My carbs are getting closer, however...

[Haarstad]

I replaced my main and pilot jets because i was having issues with my carbs. I have a 1979 XS1100 Special with 78 carbs on it.

I put in new 147.5 main jets and 45 pilots and synced the carbs, it runs good at idle and when i twist the throttle quick, it no longer stutters or backfires out the intake. However, it does seem a little slow.

So i took it out on a quick spin and it doesn't have much for power. It is extremely slow when going 55 in 5th gear and trying to accelerate. Could this be a float height problem?

It barely speeds up at all when trying to accelerate at low rpms, the more throttle you give it, slow slower it goes.

I don't know much about it my guess it either the jets are still off, or the floats aren't right.

Any help is greatly appreciated, thanks

Edit: The bike has pod filters and 4 to 1 kerker exhaust and i uses the formula to figure out what jets i should use for a starting point. In case anyone is wondering. Thanks

[crazy steve]

My guess is the jets are too big. That's five sizes over, you really shouldn't be more than three over....

[Haarstad]

Well jets go in sizes of 2.5, so this should be 4 sizes larger...
I was told that for:
4/1 exhaust= +2 sizes
individual air pods= +3 sizes
You are suppose to minus 1 size from the total, which would make the total +4.
Stock jet size is 137.5 and when you add 4 sizes, it should be 147.5

Is there another way to do this that will work better?

[natemoen]

I agree, jets too big. I always think that chart usually ends up at least 1 size too big.

[crazy steve]

I think a 3-size add for the pods is overkill personally. What do your plugs look like? What's your mileage? What jetting did you change from?

[Haarstad]

I didn't ride it enough to get a good reading on mileage, just enough to see what it was doing.

My starting jets were 137.5 and i moved up +4

So do you think i should get 142.5 and 145 on my jets or just try the +3 first and buy the 145 jets?

[mjpxs11sh]

eric

with all the carb work we do on these bikes, ive never heard that the tech tip on jet sizes was wrong, but each bike can be diff according to what the supplier of the mods u have. the Kerker may not really even need to be included in the formula cause i have one and i didnt change from the factory jets. the bike runs great! my understanding when i bought my Kerker was you dont need to do a jet change. just my experience, anyone else have carb jetting exp. w/Kerker?

[natemoen]

It's not that the tech tip is wrong, it is only a rough estimation of where to start. I especially think that the more things that you stack on the farther off it ends up being.

with some of the stuff I had on my bike previously that chart said i should have went up 3 sizes, I only went up 2 and my plugs showed that I was still running rich.

[jetmechmarty]

Think of the engine as an air pump. You can't move more air out than you take in, or vice versa. In theory, if the intake and exhaust are a perfect match and you change one of them, you shouldn't need to make carburetor changes because you can still only move the same amount of air. I know this is over simplified, but a good place to begin understanding the system.

[TADracer]

I put in new 147.5 main jets and 45 pilots and synced the carbs, it runs good at idle and when i twist the throttle quick, it no longer stutters or backfires out the intake. However, it does seem a little slow.

OK, I keep spec sheets on my bike. I looked at one from a few years ago before I upgraded my carbs to Keihins. I was completely stock except for pod filters and 4-1 header. My sweet spot with the Mikunis was 142.5 mains and 42.5 pilot jets. I do think your jetting is a bit large right now.

[Haarstad]

Well, I double checked the mains that I started with, and it turns out they were 140's, not 137.5. Do you guys think I should go ahead and get the 142.5 and the 145's? I was thinking I should just get them both so I don't have to wait for the other ones to ship if one is wrong.

[skids]

If all of the carbs are running the same way (color the plugs) then this is probably not an issue... The pressed-in air jets are easily plugged and can be difficult to clean. Those are air bleeds for the emulsion tubes over the main jets. It stands to reason that if they are restricted, the mains will pull fuel faster, leading to an over-rich condition. But it really isn't likely to have them all fouling at the same time because of this (maybe).