Running Rich?

First of all I would like to thank everyone for all their wisdom since this is my first post. I'm a newbe to the forum thing all together.
I've read a lot of posts on this site and there is a huge amount good info here. so thanks again!

OK here's the deal
81 XJ750RH SECA
4 into 1 supertrap exhaust (13 plates)
pods
Hitachi carbs model HSC32
#126 main jet (#120 is stock)
#43 pilot jet (stock UK spec)
air jets are stock UK spec as well
main air jet #80
pilot air jet #225

With this setup I would expect it to be running lean, but it runs rich.
I have had the carbs of a couple of times & gave them a good cleaning.
diaphrams are also in great shape. replaced the o-rings on the pilot screws, set float hights & bench sync them before putting back on the bike.
I should also mention that all valve clearances are within spec.
Then did final sync using homemade bottle sync & YICS tools (from some posts i found on this site)

The bike seems to idle fine & top end seems OK.
The big problem seems to be 0 to 1/4 throttle (taking of from stop)
It just coughs & spudders untill i get enough RPM's to use up all the fuel.

Today i pulled the pods off while the bike was running & noticed the slides in the carbs flutter as i give it quick shots of gas. I 'm not sure if this is normal or not?

Another possibilty is someone has drilled the pilot jet to larger diameter?
Does anyone have a #43 jet they could measure the hole diameter on so i could compare it to mine?

Any other suggestions I could try?
I am running out of ideas!

THANKS MUR

 

Madmur, welcome. Have you adjusted your mixture screws at all. After carb cleaning/bench sync you need to turn the air mixture screw in to a soft stop then turn them out 2 - 2.5 turns, depends on if they are course or fine thread. this is you base starting point for fine tuning. With the possiblity of that jet being bored oversize what I said may be meaningless but it is just my thoughts. Let us know & keep asking somebody will get you straightened out.
Gregg

 

Yes I have the idle mixture screws set at 2 1/2 turns out

 

It may be that bored out jet. Have you tried turning the mixture screws in 1 or 2 degrees to see if that makes a difference? As far as those fluttering butterflys, I don't have a clue on those.

 

I have turned the the screws in a bit they were originaly set at 3 1/2 turns.

Its actually the slides that are fluttering not the butterflies.

 

Sorry about the slide butterfly mixup. The slides fluttering may be normal. I can't say for sure, but may be a process as vac is created by cyls thru their cycle. Have you been able to do any plug chops & if so, what color are they. Ideal is light tan color.

 

No, haven't done any plug chops. But after a drive around town with all the stop & go the plugs get pretty sooty. On the other hand if I've been on a good tear of the back roads the plugs look not to bad.

 

In regards to jet sizes is the number stamped on the jet a reference to the hole dia or the flow in cc's/min

#43 jet = .43mm or ??

 

"The bike seems to idle fine & top end seems OK.
The big problem seems to be 0 to 1/4 throttle (taking of from stop)
It just coughs & spudders untill i get enough RPM's to use up all the fuel."
Mine is doing the same thing!!!

 

"The bike seems to idle fine & top end seems OK.
The big problem seems to be 0 to 1/4 throttle (taking of from stop)
It just coughs & spudders untill i get enough RPM's to use up all the fuel."
Mine is doing the same thing!!!


MINE TOO!!

I am stumped, although I'm pretty sure the problem started when I started messing with my mixture screws...a necessary evil I suppose. I'll be keeping a close eye on this post.

 

Got a hold of an infrared thermometer today and checked my headers. The fourth is way behind the other three, about 80 degrees cooler. I suspect that I may be running on 3 cylinders upon takeoff and that is what is causing my stumble. Could be a synch problem maybe, or valves, or...what? Need to do a compression test. What would low compression tell me about that cylinder, in terms of repair?

Damn bike.

 

Low pressure could be valves or rings.
Pull all plugs and hold throttle wide open for comp test.
If low put a little oil in the cylinder if pressure comes back, rings, if still low valves not seating.

As far as the stumble off the line check to see that the enrichment plungers are all letting the circuit close.
#4 could be hanging causing that cylinder to foul.

 

First, the simple answers.

* #43 jet means a hole diameter 0.43 mm. That's true for Keihin and Hitachi. Mikuni using a different type of measurement.


* "With this setup I would expect it to be running lean, but it runs rich."

Not. She's running rich at a given RPM range under certain conditions (from 0 to 1/4 throttle when you're accelerating). It does not mean she's running rich. Note a thing: we got at least 4 different RPM ranges what we can tune individually. In other words: the conditions (lean - rich) may be different for all the different RPM ranges.

* "The bike seems to idle fine & top end seems OK.
The big problem seems to be 0 to 1/4 throttle (taking of from stop)
It just coughs & spudders untill i get enough RPM's to use up all the fuel."

Possbile causes (if we assume that she's running rich instead of lean, but it's fine for now):
- worn spark plugs. Get a new set of plugs and see what's happening. Worthwhile to do, it can save a lot of money and time.
- too much fuel in the carbs. Lower the fuel level by 1 mm. It should solve the problem. If not ... well. You f***ed up Sorry.
- worn diaphragm springs. No question, replace them. Buy or find a shop where they able to do a new set. Order a set with the original wire diameter and order another one(s) with larger wire diameter(s).
- lower the needle. Unfortunately it's not so easy. You have to have the Dynojet kit or you have to ruin the original plastic things at the end of your needles.

By the way, carefully check the needles. The Japs designed the Hitachi carbs to worn out with time. The needles are forced to hit the side of the needle holders by design causing excessive wear over time.

Oh, by the way:

Welcome in the club What you're trying to address is the most frustrating, time consuming and expensive things to do on these bikes. You're trying to fine tune all the engine parts, carbs and exhaust to each other to end up with a result you can live with. It's not easy, it's time consuming, it's frustrating and burns a lot of money and time. Did I write this ... ?

Anyway, to achieve your goal please take a few advices.
- Don't rush. Be patient. It's time consuming, frustrating, etc.
- Do one thing at a time. For example: change the spark plugs first. If it doesn't help (she will work fine for a good 30 or so miles), lower the fuel levels. Then if it doesn't help try something else but don't change more than one thing at a time!
- Try to understand how our carbs are working. If you understand what does what and why, you should be able to become the master of the very fine art:

!CARB TUNING!

 

There's something I haven't checked.
I can't find the spec for compression in my haynes manual. Does anyone know what it should be for a 81 xj750 seca?

 

Sorry didn't see your post zsoltk before i posted my last one.
Sounds like some good Ideas.
"Worn Diaphram springs" this would explain the slides fluttering!
"lowering the needles" How much?
First of all i guess a new set of plugs wouldn't hurt and this is the easiest thing to try.

 

Good thinking! Let's see the result after the spark plug change.