Topic: CB750 K7 running rich with MAC 4 into 1 exhaust

[mazingerzeca]

Hello all.
I've just installed a black MAC 4 into 1 exhaust in my CB750 K7. Before the MAC exhaust it had the standard 4 into 4 and it runned perfectly perhaps a little on the rich side, but after changing to the MAC exhaust it runs rich, when I start the bike and until I get high revs, the @ collector doesn't run well, it is even a bit cold. On higher revs it runs better, so I suspect that the problem is on the slow idle circuit. As you know, the K7 carburettors have a pressed-in slow jet, so I think I can't change them. Please, tell me that I only have to touch the pilot screw to solve it!

[AintNoEasyWay]

I have the same bike & same exhaust. Work with the pilot screws a bit, but the problem could require more work. Also, not sure what you mean by "@ collector"...

But, yes, the slow jet is pressed in. You can still remove & replace it, though. Spray penetrating catalyst around the jet, wrap a rag or thick paper towel around the jet, & use vice grips or pliers to gradually ease it out.

[dgedinyte]

Make sure your float level is right. Sounds like your pilot jet is swimming in gas. Seems though,like a switch from stock to mac exhaust should make your bike need more gas not less if the needs changed at all. Check your plugs to make sure what your detecting is a rich condition.

[mazingerzeca]

Thanks for the replies. I will check the float level. I  checked the plugs and they are almost black, so the mixture must be rich. By collectors I mean the exhaust canisters. Another sympthom is that when I cut off the throtle from high revs I hear some exoplosions.

[dgedinyte]

If you mean that the exhaust header is cold where the individual pipes meet the engine, then that's something else altogether. If your cold close to the engine then that cylinder is not firing or intermittently firing. Pull and examine your plugs. Clean if fouled and check for damage and spark.

[Buttmask]

I had a mac and and currently a motogp works yoshi replica, my bike always ran rich, even with stock jets and air screws out way more than normal range, and it never went away til I moved my needle clip up one position.

[Jerry Rxman Griffin aka MuthaF'er]

Remove your baffle and see what that does.

[mazingerzeca]

Yes, I cleaned the spark plug, which was black due to a rich mixture. Í unscrewed the pilot jet and now the bike runs well, although I can still hear some explosions when I cut off the gas. My needle jets don't have different positions for the clip.

[harisuluv]

Once spark plugs are fouled, they are done.  You can get some very limited short term use out of them by "cleaning" them but they are fouled. 

Unscrewing the pilot jet on PD carbs makes it run richer, so that would conflict with it being rich and then fixed by going richer.

K7 should have adjustable clips.  It's mostly the K8 that doesn't have them.

I still don't understand by this:  "By collectors I mean the exhaust canisters."  If you have a 4-1 then you have one canister.  That is the "1" in the 4-1.  If you are talking about the pipes that attach to your head and go to the canister/muffler then those are your headers/down pipes.  If those are cold then you have cylinders that aren't firing.

[mazingerzeca]

Yes, I was wrong twice. I screwed on the pilot screw to reduce gas on the mixture. About the canister, yes, I mean the tube that connects the pipes to each of the exhaust ports. I apologize for my english

[dgedinyte]

Once spark plugs are fouled, they are done.  You can get some very limited short term use out of them by "cleaning" them but they are fouled. 

Unscrewing the pilot jet on PD carbs makes it run richer, so that would conflict with it being rich and then fixed by going richer.

K7 should have adjustable clips.  It's mostly the K8 that doesn't have them.

I still don't understand by this:  "By collectors I mean the exhaust canisters."  If you have a 4-1 then you have one canister.  That is the "1" in the 4-1.  If you are talking about the pipes that attach to your head and go to the canister/muffler then those are your headers/down pipes.  If those are cold then you have cylinders that aren't firing.

Wow! I've never heard that. I've even got one of those mini sandblast spark plug cleaners. Guess that was a waste of money. Learn something new everyday. Thanks harisuluv.

[harisuluv]

Like I said, you can still get "limited" use out of cleaned spark plugs.  It depends on how you do it.  There are different ways people do it, so your mileage will vary accordingly.