Topic: 750F cam guys ( 2 pics of cam)

[KRONUS0100]

factory service manual fails to give the base circle and lift measurements for a 1978 750F3 motor.   I have 2 cams on the table that came with this motor.


Cam #1...........base circle is 28.01mm
                        intake height is 36.19mm for lift of 8.18mm
                        exhaust height is 35.87mm for a lift of 7.86mm
this cam looks to be marked RC in the usual spot next to cam gear boss.....the R is very clearly marked, the C looks almost like a fancy cursive L.  find it hard to believe this an actual RC camshaft as there are no other markings.


Cam #2.............base circle also 28.01mm
                         intake height is 36.42mm for lift of 8.41mm
                         exhaust height is 35.75mm for lift of 7.74mm
this cam is marked R1 in usual spot


which one of these cams should i be using in a bone stock rebuild of this motor?
thanks in advance guys.


ps---will try an attach pics

[HondaMan]

Hmm...as for 'bone stock', neither of those cams fits that description? The stock lift intake was 7.96-8.02mm, exhaust 7.62-7.82mm. There are/were many cams made for the engines, though, by folks like R/C, Action Fours, Megacycle, and others now gone. The lift numbers look like one of R/C's mild street cams on the #1 cam, the #2 cam is considerably more lift and may require increased springs to keep things in line.

Generally, the stock springs on the "F2/3" engines are the strongest of the SOHC4 750 springs, and are good to 11k RPM with lifts of 8mm, if in good shape. Once the lift gets higher than that, the pistons may need flycutting in the valve pockets a little more. You want to have at least .080" clearance in the intake pockets, which can be measured with some modeling clay stuck in them and a few turns of the engine (all bolted together with an old head gasket is OK). Just slice thru the clay after re-disassembling the engine to see how thick it is where the valve(s) dimpled the clay in the process. They get closest to the pistons during overlap, between intake/exhaust strokes.

[KRONUS0100]

those are the best 2 pics i could get of the Cam #1 which is the suspect RC Engineering camshaft.

[HondaMan]

those are the best 2 pics i could get of the Cam #1 which is the suspect RC Engineering camshaft.

That "RC" is actually "R6", which is a mold number from Honda's days of old. That means it is at least a stock cam base. The Megacycle folks can take a cam and build it up, then grind it back, into a "hardweld" cam, with many lift profiles. This sometimes confuses things when checking cams, because they look like stock Honda cams, but have differing profiles and lifts.

On the F2/3 cams (and some hi-perf aftermarkets), I have sometimes tried the measurement technique of measuring peak vs. base circle and subtracted to get the 'lift' difference, and come out with some unusual numbers that don't really exist. Usually, it appears to have more lift than it really does on these particular cams, because the closing ramp is very steep and bottoms out before the base circle. This has also been the Achilles Heel of these cams, causing the valve lash on cylinders 2 and 3 to become too loose when they are set in the normal method shown in the earlier manuals: they can end up with more than .005" clearance sometimes, which over time causes a small divot to appear near the base circle settling ramp, often ruining the rocker foot. I even have 2 of those here from bikes I've rebuilt since 2006, same problem. One day I will send them to Megacycle for rebuild into one of their 125-05 profiles, as they are otherwise quite good cams.

Back to my point before the rambling...checking the lift is best done with the cam in the bearings, with dial indicator on the cam or on the rocker end near the valve, to see what the lift actually reaches. It might surprise you with the "R6" cam and come out closer to the 8.15mm max spec of the F2/3 cam? I have seen an occasional lobe reach 8.23mm on a cam, but it usually is only one lobe when that shows up. The K cams have less lift, even the K7/8 versions, so their valve guides last much longer.

[KRONUS0100]

Thanks HondaMan.  I had been told that MegaCycle and Webcams would grind a cam for me to RC specs if i wanted.  i dont have a set of v blocks.....but i think i have a spare set of cam holders off a blown motor, i will find my dial indicator and see what that R6 cam actually is.  both these cams were sposed to be F3 cams.  I will post some numbers when i find my dial indicator, havent used it in a while.


ok measurements with dial indicator   R1 cam....................intake  .321 or 8.12mm
                                                                                            exhaust  .302 or 7.67mm

the R6 cam intake  .320 or 8.13mm
                   exhaust  .305  or 7.75mm

these are averages per cam of all the intake and exhasut lobes, measured on the unworn spot at edges of lobes.  they both do appear to be stock F3 cams for 78

[HondaMan]

Thanks HondaMan.  I had been told that MegaCycle and Webcams would grind a cam for me to RC specs if i wanted.  i dont have a set of v blocks.....but i think i have a spare set of cam holders off a blown motor, i will find my dial indicator and see what that R6 cam actually is.  both these cams were sposed to be F3 cams.  I will post some numbers when i find my dial indicator, havent used it in a while.


ok measurements with dial indicator   R1 cam....................intake  .321 or 8.12mm
                                                                                            exhaust  .302 or 7.67mm

the R6 cam intake  .320 or 8.13mm
                   exhaust  .305  or 7.75mm

these are averages per cam of all the intake and exhasut lobes, measured on the unworn spot at edges of lobes.  they both do appear to be stock F3 cams for 78

Yep, those look like spot-on stock!

[KRONUS0100]

new question....wanna make sure i'm doin this right.

using the Cam #2 from above....    8.15mm intake lift and 7.67mm exxhaust lift.

its installed according to factory service manual specs using factory cam gear.

cheackin measurements where its at in motor to find centerlines of lobes here is where im at:

true top dead center using piston stop done
dial indicator set up on valve retainer and zeroed done
rotate motor until dial indicator reads .050 indicates valve opened 5 degrees ATDC  (actually -5 BTDC).....becomes a negative 5 degrees due to it opened ATDC
continue rolling motor around until dial indicator again reads .050 indicating 32.5 degrees ABDC

now the math..............-5 plus 32.5 plus 180 equals 207.5 degrees duration
207.5 divided by 2 minus the opening values of -5 equals 108.75 degrees for centerline of intake lobe.


have i done this right so far?  have not exhaust valve yet because i wanna make sure this is right.
I am doing this according to article on WebCam's website.

[HondaMan]

new question....wanna make sure i'm doin this right.

using the Cam #2 from above....    8.15mm intake lift and 7.67mm exxhaust lift.

its installed according to factory service manual specs using factory cam gear.

cheackin measurements where its at in motor to find centerlines of lobes here is where im at:

true top dead center using piston stop done
dial indicator set up on valve retainer and zeroed done
rotate motor until dial indicator reads .050 indicates valve opened 5 degrees ATDC  (actually -5 BTDC).....becomes a negative 5 degrees due to it opened ATDC
continue rolling motor around until dial indicator again reads .050 indicating 32.5 degrees ABDC

now the math..............-5 plus 32.5 plus 180 equals 207.5 degrees duration
207.5 divided by 2 minus the opening values of -5 equals 108.75 degrees for centerline of intake lobe.


have i done this right so far?  have not exhaust valve yet because i wanna make sure this is right.
I am doing this according to article on WebCam's website.

Well, first, remember this: the Japanese cams use 0.1mm (0.040") lift start instead of SAE's 0.050" lift numbers. To make it even more confusing: the 1976-1978 bikes used .020" (0.05mm) instead, for some reason I still don't understand, to get their inlet opening at 5 BTDC.

Here's how I do it when dialing in the stock cams:
1. Use a new cam chain. A stretched one will be late.
2. Use a new front cam chain slipper tensioner. A used one will also make it late, as the arc is worn in and flat.
3. If the cam does not come within 3 degrees of the stock numbers, consider using one of the APE slotted sprockets instead, as the Honda sprockets on the post-1976 bikes were lightened and don't have much room to widen their slots.

Then, it actually works out easily! The cam timing should open at the intake valve at 3-7 BTDC. They are often "off" by as much as 3 degrees one way or the other (usually they are late, opening at 0 degrees with a used chain and tensioner) in production tolerances.

So, set up your indicator at the #1 intake valve, turn the crank until it opens 0.1mm (0.040") at the stem, check the degree wheel and note the degrees BTDC. Then rotate until it returns to this same 0.1mm (0.040") lift value, check the crank degrees again. Repeat for all 4 intakes, then split any difference between them for the best smoothness. For example, if you find one is opening at 4 degrees and another at 2 degrees, you can pull the cam forward to have it open at 6 degrees on the first one and the second one will then be 4 degrees. This is typical of these cams. Always measure the intake valves on the same "spot", near the valve stem, so the geometry doesn't vary from one to the next: this is the tricky part!