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Discussion Starter #1 (Edited)
My ZZR400 (1993 ram air) has been a bit under powered, hesitant, noisy and lacking power at speed. Just had my mobile mechanic mate round and he's discovered that the air box just isn't seated nicely against the carbs. Huge gab along the bottom edge at the back. Can see daylight through. Reading around, this lack of pressurisation sounds like the cause of issues that I've been having, which are especially pronounced at high speed.

So, any ideas, what could be up? Warped air box? Missing a seal/gasket? Carb/throttle bodies not seated well? They look like they'll line up better if flexed up slightly toward front of bike. Two bolts holding the back of the box down are in tight. Or is it just bad design and needs bodging with some silicone sealant?

Pipes going into front of box appear to line up fine (once wrestled in). K&N filter is clean. Access holes at back are taped up. Tighten bolts even more?
 

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Pursebully
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Wouldn't have a mass air sensor on a carbureted engine. My guess is the plugged hole is from a crankcase breather or similar.
 

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The Indifference Engine
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Take it apart and look around the airbox. See how to make things line up.

Breather port is correct.
 

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Discussion Starter #5
Just dunna line up. With bottom of air box bolted tight to throttle bodies, the base of the box has a gap, ~2-3mm along the rear edge. Flexing the throttle bodies forwards, makes things line up better, but they just flex back, nowt to hold them lined up better. Similarly, pulling the air box forwards slightly, lines things up better, but again, no way to secure it in that position, so it just flexes back.

There's the rubber trim on the top of the throttle bodies (which is in good nick), but is there supposed to anything else between this and the bottom of the air box, and extra gasket or something?

Only ideas we had were to try a new air box, melt the current box into a better shape, make up some kind of extra foam gasket, put in some extra self tapping screws on corners to pull two together or just go mad with silicone sealant.
 

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Discussion Starter #7
Nope got the 4 ducts coming up from throttle body.

That diagram you have linked looks pretty damned close to what I have TBH. Now the part I'm wondering if I'm missing is 92093 (SEAL,DUCT HOLDER), don't recall see this part at all.

The gap is between part 13091A (HOLDER,DUCT) and the bottom of air box 11011B (CASE-AIR FILTER,LWR). The two just don't marry together flat against one another. The top edge of 13091A is flat against the box, but at the lower edge of 13091A there's a gap of a few mm. In fact, if I look through the gab in the frame horizontally at the bottom of the air box, I can see through this gap and can see the the 4 ducts inside the box!

Maybe that part 92093 (SEAL,DUCT HOLDER) would fill this gap, how thick is it?

Anyway, ordered a new air box off ebay (was only £20), will try that first.
 

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Yup, I bet it's that "Seal,Duct Holder" part that you are missing (I'm not sure how thick it is). The air box you ordered probably won't come with the seal, so you will probably still have the same problem. Try to find that seal for your bike model, and then you should be good to go.
 

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Discussion Starter #10
So, finally got round to swapping airbox and duct seal on the bike. Got another airbox off ebay which seemed to sit better on top of throttle body. Replaced the duct seal with a new one too. It was there, just heavily squashed into plastic surround on top of throttle body and wasn't really doing much.


New air box came with correct bolts too which seemed to clamp it down better too. Also had correct rubber seals to cover access holes, much better than the gaffer tape used previously...

Aside from managing to slice the fuel hose with one of the clamps and having to snip it a spot shorter, everything went well. Doh!

Running, it feels a bit better. Slightly more responsive at low revs. However once I hit 6000 and above it loses all power, and starts charging and fading again and again.

So couple of questions:
The new(er) airbox has a hole in top at the front on left. Currently plugged with a rubber seal that also a hole in it... Part: GROMMET,AIR FILTER, 92071-1042 looking at the diagram. Old box didn't have this, just a guide on plastic to cut hole out if needed. What's this for, what's should go in there? Believing it shouldn't be open I gaffer taped it up. Best plan?
Did try using top off old box, but it didn't quite marry up snug, ever so slightly smaller. Maybe I could try brute forcing it.

And the pipe coming out of the top middle rear of the air box is the TUBE,BREATHER
92190-1346. Again, I'm missing this. Old box had it gaffer taped up, so I did the same on new one. Again, will this do, possible cause of no power?

So ultimately the big question is, what to look at next? Carb main jets?
I've also tried running a tank of Forte Gas Cleaner stuff through, hoping that'd give the carbs a bit of a clean, but has made little difference.
 
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