testy

Author Topic: Grinding stem on valves question 78 750 F3  (Read 605 times)

0 Members and 1 Guest are viewing this topic.

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 25,383
Grinding stem on valves question 78 750 F3
« on: September 19, 2018, 05:44:51 pm »
 I put in a rebuilt F3 motor in my 750.
 Not sure what was replaced or not, but it was very clean with assembly lube under valve cover and no carbon inbthe ports.
 So 2500 miles mostly hiway, and I have adjusted valves  several times.
It seems noisy, was very  quiet at first.

 So I have a theory.  I was told by an engine shop you can take  a lick off the stem, not a lot, hard portion is thin.
But I don't know if valves were ground or replaced

 So I have to wonder, if I am maybe having a lot if stem  wear. I pulled one adjuster and nut out  stem appeared to be pocked. End of adjuster had lines on it, radially.. like a sort of star look.
 Anyone seen a lot of stem wear on reused valves  or re ground valves..?

 I just hope I did not bugger yet another cam .  Every time I adjust  each valve looks wet. Don't think it's low on oil supply.
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 kmb69

  • Master
  • *****
  • Posts: 1,871
Re: Grinding stem on valves question 78 750 F3
« Reply #1 on: September 19, 2018, 05:51:31 pm »
Frank, If you see boogers on the valve stems, they're toast.
You might be able to grind them flat and use some stem caps.
I would be suspect of the adjuster screws as well.
Mike might be able to point you to some stem caps.

Offline scottly

  • Really Old Timer ...
  • *******
  • Posts: 12,299
  • Humboldt, AZ
Re: Grinding stem on valves question 78 750 F3
« Reply #2 on: September 19, 2018, 07:53:05 pm »
Valve grinding machines have a fixture on one end for facing off the ends of the stems, but that would require disassembly of the head, which I doubt you want to do. ;)
I've seen those radial lines on the ends of the adjuster screws; they get formed as the end of the adjuster sweeps across the stem, forming a "flat". When you re-adjust the valves, it puts the worn edge into contact with the stem, and the narrow contact area wears quickly, and so on until the adjuster gets to a less worn area.
If the worn areas on the end of the valve stems are deep enough, you won't get a good clearance check with feeler gauges, as the gauge will bridge the pocket in the stem. If lash caps can be fitted, that will solve that issue. You could also set valve clearance with a dial indicator.   
Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 25,383
Re: Grinding stem on valves question 78 750 F3
« Reply #3 on: September 19, 2018, 08:40:02 pm »
Scott ain't much there to grind m you get one go I think  if it's very much depth it's into  softer metal..
 It runs fine,  aside from poor mileage and a bit of valve noise m it's fine for now.
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 scottly

  • Really Old Timer ...
  • *******
  • Posts: 12,299
  • Humboldt, AZ
Re: Grinding stem on valves question 78 750 F3
« Reply #4 on: September 19, 2018, 09:36:34 pm »
Scott ain't much there to grind
That's where the lash caps come in. ;) As far as mileage, the last time my bike only got 40ish MPG was bucking a 45 MPH headwind for 100 miles. ::) Not sure where you should look, but if the plugs aren't sooted up I would suspect retarded ignition timing?
Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Online PeWe

  • Really Old Timer ...
  • *******
  • Posts: 6,873
  • Bike almost back to the 70's 2015
Re: Grinding stem on valves question 78 750 F3
« Reply #5 on: September 19, 2018, 09:46:09 pm »
My K6 head had F2 inlet valves. They mushroomed really quick. Adjuster screw carved a dimple in the tops. Last set were a little bit grinded for good surface, lash caps from Mike. I thought that the lower valve got dimple easier due to the different angle of adj screw that is rather flat.

I had to grind the sides to get out the valves thru the guides. Valve on photo had not been used for long.
Are these Honda valves of a softer material?
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 stock looking project started 2017

Offline scottly

  • Really Old Timer ...
  • *******
  • Posts: 12,299
  • Humboldt, AZ
Re: Grinding stem on valves question 78 750 F3
« Reply #6 on: September 19, 2018, 10:28:35 pm »
Per, the greatest wear is well off center of the valve stem, (and that is some considerable wear!) which indicates improper valve/adjuster geometry. IIRC, the F2 valves have shorter stems, which, when installed in an earlier head, would require the adjuster screws to be threaded further down into the rockers, shifting the contact point away from the center of the stems.
Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Online seanbarney41

  • possibly slightly younger than an
  • Really Old Timer ...
  • *******
  • Posts: 7,675
Re: Grinding stem on valves question 78 750 F3
« Reply #7 on: September 20, 2018, 01:51:17 am »
My f3 had pitted valve stem tips which would chew through a fair amount of tappet adjusters.  I just dealt with it as best I could.  I got about 10,000 miles out of it before the valve guides were consuming excessive oil at about 28,000 miles.
« Last Edit: September 20, 2018, 01:57:01 am by seanbarney41 »
If it works good, it looks good...

Online PeWe

  • Really Old Timer ...
  • *******
  • Posts: 6,873
  • Bike almost back to the 70's 2015
Re: Grinding stem on valves question 78 750 F3
« Reply #8 on: September 20, 2018, 02:54:16 am »
Per, the greatest wear is well off center of the valve stem, (and that is some considerable wear!) which indicates improper valve/adjuster geometry. IIRC, the F2 valves have shorter stems, which, when installed in an earlier head, would require the adjuster screws to be threaded further down into the rockers, shifting the contact point away from the center of the stems.
Yes, that what I noticed. Lash caps prolonged the stems a little.
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 stock looking project started 2017

Offline kmb69

  • Master
  • *****
  • Posts: 1,871
Re: Grinding stem on valves question 78 750 F3
« Reply #9 on: September 20, 2018, 06:52:15 am »
Per, the greatest wear is well off center of the valve stem, (and that is some considerable wear!) which indicates improper valve/adjuster geometry. IIRC, the F2 valves have shorter stems, which, when installed in an earlier head, would require the adjuster screws to be threaded further down into the rockers, shifting the contact point away from the center of the stems.

That is correct. They shortened the stems on the F2 & F3 because they sank the valves deeper in the combustion chamber.

Offline Old Scrambler

  • My CB750K3 has been in 39 States & 5 Provinces
  • Old Timer
  • ******
  • Posts: 3,221
Re: Grinding stem on valves question 78 750 F3
« Reply #10 on: September 20, 2018, 08:18:40 am »
I measured the F2-3 chamber roof to be 1mm taller than the K models and correspondingly the valve-stems are 1mm shorter. I have 4 F2 motors and all have dimpled stems. The valve angle is the same when all parts are new.

Frank, if you get some lash-caps, be sure to use fresh adjusters to avoid rocker contact.
Dennis in Wisconsin
'64 Triumph Cub & '74 Honda CB750 Bonneville Salt Flats AMA Record Holder (6)
CB750 Classic Bonneville Racer thread - http://forums.sohc4.net/index.php/topic,135473.0.html
'63 CL72 Project(s)
'66 CL77 Red
'67 Triumph T100C
'73 750K3 Owned since New
'77 750F2 Cafe Project
2000 Aprilia Pegaso

Offline simon#42

  • Master
  • *****
  • Posts: 1,946
  • liverpool
Re: Grinding stem on valves question 78 750 F3
« Reply #11 on: September 20, 2018, 10:41:14 am »
i have 5mm stem valves in mine with lash caps , they have been in for about 35 thousand miles and are still ok , they have some wear but are still fine . the beauty of them is you can easily change them when they are worn out

Online PeWe

  • Really Old Timer ...
  • *******
  • Posts: 6,873
  • Bike almost back to the 70's 2015
Re: Grinding stem on valves question 78 750 F3
« Reply #12 on: September 20, 2018, 11:36:30 am »
I measured the F2-3 chamber roof to be 1mm taller than the K models and correspondingly the valve-stems are 1mm shorter. I have 4 F2 motors and all have dimpled stems. The valve angle is the same when all parts are new.

Frank, if you get some lash-caps, be sure to use fresh adjusters to avoid rocker contact.
Finally got it confirmed what I first saw in the middle of the 80's! F2 stems really soft.
I thought the local shop had made a mistakes with the valve stems. Understood later that stems must be softer.

If using lash caps, stem must be shortened with same lenght as cap is thick for the correct geometry if correct from the beginning.
« Last Edit: September 20, 2018, 11:39:20 am by PeWe »
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 stock looking project started 2017

Online seanbarney41

  • possibly slightly younger than an
  • Really Old Timer ...
  • *******
  • Posts: 7,675
Re: Grinding stem on valves question 78 750 F3
« Reply #13 on: September 20, 2018, 02:25:18 pm »
I was always under the impression that these problems were the result of Honda not getting the geometry right in the first place.
If it works good, it looks good...

Offline BPellerine

  • Master
  • *****
  • Posts: 2,024
Re: Grinding stem on valves question 78 750 F3
« Reply #14 on: September 20, 2018, 02:39:37 pm »
at one time I was going to put an f3 top end on a k motor to get the bigger valves and better breathing,untill I read about all the issues with them,cancelled that idea.bill
1978 CB 750K ard and webers
another anfob

Offline kmb69

  • Master
  • *****
  • Posts: 1,871
Re: Grinding stem on valves question 78 750 F3
« Reply #15 on: September 20, 2018, 03:16:58 pm »
I was always under the impression that these problems were the result of Honda not getting the geometry right in the first place.

You are 100% correct.

Online seanbarney41

  • possibly slightly younger than an
  • Really Old Timer ...
  • *******
  • Posts: 7,675
Re: Grinding stem on valves question 78 750 F3
« Reply #16 on: September 20, 2018, 04:24:03 pm »
I was always under the impression that these problems were the result of Honda not getting the geometry right in the first place.

You are 100% correct.
and that new valves and guides from a supplier that is aware of the correction necessary, such as APE, solves the problem once and for all
If it works good, it looks good...

Offline kmb69

  • Master
  • *****
  • Posts: 1,871
Re: Grinding stem on valves question 78 750 F3
« Reply #17 on: September 20, 2018, 06:00:47 pm »
I was always under the impression that these problems were the result of Honda not getting the geometry right in the first place.

You are 100% correct.
and that new valves and guides from a supplier that is aware of the correction necessary, such as APE, solves the problem once and for all

Doesn't solve the geometry problem but they use materials that solve the wear problems associated with the bad rocker geometry.
Honda fudged on the rocker geometry to keep the engine as short as possible. Pretty much got away with it until the F2-F3.

Offline bwaller

  • Really Old Timer ...
  • *******
  • Posts: 7,041
Re: Grinding stem on valves question 78 750 F3
« Reply #18 on: September 21, 2018, 04:39:16 am »
I've measured valve angles, and as Dennis mentioned, that didn't change in any of the 750 heads including the later F's.

I'm not sure why duplicating earlier stem height with better valves/guides wouldn't return reliability??

I still don't care for bigger chambers & the needed huge domes to match.

Offline MRieck

  • Really Old Timer ...
  • *******
  • Posts: 9,571
  • Big ideas....
Re: Grinding stem on valves question 78 750 F3
« Reply #19 on: September 21, 2018, 04:03:07 pm »
 A lot of good points here. Personally I like lash caps as you can easily replace them. All the 5mm conversion kits use them and Brent runs them on his race bike. I have 6.5mm caps on hand if anybody needs them. The stems need to be ground -.040. The tip also need to be flat and perpendicular to the stem.....you can't get in there with a Dremel and expect them to work.
Owner of the "Million Dollar CB"

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 25,383
Re: Grinding stem on valves question 78 750 F3
« Reply #20 on: September 21, 2018, 04:29:53 pm »
 If my valve stems were ground and are now soft, then I expect it to continue to wear.  And I may not be able to run lash caps, unless I tear head down.
 Then I would need a length measurement to know how far to grind them.
I suspect if they ground them,  they took around 15 to 20 thou off.
 if I were to try to install caps without removing valve I would mask it off and stone them to knock off any burrs..
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 simon#42

  • Master
  • *****
  • Posts: 1,946
  • liverpool
Re: Grinding stem on valves question 78 750 F3
« Reply #21 on: September 23, 2018, 08:41:56 am »
if you did decide to install caps without removing the valve it would be a mistake .

Offline MRieck

  • Really Old Timer ...
  • *******
  • Posts: 9,571
  • Big ideas....
Re: Grinding stem on valves question 78 750 F3
« Reply #22 on: September 23, 2018, 10:24:41 am »
if you did decide to install caps without removing the valve it would be a mistake .
Yes....I have personally seen that AND the lash cap quality was blamed (which was total BS). The cap will tiddly wink around and all sorts of bad stuff starts to happen.
Owner of the "Million Dollar CB"

Offline gschuld

  • Master
  • *****
  • Posts: 1,691
Re: Grinding stem on valves question 78 750 F3
« Reply #23 on: September 23, 2018, 11:00:54 am »
if you did decide to install caps without removing the valve it would be a mistake .
Yes....I have personally seen that AND the lash cap quality was blamed (which was total BS). The cap will tiddly wink around and all sorts of bad stuff starts to happen.

Mike,

Is “tiddly wink around” a technical term?🤔.  😁

George

Offline MRieck

  • Really Old Timer ...
  • *******
  • Posts: 9,571
  • Big ideas....
Re: Grinding stem on valves question 78 750 F3
« Reply #24 on: September 23, 2018, 04:30:33 pm »
if you did decide to install caps without removing the valve it would be a mistake .
Yes....I have personally seen that AND the lash cap quality was blamed (which was total BS). The cap will tiddly wink around and all sorts of bad stuff starts to happen.

Mike,

Is “tiddly wink around” a technical term?🤔.  😁

George
Pics to follow....you'll see.
Owner of the "Million Dollar CB"

 

;
Honda