Engine bogs under 3k, then runs fine!?

Bufoalvarious

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Hey everyone! I'm new to this whole community a appreciate the amount of help I've gotten. I'm currently putting a 79 back together and I'm to the test drive point if things. The bike idles great, and revs out normal sitting, but on the street it seems to break up under 3k rpms then run perfectly fine after. I've tried turning the choke on but it seems to flood it out. I have pods and my exhaust is cut below the foot pegs, carb has already been rejeted.
 
..... and re-jetted from what to what? Also, with bigger jets, you often need to lean the needle setting. In particular, this is almost always needed with a '78-'79 carb set.
 
The slides are a crucial component in governing mid throttle performance and throttle response.
Try removing your filters and applying the slide test.
Check that both slides drop with a audible click to the bottom and they are not hanging or sticking at any part of their travel.

Using your finger lift each of the slides in turn fully . Place your thumb over the top vent and let go the slide and see how quickly it drops . You should be looking for around 15-18 seconds if the diaphragms have no holes and are sealing properly.
 
Slides are good and are falling at the same rate! Any other ideas? Should I RIP the carbs apart and give them a good cleaning? I've also read some things about maybe a needle jet or a pilot jet for low range. Not sure
 
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Slides are good and are falling at the same rate! Any other ideas? Should I RIP the carbs apart and give them a good cleaning?

I understood that you had already cleaned the carbs to within an inch of their lives ;)

Lets have a little more information and detail about the performance ,
Can we see an image of the front of the carbs so we can ID the model 76-77 or 78-79 there are key differences and parts often get mixed.
What needle jets and jet needles are fitted ?

What happens when you throttle up slowly ?
What happens when you are cruising along at a steady pace on half throttle and then open it wide ?
Does it bog ? does it stumble ? or cough ? or does it just accelerate cleanly but slowly ?
Is there a particular rpm or throttle position that the engine suddenly performs normally?
 
Yes, there were 5 different sets of carbs used on these bikes, similar to each other but jetted differently. You base your re-jetting for mods on what was stock for the year of the carbs you're using, not the year of the bike. So, 1st task is to positively determine what year carbs you have. Then yes, some disassembly will be needed to check and record your current jetting. The carbs don't have to come out. The main and pilot jets are mounted in the float bowl on the BS38s so just removing that, carbs still in place, will give you access to those jets. The needles are in the slide/diaphragm assemblies. You have to take the carb tops off and pull the slide/diaphragm assemblies out, but that too can be done with the carbs still in place.
 
Slides are good and are falling at the same rate! Any other ideas? Should I RIP the carbs apart and give them a good cleaning?
I dont have experience with carburetors, I just have an "idea" of how they operate. They are 79 carbs that I knowledge came about while learning to install my fuel lines and figure out vaccum and what not. I waited for the right first bike and found a box of pieces for $200 and well it's been a puzzle since then. I'm almost there but stuck at a wall. Slow throttle is okay some of the time and when it gets past 3k it runs perfect, no matter what gear or speed. It sputters and coughs like it doesn't have enough fuel I've tried turning the choke on and it dies on me, I haven't tried holding at the bad spot with choke though. I've also tried taking off my pods completely and it will also die. I hope that is enough info and I will post pictures in about 5 minutes
 
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I dont have experience with carburetors, I just have an "idea" of how they operate. They are 79 carbs that I knowledge came about while learning to install my fuel lines and figure out vaccum and what not. I waited for the right first bike and found a box of pieces for $200 and well it's been a puzzle since then. I'm almost there but stuck at a wall. Slow throttle is okay some of the time and when it gets past 3k it runs perfect, no matter what gear or speed. It sputters and coughs like it doesn't have enough fuel I've tried turning the choke on and it dies on me, I haven't tried holding at the bad spot with choke though. I've also tried taking off my pods completely and it will also die. I hope that is enough info and I will post pictures in about 5 minutes

you need to be very precise about your description as its rather ambiguous at the moment.

When you say 'it coughs and splutters ' when is that ? you don't say is it on acceleration , steady thottle ? deceleration ? idle ?
We can only advise on the information you give us as we cannot see and hear the engine running.

Who told you that the carbs are 78-79 BS38s ? could we have a picture of the mouth of the carbs to be certain
EDIT
Ok our posts crossed.
Those are indeed 78-79 BS38 carbs .
Let us know exactly what needles and jets are fitted when you can.

I can see that the carbs have not been removed stripped and cleaned in some time;) I suspect that is the problem right there ...

if you are using pod filters and open headers or no silencers then the carbs will need rejetting
you should have 502 jet needles Z2 needle jets and probably need to start with 35 pilot jets and work down to 32.5 or possibly 30 but standard 27.5 will be too small for sure.
Main jet will probably need to be as large as 140 perhaps slightly less its all trial and error to get it right
 
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On acceleration from about 2k to 3k the engine will fight me to get past it, trying to die. It will break past or die if heavy throttle is applied. Steady throttle in the "bad zone" causes the bike to die completely. I will check all my jetting sizes as soon as possible but as everyone says "the previous owner" says main jet and pilot jet were changed but of course cant remember to what. Bike idles perfect and decelerates fine through said "bad zone"
 
On acceleration from about 2k to 3k the engine will fight me to get past it, trying to die. It will break past or die if heavy throttle is applied. Steady throttle in the "bad zone" causes the bike to die completely. I will check all my jetting sizes as soon as possible but as everyone says "the previous owner" says main jet and pilot jet were changed but of course cant remember to what. Bike idles perfect and decelerates fine through said "bad zone"

Forget what the PO has said..........You need to determine the jet sizes, jet needle and clip position, for yourself and us.......
 
2 to 3K is right in the idle to midrange transition area. That means it could be pilot jet size and/or needle setting. Make sure your mix screws are set at least close to correct too. Starting setting spec for them is 2.25 turns out. If they are set at like 1/2 or 1 turn out, that could cause issues.
 
Your main is most likely a 142.5. I doubt the bike would even run with mains as small as 42.5. A 45 pilot is huge and may account for your 2-3K issues. Try something in the low 30s.
 
I will also mention that the air pods you're running are no good for CV carbs and they may be contributing to your problems.
 
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