|
|
Post by wotavidone on Aug 2, 2007 20:36:50 GMT -5
My suggestions:
Check float height. Not as critical as you might think, my manual says the tolerance is +/- 2mm, so you got a bit of leeway,
Make sure the very small holes in the emulsion tube are clean. That's the tube the main jet actually screws into.
On my '78 model, the standard position for the needle is one notch below the middle notch.
Counting from the blunt end of the needle, it is the 4th groove.
That is, you have actually got the needle on a leaner than standard setting, if it is on the middle notch.
Given you have a K&N filter, which is supposed to be free flowing, and thus likely to make things leaner, you might even want to try the last (5th from the blunt end) notch.
I have ended up with a larger main jet as well- #330. This works well. I have the stock air filter with the old foam replaced with a layer of open cell foam, oiled with engine oil.
Mick
|
|
|
|
Post by brokenicarus on Aug 2, 2007 23:02:29 GMT -5
Thanks mick, I'll check it all! I'm really itching to get the bike back on its best performance so I can continue with cosmetic mods. I just don't understand why I've got no fuel at all. can a vacuum problem lead to a dry cylinder? if the float bowl's wet does that mean the vacuum isn't an issue? I'll go in tomorrow and buy a brace of larger pilot jets as well. couldn't a small pilot mean no starting? Also, what's the deal on the air mixture screw? the manual doesn't mention it, neither do many people on this forum. but here it is on the Mikuni VM34SS tour:  my bike has that screw as well, anybody know about it? like how much it effects the mixture, whether in or out leads to more fuel, or if it has a default setting. thanks! Davey |
|
|
|
Post by miker on Aug 3, 2007 8:56:14 GMT -5
Try this - with the float bowl removed, put the pet-thingy on "prime" and hold the float up with your finger. You'll see fuel stream out if its getting into the carb.
I noticed your comment on main jets. It's my understanding that ALL of the fuel that the needle jet passes comes to it thru the main jet. When the throttle is all the way open the needle is withdrawn and only the main jet limits fuel. So having a clear main is important even at 1/4-3/4 throttle.
Air screw - I'm surprised to see it called that because normally a screw in this position (on the engine side of the carb) is an idle fuel screw. Clymer shows only the later capped screw and doesn't mention it. Haynes calls it a "pilot adjust screw" and indirectly suggests perhaps 1-1/4 or 1-1/2 turns out as a starting point. They say to adjust it for highest idle speed.
If it's really a fuel screw (as would be normal) then turning it in will lean the idle mix, turning it out will richen.
miker
|
|
|
|
Post by brokenicarus on Aug 3, 2007 14:04:16 GMT -5
Ok miker, I struck out on your first diagnosis! full tank of gas, took the float bowl off, it was dry, turned the petcock to prime, pushing up on the float yielded no fuel. Not at closed throttle, not at open. however, take the fuel line off at the carb and it sprays away happily.
what does that indicate?
This might help me diagnose: A non-vacuum fuel petcock off of an IT400 that I was going to use on my new gas tank. can I circumvent the vacuum system and use the non-vacuum petcock? would that solve/ isolate the problem?
Thanks,
Davey
|
|
|
|
Post by canucksr on Aug 3, 2007 16:36:11 GMT -5
This may sound stupid...but is the fuel line itself clogged up? Or the inlet on the carb? On prime...the gas should be flowing when the float is down.
|
|
|
|
Post by brokenicarus on Aug 3, 2007 17:03:39 GMT -5
don't worry about stupid responses, I'm hanging on every input at this point, and if a solution sounds stupid it's an easier fix for me  . the fuel line isn't clogged, I'll check the inlet right now. no gas at all with the float in any position, petcock on prime.... Wow. you might win the prize canucksr! When I spray carb cleaner into the fuel inlet it comes right back at me, doesn't come out of the float bowl anywhere, even with the main jet and emulsion tube out! Now it's time for my dumb question: where is fuel supposed to come out once its gone through the inlet on the carb? can I just spray compressed air into the inlet, or will that ruin everything? Thanks! |
|
|
|
Post by brokenicarus on Aug 3, 2007 17:35:10 GMT -5
Heh. I think I solved the problem, I haven't put it back together yet but... It looks like the little clip that holds the float needle in place was keeping it from moving. the float needle has to move between these two positions, no?   That little clip was getting in the way! any proper way to insure it minds its own business? Thanks for all the help, I'll post soon if this works! Davey |
|
|
|
Post by canucksr on Aug 3, 2007 17:35:14 GMT -5
The fuel should come out at the needle and seat and fill the float bowl until the float rises enough to seal the "Needle" into the "seat". Must be some crap in there somewhere
|
|
|
|
Post by canucksr on Aug 3, 2007 17:36:57 GMT -5
Was that little clip jamming the neele "closed"?
|
|
|
|
Post by brokenicarus on Aug 3, 2007 17:38:33 GMT -5
yeah, I believe it was. it seems a little bent, I had no idea it was so important. does your carb have a similar clip?
|
|
|
|
Post by canucksr on Aug 3, 2007 17:48:28 GMT -5
No...my carb has a crescent shaped tab holding the "needle seat" in place. I don't know what that wire clip is? Is it the wire clip that should attach the needle to the float?
|
|
|
|
Post by canucksr on Aug 3, 2007 17:51:05 GMT -5
Yes...that's the wire thingy that attaches the needle to the float arm.
|
|
|
|
Post by canucksr on Aug 3, 2007 18:00:46 GMT -5
Oh...and my carb also has that mystery screw that was mentioned at the top of this posting, that's called a "Air adjust screw" in the diagram
|
|
|
|
Post by brokenicarus on Aug 3, 2007 18:29:48 GMT -5
yay! my bike runs! it just needs to be fiddled with, all the above advice will be taken Thanks so much guys, my motorcycling dreams would not be at all possible without this website. Wheeeee! |
|
|
|
Post by wotavidone on Aug 5, 2007 22:08:10 GMT -5
Interesting photo of a needle and seat you have there. I am not convinced my own carby actually still has the little clip. Can't recall - the good news is once you get the thing set up right you won't have to touch it too often, but then you tend to forget what's inside.
I would guess its main function is to make sure the needle doesn't fall out of the seat while you are fitting the floats. Once the floats are fitted, I reckon the needle can't fall out anyway, then the clip would be redundant.
The "air adjust screw" is called that because on lots of mikunis the idle mixture is adjusted by regulating the amount of air bled into the idle circuit.
However, I believe on the VM34SS its actually an "idle enrichment screw", which controls fuel flow at idle.
i.e. Turn it out to enrich the idle mixture, and in to lean out the idle mixture, as per most carburettor fed cars. This would be opposite to an "air adjust screw" where winding iot out lets in more air, thus leaning the mixture.
There should be a range of adjustment where the bike runs OK, but if you turn the screw in it starts to run rough, and if you turn it out it also starts to run rough, and if you go too far either way the bike stalls. You need to find this range of adjustment.
I reckon you turn the screw in until LIGHTLY seated, don't forget to count the turns so you can put it back where it was if you lose the plot altogether. Then turn it out 1 - 1 1/2 turns. Get bike running and warmed up, set idle speed at 1200 rpm.
Turn the screw out until the bike starts to stumble and run rough. Then turn it back in until the idle cleans up just. Check idle speed and repeat if necessary.
i.e. in that range where the bike will idle reasonably well, you want the mixture to be slightly on the rich end of that range of adjustment.
Almost all of the fuel does indeed go through the main jet, with the exceptions of the tiny bit that goes through the idle mixture circuit, the overrun enrichment circuit - that's the diaphragm on the side of the carb, and the accelerator pump, the diaphragm on the bottom of the carb.
Looking back over my words, I begin to see whay lots of guys just ditch the lot and go for a nice new simple round slide Mikuni. You'd still have to learn all about jetting though.
Mick
|
|
. the fuel line isn't clogged, I'll check the inlet right now. no gas at all with the float in any position, petcock on prime....