cagimaha Posted January 12, 2011 Report Share Posted January 12, 2011 What I had previously taken to be quite the bargain is quickly becoming a recurring nightmare! The bike is a 750L with the original Mikuni BST38SS carbs fitted. I've got the carbs in bits to see what jets it's got in it and there seems to be a small brass tube floating about in each of the float bowls. I've looked for it in both the parts manual and also the genuine workshop manual but to no avail. It's not threaded, looks like it should be pressed into somewhere but I'll be buggered if I can see where! It's about 4mm long and maybe 2mm diameter with a slight taper at one end. They're almost identical in size to the powerjets that screw into the float bowl but without the thread and with a bigger hole through them. I'm guessing they're a guide or spacer for something but what? Also the jetting is still bog standard even though the previous owner has fitted a Micron 4-2-1 full system and pod filters, that'd be the cause of the flat spot then... Any ideas of what jets it should have in for this setup. I'm guessing around 4 sizes bigger on the mains and raise the needle a notch? Any help much appreciated, I'll post a pic of the brass widget when I get chance. Cheers, Ben. Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 What I had previously taken to be quite the bargain is quickly becoming a recurring nightmare! The bike is a 750L with the original Mikuni BST38SS carbs fitted. I've got the carbs in bits to see what jets it's got in it and there seems to be a small brass tube floating about in each of the float bowls. I've looked for it in both the parts manual and also the genuine workshop manual but to no avail. It's not threaded, looks like it should be pressed into somewhere but I'll be buggered if I can see where! It's about 4mm long and maybe 2mm diameter with a slight taper at one end. They're almost identical in size to the powerjets that screw into the float bowl but without the thread and with a bigger hole through them. I'm guessing they're a guide or spacer for something but what? Also the jetting is still bog standard even though the previous owner has fitted a Micron 4-2-1 full system and pod filters, that'd be the cause of the flat spot then... Any ideas of what jets it should have in for this setup. I'm guessing around 4 sizes bigger on the mains and raise the needle a notch? Any help much appreciated, I'll post a pic of the brass widget when I get chance. Cheers, Ben. Could they be starter jets? Often they are pressed in parts? Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 I don't think so, the pilot jets are screwed into the carb body and the pilot air jets are screwed into the front of the carb (air inlet side). They look more like spacers than jets. Also on closer inspection the powerjets aren't jets at all they're blanks... The parts microfiche also lists the jet size as zero so it looks like they should be that way standard? Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 I don't think so, the pilot jets are screwed into the carb body and the pilot air jets are screwed into the front of the carb (air inlet side). They look more like spacers than jets. Also on closer inspection the powerjets aren't jets at all they're blanks... The parts microfiche also lists the jet size as zero so it looks like they should be that way standard? Not pilot jets, starter jets... Only used when the choke is on. Yeah, zero size jets will be undrilled. Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 Ahh sorry, I've accounted for them as well though! They're above the floats. Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 Ahh sorry, I've accounted for them as well though! They're above the floats. It does ring a bell that they unaccountably put powerjets on those and didn't actually use them... I bet that's where they should be. Is there evidence of anything else that controls fuel flowing through the powerjet circuit? Maybe it was a homologation thing? Link to comment Share on other sites More sharing options...
wazrs257 Posted January 12, 2011 Report Share Posted January 12, 2011 I've just had a quick look in the float bowl of one of my 750L's carbs & can't see anything like you describe Ben. Pics might help. Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 Here's a few pics of the part in question. Sorry for the poor quality but I've only got the camera on my phone. It's sat on top of a pot of grease to give you a rough idea of the scale. To be honest I wouldn't be surprised if it's meant to go somewhere else in the carb and the monkey who had the bike before me had just thrown them in the float bowl. Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 Is it me or does half of it look a different colour as though it was pressed into something? Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 Yeah I think it has been pressed somewhere, there's a slight taper which makes it a bit more noticeable as well. If you want I'll take a pic of the float chamber and you can see if you can spot where it goes? Link to comment Share on other sites More sharing options...
BudgetBoy Posted January 12, 2011 Report Share Posted January 12, 2011 I might be dumb nd make myself look a complete but this "LOOKS" like it is where the needle valve goes through to get to the main jet. If you look through the carb there should be a brass bit like yours in the center of the carb (as you look DOWN from the top) These are pressed into the carb. neddle goes thought that into main jet. Is that what it is?? Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 I know the bit you mean and I've accounted for that as well! On these carbs the emulsion tube or jet holder is huge and is actually pressed into a big plastic piece that the slide runs in. Here's some pics of the float bowl and the float chamber: Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 Yeah I think it has been pressed somewhere, there's a slight taper which makes it a bit more noticeable as well. If you want I'll take a pic of the float chamber and you can see if you can spot where it goes? Yeah, cool.... Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 Looks like it did come out of there... Where do those drillings go? That's the key to understanding it.... I bet it's a jet that works on the pilot bypass thingamy... Link to comment Share on other sites More sharing options...
BudgetBoy Posted January 12, 2011 Report Share Posted January 12, 2011 Take a photo from the top of the carb downward. So you are looking at it with the float bowl underneath. Camera shot V (downwards) Top -- -- -- Float bowl Take the top off the carb to get the diaphram out and needle (dont bend it keep it safe) then look inside and take photo mate. Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 Right on the bottom pic, from left to right the drillings are: fuel inlet (float valve), main jet, pilot jet, overflow and locating recess for the dowel on the bowl. I'll get some pics of the top half disassembled now. Thanks for the speedy replies by the way, much appreciated! Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 Right on the bottom pic, from left to right the drillings are: fuel inlet (float valve), main jet, pilot jet, overflow and locating recess for the dowel on the bowl. I'll get some pics of the top half disassembled now. Thanks for the speedy replies by the way, much appreciated! Nah, I mean the tiny drillings in that bit where it looks like the brass part came from. On the third pic, towards the front of the carb but on the centreline... Link to comment Share on other sites More sharing options...
BudgetBoy Posted January 12, 2011 Report Share Posted January 12, 2011 Nah, I mean the tiny drillings in that bit where it looks like the brass part came from. On the third pic, towards the front of the carb but on the centreline... If you mean the bit with 3 tiny holes then thats normal mate. Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 If you mean the bit with 3 tiny holes then thats normal mate. I'm not saying it's unusual, just asking where they go...... Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 Nah, I mean the tiny drillings in that bit where it looks like the brass part came from. On the third pic, towards the front of the carb but on the centreline... That's just a locating hole for the dowel on the float bowl. The mystery piece is less than a third of the size of that hole. Pics of the carb body from the top and also the jet holder: Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 That's just a locating hole for the dowel on the float bowl. The mystery piece is less than a third of the size of that hole. Doh! Could it be a jet for the vent up in the diaphragm chamber? From the 1st of these last two pics there looks to be a bit of a hole it might fit? Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 That's already got something pressed into it! I'm tempted to put it back together with them left out and see how it runs. As far as I can tell it's been running with them just floating round in the bottom of the float bowls anyway! All four of them were just loose in the bowl when I took them off. I'm actually amazed at how well it runs considering how much has been altered induction and exhaust wise whilst the jets are still standard. Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 Yeah, if it runs well then it's not worth stressing over I suppose... Could it have fitted on the end of the brass tube sticking out of the body? Or could it have fallen out of that plastic float assembly? Link to comment Share on other sites More sharing options...
cagimaha Posted January 12, 2011 Author Report Share Posted January 12, 2011 Well it doesn't exactly run "well" which was why I was in there in the first place. I'm guessing this is to do with standard jetting and modified intake and exhaust though rather than any missing components though. I'll give it a go and let you know how I get on Link to comment Share on other sites More sharing options...
spacemonkey Posted January 12, 2011 Report Share Posted January 12, 2011 Well it doesn't exactly run "well" which was why I was in there in the first place. I'm guessing this is to do with standard jetting and modified intake and exhaust though rather than any missing components though. I'll give it a go and let you know how I get on What've they done to it? Look at the needle jets and the white plastic blocks with a suspicious eye...... Any wear? Link to comment Share on other sites More sharing options...
BudgetBoy Posted January 12, 2011 Report Share Posted January 12, 2011 I can honestly say mate it does not come out of your carbs as they are. Everything is there that needs to be there.i would say it is some sort or rejet that was used in the past. Stupid place to leave them mind.. On the bad running... Check your emulsion tubes to see if these are worn. Also make sure all jets are clear. The needle valve on the float might be sticking so make sure this is not doing it. Once you have given the carbs a good clean see how it runs then. If still running badly, check your plugs to see if they are wet (overfueling) and get the carbs balanced while you are at it. Link to comment Share on other sites More sharing options...
BudgetBoy Posted January 12, 2011 Report Share Posted January 12, 2011 oh this might helpyou set them up... BST38SS Main jets...................117.5 Main air jet................0 mm Jet needle.................6ZD7-3 Needle jet.................O-8 Pilot jet....................32.5 Pilot air jet 1990....................1.2 mm 1991 on................Not specified Pilot screw setting......Preset (1-1/8 turns back) Starter jet.................40 Float height...............14.7 +/- 1 mm (0.58 +/- 0.04 inch) Link to comment Share on other sites More sharing options...
cagimaha Posted January 13, 2011 Author Report Share Posted January 13, 2011 Thanks for all the help guys, as ever the response has been awesome. After looking at everything again I finally realised that the unknown part is something to do with the overflow circuit and actually presses into one of the plastic arms that go under the floats! I've fitted them all and put it back together, I'll wait until tomorrow before treating the neighbours to a trial run! As for the bad running the carbs are currently set as per the manual. Nothing seems to be worn with the internals, everything still has a nice gold coating (I'm assuming the emulsion tubes are ok, I can't see inside them but judging by past experiences they usually wear at a similar rate to the needles and they're fine). I'm pretty sure it's the jetting that is out, standard jetting + engine mods is usually not a brilliant idea. Once again thanks for the help! Link to comment Share on other sites More sharing options...
ronenige Posted January 13, 2011 Report Share Posted January 13, 2011 I'm not saying it's unusual, just asking where they go...... Progression jets , either side of the throttle butterfly controlled by the pilot circuit Link to comment Share on other sites More sharing options...
spacemonkey Posted January 13, 2011 Report Share Posted January 13, 2011 Progression jets , either side of the throttle butterfly controlled by the pilot circuit You'd think they'd be capped by something other than that dowel... Glad the mystery is solved anyway.... Link to comment Share on other sites More sharing options...
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