DIY carb synch tool problem.

Hi Guys, a little help please.

I have a 1981 XJ750A and have been sorting it out over the last few months.
Valves are in spec, carbs cleaned and float levels correct, bench synched, and bike was running well. Better than when I bought it.

I decided to build a DIY carb synch tool to fine tune the carbs. The problem is I used ATF fluid for the ease of reading, but No 3 cylinder draws the fluid up th e line before I get a chance to adjust the other cylinders to suit.

I have small tap like restrictors at the top of the tubes to slow down the action, but surely these would have to be adjusted to exactly the same point on all lines or I wont get a correct reading? Should I use a heavier grade of oil? See image of tool below.

http://xjbikes.com/coppermine/displayim ... ize=1.html

 

Sounds like your sync tool is trying to tell you something.

The "restriction" should match, yes; you might want to revisit the bench sync or it may be that a slight re-think on the tool is needed. I looked at the pic in your gallery, it might need to be "taller" yet, and I can't figure out exactly what you've got going at the bottom there.

 

Thanks for your reply BigFitz.

The tool is over three feet tall, so that shouldn't be a problem. I will re bench synch this evening.

The bottom of the tool is just two "T" pieces linking the pipes? Should there be something else? What have I missed?

 

Two "T" pieces linking the pipes? So you have two "loops?"

So in it's present configuration, it's "comparing" 1/2 and 3/4?

 

The T pieces link all four pipes. Any fluid rise or fall will affect the other three pipes.

Is this not correct. I thought I had looked at enough DIY tools on the net to make it correctly. My understanding is that all four pipes should be inter connected to correctly synch the four carbs.

 

Your apparatus is plumbed properly. Differences in the restrictors don't matter. Your problem is it's not tall enough for the working fluid.

Here is a little thought experiment for you (British Units):

Many manuals specify sync within 2" Hg which is roughly a 2 foot water column. If your apparatus used water it would be just barely tall enough to finalize a set of carbs that were already pretty well synced.

ATF is less dense than water so it'll be even more prone to being too short.....

The restrictors don't compensate for the height problem, they are to damp out pulsing of the columns. A heavier oil is more viscous, not necessarily more dense, so it won't solve your problem.

 

Not easy to tell from the picture in your gallery, but it seems there is a connection between yours T links, so lines 2 and 3 are connected to each other, right? If so your sync tool should be OK.

I suspect your bench sync was a bit too much approximative. Use narrow strips (1/8 inch) of a business card to make sure that the butterflies are closed evenly on all four carbs.

ATF should be heavy enough, that's what is recommended with this kind of tool.

 

If you use a two bottle sync tool that sort of thing can't happen

 

Thanks for the tips everyone.

I'll re do the bench synch tonight and post results tomorrow.

Long night ahead!

 

I'd say your tool is correct, but you're bench sync is WAY off if it's pulling like that on #3

I had an almost exact setup, last time i built one. If i had the issue you do, basically you give her a couple cranks, then kill switch it, to see which is rising too fast, adjust, repeat, until you can get it to a point where you can run the engine and none will suck fluid too quickly.

 

On mine, lines are sbout 6 feet long each, with about 2 feet of oil each, but I haven't had the chance to test it yet.

 

Keep in mind that if each "set" of tubes are only connected to each other (so that there are two "loops") each loop is only going to "compare" the two carbs it's hooked up to.

When doing the vac sync, you need to compare (and sync) #1 to #2; then #2 to #3 (#2 brings #1 along with it) then #4 to #3.

So even though you have 4 tubes, because they are in two loops, you'll need to swap them around a bit to get all three comparative synchronizations accomplished.

In your current configuration, #3 is only comparing to/fighting with #4; and #1 is only being balanced against #2. At some point you'll need to move one of the "loops" to compare #2 to #3.

 

bigfitz

there is a very short piece of tube between the two Ts at the bottom, so the way i understand the set-up, there aren't two loops but four lines that are connected to each other.

 

Well, then--- nevermind. It looked like two discreet loops.

Except for it to work, wouldn't the tubes all have to be the same exact length and have exactly the same amount of fluid in them? From what I can tell, the outer two tubes are slightly longer.

I'm "conjecturing" since I have a Morgan CarbTune, and was using a borrowed set of old-school mercury "stix" prior to that.

 

I bought one of these, mostly because I am cheap. it works very well and there is no fluid to spill or get sucked up into the engine.

http://www.ebay.com/itm/UNIVERSAL-MOTOR ... 4d18dbefaa

CN

 

Nope. Since it's all interconnected the fluid will be self leveling. Length of tubing above the fluid is irrelevant. The only problem with his device is it's not tall enough unless the sync is very good when he begins.

What he's built I call a comparative manometer - it compares the cylinders to each other. So it will tell you if they are balanced, but not what the absolute vacuum is. Since it uses a light fluid it should give very precise results.

Dial gauges compare to the atmosphere, so you also know the absolute vacuum. This is helpful because a well running, synced engine pulls about 10" mercury. Drop one cylinder for some reason and the vacuum falls to about 6". So you get information not just on is it synced, but is it running well.

I've never used the CarbTune or Mercury sticks. I'm pretty sure they show vacuum relative to atmosphere or to a fixed air pocket built in. If so, they would also indicate how well the engine is running.

BTW, I used a MotionPro SyncPro in the shop for awhile. It's got a similar problem to his manometer. The first time I put on a bike that wasn't real close it sucked the fluid out. After having that happen a couple times I junked it.

 

Cool. I was referring to the length of the various tubes NOT above the fluid, but the lengths with the fluid in them. More fluid=harder to move, no?

The Morgan is vacuum relative to atmosphere. And with three XJs in the stable it was worth the investment.

 

Bigfitz

This is an open circuit, if there is more fluid in one line, that will be a very temporary situation because all four of them will equalize themselves in a few minutes.

 

Sorry guys, I only got the carbs removed last night.

Visitors! I'm off work tomorrow Friday so will re synch on the bench and let you all know how it goes.

 

Me too, so simple to use

 

I'm not quite sure, but I think I saw a post here several months ago about a sync tool using only a long loop of clear tube with a small steel ball in it, you plug each end of the loop to one carb, if the ball stays exactly at the bottom of the loop, the carbs are sync'ed.

I just can't remember the diameter nor the length of the tube, neither the size of the ball...

 

Agreed.. I had a similar problem with my when I first started out. I built the 2-carb version out of a yard stick. Even with a real good bench synch one of the carbs (I think it was #3) was pulling hard enough that it damn near sucked some fluid before I could shut it down.

I got a long section of smaller diameter tubing, cut 2 equal lengths and shoved them in the ends of the larger tubing. Then I just hung the manometer from a hook on the ceiling and ran the tubes down so the whole thing was about 8 feet tall but it gave me plenty of time to back that carb off and level it out. Worked like a charm.

 

Well Thanks everyone. I feel bad. I have given up.

I took on board all of the suggestions above. Firstly, I re bench synched the carbs using the card method. Bike ran well but tick over was a bit rougher than I liked. So I extended the balancer tubes by the same length again and re tried to synch. Got 3 and 4 synched ok. Then 2 and 3, but as soon as I started to adjust no 1 the bike started to backfire and spit back through the carbs, then as I turned the adjustment to synch, the bike revved to 4k and would not come back down.

I changed the fluid to coloured water then, but the whole system filled with bubbles when I started the engine so there was no definite level to gauge by?

I stopped for tea.

Then I made a 2 bottle synch tool as suggested by pollock. Took carbs off a second time. Bench synched again. And started with the new 2 bottle tool.

Guess what, exactly the same thing happened. 3 and 4 first ok. Then 2 and three ok. then 1 messed it all up again.

I gave up. I switched off by rewiring a friends KZ1000P Police special and got it running for him. I decided not to offer to balance his carbs!

During the coming week I will re bench synch my own carbs and leave it at that for now!

I'm stumped!

 

Here is the right sequence to sync the carbs on XJs:

...**Now check the vacuum gauges and see how well balanced they are.
If they are not the same start with number 1 and 2 carb. Adjust the screw between the two carbs on the butterfly shaft and set it so the same reading is on each gauge for one and two carb.

**Then adjust the number 3 and 4 carb the same way

**Now adjust the two pairs of carbs with the screw between number 2 and 3
carb to give the same readings over all 4 gauges.

**You may need to redo the settings sequence to fine tune the readings...

 

Just an update.

I removed carbs yet again. I bench synched using a ten thou feeler blade. They were only slightly out. Then re checked the idle air screws, three turns out on each carb. Refitted. She runs like a dream again.

I have ordered a colortune plug kit, so I'll check that and try to re balance with the two bottle method soon and post results.

I think I'll just enjoy riding it for a couple of weeks first. It's St Patricks weekend so a long weekend, and weather looks good for all of it so far.

Happy days!