By the way how did you come to find out about the after market diaphrams being no good? just wondering
Last year, there seemed to be a bunch of threads saying that people were having hard starting issues after a rebuild, but the engine would run fine after it got re-started.
Several of the big boys suggested that the needle and seat were leaking, and after a while, the consensus was that the pin on the aftermarket diaphragms was a little too long. (discovery one)
So... I was doing a restoration on my Sportster, and the guy I bought it from put a brand new engine in it, but couldn't get it to run. So... I went right out and bought 2 "Windrosea" rebuild kits. Once I got the carbs off... they were rusty and nasty, so I work my regular magic on them using these kits. The diaphragms in them were OEM Mikuni. (red nipple) I measured them, and I measured the Windrosa kit, and the nipples were within an acceptable tolerance to each other. (Seeing that we just bend the spring arm to make adjustments)
After putting them back together... the boat ran fine, but guess what... I had to crank it for 10 seconds with the throttle half way up to get it to fire. And when it did fire... it would cough, and chug on one cycl for a couple seconds, and then finally clear out, and run spot-on. (idle, transition, WOT were all good) So... it was clear that the carbs were leaking while sitting... but the flow wasn't so bad that it wouldn't idle smooth.
Later that week, I pulled the carbs and went though them again, taking the "Pin is too long" info with me. Once again, I measured the OEM pin and the Windrosa pin, and I filed it to the exact same length. I put the carbs back on the boat, and went for a ride.
While at the lake... I still had the same problem, but since the boat ran... I just didnt' kill it until I was ready to be on the shore for a while, and knowing about the issue, it was easy enough to get it to re fire. (half throttle and crank)
SO... one more time... the carbs come off. This time, I double checked the pop-off pressure, and I did a leak test on the needle. It passed just fine, but as I was putting the diaphragms back in... I hear a "PHSSSssss" when I tightend down the screws. Sure engouh... the needle wouldn't hold pressure at all. (FYI, my pop-off in that carb was 44 psi, and now it wouldn't hold 3 psi) But... as soon as I loosened that screw again, all was OK.
Knowing that the pin was exactly the same as the OEM pin, I checked for other problems.
My conclusion was that the bellows of the aftermarket diaphragm was too thick, and I think the bellows was contacting the back cover, and it was putting pressure on the pin.
I even tried to bend the spring arm down a little, but it would still lift my needle and seat once the cover screws were torqued down.
The easy solution.... I put in new mikuni diaphragms, and the carbs could then hold pressure with the diaphragm and cover in place.
I'm sorry that got long... but I figure I would put that story out there so people would know that I had first hand issues with this problem.
The kits may have gotten better because this summer, I've only seen one or two people reporting hard starting, and it being a rebuild issue.
