1997 sportster runs well for 15-20 minutes then surges.

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jwill19028

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Good Evening,

I have a been working on a 1997 sportster for the past few months and have been recently testing it on the water. The carburetors are rebuilt and I am having problems with surging. I had a day where it ran fantastic (ran it for about an hour cruising around a lake) but the next day it kept surging and dying. I took apart the fuel system, (I don’t have any grey lines) made sure everything was clear, took apart the fuel selector, cleaned it out. I had the same problems with surging again. I then bypassed the entire fuel system today (including the selector) and ran it in the water off of a clean 5 gallon fuel tank. It ran well for about 15 minutes then began to surge again. While it was running well, I did a spark plug test, wide open throttle for about 2 minutes then pull power to idle and immediately shut off the engine. When I pulled the plugs it looked like it was running a bit lean (I’ll include a photo). Any thoughts on what my next step should be? Thanks!
 

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Check your exhaust connections - I had a similar issue and the purple muffler part (see below - #27) wasn't tightly connected to the exhaust (#29), apparently without proper back pressure you will get the seeking...

Worked for me...

1661796665346.png1661796665346.png
 

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Good Evening,

I have a been working on a 1997 sportster for the past few months and have been recently testing it on the water. The carburetors are rebuilt and I am having problems with surging. I had a day where it ran fantastic (ran it for about an hour cruising around a lake) but the next day it kept surging and dying. I took apart the fuel system, (I don’t have any grey lines) made sure everything was clear, took apart the fuel selector, cleaned it out. I had the same problems with surging again. I then bypassed the entire fuel system today (including the selector) and ran it in the water off of a clean 5 gallon fuel tank. It ran well for about 15 minutes then began to surge again. While it was running well, I did a spark plug test, wide open throttle for about 2 minutes then pull power to idle and immediately shut off the engine. When I pulled the plugs it looked like it was running a bit lean (I’ll include a photo). Any thoughts on what my next step should be? Thanks!
Thank you for taking the time to include a diagram! I’ll take a look at it.
 
I have to agree with this. Based on my experience a few days ago where that #29 rubber hose melted a big ole hole in it because the water regulator hose was clamped, I had very poor, surging, bogging and sluggish engine performance trying to get back to the dock before I sunk. I made it, but the engine was not please with this experience. Very grumpy.

So yes, those need to be sealed well or you'll have awful performance.

The surging is interesting, as though it's getting too much air and not enough fuel. Perhaps you have a blockage that only manifests itself after running for a bit causing it to starve for fuel. Looking at the plugs, they look like you are in fact running lean. Those should be a darker chocolate brown I believe.

What I just did was readjust all my settings. I have a different thread I'll post here with my procedure to get the linkage, jets, idle and oil pump set properly. I'll post the link here. I'd suggest going through your throttle and carb adjustments to dial them in. You don't want to run lean because you'll fry the engine.

You may also want to pull the fuel selector and inspect that it isn't blocked. I'm sure you've checked but make sure you don't have the old gray temp fuel hoses either.
 
You might also consider an air leak. As the the motor heats up and parts expand it can aggravate weak spots in gaskets and seals. Next time it is causing you troubles Maybe shoot a little starting fluid around and see if you can induce a surge. Just a thought.
 
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