 11:21:30 AM by stikman »
1977 CB750A "High Horse": 836cc, mild porting, custom cam, K5 roundtops, v-stacks...
1973 CB750K D5 plunger frame chopper currently building..

Offline Sam Green Racing

  • Moderator
  • Really Old Timer ...
  • *****
  • Posts: 16,068
  • I REALLY? hate black rims.
Re: Building a New Engine - Advice Needed
« Reply #1 on: December 09, 2017, 03:35:04 PM »
The DP295 cam is ideal for what you want, stacks of low to mid range power.
C95 sprint bike.
CB95 hybrid race bike
CB95 race bike
CB92
RS 175. sprint/land speed bike
JMR Racing CB750A street ET drag bike

Offline Jerry Rxman Griffin aka MuthaF'er

  • This MuthaF'er is getting to be a
  • Really Old Timer ...
  • *******
  • Posts: 14,902
  • Bought her new 4/75
Re: Building a New Engine - Advice Needed
« Reply #2 on: December 09, 2017, 03:48:17 PM »
Can't go wrong with bigger valves. Bigger valves = more air = more gas = bigger bangs
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline HondaMan

  • Someone took this pic of me before I became a
  • Really Old Timer ...
  • *******
  • Posts: 13,783
  • ...not my choice, I was nicknamed...
    • Getting 'em Back on the Road
Re: Building a New Engine - Advice Needed
« Reply #3 on: December 09, 2017, 07:16:17 PM »
With the Dyna ignition (I'm presuming a Dyna S?) and Dyna coils (if 3-ohm type), you will NOT be able to run around under 5k RPM. It will kill the battery. At the very least, you will need 5-ohm coils, or perhaps one of my Resistor Packs if you have the 3 ohm coils.

If you're going to the 4.00-16 rear wheel look, consider the 19T countersprocket from Cycle X. You must trim (grind) about 1/8" (3mm) from the tips of the teeth to prevent it tearing off the rubber chain cushion inside the sprocket's side cover, though. I ran this on my 750 for 2 years and liked it, but am going to larger tires now, so don't plan on using it until I hit the road for a longer ride.

For improved low-end torque...retard the intake valve opening timing, whatever cam you use. The max low-end torque in this engine can be found at no more than 5 degrees BTC opening for the intake valve, if the exhaust valve timing matches that number: if the exhaust opens sooner than the intake, it will be for higher RPM. If the exhaust opens later than the intake, it will spread the power out and lower the torque RPM curve in RPM. For example, the 750A cam opens 5-8 degrees AFTER TDC (at the least, many are more than this) to improve the low-end torque for the Auto. It works well, but limits the top RPM for asthmatic reasons: the "A" doesn't even come with a tach because it can't overrev itself in 1st gear, rather like Honda's S90 engine. ;)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline MRieck

  • Really Old Timer ...
  • *******
  • Posts: 10,560
  • Big ideas....
Re: Building a New Engine - Advice Needed
« Reply #4 on: December 09, 2017, 07:33:38 PM »
 Larger valves only work if the seats are opened up to accommodate them. If you want torque use the 41 cam. The exhaust valve always opens before the intake valve closes with any aftermarket cam. With fixed lobe separation you cannot get creative.
Owner of the "Million Dollar CB"

Offline stikman

  • Bit of an
  • Enthusiast
  • **
  • Posts: 189
Re: Building a New Engine - Advice Needed
« Reply #5 on: December 09, 2017, 10:32:51 PM »
Larger valves only work if the seats are opened up to accommodate them.

I'm sure the machine shop can open them up, and I should be able to smooth them in.  I can't seem to remember if I've read anything saying whether the 33.5mm intakes and complimentary porting will add more everywhere, high to low, or if they just kill low end power while allowing better breathing up high.  Most guys seem to be looking for all they can get in the higher revs on these engines, which doesn't exactly suit my program.  So you really figure the 41 will out perform the DP295 in the low/mid eh?
1977 CB750A "High Horse": 836cc, mild porting, custom cam, K5 roundtops, v-stacks...
1973 CB750K D5 plunger frame chopper currently building..

Offline PeWe

  • Really Old Timer ...
  • *******
  • Posts: 15,531
  • Bike almost back to the 70's 2015
Re: Building a New Engine - Advice Needed
« Reply #6 on: December 10, 2017, 04:26:29 AM »
1000cc give nicer low end ;) It start a domino effect of other mods...

BUT, my bike worked really good with 836 stock carbs, ported head with 34mm inlet, cam Action Fours SS-1, stock rods, lightened alternator rotor.  17-48 gearing.
A good touring machine.
Maybe not perfect for a chopper since the engine started to scream and pull hard from 4500-6000 to 9500 rpm. It could cruise at around 100mph all day long as it had to on my holidays going to the sun and blue warm sea 2000 km away south.
« Last Edit: December 10, 2017, 04:33:21 AM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline stikman

  • Bit of an
  • Enthusiast
  • **
  • Posts: 189
Re: Building a New Engine - Advice Needed
« Reply #7 on: December 10, 2017, 11:23:21 AM »
With the Dyna ignition (I'm presuming a Dyna S?) and Dyna coils (if 3-ohm type), you will NOT be able to run around under 5k RPM. It will kill the battery. At the very least, you will need 5-ohm coils, or perhaps one of my Resistor Packs if you have the 3 ohm coils.

Running Dyna S and 5 Ohm coils.  Will be starting out with 18/48, go from there.


1000cc give nicer low end ;) It start a domino effect of other mods...

BUT, my bike worked really good with 836 stock carbs, ported head with 34mm inlet, cam Action Fours SS-1, stock rods, lightened alternator rotor.  17-48 gearing.
A good touring machine.
Maybe not perfect for a chopper since the engine started to scream and pull hard from 4500-6000 to 9500 rpm. It could cruise at around 100mph all day long as it had to on my holidays going to the sun and blue warm sea 2000 km away south.

Yeah, no doubt  ;D.  Thanks for that info, Per.
1977 CB750A "High Horse": 836cc, mild porting, custom cam, K5 roundtops, v-stacks...
1973 CB750K D5 plunger frame chopper currently building..

Offline Jerry Rxman Griffin aka MuthaF'er

  • This MuthaF'er is getting to be a
  • Really Old Timer ...
  • *******
  • Posts: 14,902
  • Bought her new 4/75
Re: Building a New Engine - Advice Needed
« Reply #8 on: December 10, 2017, 06:09:39 PM »
17/48!! You are talking about performance, right?! Best/easiest/cheapest place to start.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 29,058
Re: Building a New Engine - Advice Needed
« Reply #9 on: December 10, 2017, 06:54:19 PM »
17 / 54   will do it..
 STIKMAN, I MAY HAVE A 410 head to sell, with the big valves before Christmas?
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline MRieck

  • Really Old Timer ...
  • *******
  • Posts: 10,560
  • Big ideas....
Re: Building a New Engine - Advice Needed
« Reply #10 on: December 21, 2017, 08:14:18 AM »
17 / 54   will do it..
 STIKMAN, I MAY HAVE A 410 head to sell, with the big valves before Christmas?
I have a complete head kit for a 410....valves, springs, retainers, special keepers and bases. Will sell for a good price.
Owner of the "Million Dollar CB"