Topic: Found this about valve reliability

[Chuck Hahn]

Found this older post when looking for some valve education.

http://forums.sohc4.net/index.php/topic,10540.0.html

Im working on a 79 CB750K and it runs absolutely flawless until I get to around 7500ish to 8000 RPMs...WOT and trying to break the ton was mu intentions here.   Anyways it picks up a stumble.  Back it down to around 6G and its rock solid again.   Synced and resynced tha carbs with the Morgan Carbtune Pro and all 4 rods are almost perfectly level.

The exhaust is the Jardine system sold back in the day..1and 2 headers go into the left muffler and 3 and 4 cylinders drain into the right muffler. When its stumbles i can hear it from the right side of the bike and the left side is smooth as silk sounding.  Thus its obviously a problem in either 3 or 4.

Anyway, the bike has a little over 19,000 on the clocks.   I had the cam cover off and checked that the .05MM feeler gauge went into all the valves..but maybe the valves arent set right still??  Got a new gasket on the way and plan on rechecking ALL the valve clearances yet again seeing its been running again after 14 years.

So my question is, from reading the linked thread , is what the OP saying is to set the valves in THOUSANDS of an inch and NOT in MM???   I also noetd that cylinder 3 plug looks a little ashy and 4 is a bit rich but not carbon fouled  I also have taken the carbs off to recheck and clogged jets and nothing..nice clean holes in all the jets.  I rebuilt the carbs after a full 24 hour soak in Berrymans with all new orings directly from Honda.

OK fire away with suggestions on the stumble at High RPMs.  I also thought of the spark advancer and am considering doing the spring mod wehre you cut off one end and rebend a loop which stiffens the springs...thus delaying what RPMs the advancr starts advancing spark.

[slikwilli420]

So my question is, from reading the linked thread , is what the OP saying is to set the valves in THOUSANDS of an inch and NOT in MM???

What unit of measurement you use is totally irrelevant as long as you set them correctly. They cannot be too tight or will not close all they way and that could be part of the issue you are having.

[KayOne]

If it’s a ‘79 750 DOHC, the valves should be set to 0.005 of an inch. Most guys set them to 0.006”-0.007” though.
The pilot circuits on the DOHC carbs are VERY susceptible to blockage and if the bike has been sitting long the carbs will have to be disassembled and cleaned. There are a number of websites that have directions on how to clean the VB carb series. I can’t remember the exact address but I’d start at CB1100F.net.

[Chuck Hahn]

Kayone.  Im no stranger to doing complete tear downs and rebuild on Mikuni CV and VM carbs and have done many sets of Honda Keihins as well.  Bike had set for nearly 14 years.  I have completely stripped all 4 and dipped in Berrymans parts and carb dip for a FULL 24 hrs.  Once they come out of the dip they are rinsed with water to flush off the Berrymans. Then they get lots carbs spray from all directions.....and I mean at least half a can per carb... minimum to flush out passages.   This is followed by a ton of compressed air. more carbs spray and then more air.  I do it to a point of excess really.


All holes in all the jets are poke with a fine wire, carb spray and air to see that they are all open too.   So just maybe I need to resoak 3 and 4 because all my problems can be heard popping from the right side of the bike.   And as you may recall 3 and 4 jugs go into the seperate muffler from jugs 1 and 2.

[Don R]

 Double check the float levels?

[Bankerdanny]

WOT is when the main-jet size really come into play (at lower RPM's the needle is the limiter of fuel flow), maybe you need to be one step richer.