Front cylinder runs rich on sub 10k 117 cu S&S - Super G carb

PAUL NOBBS

Member
Hi all, My bike is 2005 Bulldog
Just wondered if any of you guys can point me in right direction of why front Cylinder plug has black appearance on relatively low mileage engine using one carb. back cyl plug looks good colour. It runs and ticks over and pulls ok but obviously less than perfect tune!! Plugs both same, gaps are same. I have yet to do a compression test though on low miles engine I am not expecting an issue . I have been used to metric inline fours so big twins are a new thing for me!!! Any ideas appreciated?
 

Th3InfamousI

Administrator
Staff member
Lifetime Supporting Member
Supporting Member
Not a damn thing you can do with a carb motor unless it's a serious issue with a blown gasket but what your stating is common.

Rear cylinder runs hotter cause these are air cooled engines and the front one gets all the cool air attention riding down the road. Hotter cylinder will make it look leaner.



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PAUL NOBBS

Member
Yeah thanks for sharing that....It did cross my mind that could be one explanation I wonder if some people have ever run a hotter plug in the front to compensate????
 

KaptinAmerika

Well-Known Member
Hi all, My bike is 2005 Bulldog
Just wondered if any of you guys can point me in right direction of why front Cylinder plug has black appearance on relatively low mileage engine using one carb. back cyl plug looks good colour. It runs and ticks over and pulls ok but obviously less than perfect tune!! Plugs both same, gaps are same. I have yet to do a compression test though on low miles engine I am not expecting an issue . I have been used to metric inline fours so big twins are a new thing for me!!! Any ideas appreciated?
Can you post a pic of the plugs please?
If there is an extreme difference then you may have a rear intake leak causing the rear cylinder to look leaner.
If everything is in order the rear plug will usually be maybe one to two shades lighter at the worst.
If the front plug is running a dark cinnamon then the rear plug should be running a medium cinnamon color as an example.
I have had a couple of BDs in the shop with the front plug running black and sooty and the rear plug running a medium cinnamon and found it was a combination of a rear intake leak and rich jetting, someone trying to compensate for the rear lean situation without considering the possibility of an intake leak.
 

Whodathunk

Active Member
I'm with Kaptin Amerika on this one, mine was doing the same thing, and it turned out to be a bad intake gasket...
 

PAUL NOBBS

Member
Hi Chaps, I can send pics but the difference is very marked the front plug is basically is all matt black and the rear is almost white so you may be correct in what you say and I should at least give this a first consideration. I am not sure what that looks like but I will have a look now . Many thanks
 

KaptinAmerika

Well-Known Member
Hi Chaps, I can send pics but the difference is very marked the front plug is basically is all matt black and the rear is almost white so you may be correct in what you say and I should at least give this a first consideration. I am not sure what that looks like but I will have a look now . Many thanks
I highly recommend you do a complete carb rebuild and replace your intake gaskets and carb to spacer o-ring then do a base tune on the carb for a starting point.
 

PAUL NOBBS

Member
I did the carb rebuild last year but not the o-ring or the inlet seals so will take your advice and get that done - Cheers Pal
 
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