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Discussion Starter #1
Hey guys.
New to me 93 GSX-R 750. Bike starts and runs. I put a heat gun on all 4 pipes and noticed #3 was cool. I successfully tested for spark. It looked a little weak, but no different from #4. Plug is clean and doesn't smell like gas. I pulled the carb rack out and drained all of the float bowls, expecting #3 to be dry, but that wasn't the case. PO claimed he cleaned the carbs and I'm not opposed to cleaning them again, but don't feel like doing it if I don't have to. I'm stumped. Any suggestions?
Thanks
Nick
 

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Just because there was fuel in the float bowl doesn't mean there couldn't be a blockage in one of the jet passageways. Also, check for any vacuum leaks aroiund the #3 that could be bleeding off your intake vacuum. If you feel real sporty you can swap the #2 and #3 carbs on the rack and see if the problem moves to #2.
 

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Discussion Starter #5
This may be a reach, but wondering if it might be a header problem. This bike has aftermarket headers and pipe. I removed them to clean up the pipes and paint them. I re-used the pipe gaskets when I put them back on. I'm reaching here, but totally stumped as I mentioned.
 

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Uhm, no; its not the header. It will idle without the header, so no.

Try swaping the coils first just to double check it's not a coil problem. IIRC those were waste spark systems, two cylinders connected to the same coil; swap the wires on the other cylinder that shares the coil with No. 3. (it's cyliner No. 2). I think based on the fact cylinder No. 2 works fine that the coil is working, but the cable might be shot. If the problem doesn't move to cylinder No. 2 that would indicate the cable and coil are fine; spray starting fluid directly on carb No. 3, if it fires that would confirm there is a problem with the carb, if it doesn't then you'll need a compression tester to figure out if it has enough compression.
 

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Discussion Starter #8
I swapped the coils and no change. After that I kept the engine running and continually hit the #3 cylinder with starting fluid through the air box and the cylinder head pipe came up to operating temperature. Looks like a carb problem so I'll pull the rack AGAIN and dive into it.
 

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When you clean the carbs, make sure you pay attention to the pilot jet, and the emulsion tube. Those have super tiny passages that are easily clogged. And make sure you set the float level correctly as well.
And as mentioned before, check the intake boot(s) for vacuum leaks. The rubber boots are subjected to a lot of heat from the engine and get hard and cracky over time.
 

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The emulsion tube presses out the top, the big plastic piece it passes through also comes out. Check out Bad_T's sticky in this forum on photo carb rebuild, it has lots of good pics. Also check out this BST40 Biblethread on ADV Rider, it is for the BST40, but the construction is the same on the 36 and 38s.
 

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Discussion Starter #16
Thanks for the info Bufftester. I was able to slide out the emulsion tube on the #3 carb and found that it wasn't clogged. I set all of the pilot screws at 2.5 turns and re-installed the rack. Bike fires right up and runs pretty good except for the #3 cylinder.
 

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Yeah, the 2.5 turns is a good starting point that should let the bike run, though it might not be optimal. I think most of my bikes have ended up closer to 4 turns when they were running good.
Agreed on checking the passages as well. There are some seriously small passages that get clogged so easily.
Be careful removing the diaphram so that you don't tear it or spray any cleaner there, as most carb cleaners are harsh on the rubber pieces. There is also a tiny 'O' ring for the synch port. don't lose it!
Check the choke plungers as well. The caps have a rubber seal inside that could be causing a vacuum leak.
 

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You have to leave those carbs soaking in carb cleaner overnight and then blow the passages with compressed air; carb cleaner spray cans do not have enough pressure to really clean the passages if they are clogged. At this point, with all the testing you've done, it's pretty clear the problem is the carburetor.
 
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