'81 450L + Pods + 2-1 exhaust = surging acceleration

[telemaster]

First, I am psyched to be an active member of this forum again. It's been about 6 years since I last posted, and that long since I've ridden my beloved '81 450L. She's back in action and I had forgotten how much I missed her.

Here is the scenario: ~6yrs ago I completed the second version of my 450L. Added pods and a 2-1 exhaust--plus other cosmetic changes. The bike ran fine... no issues except the massive drop in MPG: from 60 to 40... Yikes! Think I over jetted it a bit with my "tuning"??? I put one shim on the needle, went from a 115 main to a 120, and upped the 17.5 pilot jet to the next size up (forget exact size ATM). So, I pulled the plugs and they are wet with gas. Never had a chance to correct the problem then, but acceleration was smooth through all gears and RPM ranges.

Fast forward to now: Dug the bike out of parents garage and moved it to my house. Cleaned it up really well (I had drained the carbs and tank before storing) and got a new battery and plugs. Added fresh gas and on the second attempt she came to life! WooHoo!!!

Next, it was time to address the jetting issue from years past. Pull carbs again and go back to factory settings to see how that works. Removed the needle shim, and replaced original jets. She runs!!!! Time for a test ride up the road. Everything going well, excited for that rush from the acceleration after all those years. I'm on the straight stretch of the road, no cars, now's the time. I crank on the throttle and nothing.... the bike kind of surges forward with intermitten pulses of speed until the RPM range hits ~6K and then she comes to life like I would expect... ripping down the road. When the throttle is pegged, the bike will surge like this in any gear when the RPMs are below 5-6K.

My novice opinion is that she needs more fuel at WOT... the main jet. I am going to replace the 115 OEM main with the 120 I have from the previous jet job and see where that gets me. The goal is to get it to a point for nice smooth acceleration through all gears at all RPM ranges. Then I'll take it my buddy's dyno for the "fine" tuning. Need to get it to run safely for inspection first!

Just wanted to put this scenario out here and see if anyone else has experienced this, and if you think I'm on the right track. What say you?

[7981GS]

Install the factory pilot jet and up the mains to 120's or 122.5's.
Leave the jet needle as is. Report back.

Eric

[telemaster]

Can try the 120s tonight or tomorrow. Just ordered the 122.5s. Update coming.

[telemaster]

I was wrong

Can try the 120s tonight or tomorrow. Just ordered the 122.5s. Update coming.

So I thought I had replaced the 115 mains with 120s.... when I went and looked at them, they are 130s. No wonder I my MPG dropped so much!

Since they're all I have today, I did put them in to see how it would affect the bike's performance. It has a much smoother acceleration in the lower end 2-6K range, but doesn't seem to have the pull it had before. Also, now it pops quite a bit in the 7-9K RPM range... the acceleration lags while it does this too.

My assumption, and correct me if I am wrong, is that it is now getting too much gas and at the higher RPMs is pushing the extra gas through the exhaust where it is combusting and causing backfires?

Waiting for the appropriate sized main jets to arrive in the mail before I can test it again... delays... the delays kill me.

[BassCliff]

Hi,

Have you replaced the intake boot O-rings and ensured the entire air intake system has no leaks? Yes, I know, you're running pod filters. But you don't want air getting in anywhere except through the air filters.


Thank you for your indulgence,

BassCliff

[Dave8338]

So I thought I had replaced the 115 mains with 120s.... when I went and looked at them, they are 130s. No wonder I my MPG dropped so much!

Since they're all I have today, I did put them in to see how it would affect the bike's performance. It has a much smoother acceleration in the lower end 2-6K range, but doesn't seem to have the pull it had before. Also, now it pops quite a bit in the 7-9K RPM range... the acceleration lags while it does this too.

My assumption, and correct me if I am wrong, is that it is now getting too much gas and at the higher RPMs is pushing the extra gas through the exhaust where it is combusting and causing backfires?

Waiting for the appropriate sized main jets to arrive in the mail before I can test it again... delays... the delays kill me.

I think you are spot on with your assumption. The 120~122.5 should put you where you want to be. The 850s come stock with a 120 main if the 122.5 seems to be a bit to much. I'm sure the 120 mains are laying all over the place around this fourm.

[telemaster]

Hi,

Have you replaced the intake boot O-rings and ensured the entire air intake system has no leaks? Yes, I know, you're running pod filters. But you don't want air getting in anywhere except through the air filters.


Thank you for your indulgence,

BassCliff

Good point. These were replaced last time I had it running.... 6 years ago. On visual inspection all looks good, but it doesn't hurt to pull them next time the carbs come off to ensure all is well. O-rings are cheap... Might as well replace them.

I think you are spot on with your assumption. The 120~122.5 should put you where you want to be. The 850s come stock with a 120 main if the 122.5 seems to be a bit to much. I'm sure the 120 mains are laying all over the place around this fourm.

I'm hoping so. Just keeping my fingers crossed I get the 122.5s before the end of the week. I want to get this thing on the road before it's too cold.

On another note, I seem to have misplaced the title in the last 6 years and 7 moves.... Grrr just one more thing to fix.... Dealing with the DMV just isn't as fun as wrenching in the garage.

[telemaster]

Happy Ending

I finally got all the jets I wanted to try. I was just working on the mains for this first round of tuning. Main goal was to get smooth acceleration through all gears/RPM ranges without fouling plugs or over heating (rich/lean).

Got the 122.5 mains.... Put them in and took her for a test ride... More of the same.... just no top end. Not sure what's going on.

Got the 120 mains... Put them in another test ride... really... it's still not running right at all. The wind has really come out of my sails by this point and I'm pretty frustrated.

Checked the intake boot o-rings... even had an extra set in my parts box, so I replaced what appeared to be perfectly good o-rings. Still... no improvement.

Next thing I know she is starting to be a PITA to start. It cranks and cranks and cranks with pops and backfires out the tail pipe. I figure this could be fouled plugs... which might make sense from the all the jetting adjustments I made.

I get new plugs and read a little more carefully in the specs. Spark plug gaps should be 0.6 - 0.8mm NOT thousands (.06-.08)... which I had incorrectly gapped the first time.

What do you know, with the appropriate plug gaps she starts right up as soon as you breath on the start button AND when I took her up the road no more sputtering and backfires... just a smooth acceleration for all gears/RPM ranges.

What do you know... it wasn't the carbs that were the problem after all. It was the spark plug gaps. My bad!

Still need to fine tune her, but at least we are in a decent running condition and I can make the 20 mile ride to my buddy's dyno shop to get some baseline data.

Anyone know what jet(s) affect which carb circuit? I can't seem to find any data on tuning these old BS34 Mikunis.

My guess is:
Pilot = Idle
Needle = Mid-range
Main = High/WOT

I do have needle shims and other pilot jet sizes, and we can always swap one out at a time and run dyno tests to see where the difference is, but if anyone has already done this, your wisdom would be appreciated.

[pete]

Glad to hear you found the source of your worries!

Have a look here:

http://mikunioz.com/jets.htm

On our BS34 CV carbs, tune the main jet first, then tune the needles, then tune the pilots.

All I can tell you really is that with my setup which is K&N pods and a professionally made 2 into 1 that is much freer flowing than stock, 8 size up mains are too rich...