So I have this 488 (87 Indy trail) that will not hold an idle for anything. I cleaned the carbs, set the main needle in the middle (it was one up before), set the idle mixture all over the place and I still can't get it to idle. It starts ok, runs at 3 grand and then slowly steps down to a point where I have to feather it to keep it going, and then it is boggy to the point where it will not contine to run unless it is over 3 grand again. So....I assume it is starving, right? I have done all the normal leak checks, compression is good....should I be looking to the fuel pump?
What are the PROPER settings for the idle mixture? 1.5 turns out?

Thanks...getting perplexed here.....

i would say maybe the air screw because that contols the ratio of air to fuel over all temp ranges

Seems quite a bit more complex then that... I am wondering if someone modded the jets to non stock at some point and it is the problem. I have 125 lbs compression, no seal leaks, I have cleaned the carbs six million times (boy, are they clean! ).....I finally found that if I put the needle jet in the furthermost up position, dial the low speed mixture to about 3/4 of a turn out, it will at least try to idle around 2400. Argh. Driving me nutso........

Hey, will other Mikunis readily fit, like the 34mm ones from an xc 440? anyone got some spares that still have the proper jetting for this motor? cheap? LOL!

crank seals

Sprayed em, sprayed the intake boots too, and no change at all in idle speed......

When I had a 488, it was 2 1/2 turns on the idle screw, and 1 turn on the air screw.
Adjust the top of the carbs so both floats have no tension on them, then slowly start to turn the top of the each cable until the float just starts to move, make sure to do this to both carbs. Make sure both floats are moving and travling the same distance inside the carb.

Small crack in the fuel pump diaphram could do it.. Pull the vacuum line from the engine to the block and see if you are getting fuel in it.. Then start with checking the resistance on your plug caps - better yet just toss a set of new caps on.. Make sure your plugs are good and gapped correct. Is that engine reed valved, if it is you might check your reeds.. Also, when you say you have cleaned your carbs are you saying you have wired out all the passages?? On the inside of the carb throat (engine side) there are two tiny holes that are the idle circuit.. Both have to be open.. Also, what kind of play do you have in your carb slides and the area that your needle jet slides into?? Those brass parts will oval over time and cause xtra fuel to pass.. Hows your float height set?? Do you have a line guage to use or are you guessing at float height? Seeping needle and seats can also cause REAL funky idling as engine loads up from xtra fuel in bowl...
Now,, what you are describing does sound like a tired engine. Do a comp check both cold and hot, open the throttle, pull the same amount of times, check both sides against each other... Take your pipe off and take a peek at the exhaust side of the pistons thru the ports, look closely for stuck rings or scrubbed pistons.. As I recall (been a while since I have done one of those) those engines were iron bore and can be rebuilt real cheap if you catch em before they shatter a piston and take out the bottom end.. If it hasnt been freshend up in a while it wouldnt be a bad idea to do so..
Ripperrrrrrr

Slides.

Not a fool proof test,... how do you spray behind the flywheel while it's running?,... remove the air box, warm up engine, place your palm over each carb, one carb at a time, RPM's should fall off or engine should kill, your hand should be wet with fuel,.. if engine speeds up or no RPM change, it's leaking somewhere, I have seen this dozens of time on my customers sleds where they had previously "sprayed" the seals and carb boots. It's unlikely to be a fuel delivery issue to the carbs, as you stated it runs ok at above 3000 RPM's, how can it be running out of fuel under 3000, but ok above 3000 RPM's? That's what happens when vacuum is low, it doesn't have enough vacuum at low RPM (air leaks) but as piston speed increases it is able to pull fuel from the float bowls through the carbs circuits. How often have your seals been replaced, it's a 21 year old sled, seals rarely last that long. You can also pressurize your crankcase to see if it's holding pressure.



Don

Small crack in the fuel pump diaphram could do it.. Pull the vacuum line from the engine to the block and see if you are getting fuel in it.. Then start with checking the resistance on your plug caps - better yet just toss a set of new caps on.. Make sure your plugs are good and gapped correct. Is that engine reed valved, if it is you might check your reeds.. Also, when you say you have cleaned your carbs are you saying you have wired out all the passages?? On the inside of the carb throat (engine side) there are two tiny holes that are the idle circuit.. Both have to be open.. Also, what kind of play do you have in your carb slides and the area that your needle jet slides into?? Those brass parts will oval over time and cause xtra fuel to pass.. Hows your float height set?? Do you have a line guage to use or are you guessing at float height? Seeping needle and seats can also cause REAL funky idling as engine loads up from xtra fuel in bowl...
Now,, what you are describing does sound like a tired engine. Do a comp check both cold and hot, open the throttle, pull the same amount of times, check both sides against each other... Take your pipe off and take a peek at the exhaust side of the pistons thru the ports, look closely for stuck rings or scrubbed pistons.. As I recall (been a while since I have done one of those) those engines were iron bore and can be rebuilt real cheap if you catch em before they shatter a piston and take out the bottom end.. If it hasnt been freshend up in a while it wouldnt be a bad idea to do so..


I changed to a new fuel pump....no difference...
I will check the plugs, throw new ones in today...I left em because they were both burning evenly, but I will change em today.
Carbs are so clean they look new...all passages have been cleaned......as for the ovalling issue where the carb needle drops, that is an interesting thought. I so wish I had an extra set lieing around here.....then I could just do a full carb swap and see what happens.
I am guessing on the floats.....is there an easy way to set them? I have the ones that are "afixed" in the bowl on the posts....not the easier ones that come out with the upper carb body. How do I set these ones?
The compression is 120 on the amg side, 125 on the clutch side.....
I will look into the pipe today also.....sigh.......
I do not know what has ben done to this motor, as I just purchased it for my girl to run.......I am pokin in the dark here. I have never freshened up a motor and I am pretty tentative about doing so....

Thanks guys....I will go take a look and see what else I can get going!

When I cover either carb it drops then tries to die out......so I am *ass*uming that there is no air leak??

Thanks!

Look through the carbs venturias to see if you have a cracked piston skirt on the intake side, (unlikely from what you are describing). Check resistance on spark plug caps, 5000 ohms, +/- 10% use a digital volt ohm meter.

New plugs let me take the idle down to around 2400. Stays there no problem, but when I ran it a wee bit it was very boggy in the mid and wouldn't get over 35mph. I stepped the needle jets back to the mid way position and my idle went to hell again.....I am suspecting that the carbs are just plain worn out....does this make sense? Could my needle seats be just plain old worn out?
I cut the plug lines fresh and re-seated the boots......I didn't get to check the resistance yet, I will later today. Thanks....I really appreciate all this!

Here is a real easy one!!

Check your choke assembly. Mine was doing the samething and when I took the carbs apart I found the choke plunger (don't know if that is the right word) was gummed up and stuck in the carb. It is the little black plastic part at the end of the choke cable. I cleaned that off really well and problem solved. Esstentially it is flooding out.