07-02-2013, 07:16 AM #1
Runaway Lean when revving over 4000 - 5000 rpms.
Helping a friend with an XR1800 (66v 1200r engine) and I just installed a new SBT rebuilt motor. I also rebuilt the carbs (aftermarket kit). I left the stock spring, needle & seat and main jet. I changed the low jet to 100 pilot. Installed D plate and i am leaving the stock air box.
When we rev the engine over 4000 to 5000 rpms, it keeps revving high. I pull the spark plug wires and turn off the engine and it keeps running. When i adjust the idle screw down, it then shuts off. Searching around, it sounds like a idle screw issue. When idling, it sounds like she wants to shut off when the idle screw is turned out.
Any thoughts where I should start looking?
07-02-2013, 09:14 AM #2
- Join Date
- May 2008
- Elephant Butte New Mexico
should have changed the spings, needles and seats and tested popoff. Why bother "rebuilding" the carbs and leaving three high wear items in place from an engine that blew?
you certainly have some fuel restriction going on, find the lean condition before you need to take advantage of the sbt warranty.
07-02-2013, 11:01 AM #3
Engine blew from oil line failure. Needle and seat looked good (no wear indication around the tip). Pop-off was about 62 with 1.2 n/s and gold spring.
07-02-2013, 12:52 PM #4
- Join Date
- Aug 2006
Honestly alot of people think they blew or melted their engine from an oil line popping off when they really burned it up from an air leak or other fuel issue, then when they pulled the airbox off they dislodged the oil line because they come off so easy. Then they wonder why their freshly rebuilt engine burns up again. Just saying, I have knocked the rear oil line off on a stock boat I was working on several times. Probably need to verify the fuel pump check valves operation, fuel tank flapper valve and reserve line length, and other fuel or air leak possibilities.
07-02-2013, 01:44 PM #5
Wondering if it's a reed issue? Could you preform a pressure test through the vacuum fitting to verify any leaks?
07-06-2013, 06:21 PM #6
May have found the culprit. There was water dripping from the exhaust manifold where it meets the middle cylinder. Ran out of time before I could dig in further.
07-07-2013, 03:13 PM #7
- Join Date
- Jun 2011
hence a quick lesson in how a diesel engine works
you might be on to something with the water leak on the exhaust side, but just becuase water is seeping from a cooling passage dosnt mean that you have a air leak going in. Either way, it needs to be fixed, but it sounds like you are running out of fuel at high demand/have a pretty major air leak that is causing the engine to diesel when it gets hot enough and run away. turning the idle screw down is just closing the butterflies and suffocating the engine, stopping the dieseling process.
seal the engine off and do a leak down, i bet you find an air leak somewhere on it.
07-08-2013, 08:32 PM #8
Ok, swapped the carb rack from the other motor and now it runs good. Put this carb rack and reeds on the opposite motor and it does the runaway lean. It's definitely a carb issue. Reeds looked identical to the ones I removed from the other motor. What should I look for? Needle and seat? I rebuilt the carbs with new aftermarket kits. N/S remained with stock spring, they looked good.
07-11-2013, 08:18 PM #9
Alright, so I removed the rack again and went thru every carb. Check the needle, pop off was about 65 on all three. Cleaned them again and reinstalled. Same problem. Ugh! So I did the lean runaway test, pulled the lanyard and hit the throttle. The first plug was hot and gray (other two were moist and Carmel colored), so i assume the issue is with the first carb. Should I reinspect the fuel pump on the carb? Possible reed issue? I am almost at the point I would rather replace the whole carb and reed assembly!
Any help would be much appreciated.
07-11-2013, 09:08 PM #10
You are lean, lean...and possibly an air leak in the carburetion chain "carbs to mani, mani to lower case" or possibly an engine leak. Check your gaskets and if they have ANY tears or torn areas, replace them. Here's good test to see if you are getting fuel to the chambers. Pull the spark plugs and remove the stock air cleaner or whatever A/C you have so you can look down into the throat of the carb. Take a clean rag and put it over one carb at a time..turn the ski over with start button, look inside the carb to see if fuel is shooting into carb throat..you cover the carb, turn it over and then immediately look into carb to see if there is fuel going in. If there is a carb that no fuel is going into, you might have a clogged jet or some restriction in that carb...you might to do a complete leak down test to see if you have leaks in the intake manifold or carb gaskets or even a case/cylinder leak. Reeds won't cause a runaway unless they are just wrecked or broken beyond repair. But you have a lean condition. Check fuel lines to make sure they are not reversed, are pulse lines attached? This is definitely a fuel issue. Now you're gonna have to locate it.
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