'76 400 Coughs and pops

[autotech57068]

Recently bought a 76 400. Didn't run when I bought it, no spark at either plug. Put a new CDI box and have good spark now. Now here is my problem. It fires up, sometimes needing the throttle squeezed to start, but runs very poorly. Will not get enough rpms to engage clutch and move machine even with wide open throttle. Will not run with the choke on though. Plugs are always wet when I pull them out. I have good pump impulse to the carb and fuel is circulating in the lines so i am assuming the PTO side case is sealed good, tried to spray carb cleaner in by the mag side seal, seemed ok, kinda hard to know for sure the carb cleaner made it that far in. It usually stops running when it gives a pop out the exhaust, kinda sounds like a shotgun. I have pulled the Walboro carb and pulled the stack of plates apart. Everything looked good and clean and in good condition. Reinstalled and set both needles to what the engine cover said to. It has a WRA-31, is there a rebuild kit for this carb? Looking for any input from someone who has some experience with these engines and can point me in the right direction. Thanks

[400brian]

The WRA-31 carb was used from '74 on. In my opinion they are a decent carb.
Gasket and diaphragm kits are still available. Here is one source: http://www.mfgsupply.com/snowmobile/sno ... 7-410.html

Diagnosis from afar is often tough, hell, diagnosis of the one in the garage is often tough.

My first impression is that you are either getting way to much fuel, or the spark is not as good as you think.

The Prestolite stator possibly could have bad high speed windings.

The carb may need the metering lever adjusted.

[autotech57068]

Is it possible the windings could be bad but still ohm good? If the high speed windings were bad wouldn't it idle ok? I did eyeball the needle/float adjustment and moved it down a hair with no change. Yea I am a auto mechanic during my day job and get people that call and think I can pinpoint there problem. Just pick the collective brain here for some new perspectives on this.

[harleysportster]

Autotech,
Here is info you should check out to understand the Prestolite CDI system.
http://www.green-sleds.com/community/vi ... e4437c6cb9
Diagnostic info as well.
http://www.green-sleds.com/community/vi ... e4437c6cb9

[autotech57068]

OK, I understand the theory of this system. I ohmed the triggers and the high and low speed coil.
here is what I got

#1 and #2 triggers: 31ohms
low speed coil: 2.6k ohms
high speed coil: 83 ohms

With the high speed being 6 ohms out of spec will that cause this problem? Have not pulled the flywheel off and did a visual, did get it to idle for 3-4 minutes, now my coils are this...starting to think the coils are going bad...


low speed coil: 2.7k ohms
high speed coil: 87 ohms

[harleysportster]

The trigger coils should be 30 ohms
Low speed coil should be 2.7K ohms
High speed coil should be 76 ohms
All these measurements have a 10% allowance.
Your measurements are fine, I would think your issues are still with the carb. There are many tiny passages that must be spotless.
Did you check compression? You need at least 100 lbs.
I would also consider replacing the crank seals.

[JDT]

Dustin where are you on this one?

I would be renewing the cranks seals even if you think they are OK.

Sparkplug wires could be another problem area.

Brian I just saw the picture of your machine. Looks pretty nice.

I am up in Milbank.

[JD600]

Sorry to disappoint Todd. I was more focused on what could be wrong with the Walbro... You know how much they intrigue me....

DE

[autotech57068]

Did a cold engine compression check here, wide open throttle and a good pull from the rope gets both cylinders in the 130-135 psi area. I think I am going to get a new carb kit, 07-410, and give the carb a good bath. May as well put some crank seals in, will have to read up on that in the manual. Have a 78 yamaha 250 enticer I rebuilt last winter and swapped the keihin out for a vm34 mikuni, half tempted to do that on this 400. My dad has another 76 400 which is only about 30 serial # down the assembly line from mine. Has been sitting in back corner of machine shed for 16 years that needs engine help since it was run on straight gas that one time ....

[harleysportster]

You have great compression. You can switch over to a VM34 but then you get into trial & error jetting it. The Throttle lever will need changed as well to a later Cyclone style. Once you get the Walbro clean, it should work well.

[400brian]

You may have to set the metering lever even lower than the book says. I know its a lot of messing around, but you may have to do a bit of trial and error. I suspect the aftermarket diaphragms are a bit different than the originals.
You may have to go so far as set the lever below the body of the carb...whatever works.

[autotech57068]

The more I think about it the less I think its fuel/carb problem. Just seems ignition to me. I tried pinching off the fuel feed line, wouldn't it lean out for a few seconds before it runs out of fuel and run good enough and get enough rpm to start engaging the clutch? When it runs it will sit there and idle for a few minutes but if you throttle up rpms hit a wall and rpm's will not build enough to wake up the clutch. I picked up the CDI box from
http://www.psep.biz/store/john_deere_sn ... _boxes.htm Part number is 11-1332 on a Sport Parts Inc box

Could the new CDI box be faulty? Going to pull it apart and take a visual of the stator today, have to check my lighting coil anyway....

[JD600]

I'm thinking you have a pressure leak in your crank case. My next step would be to pressure test the engine. If you are going to pull the flywheel, it would be best to change the seal while you're in there.

DE

[400brian]

wouldn't it lean out for a few seconds before it runs out of fuel and run good enough and get enough rpm to start engaging the clutch?

You would think so.

[autotech57068]

Seals are good. Going to go find the parts sled in the back of the shed this weekend...swap the cdi first and go from there.