GT750 won't idle on left cylinder.

[Striker1423]

I had the bike out today. All was normal until it decided to start idling really low on the way home to where It's basically a T500 now. It idled kinda low, but seemed fine at speed. So, I was giving it some extra throttle when a car in front decided to slow down and turn quickly, so I rolled off and it died. A few cranks later and it was running again. The left cylinder isn't running at idle. It has compression. Around 105psi. It has spark. It's getting fuel. The plug will come out wet.

If I throttle it up the cylinder comes alive again. As soon as I let off, it's bupkiss. Also, strangely enough if I hold the cap a little bit away from the plug I can get that cylinder to run sporadically, and it's back to nothing with the cap seated. Rechecked timing and it hasn't moved.

Only thing I recently did was replaced the oil pump and set the float heights in the carbs. Which actually raised the fuel in the bowls a bit. The idle circuit was clear as well.


It has the original coils on it. 

[Striker1423]

Crank seal perhaps?

Double post. Oops.

[teazer]

Sounds like it's flooding on that cylinder.  So much fuel it can't fire at idle. Increasing the gap means the coil voltage has to build higher before it jumps the gap.  Stronger spark helps it run.

Are all 3 as low compression as that one? It should be north of 120 and possibly closer to 135-140. I like to run at least 140 on mine with a few tweaks.

test compression and do a leakdown to see if the seals are also shot and pull the carbs again.  Fortunately you have to remove them to do a leakdown test.  And fortunately you can test one cylinder at a time without having to blank off the other cylinders - No lab seals on this high tech beauty.

[1976RD400C]

I would start by trying a new spark plug in that cylinder. A combination of low compression and weak spark could make the cylinder drop out at idle.

[m in sc]

lower fuel level on that one, or look for slight seep at needle. id bet its carb related when warm. drop it like 1-2mm and i bet it clears up.

or, the pilot jet fell out.

[Striker1423]

lower fuel level on that one, or look for slight seep at needle. id bet its carb related when warm. drop it like 1-2mm and i bet it clears up.

or, the pilot jet fell out.

The fuel level is definitely higher than it was... Also, the cylinder dropped out just like the snowmobile did when the float was set too high... Worth a shot as it ran fine before I messed with the carbs. Which figures lol.

Teazer,
All 3 are matched at about 105-110 PSI. She's a tired old girl that's for sure. I'm also fully aware that it's past due for a rebuild. I just need to acquire a set of cases. As paying a welder to repair my broken one seems like it could be a big pain in the dingus (though I do know a wizard with a welder, it takes months to see your stuff again). I have a line on a local set of cases, same year range and everything, including the cylinders AND potentially all the engine covers for around $600. Just figured I'd wait and try to get the last of the Fall season on this motor before baby arrives and kills my engine rebuild timeline. I figured the cost and promptly buried it into the recesses of my mind. 

[teazer]

Drop the fuel level a little and fit hotter plugs to see if that cleans it up a bit.

BTW, don't ever try to build a budget for rebuilding it.  You may go into shock and never get around to it.  I prefer the opposite approach of buying parts when I have some cash and eventually it gets done.   

With an older motor, it's really important to tweak things to get it to run as well as it can. A healthy motor can be way out of spec and still run OK but a worn motor needs more attention. If the exhausts have much carbon in the baffles, that could contribute to the symptoms.

If you are not running it hard, you go to a B7 or even a B6 plug which will tend to burn off excess fuel. I ran B8 or B9 in mine at the drag strip and dyno testing and they were fine.

The fact that it runs at higher loads and fouls at idle strongly suggests excessively rich mixture.  Check the pilot jets, but also check the chokes.  The plungers or whatever the correct Mikuni nomenclature is go hard over time and no longer seal properly allowing the "choke" to be active even when you have closed it.  MikesXS has a plunger for a BS38 that is the same part at a reasonable price.

With the cracked cases, it's not hard to build that area up with weld but heat the cases before welding and obviuosly that has to wait until the motor is stripped. I have had a couple of cases done.  For sure it's a lot of messing around to dress the case back so it looks perfect but worth the work.

And just a heads up.  Check that the points and alternator leads are well clear of the exhausts.  If the leads droop and melt on the pipes, the trail of destruction can be interesting to say the least.

[Striker1423]

Ok, I totally needed to put this info here but failed.

It has an Accent Ignition on it now. It's tucked up nicely in the motor mount points. The motor ran great until I set the float heights. They have around 2mm more fuel in them now. So, I'm gonna say that both you and Mark are correct. I think I dicked it up by setting the floats to stock with aftermarket needles and seats. As the bike ran absolutely fine before I did that. The chokes are all originals, but the rubber was good on them. Squishy, not worn in. I do have keyster replacements if necessary. But, running it on choke kills it, as it should.

I'll re-set the float heights back to where they were and throw a new set of plugs in it. Meanwhile, I stretched the Rd's legs today. Felt good getting on a bike that pretty much runs as it should... heh. Pretty much.

[ajb]

Try switching the carbs around if you can and see if the problem follows.

On my H1 500, it didn't fix the problem and I spent ages diagnosing poor running / idle from the left pot.  A chap on a forum suggested to take the baffle it. That was it, a lump of carbon got dislodged. Clearly my efforts to decoke weren't good enough.

[Striker1423]

The carb trick would work if they were single carbs, but this is a bank of three, with the center carb tied to the other two.

[Striker1423]

Put the carbs back to about where they were and still won't run. New plugs showing up tomorrow.  Leaving the old plugs out of the bike overnight, and disconnecting the fuel. I'll give it one last try tomorrow, but I think it might be a dead soldier.

[teazer]

.....). I have a line on a local set of cases, same year range and everything, including the cylinders AND potentially all the engine covers for around $600.

I don't think I paid that much for a complete bike... Just don't ask how much I sank into those cheap bikes though..

Going back a step, is it possible that the pipes are now full of fuel along with the crankcases?  Pull the SRIS valves or plugs and see how much black oil/fuel mix drains out.

If the airbox is off, apply air flow to the carb inlets and see if the slides all lift at more or less the same rate. Did you do a leakdown test?  It's possible that when it was running that a crank seal tore.

If none of the above work, please wrap it up and send it to me and I promise I won't charge you much to take it off your hands. If you happen to be headed to WI or states west or north anytime, drop the bike off with me and I'll see if I can work out what's wrong before you have to head home.

[Striker1423]

I don't have a leakdown tester. That's on the list too. But, yea, I can try pulling the SRIS plugs to see what happens...

[sav0r]

Pull the plug on the questionable cylinder and crank the engine. At a minimum you can check for spark and see if it blows a bunch of fuel out the spark plug hole. Maybe not the worlds most decisive test, but you might learn something.

When you break it down, you need fuel, spark, and compression. Of course too much fuel won't work, but if that cylinder is blasting fuel like Moby Dick, then you know it is a fuel issue.

[Striker1423]

I tried this for a solid 30 seconds. Nothing came out of the hole except air. Put plugs in and the plug gets wet within a minute or two. Might be coolant. I'm going to see how much coolant left the overflow bottle later today too.