GT550 mis-fire at part throttle opening

[losclarkos]

I've completely rebuilt my '76 GT550 and although I'm really pleased with the result I still have a mis-fire problem.
The bikes starts well although idle is a bit erratic but better than it was. She pulls fine until I open the throttle wide (past about 1/3rd to 1/2 opening). If I do this at low revs, below about 4000, the bike just bogs down and gets slower and slower. She will then pick up again if I change down then open the gas slowly. If I open her up above 4000, there is a definite mis-fire although the speed will increase but I can feel a her coughing.
I have, as I said, done a full rebuild and have done around 1000 miles since. New crank seals, new pistons and rings, carb repair kits, new battery, new plugs, points and condensers, new inlet rubbers. Timing and points gap has been set, carbs balanced and cleaned so many times I could do it my sleep. I just don't know what else to try now. Plug colour is good which suggests that the idle mixture is fine. Adjusting the air screws doesn't make any difference (which it shouldn't do as the problem is not with the idle/slow running circuit). It feels like fuel starvation but it's really difficult to tell. I've tried lifting the needles, dropping the needles, running without air filters but nothing makes much difference. The exhausts are clean (stock) but I'm not running any wadding round the baffles. Could this make a difference?
I also read that the middle main jet should be a slightly different size to the left and right but mine are all the same. Would this be the problem?
Number 1 cyclinder produces a bit more smoke than 2 and 3 but not a massive amount. A small amount of unburnt oil residue is finding it's way out of the exhaust flanges on all cylinders but, again, not huge amounts.

The bike is rideable and still enjoyable but........................

Any ideas or suggestions as to what else I can try would be greatly appreciated.
Thanks

[Craig380]

Hi, try turning the fuel tap to the PRI (prime) setting when the bike is running, and see if it shows the same behaviour. I wonder if the fuel tap is not opening fully on the normal vacuum-actuated setting.

Also make sure the fuel cap vent is clear ... that can cause fuel starvation too if it is partially blocked.

[ConnerVT]

+1 on what Craig said - Confirm there isn't a fuel supply issue.

Then, some good reading: http://www.klemmvintage.com/mikuni-tuning.htm

[Cliff]

Check your float height. Try raising the floats 1mm (0.040in). Measure the float height without the bowl gasket and with the carbs on an angle so that the spring isn't compressed in the needle. Check each float on both sides to make sure it isn't bent/twisted. My floats are set at 25.0 mm.

[GT750Battleship]

Hi,everything sounds good...even if baffles are clean,the pipes themselves maybe full of carbon & oily sludge.My GT750 recently drove me nuts with bad running & not pulling properly...one pipe so badly choked I couldn't get it cleaned out,hot chemical bath,also left in a drum of acid for 3 weeks.I have now fitted a set of Delkevic Replica Exhausts & she is running sweetly again
Cheers,
Roger

[Alan H]

I have exactly the same on the 550k with separate carbs. There is a partially blocked jet or orifice in the slow running circuit of one carb, so it ticks over on 3, then as you open up a bit, is actually on 2 until the main jets come in. Then off she goes. I need to get my carbs off again and find what must be the only bit I missed.
When you get to the 'bog' bit, flick the choke part on for a second. If I do that it picks up fine on 3, so it must be something on the pilot/slow running circuit.
Tickover has always been a bit erratic, I have decided to live with it - it's a 43 year old 2 stroke after all.

[GTandcbr]

your middle jet should be different look here
http://www.3cyl.com/tripleed/suzigtcarb/27.jpg" onclick="window.open(this.href);return false;

[Vintageman]

your middle jet should be different look here

But, that difference is trivial.

It sounds like it can't handle large throttle until RPMs are up there and still not great. Verify timing via TDC method not marks wheel.

Did you double check carbs for clean. Including all passages in pot metal body like needle air jet passage, float vent tube. Is you gas cap plugged not venting ? not Did you verify choke is closed when closer. I assume you verified exhaust not plugged.
Did you check the condition of the needle jet bore for smooth inside and round at the exit. Was the air jet hole plugged. It would have to be bad to run as you say.

You’ll figure out. Good luck, let us know either way

[Suzukidave]

+1 on checking those small openings and jets , i havent pulled apart 1 carb that the needle jet wasnt nasty and you have to pull them from the carb to clean them properly .

[losclarkos]

Thanks guys and apologies for not getting back here sooner. I decided to go the cheapest route and do another clean, just in case. I am in Spain and the high temperatures can evaporate fuel quicker than I can burn it. So, I stripped them down again, took out all the jets and put them into soak. Ultrasound clean of the carb bodies. Then check out everything. I bought carb repair kits a while back and just assumed they were good but when I looked closely the jet needles are different to the originals. They are 1mm longer but, more importantly, the profile is quite different. The taper is more than than the originals which would make it run richer at any given slide opening. So I tried dropping the needles a notch and this has definitely improved things although it still " 4 strokes" at lower revs but high throttle opening. But she starts really easily and pulls well at lower throttle opening. The original needles are quite warn where they sat in the top of the needle jets but I think I'll try popping them back in and see what happens. If that improves the running at higher throttle openings (even if the idle then goes awry) I then know I have to replace the needles with "proper" ones. Lesson learned. "don't buy the cheapest".
I'll keep you up to date on results.

[rngdng]

There isn't a good carb kit on the market. Jets are usually wrong, and the needles are ALWAYS wrong. Now you know.



Lane

[Coyote]

+1. Put the original slide needles back in. That's probably the entire problem. Parts may look similar, but similar don't cut it.
It's a shame. Thousands of carb kits sold and even more thousands of perfectly good parts have been trashed.

[Suzukidave]

Also take a very close look at the opening of the needle jet , if the needles are worn chances are the hole in the needle jet where the needle slides through will also be worn oval .