Joined
·
105 Posts
I’ll get the apology out of the way first. I owned a 2002 ZR-7 for several years. This bike ran great right out of the box, no hesitation issues. Factory pilot screw settings were 1 1/4 turn out on all 4 carbs. I bumped them up to 1 3/4 turns as per the service manual and noticed the bike being much more eager off the line. Having read all the discussion about jetting on this forum I figured you guys were either nuts, or just needed something to complain about.
My sincere apologies…I now understand.
I picked up a 2001 ZR-7 as my replacement bike. This bike has HUGE amounts of hesitation at anything below 1/4 throttle position and an erratic floating idle. Also I noticed lean surging at cruise with throttle settings below 1/4. I removed the carbs and spent a day cleaning and checking them. Everything was stock and spotless, except one of the pilot jets was partially plugged, but I got it cleared out. The pilot screws were set at 0, 1/2, 0, 1/2 . I also noticed that when completely closed, the #2 pilot screw protruded nearly 1mm further in to the carb throat than the other 3. I set them all to 2 turns out. Idle was now solid, but the hesitation and lean surging persisted. I also noticed that the header pipes are blue where as my 2002 pipes had a nice straw color. I used the carb spray test to check for any airleaks around the airbox, carb boots, etc. None found. Once throttle gets past 1/4 things are pretty much as I remember on my other ZR-7, a nice smooth pull to 7k, and then a slower climb from there.
HOW CAN 2 “identical” unmodified completely stock bikes run SOOOO differently? Has any root cause been identified for this variance in jetting requirements for the ZR-7?
What I have done:
I used tape on the throttle grip to mark off 0 thru 100% throttle positions so I could identify the areas where hesitation occurred. As I mentioned above, all hesitation occurs at less than ¼ throttle. Most prominent at 1/8. As suggested elsewhere, I plugged up 1/3 of the intake and the bike ran great at low throttle settings but bogged down above 1/4 throttle. Based on these observations I ordered #38 pilot jets to richen up the pilot circuit. The bike is running better at idle and pulls a little better off the line(don’t have to rev and slip the clutch to get moving) but still hesitates at 1/8 throttle and some lean surging at cruise persists. The pilot screws are at 2 turns out except #2 is at 2 1/2. I set this based on pulling plug wires at idle and looking for equal RPM drop for each cylinder. With #2 set to 2 turns like the rest, the RPM didn’t drop at all when #2 plug wire was pulled. Smoothed out the idle a bit. Oh, and of course a careful carb sync has been down.
What I will be doing:
1) Tonight I will be trying 2 1/2 turns out on all carbs(maybe 3 on #2). This will probably be too rich at idle as blipping the throttle already results in RPM dropping slightly below idle momentarily and then recovering.
2) I have an dyno appointment for tomorrow to get some fuel/air mixture reading information. I will post my findings.
Questions:
1) Has anybody ever needed to use a pilot jet larger than #38 to cure hesitation?
2) I am beginning to think that the larger pilot jet is just helping out the real problem of a too lean setting for the straight section of the stock jet needle. Is it possible that there is a variance in stock needle diameter? I will measure mine next time I have them out.
3) Has anybody tried to modify the straight section of the stock needle to richen up the mixture for low throttle settings? I have done this before on a jetski carb that had problems with lean running. Of course it only had 1 carb.
Thanks for listening,
Steve Bolser
Saginaw, TX
My sincere apologies…I now understand.
I picked up a 2001 ZR-7 as my replacement bike. This bike has HUGE amounts of hesitation at anything below 1/4 throttle position and an erratic floating idle. Also I noticed lean surging at cruise with throttle settings below 1/4. I removed the carbs and spent a day cleaning and checking them. Everything was stock and spotless, except one of the pilot jets was partially plugged, but I got it cleared out. The pilot screws were set at 0, 1/2, 0, 1/2 . I also noticed that when completely closed, the #2 pilot screw protruded nearly 1mm further in to the carb throat than the other 3. I set them all to 2 turns out. Idle was now solid, but the hesitation and lean surging persisted. I also noticed that the header pipes are blue where as my 2002 pipes had a nice straw color. I used the carb spray test to check for any airleaks around the airbox, carb boots, etc. None found. Once throttle gets past 1/4 things are pretty much as I remember on my other ZR-7, a nice smooth pull to 7k, and then a slower climb from there.
HOW CAN 2 “identical” unmodified completely stock bikes run SOOOO differently? Has any root cause been identified for this variance in jetting requirements for the ZR-7?
What I have done:
I used tape on the throttle grip to mark off 0 thru 100% throttle positions so I could identify the areas where hesitation occurred. As I mentioned above, all hesitation occurs at less than ¼ throttle. Most prominent at 1/8. As suggested elsewhere, I plugged up 1/3 of the intake and the bike ran great at low throttle settings but bogged down above 1/4 throttle. Based on these observations I ordered #38 pilot jets to richen up the pilot circuit. The bike is running better at idle and pulls a little better off the line(don’t have to rev and slip the clutch to get moving) but still hesitates at 1/8 throttle and some lean surging at cruise persists. The pilot screws are at 2 turns out except #2 is at 2 1/2. I set this based on pulling plug wires at idle and looking for equal RPM drop for each cylinder. With #2 set to 2 turns like the rest, the RPM didn’t drop at all when #2 plug wire was pulled. Smoothed out the idle a bit. Oh, and of course a careful carb sync has been down.
What I will be doing:
1) Tonight I will be trying 2 1/2 turns out on all carbs(maybe 3 on #2). This will probably be too rich at idle as blipping the throttle already results in RPM dropping slightly below idle momentarily and then recovering.
2) I have an dyno appointment for tomorrow to get some fuel/air mixture reading information. I will post my findings.
Questions:
1) Has anybody ever needed to use a pilot jet larger than #38 to cure hesitation?
2) I am beginning to think that the larger pilot jet is just helping out the real problem of a too lean setting for the straight section of the stock jet needle. Is it possible that there is a variance in stock needle diameter? I will measure mine next time I have them out.
3) Has anybody tried to modify the straight section of the stock needle to richen up the mixture for low throttle settings? I have done this before on a jetski carb that had problems with lean running. Of course it only had 1 carb.
Thanks for listening,
Steve Bolser
Saginaw, TX