Rejetting Mystery?

djg42

djg42

Been Around the Block
Hello and thanks in advance for your advice!
I just put in pods and rejetted. I got it to a point where it was running great, if let it rip in first or second gear my front wheel jerks up into the air which is awsome.
I took it out for a drive and after an hour i started to notice the performance was dropping. Mid throttle range would kind of hesitate and then surge. I also hear some subtle soft type of popping/thumping from the exhaust.
Low throttle is un-effected.
I remember reading that a rich setting will get worse when the engine gets warm, so i figured i was just a bit too rich. I tried it again the next morning to see if when it was cold it would run better, but it was the same. Next, i de-shimmed the needle back to stock to see if leaning it up would help, it didnt.

What could have happened?What could have changed during riding? It was running so good!

btw,
when i rejetted u went up on the main and pilot, and shimmed the needle
 
What bike exactly?

And just checking, have you cleaned the carbs / tuned the motor lately?
 
Thanks,
Its a 1980 GS450. I just cleaned the carbs, put fresh spark plugs in.
Its not better with the choke up a little,
Its not better with a size smaller main jet.
Thats all ive done lately, i havent adjusted the valves or anything like that.
It was running so good at first, i dont know what happened?
 
heres a picture of the plugs,
http://i837.photobucket.com/albums/zz293/djg42/sparkplugs.jpg

Should i try adjusting the float height?
 
Main jets too small
what size is in there? (Mikuni 'round head' 115 or bigger?)
Fit at least bigger mains and drop needle to clean up mid range.
As posted in various forums, mid range will never be right but will be ridable

PJ
 
thanks! Im gonna order a few next sizes up, stock are 117.5 i have 130's in now.
Is just weird that this configuration was woking great at first, and then suddenly dropped after an hour of riding
 
What did you do to exhaust?
If its stock, 130 should be too big.
May have lost the vacuum plugs on intake?

PJ
 
Sounds like you may be running lean just on jet sizing, and the description you give definitely sounds like its that.

Skipping more than, say, 2 jet sizes at a time over stock is always a bit iffy. 130 roundheads should be starting to get close though - my GS850 with pods, 4-1 and 32mm Mikuni CV's is running 130 mains and still needs more. The stock airbox is very restrictive so don't be surprised if you end up with mains in the vincinity of 135 or even 140.

Only change one thing at a time though, and only chop plugs at full load (ie flog it on a backroad stright to top speed, then hold that for a few seconds and hit ignition kill and pull clutch, coast to side of road and check plugs). Make sure you've been riding for at elast 10 minutes before chopping so as to make sure the bike is warm.

Most of all, be patient. It might take you a whle to find your exact spot-on tuning, but it'll be worth it in the end.

Cheers - boingk
 
thanks for the encouragement boingk!!
Thanks PJ, what are vacuum plugs ??


crazypj said:
What did you do to exhaust?
If its stock, 130 should be too big.
May have lost the vacuum plugs on intake?

PJ
Click to expand...
 
depending on carb/year, vacuum plugs block carb sync 'holes'.
May be rubber caps on spigots or 5~6mm screws on carb/manifold.

PJ
 
If it was running good at the start and then got worse, it could be fuel delivery issues.

Is the tank venting properly?

Is the vacuum tap working properly?

Did you clean the carbs / tank / fuel lines?
 
Thanks hillsy! I cleaned the carbs, the tank is rusty-but the fuel flow from the petcock seems to be good. How do I check if the tank is venting properly? Thanks again,
DG
 
djg42 said:
How do I check if the tank is venting properly? Thanks again,
DG
Click to expand...

Safest way is to put the tank on a bench and drain through the tap into a bowl (with the cap closed). If it is not venting the fuel flow will slow down (and probably stop) as the tank becomes a vacuum. Another way is when the problem occurs, open the fuel cap and see if the problem goes away. Obviously this is not the safest thing to do while you are riding.
 
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