Stumble when letting off the throttle and other fueling problems

[Leo44]

Hi everyone,

My 1VJ makes sure that I'm never bored and with my hands clean, so here we are again.

When I did a top-end rebuild, I also swapped in a freshly rebuilt pair of carbs. How do I know they were freshly rebuilt? Well... I got them from a reputable friend of mine and trusted him on his words... Never even removed the float bowls or bench tested them, huge mistake on my part.

Anyhow, I set the idle mixture screw and idle stop screw, bike starts and (for the most part) runs just fine.

The two major issue that are occuring right now are:
1- If I go WOT starting from around 2500-3000 rpm, the bike would stumble a couple of times before picking up (I don't really know if "stumble" is the right term, but basically it's as if she looses power for a split second, "pulling back" even, if that makes sense)

2- Whenever I'm at mid to high rpm (say 5000) and mid to high throttle (say 50%), if I throttle down (to something like 15-20%) somewhat rapidly, the bike does the same sort of stumble/pull back/cut out as described above, just one time, then picks up to the "desired" throttle level.

In every other scenario the bike runs fine, cruising, slowly going WOT, slowly letting off the throttle, no problem there.

As always, thanks in advance and any suggestion/advice is gladly appreciated!

[charliext1vj]

Have you checked the linkage is properly engaging between both carburettors - it sounds as if it is out of adjustment between the both carburettors. Sometimes they can be connected wrongly? Also is the CV type carb free in its operation does it slide up easily by hand?

[Leo44]

Have you checked the linkage is properly engaging between both carburettors - it sounds as if it is out of adjustment between the both carburettors. Sometimes they can be connected wrongly? Also is the CV type carb free in its operation does it slide up easily by hand?

That’s exactly what I thought as well, either a stuck/binding secondary or a un-synced butterfly again in the secondary.

I’m not a carburetor expert, but that stumble feels like a lean condition, so possibly the secondary slide is having trouble lifting, thus only letting air through.

In any case, the carbs have to come off so I’ll have time to check everything

[charliext1vj]

Don't forget to check those dodgy inlet rubbers...I had a new set fail within a month!

[Leo44]

Don't forget to check those dodgy inlet rubbers...I had a new set fail within a month!

Well considering that I bought a set off of Amazon (so probably chinese stuff), sounds like a possibility. I'll do a vacuum leak test

[donnybach]

Pick up from low revs is massively dependent on float height and pilot jet size.
Took a while of experimenting with little adjustments ( only problem with carbs over injection) and test rides. Mine would need 2200 rpm at least. Got it so it pulls cleanly from as low as 1700 after.
Starting point is factory spec as per the manual.
I upped the pilot in the end. Several threads on here including my set up.
Inspect for damage etc as charliext1vj said. Pull the carb. Order all new seals (two or three of the float bowl ones, they distort every time so have to rest between removing and reusing, then fit a new one once sorted finally).
Order a manual if you don’t have one. Great investment.