Carbs only get Gas when the choke is on

[Karl squire]

First Sorry everyone for having so many threads, I cant figure out how to delete old threads, if someone can tell me how to delete I can delete all the other questions that have been answered

Anyway 73 RD350 with VM28 400 Carbs, everything from what I can tell is clean and nice, all the jets everything is good took them both apart again and double checked everything but seems like the carbs only get Gas if they are choked, there has to be a simple answer to this, a blocked vent maybe?

I start it, it runs good then gets boggy, turn the choke on starts to run good, turn the choke off continues to run good for a min or 2 or 3 then starts to bog again then turn the choke and so on. I was thinking maybe it was sucking air from the carb boot or reed block but wouldn't start to die as soon as I stopped choking it?

[m in sc]

i need to ask, when you rebuilt these carbs, did you us e a rebuild kit, and did you use the brass that was int he kit? if so, that might be part of the issue.
if its boggy (falls on its face) giving it throttle, (and this will contradict what i said earlier) thats a classic lean symptom on the needle. 

also, remember, vm28 rd400 carbs use a different float setting than the 350 carbs, they need to be 23mm-ish , not the 15 350 carbs use.  if the float level is way off, then this could explain a lot of the issues. .02

(users can not delete their own posts just fyi, no harm in having them here)

[Karl squire]

All the parts are Mikuni parts, I'm pretty diligent about only using Mikuni parts and not other manufacture parts, if that is what you mean, however that been said, I should double check but 95 Percent positive they are all mikuni parts. I should double check the slides, they are 2.5, pretty certain that is what the Dave mod calls for.

[m in sc]

2.5 slide is fine on it.

[1976RD400C]

With the Dave F mod it should have a needle jet like the one on the right, has that been done? If the bogging is acting like you shut the key off, then try making it richer.

[m in sc]

to the right.  should be a 169 p0 or 169 p2

[m in sc]

fyi:  https://www.2strokeworld.net/wp-content/uploads/general-tech/tech-jetting/davefmod.pdf

[RustyRD]

check the entire fuel system, as in flow from the tank through the filters if any. there can be sneaky culprits such as a plugged vent in the gas cap. opening the tank to take a visual check of fuel close and after the air in the tank is not equalized flow starts to decrease and carbs starve for fuel. I purchased a replacement petcock on my rd and found  that it was a major restriction in flow to the carbs. Hard to trouble shoot when you get caught in the mind set, it can't be that I just replaced it.

[Karl squire]

to the right.  should be a 169 p0 or 169 p2

The needle jet is  a 160 p0, The needle is a 5Dp7

Ok so first YOu all need to know I have a phobia of over tightening shit, frigg I have stripped more bolts and screws then I care to imagine, that been said.
the last time I took the carbs off Saturday to make sure the air jets were good, I remembered the float bowls had lots of gas in them, Sorry I didn't mention that soon my bad, I was thinking about Air jets. So the bowls are in fact getting gas.

With the above in mind I wanted to check 2 things this morning, I started the bike, checking for air leaks (remember my phobia) I started the bike and held an unlit torch to the engine, I could not find any specific air leaks but when I held the torch to the mouth of the carb the RPM's picked up, confirming lack of Gas, so to me that means I'm sucking air somewhere, I waited only a min or so and the bike stalled (fuel starvation) and I took the carbs off to confirm the bowls were full and yes they are full of fuel. So maybe you can chime in on this, it seems like I'm sucking air. I tightened the read blocks and the fuel bowls as I think they need to be air tight as well and when I install the carbs again I"ll make sure they are tight. I also confirmed the slides were Mikuni's. Feel free anyone to offer your thoughts
Thanks again everyone I appreciate you time and thoughts.

[m in sc]

should be fine there then. I've probably done 10-15 dave f mods, and done them on 400 carbs on a 350, so you should be ok.

since its idling, the pilot jet is the 'highest' in the fuel system so float level 'should' be ok.

id start at 1.5 out on the airscrews, and set the needle in the middle. fire it up, and warm it up. get it to idle, and adjust the air screws IN until the idle starts to just drop, then lean it back out 1/8 turn. thats a good place to start.

if it falls on its face giving it throttle over 1/8~1/4 up to 1/2 throttle, it usually indicates lean on the needle.

go richer until it revs cleanely . if it burbles and sounds fat, go leaner.

the main can only be set after riding it, but that's mostly going to be 3/4-and up throttle.

*note none of this mentions rpm, its all about what the motor does at certain throttle positions.

[Karl squire]

check the entire fuel system, as in flow from the tank through the filters if any. there can be sneaky culprits such as a plugged vent in the gas cap. opening the tank to take a visual check of fuel close and after the air in the tank is not equalized flow starts to decrease and carbs starve for fuel. I purchased a replacement petcock on my rd and found  that it was a major restriction in flow to the carbs. Hard to trouble shoot when you get caught in the mind set, it can't be that I just replaced it.

Thanks, so I also have a leaky fuel tap, what did you end up doing to resolve your fuel tap problem?. Also my fuel tap is not the problem of this thread as it is doing the same thing with the gas tank and a feul bladder but I'd like to know what you used to fix the fuel tap problem?.

[Karl squire]

should be fine there then. I've probably done 10-15 dave f mods, and done them on 400 carbs on a 350, so you should be ok.

since its idling, the pilot jet is the 'highest' in the fuel system so float level 'should' be ok.

id start at 1.5 out on the airscrews, and set the needle in the middle. fire it up, and warm it up. get it to idle, and adjust the air screws IN until the idle starts to just drop, then lean it back out 1/8 turn. thats a good place to start.

if it falls on its face giving it throttle over 1/8~1/4 up to 1/2 throttle, it usually indicates lean on the needle.

go richer until it revs cleanely . if it burbles and sounds fat, go leaner.

the main can only be set after riding it, but that's mostly going to be 3/4-and up throttle.

*note none of this mentions rpm, its all about what the motor does at certain throttle positions.

Thank you for the advice, honestly I really appreciate everyone one pitching in with their help.

[Karl squire]

should be fine there then. I've probably done 10-15 dave f mods, and done them on 400 carbs on a 350, so you should be ok.

since its idling, the pilot jet is the 'highest' in the fuel system so float level 'should' be ok.

id start at 1.5 out on the airscrews, and set the needle in the middle. fire it up, and warm it up. get it to idle, and adjust the air screws IN until the idle starts to just drop, then lean it back out 1/8 turn. thats a good place to start.

if it falls on its face giving it throttle over 1/8~1/4 up to 1/2 throttle, it usually indicates lean on the needle.

go richer until it revs cleanely . if it burbles and sounds fat, go leaner.

the main can only be set after riding it, but that's mostly going to be 3/4-and up throttle.

*note none of this mentions rpm, its all about what the motor does at certain throttle positions.

So Seems you and everyone has been a victim of my phobia of tightening things to Tight, I tightened everything up its running as it should as fare as I can tell, so my apologies to everyone that has taken the time to read and contribute to my thread.
should be fine there then. I've probably done 10-15 dave f mods, and done them on 400 carbs on a 350, so you should be ok.

since its idling, the pilot jet is the 'highest' in the fuel system so float level 'should' be ok.

id start at 1.5 out on the airscrews, and set the needle in the middle. fire it up, and warm it up. get it to idle, and adjust the air screws IN until the idle starts to just drop, then lean it back out 1/8 turn. thats a good place to start.

if it falls on its face giving it throttle over 1/8~1/4 up to 1/2 throttle, it usually indicates lean on the needle.

go richer until it revs cleanely . if it burbles and sounds fat, go leaner.

the main can only be set after riding it, but that's mostly going to be 3/4-and up throttle.

*note none of this mentions rpm, its all about what the motor does at certain throttle positions.

Ok its running much better I needed to tighten everything up so it was not sucking air.
That been said, the Air screw as you mentioned above. I had it at 1.5 turns when I set the idle and synced the carbs, but when I blurp the throttle, bog, I tuned them in, idle picked and I went all the way in running much better maybe not the way it should but I could blurp the throttle as soon as I turned them out 1/2 a turn and when I blurp with the  Air Screw 1/2 turn, Bog, just tiny blurps bog. Pilots 27.5, if I'm not mistaken that would be a sign that the pilots are too small?

[m in sc]

exactly. id jump up to a 32.5 or a 30.

[Karl squire]

I think I'll run out to Winner Circle now and pick up a set of 30's, I dint think 32.5's would be necesarry but its a 35 min drive, might as well save my self a trip just in case. Thank you and I'll definitly keep this thread updated. Cheers.