Engine running lean? Some popping. Got a couple questions.

Okay guys, I rebuilt the top end with new rings, new exhaust valves and set the valve clearances to within spec, most are right at .003", a couple are at .0025" and one is at .005 and .006". The ones that were really out of spec I replace to bring them within acceptable ranges. I just synced the carbs and tested compression. I got 135, 130, 125 and 135 for compression. I have riden it around for the last week and took a picture of the plugs.





This was from cylinder one but all four look exactly the same. I'll get a picture of all for together. I have a MAC 4-2-1 exhaust. Does this look lean or rich to you? I get a little popping on decelleration and at high rpms, like around 7000 or 8000 I fell like the bike doesn't get enough gas to it like it chokes very slightly. There is a slight rattle as well. What do you think? 1980 cb750k.

Well, I did some more research on my own and still can't tell if my plugs look too lean or too rich. It looks close to either one really. Picture for comparison.



So, I have a Dynojet jet kit and according to them I will only use their Stage 1 kit, since I have stock airbox and stock type K&N filter and stock carbs with a MAC 4-2-1 exhaust. Do you guys recommend using this kit or just simply going up a size on the jets?

I have popping at times (not all the time) on deceleration only and when getting into the higher RPMs sometimes it feels like the engine chokes a little or cannot get the juice it needs to be there. It is just very very subtle. Other than that the bike starts right up, warms up and idles right at 1100 no problem. Thanks guys.

Popping on deccel says lean. plugs look a little rich.
Check for air getting drawn into the exhaust.


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Nebr_Rex said:

Popping on deccel says lean. plugs look a little rich.
Check for air getting drawn into the exhaust.


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So I will be checking the exhaust tomorrow. It is a one piece MAC 4-2-1 exhaust and was very difficult to get on and in position. I've got it tightened down, but can still see some brass exhaust gasket peeking out from behind the headers. Is there any other way to tell if your exhaust is not flush with the exhaust ports other than sticking your hand down there and feeling for air coming out? I do that and can only feel heat, no puffing air etc.

If the carbs are running rich, Do I need smaller jets? I rebuilt these carbs back to stock last year. I am pretty sure they were running rich and all kinds of other bad last year before I rebuilt the top end. Here is the plug last year:

Thanks for the reply.

Yep, that's rich. Re-check float levels and set 0.5mm higher than stock (I think 14.5~15mm ?)
The MAC has a highly restrictive baffle but running without it 'makes your ears bleed' (way too loud)
I think they do a much less restrictive 'race only' that works properly

crazypj said:

Yep, that's rich. Re-check float levels and set 0.5mm higher than stock (I think 14.5~15mm ?)
The MAC has a highly restrictive baffle but running without it 'makes your ears bleed' (way too loud)
I think they do a much less restrictive 'race only' that works properly

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Okay I am pulling the carbs off this weekend. On the MAC exhaust I have a cone engineering cone muffler. It cuts the noise down quite a bit but is not as restrictive as the MAC muffler. I need to replace the float bowl gaskets (I got some Randaak gaskets) because I've got some gas slowly seeping out, and I will be replacing the air cut off valves and the ones I have in there are basically disintegrating. Should I use the Dynojet stage 1 carb kit I have?

IIRCC the Dynojet kit comes with a bunch of different jets, drill bits and various other stuff?
Probably a good idea to use it, those carbs are pretty horrible with K&N filters

crazypj said:

IIRCC the Dynojet kit comes with a bunch of different jets, drill bits and various other stuff?
Probably a good idea to use it, those carbs are pretty horrible with K&N filters

Click to expand...

I'll just use the stage 1 set up because I am using the stock air box and the aftermarket exhaust. I'll see if that helps.

Poping on deceleration is unburnt fuel/air in the header poping off. Not necessarily a lean issue and very common with after market exhaust systems. I would have said that first plug picture looked good to me.

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Psycrow said:

Poping on deceleration is unburnt fuel/air in the header poping off. Not necessarily a lean issue and very common with after market exhaust systems. I would have said that first plug picture looked good to me.

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Huh okay. Man there is so much to learn on these vintage bikes, like this noise and that noise etc. Okay Psycrow, what do you think, besides the popping on deceleration it feels like it runs rough in the lower gears (1-3) under 4000 RPMS and then in the higher gears sometimes I get some hesitation as I get over 5000 RPMS. Do you think that could be jetting/carb issues or something more? thanks for the reply!