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bad mileage after rejet

3K views 23 replies 5 participants last post by  MikeB 
#1 ·
2001 vt600

did some searching around before posting and need some more help. I'm running straight pipes with baffles and a k&n air filter in the stock box. Everything i've read has said rejetting is a must for changing the exhaust and air filter. Ordered the calculated kit from CJK going from 125 main, 45S pilot jets to 130 main, 48 pilot. It runs great! only problem is I'm getting about 38mpg! That's horrible compared to what I was getting; 43+!

I took the plugs out and they're black on the tips and wet so its definitely running rich. Adjusted the mixture screw but still no increase in mpgs. Should I go back to stock jets? It was running fine but a tad on the lean side, still ran like a beast tho...
 
#2 ·
Did you let the bike return to idle before shutting the engine off and pulling the plugs? To read them you should shut the engine down while running at speed, pull in the clutch, roll to a stop and then pull them. Might be overly rich on the pilot jet, but fine on the main jet, or vise versa.
 
#4 ·
I'll take her out right now and check

If it just needed a bit of enrichment, you could get jets a little richer than the ones that were in there and it may be perfect.
I have an old 83' 750 and I get about 52+ MPH, shouldn't a newer 600 get better than 43? But I don't ride hard all the time.
Maybe some of the guys with 600's can chime in.
I admit I do ride it hard quite often but after filling her up monday morning and doing a 90 mile round trip to class, I ran her gently to see the mpg id be getting. Don't think I've ever gotten 45+ tho, even when I'd just cruise along at 55 for a 300 mile trip
 
#3 ·
If it just needed a bit of enrichment, you could get jets a little richer than the ones that were in there and it may be perfect.
I have an old 83' 750 and I get about 52+ MPH, shouldn't a newer 600 get better than 43? But I don't ride hard all the time.
Maybe some of the guys with 600's can chime in.
 
#5 ·
Was it already modded with the pipes and K&N when you got it? Maybe someone had rejetted a little before you?
A lot of 600 guys on here so maybe they can advise of their mileage and mods done.
 
#7 ·
I know there is a fuel comparison website that some of the guys here go on to report their mileage for a comparison, but I can't find it now. Maybe someone else has the link to that site. Good to see where where your mileage averages in.
 
#12 ·
Ida has Cobra Drag pipes, K&N air filter and has been re-jetted. I installed a Cobra stage one jet kit. I removed a 125s stock { Hondas number} jet and replaced it with a {DJ126 Cobras number}. I also replaced the needle with one that came with the kit. I placed a e-clip on the needle in the 3rd groove from the top of the needle. She will get 55 to 60 miles per gallon tank after tank.


There is no standard for jet sizes. Each company uses their own numbering system. Check out the link below.


 
#14 ·
Ida has Cobra Drag pipes, K&N air filter and has been re-jetted. I installed a Cobra stage one jet kit. I removed a 125s stock { Hondas number} jet and replaced it with a {DJ126 Cobras number}. I also replaced the needle with one that came with the kit. I placed a e-clip on the needle in the 3rd groove from the top of the needle. She will get 55 to 60 miles per gallon tank after tank.


There is no standard for jet sizes. Each company uses their own numbering system. Check out the link below.


https://www.youtube.com/watch?v=nPCZHxmYpPQ
that's TJ! I just watched his vid before I changed out my jets (first time). I didn't get a needle with my kit tho and I'm curious...did your original needle have shims on it when Ida was stock? Mine had 1 with the K&N and it looks like the only other change was the mixture screw

Tearing apart my carbs is therapeutic for me so I went ahead and put the old jets back in after cleaning em yesterday. Bike still runs fine, and after plug check, tan/brown on rear cylinder and black/wet on front again. Maybe its just unburnt fuel from pulling clutch and killing it maybe?
 
#17 ·
Just to clear up my own curiosity does this have 2 carbs? I thought the newer models had only 1 carb. But then how could 1 cylinder be too rich and the other not, unless there was a vacuum leak on the leaner cylinder. Just pondering. Trying to be helpful.
 
#20 ·
Get it running and spray carb cleaner around the intake gaskets and head surface where it connects to the boot going to the carb. Use a straw on the can to direct the spray very precisely so as not to get a false reading from having to much spray in the air going to the filter. If there are vacuum hoses ( such as emission equipment) going to the intake, pinch them off and see if it changes the idle sound.
I don't know about your engine so I am guessing how it could be different from one cylinder to the other. If it is too rich in general, but has a vac leak, it could make one cylinder show leaner. -- ???
 
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