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Discussion Starter · #1 ·
1994 Thundercat 900

I reconditioned the carburators, syncd the carbs, 1 turn on the pilot, 1/8" gap on the slider, emptyed the old gas and filled it with new 87 octane, installed new spark plugs.

The problem I am having is it will idle, but if I open the throttle it will instantly die. This is of course with the choke off. It seems to rev ok with full choke (3 position), but obviously this isn't right.


Heres the detailed story of my journey:

-In the beginning, it took 100 pulls to get it primed.

-I lost spark for a bit, but then regained it after wiggling the stator clump'o'wires and repositioning some eyelets.

-Then I filled the cylinders one by one with brake clean and ran it on that about 5 times (since it would die in 4 seconds).

-Finally she started to idle on full choke and 2.5 turns on the pilot...if I touched the throttle, it died.

-I returned the pilot to 1 turn and continued to warm up the engine on full choke.

-If I turned off the choke it would slowly die until I choked it again...once again if I touched the throttle, it died.

-So I contiued to mess with the pilots with no headway, and kept them on 1.

-After about 15 minutes of searching here for this bog issue, I found nothing and started the sled up again in full choke, now it was revving and engaging the clutch. It would rev a couple times with no choke, but die sometimes. It would work on full choke though.

-I unloaded the sled and took it for a spin, it ran ok with full choke, I was able to do some WOT runs with "full power", but if I turned off the choke it will not run.

The only mods to this sled that I can see is an aftermarket can (loud as hell!), I don't know if it has reeds, and for sure it does not have spacers. Any ideas?
 

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are yu running a 35 pilot?? and is this a 38 stock carb??


BM...
 

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I would check your pilot to see what size it is... it seems your to lean, needs more fuel to rev up... I dont think its a stator problem as you can get it to run WOT with the choke on... pull the wire plugs apart at the stator a put dielectric grease in them and check your ground there too, you said you lost spark before... turn your pilot screw out 1/2 turn from seat and see what happens... when you cleaned the carbs were the floats level?? is this the first time this year you run the motor??? there is alot of carb masters here, you'll be burning the snow yet... :thumbsup:


BM...
 

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Discussion Starter · #5 ·
I just bought the sled, but the previous owner had it sitting throughout the winter. I didn't mess with the floats, but they weren't sticking or nothing, and the little fork wasn't bent or nothing.

I can't work on it until Saturday, but I guess I will pick up some larger main jets to play with. All the choke does on these carbs is let in more fuel...right? I'm use to chokes that restrict air flow, but this choke is a tiny plunger in some valve.
 

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for stock jetting for a 900 would be 35 pilots, 440 mains jets Q0 nozzles and clip #3 on needle.... on a 38 carb


BM...
 

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Discussion Starter · #7 ·
<div class='quotetop'>QUOTE (blackmax @ Feb 2 2007, 08:17 AM) <{POST_SNAPBACK}></div>
for stock jetting for a 900 would be 35 pilots, 440 mains jets Q0 nozzles and clip #3 on needle.... on a 38 carb


BM...[/b]
I will have to remove the carbs again and check these settings. I guess I take apart the slide to get to the needle settings, but What is a Q0 nozzle?
 

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Check to make sure that all of your fuel lines are in good shape, I had a crimped line that caused a similar condition and would restrict fuel flow. Found the bad line, replaced and she ran like a top.
 

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When you have the carbs apart to check the jetting make sure you pull out the pilot jet and pilot air screw and flush that circuit with carb clean. Check the pilot jet for lacquer blockage, replace if you can't clean them. Sounds like your starter (choke) and main circuits are functioning correctly but your pilot circuit is plugged. The pilot works from idle to about 1/4 throttle. The 'choke' is really a misrepresentation, you are correct that is does not restrict air flow as is typical in the older 4 stroke vehicles (chevy etc) but adds fuel to richen the mixture. Larger main jets will not fix this problem
 

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It wouldn't surprise me if you have a few issues.

Won't idle with choke off sounds typical of a plugged pilot jet. Larger main jet won't help. You can check the parts fiche here to see which is the pilot jet. The screw you're turning is an air screw and not the pilot jet.
http://www.brownsleisureworld.com/

More importantly get the elctrical solved. As mentioned check the ground coming off the CDI box, wiggle the wire near the eyelet. Also check the eyelet coming out of the stator (magneto) wiring harness as well.

To make sure you don't have a key/kill/tether/throttle safety switch problem you can remove them from the circuit by unplugging the connector coming from the stator and putting a jumper across the 2 non-yellow wires on the stator side of the connection. The other half of this connector goes up to the handle bar switches. You won't have headlights, but the engine should run. If it runs than your coils, CDI or the stator are all good.
 

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Discussion Starter · #11 ·
<div class='quotetop'>QUOTE (Wayne from Pontypool @ Feb 2 2007, 11:18 AM) <{POST_SNAPBACK}></div>
When you have the carbs apart to check the jetting make sure you pull out the pilot jet and pilot air screw and flush that circuit with carb clean. Check the pilot jet for lacquer blockage, replace if you can't clean them. Sounds like your starter (choke) and main circuits are functioning correctly but your pilot circuit is plugged. The pilot works from idle to about 1/4 throttle. The 'choke' is really a misrepresentation, you are correct that is does not restrict air flow as is typical in the older 4 stroke vehicles (chevy etc) but adds fuel to richen the mixture. Larger main jets will not fix this problem[/b]
When I cleaned the carbs I did not remove the pilots, but I did blow full air through them and they did flow. I washed the carbs in an ultrasonic cleaner too. I will have to get those pilots out I suppose and inspect them.
 

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Discussion Starter · #12 ·
I think I fixed the problem...I raised the needles to their highest (richest) setting, she runs without the choke now. Although the plugs didn't show a rich condition, which is making me wonder if something is wrong (assuming I have stock jets installed).
 
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