GSX750F running problem

[GavintheTazma...]

I have a 1998 GSX750F which has a running problem, it seems to be misfiring and can only get 60mph out of it.

I have had it in at my local bike mechanic and he has stripped, cleaned, rebuilt and balanced the carbs, fitted new plugs, checked the resistances of the coils and HT leads (all ok), replaced the carb diaphrams and set the float heights.

He told me that the problem was still there and told me to try the following: change the exhaust (non standard), replace fuel for new, replace air filter to genuine (pattern fitted at present) and then check for an electrical fault. I think he couldnt be arsed with it any more and charged me £300 for the privilege.

I have changed the fuel, on removing the tank I noticed that the plug leads werent even attached to the plugs properly (not pushed on) and that one of the carb rubbers had been fitted incorrectly (trapped in air filter housing), replaced the exhaust for a full stainless Delkevic system and as for the air filter it has been in for months and have never had a problem with it before. I am yet to go for a test ride after completing all the work mentioned but thought I would ask on here for any advice to cure the problem. All advice/help will be very much appreciated.

Thanks, Gavin.

[Pete.]

'Mechanic' doesn't know what he's doing, he's just guessing and changing random stuff. Keep away from him.

You say the air filter has been in for months and you've had no trouble with it but you think that suddenly the exhaust might be at fault? doesn't make any sense.

If you were nearby I'd offer to look at it except that I have a very full schedule of work for the next month.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[jaffa90]

What mileage and when were the valve clearances checked?

[pepperami]

First question: did this fault come on slowly or did appear all of a sudden?
This may help indicate what it is.

Coils are good, plugs are good, what about ht leads breaking down under load?, I've had that before.
Is it happening at a specific speed or in a specific gear?

Will it rev highly in lower gears or can you not get the revs passed a certain rpm? That may indicate something wrong with the electronics? Maybe? Possibly?

Is the fuel tank hissing when you turn the engine off = possible partially blocked breather, possibly, maybe,

You said you've changed the exhaust and the air filter ?
Has this made the problem worse or better?
Now things have been changed there may be a jetting issue, possibly? Maybe?
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I am the sum total of my own existence, what went before makes me who I am now!

[GavintheTazma...]

@Pete: I changed the exhaust as the joint between the down pipes and link pipe was corroded and blowing (only held on by 2 pieces of the metal flange)

@Jaffa90: The mileage is 29,000 and as far as I`m aware the valve clearances where checked??

@Pepperami: I took the bike in to the garage as it was running erratic (revs holding on) and to have the carbs balanced. The fault was there after the garage had completed the carb balancing, apparently the HT leads are good according to the garage???, Its happening at all speeds and gears but only under load, its revs fine in neutral. There`s no hissing coming from the fuel tank and the fault is the same before and after the exhaust change.

[pepperami]

[lilredmachine]

[lilredmachine]

One thing I will say, the delkevic pipes you have installed are larger than the stock. Considerably.

I'm running similar on my 1200 motor and I had to stage 3 dynojet to make any sense of them. 155 mains. I am running an open pipe (111.7db ) though.

You should be able to get over 60 though. Have you checked the motor side carb rubbers? An air leak would explain some of what you are describing. Look for deep cracks or if you can get it idling and spray easy start or wd over the rubbers, if the revs rise you have an air leak.
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Bikes: too many, too much for one man to maintain anyway.

[GavintheTazma...]

POST UPDATE: I have found a few more faults my bike: 1, one of the air box rubbers was incorrectly fitted (trapped) so creating an air leak, I came across this by spraying WD around the airbox rubbers which caused the revs to rise. there are no longer any air leaks. 2, The sealing foam on the air filter itself was partially missing, so created a temporary seal with silicone. I have fixed these faults and also changed the inline fuel filter for a new one, cleaned out the fuel tank (again), and filled with fresh fuel today (27/7), It runs better at low speeds but still feels like its missing and lacks power at top end speeds. I have a genuine air filter on order so will fit that and see what happens when it arrives. Any further advice/help would be appreciated. Cheers

[Snowdonia Rider]

This sounds exactly like what happened to my '98 GSX600F. Turned out the HT leads weren't connected to the the plug caps properly and some of the electrical wiring had gone shit. With that sorted it ran great. So I'd deffo start looking into your wiring.
____________________
I want your clothes, your boots and your motorcycle.
Suzuki GP125 Suzuki GSX600F Suzuki SV650S KTM EXC250F SkyTeam Bongo 125
My YouTube channel: https://www.youtube.com/c/SnowdoniaRider

[GavintheTazma...]

SNOWDONIA: Thanks for your reply to my post, I will try out what you have suggested and will give an update. Cheers again, Gavin.

[GavintheTazma...]

Its now been over a month of trying to sort this problem out , I have fitted a genuine air filter, removed the inline fuel filter and replaced with a brand new fuel line WITHOUT an inline filter. I have checked all the wiring and all seems fine. I am getting really frustrated with this fault, as I have never had any problems with the bike in the 10 years of owning it. I am know thinking it could be the coils??? as the symptoms are no top end speed (feels restricted) and feels like its running on 3 cylinders over 4000 revs. I don`t want to keep spending out on "it could be this, or try that". I would be very grateful for any advice on what to do next. Cheers

[jaffa90]

Run the bike on 3 cyls or top speed for a while and then stop and check / feel or spit on all the down pipes.

[GavintheTazma...]

Jaffa90: what am I looking for by doing this???

[jaffa90]

WHAT??? Come on, a non running cyl will have a cool down pipe,
if all 4 are the same temp then it`s a restricted fuel or exhaust system providing the spark timing is advancing correctly.

[GavintheTazma...]

Ah ok, just wasn`t sure why thats all. how do I check if the spark timing is advancing?

[jaffa90]

A strobe light but the spark plugs will get black if that`s the case.
The spark plugs tell a tale.

[Pete.]

Forget the timing advance it would affect all cylinders not just one.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[kev2b4]

I used to have a GSX750E - silencer got modified ( cut and loose baffles removed and welded back up) - and found I had to up the main jets a couple of sizes ( I'll have to have a dig to see if I've still got thhe 2 or 3 sets of jets I changed out)
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previous C50,CD175,GT500,GSX250,CB750KZ,GSX750E,GSX1300R

[jaffa90]

[Pete.]

You don't think running on three will affect the top end speed?
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Tdibs]

Ive had the same symptoms happen to me on 2 bikes, idles fine, roll on the throttle and losing a cylinder and having no power. One time it was the HT lead, hairline cracks near the plug head and the lead was arcing to the head.

Second time It was a loose connection from the lead to the coil. Would be double checking over all that section.
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Previous : 09 Vanvan 125| 02' Sv650s || Current: 1999 Xj600n | 1992 DR650 RSE | 2005 Fazer 1000

[Robby]

If this all happened after asking a shop to balance the carbs, I would ask them to fix what they broke.

Assuming this doesn't happen, make sure all of the blanking plugs or rubber covers were reinstalled on all the vacuum ports on the inlet rubbers.

[GavintheTazma...]

Thanks for all your replies, I have decided to change the coils and will see if that cures the fault. As for the shop who carried out the work, I went back today to complain only to find out the shop has closed down and is up for sale because the owner had passed away (Bike accident). I will post an update when I have fitted the coils.

[Robby]

Looks like a fuelling problem.

Started happening after a bike shop was messing around with the carbs.

So you're changing the coils.