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Discussion Starter · #1 · (Edited)
Hi Guys,

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 its quiet and 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 ( can only think detonation in the header).

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.

Thanks fellas !!
 

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Discussion Starter · #2 ·
Well im right back where I was a month ago, every time I run it it gets worse than the time before.

I did swap some plugs around and it made no difference

tested the coils and they seem ok ( a wee bit high on the primary side 3.9 ohms, but perfect on the secondary side)
Compression is Good

today I ran it for 10 minutes and #3 wouldn't fire at all today so it didnt get hot enough to make the valve rattle. ( I have spark when I test the plug against the valve cover).

Later on when I fired it up I also lost cyl #4

Im gonna put 4 new plugs in it and pull the carbs again for a cleaining

Im still running on a remote fuel source, haven't put the tank on it yet so I know there is no shyte from a tank getting in there.

Im guessing the rattle is a valve hanging up some how when #3 exhaust valves get hot (but not today), valve cover will have to come off for a look see. First is to get it running on all 4
 

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Discussion Starter · #3 ·
Sometimes its the simple things.

Fired the bike up twice the morning and just like last month, it continued its dropping a cylinder cycle. Dropped 3 and 4 on the weekend and it dropped another cylinder each time this morning and in the end wouldn’t run at all. All 4 gone.

So I went to our local Bombardier dealer this morning and they happened to have 4 CR9E spark plugs in stock

Came home installed the plugs and Bingo, all 4 roared to life and it ran great.

I let her run a few minutes and that rattle came back from Cyl 3 pipe, the good news it its not a valve. Its the header pipe rattling in the head. The V&H headers only have a bolt down collar on Cyl 1 and 4, cyl 2 and 3 just sit in via pressure. I started pushing on the pipe with a long screw driver and when I pushed right at the spot it enters the head the rattle stopped. Why do they only have a collar on 1&4 ? I dont know for sure but I suspect to make removal easier since you cant change the oil filter with the V&H headers in place (you can with OEM headers). I did notice that cyl 4 pipe is 100% seated and the rest are slightly out, so ill mess with that happily rather than pulling a head off to free a sticky valve.

Ill still pull the valve cover tho and check all the valves

Ill see if I can seat the. headers better, and maybe a bit of red RTV Heat Silicone may help the vibration (?)

Happy Happy Happy
 

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I would guess that the previous owner broke the centre exhaust collars. Find someone local to make replacements using the others a template.
 

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Discussion Starter · #5 ·
Did some research and seems the V&H didnt have them on the centre headers. In fact the o/s headers also have a welded lug on the pipe to stop the collars sliding down the pipe. the centre ones do not have these lugs either.
 

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Unlikely, but suit yourself. Those pipes will not seal, even with silicone. Get some collars made up.
 

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Discussion Starter · #7 ·
Im not sure how you get collars on pipes that are already welded up. you cant slide them on over the header end due the the flange they grab onto. If V&H built them that way im sure they should seal. none of them leak, just one vibrates a bit, but im going to put new Gaskets in and see how that goes.

Do you have a way to get a full collar over an already built header flange without chopping the pipe and re welding it?
 

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Machine clamps in two halves and butt weld together. Not difficult at all. It's your ride, do as you wish. If they seal OK you should not have problems with exhaust gas leaks, mixture leanout on the centre cylinders, or decelleration pops. In my experience this normally happens when one bolt comes loose, let alone all 4 middle bolts.
 

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Discussion Starter · #9 ·
OK, thanks.
 
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