GT750 idle issue

[sfront]

I'm working on a 1977 GT750 thats giving me fits.
Center cylinder doesn't fire at idle.
As soon as it hits 3k it comes on board and runs great.
New plugs, carbs gone thru (multiple times!), no clogged jets or air bleeds, no vacuum leaks.
New floats set at 1.090.
Stock engine and all Mikuni jets.
Timing checked too.
Suggestions?

[Alan H]

Sorry, but you either have a blocked pilot jet, the carb butterfly is out of synch or the diaphragm is perforated.

[sfront]

I absolutely agree! But I'll be damned if I can find it.
Carbs are on the bench again......

[sportston]

I agree with Alan, that the diaphragm may have a perforation or the rubber may have decayed, or be poorly seated and not sealing (if you are running with CV carbs). If unsure, try swapping the diaphragm with one from its neighbouring carb and see if the problem persists or transfers to the neighbouring cylinder.
But also the following checks may help if you can see no problem with diaphragm.
Is your mixture screw badly adjusted or poorly sealed on that carb?
Is your float jet sealing well? Worn float jet could cause over-fueling at idle.
Is your jet needle damaged and is it set to the same height as the other carbs?
While carbs are the most likely source of your problem......
Have you checked quality of spark?
Is it running on points? If so check dwell angle and points contact surface.
Is your plug cap faulty?

If fueling is all good and spark is good too then suspect mechanical problems.
Do you know for sure that you don't have a crankshaft seal leak?
Are your crankcase halves sealed perfectly?
Do you have compression that is equal to the other two cylinders?
Do you have a head gasket leak?

[tz375]

Certainly sounds like a pilot circuit not flowing. If teh carbs are off, I would start by spraying WD40 or carb cleaner through the passageways in the body and float bowl. Spray in one drilling and then repeat in that same hole in the other two carbs to compare flow. The airflow to the bowl is often blocked.

Interestingly enough I had a similar problem recently with one pot not firing until the revs were higher and that turned out to be a battery on the way out. It tested OK on my little multi tester but under load the voltage dropped too low at low revs to run cleanly. As revs rose, the alternator was able to push out enough of those pesky electrons to get all three firing again.

[rngdng]

Look very closely at your pilot jets. There are two Mikuni pilots that will fit the BS40 carbs. If you see them, you can see the difference. Make sure they are the same. I'm wondering if someone got one of the wrong jets in that carb. Coyote had that problem about 10 years ago and he sent his carbs to me, and one of them had the wrong pilot. I don't remember what his original symptom was though. I used to have some pictures of the difference, but I can't find it right now. I will look.

[tz375]

Good point Lane.

That picture is on this page: http://pinkpossum.com/GT750/carb/BS40carbs.htm" onclick="window.open(this.href);return false;

[sportston]

Look very closely at your pilot jets. There are two Mikuni pilots that will fit the BS40 carbs. If you see them, you can see the difference. Make sure they are the same. I'm wondering if someone got one of the wrong jets in that carb. Coyote had that problem about 10 years ago and he sent his carbs to me, and one of them had the wrong pilot. I used to have some pictures of the difference, but I can't find it right now. I will look.

The difference being the amount of holes in it.

[tz375]

Not just the number of holes or if it has holes. The metering orifice is in a different place as as such it flows very differently. Either pilot jet style will fit, but only one will work properly.

[sfront]

This was my bike which ran perfectly about six months ago. The new owner travels alot and his bikes sit for months on end. I've cleaned more carbs for this guy than I care to count! They always come back green and sticky. I've told him to stop using whatever fuel he's been using but he says its non ethanol with stable in it........The bike lost low rpm power a month back. Got the carbs opened and found that whatever he's using for fuel ate the solder on the floats and all three were leaking. New Suzuki floats installed, all carbs cleaned and reassembled. Since that time the bike has a very weak low end. He's since put a set of Higgspeed pipes on it and the problem has only gotten worse. David at Higgspeed told him these pipes were designed to use the stock jetting and are a direct replacement.....If anything, the plugs look like the bikes too rich but I've never put a set of pipes on a bike and gone down in jet size.....I'm about to put the original pipes back on and have just ordered a complete new set of jets. Just trying to eliminate possibilities!

[jabcb]

Was the tank lined?

The tank on my 71 T350 was lined with something that caused the gas to turn red. Would have been fine if it was a daily rider that went through a lot of gas because eventually the liner stops turning the gas red. But I only rode it ocassionally so the red turned to muck in the float bowls.

[sportston]

If anything, the plugs look like the bikes too rich but I've never put a set of pipes on a bike and gone down in jet size.....I'm about to put the original pipes back on and have just ordered a complete new set of jets. Just trying to eliminate possibilities!

So the plugs look too rich and the problem appears to gradually worsening. You might have a problem with the Variable Venturi side of things. Especially if, as you say, it ran ok with the current jetting while it was in your ownership, again I would question if the diaphragms are over-stretched, perforated, not seating correctly or not being actuated well due to blockage in the carb.
I have come across many problems in carbs caused by old diaphragms. Sometimes they go too stretched and soft with age and cause a rich mixture even though they are not perforated.
Also worthy of consideration; Are the carb lids rusty? Are they corrosion perforated? Do they seal well? (Are they warped and failing to seal?)
I take it you have checked the air filter is not clogged/disintegrating and that air flow is not impeded in anyway by obstruction of the airbox intake. E.g New owner might have thought it was a handy place to put the rag he uses to clean the dipstick, or fitted a nice new alarm right next to it.

[tz375]

Or wasps or rodents moved in.......

Could also be wasps etc in the exhausts.

[sportston]

Or wasps or rodents moved in.......

Could also be wasps etc in the exhausts.

Lol. Funny you mention rodents. My mother had that on her bike many moons ago. A Honda CM200T. Always ran like a swiss watch and never failed to start after a winter laying idle, except one spring-time..... It fired briefly then stubbornly refused to run. We found that mice had used the airbox to hide their illicit stash of peanuts (originally intended for the bird-feeder). After removing the peanuts from the airbox the bike ran fine. We moved the sack of peanuts to a less accessible place than on the shed floor. Doh!
Never had wasps in the exhaust pipe though. Hahaha.

[Madbuffalo]

David at Higgspeed told him these pipes were designed to use the stock jetting and are a direct replacement.....If anything, the plugs look like the bikes too rich but I've never put a set of pipes on a bike and gone down in jet size.....I'm about to put the original pipes back on and have just ordered a complete new set of jets. Just trying to eliminate possibilities!

Interesting, When I bought my Higgspeeds back in 2010 David said I would need to experiment with jet sizes. I did extensively and found the best combination to be 47.5 pilots, 117.5 mains & needles on center setting, though I believe it will vary slightly from bike to bike. That's likely a side note to this conversation but may be helpful if you're replacing the jets.

Josh