Stuttering at half throttle and more

OK, so here's the bike my wife has been moaning about for the past 2.5 years. If arrived in the back of my Golf-sized car in about 4 boxes plus frame, tank and bare wheels - there were also quite a few parts missing and really I shouldn't have bought it. However I was nostalgic over having the same bike as in my youth (aged 19).

It came with a 550 engine which is currently undergoing a top-end refresh with some 600 parts. The engine in the picture is a Radian and it is fitted with Fz600 (BS30 carbs) after several false starts. It is almost ready now for it's road worthiness test (MOT in UK) so I took it for a sneaky run up the village roads. this afternoon. only just about 4 miles.

It burbles quute nicely at low revs/idle and even sound pretty good through the barely baffled exhaust whe you rev it at stand-still. However under load it stutters badly at just under half throttle which restricts top speed to about 60 mph. I cant tell you what RPM that is as the electronic counter is not yet working properly..

Carbs were cleaned balanced by me and shims set when I rebuilt this one's top end. One bore is slightly down on compression due to bore wear (135 vs 150). The fuel tap vacuum works OK but I have an awkward arrangement with an in-line fuel filter and a slightly kinked fuel pipe that I think could be restricting flow. The airbox is also standard 550 and it has a fresh filter in.

Does this sound like fuel starvation ? or blocked jet(s) ? As I say its fin e on pick up a quick off the mark for the first 25 yards then the spluttering/misfiring starts..

 

sounds like fuel starvation, try upping the needle position to allow more fuel to flow in the midrange. This is provided the carbs are clean. Mind you if the main jet isn't set correctly upping the needle isn't going to help much. Bike looks fantastic, what 4-1 pipe is that?

 

Thanks next step will be to fill up the tank (more pressure) and bypass the filter..

The photo is VERY flattering as teh bike has plenty of chips and scratches - but hey.. The pipe is an Alfa (by Micron) made in UK. No longer available but I found this one off ebay fairly cheap 45 quid - it was the same as I had in the 80s !

Cheers

James

 

could be the fuel line kink gets worse when line is heated restricting fuel flow

 

Agree fuel line . You say you did sync was that bench AND vacuum gauge? I did bench sync on mine, but found I was off when vacuum gauges hooked up now runs much better after doing this.

 

Yep - gauge sync'd...

 

Pulled carbs to check/adjust float heights. Found #2 main jet loose in the bottom of the bowl - oops. That would do it right ?

 

yup that would flood out the carb when you hit main jet rpm unrestricted fuel flow

 

Yeah, I was lucky in a way, the short throttle pull link cable snapped so I HAD to pull the carbs and while I was at it I decided to check the floats - I almost didn't though..

 

That exhaust is cool as shyte!

 

you would have been it the carbs sooner or later trying to find this problem, as a general idea it is not a bad idea to open the bowls every time you remove the carbs just to see how things are going

 

Those header pipes look pretty blue, overheating due to lean condition?

 

Pretty sure they were like that when I got 'em. I've only done 2 x 4 mile trips so far..

 

Does this also mean you checked the wet set of the float levels?

Gary H.

 

More or less. I checked them before removal and they seemed a bit low - about at the bowl screw flat (say 4mm). I then tweaked them by approx 1mm by measuring before and after with a vernier depth gauge before reassembly. I guess they are now 2-3 mm.

 

I know it's a PITA but there's only one way to know for certain.

 

spec is 2mm +-1mm

 

Single-wall pipes will do that, though usually not that dark when the A/F mix is right.