Suzuki GSXR Forum banner
1 - 20 of 46 Posts

·
Registered
Joined
·
86 Posts
Discussion Starter · #1 · (Edited)
Having a problem with my 750 dropping multiple cylinders then coming back to life, only to drop them again. Sorry for the long winded descritopn but Im trying to provide as much relevant info as I can

Here’s the back story:

-When I got the bike home after buying it in May, she wouldn’t start or run at all (I bought it as a non runner),
-I removed the fuel tank and filled both fuel lines with fuel, the bike immediately fired up and ran Fine. I was able to Rev it up and it sounded Good.
-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 ran worse and worse (dropping cylinders one at a time) 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
-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 and here’s what I got:

-She fired right up on a cold start with choke
-Once its was running for a few Minutes and was getting full warm/hot, it seemed to start misfiring, snap the throttle and its no longer smooth, it poped and snaps etc, flames out the pipe.
-Using my infrared thermometer on the header pipe, I saw that it cyl 2 first wasn't hot, 5 min later cyl 4 started to cool off and of course it ran worse again. with each re-start and run cycle it dropped a cylinder.

After it went down to 1 cyl again, I changed the plugs , fired it up and it ran on all 4 cyl again, But...again, after 10 min it started dropping cylinders again and now im back to running 1 or 2 cyl's. I changed plugs again (new ones each time) and this time its not making a difference, back to 1 or 2 cylinders.


What ive done /noted so far:

- Noted1&4 cylinders run off one coil and Cyl's 2 and 3 run off another - wasted spark system
-Cleaned the carbs (ultrasonic)
-Running the bike on an external fuel source with fresh fuel
-Tested the Pulser/trigger Ohms are good
-Tested the Coils - secondary is good, primary a bit high at 4.0 and 3.8
-Trimmed the HT leads where they attach to the plug caps for a better contact
-No difference with air-box cover /filter on or off
-Spark is present on all 4 cyl
-Changed spark plugs twice
-Bike runs like this be it hot or cold, once the cylinders drop, I can leave the bike overnight and fire it up again the next day and I still have the problem, so it does not appear to be related to how hot the bike is or the heat cycle, although run time does seem to trigger the problems, iethe problems dont "reset" or go away when its cold, ie its almost a cumulative issue.
-Cyl's 2 and 4 go out most often - they're on seperate coils and run off a different fuel line to each other

I've ordered a Carb kit in case I have bad o-rings and or worn needles /emulsion tubes (30 years of fuel isnt kind to OEM o-rings) - so ill pull the carbs again and "rebuild them"

Im contemplating Dynatek coils and wires (just because theyre are a bit out of spec on the primary resistance) , but ....I do I wonder why I can get a "no run" condition on cyl 2 and 4 (when both have spark) when they're on different coils and cyl 1 and 3 can be running fine (granted cyl 1 or 3 can also drop on me as well, its pretty random). --------Is the higher primary resistance in the coils (3.8, 4.0) frying plugs ?

Looking for any added insights here, Im reluctant to keep throwing $ at it without having a diagnosis pinned down. not looking to go broke on this

thanks in advance
 

·
Mod of the North
Joined
·
29,107 Posts
You've done everything right so far that I can see. How do the plugs look when you pull them out? Are they the stock plug or are you running a different temp? I am thinking fuel fouled - possibly needle height or improper jets? A rich or lean condition will help with the troubleshooting. Did the past owner jet it for some reason?

Are you running the stock air box and what exhaust? If they are CV carbs they will be sensitive to changes there. My BST 40's hated the aftermarket stuff and i went back to stock.

One thing you could do is switch the spark plug wires around. Not sure if 1 & 4 will reach to switch but if you are losing 2 & 4 then switch and you are losing 3 & 1 you have an electrical problem. If the issue stays it is carbs.
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #3 · (Edited)
-CR9E plugs (stock is CR9EK I think ?) Plugs look "normal" nothing obvious there, either on the old ones or the new ones that have only about 20 min run time on them
-No obvious fuel soaking on the dead cyl's either.
-Stock Airbox with K&N filter
-Stock CV carbs
-Nothing obvious on internals changed with jetting, but ill re check that when I pull the carbs again. ( I recall looking when I had them apart last time but forgot to write down the jet sizes).
-Carbs have been opened up before for sure, as someone changed the top caps screws and bowl screws for Allen style ones.
-Stock Exhaust was on it when the problems started (although I do have different one on now, it makes no difference to the problem when I swap it back).

Good point - I overlooked that obvious test - Ill try and swap the wires if they reach, but I think #1 cyl has Sh&t the bed as well and now im left with #3 alive, so may try swapping 2&3 if they reach. trouble now is its a bugger to start with 1 cyl only. Fun times. LOL

be easier if the problem was isolated to 1 cylinder or 2 cylinders that are paired to one coil, or if it was ALWAYS 4 &2, but sometimes #1 or 3 goes out as well.

It seems to me that it would be unlikely that 1) both coils would crap out at the same time, and 2) in both cases each coil would only fire one of its 2 plugs sufficiently to ignite the fuel and 3) that the problem would progress from 4 running cyl to 1 running cylinder over several start/run cycles,.... but anything is possible I guess.

Its the Primary resistance reading on the coils that has me worried that its the coils during the plugs, but I read that the primary only comes into effect when starting the bike, then the secondary takes over....but im not educated enough on coils etc to know if thats correct.

Frustrating now, but satisfying once I get it figured out I guess.

Thanks for the reply and input !!
 

·
Mod of the North
Joined
·
29,107 Posts
I am just spit balling - I don't know the lightning voodoo the inside of the coils holds either. I don't think it is the issue for this but the K&N may be a bit free flowing, I found my CV's liked to have to draw hard on the intake. I think the pressure differential across the carbs helped it operate and pull fuel better. It's a small thing you may address in the tuning stage and not likely gonna take out three cylinders.


Something else that popped in my head was float heights. You just did a petcock rebuild - If it still isn't passing fuel as it should and you have some low float heights in those bowls you might be simply running out of gas. Try it with the petcock straight down to open the valve manually and bypass the vacuum tube. Also check the vacuum tube to the petcock from intake boot 3 and make sure there are no cracks or leaks. An air leak around that boot will also reduce how well it works.

Also - when it does get limping on 1 or two cylinders give her a shot of WD40 and see if she perks up. That would indicate a fuel delivery probelm and confirm you still had spark.
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #5 · (Edited)
Thanks Tinsnips, im not currently running the fuel tank on it. Im using a remote fuel delivery bottle to eliminate the tank and petcock from the equation. Fuel starvation occurred to me as well when I first got it and it was ok after I filled the fuel lines with the tank off initially, but then it started misbehaving again without the tank.

Ive used both a remote fuel feed bottle and i've fed the fuel lines individually from a squirt bottle as well. With cyl's 4 and 2 being out of sorts most often, I initially thought that with the bike on the side stand, those 2 carbs are on the uphill side of the fuel feed and many be prone to starve first. However even with the bike on the lift stood vertical, im not able to remedy the issue.

I will for sure check float heights when I pull the carbs for rebuild, 15mm I believe is the spec for them, I didnt check that when I had them out before, lesson learned, I just did an ultrasonic cleaning and really should have taken more time to investigate and record more data. would have saved me time in the long run.

I did try a shot of ether/quick start a while back when she was limping and I got no response, my thought then was over fuelling or electrical.

Appreciate the ideas, I really do. its helpful to be quizzed on what ive done and what I haven't. some of the symptoms seem to be counter to one another.

At this point im down to the following possible culprits :

1) Carbs (worn internals, Orings etc, or improper calibration of jets etc)
2) Coils (HT leads/Plug Caps would get replaced with coils) either breaking down and only providing sufficient power to ignite one plug with fuel in the equation, or out of spec coils toasting plugs prematurely ( I read this is possible, but I dont know)
3) CDI is F'd
 

·
Super Moderator
1995 GSX-R 750
Joined
·
1,770 Posts
A couple thoughts after reading ALL (lol) of this.
Carbs are severely out of sync.
Carbs are NOT completely clean. Ultra sonic all by itself is not good enough. Need to run a guitar string (get a few different sizes) through ALL holes, and passages. Especially the emulsion tubes.
could possibly have bad coils. I’ve seen old stock coils do really weird shit when they go bad.
the coils will not “kill” plugs.
you haven’t mentioned anything about pulling the valve cover. Need to check the cam timing, and the valve clearances.
and finally, need to get a compression test across all cylinders. Need to rule it all out at this Point.
keep us posted…
 

·
Super Moderator
1995 GSX-R 750
Joined
·
1,770 Posts
Another thing that needs to be checked is the 12 volt feed to the coils. Need to be sure they are getting at least 12 with key one, and also, at least 10 while cranking.
when the CDI box goes bad, it’s typically dead dead. Never heard of one being intermittent. Ever. So it’s LIKELY not the issue.
get one of those “in line” spark testers. The ones that connect to the plug, and you put your plug wire on the other end and it lights up. It will give you confirmation that you are getting consistent spark one cylinder at a time.
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #8 ·
Cheers Spyder

I have a spark tester but unfortunately it wont fit in a plug cap that takes a threaded post, ill have to find a better one.
Compression was tested, all good (id have to go back to my notes on the PSI ratings)

Acknowledged on the other tips, ill get on those

Appreciate the input here !
 

·
Super Moderator
1995 GSX-R 750
Joined
·
1,770 Posts
Oh, yeah, I forgot about the threaded tips. I haven't had them for a few years. Not sure they make one for those. If they do, it may be expensive.
Did you do a bench sync on the carbs before you installed them?
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #10 · (Edited)
Yes, they were bench synced, initially I dint take them off the rack, but they were sync'd ok. this time since im taking them apart to cleaning rebuild, they will be re-sync'd before they go back on. Im just waiting for parts to arrive before I get back at it.

I also observed all 4 slides moving simultaneously when the throttle is operated/bike rev'd.

I take your point on the cleaning, I do have spare or old guitar strings here ( I play, albeit poorly) ill use those. Truth be told I rushed the cleaning a bit (left them on the rack, didnt check float heights) and now thats bitten me in the arse as I have to do it again now.
 

·
Super Moderator
1995 GSX-R 750
Joined
·
1,770 Posts
That would do it. Float height is crucial with the RS carbs, as well as the emulsion tubes. They get worn into an oval from the needle over the years and leak. Tuning nightmare. All the tiny holes in the tubes need to be cleaned out. You can get it to run decent with worn needle jets, just impossible to get it to run perfect. Do you have a vacuum sync tool? (With that collection, I would think that you do).
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #12 ·
Im embarrassed to say "No" I dont have a sync tool. I need to get one. I borrowed one off a buddy a couple of times when needed, but I need to buy one, its on my shopping list.
 

·
Mod of the North
Joined
·
29,107 Posts
Get a couple of vacuum gauges and some hose. You only really need two at one time for the GSXR. I was $40.00 for my set up.
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #14 ·
Just bought a set online. thanks for the push. been in my shopping cart for a while and never pulled the trigger
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #15 · (Edited)
Just popped out to the shop to the check the voltage at the coils as suggested.

Battery static volts is 12.6 (its not been charged in a while as ive been doing alot of starts and short runs while trouble shooting - battery is a good one tho)
Coil Volts, ignition on , one probe on the coil terminal one to ground is 11.71
Coil Volts when cranking over is about 9- 9.5 volts before the bike fires up.

so, since im not an electrics wizard, is this OK ? if not whats my potential issues ? is that 0.9v static drop acceptable drop acceptable, the 3.x v drop on the starter OK? ( I imagine resistance from warring and switches etc will amount to some sort of drop, the question is how much is too much)

Ive put the battery on the tender for the night, juice it back up a bit. I imagine if its fully charged and putting out 13-13.5 volts I'll get my 12v at the coil and 10v when cranking if I look at the current voltage drop #'s.

the optimist in me says im likely ok. (?)

UPDATE :

After charging the battery overnight, im getting 12.6 at the coils with the Ignition on and 10.4 when cranking. -- All good
 

·
Super Moderator
1995 GSX-R 750
Joined
·
1,770 Posts
I forgot to ask you to also check the voltage drop at the battery when you hit the starter button. I agree, I don't think this is your main issue. BUT, you could use to wire up a relay for your coils. It's a pretty simple mod to by-pass the old factory wiring to the coils, and will give you true battery voltage at the coils at all times. You just use one of the + coil leads as the trigger for the relay. Lemme know in the future if you have questions about this mod.
I am still thinking you have a fueling issue. Another thing I don't think I asked about, are you using the stock airbox? These bike do not take kindly to the pod filters, or no filter at all for that matter. It is possible to tune for them, but it's a trial and error nightmare.
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #17 · (Edited)
Yes, stock air box, with a K&N filter (how it came)

thanks, I will hit you up on the relay mod at a later date

I suspect your correct on fuelling as my issue, although It may be worth ordering the new coils just because the primary Ohms are a bit higher than spec on my OEM ones, im not sure what effect that has but...
 

·
Super Moderator
1995 GSX-R 750
Joined
·
1,770 Posts
I'm sure they are pretty worn out as old as they are. There is noticeable performance gain with the aftermarkets, and another gain by doing the relay for them.
 

·
Registered
Joined
·
18 Posts
Having a problem with my 750 dropping multiple cylinders then coming back to life, only to drop them again. Sorry for the long winded descritopn but Im trying to provide as much relevant info as I can

Here’s the back story:

-When I got the bike home after buying it in May, she wouldn’t start or run at all (I bought it as a non runner),
-I removed the fuel tank and filled both fuel lines with fuel, the bike immediately fired up and ran Fine. I was able to Rev it up and it sounded Good.
-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 ran worse and worse (dropping cylinders one at a time) 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
-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 and here’s what I got:

-She fired right up on a cold start with choke
-Once its was running for a few Minutes and was getting full warm/hot, it seemed to start misfiring, snap the throttle and its no longer smooth, it poped and snaps etc, flames out the pipe.
-Using my infrared thermometer on the header pipe, I saw that it cyl 2 first wasn't hot, 5 min later cyl 4 started to cool off and of course it ran worse again. with each re-start and run cycle it dropped a cylinder.

After it went down to 1 cyl again, I changed the plugs , fired it up and it ran on all 4 cyl again, But...again, after 10 min it started dropping cylinders again and now im back to running 1 or 2 cyl's. I changed plugs again (new ones each time) and this time its not making a difference, back to 1 or 2 cylinders.


What ive done /noted so far:

- Noted1&4 cylinders run off one coil and Cyl's 2 and 3 run off another - wasted spark system
-Cleaned the carbs (ultrasonic)
-Running the bike on an external fuel source with fresh fuel
-Tested the Pulser/trigger Ohms are good
-Tested the Coils - secondary is good, primary a bit high at 4.0 and 3.8
-Trimmed the HT leads where they attach to the plug caps for a better contact
-No difference with air-box cover /filter on or off
-Spark is present on all 4 cyl
-Changed spark plugs twice
-Bike runs like this be it hot or cold, once the cylinders drop, I can leave the bike overnight and fire it up again the next day and I still have the problem, so it does not appear to be related to how hot the bike is or the heat cycle, although run time does seem to trigger the problems, iethe problems dont "reset" or go away when its cold, ie its almost a cumulative issue.
-Cyl's 2 and 4 go out most often - they're on seperate coils and run off a different fuel line to each other

I've ordered a Carb kit in case I have bad o-rings and or worn needles /emulsion tubes (30 years of fuel isnt kind to OEM o-rings) - so ill pull the carbs again and "rebuild them"

Im contemplating Dynatek coils and wires (just because theyre are a bit out of spec on the primary resistance) , but ....I do I wonder why I can get a "no run" condition on cyl 2 and 4 (when both have spark) when they're on different coils and cyl 1 and 3 can be running fine (granted cyl 1 or 3 can also drop on me as well, its pretty random). --------Is the higher primary resistance in the coils (3.8, 4.0) frying plugs ?

Looking for any added insights here, Im reluctant to keep throwing $ at it without having a diagnosis pinned down. not looking to go broke on this

thanks in advance
I assume this bike starts missing while riding? It sounds like it could be a fuel supply problem and having seen this same thing before, it was a non-venting gas cap. Try riding until it starts to miss and then using a spare key pop the gas cap and see what happens. I spent two weeks chasing this issue in my shop.
 
1 - 20 of 46 Posts
Top