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Thread: Stock Carb Setup Help

  1. #1
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    Stock Carb Setup Help

    Alright, so decided to take the carb apart to check what the jets are and see if I can get it to stop popping on decel (signs of being lean). The manual says it is stock for a 300 main, 25 pilot, 6fl25-2 needle and p-8 needle jet. When I took mine apart there is a 270 main, 25 pilot, 5fl31 needle, and p-8 needle jet. Records of the bike show it hasn’t been jetted, but that’s a few pieces of paper and word of mouth from previous owner. The bike has an aftermarket exhaust, and a k&n air filter in the stock box. I know I can buy a new carb but wanted to try and get the stock one running well if at all possible. It wasn’t running terrible but would sometimes after being warm have a hard time idling. Also sometimes would bog down from an idle if I snap open the throttle.

    Should i try a stock 300 main? Or maybe just go up one or two sizes? Also not sure why the needle is different. Not sure if that needle with the 270 main gives you a close to stock setting? Any advice/ experience from anyone who has a similar set up and a good place to start would be great! Thanks.


    Edit: Sorry, I have a 1980 SR500G US model. I found a specs page online that says 260 main and 20 pilot are stock. the 5fl31 needle is also stock. so i guess that helps me out a bit, seeing that someone must have upped the jets somewhere along the line. so closed throttle popping on decel do i need to up the pilot jet?
    Last edited by Thatiltlyfe; 11-11-2017 at 01:43 PM.

  2. #2
    I'm surprised that some of the guys here, who have forgotten more about carbs than I will ever learn, haven't yet commented. But here's my 2 cents. First, general stuff: check to see if there is a vacuum leak between the carb and the petcock. Also, when you have the carb apart, check the diaphragms carefully for pinhole leaks. I can't speak to a 1980 SR, but I thought the "usual" main jet was around 230-240, and pilot 25. If your aftermarket exhaust is less restricted than stock, then it probably makes sense that someone upped the main sometime in the past. If you have other jets, it's not that onerous to change them out (only make one change at a time) and see if it helps or gets worse. I know that at least ONE of the causes of popping on decel can be exhaust leaks, so you might also want to inspect exhaust joints for tightness. If you have the original header, it is/was double-walled, and if the inner wall has rusted away, it can allow air into the exhaust stream via the 2 vent holes you may find in the outer wall, up by the clamps that connects it to the head. You'd have to remove the header to inspect the inner wall. OK, there's my pocket change. :-) I'm sure the guys who really know what they are talking about will chime in with proper advice! :-)

  3. #3
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    Hi That,
    The carb specs vary considerably. Referencing Partzilla.com, my 1978 SR500E uses a STANDARD 300 main, with 290 and 310, optional. This is for the VM34SS pumper carb. Now, for a 1980 SR500G, Partzilla shows main jet 260 as standard. It behooves all of us to check several references to get a handle on what should be in the carb.

    Now, since you have a less restrictive air intake, coupled with a presumeably less restricted exhaust, causing the mixture to be a bit leaner, some previous owner has changed the main 260 to a "richer" 270.

    For my 1978 SR500E with Supertrapp exhaust and K&N Air filter, I changed my main to a 310, and raised the needle 1 notch, to eliminate the overly lean mixture I was running. I don't have all the specs in front of me right now, but if I remember correctly, the 1980 carb was a 32 mm unit, rather than the earlier 34 mm unit, that I have. The means of adjusting needle height I believe is different in the carb you might have.

    Now, if my carb rebuild of the original VM34SS carb had failed to perform satisfactorily, I was going to try the carb from my great-running 1980 TT500G, on my SR500. That carb is a much simpler carb, without all the diaphrams and accellerator pump. Others have tried this, apparently successfully.

    Above all, the more carb work you do, the simpler they become.

    George

  4. #4
    Senior Member Simon's Avatar
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    The quickest and easiest way I found was after rebuilding my stock carb with new diaphragms etc to order a jet kit from 6sigma, they got my jetting spot on first time. Try a search on here or the old forum for them.
    Simon.

  5. #5
    Senior Member MadMax's Avatar
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    Where was the bike built for when new or which state was it 1st registered in? I'm sure I read somewhere that Colorado market bikes were jetted leaner for the thin mountain air?

  6. #6
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    Thanks all. So i got the carbs off and upped my main jet one size to 280 and cleaned everything. I am curious about the two different adjustment screws that I have. there is on on the right side of the carb and then one under the carb in front of the float bowl. What is each one for and what are the standard adjustments for each of these screws as a baseline for me to start at?

    Thanks.

  7. #7
    Senior Member MadMax's Avatar
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    Quote Originally Posted by Thatiltlyfe View Post
    Thanks all. So i got the carbs off and upped my main jet one size to 280 and cleaned everything. I am curious about the two different adjustment screws that I have. there is on on the right side of the carb and then one under the carb in front of the float bowl. What is each one for and what are the standard adjustments for each of these screws as a baseline for me to start at?

    Thanks.
    Here's a guided tour of a VM34SS, http://www.siue.edu/~rsutton/SR/mikuni.htm ignore the modifications suggested.

    The adjuster on the side of the carb controls idle speed, should be tweaked to around 1100rpm with a warm engine.

    The screw under the float bowl controls idle mixture, usually preset at the factory but should be somewhere around 1.5 - 2 turns out from fully seated, only you can decide where exactly by determining what gives you the smoothest idle, make small adjustments (0.25 turn) at a time until you're happy.

    Max
    Last edited by MadMax; 12-11-2017 at 08:04 AM.

  8. #8
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    {Quote, ignore the modifications suggested.} Why do the mods not work? I was getting ready to do them?????

  9. #9
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    So here are the two pilot air screws I am talking about. Iíve never seen 2 of these on a single carb, and Iím wondering what the stock setting is on each.

    I also ended up ordering a keyster rebuild kit (which is why the pilot adjuster screw in the bottom is a knob type and not set in the hole, makes adjusting it way easier) and it comes with a few set ups invluding new needles that are adjustable and jets. I got the bike set up fairly well using his kit. I also used this kit on my srx600 and it worked well on it too.


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