----- Original Message -----
Sent: Monday, September 03, 2007
11:32 AM
Subject: [FlyRotary] Interface
between Primary fuel rail and diffuser (air bleed) 20B
Background
When installing the primary fuel rail on
my 20B, I noticed a gap between the primary fuel rail and the diffuser
(air bleed.) One of the original air bleeds was damaged when the
rebuilder was removing. He could not locate a replacement. He
supplied new air bleeds designated for a 93-95 13B, which he was informed
were usable in the 20B. The gap between the fuel rail and the
diffuser seems to be between 0.110" and 0.145". This measurement was
determined by inserting the diffuser and measuring its depth. Then
the fuel rail was bolted in place. Through the intake port, the
injector was pushed up until it contacted the fuel rail. The fuel
rail was then removed and diffuser depth was measured. The
difference between the measurements was between 0.110" and 0.145".
The original diffuser returned by the rebuilder has no apparent seal, the
replacement has a rubber seal molded into the top.
I took some measurements and drew up the
primary, fuel rail / injector /diffuser (see attached pdf.) It looks
like there are 2 differences between the diffusers that came on the
20B and the replacements recently purchased. The height of the
diffuser above the seating surface is much lower on the replacements and
the length of the area that projects into the port/air stream is
longer. The extra length extending into the port is probably not an
issue.
The shorter height of the replacement seems
like it will be a problem, the problem being a vacuum leak around the
injector. A secondary issue from a vacuum leak in this area may be
an effect on fuel droplet size. By looking at the injector, it
appears that a metered amount of air is intended to flow into the pintle
cap, presumably to aid in reducing the fuel droplet size. A vacuum
leak in this area may reduce the air flow through the pintle cap and
the resulting droplet size reduction.
I would speculate that at medium or higher
power levels the vacuum leak would probably have little noticeable
effect. I would expect that the most significant effect would be at
or near idle conditions. Since smooth idle seems to be difficult to
attain in many installations, it would certainly be prudent to try and
eliminate this issue (not to mention that it would be good practice to get
it right.)
Questions
1. There seems to be a very small gap
(~0.010") between the fuel rail seating surface and the top of the
original diffuser. It looks like there should be a grommet that
makes up the difference and provides a positive seal. To anyone who
has disassembled a 20B, is there a grommet/seal the sits on top of the
diffuser?
2. Same question as 1, for anyone who
has disassembled a 93-95 13B (13B-REW, I believe)?
3. Anyone have a source for the proper
20B diffusers?
Any opinions and/or remedy suggestions are appreciated.
Thanks,
Joe