2 into 1 intake problems

fishman

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I made 2 into 1 intake. I'm using a VM36 carb. The bike starts right up, but the left cylinder is overfeuling. I tried the intake on a different motor with same result. Swapped ignition from left to reght -> same result. Made a different design intake -> same result.
Both intake runners are the same length with no sharp bends.

I'm using this setup:
180 main
6f9 needle
25 pilot
2.5 slide
159 p6 needle jet

Does anybody have a clue what is going on?
 

grizld1

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Check compression and valve lash. A dark spark plug doesn't necessarily indicate a rich mixture. Symptoms?
 

fishman

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Check compression and valve lash. A dark spark plug doesn't necessarily indicate a rich mixture. Symptoms?
Comp left 110 right 120. So a little low. Black smoke from left exhaust.
Sparkplug is wet on the left.
 

fishman

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gggGary

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Just a guess, the low side runner is getting raw gas pooling. was the LH the low side runner in both your designs?
That seems to be a common problem on 2 into 1 intakes.
 

grizld1

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In your "other design," were the intake runners equal length as well as level? Don't know how you'd accomplish that. Anyway, you have two things contributing to your problem. First, condensation of fuel at idle and low speed is a chronic problem with single carb conversions. Second, one factor that keeps the fuel atomized is engine vacuum. Lower compression=lower vacuum. I don't know if your compression was measured on a hot engine or a cold one, but here's my personal standard. If cold, 110 psi is marginal. If hot, unacceptable.

Bear a couple of things in mind. First, a 2-1 intake can work just fine on the open road but will never be quite right in urban start-and-stop traffic. Next, the too-fat mix resulting from sucking in badly atomized fuel can cause fuel scouring of the cylinder and rings.

Feel free to yes-but to your heart's content, I have no personal stake in this.
 

fishman

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In your "other design," were the intake runners equal length as well as level? Don't know how you'd accomplish that. Anyway, you have two things contributing to your problem. First, condensation of fuel at idle and low speed is a chronic problem with single carb conversions. Second, one factor that keeps the fuel atomized is engine vacuum. Lower compression=lower vacuum. I don't know if your compression was measured on a hot engine or a cold one, but here's my personal standard. If cold, 110 psi is marginal. If hot, unacceptable.

Bear a couple of things in mind. First, a 2-1 intake can work just fine on the open road but will never be quite right in urban start-and-stop traffic. Next, the too-fat mix resulting from sucking in badly atomized fuel can cause fuel scouring of the cylinder and rings.

Feel free to yes-but to your heart's content, I have no personal stake in this.
Thanks for your answer. Mu other disign was with a short left runner, like most use. Compression was meassured cold. Will do with a hot engine tomorrow.
 

fishman

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Did some testing today. Took the bike for a drive. From 2500 rpm, the bike runs real smooth and pulls great. After a 10 minute drive i measured the temperature of the exhaust. Left is about 15 C colder than right. Took the aparkplugs out, both are dry. Left a little darker than right. Compression was 125 left and 135 right.
IMG_20220708_104857.jpg
 

arcticXS

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Fishman, is your XS stock, or rephased?
I once read that 180 degree twins with a single carb tend to run richer on one cylinder, and leaner on the other, due to the uneven timing of intake events. This may also be the case, to a lesser extent, for a 277 degree twin.
H-D bikes, having a 315/ 405 degree firing order may not have this issue, as they run fine with a single carb. (XR 750 and XR1000 being the dual carb exceptions)
 

fishman

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Fishman, is your XS stock, or rephased?
I once read that 180 degree twins with a single carb tend to run richer on one cylinder, and leaner on the other, due to the uneven timing of intake events. This may also be the case, to a lesser extent, for a 277 degree twin.
H-D bikes, having a 315/ 405 degree firing order may not have this issue, as they run fine with a single carb. (XR 750 and XR1000 being the dual carb exceptions)
It's stock.
 

X77S

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These engines are 40ish years old. Buy a good set or two of regular spark plugs. Iridium plugs seem a waste in these motors.
 

fishman

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Removed the carb after idling. There was condensed fuel in the inlet. Since the left runner is the lower 1, i guess that one aucks up all the condensed fuel. So grizld1, i think you were right!
For now, back to the BS38's!
 
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