Carb Tolerance issue ?

Hi All

I'm hoping that someone can help with this issue, I have a 1980 XJ 650 4K0 (UK model ) in std trim and unmolested with a complete original exhaust system.

I have an idle issue ( wanders dependant on throttle shaft position ), caused I suspect by worn throttle shafts / carb bodies the carbs are Hitachi HSC 32's.

Is this a common issue ?

How much movement is acceptable ?

Is it normally the carb body that wears or the throttle shaft or both ?

Is there a fix for this issue ?

ie, are new throttle shafts available, is it possible to rebush the carb bodies ?

I have not yet removed the carbs for dismantling as I see no point in disassembling them if there is no remedy other than to replace the whole set, as the bike is running - just not the way I want it to be.

TIA

 

Throttle shaft seals are most likely. Very common issue, replacement seals are available. Read the writeups here.

Dave

 

Throttle shaft seals are most likely. Very common issue, replacement seals are available. Read the writeups here.

Dave

 

Hi Dave

I intend to replace the seals but, what I need to know is how much wear is acceptable in the throttle shaft / carb body as replacing the seals is only going to hide the issue temporarily if the wear is excessive allowing the throttle shaft to lift under throttle application causing the seal to pull away from the shaft creating an airleak as this is the current problem.

TIA

Nigel

 

If anything at all wears in that area, it would be the shaft BORE in the carb body rather than the hardened stainless steel shaft. We've never seen an indication of any meaningful wear in either of those areas (besides the seals, of course!) and due to the rather minimal radial loading on these items, it's doubtful that it would be an issue unless they were "million-mile" carbs......

 

Thanks Chacal

The reason I ask is that there is approx 0.005"-0.010" vertical lift on both 1 & 4 carbs at the outer end of the throttle shaft.

This feels quite a lot and given the size of the seal I was wondering if new seals would accommodate this amount of movement for long ?

Carbs have around 45,000 miles on them as far as I know.

I'll get to ordering the seals ASAP

Thanks

 

New, correct seals, carefully installed, will fix the issue.

 

Well I'm going to be the dissenting voice on a couple of things...

First, the throttle shafts on the carbs I've messed with are not hardened. They appear to be a magnetic grade of stainless, but they are definitely not hardened.

Second, I'm of the opinion that .005 to .010 (assuming those are thousandths of an inch) is too much slop in the throttle shafts and I'm curious as to how you measured that.

I haven't measured the play myself in the cycle carbs, but from other similar applications that would be considered excessive. If you're sure about that number, I'll measure a set I have here and see what I get?

 

You'd be comparing Mikunis with Hitachis.

While it's always possible the carb bodies are worn to the point where new seals won't resolve the issue, it's highly unlikely.

 

Hi rice_burnarr

Yes the measurement is in thousandths of an inch, I am more comfortable with that than tenths / hundredths of a millimetre, showing my age LOL.

I checked the movement by eye first of all, then having seen what I thought to be excessive movement I got out a dial gauge and mounted it to a fixing bolt on a adjustable arm and rechecked my original opinion by moving the shaft downwards as it appears to be lifted at the end possibly by the action of the throttle adjusting screw.

All measurements were carried out with the carbs in situ as they are still mounted on the bike.

Having no previous knowledge of these particular seals in this application is why I posed the question re the movement allowable in the shaft / housing without causing the seal to pull away from the shaft there fore creating an air leak.

Which is what I am attempting to stop at present, I can see that with the current OLD seals that this is happening and was concerned that even if the NEW seals would seal this distance when new that they wouldn't seal for long and in short space of time I would have a return of the original fault.

No point in wasting money if I need to replace them with a better set first money is just too tight to be throwing it away.

TIA

 

Wow.. Z I wish MY eyes are god enough to see " approximately .005-.010"

I can see 1/64" though, for measuring frets.....

Dave

 

Well, having been an engineer for over 40 years I am well used to being able to see and judge small increments.

Try looking at the thickness of a feeler gauge .... say the one you use to check your valve clearances ....... are you saying that you cant see the clearance between the base of the cam and the shim sat in the bucket ?

Maybe try putting some light on the subject from behind to help.

5 - 10 thou is well within the normal range for valve clearance settings on a lot of vehicles and is very easy to feel / see even a few thou of variation !!

BY the way 1/64" is 15.6 thousandths of an inch !!

 

OK, I'll take a look at the carbs I have here. I really think that .005 - .010 is excessive.

In a geometry like this, it's very easy to hold tolerances much tighter than that when new. The people who made these carbs are quite capable of doing much better than that even without really "trying".

Of course, I have no idea what the 45K miles have done to the tolerances involved. Maybe the holes in the carb bodies have worn oval? And if they're worn to that point, I'm thinking that you might be in the market for a set of carb bodies or maybe someone to bore and bush the original bodies.

Are you sure you weren't including flex in the rubber carb joints to the engine or somewhere else in the measuring system?

 

Hi Rice_burnarr

I am suspecting that the problem is in the wear pattern of the bores for the throttle shafts and I was very careful not to allow any flex in the carb mounting when taking these measurements.

The scenario you have put forward re boring the holes and rebushing is what I was looking at maybe having to do if the seal is unable to take up that magnitude of movement long term.

That really is a small seal and a lot of movement at 10 thou and I don't want to go down the road of fitting the seals only to find in a short period of time that the seals are leaking again and having to strip & rebuild the carbs again doing further expensive work and replacing seals again.

I want to do the job once and be done with it. This is not my first XJ, I had one from new back in 82' and as I remember I was seeing the beginings of this sort of issue on that bike then after only about 15-20,000 miles, just didn't realise what it was back then !!

 

I'm not sure how much slop the seal could even account for, but something else to think about is that if the end of the shaft is wobbling up and down with 10 thou tolerance, then so is the throttle plate inside... And there's no way I would want that to be happening.

So I'll take a look at what I have here today. Keep in mind that I've got Mikunis and you're running Hitachis, right?

 

Yes I have Hitach's but that movement in either system is I feel significant.

 

Me too. I wouldn't want to see that much movement in any carb shaft. Just wanted to let you know that I unfortunately cannot do a perfect "apples to apples" comparison for you.

 

Imo getting the carbs correct is the "hat trick" to making a xj a solid runner once every thing else "running" related is in order. Any air leaks will cause her not to idle correctly. Even if you source another rack the seals will need to be replaced. The seals cost a lot less than a rack. Hope this helps.

Gary H.

 

OK, I measured my Mikuni throttle shafts and holes.

Throttle shaft = .3139 dia
Bore in carb body = .3150 dia

So I get just about one thou clearance between the two and with that one thousandth clearance I get about .002 up/down play in the throttle shaft as measured on the little stub that sticks out the side of the carb where the "C" clip goes.

I also inserted a .3114 rod through the carb body and measured .006 up/down play with that rod. So it seems the geometry make it such that the radial play is about twice the clearance between the two diameters. IMHO, that .3114 rod felt quite sloppy to me and I would immediately flag it as a "problem". And you're saying that you've got more play than that??

To be honest, I respectfully still suspect a measurement method oversight. How about you move the indicator off to the side a tiny bit and put it against the carb body instead of the shaft and repeat the test. In theory, it should read 0.0 deflection, right? Does it?

If not, you need to rethink your measurement scheme.

If so, you need to rethink your carb bodies.

 

With all this said, I would break the rack, replace the throttle shaft seals, & fuel rail o-rings....to see if this resolves your problem......then you will know for sure......then you can think about replacing or reboring.....why not take the easier approach...replace the seals & o-rings....