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Buell Motorcycle Forum » Old School Buell » Archive through October 21, 2007 » Jetting Question for anyone with a 1250 + Mikuni « Previous Next »

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Nocompromise
Posted on Tuesday, October 09, 2007 - 01:31 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

So, here is my setup: 1250 kit, Mikuni HSR42, 536 cams, Stage 2 XB heads, SE Ignition, Force exhaust, CF Ham Can filter. The symptoms are - 1.) pinging at around 3500 rpm in 4th and 5th gears under load 2.) it backfires continuously when I am decelerating with the throttle closed. I adjusted the clip on the needle 2 positions richer at Fireman Jim's suggestion (after he test rode my bike). The plugs looked like the mixture was lean. If you number the clip positions 1-5 with 1 being leanest, the clip was set to 2. Now it is set to 4. I checked the jets and I have a 162.5 main jet and a E20 idle jet.

Does anyone have any suggestions on the proper jetting? The previous owner had a shop in AZ do all the work and dyno tuning, but I don't know what re-jetting (if any) was done. I know there are other guys running similar set ups - what size main and idle jets are you using? The pinging seems to be a little better with the richer needle setting, but it is still there. Do I need to advance the timing? Thanks for any help!
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Xldevil
Posted on Tuesday, October 09, 2007 - 01:42 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

Hi.
My setup is 1250cc,which is set to a close squish,stage1 TS heads,536 cams,Buell race kit,(ignition module,header/muffler and ac)
My Mikuni performs well with a 27.5 idle and a 167,5 main.
Needle is stock in stock position.
Ralph
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Cyclonecharlie
Posted on Tuesday, October 09, 2007 - 08:15 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

There are more things that will cause the pinging than carburetion. There's plug heat range and timing, two of the main ones.Too much compression and octane and more.
Eliminate the easy one's first, what plug's are you running? What are you using for fuel? If the plugs don't look lean,move the timing back a couple of clicks.
And if you really want to know, put it on the dyno...Charlie
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Road_thing
Posted on Tuesday, October 09, 2007 - 10:07 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

This is what I got from Aaron Wilson. My motor is a 1250 kit with stage 3 heads, red shift cams, and a race pipe:

On that motor, and your elevation, I'd probably stick a 175 in it to start. Take plug readings or better yet, get to a dyno.

Use a 70 nozzle and a 25 or 27.5 pilot, again those are starting points only. Oh, and a 97 needle, either in the middle position or raised one notch, you should be close.

It runs real nice with that set-up, I haven't had it on the dyno yet.

rt
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Chasespeed
Posted on Tuesday, October 09, 2007 - 11:23 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

I agree with Charlie...

If richening up, minimized the pinging...

Try a cooler plug.. Again, what plugs are you using?

If richening it up, started to quell the ping, it very well could be pre-ignition..

The chamber were hot, and the additional fuel is being used to cool them... but, the plug could still be holding heat, which can still lead to pre-ignition(ping, spark knock, etc)

Went back to cold plugs on mine, finally got it to stop pinging...

Chase
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Firemanjim
Posted on Wednesday, October 10, 2007 - 11:57 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

Hey,Kevin.I just found one of my old NGK plugs and they were 9's.IIRC you had 8's in your bike. Should change to a cooler plug.And my jetting sounds just like RT's.
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Nocompromise
Posted on Sunday, October 14, 2007 - 09:36 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

Thanks for the responses guys! I swapped the 8 plugs for NGK DPR9EA9's. The pinging was still there on the last ride I went on, but it seemed better.

If that jetting is working for both RT and Jim, then that's where I'll start. I think I must've been reading the idle jet upside down (E20 = 023 right side up?). If it still pings after rejetting then I'll try advancing the timing. I would like to run it on the dyno for tuning and to see how much power she puts out, but I'd like to get her in the ballpark first.

By the way Jim - I *think* that leak might be coming from the rear cylinder pushrod tube base. It looks like there is some oil pooling around there and then dripping down. The pushrod base seal was torn on the front cylinder, so maybe I have the same problem on the rear? When I get back from vacation I'm going to change them out. Thanks for your help diagnosing the problems!
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Firemanjim
Posted on Sunday, October 14, 2007 - 01:29 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

Timing needs to be retarded,not advanced.
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Nocompromise
Posted on Saturday, October 20, 2007 - 06:46 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

What is a "70 nozzle"? Also, is a "97 needle" stock? I ordered the main and pilot jets - I hope those make an improvement.
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Nocompromise
Posted on Friday, November 02, 2007 - 10:36 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

Ok, here is my update - I installed the 175 main jet, 27.5 pilot jet, and have the needle raised to the #4 position (with #1 being leanest and #5 being richest). I also adjusted the Pilot Air Screw, which helped a lot with the rough idle and backfiring on deceleration. The new main jet has cured the pinging, but I seem to have another problem now. When I open the throttle quickly at 3000 rpms or below in 4th or 5th gear, the engine bogs and stutters really badly. If I open the throttle at a slow or moderate speed from 3000 rpms or below, the engine will behave normally and gradually pick up revs. Everything seems fine above 3500. This was not the case before I rejetted everything. I adjusted the needle 2 positions richer (from the stock #2 position to #4 position) first while I was waiting for the new jets. I didn't specifically notice the stuttering/bogging problem while running the stock (default might be a better term) jets and the richer needle position.

So, I read through the HSR tuning manual and it sounds like it might be the main jet. I was guessing perhaps the needle might be in too rich a position so it allows too quick of a transition to the main jet. Could a 175 main jet be too rich for my engine? When I had the default 162.5 main jet it didn't exhibit this behavior. Any advice would be greatly appreciated!
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