Looking for opinions on pistons and cylinder condition

[ampzip]

Not sure if I should keep posting to this thread or start new ones with new issues.. In any case, for whatever reason I can't get the bike to reliably start on it's own now.

From cold start, it will start with choke on, but will die out before it warms up at all. Any throttle at all kills it. If I spray carb cleaner in the intakes, it'll rev up and run well, so I keep spraying till it runs without choke, and then it runs great. It'll run, idle and drive really well.

Again, carb issue? I've cleaned both several times. Idle jets, jet tubes, main jet are all spotless and spray well.

I've tried air screw down to 1/2 turn and it reacts the same.

[SoCal250]

Fuel delivery / carb issue

[1976RD400C]

Does it die out even with the choke on? Do you have the washers on that fit under the main jet? Is that a tight fit?

[ampzip]

Does it die out even with the choke on? Do you have the washers on that fit under the main jet? Is that a tight fit?

Yes it will die with choke on. Yes, washers are on under the main jet. The washers appear to be factory, they fit perfectly under the main jet.

I measured float arm and it is at correct height as well.

[quocle603]

Check for some air leaks. Sounds like its running really lean at a cold start. Strange that it will run after you do your method. Is the choke crossover tube installed when you put the carb in? Stock airbox? Double check if the carbs are synced?

[ampzip]

Am I correct in stating that there should be a hole in the bowl of the carb with the pickup tube? Both bowls have a "2" stamped in them. It looks like there is supposed to be a "1" bowl which has a hole...

I guess that means the pickup tube isn't doing anything.

Would it be possible to drill that hole rather than buying a new bowl?

[quocle603]

Ahhh thats a culprit of the choke circut not being engaged due to how its not getting any fuel! You could drill it, just be very careful. Do you need a drill.size and the depth? I can go get that info for you with my spare carb

[ampzip]

Ahhh thats a culprit of the choke circut not being engaged due to how its not getting any fuel! You could drill it, just be very careful. Do you need a drill.size and the depth? I can go get that info for you with my spare carb

That would be super helpful!
Either way it'll need to be replaced so I figure I'll give it a go.

[quocle603]

Sorry for the delay. Having a newborn really put a damper on your ability to do things. I was able to fit a 3/32 drill bit into the hole and measured about 9.5mm down into the hole. It does not go into the port from what I can see, I tried to get other bits into it but couldn't see anything. I assume theres a smaller hole for the choke circuit to kick in.

Good luck!

[1976RD400C]

The service manual lists the starter jet as a #70 so I would imagine that hole is equivalent to a #70 jet.

[teazer]

It looks like you have two of the same bowl.  In a "choke" bowl, there is a cross drilling plus a small #70 jet in the vertical hole.  If the small brass jet is in place, the port can be opened and the size is not critical. 

That's how most Mikuni carbs are set up.  https://pinkpossum.com/GT750/carb/BS40carbs.htm  has  nice cross section.

[m in sc]

just carefully drill in a 1.5 to 2mm hole. dont go all the way thru the bowl though (ask how i know), and yes, that def explains it