Persnikity 94 YZF750R and Mikuni BDST 83 Carbs!

[flyboyn2144p]

Ok this is my first time posting here so please bear with me.

I have a 1994 Yamaha YZF750R who when running rite is a wonderful bat out of hell. Rite now she is being a pain in the rear!

A brief history about her. I got her with 9200 miles from a Yamaha Dealership in South Carolina. She was owned by the shop owners son who didnt have any intrest in bikes so parked her in the warehouse of where she sat for approx 7 years. Well about 2 years ago the carbs got pulled to try to get her running by one of the TRAINED(Yes he was, I worked with him) mechanics. Brilliant fellow. Any way, he rebuilt the carbs and "made ready" the bike, but never got arround to putting fuel to or even installing the carbs before other projects were handed to him. So off she went to the "shed" to sit 2 years with her carbs in heavy plastic bags. Till I happened upon the place and found her. Paid $25.00 USD for the title and removed the COMPLETE BIKE off the propetery.

So over the past 7 months I have fiddled and tinkered like a mad scientist over her getting a part here and there, and out of salvage and from ebay. I have been able to ride her once but was not rite plus found to have other issues that have been corrected.

OK NOW DOWN TO THE PROBLEM>

At first when I got her running I had to reinstall the jets in the #1 carb because the clamp had come unscrewed in the bowl. So fixed that, plus added new oversized o-rings and pulled all the other bowls and found them to be ok and all were CLEAN AND BRAND NEW LOOKING AS WERE THE CARBS IN AND OUT! So I installed them and with a can of Ether off I went to get her purring again. It was not until I put the airbox on was I able to get her running. Any way she would not idle and I adjusted that but still wanted to die. she would run like a bat out of hell from 1400 to 13K with out thinking twice. I had fuel dripping from the bowl gaskets so I replaced new gaskets and no more leak.

Now, she will idle and rev out when first started but anything after that she only will go to 4000-4200 and no further. The Slides in the carbs 'hammer" bababababababaaba All 4 of them. Let her sit and cool a little and she will idle like a dream and crack her again and she will open up for a second and then back to the "hammer" of the slides. I am also getting light fuel spray comming out of the airbox side of the carbs.

I set the needle to the recomended 3.5 turns in the haynes(thinking this is a crap book at this point!), and have a brand new K&N in the box plus a Yoshi RS3 with factory header and no EXUp valve! but the servo is still there to make the computer think it is there. I also went through every passage way whith my air compressor and checked every emulsion tube, jet, orfice, etc. I checked all the slides, needles, diaphrams and o-rings for wear. Also replaced all the intake rubber boots so there was not any cracks. Installed a new fuel pump that carrys just under a lb of pressure plus a new controller and tank. I installed and gapped new NGK cr9E's, Topped off all fluids and NEW Oil change, Brand new Battery, New inline fuel hose and filter,

I am stumped and lost as to what to do from here. I am desperate and want to ride!! Plus she is transportation!

Nathan Fly_n_low1260@yahoo.com

[T1z3R]

im no expert but with no exup valve and the RS3 can & K&N filter i believe this bike will need rejet/setup to match to run with those items installed.

i have the same bike, recently returned to road ready condition, although a '97 one which is stock. ive had numerous running problems over the time ive had it which were all to do with a poor fitting airbox and a seized exup valve system.
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duck my sick!

[NSR Mick]

An EXUP servo motor still connected with no valve attached could make it like you say, I remember my servo motor going faulty and it ran like crap.
____________________
If you dont like the way that I ride.......Stop trying to keep up!!!

[flyboyn2144p]

I will unhook the exup this aft and see how she does. its a coupple of wires.. I thought the way the schmatics were that I needed it hooked up for the computer to function properly. I am thinking it is more carb related though. because the EXup was the same way before when it ran like a scalded dog.. Its all been carb changes.

Nathan

Will try it this aft though.. I truly need another mechanic here in jacksonville FL area to holler my way way with a set od gauges and adapters to rule vacuum out..

[T1z3R]

check out the exup brotherhood or exup100 . both good sites full of exup fanatics.
____________________
duck my sick!

[flyboyn2144p]

unhooked EXUp and still the same issue tacking out at 4K.. I also removed the new inline fuel filter I put in and still the same issue as above.

On inital start it will tack out to 13-15K but then the second or third rev it only hits 4000rpms..

AHHHHHHHHH

[flyboyn2144p]

I am going to compresion test and check all the diaphrams and o-rings for the slides


any Ideas?? Any one else??

[flyboyn2144p]

propane leak down test is tomorrow

[flyboyn2144p]

latest as of today April 22nd

I took the Mikinu BDST38's out

1. took off the slides and checked to make sure the diaphrams were all seated and o-rings were good. (all checked out)

2. Took air compressor and blew all jets out seeing as how there has been fuel ran through the system for several weeks and all trash should have been desolved by now. (nasty crap but clean fuel now)

3. inspected all intake boots and checked clamps on the carb side and all were tight

4. Checked Choke operations again (good and idles till 2500 then take choke off and idles @1200-1500

5. checked and tightened all bowl gaskets and checked float levels (ALL GOOD)

6. Started and let choke out and warm up closed choke and wound tach out several times.. got on to ride and had good take offs BUT CRUISE WAS ROUGH AND JERKY>>> 8K and up was a bat out of hell. stand up materal.. between 1500 and 6000 it is rough.. any ideas?

Nathan

[NSR Mick]

I take it the 38's arent standard kit?

Sounds like a serious jetting issue if that's the case.

When I ran my 1000 with open carbs it was very lumpy low down and would cough and splutter until it reached certain revs (could have been around 8k like yours) then it would take off like a scalded cat.
____________________
If you dont like the way that I ride.......Stop trying to keep up!!!

[Robby]

This could well be a simple fuel suppy issue.

You say its running fine when you start it and will rev right up to the redline, but after that it won't rev past 4k.

Simple things first - you changed the fuel pump, did you change the fuel line? If the fuel line has 6 years of fuel gunge in it, you could well have constricted supply to the carbs. Is the fuel pump supplying the correct pressure? If its too high pressure you may be overfilling the float bowls, needle valves on bike carbs are always crap.
This could give you an excessively rich mixture, which would explain poor high rev running.

Unfortunately, you have also messed around with the exhaust and air inlet, and the exup valve. You've disturbed a black art, so it will probably need dyno time for a proper set up either way.


Then there is also the problem of an electrical fault. The faults would point towards pickup coils or ignition coils, but it is unusual for these to break down so quickly - they usually need to get warm before stopping working, and that would cause the bike to run on 3,2, or possibly 1 cylinder.


What would I do?
First, check/replace the line from fuel tank to carbs. If it goes into a metal pipe that distributes the fuel along the carb bank, ensure that this pipe is clear.
If that doesn't do it, I would probably go for new standard size jets. I don't know how your cleaned the ones you have, or is you can be sure they are spotless. If you cleaned them by poking a wire through them, you can alter the size and shape of the opening. This can cause all kinds of problems, and jets are cheap.