Questions and Advice on Carbs: Jet sizes, Air leak and Cleaning on a XJ750J

This is an 82 Maxim XJ750J.

The bike has plenty of smooth power and idles pretty smooth if I hold a little pressure on the throttle or else it dies. (This has taken time to get worse.) I have massive air leaks which I have determined by;

1. Spraying carb cleaner on the boots to the engine and get a rise in RPM's.
2. I put my hand over the airbox opening (while running), and absolutely nothing happened when I blocked the air flow off.
3. The boots are 40 years old and cracked.
4. Only get half way on choke, then no change (when running cold), which makes cold starting very difficult. (Probably not good for battery life either.)
5. All vacuum hoses look fine including sync caps (the rubber is pretty hard though).
6. Air filter doesn't get very dirty anymore, at least the last one.

That was enough for me to pull carbs to replace boots, clean and rebuild.

Note:
Using a heat gun/hair dryer on the boots from airbox made them very easy to maneuver out of the way. Carb bank came out easily. (Rubber on engine boots were actually overly flexible. The end against the engine were pretty brittle and have multiple cracks inside)

Also, I sprayed penetrating oil on the allen bolts securing the boots to the engine for a couple of days, then heated the area very carefully/slowly with a propane torch and used a snug fit allen wrench, putting even preload finger pressure on the wrench and tapping gently with a hammer for removal and they all came out fine. ( I had read of guys snapping these and having to drill and tap them out.)
I'm in the process of reading around this site and have found a lot of good info.

Brief History:
I've had the bike for 25 years and don't think the first owner did anything other than usual maintenance(?) and put gas in it. He only sold it cause he got drunk one night, rolled it out in the driveway to ride it, and fell over. His wife made him sell it. (One can only imagine the threats she used or just plain screaming) I came along at just the right time to buy it from the buddy he sold it to who only had it a very short time. Bike is in visually great condition. Ok back to the carbs...
I got new boots (OEM grade). So far I have pulled the carbs off and started the teardown.
Pulled off each float bowl and everything was remarkably clean. (I did turn them upside down when spraying cleaner on the outside and dumped all the fuel out so maybe not so surprising.)
The mains are 120 and pilots are 40. The kits I got have 110 mains. Not sure if this is a big issue? (I'm at about 7300' altitude and plugs have always looked fine so I think the mixture is ok and power is good.)
The jets look great, no scoring or dirt and the seats look good too and float needles. I was surprised. Should I change them anyway or just swab with Q-tip?

When I went to remove them they were barely fingertight and one float needle seat was actually loose. There was some granular particles in back of the screen up in the chamber. Not sure if I should spray carb cleaner in from the other end (fuel inlet hose) or hold them upright and spray the area directly?

I have not taken off the tops to look at the diaphrams etc yet.
Any advice at this point is appreciated. Since the main jet sizes are not matching up I thought about ordering some additional kits from Len.
I did drop one seat on the floor and bent the screen a bit but no tears. The kits I have don't have screens so if I needed to replace one not sure if Len's kits would have them or if there's a replacement part.

Again TIA for any help.

 

First thing I would have done was just turn the main idle speed screw in a little more, but since you have the rack out and apart, it’s time to do a total Teardown, clean, rebuild….AND check shim clearances. New boots, new seals, synced, in-spec shims, etc…….will work wonders

 

Thanks for the info. I did screw in the idle adjustment a while back but it left the idle hanging way up.
I'm not a wizard...maybe wizard apprentice in training, so I'll have to ask what seals?
Also wondering if 110 mains are a big drop from 120.
I still have three diaphrams to inspect and pull the cold start plunger assy's.
Everything seems to be abnormally clean and look good except for the one loose needle seat.

 

Throttle-shaft seals (and also all other gaskets and o-rings…….and fuel valve rubber tips—which is why I prefer the metal tips)

idle speed— you may have turned it in too far too fast. You have to turn it just a tweak, lip the throttle and let it settle down, then turn a tweak, blip….rinse, repeat, etc….. But it could also be other things……. If the sync is off, you can get a run-away idle;if there are vacuum leaks, you can, too.

So if you have the carbs off, do EVERYTHING. Otherwise you start chasing yourself around the tree until you DO do everything
dfox

 

I did spend some time micro-adjusting the main idle screw under various conditions. It was high idle or die out, regardless, and it never was quite right telling me something else was wrong. My theory is that I will do the carb rebuild, as much as seems appropriate for the circumstances to jusitify the effort (namely the huge air leaks from the boots - plus tearing down almost everything in the carbs), to see where I'm at, and after that and hopefully/maybe, not having to separate each carb unit to change out the throttle shaft seals. I don't mind isolating the actual fault, even if it takes some effort and time to do so, considering the weather right now, and no, I don't really have that much time to waste but I do want to see how fixing one particular major issue may actually cure the problem without 'fixing everything' that may have contributed to the problem and then never knowing what the actual problem was. (Yeah, I get it, just do a total rebuild and it will be fixed.) But, then I will know how bad the air leaks were and how that specifically affected the performance. (Just switch out the boots and see how it works approach) You could call it the process of elimination, or something else (go ahead, I won't be insulted), but I consider it a 'test case' or 'the cost of not doing proper maintenance',...nolo contendere there.

Considering the abnormal cleanliness of the carb system of 40 years with no maintenance (which looks like most of the 'after' pics of what I've seen), I have gone as far as I think is worthwhile to isolate the issue, if not for functionality, but for others to save time that have rotting boots and don't need to rebuild their carbs needlessly. This incremental approach may seem less efficient than the all-or-nothing treatment, but there is some value in determining the specific causitivie contributing faults, or seemingly symptomatic indications of a rich or lean mixture or the travel of the main throttle piston, or any hang-ups in the system due to varnish or scoring. My theory is that I will do the rebuild, as much as seems appropriate for the result to jusitify the effort (namely the huge air leaks from the boots), to see where I'm at, after that. I don't mind isolating the actual fault, even if it takes some effort and time to do so, considering the weather right now, and no, I don't really have that much time to waste but I do want to see how fixing one particular major issue, may actually cure the problem without 'fixing everything' that may have contributed to the problem and then never knowing what the actual pronlem was. Then I will know how bad the air leaks were and how that specifiaclly affected the performance or masked it. Hmmmn...

Considering the abnormal cleanliness of the carb system (which looks like most of the 'after' pics of what I've seen), I have gone as far as I think is worthwhile to isolate the issue, if not for functionality, but for others to save time. This incremental approach may seem less than the all-or-nothing treatment, but there is some value in determining the specific causitivie contributing faults.

I still need to figure out how to rebuild my left fork seal (I'm only three bolts away from pulling the fork), which has caused multiple issues with with weak front braking not to mention soaking my pads, brakes lines dated from 12/81 (yeah, shoot at me later but help me now), and having to clean everthing out in proximity to it in excrutiating detail. Any refs to info on how to do this would be appreciated. If you can save me the time it took to write the above it is appreciated.
Again, after all this discussion, are the 110 mains a big drop from the 120's??

Also, many thanks to Dwayne Verhey of xjcd.org and his effort to post a detailed breakdown of the carb rebuild/cleaning instructions and pics for the XJ750J Hitachi carbs.