Hey all,
I've been working on my bike for some time now and am having a problem that is not making much sense to me. The bike only runs on 2 cylinders. The middle as well as the right. The left pipe is cold, and will drip raw fuel out of the exhaust cross over. When I cut the fuel for the left carb, and drain the bowl some, it kicks in and starts running until the bowl is empty. The float height is perfect, the needle and seat is functioning as it should, and the float is not sunk. The carb is spotless, and I made sure that I have good flow through all of the little ports within it. When the engine is off. I cupped my hand over each intake, and have good suction through all carbs which tells me that the crank seals are good. Also, there is no loss of transmission oil. The spark is strong and consistent. The spark timing and points gap is spot on. I'm not sure where else to look. I will try swapping the carbs from one side to the other just to see if the problem follows the carb, but otherwise I am lost. Any input?
Thank you,
Christian
1972 GT750 flooding left cylinder
-
teazer
- Posts: 1035
- Joined: Mon Mar 07, 2011 8:10 pm
- Location: Chicago
Re: 1972 GT750 flooding left cylinder
Are the pilot jets new or used? Sometimes a PO may have dug around in a jet trying to clean it up and they end up being way too large. The other possibility is that the fuel level is too high on the left side or the pilot air jet may be blocked. Not just the air inlet side but also check teh small outlet holes under the front edge of the slide.
I would take at least the two outer carbs off and strip them down to compare them. Grab a can of carb cleaner or WD40 and blow it through one drilling in one carb and then repeat on the other carb to compare flow. Then go on to do that for every orifice in the carbs - not your own.... One carb then the other to compare them.
It is very possible that the flow through the pilot air jet to the pilot jet may be blocked. Also do that comparison test on the jets too.
One things that bugged me for ages on a GT750 was a similar issue and it turned out to be the "choke" plunger which didn't seal properly so that cylinder kept flooding.
You should also check the boot to the airbox just to be sure that it isn't inhabited with small rodents or nuts for the winter, and check that the exhaust baffle is clean.
I would take at least the two outer carbs off and strip them down to compare them. Grab a can of carb cleaner or WD40 and blow it through one drilling in one carb and then repeat on the other carb to compare flow. Then go on to do that for every orifice in the carbs - not your own.... One carb then the other to compare them.
It is very possible that the flow through the pilot air jet to the pilot jet may be blocked. Also do that comparison test on the jets too.
One things that bugged me for ages on a GT750 was a similar issue and it turned out to be the "choke" plunger which didn't seal properly so that cylinder kept flooding.
You should also check the boot to the airbox just to be sure that it isn't inhabited with small rodents or nuts for the winter, and check that the exhaust baffle is clean.
-
TheStone
- Posts: 6
- Joined: Thu May 27, 2021 2:14 am
Re: 1972 GT750 flooding left cylinder
Thank you for your input. I will tear into the carbs again when I get home from work. I will also check the choke plunger, however I do know that the cables are adjusted properly. The airbox is clean, and I cleaned the exhaust pipes and baffles last week. I'll let you know how the carbs compare!teazer wrote: ↑Tue Aug 17, 2021 3:43 pm Are the pilot jets new or used? Sometimes a PO may have dug around in a jet trying to clean it up and they end up being way too large. The other possibility is that the fuel level is too high on the left side or the pilot air jet may be blocked. Not just the air inlet side but also check teh small outlet holes under the front edge of the slide.
I would take at least the two outer carbs off and strip them down to compare them. Grab a can of carb cleaner or WD40 and blow it through one drilling in one carb and then repeat on the other carb to compare flow. Then go on to do that for every orifice in the carbs - not your own.... One carb then the other to compare them.
It is very possible that the flow through the pilot air jet to the pilot jet may be blocked. Also do that comparison test on the jets too.
One things that bugged me for ages on a GT750 was a similar issue and it turned out to be the "choke" plunger which didn't seal properly so that cylinder kept flooding.
You should also check the boot to the airbox just to be sure that it isn't inhabited with small rodents or nuts for the winter, and check that the exhaust baffle is clean.
Thank you,
Christian
-
teazer
- Posts: 1035
- Joined: Mon Mar 07, 2011 8:10 pm
- Location: Chicago
Re: 1972 GT750 flooding left cylinder
Just a thought, but it would be a good idea to check compression on all 3 just to be sure. I have a CB160 that floods on the right side and after checking the carbs multiple times I re-checked compression and it's low on that side, even though the motor has been rebuilt with new pistons, rebore etc etc. Not clear why low compression would cause it to flood but the current theory is that it doesn't fire at low revs but it does suck in air and fuel which accumulates and wets the plug on that side.
-
TheStone
- Posts: 6
- Joined: Thu May 27, 2021 2:14 am
Re: 1972 GT750 flooding left cylinder
Hey All,
Just following up to let you guys know that I got the bike sorted out. To try and make a long story short... The original float was sunk, so I replaced it with one that I found in my stash. Visually it looked the same at the time. Come to find out after months of head scratching that it didn't sit centered in the carb body. So although it was shutting the needle and seat with the float bowl off the carb, with the bowl on, it was forced open. Being brass, I bent the floats out so that they now are centered. The problem went away. I also had a problem with the middle and left cylinder killing spark plugs. A new set of points and condensers fixed that issue. Anyways, thank you for all the help guys.
Much appreciated,
Chris
Just following up to let you guys know that I got the bike sorted out. To try and make a long story short... The original float was sunk, so I replaced it with one that I found in my stash. Visually it looked the same at the time. Come to find out after months of head scratching that it didn't sit centered in the carb body. So although it was shutting the needle and seat with the float bowl off the carb, with the bowl on, it was forced open. Being brass, I bent the floats out so that they now are centered. The problem went away. I also had a problem with the middle and left cylinder killing spark plugs. A new set of points and condensers fixed that issue. Anyways, thank you for all the help guys.
Much appreciated,
Chris