Carb Tuning Woes

[senrak]

Hey Guys,
I know that there are a number of smart guys here with a lot of experience so once again I’m going to call on you.
I am trying to tune my 82 XJ. Have done a good cleaning, re-synced the carbs a couple of times, each time with varied success, mostly improvements.
Engine wants to bog when the throttle is opened. I stretched the springs above the diaphragm piston making them about an inch longer. That seemed to help lessen the bogginess. If I open the throttle slowly the RPM will rise to 5-6 grand easy. Close the throttle completely, and the RPM drops to 3,000 and stays there. The linkage is not sticking, it comes back too and seats on the adjustment screw completely. The RPM will not drop to idle unless I put a thumb and a finger over the idle air jets inside the intake throat on two carbs, any two, (Air cleaner is off at this point.) After that it drops back down to 1000 rpm, and will stay there until you again open the throttle. Once again it revs and will not drop below 3 grand.
I’ve been scratching my head on this one. If it was a vacuum leak, it would be there all the time, or maybe only when hot. No signs of a vacuum leak. I have used the propane torch trick. The only time the engine reacts is when I point it down the carb intakes.
Any help here would be greatly appreciated. Thanks, Dale

[MPittma100]

Carb Tuning

Sounds like one of the rubber diaphragms may be leaking to due a crack or pin hole. Check them very carefully if you have not done so. Stretching the springs will work adversely to your cause.

Mike

[motoman]

Well, unfortunately stretching the diaphram springs was first mistake, as they don't collapse over time. That would work the same as having pin-holes in diaphrams. Since it was a guesstimation as to how much, you may or may not get a couple of coils to give up enough to get them back to original 'resting' height, whatever that was. Someone here may have a set of the later carbs apart and give you that measurement.
I'll have mine off again in bout a week for final 'going thru' and can post that length if you don't have it by then.

[motoman]

BTW, 'a good cleanin' is a pretty broad statement. Were ALL the jets removed....emulsion tubes removed and cleaned throughly including toothpicing all the tiny sideholes of them AND the pilot jets?
If not, any 'running issue' thereafter will be an act in futility.

[mack]

Springs

I think once they are stretched they are garbage. Steve might be able to make a new set.

[BA80]

When you went through the carbs, did you get kits with new jets included?

If you did you may have inferior replacements in there.

Recheck those diaphragms for pinholes too. Hold them up to the light and check them. Especially all around the outer edges.

Did you remove the emulsion tubes and clean them really good and the wells they sit in?

Have you synced the carbs with the YICS tool installed?

[senrak]

Thanks for the response. I appreciate the help.
The slightly stretched springs was an idea that came from the tech advise on this site. The guy replaced 1100 springs with 650 springs which were shorter, and his performance fell off. His theory was that they could in fact lose tension over time. I would say that in my case stretching them by 1/2 inch actually improved performance. I don't believe for a minute that this slight change has made them junk. Proof is the increased performance.
The high idle problem was there before and after the spring mod and remains the major problem.
All jets and tubes were removed and cleaned. There was no kit, and no new jets. They are all stock and original. Sync was done with YICS tool installed. The diaphragms were inspected closely and I don't think they are leaking, but I will go back and double check them.

[senrak]

Playing with the XJ this afternoon and found that it doesn't hold that high idle speed until the engine reaches normal operating temperature. I would say vacuum leak when hot. But where ? Any ideas are welcome.

[BA80]

I was thinking about it this morning and The first thing I would check is where the boots meet the head.

[MPittma100]

Thanks for the response. I appreciate the help.
The slightly stretched springs was an idea that came from the tech advise on this site. The guy replaced 1100 springs with 650 springs which were shorter, and his performance fell off. His theory was that they could in fact lose tension over time. I would say that in my case stretching them by 1/2 inch actually improved performance. I don't believe for a minute that this slight change has made them junk. Proof is the increased performance.
The high idle problem was there before and after the spring mod and remains the major problem.
All jets and tubes were removed and cleaned. There was no kit, and no new jets. They are all stock and original. Sync was done with YICS tool installed. The diaphragms were inspected closely and I don't think they are leaking, but I will go back and double check them.

Check the tightness of the diaphragms at the center attachment point. If one is loose (turns easily), it will leak even if there are no pin holes.

Mike

[senrak]

I'm going to do that now. I have hesitated because the boots and gaskets are brand new. I didn't use any sealer when I assembled them (I dislike getting things all gooped up), but maybe they need some HiTack.

[natemoen]

Or it could just be a lean condition that occurs once you turn the choke off.

What are your idle screws set at?

[senrak]

Okay, Pulled the intake boots off. Only thing that accomplished was to ruin the gaskets. They were all sealed perfectly. Can I put the boots back on with hi temp (red) RTV, or do I need new gaskets?
Pulled all slides, the diaphragms are tight at the center mount with no pinholes or cracks. All four look in very good shape.
Were can I find throttle shaft seals. I haven't looked at those yet, but maybe the carb body heats up faster than the shaft and leaks in air.

[BA80]

Scrap all that old gasket material off really good. Those gaskets are a constant point of failure.

Put the boots back on with a thin coat of Yamabond or Threebond.

I'll almost bet those paper junk gaskets were leaking.

[BA80]

BTW, JAT (don't you just love our short hand) When you cleaned those carbs did you make sure those 3 little holes in the top of the venturi just behind the idle mixture orifice and the top of the butterfly were clean and clear?

That's where the fuel for midrange acceleration comes from.