How pop off pressure affects throttle response?

[SPX_N_NEED]

I rebuilt my carbs and installed new springs that came with the rebuild kit.

The pop off pressure was close but not the same. it was slightly higher.

I am trying to diagnose a slow throttle response.

I have to ease into the throttle or else it bogs down and seems to flood the carb.

Once I am up and running no issues. Great response from 1/4, 1/2 to WOT.

Just from a dead stop to WOT it hesitates and sometimes dies.

Also if I am out riding at full throttle and get thrown off, When I start it back up the carb seems to be full of gas still and flooded.

I have to start and idle rough for a bit or with the choke on and burn the fuel out of the carb and ease into the throtle to build up some steam.

I wanna just be able to go wide open and get on plane asap. Before the carb rebuild it was opposite.

I could go zero to WOT and once i had some steam built up it bogged and hesitated.

Thanks in advance for any solutions.

 

[OldSchool]

I rebuilt my carbs and installed new springs that came with the rebuild kit.

The pop off pressure was close but not the same. it was slightly higher.

I am trying to diagnose a slow throttle response.

I have to ease into the throttle or else it bogs down and seems to flood the carb.

Once I am up and running no issues. Great response from 1/4, 1/2 to WOT.

Just from a dead stop to WOT it hesitates and sometimes dies.

Also if I am out riding at full throttle and get thrown off, When I start it back up the carb seems to be full of gas still and flooded.

I have to start and idle rough for a bit or with the choke on and burn the fuel out of the carb and ease into the throtle to build up some steam.

I wanna just be able to go wide open and get on plane asap. Before the carb rebuild it was opposite.

I could go zero to WOT and once i had some steam built up it bogged and hesitated.

Thanks in advance for any solutions.

You used the same spring you took out? Usually they come with 4. In any case, I have learned that it is always better to be on the low side of your pop off range rather than the high.

Immediate off idle response is usually related to the LS screw and pilot jet if your pop-off is good. If it worked good prior to the carb rebuild and you used the same jets I would check the LS screws, but a high pop off will certainly cause low to mid level hesitation.

I would do whatever it took to make that pop-off lower if it was me, I would have never put the carb back on even if it was one pound higher.


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[Mekanix]

There is something else going on.

Sounds like your running lean instead of flooding.

If it was flooding, It would stay running and work its way through it. Unless the needle is being held open and allowing full flow all the time.

Where do you have the low speed screws set at ?

What was the new number you got with this setup?

Did you check pop off after they were fully reassembled?

 

[SPX_N_NEED]

should i just put the old springs back in?

When I stand them up next to each other, the old ones are slightly shorter

I havent checked the pop off pressure since I put it all together.

Just took it out for a run and it ran fine then started to hesitate from a dead stop to WOT.

How can I richen it up if it is running lean?

 

[Mekanix]

By setting the pop off to the correct number. This is the adjustment you make for up to 1/4 throttle where its hesitating and to help the transition to main jet.

The old spring would make it rich being shorter.

I've noticed that a change of about 1-2mm in spring length can change pop by 10 psi! I'd say you made it leaner by using that spring.

 

[Strizzo]

unless you used all OEM mikuni parts in your rebuild, the new parts could be what is giving you trouble. you need to check the pop off any time you mess with the inside of the carbs, otherwise you have no idea where your mixture is between idle and 1/3 throttle. you also need to do a few tests to make sure the needle and seat aren't leaking while you have things apart, or your rebuilt carb can end up worse than before. if there is a hesitation from idle to WOT, you can try opening up your low speed needle 1/8 to 1/4 turn and see if it helps, but i think you still need to pull the carbs and verify everything.

 

[Sportster-2001-951C-Stock]

I rebuilt my carbs and installed new springs I have to start and idle rough for a bit or with the choke on and burn the fuel out of the carb

Holding the choke on adds fuel, so it sounds like you're describing a lean condition at low throttle positions.

pop-off has it's greatest impact at low throttle position, pop-off is set so that the engine vacuum signal entering the idle screw port pulls the metering diaphragm into the metering lever which opens the metering needle valve to allow fuel into the fuel chamber.

If the spring tension is too great, you may not have enough vacuum to draw fuel properly.

Assuming the low speed mixture is too lean now(choking increases carb bore vacuum), I think the springs from the kit are likely too strong and the originals were a better match to balance the fuel regulator (low speed metering) pop-off.

Once the carb is past 1/3 throttle and assuming something's not completely haywire, pop-off has a minimal effect.

You might be able to tune enough fuel by opening the low speed screws a bit more, but if the lean condition off idle persists, you'll probably find it necessary to return to the original spring tension. I think much more than a couple of turns out on the idle mixture screws warrants a pop-off adjustment.

By confirming pop-off before final carb assembly, you can avoid having to guess and possibly having to remove the carbs to adjust the pop-off.