Hello, I have a 1980 XS650 that is stock except for UNI pod filters. I am having trouble with the jetting on the bike it has stock carburetors as well. The previous owner changed the main jet to a 140 when he did the filters. At first, it ran well then it developed a surge taking off from a stop and seamed a little rough between 2000 and 3000 rpm then it clears up. I have tried other jet combinations with no luck. I am running 91 octane Non-ethenol gas in it as well. I have done fresh plugs a few times as well. If anyone has a suggestion on what might get rid of the surge that would be great. Thanks
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Carburetor Jetting Need Help
- Thread starter SRosso
- Start date
Use this as a guide, stock settings.............Uni filters make it breath easier so the mains need to be larger,
Read through the Carb Guide. First link in this thread..........
http://www.xs650.com/threads/carbs-carburetors.43/
Read through the Carb Guide. First link in this thread..........
http://www.xs650.com/threads/carbs-carburetors.43/
welcome to the forum SRosso
do you mean that the roughness between 2000 & 3000rpm goes away occasionally at random or do you mean that the roughness disappears above 3000rpm ?
do you mean that the roughness between 2000 & 3000rpm goes away occasionally at random or do you mean that the roughness disappears above 3000rpm ?
In most cases, re-jetting these 650 carbs for mods involves more than just changing the mains, you need to change the pilots too. 1st off, if the UNI pods are your only mod, 140 mains are probably a bit much. 135 or 137.5 mains would probably be better. Along with that, bump the pilots up one size to a #45. Your issues at 3K and below seem to point to the pilot circuit as the area needing attention. Also make sure your mix screws are opened up enough. The BS34s like to see between 3 and 3.5 turns out on them.
Today I tried the main jets at 138 and a pilot of a 49 with the mixture screws just over 3 turns. It is no longer rough when revving the rpm but when I go down the street and stop then go to pull out the engine surges 3 or 4 times and then will go away once I shift to the next gear. Before the 49 I had a 42.5 pilot jet in with the same result maybe the 49 is too much and I need to order a pilot in the middle.
138 mains?
https://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_N102-221_main_Small_round.htm
49 pilots????
42.5 45 47.5 50 would be normal steps.
https://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_BS_30-96_pilot.htm
Having changed a jet or 2 I will only use genuine mikuni jets. Actual flow vs the stamped number can vary a bunch with "other" brands.
Are you drilling them?
Surging might point at carb synch or a leaking float needle.
https://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_N102-221_main_Small_round.htm
49 pilots????
42.5 45 47.5 50 would be normal steps.
https://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_BS_30-96_pilot.htm
Having changed a jet or 2 I will only use genuine mikuni jets. Actual flow vs the stamped number can vary a bunch with "other" brands.
Are you drilling them?
Surging might point at carb synch or a leaking float needle.
a hunting or surging engine can sometimes be due to a constantly changing, lean fuel mixture ie fluctuating vacuum ........ not necessarily a fixed fuel /air delivery issue as in wrong jet size or idle mixture setting
Changing jets might not improve the situation in which case it might pay to double check that there is no unwanted air leaks on the carb or inlet rubbers causing a lean mixture .
Also check that you have the float height set correctly,... a slight increase in float height setting in conjunction with a larger pilot jet and or increased idle mixture setting may improve things .
Replace old inline fuel filters if fitted and fit clear fuel lines to check for air bubbles indicating leaking fuel line connections. Might also pay to check fuel delivery rate from the petcocks and clear tank vent.
Also check both diaphragms are sealing correctly by applying the slide drop test
What happens to the surging if you apply a very small amount of choke ? does that reduce the surging ?
Changing jets might not improve the situation in which case it might pay to double check that there is no unwanted air leaks on the carb or inlet rubbers causing a lean mixture .
Also check that you have the float height set correctly,... a slight increase in float height setting in conjunction with a larger pilot jet and or increased idle mixture setting may improve things .
Replace old inline fuel filters if fitted and fit clear fuel lines to check for air bubbles indicating leaking fuel line connections. Might also pay to check fuel delivery rate from the petcocks and clear tank vent.
Also check both diaphragms are sealing correctly by applying the slide drop test
What happens to the surging if you apply a very small amount of choke ? does that reduce the surging ?
Hello everyone, today I finally figured out the problem. It was not the carburetors it actually was the blinkers. They must be drawing on the system and cutting spark so now I know I need to trace some wires, but there is no surging after I turn the blinker off. Thank you for the input and help with the problem.
I had a similar issue with electrical and it ended up being a battery. It would pass a load test at the parts store, but I asked them what the cranking amps were and it was way too low. I hope you fixed the problem, but most of my head aches have come from the xs650 electrical system.
