----- 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