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A carb question for those who might know the answer

Discussion in 'XJ Technical Chat' started by imnotamonster, Mar 20, 2010.

  1. imnotamonster

    imnotamonster Member

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    Hi, I don't post much mostly because I find what I need through the search but this one has too many variables to do a good search.

    I am bringing an 81' seca 750 with only 6,361 miles back to life after a long sleep and am completely stumped. I have the carbs pretty much spotless and added the rebuild kits while I had them apart. Can't get the seca to start (electrical I think) so I put them on my maxim 750 (daily ride running perfect) to be sure I had put them back together right.
    My maxim fires right up with a little choke but it runs all gurgley sounding. As soon as I back the choke off it revs real high and smooth for about 3 or 4 seconds then dies. I have triple checked the float heights and made sure the new float needles weren't sticking. The carbs are only bench synced because there is no sense to sync them on the maxim when they are for the seca. The slides clunk nicely and the enrichment holes spray quite a distance so I know it's not that. I have the air fuel mixture screws at 2.75 turns, tried them at 2.5 makes no difference. I must be missing something but can't think of what it is.
    I am hoping that someone on here can give me a couple of ideas

    Thanks :D
     
  2. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    HOW did you check the float heights?
     
  3. Bushy

    Bushy Active Member

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    They are both 750's so if you synch on th Maxim it's gotta be damn close for th Seca..
     
  4. jvswan

    jvswan Member

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    Well, you've probably already checked it, but is it possible your main jets are the wrong size? When you got the rebuild kit, did you get the right size? Are the jets included in a rebuild kit?

    Anyway, I've got oversize jets in my carbs right now. I'm riding it while I wait for the new stock jets to get here from Len, but until then, I have to be careful because all sorts of wacky things are happening at idle. Jumping, hanging, and even dropping off.

    Just a thought.
     
  5. imnotamonster

    imnotamonster Member

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    Thanks for the quick responses...

    As for setting the float height, I used this thread.

    http://www.xjbikes.com/Forums/viewtopic ... t+dry.html

    I used a paper clip that I measured and marked with a file at 17.5 mm then I measured the floats just as recommended with the carbs at an angle so the floats are barely touching the needles and I measured from the gasket seat to the bottom/top of the floats at their center visually in line with the needle

    As for the jets they were not included in the kits I bought. I believe the jets are stock in the carbs I didn't actually look at the numbers but I know the history of the bike and I doubt they had been changed.

    I don't think syncing is possible at this time now that I think about it because it only stays running with the choke partially on or can I?

    Thanks again
     
  6. imnotamonster

    imnotamonster Member

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    Any other thoughts?
    It's obviously running out of gas when I turn the choke off. But why does the engine race nice and smooth right before it stalls. Hmm, it seems as though with the choke partially on it's getting too much fuel hence the gurgling kind of sound at idle. Then with the choke off it's not getting enough. What could cause those conditions?
     
  7. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Clogged pilot jet circuit, perhaps?

    The racing when you close the choke may be due to a sudden lean condition as you're cutting off the only source of fuel to the engine....
     
  8. RickCoMatic

    RickCoMatic Well-Known Member

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    There are a few things you ought to check.

    Remove the Inspection Plate and insure that both AIR Jets are:
    1. The Correct Value
    2. Properly Placed
    (AIR Jets are opposite Fuel Jets. Large O = Pilot. Small o = Main)

    Choke Linkage's and Top Hats.
    Look at the Enrichment Valve "Top Hats" for and Kinked or Bent.
    Heat and Straighten with Needle Nosed Pliers. Slowly.

    Watch the Enrichment Valves LIFT.
    The Enrichment Valves need to be in Sync, too.
    Bend Lifting Forks to Match
    Look for Valves to SEAT when closed.

    Know that the VALUE of every Fuel JET is: Correct!

    Know that the Pilot Mixture Screw Parts are ALL in CORRECT Order.
    I don't know HOW it happens BUT ... sometimes the Flat Washer winds-up UNDER the O-ring.

    Re-bench Sync:
    Use a narrow strip of 3X5 Card to assure that after the Bench Sync, ... the Throttles will be nearly Closed.
     
  9. streetbrawler750

    streetbrawler750 Member

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    My air jets under the top of the carb do not have a plate like the one that is shown in the haynes manual that covers them . Would that affect the idle a lot, or any? I am having idle problems similar to the aforementioned.
     
  10. RickCoMatic

    RickCoMatic Well-Known Member

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    Yes, Boss ... BIG Time.

    You got burned by the famous Haynes mess-up

    [​IMG]

    Change the AIR Jets to look like the Photograph.
    You are going to have too RE-Tune because of the hiccup.

    You'll be healed, soon.
     
  11. streetbrawler750

    streetbrawler750 Member

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    I double checked and they are correct as your photo rick, but what about the cover that is suppose to be there in the haynes manual pics none of my carbs have that. Does it make a difference? (I don't mean to hi jack the thread just a good place to ask it seems) Also I just read your pevious post rick about the 3x5 card trick. All of my butterflys are completey shut when it is on the bench. I synched them all by looking at the spaces under them, then I used a 4 vaccuum gauge set with out the yics tool and got them pretty darn good. but it will slowly die at idle
     
  12. RickCoMatic

    RickCoMatic Well-Known Member

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    Can you prevent it from stalling?
    Do you have an "Airbound" Inline filter?

    What happens if you get it running and put the Petcock on PRI?
     
  13. skeeter

    skeeter Member

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    my instinct tells me that missing the little "plate" over the air jets wont matter. my my instinct also tells me that, if it doesn't matter, then why is it there??

    i'm gonna go out on a limb and guess that if the plate is missing, you'll get slightly more airflow to the airjets which equates to a leaner mixture, i think. . .
     
  14. streetbrawler750

    streetbrawler750 Member

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    Ya sometimes I can feather the throttle and get it to rev up but not always. I don't have an inline filter, and the petcock has been modified so there is no off, I have an inline off. I am going to take the rack back off and check float hieghts with the clear tube method. I have variables that could also be messing with it. PVC intake, Dyno jet kit. but I have tried almost everything so far(that I can think of), I maybe will go up another cick on the main needle? Rick, how do you bech synch the carbs with that piece of cardboard paper? Do you set the idle so the butterflies are up some? I also changed my vavle shims yesterday and think it took the carbs out of synch the higher I try to turn the idle the farther out of whack the get with a slow to rev down when revved.
     
  15. RickCoMatic

    RickCoMatic Well-Known Member

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    You don't need them.
    You can run just fine without them.

    I rarely see them on Carbs any more.
    I find them on Carbs that still have the Pilot Mix Hole plugged.

    There's none on mine.
    What good is it?
    It's not air tight?
     
  16. ManBot13

    ManBot13 Well-Known Member

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    I've got an idea on that one. If you put the oring in, and the washer, and have to push them down with a small implement, the denser washer will "spin" the oring-washer pair over. So before you put the spring in, make sure both are at the bottom, and the washer is on top. The only reason I don't put them all on the screw is that I'm afraid something will fall off and I'll lose one or more small parts.
     
  17. imnotamonster

    imnotamonster Member

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    Phew, what a couple of miserable days we had. I am working under a carport and the rain was just too much for me to do anything till today especially after being out in it all day at work.

    My main jets are correct I believe at 120 for the 750 carbs. Hard to see with these old eyes but I think the pilots say 40 and that is also correct right? They are all in the right places but they are pretty chewed up at the screwdriver slot, should this cause a problem? I can see light through them so they are not plugged.

    They all appear to be in sync like you said, they all become engaged by the lifting forks at the same time and nothing else seems to be amiss...nothing broken bent or twisted. they all seat at the same time

    Mine have the inspection plate
     
  18. RickCoMatic

    RickCoMatic Well-Known Member

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    Well. ... while you are getting to the bottom of your Carb problem.

    I've got mine off the Bike and on-the-bench setting them up, ... Purr-feck-toe ... for the upcoming good weather.

    I refined a few techniques:

    Flushing the Enrichment Circuit Siphon Tube.
    I put a small piece of Heat Shrink ... shrunk onto the end of the Carb Cleaner Tube.

    I even measured the Depths of the Pilot Mix Screws with a Depth Tool to reset the Screws just right after blowing-out the passages and wrapping the Screw Barrel with Teflon.

    The good news is that a tiny O-ring came-up M-I-A and it was in the first spot I looked to find it.
     
  19. OnTheStorm

    OnTheStorm Member

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    compression??? make sure intake manifold boots and airbox boots are on properly w/o leaks, hoses are clamped on good and tight, maybe if they are old they might need to be replaced. Did you screw with the tank/petcock/vacuum hoses? this could also screw things up

    I have a compression problem and my temp fix for the next few days is playing with the idle screw, the big one in the middle of the carbs. my bike would not idle, even with the choke, without cranking this idle screw up.

    it sounds like you did the carbs right. have you checked spark? bad plugs, boots, battery are all things that could kill your bike and are easily overlooked
     
  20. imnotamonster

    imnotamonster Member

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    Well I am going to start from the beginning again. I looked at all of the suggestions and could not find one thing amiss. I know it's not the bike because I am using my daily ride to get these straight for the 81' Seca I am working on and my maxim is running grrrreat with it's own carbs on it.

    Anyways I have the Seca's carbs all stripped down again and I am going to clean them again wish me luck

    [​IMG]

    Here is the inspection plate if the pic works
    [​IMG]

    One last thing...What are these? They look like jets, there are two on either side of the butterfly
    [​IMG]

    Hope these pics work
     
  21. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Those are called air compensator jet and you don't need to remove them, they're so large that unless a huge bug or a small hairy mammal squeezed in thru there, they'll be clean. They only pass air, allow the carb bowls to "see" atmospheric pressure.

    When your carbs "flood" due to a stuck needle valve that is wher the over-flowing fuel goes.......
     
  22. ManBot13

    ManBot13 Well-Known Member

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    Really? So how does gas get into your crank case? I had a float needle stuck open, and gas poured out of the airbox drain. I freaked a little and changed the oil believing there would be gas in it. I would think that the gas would leak out the emulsion tubes (or are those completely sealed off when the slide is down?), and some would flow forward into the intake, and some backward into the airbox.
     
  23. petersja

    petersja New Member

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    Hey imnotamonster- what did you finally end up doing? I have this exact same problem with my bike (85 Maxim). The carbs needed cleaning and so I pulled them off and just sprayed carb cleaner (I didn't even take anything apart). I put it all back together and now it revs way up and dies. I got it to idle today, but not for very long. Any suggestions? Also, where do the vacuum hoses (I think that's what they are called- the ones that come out the bottom side) attach? Thanks.
     
  24. RickCoMatic

    RickCoMatic Well-Known Member

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    petersja:

    You need to do considerably more that that to Clean your Carbs.

    You need to Flush Passages
    Clean Emulsion Tubes
    Fuel Bowl Enrichment Circuit Ports
    Clean and Polish the Main Bore
    Open and Flush-out Pilot Passages
    ... and some more.
     

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