Mac 4 into 1 ...now carb issues

Put the new exhaust on and now I my bike is stumbling on acceleration. I believe this is due to the jetting. I still have factory jetting in carbs. Does anybody know what jets I have to replace and to what size?

Thanks

 

Personally I would go for up 2 sizes main jets, but you may all so need to change the pilot jets and air jets.

 

Before you go changing jets how clean are the carbs? Valves in spec? float levels, etc? All those contribute to running issues.

 

Bike ran great with old exhaust. Carbs are clean. Going to change jets...hope that works

 

If you're spending money, just get the 124's - - it's unlikely that they would be too rich, whereas the 122's might have you wanting more.

Or - buy the 122's and DRILL the original (now junk) 120's to taste.
{using RickCoMatic's nifty drill/jet guide}

 

Not by itself it probably won't. Unfortunately you've upset the delicate balance between intake and exhaust and this will have affected the motor's "airflowing" characteristics and therefore its "vacuum signals" to the carbs.

Carbs are at the very least going to need to be adjusted (probably re-jetted and not just mains) and re-sync'ed; for that the valves need to be in spec.

You can do all the steps in order; or you can fight with it for a long time.

By changing the exhaust, you've hit the "reset" button for the state of tune of the carbs; nothing to do to fix it except do it right.

 

Fitz, is that theory or conjecture?

 

I'm running standard carbs with a Mac 4 into 1 no problems. (750 Seca)

 

I had a 4-1, not sure of brand, and stock carbs and jets (81 650 max)

 

my 750 seca with a mac 4-1 liked 126 mains but it likes 128 mains and 41 pilot better with stock intake

 

I have a Mac 4 into 1 with stock carbs....no problems here.

 

Neither. Science, pure and simple; backed up by real-world experience.

Adjusting mixtures, whether via re-jetting or tweaking screws, will throw the sync off a bit and a re-sync will be necessary.

Synchronization with out of spec valves is impossible.

Since the rack will be off for any jetting changes; it's a good time to verify float levels.

Hence my statement about a "reset" in regard to the state of tune. No theory or conjecture, just inescapable truth.

 

I see so what is it about adding or removing fuel from the air that changes the sync.

 

easiest thing to do is to go back to stock and save yourself from buying costco sized bottles of asprin

 

Becauses it changes how each cylinder runs! If a particular cylinder runs stronger due to more fuel, or due to less fuel (the XJ motor loves lean right until it blows up) then that cylinder is no longer going to be "pulling" (and making vacuum) like the others. The "change" will affect each cylinder slightly differently, and there goes your sync. Remember, you're not really sync'ing carbs, you're DE-tuning each cylinder so that it's vacuum signal ends up matching the weakest one, or as close as possible. Because we use the carbs to do it, we call it "syncing the carbs." It's really not. We're really sync'ing the cylinders.

Anybody on this forum who's actually "sync'ed his own carbs" AND used a ColorTune knows first-hand that when you tweak the mixtures you throw the sync off slightly and need to revisit.

You've never actually done this, have you? Remember where I said "real world experience?" I have two running XJs, that have had a lot of miles hammered on them, especially the '81. The carb sync is part of the 5000 mile service, like valve adjustment. So I've done this a few times. Which is why I posted what I did, I was speaking from experience. You?

 

That does not sound very scientific at all Fitz. Yes I have done the carb sync a few times and I to have a few of those running xj's.

 

Well I'm running stock carbs and jets on a 83 XJ750 Maxim, Mac 4to1 exhaust, with no problems. But I do have my air/fuel mixtures open more then the manuals suggested number of turns. But she is set right neither rich or lean.