How to fix lean surge

[KoboldWrangler]

So, got an interesting issue I am trying to tackle. Bike is an '84 xvz1200 with 1300 carbs (1200 carbs were beyond trashed). Got the bike running beautifully, idles smoothly, starts right up, accelerates like a freight train...accept when I am cruising at or above 55mph, I've got a real bad lean surge.

Carbs are clean as a whistle inside, float heights were set dry (28.5mm, and checked alternate height method from top of jet block to float, which is 16mm), idle air screws are 2.75 turns -- each cylinder responds nicely to the idle drop test, diaphragms are not torn, valves adjusted, coils and TCI were replaced with other known good ones, and the tank, filter, and pump are clean and flowing well. 

Seeing as this is only at a small throttle input, should I turn the air screws in or out?

(edit: updated post to reflect me learning of lean surge)

Edited March 26 by KoboldWrangler

[Chaharly]

Hey there! I've seen your bike posted on one of the facebook groups sometimes. Before you get too crazy re-adjusting idle screws etc, did you replace the jets from the 1300 carbs to the 1200 carb jet sizes? Some of the jets are different. There's a chart somewhere on this page that has different jet sizes for different years etc. 

 

If your jet sizes are okay, you may need to turn your pilot screws out an eighth turn at a time until it quits, that will richen the mixture.  

 

Also, in my experience, I believe that I have an air leak at from the boots that go from the airbox to the carburetors on the bottom side. That might be leaning you out as well.

 

Another thing to think about. If you're running stock main jets from your 1200 carbs, your emulsion tubes may be worn out of spec. The needle rubs against them and can oblong them. Maybe you're actually running a little too rich. You can get some smaller thickness washers ( I dont remember the size i replaced mine with) and drop those needles a little more into the emulsion tubes to lean it out and see what you get there.

 

What are you getting for fuel consumption??

[KoboldWrangler]

46 minutes ago, Chaharly said:

Hey there! I've seen your bike posted on one of the facebook groups sometimes. Before you get too crazy re-adjusting idle screws etc, did you replace the jets from the 1300 carbs to the 1200 carb jet sizes? Some of the jets are different. There's a chart somewhere on this page that has different jet sizes for different years etc. 

 

If your jet sizes are okay, you may need to turn your pilot screws out an eighth turn at a time until it quits, that will richen the mixture.  

 

Also, in my experience, I believe that I have an air leak at from the boots that go from the airbox to the carburetors on the bottom side. That might be leaning you out as well.

 

Another thing to think about. If you're running stock main jets from your 1200 carbs, your emulsion tubes may be worn out of spec. The needle rubs against them and can oblong them. Maybe you're actually running a little too rich. You can get some smaller thickness washers ( I dont remember the size i replaced mine with) and drop those needles a little more into the emulsion tubes to lean it out and see what you get there.

 

What are you getting for fuel consumption??

Yep, thats me on there as well. FB is nice for quick responses, but rarely does it ever net a quality response. 

nope, running stock 1300 carbs as the 1200 carbs were beyond trashed. I found this comparison chart, and both the '84 and '86 venture use the same pilot jet (37.5), only the main jet changed. (https://web.archive.org/web/20130904201220/http://www.bergall.org/temp/venture/venture-carb-part-differences.html)

ill check and maybe seal the boots on the air box. i know they can be easily rotated in the box. I dont think itd affect fuel ratio, but its worth checking. When looking up what lean surge is, it matches exactly what my bike is doing -- "Lean surge is typically related to a lean mixture and occurs when cruising along at steady state speeds with small throttle openings. You can actually feel the bike pull and then pause, pull and pause..."

I am leaning towards it being the idle air screws. theyre closer to 3 turns than 2.75, and i know you cant really go much past 4 before they come out.

I have looked at the gunson colortune, but not only are they getting hard to find, they are pricey as well...

not sure on fuel consumption -- i havent really ridden it much as i just got it all together, and the lean surge bothers the shit outta me when im on the expressway.

Edited March 26 by KoboldWrangler

[Chaharly]

Yeah I’d just start with turning the pilots out 1/8 of a turn at a time. When you have time put 120 miles on it and check your mileage.

 

 I had similar symptoms to the lean surge but mine was rich.

 

 Let us Know what you find out!

[KoboldWrangler]

Ugh. Decided to wet check the carbs with them split. Only got one checked so far and it's 20mm below center, (14mm is stock). I made sure to tap on the carb and repeatedly prime the fuel pump to ensure the level is accurate. It's so weird cause the floats are set right at 28.5mm. this would explain why it's running lean I suppose. I'll get them reset and report back.

[Chaharly]

The book says to test the float level while the bike is running and on the center stand. Try that before you tear everything apart!

 

[KoboldWrangler]

On 3/29/2025 at 4:07 PM, Chaharly said:

The book says to test the float level while the bike is running and on the center stand. Try that before you tear everything apart!

 

not for the 1200's. it says to warm up the bike, shut it off then check. regardless, i still did a static wet check. two carbs were smack on, the other two were way off. Fixed them aaaaannnnd....nothing changed. 

 

checked spark plug boots. Found one of the resistors had a carbon track on it, bypassing the whole resistor. Changed the boot aaaaaaaaand....nothing changed. 

 

So, this scenario is very unique to me. I have found the issue for my bike. You see, I wasnt running almost any of the plastics due to them being mostly smashed or cracked, and me wanting to be stupid and do adventure riding with it. Which means the air box is out in the open. From a hunch, while on the highway when it was surging, I decided to cusp the air inlet on the box to help stop the wind shear blowing across it aaaaaaaand...the surging stopped. Removed my hand, surging started again. So, I went back home, reinstalled the fake tank lid, and what do you know, that was it. Turbulent air was causing my lean surge. At least I can ride the bike without the annoying lean surge. 

 

Also threw on the messed up side covers. Makes it look like a big Seca~

[saddlebum]

Try draining the carbs and then close the drains. Remove fuel and vent lines from the carbs shoot carb cleaner in through the supply port then in through the vent or overflow ports. Drain the carbs again and if you have air turn down the pressure right down to 5 psi and with the drains open gently blow into the supply fitting. Close the drains reconnect the hoses and go for a test ride. See if that helps. Saved pulling a few carbs doing this. There is also the hot shot method described, in the tech section, you can try.

Edited Wednesday at 01:59 PM by saddlebum

[KoboldWrangler]

14 minutes ago, saddlebum said:

Try draining the carbs and then close the drains. Remove fuel and vent lines from the carbs shoot carb cleaner in through the supply port then in through the vent or overflow ports. Drain the carbs again and if you have air turn down the pressure right down to 5 psi and with the drains open gently blow into the supply fitting. Close the drains reconnect the hoses and go for a test ride. See if that helps. Saved pulling a few carbs doing this. There is also the hot shot method described, in the tech section, you can try.

i take it you did not read my latest post, eh?