How to Modify Book - Wrong Float level?

[inline6]

I am working through my Mikuni tuning issues... and I think I found something that anyone still messing with Mikuni's and who is using Honsowetz's How to Modify your Nissan/Datsun OHC Engine should know.  On page 112 and 113, it says:

 

"To check fuel level without disassembling the carburetor, remove the jet cover - center top of the carburetor and remove one jet block.  The fuel level should be visible in the jet-block hole.  The recommended fuel level is 0.826-in. (21mm) below the float chamber top surface - horizontal surface at the top of the jet-block hole.  This directly relates to a fuel level that's 0.787-in. (20mm) above the horizontal center of the throttle bore on 50mm and early 44mm models and 0.940-in. (24mm) above the throttle bore on later carburetors."

 

My Mikuni PHH Carburetor Manual has a drawing (Fig. 3) on page 3 that shows the fuel level in the carb body.  It does indeed indicate that the fuel level should be 24 mm +/- 1 mm (this is for 44 PHH - the dim is 24.5 +/- 1 mm for 40 PHH) above the horizontal center of the throttle bore.  However...

 

I have a 44 PHH carb body in my hands at the moment, and measuring with a caliper, I am coming up with a different measurement than published in the Honsowetz book.  On the front face of my carb, between the two throttle bores, there is a step in the casting right at the throttle bore horizontal center.  This makes for very easy measuring.  From that step ("the horizontal center of the throttle bores") to the "float chamber top surface", which is also the same surface at the top of the jet-block hole, I measure 1.792".  Now...

 

If 24 mm, when converted to inches, is 0.945", then the recommended distance to the fuel level below the float chamber top surface should be 1.792" minus the .945".  So, the distance down to the fuel should be 0.847".

 

In other words, instead of using 0.826" for the measurement from the jet-block hole down to the fuel level, I should be using 0.847"   

 

Can somebody else confirm this?  

 

Interestingly, I have the jet block holder... and an OA jet block in hand also.  With the jet block installed in the holder, the jet block stick up .285" inches above the jet block surface.  If I add the .285" and the .847"... I get 1.132" which sure looks to me like it puts the float level exactly in the middle of the "fat" barrel portion of the jet block - the part that is widest in between the 4 elongated, lower holes in the jet block and the eight round holes above.

Edited March 31, 2015 by inline6

[ryant67]

That's a difference of 0.021, or half of a millimetre... I'm not sure the floats need to be quite that precise. Your math works, but perhaps the number was rounded within margin of error since most people would lack the ability to measure quite that accurately?

 

I actually made a little float measuring device today using some clear acrylic rod and a nylon spacer. Copied this design, which seems useful:

 

http://www.webstore.com/item,pgr,Optical-Fuel-Level-Measuring-Tool-Indexed-at-25mm-for-DCOEs,name,44933341,auction_id,auction_details

 

Just mark it at the desired depth (21.5mm), place it over the jet block hole, and slide it down until it hits fuel and the rod goes dark. With the floats set properly, the mark should be where the change occurs. Simple but effective tool for very quickly confirming float levels.

[madkaw]

Todd at Wolf Creek had me remove the cover and rest it against something and measure the distance between the cover and the top of the float =12mm

 

Not sure is 44's are different then 40's

[ryant67]

Todd at Wolf Creek had me remove the cover and rest it against something and measure the distance between the cover and the top of the float =12mm

 

Not sure is 44's are different then 40's

Like on this page of the mikuni phh booklet:

http://www.mikunipower.com/PHHCarbs/PHHservice18.jpg

 

Whole booklet here:

http://www.mikunipower.com/PHH01.htm

[madkaw]

Exactly

[Tony D]

How to Modify is an expedient check.

It's supposed to be the same as the Mikuni Method mentioned immediately above.

 

As I often say in my training classes, "I will teach you the right way to do it so you know how....I will also teach you handy shortcuts. Remember, if a problem arises, stick to the proper method, no shortcuts so you know there is no question it's been done right!"

[inline6]

That's a difference of 0.021, or half of a millimetre... I'm not sure the floats need to be quite that precise. Your math works, but perhaps the number was rounded within margin of error since most people would lack the ability to measure quite that accurately?

 

Yes, that makes alot of sense. It is just half a millimeter, but I had a few thoughts about this.  

 

First, siphon action is what delivers the fuel to the auxiliary venturi.  Logically, a "very low" level of fuel will require more siphoning "action" to pull the fuel than a "very high" level.  I'm guessing that siphoning action difference between these two different levels can be compensated for via changes to the jetting, but I do not know this.  

 

When I attempted to set my fuel level to the Honsowetz spec, I actually used a section of wire that was .815" instead of .826", because my attempt to mark it was a bit off.  That resulted in a fuel level that was .032" "high" vs. the .847".  It is still within the +/- 1 mm range in the manual, but given the problem I'm having, I'd rather be on the low side than the high side.

 

In my particular circumstance, I am experiencing AFR's in the low to mid 10's across a very wide range of main fuel jets.  Right now, with main fuel jets of 130s to 180s, I get 10's when I apply fuel throttle at low rpms (I do this in such a manner that I can rule out the pump nozzles being the culprit). 

 

So, my "high" fuel level may not be part of my problem, but I'm going to set the level to ~ .847" below the jet hole surface, and see if it helps the problem I am having at all. 

Edited June 10, 2015 by inline6