XJ750 Seca stalls when braking (only when cold)

Hello fellow XJ riders,

First, many thanks for the insanely helpful information on this forum.

I have a 1981 XJ750 Seca that I have been working on for about two years, and I have a problem.

My current setup:

- pods (PO, not my doing!)
- a very clean set of carbs with a bench sync (church of clean level clean)
- valves that are all in spec
- a fuel level that I haven't measured precisely, but seems about right (using the clear tube method, maybe 4mm from the base edge of the carb body when the carbs were installed on the bike...?)

The engine stars up just fine, and when warmed up, the engine has a great throttle response, up and down the rev range. The engine and carbs sound "crisp" and "precise" and not "soggy" at all.

The problem is this: when the bike is just warming up, she will hesitate and then stall under very moderate braking. Any firm braking will stall the engine quickly.

Also, I want to brake and NOT stall, I must brake as I pull the clutch and give her just a bit of throttle (say 1/16th only).

(Also, if I have enough speed and road ahead of me, I can brake hard, the engine will stall, but if I continue to coast with the clutch out, the engine will keep turning over enough to "clear out" the problem, and then recover.)

During this warmup period, the engine sound at low RPM is often "wet" or "soggy" or very "gurgly", and it's slow to respond to throttle. It feels very "imprecise" and "sloppy" to respond.

However, these problems ONLY happen for the first few minutes after a cold start, and as I said, when the engine is warm, it's wonderful.

After the braking stall, I have noticed that the bike will start back up in 2-3 cranks if I hold a lot of throttle open while press the starter button to crank. If I do NOT touch the throttle, however, it takes more like 7-10 cranks for the bike to start back up.

All of this makes me SUSPECT that the stalling is very related to the fuel level in the carbs.

Perhaps the fuel level is too high, and the fuel is pouring into the cylinder, causing a super rich condition.

However, I don't have a good theory on why the problem disappears when the bike is warmed up. If the fuel level is causing a super rich condition, why would a warm engine respond that much differently?

Does anyone:

1) also suspect the fuel level is too high and it is "flooding" the engine?
2) know if the fact that she will start back up faster with the throttle open confirms that the engine is "flooded" the fuel level is the issue?
3) know why this disappears when the bike is warm? I thought that as the engine warms up, it would still be sensitive to flooding/super rich conditions.
4) have a suggestion on how much fuel level lowering is worth trying when I adjust? 1mm? 5mm? I don't have any experience on how sensitive the fuel level is on these bikes.
5) have any alternate theories? Am I missing something here? Is it possible that the fuel level is too LOW? Or this isn't related to the fuel level at all?

Many thanks!

Conrad

 

at 4mm fuel is low lean
pods run lean

lean runs hot

 

not good enough, you need a real running sync

maybe extend your warmup time
is the choke on when it stalls?
what rpm is it stalling from?
pods

 

The fuel (float) levels are low.
Did you rejet for pods?
Can you reinstall the airbox?

 

I concur, floats low and you need a better sync.

 

Thanks for all the replies!

I will do a running sync. I was hoping to not have the spend the $$ for that tool, or try to make it myself! I'm not super handy...

Polock, it happens at all sorts of RPMs. I brake moderately or firmly, and it feels like I turned the key off. The engine keeps spinning from the wheels.

K-Moe, I did not rejet for pods. The PO didn't, either, and because I didn't seem to have any problems, I didn't want to mess with it unless I was SURE. I hate taking off the carbs, and I don't have an airbox, and was hoping to not have to buy one!

Krs14, thanks for the input.

Here's something you all might be interested. I did as a test and changed one thing with a pretty dramatic result.

The bike is cold -> I sit on the bike -> apply full choke -> hit start button -> bike quickly fires up -> RPM climb past 2k around 8-10 seconds -> I close the choke (no choke now) -> I shove the bike downward with the front brake on -> bike will stall, IMMEDIATELY

Same as above, but keep the choke ON -> bike will stutter but never stall.

I think that this confirms that when the bike is braking, the carbs are "running dry"?

Question: what is the next action step? Does this clearly mean that I should correct the fuel level ONLY, or remove the #40 pilot jet and raise to #42 ONLY, or do both? Or do a running sync first, and see what happens?

Thanks!!

 

Since you are running pods you should rejet for them while you have the carbs off to set the fuel levels, otherwise you're just going to have to take the carbs off again (which you might have to do anyway, because pods). With the stock jets the bike will run lean as long as you have the airbox and the stock filter removed.

Setting the fuel levels

A running synch won't correct the obviously low fuel levels, but will need to be done after rejetting and setting the idle mixture.

The reason she stalled with the "choke" off, but not with it on, is that the enrichment circuit draws fuel from the bottom of the float bowls, where the pilot and main draw off of the top.

Inside your Carbs

 

K-moe, many thanks. Very helpful. I didn't realize the difference between pilots and enrichment drawing off a different area, even though now that I can think of it, that makes 100% sense! Conrad

 

If the carbs had a true choke then we'd be talking about a different problem.

 

When you say it's like you turned the key off, I'm wondering about an electrical issue. Maybe take a little time to check/clean electrical connections while you're working on things.

 

Sidestand switch maybe, but low fuel height kills the engine just as quickly as an electrical fault will. Also note the lack of stalling when on enrichment.
Still a good idea to clean all the electric contacts and switches. Regular maintenance that hardly anyone ever does.

 

there's a tang on the floats to stop them from going low enough to let the low speed jet run dry.
i don't think you can push the forks down far enough to change the fuel level in the bowls, if the front end goes down gas would go towards the low speed jet.
the floats might be upside down
you got pods

 

^+1 on all of the above

maybe you have sticky clutch discs and the drag is stalling you. maybe they free up with the heat

would not hurt to put some seafoam in the crankcase to degunk anything follow directions on can