Mysterious carb behavior

I rebuilt the carbs on my "81" xj750 seca this summer. The bike starts right up with choke pulled
all the the way. It warms up OK and idles faster after a while, but I can only ease the choke off about
half way before it starts to stall. When I turn the idle knob nothing happens, but I can pick up the rpm's
with the throttle grip.
I did a thorough rebuild, disassembled soaked and cleaned all the parts in laquer thinner ( not the diaphrams ),
blew out the passages with compressed air, new mixture screws springs and "0" rings, cleaned the choke parts
with new "0" rings, polished the slides and cylinders and cleaned the accelerator needles, seats and jets. I cleaned out the pilot tubes in the float bowls and replaced the float needles and seats. It got new float bowl gaskets and I set the mix at 2.5 turns out. I set the float drop to specs and the wet test was close enough to
leave things alone.
For some background, the bike had pods on it, which I replaced with an airbox from a 650 seca. I looked up
the jet sizes and they were within specs for an air box at this altitude (7,500'). standard carb manifolds fit the
airbox just fine and the carbs installed easily ( new carb holder rubbers).
When I took it for a ride, I left the choke half way and eased it off after a mile. the bike was balky but didn't stall.
When I stopped at a light the revs stayed high without any throttle. I got back home right away.
Any tips ? too lean, too rich, wrong jets, need throttle shaft seals ? Wrong accerator cable etc. ?

 

Once it is fully warm, does it idle and respond to throttle well without stalling with no choke?

 

to set idle properly you need to go for a ride. about 10 miles should do it. pull over somewhere safe and adjust idle to 1050 rpm
look to see that the idle knob is moving throttle shafts.
did you bench sync carbs?

stock paper filter or one of those problem laden K&N filters use a stock paper filter.

here are the jets you should use unless you are in Canada. Canadain jets Pilot FUEL Jet Size: #43 Pilot AIR Jet Size: #195 Main jet NEEDLE ID: #Y-14
XJ750 Models:

Years & Models: 1981-3 XJ750 Maxim, Midnight Maxim, and Seca USA models
Carb Manufacturer: Hitachi
Carb Series: HSC32
Carb Model ID: 15R00 (Maxim, Midnight Maxim)
Carb Model ID: 5G200 (Seca)
Main FUEL Jet Size: #120
Pilot FUEL Jet Size: #40
Main AIR Jet Size: #80
Pilot AIR Jet Size: #225
AIR COMPENSATOR Jet Size: #36
Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #Y-13
Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 3mm +/- 1mm (.118" +/- .039")
Idle RPM's: 1,050 rpms

air jets go in like this which is the correct way Haynes manual shows them in the incorrect position


sounds like your pilot jets are still clogged if it will not run off of choke. they run the bike until 2000rpm

 

mixture screw assembly put spring then washer then o ring on mix screw. 2-1/2 turns out from soft seat. where did you get the orings for mixture screws?

with your reves high try backing out on the idle knob of course the choke must be off or rpms will climb to about 4k rpm

did you change the throttle shaft seals? did you soak carb bodies?

 

So, senior dog face do you see any problem with using an airbox from an xj650 seca in an xj750 seca ? I will most likely pull the carbs
and check the jet sizes against your list, thanks. But would those jets be too rich for Santa Fe's altitude of 7,500 ft. ? I was also wondering
if my throttle cable is the proper length ? Do you know how long the throttle cable on an xj750 seca should be ?

 

I did install the mixture screws, spring, washer "0" ring in the proper order but I didn't install new throttle shaft seals ( I will do that) as well as a better wet set job.

 

all i can say about the air box is if you put a 750 motor in a 650 frame you use the 650 airbox so it should be good.

throttle cable has an adjuster at grip should be a little slack at carbs not tight and cable should be lubed or it can stick and properly routed


from
XJ PARTS CATALOG, SECTION M: CONTROL CABLE AND HAND LEVERS

HCP2696 OEM original THROTTLE CABLE, with black curved plastic tube, adjuster nut, and thumb-wheel on the throttle end of the cable, for all 1981-83 XJ750 Seca and XJ750 Euro models. Wire length = 41".


HCP2655 Aftermarket THROTTLE CABLE, with bright plated curved metal tube, adjuster nut, and thumb-wheel on the throttle end of the cable, for all 1981-83 XJ750 Seca and XJ750 Euro models. Wire length = 40-1/2".

 

bad throttle shaft seals will cause a high idle if you soaked the bodies you need new seals you can use 2 #11 buna orings as throttle shaft seals in each location. I go with new seals to remove doubt.
improper wet set will also cause idle problems
how to with specs
Setting the fuel levels

low levels will starve motor of fuel and high will flood engine run rich

I do suggest you check cable and if idle knob is resting on throttle shaft first

 

as far as altitude goes I live in the valleys of mountains and when I ride in the higher altitudes my idle rpms go up 100 rpms and corrects when i am back in the valleys 600 feet above sea level.

 

^^^^+1 what XJ550H says ...of you soaked carbs without removing shaft seals...you will have vacuum leaks after 30 years they loose the sealing ability ..this is one of the problems from the sound of your post. Do a full tear down spate carb bodies fro the rack ,remove butterfly plate and pull the shaft this is the ONLY way to replace shaft seals there are one on each side of the carb body .

 

Yes indeed it looks like another carb. party. This is the fourth set I've done and it's getting easier each time.
as soon as I figure what's killing my Atari and blowing the signal/horn breaker, the carbs will be next. by the
way, I've got a really good system for removing and reinstalling carbs. If I can figure out how to post pics from my
picasa library I'll document the process.

 

I've said it before but will say it again ...do not take short cuts doing the carbs...spray carb cleaner will not get in to all the passages...and do it right the first time and save yourself the missery of doing asecond or third or fourth time

 

Where did you get your jets from?

 

I cleaned and re-used the jets that were already in the carbs. When I purchased the bike it had pods on it. I switched back to an airbox
but didn't change the jets. What would be the proper size main and pilot fuel jets and main and pilot air jets ? I am at 7,500' in
SantaFe, NM. I didn't mean I pulled this set of carbs four times, The xj750R that I'm working on presently is the 4th xj bike
I have rebuilt from junk over the last three years.

 

carb specs
http://xjbikes.wikidot.com/carb-specs

Years & Models: 1981-3 XJ750 Maxim, Midnight Maxim, and Seca USA models
Carb Manufacturer: Hitachi
Carb Series: HSC32
Carb Model ID: 15R00 (Maxim, Midnight Maxim)
Carb Model ID: 5G200 (Seca)
Main FUEL Jet Size: #120
Pilot FUEL Jet Size: #40
Main AIR Jet Size: #80
Pilot AIR Jet Size: #225
AIR COMPENSATOR Jet Size: #36
Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #Y-13
Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 3mm +/- 1mm (.118" +/- .039")
Idle RPM's: 1,050 rpms

numbers are on the jets but that does not mean the po did not drill them out to tune for pods

 

case in point--- I did a rack for another person a few months ago.....one of the 36's was drilled out to about a 52

 

thank you guys, super helpfull

 

Bigger is always better, right?

 

while we are on the subject how are the drill bits sized. obviously not 52mm would a 52 jet be .52mm or .052mm? I remember a thread where it was mentioned.
also Hatachi carb jets are measured in mm but mikuni jets are measured in fuel flow if i recall correctly from that thread.

 

Also, if you’ve ever wondered what the numbers on your jets refer to:

- on Hitachi fuel jets, the numbers refer to the actual physical size of the passage hole expressed in millimeters, i.e. a #114 fuel jets is 1.14mm in diameter, a #42 fuel jet is 0.42mm in diameter, etc.

- on Mikuni fuel jets, the numbers are a measurement of the volume (cc’s of gasoline) that will flow thru the jet when poured in from a height of 50cm.

- we do not know what air jet numbers refer to…….although we suspect it is a volume of air that can pass through the jet at some level of vacuum. Anyone who has insight into this issue please let us know more!

 

So, in purchasing jets, one would have to be sure that they are for hitachi rather than mikuni carbs, or the numbers would be totally misleading. correct ?

 

mikuni the pilot jet is completly different style

 

.

 

And they won't fit, as mentioned above.

 

But if I recall correctly, the main fuel jets will interchange, the head sizes are different between Mikuni and Hitachi carbs, but the main fuel jets will physically interchange.