HELP! Nomad Overheating and Backfiring.

[sfisher59]

Hi All,
I'm really pulling my hair out on this problem. I've made some changes that have incrementally helped, but the problem still occurs.
My Nomad is a '99, completely stock. It had a problem of running lean at idle (i.e. I needed to keep the choke on for a couple miles), then about 8-10 miles out, the overheat light comes on, I felt a momentary drag, and then the backfire. If I roll off and ride slow (55-60), I can go probably 10-12 miles, before it acts up.
I have changed the pilot jet from a 45 to a 48, made sure the rubber oring/washer/spring was in the right place. The main jet is a 42 (original). I reset the float, as it looked like someone set it too low. It now rests on the needle with the float arm parallel to the carb body. The pilot air screw is at 1 5/8ths turn.
I want to go to 2 turns with the pilot air screw (with the larger pilot jet).
The thermostat seems to be working normally, and I've flushed, and refilled with fresh coolant. The coolant is flowing, and the fan cycles normally, and I checked the thermostat last night. It starts to open about 155 to 160, and is full open by 180.
What am I missing?

[cactusjack]

Misunderstood was having a similar issue, hopefully he'll see this and chip in his 2 bananas worth.

[blowndodge]

If your bike is stock with no air or exhaust and this just started then I would suspect an air leak around the intake manifold. this would cause overheating and a backfiring lean running motor.

Crap in the fuel tank could be plugging up your jets in the carbs but you've dismantled them and changed the associated jets so if it was dirty in there it should be clean by now. Air leak somewhere is my best guess.

Yes, quite likely a leak where the carb bolts to the manifold or where the vacuum hose attaches.

I usually look for air leaks (also called vacuum leaks) by one of two methods. You can spray WD-40 around the suspected leak, or shoot propane at it from an un-lit propane torch. If the extra fuel gets sucked into the engine the RPM will change for a second and you'll hear it. Bad vacuum hoses are also a common source of these leaks. Look for splits and cracked or swollen ends.

There could also be a split diaphragm in the vacuum dashpot, which is above the front cylinder.

[sfisher59]

Thanks for the ideas. I already have the tank, fuel pump , and upper airbox off, to get to the thermostat. I might as well keep going and get a look at the manifold. I should probably just take it off, and replace the seals if I can't find anything else obvious.

[misunderstood]

Originally Posted by sfisher59

Thanks for the ideas. I already have the tank, fuel pump , and upper airbox off, to get to the thermostat. I might as well keep going and get a look at the manifold. I should probably just take it off, and replace the seals if I can't find anything else obvious.

My issues were from a radiator that wasn't flowing well and overheating to the point it cracked the carb and was getting water in the fuel which was when it completely died and a mechanic friend isolated the issues. Also make sure you have the right heat range plugs. Mechanic changed mine to proper ones, but I don't remember what they were. Should be able to look that up. Sounds like you cleared all the heat issues I had already. I'd follow through with the suggestions from CQ and BD. Wish I could help more, but I'm sure the more able minds will help you see this through. Keep us posted.

[macmac]

Just to add under over heating only... Use the back of your hand to feel around the rad when it is hot.. You are seeking cold spots and probably low on the rad... I hope you don't find any....

I think BD has it over all...

[sfisher59]

I may have found the leak. I got the carburetor and manifold off, and it looks like the o'rings have shrunk and flattened out. See the photo's. Of course, the local dealer has none in stock, and can't get them till Wed or Thurs. I might try to find some high temp ones at work on Monday. Otherwise, I'll have to wait for the dealer to get them in stock.

[sfisher59]

Hey Guys,

I finally got around to installing new o'rings on the manifold. I used a bed of red silicone and placed the orings in it, and put a faying seal of silicone on the manifold and installed it. I let the silicone dry 24 hours before reassembly, and removed the excess from the inside, and outside of the manifold.

I also installed the blocking plates, and removed all the hoses valves, and plugged the carbureutor vacuum point.

2 turns out with a 48 pilot, stock 42 main jet. New NGK plugs, one heat range colder (the closest my parts store had on hand), and it works well. No backfiring at all, or stumbling. My overheat light comes on running 55-60. Doesn't seen to come on running faster, or slower than that. I have to put on some more miles to verify this.

I'm thinking of adding a thin washer under the needle, if it's possible, though probably over the winter or before next spring.
There's no clips, so I think it's a stock needle.

What do you guys think?

[Cajunrider]

I don't know if you guys have ever tried this for locating vacuum leaks but it works very well on industrial surface condensers in our plant. Just regular old cheap shaving cream. Spray it on where leaks are suspected and it sucks right in. The exterior foam all dissipates in a matter of a few minutes. Surely wouldn't be a good idea around a gasoline engine intake or carburetor though. Just a suggestion for any leaks where this may apply.

[sfisher59]

Hi All,
Just wanted to post a final reply in the hopes that it will help someone else.

The cure for me was to replace the fan switch. I bought a cheap ($13) infrared thermometer off ebay, and measured temps across the radiator.

I had temps of 235-240 at the top (intake) and exiting the bottom at 215-220.

After replacing the fan switch the temps dropped to approx 220-225 in to 195-205 out. A considerable difference.

My advice: if your overheat light flickers, change the fan switch !

They are about $95 from the dealer.

Hope this helps.

[mobandy78]

Glad you got it figured out! I'm sure others will find this helpful.

[macmac]

3 years it took to get this fixed?????????????

[ringadingh]

You could have bypassed the fan switch and had it run to check it. Im surprised your light would come on at 55mph, usually they heat up at idle on a hot day.

[Dave]

Originally Posted by macmac

3 years it took to get this fixed?????????????

Why rush? :)