Cannot get a proper idle

[ninskrillz]

Ok guys, I'm having some serious headache inducing rage instigating trouble trying to tune the vm34 carbs on my 71 Xs to a strong idle. I've read both the mikuni manual and the other member written manual everyone suggests. I've adjusted the cam chain guide, valve lash, and timed the bike.

The internals of the carbs are as follow:
PJ-25
MJ-185
Slides-2.5
NeedleJet-P6
Needle-? dunno but clip is in position 3

I cannot seem to get the bike to idle. Well, it was idling ok last week, would periodically hiccup and the idle speed would slow until the bike died if I left it alone and didnt pepper the throttle periodically. It idled at 900-1000 at this point.

Whenever I get it to a decent idle speed, I'll give it throttle and then it'll race to ~2400rpm and won't go down untill i lower the idle screw about 1/4 turn then it drops to about ~800rpm and dies. A strong, constant idle just isn't happening regardless of mixture screw or idle screw position. Well when the idle races to 2400 rpm it is rather strong steady, but obviously is too fast.

Any help appeciated!

 

[TwoManyXS1Bs]

This would be my country boy 'shadetree' test method:

I'd get a timing light, hook it up.
Then remove the round alternator cover, and the points cover.
Fire-up the engine, get it to idle.
Flash the timing light at the rotor to see the idling mark.
Rev the engine and watch the timing light marks.
Look for smooth timing transition from idle mark(s) to full advance.

Then, if idle hangs high:
See if the timing mark is at full advance.
Then (this is the 'shadetree' part), grab the nut on the end of the points cam with my fingertips, add a little drag, and see if the timing retards, and if the idle returns to normal, and stays there.

If it does return to idle rpm and stays there,
And, the timing transition looked to a little too quick,
Then, I would suspect that the advancer has weak springs.


I'd also be looking to see if the timing is correct, for both cylinders. Sometimes, this difficult-to-idle and high-reving condition is caused by too much advanced ignition timing. Setting the timing a little retarded, like at the mark to the right of the 'F', and a tad right of the full-advance mark, can stabilize the idle. You'll also get a little more low-end grunt.

Clear as mud?

 

[ninskrillz]

This would be my country boy 'shadetree' test method:

I'd get a timing light, hook it up.
Then remove the round alternator cover, and the points cover.
Fire-up the engine, get it to idle.
Flash the timing light at the rotor to see the idling mark.
Rev the engine and watch the timing light marks.
Look for smooth timing transition from idle mark(s) to full advance.

Then, if idle hangs high:
See if the timing mark is at full advance.
Then (this is the 'shadetree' part), grab the nut on the end of the points cam with my fingertips, add a little drag, and see if the timing retards, and if the idle returns to normal, and stays there.

If it does return to idle rpm and stays there,
And, the timing transition looked to a little too quick,
Then, I would suspect that the advancer has weak springs.


I'd also be looking to see if the timing is correct, for both cylinders. Sometimes, this difficult-to-idle and high-reving condition is caused by too much advanced ignition timing. Setting the timing a little retarded, like at the mark to the right of the 'F', and a tad right of the full-advance mark, can stabilize the idle. You'll also get a little more low-end grunt.

Clear as mud?

I've got a Pamco ignition in it, I'll try timing both cylinders and retarding it a bit. . Thanks for the reply!

 

[pamcopete]

nin...,

Sync the carbs. Stiffen the advance springs.

Idle hover is usually one or both of those two items. If the carbs are not synced, then one of the carbs is right on the edge of working or not working at idle. When it works, the idle increases. When it doesn't work, the idle decreases. Weak springs in the mechanical advance can cause more advance than needed at idle which increases the idle speed under no load.

If you have the PAMCO E-Advancer, then you can eliminate errant advance as a cause because the E-Advancer is not capable of producing an advance that does not match the engine speed.

 

[ninskrillz]

nin...,

Sync the carbs. Stiffen the advance springs.

Idle hover is usually one or both of those two items. If the carbs are not synced, then one of the carbs is right on the edge of working or not working at idle. When it works, the idle increases. When it doesn't work, the idle decreases. Weak springs in the mechanical advance can cause more advance than needed at idle which increases the idle speed under no load.

If you have the PAMCO E-Advancer, then you can eliminate errant advance as a cause because the E-Advancer is not capable of producing an advance that does not match the engine speed.

Thanks for the reply Pete, when you say sync the carbs you mean idle screws mixture screws or both? They have been synched numerous times so I suppose I'll try advance springs next. Are they able to be sourced locally or can they only be found online?

edit: just realized you said stiffen the springs, ill try taking a loop out of one of the springs at a time and see if that helps

 

[ninskrillz]

Okie dokie,
I took out one of the springs out of the advance and clipped about one loop off of the spring. It seemed to help. So afterwards I went to ensure the mixture screws and idle screws were at least in sync. Bottomed out both air screws and let them out 3/4 of a turn. Used a 13/32 drill bit to ensure both idle screws are synchronized.

Start the bike and it is idling ok at 1k. The first cylinder is firing harder and more sporadically than the second. The second cylinder is firing at a steady tempo. From the smell of the exhaust the second cylinder seems fine, and the first firing a bit rich. I turn out the left mix screw a quarter turn to lean it out a bit and hit the throttle and the thing skyrockets again. Not sure if these two events correlate or not.

Could use some more input!

 

[ninskrillz]

I forgot to add the left cylinder seems to be coughing air back out of the air filter. Weird.

 

[TwoManyXS1Bs]

You lost me with the first/second transition to left/right...

 

[ninskrillz]

You lost me with the first/second transition to left/right...

Derp I thought the left was considered number 1 and the right number 2 maybe I was just making that up lol

 

[TwoManyXS1Bs]

Heard them called both ways, but 'left' and 'right' works for me.
Sounds like a problem with the left carb.
A simple (?) test is to switch carbs, see if the problem follows...

 

[girard]

I was having idleing issues, found it to be a air leak on the vac barb on the carb holder. check for air issues?

 

[fredintoon]

Heard them called both ways, but 'left' and 'right' works for me. - - -

Hi 2many,
sometimes left & right don't work neither,
"No, I meant the left one when I'm in front of the bike looking back."
To be sure that what the reader understands is what the writer meant, go the navy/airforce way:-
Use port, starboard, fore and aft.
Dunno if the port cylinder on a parallel twin is #1 or #2 although on V-twins the aft cylinder is #1.
Except on Guzzis, eh?

 

[weaselbeak]

Sounds like air leaks to me.

 

[pamcopete]

The left side is where the left turn signal is. The right side is where the right turn signal is. The front is where you are going, the back is where you have been.

 

[Royboy]

The left side is where the left turn signal is. The right side is where the right turn signal is. The front is where you are going, the back is where you have been.

Now that's funny!

roy

 

[ninskrillz]

jammed up the advance mechanism to disable it, and it fired up and once I synched the idle it idled pretty good. i took the filters off because one of them caught on fire and shmelted. still a slight backfire in both cylinders with filters off, im hoping once i get the filters back on it'll richen up a bit and stop backfiring, ill keep yall posted!

 

[hooser]

Syncing the carbs is more the adjusting the air screw.. remove airfilters.. turn throttle open and put your fingering one side until the bottom of the side is at the top of the cutout opening.. both side should get there at the same time.. adjust the adjusting screws on top of the carb until the match.... once that is down now adjust the idle screws.. with finger on one side watch the other.. when you turn the throttle they should start to open at the same time.. adjust until they do.. from now on adjust the idle screw the same amount until it idles where you what it.

 

[ninskrillz]

Syncing the carbs is more the adjusting the air screw.. remove airfilters.. turn throttle open and put your fingering one side until the bottom of the side is at the top of the cutout opening.. both side should get there at the same time.. adjust the adjusting screws on top of the carb until the match.... once that is down now adjust the idle screws.. with finger on one side watch the other.. when you turn the throttle they should start to open at the same time.. adjust until they do.. from now on adjust the idle screw the same amount until it idles where you what it.

Ok. The slide lift was already set and is spot on. I use the drill bit method on the idle screws.

 

[hooser]

I saw you had 25 pj, what is you needle..depending you may need to go higher on your pj. what does it do when you blip the throttle. If it stutters or wants to die go up on the pj..

 

[ninskrillz]

I saw you had 25 pj, what is you needle..depending you may need to go higher on your pj. what does it do when you blip the throttle. If it stutters or wants to die go up on the pj..

the needle is a 6f9, ill try a higher PJ after I test everything with pods.