This Gt sounded Ok until about 6 months ago. Then top end started tapping on the right. Checked valve adjustments. They're Ok. Checked chain tensioner. Ok. No loss of power. Before I dismantle could it be something like unleaded gas? I brought that up 6 mnths ago about using leaded additive (which I always used to put in) and everyone said no...So I quit that. Rode it yesterday. It runs fine. Tapping subsides when the engine is hot but tapping is still there. I have been riding this one for 25 yrs now. Has about 50,000 miles.
gt 750 15538 top end tapping
- Thread starter Evan
- Start date
Laverda SF
Hero member
Suggest you do a Leak Down Test on the cylinders first. That is pumping a constant 135lbs of air of air pressure in each cylinder and shut it off with the crank held solid at about 10 degrees after TC with both valves closed then with a stop watch time each cylinders time to leak down from say 120 lbs to 80 lbs.
The shortest time of a Cylinder leak down may point to an issue.
This usually doesn't indicate a TAPPING Sound but could diagnose a bad valve, a leaky head gasket or a bad cylinder which does sound like a tapping noise.
Assuming you have carbs to specs and sync'd and not licking that cylinder. Which I doubt as you say maintains power.
Broken Valve Spring and valve slap ?
SF's do like fresh valve springs about every 2 or 3 seasons and much before 50,000 Km's - Especially if you like to rev her ;o)
Other then what Haggis said, other possibilities are either a bad wrist pin or insert rod bearing journal on that cylinder or possible piston slap due to a worn out cylinder bore due to insufficient lubrication or that cylinder swallowed dirt.
Leak down testing reveals bad Intake Valve through hearing air flow from the Carb, Bad Exhaust valve from the Exhaust pipes, Bad cylinder from air flow from the oil pan and a Blown head gasket with soapy water bubbles around the head gasket for a particular cylinder ;o)
Sure you don't have a leaky Exhaust Donut ?
At 50,000 Km I would definitely do a leak down test on a motorcycle engine irregardless.
The shortest time of a Cylinder leak down may point to an issue.
This usually doesn't indicate a TAPPING Sound but could diagnose a bad valve, a leaky head gasket or a bad cylinder which does sound like a tapping noise.
Assuming you have carbs to specs and sync'd and not licking that cylinder. Which I doubt as you say maintains power.
Broken Valve Spring and valve slap ?
SF's do like fresh valve springs about every 2 or 3 seasons and much before 50,000 Km's - Especially if you like to rev her ;o)
Other then what Haggis said, other possibilities are either a bad wrist pin or insert rod bearing journal on that cylinder or possible piston slap due to a worn out cylinder bore due to insufficient lubrication or that cylinder swallowed dirt.
Leak down testing reveals bad Intake Valve through hearing air flow from the Carb, Bad Exhaust valve from the Exhaust pipes, Bad cylinder from air flow from the oil pan and a Blown head gasket with soapy water bubbles around the head gasket for a particular cylinder ;o)
Sure you don't have a leaky Exhaust Donut ?
At 50,000 Km I would definitely do a leak down test on a motorcycle engine irregardless.
Tippie
Hero member
- Location
- Dr?bak, Norway
"Tapping" is a very vague description. Do you mean pinking under load, or a mechanical tap, is it only under hard throttle, or is it there regardless of revs and load. On one of my bikes it had an annoying "tap" when cold that turned out to be side/end float of a rocker arm on it's spindle which I shimmed out. Another problem can be the tappet screws digging a hole in the valve tip and giving incorrect valve clearance. Loose cam halves won't stop once it appears, just get progressively worse.
Laverda SF
Hero member
"the tappet screws digging a hole in the valve tip" There's were use of a Dial Indicator to adjust the tappets is essential or file the valve tip flat and trust feeler gauges - Then suspect insufficient oil flow. SOHC Hemi engines are subject to these issues and subject basic maintenance when you get to know them.
I may have lost a few 1/4 and 20 mile races but never, to my recollection, NEVER lost an 800 mile race from Northern Ontario to Toronto against any 1000cc moped with my 70 750 Breganzie SF0 back in the day during the 70's for a week end of light to light street racing ;o)
The most potent machine of that era was the late 70's introduction of the Suzuki 1100 GWX-R Kantana that forced closure of the Breganzie factory and kissed the the 3 Cyl Jota Good-Bye. I had an 82 GWX-R for one season and know dam well.
What's really sad is that Breganzie was about the last hands on old mechanics racing enthusiasts motorcycle!
I may have lost a few 1/4 and 20 mile races but never, to my recollection, NEVER lost an 800 mile race from Northern Ontario to Toronto against any 1000cc moped with my 70 750 Breganzie SF0 back in the day during the 70's for a week end of light to light street racing ;o)
The most potent machine of that era was the late 70's introduction of the Suzuki 1100 GWX-R Kantana that forced closure of the Breganzie factory and kissed the the 3 Cyl Jota Good-Bye. I had an 82 GWX-R for one season and know dam well.
What's really sad is that Breganzie was about the last hands on old mechanics racing enthusiasts motorcycle!
1200ts
Hero member
- Location
- USA - Riverside CA
I am betting that one of the camshaft flanges is loose on the cam. I have seen this more than a few times on the 750's. The flange should be a very tight press fit on the cam. If it is not the key that aligns the two cams will start to wear and come loose. That is when the noise starts and it will get louder over time until the key breaks$$$$$$
Take the covers of each side of the head. Put a finger on the end of each cam and have someone turn the motor over slowly. If the flange is loose you will feel it.
Take the covers of each side of the head. Put a finger on the end of each cam and have someone turn the motor over slowly. If the flange is loose you will feel it.
motoddrob
Hero member
- Location
- S/W Australia
I'd put money on the the cam flanges/keyway. Happened to my sf2 way back when as well. Easy to see/feel when turning the motor over.
piranha-bro2
Hero member
- Location
- Melbourne
motoddrob said:
Yep, agree with that, and that's where I'd look first. But Tippie does point out a few other distinct possibilities.
alan james
Senior member
Happened on my 750SFC back in the day. Returning from the Paul Ricard circuit south of France, the tapping got steadily worse made it to within 20 miles of Calais then bang. You know the rest.....
I certainly hope it is not the cam-halves. A quick check - esp if you are on you own and cannot get to feel the cam looseness as the engine turns, take the plugs out and turn the engine (even by the back wheel if you don't want to take any covers off). As the spring force goes to the backside of one cam face, that cam will be shot forward, if there is play in the joint you will hear it click.
There are many other things to check through. Rocker end float? Rocker play on shaft? Carb slide rattle? etc
There are many other things to check through. Rocker end float? Rocker play on shaft? Carb slide rattle? etc
Thanks! Gives me something to do...Now the sf2 had piston slap back in 1995. But no tapping it lost compression when riding. Then when I got it apart the pistons were a little different. The ring grooves were not exactly the same. I saved them. Was surprised as I thought they would not do that in Breganze in 1974. Bottom end Ok so the plan is rebuild sf2 w/ early head to fit gt frame and put that on the gt for the meantime...