750 seca carb mystery

What should come first ? the colortune or the running synch ?

 

You can't synch properly until the mixture is right. Colortune first.

 

I followed the video directions and swapped #1 plug for the colortune plug. Put a sheet over my head
to block the daylight and started up the bike ( xj750 seca ) there was a bluish flame right off, but with intermittent
pops of white. It never looked red when opened up or white when I closed the mix screw down. I did see a more
even pulsing in blue right at the point where that cylinder started run more evenly. Is this the balance point I'm looking for ?
Would the intermittent white pops be from an air leak ? The carb holder/engine manifolds are new and the airbox boots are flawless.
The holes in the airbox are a little rough at the edges but the boots popped into place ok. I already shimmed the valves. Could this be a case
of old engine crankiness ? Carbon on the intake valve etc. ?

 

The way I did it was to close the pilot screw until I started seeing some white. Then I opened the screw until the white disappeared. Repeated for all cylinders. Then installed new plugs and took a 15-20 mile ride after which I removed the plugs to "read" the color---I was looking for a light gray or tan on the insulator tip. If it was still all white, and it was, I opened the pilot screw about the width of a dime and repeated the ride and read process until the plugs had a nice light tan color. It took me a number of rides over a couple of weeks, but it really improved my bike's starting, running, and mpg. I only used the colortune plug to get the initial setting and only at idle. Installing new plugs simply made it easier to read the plugs. I think it is fairly common to get a little white when you pop the throttle, if that's what you are doing.

 

I had problems a little too... instructions say to open to yellow and then close to blue, and I thought I landed a little rich. After reading the plugs a few times closed the rich ones slightly and all good now although I could probably be just a hair leaner.

Also, the instructions say if you can't get open enough for yellow you are too far out of sync with other cylinders and this happened to me when I went from #1 cylinder to #2 with the colortune. I put the gauges on and synced, put the colortune back in, and then it adjusted normally like #1 did.

So I ended up colortuning by:

• Colortune 1
• sync
• Colortune 2
• sync
• Colortune 3
• sync
• Colortune 4
• sync
• Ride and read plugs (about 100 miles) / adjust mix very slightly
• sync.

That's where I'm at now...

 

Isn't it easier doing an old fashioned plug chop??, i mean you can do WOT or medium throttle, a few runs, easy to get the plugs out on the side of the road.....unlike modern Bikes, and bobs your uncle as they say, I've never used a color tune, is it absoloutely necessary?..we used to 'plug chop' everything, in the old days, both 2 and 4 strokes....

 

No. Plug chops or cuts only work at the speed and load you do them, and plug colours take time to develop, and ultimately, could be changing costantly as you're riding along. From this, you should gather that if you want to set the idle mixture (with the mixture screws, because thats all they do, well and a small bit at the smallest throttle opening), you need to do a plug chop at idle. The issue with this is that it would take eons for the plug colours to give you an accurate signal that is anywhere near, let alone better than you can get with a colourtune.
I must say here that I had a colourtune years ago and binned it, I just felt it was too much faff for the results it was giving me (possibly because my old a series mini engines were allways burning abit of oil)
Anyway, I do it by ear, starting from 2.5 turns out, one by one, slowly screwing in and out, listening. I really could do with an IR thermometer to point at the header pipes - must get one....

 

Well, kind of like my point, since we are only at idle, briefly, and mostly Mid to WOT, I've always set idle by ear....and pretty much sync too, and then done a few plug chops, mid throttle cruising , and WOT...since you aren't really WOT a lot either.......then after a few longish runs, you can certainly see your plug 'running range' on the ceramic......it's worked for me, for nearly 45 years!

 

I tried a (cheap) IR/thermometer, i found it hard to get accurate.....well consistent readings......, from the downpipes....

 

The colortune gets you right where you need to be for idle, and does that much quicker than doing a chop (I say this being someone who did plug chops for a couple of decades, and also used to synchronize carbs by ear). If the engine is stock, and the carbs are stock, and clean, and the needle jet isn't worn to an oval, and all of the other parts are in their proper condition, then the mix at all other speeds will be correct by default.

Chops are still a great tool for adjusting fueling on modified bikes.

 

what's a plug chop? I'm the self taught lone wrencher.

 

So the consensus is start at 2.5 turn out. Turn the mix screw in until the colortune turns white, then turn out until it turns blue,
then synch the carbs take the bike for a ride then colortune and synch again ?

 

The colortune is only used to get the initial setting for the pilot screws. From there you open the pilot screws —no more than the width of a dime— and ride, read and adjust as needed—never more than the width of a dime. I think I synced each time.

 

Running the machine in the rev range you want for the fuel delivery you want to check, then cutting the engine off (ideally) and pulling the plugs to read. How you read them depends on what tools you have, and what sort of engine you have. For 2-strokes the typical mehtod is to cut the threads off so you can see the smoke ring deep in the insulator. For 4 strokes you can just read the ground strap and the base ring of the threads (insulator color can be read too, but only on engines that have run for a good while). There was, at one time, a magnyfing flashlight that let you see down into the deep recesses of the insulator without cutting the plug. Dad had one, but it went missing many years ago

 

I use it to set the mixture. No more turning needed.
Bunsen blue my friend. Bunsen Blue.
A tad more rich won't hurt though.

 

Yesterday, I pulled the carbs and replaced the aftermarket jets ( aftermarket 122+43 for stock 120+40) with the stock jets for xj750.
With the carbs back in, it was harder to start and the engine quickly surged to 5k rpms. I tried to control the surge by backing of the enrichment
but the engine quit. Today I'll pull the carbs again and replace the aftermarket jets (122+43) . The mix screws were 2.5 turns out during the test.
What would the surge be from, too much air or too much fuel ? I've gotten to the point where I can pull the carbs, exchange parts and
get them back in less than 2 hours I'm retired anyhow, so I can take the time to experiment.

 

Too much air. Lean makes her race, rich makes her bog.

 

Are you running pods or modified exhaust? Sante Fe is pretty high up there (7200 ft), and normally would require a reduction in jet size.

Decrease main fuel jet size by 2 sizes per every 2000' above sea level.

Decrease pilot fuel jet size by 1 for every 6000' above sea level

Probably not enough to affect the colortune issue with the pilot, but might be overly rich once you are up and running if you are running stock.

 

It has an air box and standard pipes. By your reconning the pilot jets should be 39 and the mains 113 since I'm at 7200 '. I just tried the 120+40 stock standard jets
and the engine surged big time. Too lean ? Where can I get 113+39 jets

 

Len has a good supply of jets, and then there is also Jets R Us. I don't think I would worry too much about the pilots as a change of 1 wouldn't seem necessary when you have the adjustment with the pilot circuit screw. Maybe a change in the mains if all of your riding is done at the higher altitude and the plugs indicate overly rich.

I was just really curious as why you had gone up in size from stock given the altitude of Santa Fe.

 

did you try low speed screws with that setup? back off the idle knob? i've tried all kinds of jets and never found that racing problem.
maybe look for air leaks or throttle linkage jams

 

I know I'm late to the party here, but don't use the original Simple Green on aluminum. It can oxidize the carb bodies. There is a purple formulation (pro HD) that is safe for aluminum. Just my experience.

 

I ordered a complete rebuild kit off ebay it had #122 main jets and #43 pilots. the stock jets were #120 main and #40 pilot.
On a past try I put in the stock jets and I couldn't get the bike to idle off choke and I had to keep feathering the choke to keep it from surging.
I swapped jets with the #122 main and #43 pilot, and blocked off any screw holes etc. into the airbox also I sealed the boot/airbox joints with
strips of duct tape, no obvious leaks. Checked compression, #1-130, #s 2-4 are 125. this time I could get it to idle off choke at 1200 rpms by playing with
the idle speed knob. But when I flicked the throttle it surged up to 5000 rpms . the colortune showed more blue and the engine sounded the best at
2+ 3/4 turns out. By your reckoning Rooster I should have #113 main jets ( #120 stock minus 7 sizes for altitude ) I can get # 114 off Jets R us. Do you
think a # 40 pilot and # 114 main jet would help ? I can order the # 114 mains and swap back to # 40 pilots. Also I can caulk the boot/airbox joint
with a non hardening gasket cement, since the tape made a difference. ( gasket cement recommendations ? ) "81" xj750 seca.

 

Actually, Jets R us offers a choice of either #112 or #116 mains Should I go with the #116 ?

 

Well I lived in Alamogordo for 5 years (4300 ft) and drove up to Cloudcroft (8600 ft) a couple of times and never had any drivability issues that I remember. That was with the stock jets 120 main and 40 pilot. Carbs had never been touched at that point and were still at their factory settings. I have to admit I never even looked at the plugs back then since I did not have any running issues.

So, I think I would try going back to stock where it should run OK, and then perhaps evaluate the plugs once you have it sorted.

Maybe others can weigh in here on needed altitude adjustments, and also on the carb to air box boots. My guess is a slight air leak at the carb to airbox would not cause the poor idle conditions you described.

 

There really should be no need for a rejet on a stock bike (even with an aftermarket exhaust) to account for altitude. There will be a small loss of power at high altitudes (above 6,500 feet if memory serves) due to increased richness, but no other drivability issues.
I didn't have an XJ at the time (the bike did have CV carbs), but when I lived in Oregon I'd regularly ride from sea level up to Timberline Lodge (6,000 ft asl) without any troubles.

 

Yes, the bike was set up with pods and the stock jets by the PO.The main jet #120 and the pilot jet #40. The frame was bent and the forks were mangled but it started
up with choke. I didn't start it up again until I rebuilt the carbs, mounted the engine in a good frame and added rebuilt forks. At that point The jets were still #120 and #40
I replaced the dried up shaft seals with the fattest "o" rigs I could squeeze in without binding the shafts. It started up with max choke. I had to feather the choke to keep it from
stalling or surging but I could never close the choke off. Later as an experiment I switched the jets with the ones in the rebuild kit. (#122+#43 ) This time I could get it to idle off choke
at about 1200 rpm . All the pipes got hot but it surged to 5000 rpm when I flicked the throttle. I was thinking of ordering new stock jets. Even though the old jets had been cleaned
well and the holes were open and clear, something may have been less than perfect. Since I do most of my riding over 5000' maybe I should go with #118+ #40.

 

What size o-rings did you use?
There really needs to be a double seal for the throttle shafts to not leak, so the usual alternative to the correct throttle shaft seals is to fit two #11 o-rings in each position.
If the o-rings you installed were distorted too much, that would cause a vacuum leak that might only show up with the throttle opened.

 

All this stuff is the very reason peple on here advise not to fit pods.
My advice is to find a std airbox, find some std jets and then if it still doesn't behave get some chacal shaft seals fitted.
Then if it doesn't behave get the carbs serviced by someone who knows.
Fiddling about with jets is for the experienced, and as kmoe has saidmore than once, you should even then only start from a well running bike. Otherwise good luck, I'll be suprised if you get it anywhere near running right, and this is from someone who has no issues with pods per-se, but appreciates how difficult it is to get them working well.

 

He already fixed that issue.

 

Forgive me, lost the thread over time. Std jets then, chacals shaft seals, and then it should be good?

 

I think so.

 

Okay I was sceptical of the colortune plug, I'm old school after all and have to admit it is the coolest tool ...my bike starts without the choke now ,plugs look great , bike was running lean before and hot . I always thought I had carbs dialed in good , well now I do

 

Last week I pulled the carbs again ( I'm getting really fast at this 45 minutes) and switched the jets back to stock. I found that 2 of the throttle plates
were installed backwards and the "o" rings I substituted for regular throttle shaft seals were too fat and were preventing the springs from closing
the throttle plates properly. Sometimes, I could get the bike to idle smoothly and at other times it would surge up to 5000 rpms. I flipped those throttle
plates around and substituted better "0" rings as throttle shaft seals. Monday I'll reinstall those carbs and hope performance improves.

 

I found that the color tune showed blue from about 1 turn out until the screw became loose at over 3 1/2 turns out.
I did notice that the blue flashing was smoother at a certain point between 2 1/2 to 3 turns out and the white pops stopped.
Otherwise the blue color was easy to see but the cylinder could be still be rough.

 

Where can a guy get #11 "o" rings. Autozone, Pepboys and O'reilly's only sell assortment boxes and none of the rings can be doubled up without
jamming the throttle shafts. I went to the Napa store and they acted like I was crazy. If there is another source like maybe a plumbing supply
store I could try that also. Do you know in mm what are the i.d., the o.d. and diameter of #11 rings are ? Or, do they have another size designation ?
I found a ring that fits both tight against the wall of the seal recess and around the shaft but there is about a 1/32" space to the plastic washer. also,
I was looking for silicone grease and they all offered me was dielectric grease. Is this the same stuff ?

 

#11 is by the U.S. sizing standard. Most automotive applications use metric sizing now (and almost all auto store employess don't know about anything that isn't listed in the compter inventory. Sad times that we live in).
This chart has the info you need.
https://www.marcorubber.com/o-ring-size-chart-as568.htm

You can buy #11 orings online all day long. My local ACE hardware store stocks them.

Dielectric grease is a type silicone grease, and is suitable for your purpose.

 

you'll get a few spares to share with friends and neighbors
https://www.mcmaster.com/#o-rings/=1dimfv5

 

I have gone over these xj750 seca carbs several times in minute detail. I've given more attention to this set than the 4
other sets I did for 650 maxims, and the maxim carbs run great. I'm obviously doing something wrong here that I haven't considered.
Two Questions. When I swapped pods for an airbox, I used a box from an xj650 seca. It had a different style cover and air inlet than
an xj750 RJ cover. Could this be a problem of too much or not enough air ? Also, I cleaned and saved the air jets existing ( #225+#80),
are those the correct size ? The new fuel jets are ; main-#120 and pilot-#40.

 

Yes.

No.

 

It could be that there is something wrong with the carb bodies. Have you carefully inspected them for cracks or defects?

When the air jets were reinstalled were they put in the correct locations, and not what the manuals show?

 

Thanks Rooster. I tried the test and it worked well. I was able to blow out the pilot circuits of all 4 carbs. I've been busy since your post
and it took me a month to try it out myself but I will use it from now on. One more Question though, Is there a similar test for the enrichment
circuit ? It would be great to be able to back flush somehow, by blocking off holes, to make sure the tube that sticks down into the float bowl
is clear.

 

This will help you see where the circuit runs.
Inside your Carbs

 

I used a similar method as with the pilot circuit using some small gas line (think weedeater) and attached it to the straw on the carb cleaner and to the brass tube. You can then block the small hole in the brass tube near the base of the carburetor and blast away (watch those eyes). Carb cleaner should exit in the choke plunger hole and under the diaphragm where the air inlet hole for the choke is located.

 

It took

OK. So, I examined a junk Hitachi carb and I see an air hole in the air chamber just over the pilot air jet. It connects directly to the enrichment plunger hole.
That connecting tube is about an inch long. A half inch along that connecting air passage the enrichment fuel jet tube intersects it from below. If the plunger
is down, and I squirt carb cleaner into that air inlet above the air pilot jet, wouldn't the carb cleaner then squirt out of the brass enrichment jet ?

 

Yes it should, but it will most likely just dribble out of there as it might be difficult to build pressure through the air intake hole.

 

To clean the enrichment jet you really need to take the float bowl off. Anything that gets blown out if it will just end up getting back in there eventually unless the bowl is removed and cleaned out.

 

here's how to modify a red or black tube to clean out those enrichment jets
https://drive.google.com/open?id=1JXXdJ4gZg8_KOzr5N2mEOTNkEltPoqms

 

Hooray ! I finally got the carbs set right and the engine runs smoothly. I pulled the carbs I had been working with and ordered another set from ebay
I cleaned up this new set and used the original parts instead of the Chinese parts off eBay. I did replace the float needles and seats though. I followed the youtube
video instructions for blowing out the pilot circuits, posted by Rooster 53 with great success. and back flushed the enrichment circuits similarly. Polished the vacuum cylinders,
cleaned and blasted all the jets w/ compressed air, bench synched, wet set and installed them. The bike started right up but high revved, I opened the mix scews to 2.75 turns.
Now it runs the way it should, idleing at 1.5k rpms and when goosed, it Quickly returned to that 1.5k idle. Thank you all for the great tips ; K-Moe, Rooster 53, Chacal, Jetfixer,
Pollock, Tabaka 45, and Minimutly. Next I'll do a running synch.