Author Topic: Need carb guru's... (Read 5310 times)

Krautwagen · « **on:** April 20, 2005, 12:41:07 AM »

Somewhat of the dreaded flat spot issue...

While constant cruising at about 4k, or just starting to take off, roll on the trottle, and it'll stumble a bit then go...

Previous research says that the needles are just too lean... But this is not my case.

Here's my current setup:
Yosh full system
K&N drop in
Factory stg 1 Jet kit
95 or 97.5 mains (forgot which currently)
Clip height 1 lean from center
Float height 14.5mm
Pilot/air screws @2 turns out before balancing (just had to lean #2 ~1/2 turn in)

Recent parts replaced:
Float assemblies
Vac diaphrams
Plastic Carb tops
Jet kit
CDI refurbished by Carmo
Signal generator
Plugs
Coolant
Gas (92oct (R+M/2)

Valve clearances good, Comression and leakdown good and even (on the low side of standard, but not even close to the low limit). Carbs thoroughly cleaned. No external vacuum leaks. Balanced via pilot/air screws. Needle jets look nice and round.

Main jet size may be one topic of discussion. Stock is 102.5, but the jet kits come with 95 and 97.5. Top end feels roughly the same with any of those, but midrange is more tuneable with the smaller jets, and the plugs look like a better mix with them in.

I started this spring's attempt at fixing it by replacing the carb tops and diaphrams (found a pinhole in 1 diaphram, and 2 tops had tiny cracks). Kept the needle at 1 lean, still had the flat spot, richend it up to the center position, problem got worse. Now it also will stall if you do not modulate the trottle when it hits the flat spot. Restarting is only possible by cranking with no throttle and no choke.

Tomorrow I am going to try full lean on the needles to see what happens, just thought I'd get some input from you other B4 guys...

Thief400 · « **Reply #1 on:** April 20, 2005, 09:43:27 AM »

raise the needles one step richer and retest

gsxr400 racer · « **Reply #2 on:** April 21, 2005, 01:52:13 AM »

one more time for the masses easiest way to jet with out ripping the carbs in and out is to take the air filter out for a extreme lean condition if bike runs better then you know wich way to jet the bike , or you can tape up the air inlet to the air box a bit (not completely) this will give you a rich condition , ride to see progress

JmuRiz · « **Reply #3 on:** April 21, 2005, 10:31:03 AM »

Cool idea, so simple. I'll have to give that a try when I start messing with my modded airbox (if I ever get around to installing it).

Bandit · « **Reply #4 on:** April 21, 2005, 11:00:42 AM »

I say drop the needles one step to make it leaner, it happened on my old gsxr750 with worn emulsion tube. I need to drop the needle every 2 months to solve the flat spot at 4-5K range. Its just a temporary fix, the problem will come back again if the emulsion tube is worn.

Krautwagen · « **Reply #5 on:** April 23, 2005, 12:34:21 AM »

Thanks for the tip gsxr400, I'll have to try that next time.

Got most of the issues taken care of...

Firs step was to go the other way on my clip height, since richening it (to center position) made things worse than at 1 lean. Bike ran much better at full lean on the needles. I tryed richening the pilot screws 1/2 turn at a time. This took care of the uneven idling and take off problems, but the hicup moved up to about 5k. I could feel it was beyond the pilot's effective range, and did some more troubleshooting. I leaned out the float height to 15mm, and then had to reset the pilots to about 2-2.5 turns out. Now it starts, idles and goes nicely. No more flat spot or hicup at low throttle openings, but there still is a bit of hesitation when below 3k under load. I guess that's just the downside of a 400...

So now I'm at:
#95 mains
Full lean on needle clip height
15mm float height
~2-2.5 out on the pilots

It runs better when cold, so looks like I'll have to lean it out a bit more... probably try 15.5mm float height.

gsxr400 racer · « **Reply #6 on:** April 23, 2005, 01:39:18 AM »

quit messing with the floatheight , 98.5 's middle clip posistion always wanna try to stay near the middle if full lean on clip/ needle you need to make a main jet change. if dull rich need to make a leaner main jet selection. always pull aifilter out to check progress. in your case pu the neddle clip in the middle and pull the aiur filter see how it rund=ns from the bottom to 80000 rpm

Thief400 · « **Reply #7 on:** April 24, 2005, 02:39:56 PM »

Set the float height to the factory setting, and I don't mean the jet kit makers. The first rule you must obey is " ONE ADJUSTMENT AT A TIME". Don't set the float height, change the main jet and move the needle all at once and then try and figure out what went wrong. Alway jet your Main jet first, I don't like the way Factory Pro has written there instructions. On the bandit anything over 6000 rpm is the main jet. If your bike was jetted right before you added the pipe you have to go up 2 to 3 sizes larger. Once you have the right main jet now you can fine tune the midrange with the needle. One last thing did you change your thermostat as per Factory Pro's instrctions, this helps big time in keeping the jetting consistant. The stock thermostat opens at 160 so the engine can run any where between that and 220 when the fan kicks in. install a 195 and the engine runs in a narrower temp range and the jetting stays more consistant.

jasonlarrow · « **Reply #8 on:** May 27, 2005, 11:12:59 AM »

Sorry to keep going over this again and again. I'm doing the FactoryPro jet kit right now. I've changed out the needles, going with the center position to start. I've put in the new springs. I haven't touched the floats and don't intend to just yet. I just put the main jets in, going with the 120s. Here's my concern. The kit says 2.75 turns out. When I back them out that far, they seem a bit wobbly. Even at 2 turns out they seem a bit wobbly. Is this normal? The jets that were in there, 107.5s, were in all the way. Any help would be appreciated.

PitterB4 · « **Reply #9 on:** May 27, 2005, 02:04:32 PM »

Jason,

Am I misreading the post or are you backing out your main jets? The 2.75 turns out is for the pilot a/f screws (that may be hidden by a brass plug). The mains should be torqued in there.

120 mains? Did you go with their Stage III kit with the pod filters?

jasonlarrow · « **Reply #10 on:** May 27, 2005, 08:15:54 PM »

Thank you. I thought it was a bit odd. Yes, I have the stage 3 w/ pods and a Cobra F1R exhaust.

sirdarkheart · « **Reply #11 on:** June 19, 2006, 08:54:21 PM »

i'm gonna revive this thread since it was hard to search for. Also b/c I noticed there are a lot of jetting questions (including one from myself) and I thought this was a good thread. Hope this helps anyone in need!