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'75 S3A 400 Mid-Range Bog / Low power
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Author:  zip308 [ Fri Jul 31, 2015 11:21 am ]
Post subject:  '75 S3A 400 Mid-Range Bog / Low power
My 400 is really low on power in the 3000-4000 rpm range (cruising speed). It idles great, is snappy off idle and pulls hard if you get it above mid range but if you run down the Hwy around 45mph-55mph it is low on power, boggy and almost surges a little. If you open the throttle it will eventually gain rpm and then start to pull again. The carbs are clean and have been checked multiple times. It has great spark but tough to check under a load. I lowered the floats a little (level) to allow more gas in the bowls and raised the needles to position 4 and that seemed to help a little (snappier off idle and a little less bog mid-range) but did not solve the problem. Plugs look good to lean (I will try to post a picture). The exhaust output feels equal on the stand and the air filter looks ok. Stock jetting other than needle position with no engine mods.

Thanks for the help!
Author:  mraxl [ Fri Jul 31, 2015 11:41 am ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
Use this: http://www.3cyl.com/mraxl/manuals/carbtune/carbtune.htm
Don't put float levels at anything but spec settings.
Adjust air screw per manual.
Then try different needle clip position.

Be sure prerequisites are met prior to any adjustment.
Author:  Ja-Moo [ Fri Jul 31, 2015 12:08 pm ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
You didn't happen to put in carb rebuild kits in the carbs?
Author:  zip308 [ Fri Jul 31, 2015 1:02 pm ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
Ja-Moo wrote:
You didn't happen to put in carb rebuild kits in the carbs?


Carb rebuild kits with the factory stock jets. Basically the needle/seat (float valve), choke plungers and gaskets out of the kit.
Author:  zip308 [ Thu Aug 13, 2015 4:14 pm ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
Still having problems with the low power/bog at mid throttle. After lots of carb work, plug readings and jetting (right cylinder always a little rich) I started looking elsewhere and decided to check spark once again. When I originally got the bike running I just checked for spark but didn't look too close other than to make sure it had spark. Bike runs good at high and low RPM just low on power/flat under a load. What I have found is weak spark on the right cylinder. The points were replaced but not the condensers I haven't started troubleshooting.

Are the coils the same so I can switch them to see if the problem follows the coil? (parts manual shows three separate part numbers)

Image
Author:  JohnnyO [ Thu Aug 13, 2015 5:38 pm ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
zip308 wrote:
Ja-Moo wrote:
You didn't happen to put in carb rebuild kits in the carbs?


Carb rebuild kits with the factory stock jets.


There are currently no carb rebuild kits on the market with acceptable jets, it has be proven that they are quite different ..
Order some genuine (Mikuni Jets) from a reputable source , and follow Dales link to the resource site on carb synch ..
That goes for the needles as well..

Edit:
With the exception of Ivan's Kits here in the vendor section ..
Author:  zip308 [ Sun Aug 16, 2015 9:50 am ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
JohnnyO wrote:
zip308 wrote:
Ja-Moo wrote:
You didn't happen to put in carb rebuild kits in the carbs?


Carb rebuild kits with the factory stock jets.


There are currently no carb rebuild kits on the market with acceptable jets, it has be proven that they are quite different ..
Order some genuine (Mikuni Jets) from a reputable source , and follow Dales link to the resource site on carb synch ..
That goes for the needles as well..

Edit:
With the exception of Ivan's Kits here in the vendor section ..


"factory stock jets. Basically the needle/seat (float valve), choke plungers and gaskets out of the kit" (original jets, main-pilot-jet needle-needle jet-air screw)
Author:  smoke1 [ Tue Sep 29, 2015 11:08 pm ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
I am having the exact problems with my s2. Was running great with good plug color. I have already replaced points and condenser with nos parts last winter. I have not checked the coils yet as this will be my next step. Please post what you found to be the problem.
Thanks
Author:  zip308 [ Thu Oct 01, 2015 7:40 am ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
smoke1 wrote:
I am having the exact problems with my s2. Was running great with good plug color. I have already replaced points and condenser with nos parts last winter. I have not checked the coils yet as this will be my next step. Please post what you found to be the problem.
Thanks


I trimmed the ends of the original plug wires back a little bit and replaced the plug ends and caps with NOS ones I found at a dealer. This fixed the issue about 80% but I still have an intermittent miss on one of the cylinders. I haven't had time to chase it down any further other than the spark gets weak but doesn't totally stop. Let me know if you find anything on your end.
Author:  mraxl [ Thu Oct 01, 2015 8:20 am ]
Post subject:  Re: '75 S3A 400 Mid-Range Bog / Low power
There may be something here that can help:
http://kawtriple.com/mraxl/tips/pointsignition.htm
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