Hesitation at cruise

Hi all, 82 seca 750 work in progress. Carbs rebuilt with new jets, seals and orings. Synced and colortuned multiple times as I’ve had the carbs off several times getting float heights just right. Wet setting fuel levels is awfully fun lol think you’ve got it and then you check it one last time and find it’s barely out on one. Still need to do valve clearances but that’s in the works once I get something overhead. So it idles just fine and revs up and sounds good standing still. To cruise down the highway it needs a little bit of choke applied to run smooth without missing, and then it wants to die at idle because the choke is on but it’ll idle just fine with the choke is off. I know everything points to being lean through the pilot circuit but I don’t understand what I could be missing with the carbs. Vacuum caps replaced with silicone ones and boots are pliable still with very minor surface cracks. Paper element filter in the stock air box. I’m kinda at a loss and thinking park it for a while until I’m ready to do valves and revisit it once my patience returns lol what am I missing?

 

What do your spark plugs look like?
At idle up to about 1/4 throttle the pilot system is the primary source of fuel, after which the mail jet is the primary source. However, except at idle, both the pilot jet and main jet are working together. So even at wide open throttle the pilot system is still supplying fuel and that's why the pilot screws must be set correctly or the bike will run either lean or rich depending on the pilot system setting. So it would seem to me that your pilot screw settings are very lean or you have a problem with the main jets, emulsion tubes or slides--or any combination of those.
When I colortune I like to start from a lean position (white flashes of light) and open the screws just until the white flashes disappear. Then I install new spark plugs (easier to read) and ride the bike about 20 miles and take a look at the plugs. If they are white I open the pilot screws about the width of a dime and ride again and again read the plugs. I continue this process until all the plugs have a nice light gray or brown color. On my XJ700 the pilots screws are open about 2 turns, but each varies a little.

 

I do keep pulling and cleaning them when I do the color tune so I’ll follow your procedure and find out. Thanks!

 

there's a thing called the "clunk test" check that out. make sure the diaphragms are seated in their groove.
next time your out, switch the petcock to reserve, see what that does. the air and fuel jets are in the right place?

 

Slides move very well every time I've checked them. Yesterday afternoon I was considering if the diaphragms could be out of allowable tolerance for stretch or something leading to an un-synced condition even after being right on at idle as I was checking vacuum at 2.5-3k and it wasn't far out (<4cm of Hg) but not near as close as sync was at idle. Situation persists during reserve or standard position on the petcock. Air and fuel jets I've checked every time I've had the carbs out but I'll at least check the air jets under the hat and diaphragm since I can do those with the carbs still on the bike. I'm also going to try tabaka45's method of color tuning as I've only tried the rich to set method, not the lean to set and plug reading to fine tune. I appreciate the help fellas, I'm normally able to make it through things with research, mild trial and error, and a good dose of patience but this thing takes what should be a good result for the effort and flips it back at me in the form of a new problem and its been getting irritating. Enjoying the chance to learn everything I can though.

 

has this ever run right? as in could a PO have changed carbs with a 650 and not changed the needle jet?
i'm reaching now
exhaust system stock?

 

Pardon my inexperience and inquisitive nature, do you just ride normally or do you attempt to focus on a particular trouble area in the power curve, like a pseudo plug-chop?

 

Purchased as a project from a kid and it would just start up. Needed the wiring fixed and carb to airbox boots installed. I went ahead and put new fuel jets and throttle shaft seals and fuel rails o-rings and mixture screws, float needles and seats, and I believe I switched emulsifier tubes as well plus Len's hardware kit. Needles were in the carb rebuild kits I purchased and as the old ones did not show wear( as in profile) I did not switch those. I will double check the parts I have and see about switching the needles. Exhaust system is as stock as I can figure, if anything PO might have drilled the circular pattern of holes larger. Not sure as I haven't been able to compare with a known good example but if he did, its not obviously hacked.

 

Are the right jets fitted in the right places? And are the right emulsion tubes fitted?

 

you should have done this first or your wasting your time with the sync and color tune

 

I understand the necessity of doing valve clearances in order for a nice healthy engine, but like I said my location is not good for that kind of longer term, involved process. I’m working on improving that and fully intend to do valves soon. I simply want the vacuum between the carbs to be equalized and for it to run smoothly, albeit with the chances of less than optimal valve timing.

 

To the best of my knowledge the jets and everything is correct.

 

As XJ550H mentioned, and you said in your first post, you really should start with the valve clearances. Assuming these are out of spec, when you balance the carbs to the lowest performing cylinder, the whole engine will be underperforming, and the sync will change once you bring them back into spec. The valve clearances directly affect engine vacuum, and thus how much air/fuel mixture enters the engine. It's impossible to PROPERLY sync the carbs if the valve clearances are all over the board.

Tool wise, all you need is a basic socket set, allen wrenches, a small screwdriver for popping the shims loose, a small pair of needle-nose pliers, a set of feeler gauges (metric is preferred but I did my first few checks/adjustments with SAE), and the valve shim tool from Len ($18.95 plus shipping). Once you figure out what shims you need, it's pretty straightforward to swap them out. You can do it outside if it's not raining, just rest the valve cover on the head to keep out debris while you're waiting for shims.

Worth mentioning, too tight of clearances risks burning an exhaust valve due to a lack of seat time - the valves shed heat by being in contact with the valve seats. If they spend too much time open, they get too hot. This *probably* isn't an issue unless you have almost no clearance on an exhaust valve, but it can cost you a lot more in the long run.

In my opinion, you're wasting your efforts by trying to tune with unknown clearances. Also - you can CHECK the clearances to see if they are all in spec without the special tool - but you will need the tool if even one is out of spec, because you have to use it to check the currently installed shim in order to know what size you need to order.

I will also mention (from my own recent experience) don't only check the correctness of the carburetor parts (like the number stamped into the jets), but also carefully inspect them to be 100% certain they are not damaged, enlarged, or in any condition except "pristine"... or you will be chasing carb tuning when in reality the issue is a damaged or worn part.

 

Thank all of you for the patience and I think I’ll get started on the valves sooner rather than later

 

check your needle jet needle for the Y # make sure it's for a 750

 

So just an update because I’m going to be away from the bike this weekend but I’ll be constantly thinking about it. Did my absolute best color tuning and syncing and it was perfect if I dare say so at idle. Suited up and went into my college here for the big arena parking lot and did several plug chops and found every plug was a nice tan golden color except for number 4. Consistently clean and bare. So at idle it’s fine but something in the carb is not well on number 4. Honestly the best possible carb to have an issue on as it’s the only carb I can access both hat and bowl on while they’re mounted on the bike. Planning on checking valves next week as well. Metric feeler gauges and all.

 

when you do the shims all that changes.
carb sync is to adjust the cylinders output so they do not fight each other.

I know chacal has an explanation of why a sync is done. posted someplace

read this
https://www.xjbikes.com/forums/threads/talkin-tech-various-thoughts-on-various-issues.14608/

 

Agreed. I’m not concerned about Having to sync and tune again, It’s the carburetor not behaving as it should that I got hung up on. But now I have a specific carb to focus on as well as doing shims so I feel better about the problem identifying.

 

Just wanted to post an positive update and thank you for mentioning the needles. It’s the only thing I guess I didn’t check when I did the carbs beyond a visual inspection. I pulled the hat and bowl on #4 and made sure the pilot circuit was clear and then I got interested in the diaphragm and slide. Looked at the needle and removed it and it was almost out of the plastic bushing. So I went ahead and replaced all the needles with ones from the rebuild kit I had and resynced and color tuned and while I plan on keeping an eye on the plugs, it’s perfect now. Runs great and I plan on checking valve in the next couple of days as well. Thanks everyone and happy miles

 

When you have it together and running try spraying the intake manifold boots ( engine side of carbs) with carb cleaner. I looked at my boots and saw some minor cracks,but when I sprayed the Boots when running the engine smoothed out some. When you apply heat and vacuum those cracks may leak and lean the mixture.

 

Someone on this forum advised me to check my valve clearances and I did and reshimmed them and it was like I had a different motor. Tight clearances open the valves too soon, wide too late. It is the vacuum from the cylinders carb balancing is about. Better checking valves first.