'82 XJ650L Seca Turbo Hesitates Just As Boost Kicks In

Hello!
I've been slowly resurrecting this bike off and on for several years - too many projects...and too little time. It's a 3,500 mile machine that was left behind by a college student, stored away for years, and robbed of a few parts when I came upon it. Thanks to the state of Vermont, I now even have a title for it! I collected parts for a year, went through the whole bike and got it running at about 90% with two pesky problems I'm now working to clean up. The first is the check valve...the first time I forgot to shut the petcock, a week or so later I had a crankcase full of fuel. Yes...when I restored her I installed a new o-ring from the BD guy, While I will try to remember to close the petcock, I believe I solved the issue today by polishing the valve piston and body. Bench testing shows it now closes fully and easily with just the spring tension. NOW MY PROBLEM that I need guidance on. The bike has always run well at idle, mid throttle, and goes like heck under full boost...but.... just as boost kicks in under load, it hesitates for a second or so before it bursts into full power. At first I wrote it off as turbo lag, but the more I ride it, the more I think it's more than that. So far I've cleaned and regapped the plugs, swapped out the coils, confirmed fuel flow and volume through the petcock, pump, regulator and check valve...all check out fine. My boost gage is fully functioning so the computer should be getting the data it needs. I've also checked the wastegate for full closure and turbo plumbing for leaks and found nothing. Is this the best it gets? Any other thoughts or suggestions. Anything would be appreciated. Thanks in advance, Chuck

 

No, that's NOT as good as it gets. The bike should pull strong right through the rpm range. Did you do a complete carb rebuild with new throttle shaft v-seals and all the rest?

 

Hello, Thanks for the reply. I did a full carb clean, rebuild and sync using a rubber kit from BD Designs. I dont recall (it was a couple years ago) if it included the shaft seals. That would make sense .... when the system flips from vacuum to boost, I could be losing pressure at the shafts... Hmmmm

 

UPDATE - THE BIKE IS FIXED! What was it?..I'm not sure. I followed at least one suggestion from the group here - the V seals on the throttle shafts which along the way led to various discoveries. Here's a quick rundown. I pulled the carbs to install the V seals...what I found was that I did replace them when I first put the bike back into service after decades of sitting...but I used the seals from BD Designs which are more like O rings versus the OEM V design. In the end, I do not think this was the issue as the V design only seals the shafts when the bike is running at no boost. Under boost pressure they surely let some pressure leak by. HOWEVER, I did find #2 carb had very fine green grit packed in the emulsion tube. None of the other carbs had anything in them. When I recommisioned the bike two years ago I meticulously cleaned the carbs, tank, lines, and flushed the pump and fuel pressure regulator. I have no idea as to where this grit came from. When I bought the bike both the pump and pressure regulator were missing. While flushed the used OEM pump and regulator I located, possibly I didn't clean them well enough. Also during the process of cleaning the carbs I noted that the small fuel line running from the check valve to the carbs had a slight kink....was it restricting fuel flow? Who knows...but I replaced it. I also noted that my fuel height was slightly low....so I used the clear tube method and bench set them to 3mm below the bowl edge. Finally, I installed the Power Up kit the bike never got from the dealer. After a quick rebalance using my nurse tank...she runs beautifully....and boy does that Power Up kit make a difference!

What was the root cause of the hesitation at mid RPM?...my guess is the clogged emulsion tube in carb #2 causing that cylinder to go lean. BTW - I also installed a 50 micron filter at the fuel outlet on the tank , and a 10 micron filter on the fuel pump exit to ensure nothing ever again contaminates the carbs. Many thanks to you all! Chuck