Will start with starter fluid but then cuts off

[nateweisiger02]

I have a 78 cb750f that sat in a barn for about 10-15 years. I totally rebuilt the carbs, making sure to spray every hole with carb cleaner and watched for it to come out a different hole, made sure the lines were clear and replaced lines that were in bad shape, then put it all back together. It will start with starter fluid and the choke on for about two seconds then cuts off. Gas also flows out of the overflow valve for carb 1 and 2, but not 3 and 4. Has anybody else had this problem or know what the fix is?

 

[nateweisiger02]

Just want to add, when I unscrew the screw for the overflow valve on carb 3 and 4, gas does comes out, so gas is getting there.

 

[jpdevol]

Have you checked ignition timing and the advance unit function? Compression readings?

 

[nateweisiger02]

Have you checked ignition timing and the advance unit function? Compression readings?

I have not checked the timing because it seems to have good compression. But hey, that's one more thing rule out, so I will check the timing in the morning! The advance unit is all good. I would like to add that it is getting spark.

 

[dirtdigger]

First off you still have an issue if fuel is flowing out the overflows on 1 and 2. Either float levels not set right, floats that are leaking or not floating same as others or junk in the carb again causing needles not to seat. If it is running on starting fluid then it is not getting gas so you still have an issue in the carbs.

 

[nateweisiger02]

Okay, checked timing and it is all good on the bike. I will check the floats and make sure that they aren't leaking. When I rebuilt the carb I put new needles in them, I will put the old needles in instead and see if that makes any difference.

 

[dirtdigger]

Double new putting old needles in will make a difference unless they have a defective rubber tip. My guess would be you got trash in the carbs from a dirty tank and not putting inline filter in. Carbs 1 and 2 get the fuel first...especially if fuel is first turned on with bike on side stand. First shot of dirty fuel would go to 1 and 2 getting junk on float needles and into jets in those cylinders first. I suspect you will find 1 and 2 pilot jets plugged....

 

[nateweisiger02]

Replaced the pilot jets as well so those are all clean, and I've been using a little makeshift water bottle tank for gas while I'm working on getting it running so I know it has been getting clean fuel since the cleaning and rebuild. I'll check them again just to double check that nothing was sucked up in there though.

 

[nateweisiger02]

Update, the floats in carbs 1 and 2 were not floating correctly must've gotten stuck or something so the needle valves were not able to seat correctly. Put the old needle valves back in as well. Maybe that only is was what fixed the problem, or could've been that the float wasn't working properly. Either way, no longer overflowing. Now back to the issue of, it still won't start lol

 

[nateweisiger02]

Getting some backfire out of the carb, could this be a tuning issue? I don't believe anyone has messed with the tuning since the bike ran last so I hadn't touched it.

 

[nateweisiger02]

Update: Cleaned the slow idle jets in each carb using a high E guitar string and it fired right up with some starter fluid. Thank you @dirtdigger for your help! Idle is pretty high, so might have to play around with the tuning.

 

[Davidson750]

Carburetor.... Clean,clean,clean,...then clean again...and again.