1987 gk71f is it still restricted ?

[mike151]

I,ve had the bike a month or so now + have cleaned the carbs twice, its had new O rings + plugs etc.. It seams to rev OK,ish but it lacks power & struggles to rev right round to red line, it also has a strange red flashing light that comes on at quite low speeds ,& although it ticks over most times,if you rev it up it drops down to tickover for a bit,, then stumbles a bit & dies, it,ll start straight up again
& tickover till you rev it up to mid range,then it,ll die again. Wondered if it was restricted in anyway before i start checking compression etc, its barely faster than my rg125. Ps its done about 23k miles & looks to have been cared for.

[mike151]

I,m now suspecting the exhaust, the bloke i bought the bike off mentioned the exhaust had been modified to fit, after taking it off i find its not been. Seating properly,not crushing the gaskets,reason was the collets were not long enough, have now got some genuine ones but now as i tighten them up it lifts the whole exhaust up so it fouls on the sump drain plug, i,m now having it modified again to clear the plug.what would this exhaust be off ? It has 2 tubes welded to the front & the exit hole of the silencer is only about 25 mm, it has Suzuki also stamped on it

[mike151]

I think the pipe,s off a gk73,, the outlet of the end can is only about 25mm & i,m guessing its one of the 2 end cans gk73,s had originally, i,m still trying to get it to seat properly, may have to sort a whole new pipe for it.hope it hasn't burned out the exhaust valves.

[bartman400]

hi mike! sounds to me like a fuel issue,,try removing tank and replace with a suspended fuel feed,,if this works,fine,if not then i would point the finger at the carbs,hope this will help you,cheers bartman...

[slabbygk71b]

The symptoms sound exactly like I had with my bike (gk71b - basically same engine, carbs etc) I chased this around for a long time including a couple of road side breakdowns.

I had trouble re-starting the bike when it was at normal temperature or higher. All sorts of carb work, fiddling with the engine didn't change anything. Fuel lines, plugs, ignition leads etc, etc, etc!!!

I would take a really good look at the ignition coils , the insulation breaks down and coil loses power as it gets hotter. Mine also sparked to the mounting bracket, check your coils function in the dark as you can see any sparking or arc to earth. The coils are mounted under the tank and get really hot from the engine as well as their own heat.

If you can get a good spare coil, fit it to 1&4 first then 2&3. I noticed straight away that the bike didn't falter any more when hot, it revved cleanly regardless of temperature and would hot-restart first push of the button.

I've fitted a set of stick coils, one per cylinder, it is faultless now and never fails to start or run as it should.

Hope this helps.....

As for de-restricting it, if you've got an kph speedo, fit an mph converter as it should stop the needle in the speedo triggering the circuit to the ECU at restricted top speed as you'll be going faster but reading lower in mph.

[mike151]

Thanks for advice you two,i hav,nt touched the bike since august, still trying to get exhaust sorted, there may be a problem with fuel but i,ve had a good go at carbs twice, the fuel tap is not original & does,nt rely on vacuum from carbs. I,ll look into the coil situation next. Ps what do you mean by stick coils ?

[slabbygk71b]

Buy a workshop type remote 1 litre fuel tank with hose and valve. Silverline and Draper sell them, about £25.00. You can work on the bike without the fuel tank and you can remove all the pipes to and from your tap, this isolates another problem you've got with poor running. Hang the remote tank high enough and gravity will feed your carbs letting you work on the engine without anything in the way.

Modern bikes have stick coils, one per cylinder and these are integrated into the spark plug cap. With bikes like the GK71F, you've got 2 coils and each supplies 2 cylinders. Get a problem on your coil and it affects 2 cylinders whereas a bike with stick coils has only one cylinder affected if the coil fails. The conversion from 2 output coils to stick coils was in Practical Sportsbike a while ago. It's a Bauer Medai mag, contact them for a back issue if you want to convert it, otherwise if your coils are faulty, replace them with standard parts if that seems easier.

If I were you, junk the exhaust you've got and source one for your specific bike. I wasted hours making a 71f exhaust fit my 71B, would never bother again even though I like welding & fabrication. The GK73a and 76 are also subtly different and what seems like an easy mod can be a real pain in rear. If you're in the UK, try DK spares for an exhaust.

Hope this helps....

[bartman400]

hi! bartman here,,, how is the bike running now? just fitted a stainless exhaust from a 400 bandit to my gk71f,,cut off silencer and fitted a stubby end can,,awesome!!!!

[mike151]

hi! bartman here,,, how is the bike running now? just fitted a stainless exhaust from a 400 bandit to my gk71f,,cut off silencer and fitted a stubby end can,,awsome!!!

Hi Bart, have dismantled the small bore silencer & am ready with some bigger perforated tube, just got to reassemble & re pack& test, prob in a couple of weeks.

[mike151]

i think i,ve sorted it ! still a bit hard to start, needs absolutely no choke, even on the coldest days. i,ve cut the ballance tubes off the headers so the headers can spread a few mm, wasn,t quite seating properly , probably because the pipe was originaly off a gk73. anyway it now isn,t blowing & doesn,t stumble when the revs drop after reving it up, isn,t too loud with the straight thru silencer either, in fact its not loud at all. set up a temp fuel tank like someone sugested so as to check for arching around the coils in the dark, anyway it all looked ok/ no wayward sparks ! need to re-assemble now & go for a ride, see if it revs properly. don,t like to just rev the bollocks off it when its not under load.