over revving gt 750j

[Monkey man]

good evening

can anyone help with the following problem

The bike has been standing over winter and was running fine before being laid up .

Now however when i start it it over revs to about 3000 plus and try as i might i can’t get it to settle down to 1000 or thereabouts

I downloaded the suzuki procedure for setting up the carbs ( thanks baz) but no amount of adjusting throttle valve screw or pilot air screw will bring it down

There is plenty of play in the throttle cables.

I suspect because i didn’t drain out the old fuel the carbs may be clogged somewhere but before i pull it apart is there an alternative problem or solution ?

Any advice would be appreciated

[GTandcbr]

Hi. It sounds very much to me that you have an air leak maybe through a split carb to cylinder rubber. Check that they are all sealed properly and in good condition. No amount of tuning will bring down the engine speed if too much air is creeping in.

[Craig380]

+1. GT carb rubbers can and do split, especially underneath the clamps where you can't easily see the split. The only way to check properly is remove the airbox, carbs and rubbers and have a close look.

[Monkey man]

cheers guys - will check it out

[Monkey man]

checked the connections and i suspect a leak at the connection at the back of the carbs where it joins to the air box manifold - the angle / joint between the two isn’t great so i have loosened the air box off to try and make it better . Try as i might i can’t stop the air intake rubber slipping off the back of the carbs
because they don’t marry up perpendicular . should i try to angle up the carbs ( difficult as i think they are fitted correctly and snuggly with the connecting block rubbers . or do i detach the air box and fit the air box rubber to the back of the carbs and work my way back from there but i suspect i will then transfer the problem to when i try to re - bolt the air box to the frame under the seat/tank

What’s the best way forward ?

[GTandcbr]

The airbox rubber manifold leak will not cause the motor to over rev. It will maybe make it run leaner. The air leak will be somewhere on the rubbers cylinder to carb. It may only be one as it will pull the other cylinders along too. How old are yours. With age they become rigid and brittle and dont seal properly. Start the motor and wait for it to settle. Then spray starting fluid around the carb rubbers one by one. If the engine speed rises that is where the leak is.
With reference to the airbox manifold these are always a nightmare to refit once off. Again with age they distort. The only tip i can give you is to heat it up using a hair dryer or put it in the sun so it becomes more supple. A tiny spot of superglue can also hold it in place whilst the clamps are screwed up

[Monkey man]

Thanks for that - i will give it a try and see how i get on

[GTandcbr]

Another method for refitting the airbox. Seperste the ducks foot rubber manifold from the airbox. Next loosen the carb to cylinder rubbers. Offer up the ducks foot on its own and slide it into position. The key here is adjusting the carbs into position so the boot fits easily. The tighten the carb rubbers up. Remove the boot then offer up the boot again see if it slides on okay. Then loosely re attach airbox and fit to carbs. Once in position tighten everything up. Dont be tempted to overtighten the clamps holding the boot to the carbs. All that does is forces them off. Tight will do. Remember the air is sucked through the carbs from the engine so it wont blow it off.
Good luck

[Monkey man]

hi guys - started the bike up and sprayed the carb rubbers etc with quick start - to my surprise the revs actually dropped rather than rise - however i’m guessing it still suggests that the spray was allowed to be sucked into the engine indicating an air leak . i took the bike out for a short test ride but it was awkward with the revs rising every time i came to junctions etc . one other thing i noticed is that the revs seemed to drop when i applied the choke.

[GTandcbr]

I cant explain why the revs would drop when you sprayed starting fluid. First time i've heard of that. It is normal for revs to drop when the choke is applied as the engine overfuels the plug gets wet and doesn't fire well so engine speed dies. I still suspect the carb rubbers and replacing them would be my first job. I know they are not cheap but they are vital to smooth running.
Just want to confirm that with the airbox off all the throttle valves are sitting on the bottom of the carbs?

[Monkey man]

just an update but no solution as yet . New manifold rubbers were fitted 2 yrs ago but i could see on one of them a cut out in the rim on the left hand side - luckily i had the old rubbers to compare and saw the originals didn’t have this cut out . Believing it to be a defect i removed all carbs and rubbers only to find they each had the cut out and so must be part of the nos design . Back to the drawing board as otherwise the new rubbers appear perfect with no splits to indicate an air leak . i did notice each choke connecting nut to the body of the carbs were slightly loose but not excessively so . my next move is to strip and clean the carbs now that they are off the bike.

[Monkey man]

Hi everyone - another update and it’s not good news - after removing carbs umpteen times and checking rubbers etc to no avail i found that the bottom gasket in the left hand cylinder has blown.

Don’t know why i didn’t spot it earlier

Fortunately i know a specialist locally who has said he will collect the bike for further examination and hopefully a repair.

Disappointing especially as covid regulations have just been relaxed but it’s one of the ‘joys’ of owning a 48 year old machine

oh well - perhaps it was time for an overhaul anyway .

[GTandcbr]

So thats where the air was getting in. Probably a good descision getting the job done proffesionaly as removing the barrells from 750j can be difficult to seperate.

[jabcb]

Now would be a good time to consider additional work if you want to keep the bike around for a long time: crank seals, plastic water pump gear & etc.

Since he will already have the motor apart, now would be the cheapest time for some preventative maintenance. Plus the parts may be much harder to find in a few years.

[sportston]

I cant explain why the revs would drop when you sprayed starting fluid. First time i've heard of that.

I've come across this many times when working on cars when I was a diagnostic technician. In my experience it does this sometimes depending upon the severity of the leak. A bad leak will cause the revs to drop, a tiny leak will cause them to rise. Obviously, either is indication of a problem that needs remedying.