StrayCats GSXR 750 Problem - Only VRF Forum Guys seem to be helpful

Discussion in '1st & 2nd Generation 1983-1989' started by straycat, Nov 20, 2021.

  1. straycat

    straycat Member

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    Hi Guys, since this is by far the only forum on the Internet with good attendance and sound advice, im going to beg forgiveness and post this problem and hope for some good insights:

    Id like to see were you’d go next with this problem I'm having trouble getting the '93 GSXR 750 to run properly - A misfire once hot and a coincidental rattle once hot.

    Here’s the back story:

    -When I got her home after buying it in May, she wouldn’t start or run at all,
    -I removed the tank and filled both fuel lines with fuel, the bike fired up and ran OK. I was able to Rev it up and it sounded OK.
    -Turned out there was no fuel flow from the diaphragm pet valve -I rebuilt the petcock valve, tank was super clean, put the tank back on
    -Initially it ran fine, but each time I started and ran the bike it got worse and worse until finally it just wouldn’t start again.
    -Removed the tank again, fed fuel direct to the fuel lines from an external source — no different, hard to start no run
    -I found it would start on full choke with a bit of throttle but ran on only cyl #2, and no matter how much throttle I gave it - it wouldn’t rev, nor would it rev or fire on other cyl if I put fuel down the carb throats
    -Last week I pulled the carbs off and cleaned them in the ultrasonic cleaner (they were quite clean as far as I could see)
    -I pulled the plugs and checked for spark from both coils - all seemed ok

    All back together now and here’s what I get:


    She fires right up on a cold start with choke

    I notice that often Cyl #3 is still cold or cooler when the other cyl's are getting warm /hot

    Eventually Cyl 3 Kicks in and the bike seems to Run fine, and Rev fine

    Once its been running for a few Minutes and is getting full warm/hot, it seems to start misfiring, snap the throttle and its no longer smooth, it pops and snaps etc.

    At the same time as this "miss fire” arrives, an audible rattle starts from cyl #3 head area as well, you can hear it and feel it with a screwdriver vibrating on the header pipe of Cyl#3 but not on the others - so for sure the noise is from the same #3 cyl Im “guessing" is misfiring.

    So, Im wondering if I have a spark issue on Cyl #3 thats appearing as things heat up ? Would such a misfire cause a rattle/vibration?. Up until it the misfire starts is as smooth as glass, no noise.

    When I pulled the spark plug and spark plug boot on Cyl #3, the HT lead just fell off the boot, so I snipped 1/8” off it and re attached the HT lead to the boot, but no real difference I can tell.

    Thoughts ? Wonder if I have weak spark on #3 to begin with which is why its taking longer to kick in and then it just shits the bed once its hot.? Maybe it starts dumping raw fuel into header #3 which when hot is detonating ? Maybe buy a new set of plugs, but thought Id pick your brains as many of you are far more experienced than me. The misfire is one thing, the coincident rattle is odd.

    Here is my other theory.... last week I changed the engine oil, it was way over full and seemed quite thin. Im wondering if Carb 3 has a needle and seat leak and im over fuelling that cyl. given that one coil fires 2 cylinders, its a wasted spark system and.. if im over fuelling on Cyl 3 than its also going to fire on the exhaust stroke causing detonation in the pipe ? the only part that doesnt add up for me there is that im not providing continuous fuel supply. Im filling the fuel lines from a squirt bottle, allowing it to run and then topping it up as the fuel is used, so not clear if that could still flood it.



    Coil 1 fires Cyl 2 & 3
    Coil 2 fires Cyl 1 & 4
    Fuel line 1 feeds carbs 1& 2
    Fuel line 2 feeds Carbs 3&4


    Thanks fellas !!
     
    Last edited: Nov 20, 2021
  2. Terry Smith

    Terry Smith Member

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    I'm far from an expert in these things, however some things I can suggest are:
    1. A cylinder not running, or running differently to the rest, will definitely cause a smooth engine to rattle. E.g. when carbs are badly synched they cause a rattly idle.
    2. How is the vacuum line to the petcock and which manifold does it connect to? Any leak within the petcock can allow raw fuel to enter that cylinder.
    3. Have you tried new plugs?
    4. What are the resistance readings for your coils/spark leads? Coils that check out when cold can fail when hot. You could switch the coils over and see if the fault also moves.
    5. When you cleaned the carbs did you pull the low speed jets and make sure they can flow?
    6. With the carbs off it makes diagnostic sense to set up an IV bottle of fuel and check that the floats shut nicely and no fuel leaks through to the carb throats.
    Good luck.
     
  3. straycat

    straycat Member

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    Hi Terry, thanks

    1. OK, yea , it rattles at all speeds once it gets hot, but just in header pipe #3 which leads me to think its raw fuel and its either detonating due to the hot header. (only does it once it gets hot) or since its a wasted spark system the over fuelling & heat in the header is allowing the spark plug to ignite on the exhaust stroke too
    2. Not using the gas tank yet, im running out off a squirt bottle filling the carb lines intermittently to avoid over fuelling if I can (narrowing the problems down)
    3. Not yet, but thats on the list after I swap plugs 3 and 4 to see if the problem moves
    4. Dunno, not checked them yet. I looked into swapping the 2 coils but i'd have to pull the HT leads off as well. the coils are in different locations and the HT leads for Cyl 2&3 wont reach cyl 1 and 4 --- I will look into the resistance check for sure
    5. Yes, all the jets, pilot and mains were squeeky clean, all the carbs and the jets all went in the Ultrasoinc Cleaner too
    6. Yea, shit, I should have checked that with my IV bottle when I had them off -- Dumb ass me, good reminder thanks Terry

    So im either over fuelling for some reason, or lack of good spark is causing unburned fuel to enter the header pipe.

    so right now my best guesses are either bad spark plug on Cyl#3 or Carb #3 is over fuelling for some reason (needle & seat, choke circuit, sticking slide ?)

    Tomorrow's plan:

    -Pull Plug 3 and 4
    -Examine bore for wet fuel in #3 (its sitting overnight with full bowls)
    -Swap plugs
    -Pull air box cover back off and observe the slide on Carb 3 to see if its moving like the others
    -Figure out how to test coils for proper resistance & check them

    Next steps after that will depend on what I uncover. I may be pulling the Carbs again
     
  4. Jim McCulloch

    Jim McCulloch New Member

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    Very good run down above, Checking resistance on coils I have found to be iffy at best. They always seem to check OK in my experience.

    I have seen crank ignition pick up coils fail intermittently also.

    Been a long time since I have worked on a Gixxer, and it was a late 80's model. My knowledge may be obsolete for a 93 model.
     
  5. straycat

    straycat Member

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    Thanks Jim,

    My bet is the coils are fine since its a waste spark system and the same coil fires 2 & 3 at the same time. Not sure what happens when the coil fails, if it may only trigger 1 plug ? may be....ill check them any way.
     
  6. raYzerman

    raYzerman Member

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    Following along, can't give you much more advice than you have.... can you check the spark plug adapter resistance, are they removable from the plug wires (which I assume are copper).
    If the rattle is detonation, I might go along with too much fuel, but thinking afterburn should be more like popping in the exhaust.... detonation rattle would be more like timing advanced, firing too much before TDC. ??
    Now, if you had overfueling.... leave fuel supply hooked up, spin it over in the morning (plug wires off) a turn or two, pull the plugs. Overfuelled should show a very wet plug (and of course, some likely went by the rings into the oil if the needle valve didn't seal).
    Anyway, carry on with your plan.
     
  7. squirrelman

    squirrelman Member

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    things can be fine at startup but fail when they get hot, ignition coils, cdi, pulse generator coils.

    maybe try adjusting mixture screws so it starts and run with minimum or no choke. also try switching #3 plug with any other.
     
  8. straycat

    straycat Member

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    Cheers, ill try that too SM

    Here's my next theory (mind is working overtime on this) -- What if I have a valve hanging up when it gets hot? that would also explain a misfire and the rattle. Doesnt explain one cold pipe to begin with I dont think but the misfire and rattle would make sense with a hung up valve. possible valve clearances too tight or a valve stem with some carbon hanging up ???

    Compression check when hot may yield some info there, failing that, its off with the valve cover if the electrics and fuel are all ok.

    Ray - I am getting the popping in the exhaust as well (a lot), I'm truly grasping at alot of straws here so at this point ill just start my next steps and try knocking each one off at a time and see where it leads me.

    Feel free to jump in and tell me my Valves are probably OK and how unlikely that is to have one hanging up, I could use some good news. LOL
     
    Last edited: Nov 20, 2021
  9. Jim McCulloch

    Jim McCulloch New Member

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    Yeah, a baseline compression test is probably a good idea. Check/Adjust valves while you are in there with the cam cover off.
     
  10. raYzerman

    raYzerman Member

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    I'll bring up a problem with some ...... there may be a tar-like build-up on the cooler intake valve stems, the only way to know is pull the carbs and get your flashlight out... two ways it can work... valves cold can be held either open or closed with this semi-hardened gum. Hot engine and the gum softens, enough of it may hold a valve from seating fully.
    However, I'd expect a problem starting the bike (valve open/low compression), rather than a hot running problem...... perhaps check compression both hot and cold, if it's an issue it will be VERY low compression.
    Ethanol fuels will contribute to the build-up issue.
     
  11. Captain 80s

    Captain 80s Member

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    I probably need to re-read this whole thread again one more time, but the very first two things in my head were, pulse generator and low compression. A valve lash adjustment would probably be time well spent too, you might have a "tight" valve that causes problems when things get hot and clearances change.
     
  12. Captain 80s

    Captain 80s Member

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    OK, re-read 1st post again. My Hawk GT had extremely similar symptoms, like almost exact. Before I swapped the exhaust and rejetted the carbs it started and seem to run fine, weirdly, so I started going down the path that I totally misjudged my first jetting attempt. But the more I thought about it, I couldn't have been THAT far off. So I fucked around and did the cardinal sin of changing more than one thing at a time.

    But the things I did would have been done any way, and if it took care of it I really didn't care what it was. So away I went. Swapped the coils, so when I was done and the issue moved to the other cylinder (and much easier to get to location) I would know that was likely my issue. While doing that, installed all new NGK spark plug wire, 4 new NGK caps and 4 new NGK plugs.

    Fucking ran so good, my jetting was pretty much perfect and was able to do some shake down runs this summer.

    Good luck!!
     
    Terry Smith likes this.
  13. straycat

    straycat Member

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    Thanks Capn'

    I tested the coils the secondary check on both was fine (the probes in the HT leads), the primary I could only reach one coil (the non problem one and it read a bit high. at 3.9 Ohms (2.6-3.2 is the range).

    I swapped plugs 3 and 4 and I ran it again this morning on a remote IV bottle, let her run for a good 10 min, Similar issue, cyl 3 pipe was cold. It popped and snapped and shot flames when on choke, once off choke it would run and rev but #3 was still cold. Since #3 never got hot, I never heard the valve noise today .

    After I ran for the 10 min, I pulled plug 3 again and it was dry, I also noted today that plug 3 has a different cap than the other cylinders (the wrong one for that bike). Anyway after letting it cool off for 15 min I fired it up again and now Cyl 4 is also AWOL, its barely warm either, different coil to Cy 3.

    Was gonna do a compression check and I found my tested wont fit, I have 4 adapters for it and all are too big, the plugs are tiny. New tester needed I guess.

    The odd thing is, what happening now is exactly what happened a month or 2 ago, I got it running (OK ish) and every time I run it it runs worse than the previous time and starts dropping cylinders . Eventually (last time) only cyl #2 would fire.

    Ill buy new Plugs, HT wires , boots, Coils etc once I've cleaned the carbs AGAIN. Its odd to me that the plug for #3 wasn't wet after running 10 minutes and that pipe was cold.

    Im off to the shop to see if I can get #3 or #4 to fire with a squirt down the carb throat.

    Damn bikes
     
  14. Captain 80s

    Captain 80s Member

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    Need to check valve clearances at this point before any compression check. They need to be verified anyway and you can then feel confident or sick about compression test results.

    That is odd about #3 plug.
     
  15. straycat

    straycat Member

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    Well i found a push in adapter for my compression tester (forgot I had it)

    compression is great.

    I did drizzle fuel into the throat of carb 3 when it was running and still no running on that #3 cyl, in fact it flooded so bad it dripped out the header connection at the head.

    CDi and pulsers seem unlikely as the issue. One pulser per coil. 1&4 together and 2&3 together. Since I have now lost cyl 3 and cyl 4, it seems highly unlikely I have an issue with the Cdi, the Pulser or the Coils (tho im on the fece with the coils).

    Spark plugs and a new boot to start with and another carb cleaning when I can muster the energy this week - I need a break from this monster

    The only thing cylinders 3&4 share is a fuel supply line (One supply line for carbs 1&2 and one line for carbs 3&4) ---

    I agree that I do need to check the valve clearances, but since the compression is OK, surely it should run ? no ? and it was running great on cyl 4 this morning and then that shit the bed after it cooled off for 15 min and I fired it up again.
     
  16. Captain 80s

    Captain 80s Member

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    Cold yes, but you might have one or two valves right on the edge that with any heat are staying slightly open.

    You know you need to verify, so it will be another box checked.
     
  17. Captain 80s

    Captain 80s Member

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    So many conflicting symptoms! Ugh.
     
  18. straycat

    straycat Member

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    Yea, its frustrating and odd. Im dropping cylinders 1 by 1 just like last time. starts off good then first #3 goes , then #4 and last time #1 was the third cyl to go. Only # 2 was barely hanging on.

    Ive also switch plugs around and the problem doesnt follow the plugs

    last time it happened I cleaned the carbs and all 4 came back to life only to slowly die one cylinder at a time between yesterday and today. Im sure 1 or 2 will give it up tomorrow if I keep running it.

    If the fuel tank was on it, id blame crap from tank (even tho its clean) but im running off a different fuel supply so its strange.

    Search for plugs tomorrow and Clean the carbs again. Im going to also get a new fuel squirt bottle, im wondering if mine is starting to decay inside and is depositing fine amounts of dissolved plastic goo in the jets. There is a wrinkled appearance to the inside of the squirt bottle. today I ran it off the IV tank but the damage may have been done? (looking at all options here). I tested the inside of the bottle with a screwdriver to see if I could get anything to come off, but I couldn't.
     
  19. NorcalBoy

    NorcalBoy Member

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    1.) Number 3 is not firing
    2.) Number 4 is firing intermittently
    3.) Noise coming from either inside the head or from the exhaust when running.

    My path of diagnosis would be the following:

    1.) Don't assume any of the electrical is good
    2.) Pulse generator is like a distributor, so I would take that info and check to make sure that the pulse generator is sending a signal to the CDI to actually fire the plugs. This is where the entire circuit starts, start at the beginning.
    3.) Once there is confirmation the the pulse generator is functioning, confirm that the CDI is actually sending a signal when it is supposed to, I.E. is the timing set right, both ignition and cam.
    4.) Once that is confirmed, I would swap the ignition coils and verify that the coil is firing the two plugs, 3 & 4.
    5.) Once any coil issues are confirmed and rectified, I would visually inspect the strength of the spark at the spark plugs, this can be done with an inline tester.
    6.) Once a strong spark is visually confirmed (consistent, fat, blue kernel) at all plugs, I would proceed to checking the valve clearances and making sure they are set. The popping in the pipe would indicate that the plug is firing the charge when the valves aren't completely closed, which right now, would appear to be on the exhaust side, otherwise you would be getting reversion and it would be popping back through the carb and into the airbox. Either that, or there is a stuck valve, broken valve spring, etc. Remove valve cover and visually inspect, turning the engine over manually, inspect cam lobes on suspect cylinders.
    7.) Only after all of this would I start putzing around with a fueling issue.

    I'm not saying this is the only way, because it most certainly is not, but this would be my flow path of choice
     
  20. straycat

    straycat Member

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    Thanks NB. ill go through your points, I "think" ive addressed some of them already. but ill look again

    The issue is, All Cylinders started off ok, then each cyl drops or starts to drop the more times I fire it up and run it over the course of several days. once the go out they dont come back (till I cleaned the carbs last week)

    2 months back it happened, started ok on 4 cylinders, then 3 then 2 finally only cyl 2 was making an effort.

    Cleaned the carbs last week and it ran on all 4 again (tho I didnt run it long), then 2 days later I ran it and #3 started to go south and finally quit altogether today, now #4 has gone out completely. Cyl 1 and 2 are still live and both run off a Different coil

    there are 2 coils, One coil for 1&4 and one coil for 2&3. (cly 3 and 4 are now not running)

    2 trigger pulsers (one for each coil), its a wasted spark system so 1&4 are firing at the same time and 2&3 re firing at the same time.

    Each time ive tested for spark on 3 & 4 it appears to be good. and since 1 & 2 are running (at the moment) I have to believe 3 & 4 are also good from a CDI and pulser perspective. I have moved plugs around and the issue doesnt follow the plug. once the cylinders die the only thing ive done to revive them is a carb cleaning.

    That said, I take your point, I remain perplexed on why when cyl 3 first died (cold header pipe) I was getting popping and flames out the exhaust when I first started and ran the bike on choke. My assumption was fuel from the choke circuit, lack of spark on #3 resulting in raw fuel getting into the pipes - However, it may be related to what ever valve issue I have ( although so far compression test good so no stuck valve when its cold/warm - So IDK, im puzzled and I'm open to any and all options to get it sorted out.

    I do suspect a valve issue of some sort on #3. I can hear the "mechanical" rattle, (its not a detonation like I first thought), it only happens when #3 runs and gets hot enough. so for sure the valve cover is coming off at some point.

    So.... im chasing 2 potentially different issues I think 1) im losing cylinders 1 at a time right now and that doesnt change when the bike cools off the next day they dont come back on line. and 2) the valve noise when #3 runs and gets hot causing a misfire

    Process of elimination lies ahead, cheapest stuff first I guess

    Thanks for taking the time to reply to my thread, much appreciated

    Screen Shot 2021-11-21 at 1.54.02 PM.png
     
    Last edited: Nov 21, 2021
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