Low Idle Indy 400 - HCS Snowmobile Forums

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Old 12-25-2016, 05:18 AM   #1 (permalink)
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Default Low Idle Indy 400

I recently bought a 1989 Indy 400 and cannot figure out the idle. I went through the sled and cleaned the carbs(twice), synced the carbs and checked the whole engine for air leaks. I also adjusted the choke cables without any change.The problem is the engine will idle for 10 to 15 seconds at 21-2200 rpms and then drops down to 1100rpms. I turned the idle screws up on the carbs and it will idle consistent at anything over 2200 but if I try to go down to factory setting of 1900rpms then it will just bog down to 1100rpms. Also if I adjust the air mixture screw on the carbs it seems to have no effect on how the motor idles at all. When I cleaned the carbs I used carb cleaner and blew through all jets and ports with air. If I had to guess I would say it's running rich because when I choke it just a little at idle it instantly dies. Running out of ideas, was thinking maybe small hole in fuel pump diaphragm, allowing more fuel in crankcase? The sled runs excellent other than at idle, anyone have any ideas?


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Old 12-25-2016, 09:07 AM   #2 (permalink)
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Assume you have tweaked the air screws on the carbs but still no change? You'll know it if the fuel pump fails.
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Old 12-25-2016, 02:45 PM   #3 (permalink)
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Yes I tweaked the air mixture screws with no change at all when adjusting the screws.


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Old 12-25-2016, 05:31 PM   #4 (permalink)
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Double check the pilots in the carbs, they still can be obstructed and need a better cleaning. Make sure the carbs are properly synced...maybe the throttle cable for the carb slides is to tight not letting the slides fall far enough down. These are really some engines to tune. These sleds are notorious for bad crank seals in the day depending how well stored.
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Old 12-28-2016, 09:46 PM   #5 (permalink)
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Make sure the engine is well warmed up before changing idle settings. 2200 rpm is fine on that engine. If it idles at 2200 rpm when cold then as warms up drops to 1100 rpm the air screw needs to come out as it's running rich after the engine is warm. 1-1/2 from lightly seated is stock. Turn air screw on side of carb out 1/4 turn at a time to see if idle comes back up when warm. When the idle is rich you can turn the idle way up and it will still drop down. Set slide gap to .210"/5.3mm and start over.
A little help from the service manual:

4. Place the drill gauge beneath the slide valve cutaway as shown in the illustration above.
5. Turn the idle screw out until there is a slight drag on the gauge. NOTE: The drill gauge must be in the center of
the carb bore. This indicates proper throttle gap. Repeat this procedure on all cylinders.
6. Hold the throttle cable junction block in the same location as noted in step #1. NOTE: This is very important,
as the synchronization changes if the throttle cable or junction block are improperly positioned.
7. With drill gauge installed under throttle slide as shown, turn cable adjuster out (up) until the gauge starts to
move. NOTE: The drill gauge must be centered in the carb bore.
8. Turn the cable adjuster clockwise (down) 1.3 turn. Repeat for all cylinders.
9. The throttle slides must raise from the idle pOSition in unison. Check by visually observing the drill gauge, noting
and comparing while lightly depressing the throttle.
NOTE: Check throttle lever freeplay. Throttle lever to throttle block clearance should be between 0.010" and
0.030". If adjustment is required, the throttle cable adjusters must be turned equally to maintain synchronization.
Tighten jam nut while holding cable adjusters.
1 O. Check oil pump adjustment.
11. Reinstall air box and correctly position throttle cable and junction block. Start engine and adjust idle stop
screws evenly until proper idle RPM is achieved. (See chart above.) Re-check throttle lever freeplay and
reset by turning cable adjusters evenly. Re-tighten cable adjuster jam nuts.
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Old 12-29-2016, 01:04 AM   #6 (permalink)
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I double checked the pilots, actually put new ones in both carbs, new needle and seat in each carb and rebuilt the fuel pump with a rebuild kit. Checked the choke plungers, seem to be closing all the way and the rubber is pliable yet. I re synced the carbs with the same issues, no change no matter how far out or in I have the air screw. The only time i see a difference is if I lightly seat the air screw then the sled wants to die. Running out of ideas, seems though that the sled does run good cold and when it warms up it's more likely to bog down to 1100rpms at idle.


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Old 12-29-2016, 06:39 AM   #7 (permalink)
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Quote:
Originally Posted by nater660 View Post
I double checked the pilots, actually put new ones in both carbs, new needle and seat in each carb and rebuilt the fuel pump with a rebuild kit. Checked the choke plungers, seem to be closing all the way and the rubber is pliable yet. I re synced the carbs with the same issues, no change no matter how far out or in I have the air screw. The only time i see a difference is if I lightly seat the air screw then the sled wants to die. Running out of ideas, seems though that the sled does run good cold and when it warms up it's more likely to bog down to 1100rpms at idle.


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compression down some? if it is it will show up on a warm motor. perform a cold compression test. remove both spark plugs, kill switch down or off, ( no spark) three strong pulls. the first pull should show 90 p.s.i. on a stock motor? and 120 p.s.i. after three pulls? if not you need to over bore and rebuild the motor first. these are iron lined cylinders so they wear out over time and mileage. they rust in the off season. then you remove that rust when you start using it. 30 + years of this and you see where this is going?

i worked for both polaris and ski-doo the 90 p.s.i. first pull shows the health of the motor with out doing a leak down test. poor mans way of doing this. it works trust me. and both use this method as a gauge of motor health.
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Old 12-29-2016, 07:15 AM   #8 (permalink)
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Quote:
Originally Posted by nater660 View Post
I recently bought a 1989 Indy 400 and cannot figure out the idle. I went through the sled and cleaned the carbs(twice), synced the carbs and checked the whole engine for air leaks. I also adjusted the choke cables without any change.The problem is the engine will idle for 10 to 15 seconds at 21-2200 rpms and then drops down to 1100rpms. I turned the idle screws up on the carbs and it will idle consistent at anything over 2200 but if I try to go down to factory setting of 1900rpms then it will just bog down to 1100rpms. Also if I adjust the air mixture screw on the carbs it seems to have no effect on how the motor idles at all. When I cleaned the carbs I used carb cleaner and blew through all jets and ports with air. If I had to guess I would say it's running rich because when I choke it just a little at idle it instantly dies. Running out of ideas, was thinking maybe small hole in fuel pump diaphragm, allowing more fuel in crankcase? The sled runs excellent other than at idle, anyone have any ideas?


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This tells me that the chokes may be slightly leaking all the time. I'd replace the ends just as a way of seeing if this is true.

Another possibility is that the float needles/seats are not the right ones and are affecting float levels or are the wrong hole size which affects fuel level. Should be 1.5 mm. Viton tipped if possible.

As Rick says though, check compression and also look for a broken piston skirt on the intake side.
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Old 12-29-2016, 09:07 AM   #9 (permalink)
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Current Sled(s): 05 600 xcsp 50th
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18-19 Mileage not much
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Age: 6
Riding Since: 70 s
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I've had issues with chokes on those older sleds, if the ends are grooved out where they seat in the carb, they will leak making the motor run rich...i seen them eventually cause a plug to foul. Pistons are single ring type....seems their more prone to failing than the 2 ring ones.
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Old 12-29-2016, 09:48 PM   #10 (permalink)
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If everything is working as it should it's a fine balance between idle adjustment and the air screw. Adjust warm. The slides are generally .5~1mm higher than the book to reach best idle without too much hang and/or too much fuel. (hang is generally to lean on the air screw) In a perfect world you would turn the idle air screw 1.5 turns from seated and adjust the slide to 5.3mm for this year and engine and it would idle perfectly no hanging or drooping to 1200 rpm, etc. It never works like that. You need to find the happy middle.
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