1973 TX650 Carburettor Woes

Ilbikes · New Member Joined: Oct 25, 2006 Posts: 6

Hello fellas.

I could really use some advice. I've got a 5,000 mile original TX650 that I'm trying to sort out. I've completely been through the carbs, soaked them, blown through every orfice and passage. I replaced all of the jets with factory new ones, I replaced both floats, new float valves and seats, checked both needles for stock size, confirmed they were in the 4th slot from the top, and put in a NOS choke valve, gasket, and cap in the left carb, plus checked the cross-over tube between the two. I can turn the right-hand air/idle screw all of the way in and the idle speed runs it's highest. Something is not right when the air bleed screw can be shut off. I'm getting air in there somehow.

Also, the right cylinder is running 20-25 degrees hotter than the left, an indication of a lean mixture. I suspect the right cylinder is trying to do all of the work (with it's slide opened fully) and that the left carb is somehow losing slide vacume through the choke system. The 2 carbs are identical except for the enrichment circuit. The air for that enrichment circuit comes from the "atmospheric pressure" side of the diaphram. My slide diaphrams are perfect and not leaking, but I do see the slide dropping faster on the left carb than the right one does when you cover the atmospheric air inlet while the enrichener is closed and the slide pushed up by hand. This is what led me to the choke valve. Before I buy NOS carbs or close off that enrichner circuit, what am I missin here. I've had 3 650's in 30 years and none ran perfectly. There has always been something amiss with these vacume carbs.

I've worked on many bikes and own/operate several today. This is the first time I've been stumped in many years. Your help/input will certainly be appreciated.

Thanks,
Gordon

grizld1 · Posted: October 26, 2006, 9:14 am Post subject: Re: 1973 TX650 Carburettor Woes

Whenever I've dealt with a Mikuni BS-series carbie that delivered highest idle with the fuel screw closed or nearly so, the underlying grief has turned out to be fuel level. The new float may be defective, and you may need to confirm float level.

You didn't mention spraying for air leaks. Throttle shaft seals are a common source of grief. If they show bad, be careful removing the butterfly plates; those brass screws are staked and it usually takes a screwdriver with a really good fit and a very firm push-and-twist to break 'em loose.

Ilbikes · New Member Joined: Oct 25, 2006 Posts: 6

I replaced those seals when I reworked the carbs using those Mike sells. We sprayed ether on those at the same time we did the intake rubbers and carb/rubber joints - no increase in rpm so we think there are no leaks.

Another question - my left carb has it's enrichener circuit on it's right side. According to Mike's site, the 1973 TX carb did not use a gasket on this assembly. Mine has a gasket and that gasket would be much different than the ones he sell for the 74TX. I carefully removed it, cleaned out all of the passages (both carb and enrichener), installed a new piston, seal, and cap. The connector tube is not cracked and is tight fitting. I don't know where to go on this. I love this bike and it's one of a collection I'll never sell. It also happens to be one of my favorite rides.

We have to get this resolved. - Thanks again for the input and all advice is appreciated.

Regards,
Gordon

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

It's possible the seat for the adjuster or the needle taper may be damaged causing excessive lean mixture.Try swapping the air/fuel needle adjuster. What did you soak carburetors in?

Ilbikes · New Member Joined: Oct 25, 2006 Posts: 6

Guys, I am not sure if it's a unique circumstance or if there is just not enough traffic here to get more inputs on the carburretor troubles. I do want to share this as it may help others with the same problem.

Anyway, I resolved the carb problem after months of trials and considerable expense to replace many un-needed parts. The carbs were taken down and cleaned a total of 5 times. I eventually replaced every component of the internals with NOS/OEM (so-called) and it still ran terrible.

How I "stumbled" onto the problem is a long story, but it turned out to be that I was using the wrong plunger (enrichner/choke plunger in left carb). The XS2 and TX650 require a plunger with a 3~4mm hole in it. This hole allows mixture to pass from left to right carbs when the choke/enrichner is turned "off". With a solid plunger, the left carb runs stupid-rich while the right is horribly-lean because this designed hole isn't there. I'm talking 20 degrees difference in running temps between the left and right cylinders after a 20 minute ride!!

The plunger Mikes sells and the one in the bike when I bought it are solid (I suspect someone else fell victim to aftermarket claims of OEM). I did order one from my Yamaha dealer early on and it was NLA, so I had MikesXS send me theirs. Well, since it looked like what was in there - I never thought more about it.

The lesson here is that aftermarket stuff is sometimes/often unacceptable. What some refer to as OEM is not the same, buyer beware. In my case, I'm glad Speed and Sport had that one, last remaining TX plunger in their inventory (guys there are no more, but a drill bit can make miracles). At $27 it was still a bargain since today the bike runs like a dream since installing the little "piece" of brass having a 3mm hole in the left carb.

While I'm venting about aftermarket claims - that crappy 6/7 plate clutch substitute Mikes sells is coming out. It accounts for the worst clutch system I've ever used on a bike. The original NOS 5/6/rubber cushion system is going in this week. The pitch lever-movement is not sufficient on the older bikes to seperate the newer 6/7 plates set-up properly, so there is drag, clunky shifting, and just unacceptable clutch performance. Again, Aftermarket "snake bites".

Ok, I'm better now. Thanks for listening.

Regards,
Gordon

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