Reviving a '91 GSXR750 - Suzuki GSX-R Motorcycle Forums Gixxer.com
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post #1 of 13 (permalink) Old 06-27-2019, 08:28 AM Thread Starter
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Join Date: Jan 2019
Location: Parkville, MO
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Reviving a '91 GSXR750

Back in January, I bought a 750M in non-running condition as a project. It's been a slow process, first because of the long cold winter (my shop is not heated) and I've also had a few other ongoing projects get in the way.

The PO mentioned that the emulsion tubes were ovaled so the carbs would need kits. So, I've rebuilt those, and in the process I've fixed a handful of other issues (hacked wiring, leaky fork seals, fuel tank/petcock issues, etc.). My main gripe though was that the frame/engine/wheels had been "murdered" out. I've spent the majority of my time cleaning up that mistake.

I've been able to get it running (not well) and have put a few miles on it. It seems to be solid and rides and shifts well. It just needs a little more tweaking of the carbs, and I should have it road ready soon.

But, here comes my first question. According to the service manual, spark plugs should be NGK JR9C's, but the existing plugs are NGK CR9EK's. The CR9EK's have a smaller thread so the JR9C's won't even thread into the head. Has there been a swap of some sort in my bike's past life? Does anyone know what engine/head takes the CR plugs? I haven't checked the engine numbers, but I guess that should be my next step.

Thanks,
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post #2 of 13 (permalink) Old 06-27-2019, 03:38 PM
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Re: Reviving a '91 GSXR750

the spark plug holes could've been helicoiled, therefore needing a smaller spark plug. no oiler head takes those, if im not mistaken

Last edited by Gixxerider86; 07-07-2019 at 11:14 PM.
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post #3 of 13 (permalink) Old 07-06-2019, 08:12 PM
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Motorcycle: 91 GSX-R 750
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Re: Reviving a '91 GSXR750

The front of the service manual is wrong you need to go to the 750m section. Stock plug for the 750M is cr10ek with cr9ek being an option
Number 20 in the list

https://www.suzukipartshouse.com/oem.../cylinder-head
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post #4 of 13 (permalink) Old 07-08-2019, 08:15 AM Thread Starter
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Re: Reviving a '91 GSXR750

Thanks for that rklontz. I was afraid I'd bought some sort of a frankenbike!
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post #5 of 13 (permalink) Old 07-10-2019, 10:50 AM
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Re: Reviving a '91 GSXR750

Running CR9E on my 750M so the manal did miss a bit :-)
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post #6 of 13 (permalink) Old 07-16-2019, 02:34 PM
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Re: Reviving a '91 GSXR750

welcome! please let us know your location, since sometimes it makes a difference in regards to these bikes specs.

yeah, the stock plugs are the CR10's and i'd use them.

good luck with the restoration. the trend is to paint everything black, and it seems like the previous owner was indeed a "murderer!" polish the frame and swingarm, if you are going to take it apart, OR when you take it apart. polished is period correct as well.

carb issues are no fun, but don't give up. you will learn much and once it is set, you won't have to mess with it anymore.
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post #7 of 13 (permalink) Old 07-19-2019, 08:46 AM Thread Starter
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Re: Reviving a '91 GSXR750

Thanks for the input 86gixxxer. I'd love to update my info, but for whatever reason, the website says I don't have permission to access and edit my account information. I've posted in the "Welcome" forum, but that didn't make a difference.

I've spent some more time with my carbs and found that two of the plastic slide guides were broken where the emulsion tubes penetrate. This allowed them to rise with the slides when the throttle opened, causing all sorts of problems. I wasn't able to find a place to buy just the guides, and the only used carbs I could find were in England (I'm in Missouri, USA). They're on order, but it'll take them a while to get here. Looks like I'm stuck on my other bike for a while longer (could be worse!).
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post #8 of 13 (permalink) Old 07-20-2019, 08:56 AM Thread Starter
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Re: Reviving a '91 GSXR750

Quote:
Originally Posted by 86gixxxer View Post
good luck with the restoration. the trend is to paint everything black, and it seems like the previous owner was indeed a "murderer!" polish the frame and swingarm, if you are going to take it apart, OR when you take it apart. polished is period correct as well.
Yup, going back with polished hard parts. There's been a lot of scrubbing with acetone to get it back to where it belongs.
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post #9 of 13 (permalink) Old 07-27-2019, 08:42 AM Thread Starter
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Re: Reviving a '91 GSXR750

I received my slide guides in the mail and was able to get my carbs back together and on the bike. Solved that problem, but it's still giving me trouble. I doesn't want to rev much past 4k rpm. This is far from my first carb rebuild, so I've checked the usual suspects. It idles nice and smoothly, so I was able to sync the carbs and get the air screws set. Floats are set correctly (clear vent tubes, so I can visually verify). All four slides move freely and with equal pressure. I've checked for vacuum leaks, and everything seems to be well sealed. I'm going to mess with slide needle height today, but I'm not expecting that to be the solution.

I'm starting to suspect an electronic issue. It's acting like maybe the ignition isn't advancing. Funny story. A few years ago I had another project bike with similar symptoms. I spent a month monkeying with carbs with no improvement. I don't remember why, but I eventually removed the CDI and found that someone had scratched "BAD" on the back of it. I replaced it and the bike ran like a charm.

I'd like to hear from the experts on the forum before I start replacing parts blindly. This project is starting to get expensive, and I still have to paint it. Thanks!
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post #10 of 13 (permalink) Old 07-27-2019, 03:54 PM Thread Starter
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Re: Reviving a '91 GSXR750

As I expected, shimming the slide needles didn't make a difference. According to the manual, it takes a special scan tool to check the CDI. Does anyone have experience with this? Is there a home brew way to troubleshoot the CDI?
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post #11 of 13 (permalink) Old 07-29-2019, 12:56 PM
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Re: Reviving a '91 GSXR750

this is where having extra parts is helpful. before buying extra parts, turn the petcock to the "ON" position, then ride it. if it still doesn't want to rev past 4K, then we know it is not a vacuum-fuel delivery issue.

besides that i'd suspect spark plugs and gap, coils, ignition timing/advancing, signal generator...again, having extra parts to swap is helpful.

if the engine is a 91 750, the number should begin with R716....

you've done good work on restoring the 91 750. looks much better! nice duc.
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post #12 of 13 (permalink) Old 07-29-2019, 01:17 PM Thread Starter
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Re: Reviving a '91 GSXR750

Kind of embarrassed, but I'm not suspecting the CDI anymore. A little background first.

I bought the bike in non-running condition with the carburetors removed. I got it from a local shop that was selling it for the previous owner. They had started working on it, but it got too expensive, so the guy just wanted to get rid of it. The mechanic said the main issue they had found was ovaled emulsion tubes (among several other minor things, most of which I've been able to address). The PO had converted to dual K&N pods and the carb had presumably been jetted for that setup. I'm not sure about the needles and emulsion tubes, but the main jets are 150s and the pilots appear to be stock (37.5). I replaced the needles and tubes with new stock units when I rebuilt the carbs. Spark plugs are also new and gapped correctly. Also, I currently have the tank off, and I'm using a remote fuel bottle (Motion Pro?) to supply gas. Pretty sure I'm getting plenty of fuel.

The problem with revving occurred with open carbs (no filters installed). Over the weekend after I shimmed the needles (mentioned above) I went out to start it and check a few other things. I forgot to pull the shop towels out of the carb inlets before I started it. Guess what? It zinged up 7 or 8k just like it should when I wacked the throttle. Then I realized what I had done. So I installed the filters, and it's back to normal (won't rev). I wrapped cardboard around ~3/4 of the filter circumference, and it's better but still not what I'd call ridable. So, I'm back to jetting or more restriction on the intake, etc. I'm assuming the PO had it running at an acceptable level, but I don't know that for sure. Any input you can give me on carb setup with the K&Ns and an unknown aftermarket exhaust would be appreciated. Thanks.

Last edited by twerth; 07-29-2019 at 01:20 PM.
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post #13 of 13 (permalink) Old 08-02-2019, 08:47 AM Thread Starter
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Re: Reviving a '91 GSXR750

I think I've got this solved. I shimmed the (stock) needles one more time and was able to get it to rev out, but it had a huge flat spot in the middle (~4500-5500 rpm). Moving in the right direction though. So I pulled the carbs, removed the needles and emulsion tubes and replaced them with the ones that came with the bike. The shop I bought it from told me the tubes were ovaled, but it wasn't visible to me. The aftermarket needles (DynoJet maybe? They have adjustment slots.) fit more loosely in the tubes than the stock ones, but I thought it was worth shot.

And it was! It revs cleanly all the way to the top, although a little dirty right off idle (I think I can tune that out). I'll ride it a couple of weeks to work out any quirks and then tear it back down for paint. Now I just have to decide on a color....

Thanks for the input!

BTW, the bike came with a spare air box and filter for a 1996-99 GSXR 750. I can't use it so I'd be willing to sell. I'll put it in the classifieds as soon as I have enough posts, but PM me in the meantime for more info and pics.
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