Hi.

I have an issue with my built 95" twin cam custom built bike. I purchased it in the spring and have been smoothing out minor build problems ever since I got it, but I have only a couple left, and this one I can't seem to resolve:

The bike had a dual mikuni carb set up on it when I got the bike that was virtually untunable (bike wasn't running well when I got it) so I junked it in favor of a s&s super g. Took it to my local Harley dealer to have it dyno tuned after I installed it, but it cam back with an issue at low rpm. at 2500 rpm or less (most pronounced in lower gears) the bike surged heavily, particularily when I tried to maintain an even low speed at low rpm. It would surge so violently it would snap my torso back and forth on the bike. It was also noticable in higher gears, but not nearly as pronounced. The current jetting was .025 intermmediate/.074 main. It should be noted that the lurching/surging at low rpm was not accompanied by any backfiring of any kind, nor did I have any significant backfiring on deaccelleration.

Initially, I thought this was due to a loose primary chain, so I tightened it up to spec. It didn't do anything to improve the condition, but it did reduce som operational noise a bit.

I checked my plugs, they were a little hard to read - the front plug was a little dark, but not sooty. The rear plug was a little brown with white flecks beneath. Clearly I thought this one was showing a lean condition, so I thought that clearly I had to increase the jet size of my intermmediate jet.

So I wiped off the plugs and replaced them, then went through the tedium of taking off my entire carb and changing the intermmediate jet to the only larger size I had, a .031. I then reset the fuel/air mixture and idle per the instructions when replacing the intermmediate, then went out for a test ride.

About two blocks from my house I found a low speed limit road and proceeded to test the new jetting by trying to maintain a low rpm steady speed. I cruised at approximately 1750 rpm at a fairly steady rate with much less lurching - it was still there, but greatly reduced. Amazing, I thought! I might have actually fixed this nightmare! Just about the time I was thinking to myself that I should get my hands on a .032 to perhaps elimitate the lurch altogether - BAM!

The bike started to loose power and was backfiring so bad I had to try to nurse it up to over 4000 rpm to hit the primary get where it did pick up a bit but continued to back fire (but not as much at lower rpm). Every time I slowed to make a right hand turn to try to get home I had to lower rpm and gears to turn and the backfiring was relentless and horrific. Rifle shots and non-stop popping. I limped home with the bike runnnig completely like crap. The back firing appeared to be exclusively located in the pipes.

At this point I don't know what the heck is going on. For a few miles I thought that the bigger intermediate jet was really improving things and then out of the blue at a steady spead and rpm things just went to hell. I did pull my plugs when I got home and checked them - the front one was dark and sooty as you would expect from what can only be an extreme sudden rich condition. The rear (which for some strange reason is always lighter than the front) was dark with a patch of brown on top.

Before I take all the time to start tearing things apart with no real direction, does anyone have any ideas as to what happened to me so I have some direction in trouble shooting this new nightmare? Any advice would be very appreciated.

 

The S&S product line is a good product as long as you are at 100% throttle. There are ways of tuning that carb to run but the biggest down fall is it is a two circuit carb, once you get the lower end to run right the top end stinks....get the top end to run Right and the lower end stinks .The intermediate jet and main jet over lap between 3000 rpm and 3500 rpm so the bike will run rich in this area no mater what you do to the two jets you have to tune with .This is where it gets complacated....you need to install a larger air bleed in the main jet circuit to delay the main jet from coming on until the intermediate stops fueling. It's way too lengthly for me to explain to you so it would be best to research it on the web.
However, I will tell you the best fix for your bike......take the S&S off and install the stock CV carb that belongs on the bike and it will run fantastic.
Good Luck
Doc

 

If the bike came with a dual Mikuni set up it is probably so over cammed for a 95",too high lift combined with too long duration/over lap that no matter what you do with that carb it will not run well at part throttle. Find out what you got and then you can start to change things that will help not just throwing parts at the bike, this will save you money, I know I probably have made more stupid mistakes than I would care to admit

 

Davinci makes an emulsion tube....really it is a booster from a holley dominator carb with a 7deg cut down, but machined to fit a G....they work very well for your surge/stubble at low rpms...google it and get one....easy install....easy fix

http://www.davinciperformance.com/ ....go to products

 

Sounds like your fighting a manifold leak. It was leaking a little before and now it's really bad! I would check that first before doing any carb tuning. If your leaking any then tuning is a waste of time. A single Mikuni properly tuned is the best way to go. The CV works well but the Mikuni works even better. S&S is good for drag racing not the street. Just my HO.