Breaking up @ 4500rpm

[NorazDad]

Hey guys, I am stymied. My '81H has been running very well...for the most part. I switched from cone pods to unis because I was experiencing a little hiccup around 3300. Thought it was an airflow problem. Rode it a couple hundred miles and no significant improvement. The bike ran well except for that rpm range. Went on vacation and did not ride it for a few weeks and then I fired up with intent to reexamine the old problem only to find I couldn't get down the road above 20 mph. It just bogs and sputters and lurches like it just doesn't want to go.
So, I took the carbs off and cleaned them thoroughly. Removed and cleaned and examined all the jets. Checked and adjusted the float levels and remounted and took it back out and same thing.
Now I want to think this is a jetting issue, but there are the same jets I've always had and they worked fine. Do jets wear out?
I had originally thought it was a timing issue so I did re check the timing and all looks good there. I even get a solid idle at 900 and I can rev to 3300 to check advance and all works well...just not on the road under load.
I have a pamco, pma, battery with perm volt meter and I am getting all the juice I need. I did do a top end rebuild last year complete: pistons, rings, chain, guides, stainless steel valves, but not valve springs.
So, have at it if you don't mind. I'd love to hear your suggestions. I am not opposed to rejetting, so I would be happy with any input there as well.
Thanks for your input.

 

[TwoManyXS1Bs]

Hey, NorazDad. Temporarily reduce your plug gaps to 0.015"-0.020". If it runs differently, suspect the ignition...

 

[pamcopete]

What type of battery do you have? If it's one of those tiny batteries, then it's possible that it has lost it's ability to smooth out the power from the PMA. Another possibility is that the PMA is producing an erratic voltage, so if the battery is OK, try fully charging it and then disconnect the PMA and regulator and go for a test ride.

 

[NorazDad]

Thanks. My plan is to get new plugs NGKBP7ES, Check the valve clearance, and try the battery test.

 

[NorazDad]

What type of battery do you have? If it's one of those tiny batteries, then it's possible that it has lost it's ability to smooth out the power from the PMA. Another possibility is that the PMA is producing an erratic voltage, so if the battery is OK, try fully charging it and then disconnect the PMA and regulator and go for a test ride.

What about my Reg/Rec? I guess the battery test would verify that.

 

[NorazDad]

Ok guys, I cleaned the carbs, changed the plugs, checked the valve clearances, timed and retimed, disconnected the reg/rec and took it for a ride. Slight improvement, but it bogs down under power between 3500 and 4500 rpm. It idles all day at 900, sounds good. Cruises beautifully between 3000 and 3400, but under power it bogs and sputters. It wants to go, but it just can't.
Ignition? It's a new pamco with E-adv...if you remember I shorted out the old one. So what causes the power band to stutter when all else runs fine?
Please help?

 

[pamcopete]

What is the battery Voltage in that RPM range?

 

[NorazDad]

14.5

I think I am going to retard the timing a couple of degrees and see if that smoothens anything out.

Hugh Owings sells a Mosfett Reg/Rec that I was thinking of switching to. Might that help with the power issue?

 

[Brassneck]

Prior to all this, could you rev well past 4500rpms? How about prior to the top end work you did?

 

[NorazDad]

Yes. I have a thread about a minor hiccup around 3300 and that led me to switch to pods from cones. This is a new stutter in the power band between 3400 and 4500. Above is not happy, but it revs above 5000. It ran great before and after the top end. The rebuild was due to a sheared exhaust valve dropping into the piston.

 

[Brassneck]

Based on the ignition you have...there's not a whole lot more you can do there. It's giving spark and the e-advancer should be working, not sure if there's a test for the e-advancer...but I would probably not focus on that too much and look at the carbs again.

Have you checked your diaphrams for holes/tears? maybe do a drop test to ensure you have good working slides.
I don't think I saw what mods have been done other than the unipod air filters...anything else? And then what jetting are you at (both pilot and the mains)?

My sense is you're getting too much fuel...have you checked your plugs? Sooty/black would indicate too much fuel. Could be that the air filters you have are blocking the air inlets on the carbs (Or they are blocked by something else and didn't get cleaned out well), and thus making your condition richer? Have you tried to run it without air filters at all...any difference?

 

[NorazDad]

Great ideas
I will check the plugs after yesterday's ride. They were new standard nickel NGK. I really didn't check the numbers on the jets. I should have, but I can do that when I do the drop test. I did examine the diaphragms and they looked fine. I will further examine. I will try a ride without the air filters.
Thanks for your input.
Rob
Thought I'd edit rather than repost. Checked the plugs and they look like twin poster children for clean plugs.

 

[NorazDad]

I did the gggGary needle washer mod. After my run I think I am running too lean. It is certainly worth a try and for.26 cents it is a fiscally easy test.

 

[5twins]

The needle washer mod will lift and richen the needle setting slightly, if you did it right. These BS34s are very lean right from the factory. Any little change you make to intake or exhaust can push them over the edge and make them too lean. You probably need to do some minor re-jetting. Keep in mind that the 3 circuits in your carb overlap slightly. Changing one will "bleed over" and effect that overlapped portion of the circuit next to it .....

Your issues in the 3500-4500 RPM range are right in the midrange to main circuit transition area. A larger main may help you here.

 

[NorazDad]

The needle washer mod will lift and richen the needle setting slightly, if you did it right. These BS34s are very lean right from the factory. Any little change you make to intake or exhaust can push them over the edge and make them too lean. You probably need to do some minor re-jetting. Keep in mind that the 3 circuits in your carb overlap slightly. Changing one will "bleed over" and effect that overlapped portion of the circuit next to it .....

Your issues in the 3500-4500 RPM range are right in the midrange to main circuit transition area. A larger main may help you here.

I am about test the needle mod right now. I still don''t know what mains I have, I will check that out if I feel the need for further modification. Thanks for the ven diagram. It makes total sense.

 

[NorazDad]

I am about test the needle mod right now. I still don''t know what mains I have, I will check that out if I feel the need for further modification. Thanks for the ven diagram. It makes total sense.

Ok, just took it out for a test ride. It ran like a rocket from 3000 to 5500, obviously midrange, so the needle shim seemed to have done the trick. Now, I have a great idle at 900, but no power until 3000. So, what's next? I had to use half choke to get home from a dead stop.

 

[5twins]

I'm currently working on an '83 and have noticed the same thing about the low end. It seems rather flat and lifeless below 2500 or 3K, seems to wake up above that. I think it's too lean in the pilot circuit so I plan to richen it up a bit. There are 2 ways you can go about this. You can change the pilot jet or you can change the air jet that feeds the pilot circuit. I plan on trying the air jet first. It's easier to get at in the bell mouth where the air filter mounts, and it supposedly doesn't have as big of an effect as a pilot jet size change. You can "fine tune" with it to achieve richness levels in between pilot jet sizes. The stock air jet is a #135 small round main. I plan on dropping that 2 sizes and trying a 130. With your pod filters, it would be very easy to access, easier than going into the float bowl. If it helps but isn't quite enough then I guess a pilot jet change would be called for.

 

[NorazDad]

I'm currently working on an '83 and have noticed the same thing about the low end. It seems rather flat and lifeless below 2500 or 3K, seems to wake up above that. I think it's too lean in the pilot circuit so I plan to richen it up a bit. There are 2 ways you can go about this. You can change the pilot jet or you can change the air jet that feeds the pilot circuit. I plan on trying the air jet first. It's easier to get at in the bell mouth where the air filter mounts, and it supposedly doesn't have as big of an effect as a pilot jet size change. You can "fine tune" with it to achieve richness levels in between pilot jet sizes. The stock air jet is a #135 small round main. I plan on dropping that 2 sizes and trying a 130. With your pod filters, it would be very easy to access, easier than going into the float bowl. If it helps but isn't quite enough then I guess a pilot jet change would be called for.

Sounds good to me. I am re reading the carb guide for the umpteenth time and now my issue is where are we buying our array of jets. It looks like my current issue is in the pilot and or air jets. I would also like to get a few mains to test out the full range of these carbs.
So, who gets our business and what array should I buy?

 

[5twins]

I prefer using genuine Mikuni jets. The jet types used are very common. If you have any local shops around that cater to dirt bikes and/or ATV's, they probably have all sorts of them for sale individually. I buy mine online usually in 4 packs but I do a lot of carb tinkering. They're usually cheaper by the 4 pack and I get them from one of the big box places like Bike Bandit or Chaparral.

To start with, you probably want one size larger mains (135) and either the 130 air jets or one size larger pilots (45). But, you should check and see what's in there now. P.O.'s often change that stuff, and not always for the better, lol. The wrong sized jets are often just as bad as no re-jetting at all, lol. These BS34s usually require some tinkering but the jet size changes from stock are generally pretty small. One size up on the pilot is usually enough and one to 3 on the mains usually does it.

 

[NorazDad]

I prefer using genuine Mikuni jets. The jet types used are very common. If you have any local shops around that cater to dirt bikes and/or ATV's, they probably have all sorts of them for sale individually. I buy mine online usually in 4 packs but I do a lot of carb tinkering. They're usually cheaper by the 4 pack and I get them from one of the big box places like Bike Bandit or Chaparral.

To start with, you probably want one size larger mains (135) and either the 130 air jets or one size larger pilots (45). But, you should check and see what's in there now. P.O.'s often change that stuff, and not always for the better, lol. The wrong sized jets are often just as bad as no re-jetting at all, lol. These BS34s usually require some tinkering but the jet size changes from stock are generally pretty small. One size up on the pilot is usually enough and one to 3 on the mains usually does it.

Ok, I'll start with these recommendations...but I'm going to re read the guide again...
Thanks