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Discussion Starter #1
Hi All-

I know this is a fairly common issue, but all of the threads I have read are for newer bikes with solutions that are not applicable to my year.

I have a 1993 GSX-R 750 that starts up and revs fine. It will overheat if I am not moving forward and forcing air through the radiator. On the road it will back fire. Finally, it will bog down around 10-15 miles into a ride, where it feels like all four cylinders are firing, but it just starts to lose power and I lose speed. This progressively gets worse and feels like I lose a cylinder and continues to bog worse with more loss of speed, until finally all cylinders are lost, I'm on the side of the road, and the bike won't start and get to hitchhike home to get my trailer.

The next day, the bike will start and the cycle repeats itself.

The carbs are freshly rebuilt by me... and then again by a professional (a friend) thinking I did something wrong. The pro bench-tuned the carps to factory spec. Fuel filter (in the tank) is clean and free from grime/rust. No kinks in the line that I could identify. The pro did a once over, and gave me the bike running great, and at about mile 10 the exact same issue happened.

I took the bike to a shop, and they just want me to throw money at it, which I am not interested in doing. I requested that they test things to verify something needs replaced, and to be honest they were not interested in spending the time on my old bike. That much was evident in the tone of voice and perceivable lack of interest in working on it and trying to save me money.

I have a few theories.... One, is that its running lean due to overheating and I may need to go up in jet size. But, then why would it back fire (isn't backfiring too much fuel?).

I am running a Yoshimura pipe, so maybe the bench-tune is not adjusted properly for my pipe. But, it did this before the bench-tune. I even thought about finding a factory pipe at a junkyard to see if it works.

One other possibility is I do have an electrical issue somewhere in the dash. It never seemed to affect performance, but something is loose and my speedometer and temp gauge will cut out. Tacometer always works. When I lose the speedo dial, the bike still runs normally (loose wire?), so I never equated that with the issues above.

I am at the point where I am thinking of just getting rid of the bike because I don't know what to do, and I can't afford to dump thousands into it by bringing it to a shop.

Has anyone experienced this problem before, or have any suggestions on what I should do? I know it's something easily fixed, I just don't know what to check, or if its multiple issues.

Thanks in advance.
 

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Welcome.

Those are not uncommon issues but not specific to the make and not all at once.

On the overheating. Is the fan coming on?

On the power loss since you went through the carbs and fuel supply the next thought would be tank venting.

The cluster on most bikes, including your bike, is not needed for the bike to start and run normally. It is just an indicator and you can just remove it if you like.
 

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Discussion Starter #3
Thank you for the reply John.

Yes, the fan does come on. It doesn't seem top be enough to cool without forward momentum.

I am going to inspect the tank vent. I have not considered that. I am looking through the manual and don't see any reference on how to check it. I assume it blows one way, and I can test with my mouth? I'm gonna pop that off now and look at it.

In reading the manual I do see a common remedy for two issues. The manual states that "sucking air from intake pipe" will cause both overheating and loss of power. Is it referring to the air filter box housing and where it connects to the carbs via four rubber boots held on by four hose clamps? I ensured that those were on securely, and I even replaced the hose brackets with new ones (old ones were a little misshapen).
 

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I like Johns reasoning about the tank vent, but if you ride it with about a half tank of gas, and leave the cap partially propped- that should diagnose a tank venting problem- no?

You sound as though you have a decent handle on the diagnostics- not sure where to start on the overheating thing. I mean, it obviously has the proper levels, was bled properly the last time it was changed, etc.

Good luck and Welcome to the 'R', people like John and others- we'll get it figured out properly.

As far as your cluster going out some- on my bike and many others- that is caused by the plug in the harness going to the front end where a common ground wire overheats, and fries the plug and breaks the connection to the common ground for the gauges ans such. Have you looked inside that plug going to the front end? On my bike it's on the Left on top, under the left side plastics. Large multiwire cluster plug.
YMMV
 

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Discussion Starter #5
The tank vent (the one with the hose that comes off the top of the tank and decends down between the handlebars) blows freely in both directions.
 

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Discussion Starter #6
Thank you Todd, and thank you for the warm welcome. I appreciate the response.

One thing I should also mention, is that the bike always ran hot. I've had this bike since 1995, and from day one it ran hot. The problems with loss of power were in recent years.

Also, and I don't know if this factors, but the bike used to live at sea level. I now live near the continental divide, at 3000 ft above sea level in the valley I live.
 

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I'm guessing that the "sucking air from intake pipe" reference in the manual refers to the intake boots also. This would cause a lean condition and hotter running engine so that sounds reasonable.

From your post, I assumed it was overheating while at low power or idle since you mentioned no air movement. A leaky boot would be a factor at high power but I don't see how it would cause that at low/idle power. Of course it would cause poor idling, etc.

Sorry, can't comment on how your tank is vented as I don't know the model. The quick check is to ride the bike till it acts up then open the filler cap to see if it improves or you might hear a sucking sound as you open the cap.
 

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Discussion Starter #8
I assumed it was overheating while at low power or idle since you mentioned no air movement
Yes, exactly! For example, if I were to get stuck in traffic or at an extended traffic light, the temp will begin rising. There had been times where I've pulled over to the side of the road to avoid overheating the bike. Once I am out of traffic, and moving again, the temp returns to normal.
 

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So what do you mean by overheating? Water starts bubbling out of the overflow tank? What temperatures are we talking about?
 

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As a for instance. i noticed a few days ago on a spirited ride in the country- My bike was under 180F.
I got stuck on the road train as the arms were coming down, I decided to wait. It was 78F outside, little to no wind.
I short time later here comes a LONG ass train. Seemed like at least 10-15 minutes!
I didn't shut off and watched my temp gauge.
My fan now comes on at 195F - a mod of my ECU flash.
It never got over 205F. I was happy.

What kind of temps we talking about here?
 

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.
 

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Discussion Starter #12
So what do you mean by overheating? Water starts bubbling out of the overflow tank? What temperatures are we talking about?
While riding the temp is in the normal range. As soon as I hit a traffic light the I can watch the temp slowly start to climb. It will continue to climb until the needle hits the red zone. I typically shut the engine off about 3/4 of the way up the gauge if it doesn't look like the light will change any time soon. If in traffic I will have to pull over to the side of the road, or ride up the middle.

I don't have numbers on my temp gauge, so I can't say for sure what the actual temp is.

I can also feel the heat on my legs coming off.
 

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Discussion Starter #13
I am able to manage temp better now that I live in an area with less traffic. The main problem I would like to address is the bogging, loss of power, and being stranded on the side of the road.

Of course, I want to fix the heating issue too, but I'm literally dead in the water until I get the above figured out.

My tank vent blows in both directions. Is that normal?

Does anything think my carb settings and pipe are out of balance and causing the issue?

Could it be coils? One coil is new, the other I think is not (this was done last season by the shop when I had it there).

Once I can get back on the road, I would love to fix the heating issue. Just not sure on the direction to take at this point. My knowledge on this subject is rather limited, so I appreciate all of your interest in helping me.
 

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All right. I have the same bike (almost). ('95)

Anyway, The 2 times the carbs were rebuilt, were they built back to the way they were? Same jetting, needle height, and float height? If so, moving to 3000 above sea level, will make you VERY rich. Running rich across the throttle range will give you the (runability) symptoms you have. You will need to go down a jet size or 2 (I'd start with one), and lower your needle, and floats. possibly a pilot jet size down as well. An easy way to confirm this, is to take your airbox cover off, and go for a ride.

Over heating... Have you checked the level in the radiator? Not the expansion bottle, but the radiator itself? When was the last time you changed the coolant? Was it bled properly?
 

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Discussion Starter #15
Thank you Spyder. That was another question I wasn't sure about (jetting). I'm going to order sets for the next few sizes down.

To answer your question, when I rebuilt the carbs (which was first) I set everything back to the way they were. Then when I had a pro do it I could not say. I would imagine that he would have set them back to what is required for this elevation, but to be honest I don't recall the discussion. He did say, something on the lines of, "I dialed everything in to factory spec". I am 99% certain that jets were not changed by him (I for sure did not), so they are the same jets that I had in the bike at sea level.

The coolant was drained and replaced and filled (in the radiator). I did it once, and had the shop do it once too. That is something I will check again when I get the jets.
 

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Mine are 115, and 117.5 stock. Your being only 2 years older I would think they are the same. It would tell you in the manual. IRC, the 117.5 go on the outside carbs, and 115's on the inside 2. I don't know if they were offset like that in '93, but it will be in the manual. The only thing I can think of besides coolant level, would be either air in the system, water pump, or t-stat. Both of my coolant sensors are bad. So, my gauge reads low, and the fan was bypassed with a manual switch. I bought a new fan sensor, but still need the gauge sensor. What I'm getting at, is that your sensors could be bad, and you're really not getting that hot. Just some more to look into. Do you have an IR temp gun by chance?
 

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Does this bike have a pedcock? Manual or Vacuum operated. Have you checked for good flow to the carbs pull the fuel line off and check flow. Like John suggested check for fuel venting maybe if possible leave the cap loose and ride. Sure sounds like it could be a venting problem. Stops let sit overnight pressure equalizes and the cycle repeats. As for the over heating are you sure the radiator cap is good and the correct pressure for said bike?
This may or may not help you
 

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I'm going with WAY too rich. As the engine heats up, it gets even richer. In this case, too rich to even run. Once cooled it can run again. I would contact 6-Sygma and order a jet kit for your bike. They require you to give them, year, make, model (of course) and also mods (exhaust and intake), and elevation. With this info they send you the correct jetting you will need for your given situation. They take the guessing out of this procedure.
 

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Discussion Starter #19
@Spyder13.... Thank you sir! I am going to contact them today.

@JETMECHANIC... it does have a petcock (manual). It's been a few years, but I have replaced the petcock. As far as I know it flows appropriately. I should check it again since I have the tank off getting to the carbs. I will certainly try it with the cap off, or if I end up on the side of the road pop the cap and listen for the vacuum break sound (as some have suggested). As for the heating issue, I am going to go through the whole system. I honestly don't have confidence that the cap and pressure are good.

Regarding my tank vent, I took it off and blew threw it in both directions. Is this correct? I ask, because I know that on my jetski the fuel primes with a few PSI in the tank. The tank vent on it blows in one direction, and if you could blow in both directions the vent needs to be replaced. I don't know if this is the case with the bike, and I can't find anything in the manual about it.

@Everyone... Thank you so much for the help and advice. I am humbled by the responses. This is a great forum, and I very much appreciate the assistance. It has been more help than even the shops were willing to offer, so I am very appreciative.
 

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Discussion Starter #20
I ordered a jet kit from 6-Sigma and I have a question for you gentlemen. First, I was a little disappointed to see that the jet size in my carbs are the same as what 6-Sigma sent me (well, almost... I will explain, and get to my question).

6-Sigma recommended the following jets:
  • Outer carbs (#1 and #4): Main Jet size 115
  • Inner carbs (#2 and #3) Main Jet size 117.5
I pulled the carb bank off the bike, and pulled the float caps off. My carbs right now all have 115 Main Jets in them.

My question is, we thought I was running too rich, but 6-Sigma is recommending I go up a main jet size on the two inner carbs. Wouldn't going from 115 to 117.5 make me run even richer? 6-Sigma also sent me 120 jets and said that if I was still running to lean I could go up a size (117.5 outer and 120 inner).

Is it possible that my problem is that I'm running too lean for my pipe (Yoshimura headers and pipe)? Would a lean condition cause the big to bog, backfire, and lose power?

This also has me concerned that jetting is not the issue at all.
 
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