Striped mixture pilot screw

I have an old bike that was sitting and I've been working to get it running, and I thought today was going to be the day. I did what I considered a first pass on the carbs which means I opened them all up, cleared the passages, cleaned the jets etc. But I did NOT pull the mixture screws because the manual says not to. After putting it back together, it ran decent, but not quite right. Some plugs were dark, and one of the pipes was only warm while the rest were hot.

So I figured I needed to clean out and reset the mixture screws. I took the carbs back off, recleaned everything I had done before, and drilled and pulled the brass plugs that cover the mixture screws. That part went surprisingly well. But when I was taking the actual screws out, I broke the head on the #2 carb. I was bummed out, but figured I better press on. I drilled a small hole in the screw, and tried to remove it with a screw extractor that I got from sears. But the screw extractor itself broke off in the screw. Now it is all still stuck in the carb, and the little piece of screw extractor is very hard to drill through, and the drill bit keeps slipping the the softer brass. I really don't want to mess up the threads on the carb body, so I've stopped for now.

Any thoughts or ideas would really be appreciated.

And another small question, what should my compression tester read for the 1985 XJ700? I got between 141psi and 148 psi for all the cylinders. I thought the consistency was good, but I don't know what they should actually be.

Thanks a lot,
-Josh

 

On the broken screw problem, you probably won't be able to drill out the screw remover, it is pretty hard steel, about the same as the drill bit. A better idea is to thermal cycle it. Heating it with a torch (or a heat gun, which is safer but not as hot) and then cooling it a bunch of times should help you work it free. And buzzing the heck out of it with a carbide-tipped vibrating marker has worked for me on very small (0-80) taps broken in aluminum.

Once the screw remover is out, try again BUT remember to use some Liquid Wrench. And heat is the ultimate friend you have here (but don't light the Liquid Wrench on fire).

On the compression test, ~145psi sounds like a good result. Since atmospheric pressure is about 14psi, a 10:1 compression ratio would yield about 140psi.

And I think you meant "stripped", not "striped".

 

Probably best to buy another set of carbs, as you might not be able to extract the screw. And will all so save a lot of time and hassle.
Al

 

The Carb Body AND Pilot Mixture Threaded Hole can be made as good as new.

Bring the Carb Body to a Machine Shop that has an E-D-M Machine and have the fragments broken and seized ... "Burned-out".

The Process is referred to as: Metal Disintegration Machining. The Tool Stock and the Mixture Screw will be disintegrated ... leaving just the Hole and the threads.

"Using graphite, moly, copper or aluminum, depending on the type of burn required, the machine becomes a high energy, low voltage, thermal shock producing source.
The electrode in the machine vibrates up and down 3,600 times a minute. Each time the electrode touches the piece to be burned, an arc is struck.

The arc has a point of contact temperature of approximately 5,300 degrees Fahrenheit. A constant supply of fresh water is pumped down through the electrode causing the molten metal to thermally break down, at the same time flushing the thermally shocked metal back out the discharge hole. The discharged pieces are normally less than 10 microns in size (less than a grain of sand).

The water also acts as a heat exchanger, i.e. the only portion to get hot is the small area that the tip of the electrode contacts, unlike drilling where heat from the bit transfers to the matted portion.

The MDM maintains everything at ambient temperature, except what needs to be removed." **

**Copyright ©2011 Orbital Tool Technologies Incorporated / 3DL

 

Thanks a lot for the encouraging words everyone. I will try jluschen's method tomorrow, and if I can't get it, I'll try to find a local machine shop to help out. If anyone else has other comments, I'd still appreciate hearing the ideas. I'll report back tomorrow after I try both (or maybe just one if I'm lucky) methods.

-Josh

 

yea go find a machine shop with a EDM machine, it only costs about 100$ a hour, including coffee breaks

 

Here is a follow-up question.

I found a good deal on a rack of carbs for a 1982 XJ650. I know that the jets will all be different, and I know that the carb body is mostly the same. So my question is: If I buy the rack from the XJ650, and thoroughly clean it out and use the jets from my XJ700, will that work? Or are the carb bodies themselves different?

Thanks,
-Josh

 

You would need to strip the the jets, the vac piston, the vac piston spring, the main jet needle, and the main jet (emulsion tube) out of the 700's, as they are all different....and then, you still wouldn't have XJ700 carbs, as they are HSC33 series carbs, whereas the XJ650 uses the slightly smaller (and narrower) HSC32 series carbs.

You need to fix that set of carbs (we have replacement used carb bodies available if that one can't be salvaged), or, find a used set of XJ700 carbs to use. Otherwise, you're setting yourself up for a lot of tuning/performance issues to have to deal with......

 

Good luck, Josh. This post is actually pointless, but I need some posts so I can move up from "Biker wannabe" to "Gear grinder".

Although how you can grind gears on a Yamaha XJ is beyond me...

 

I erred on my previous compression test comment.

Compression ratio is the ratio of the cylinder's biggest-to-smallest volume, and so a 10:1 compression ratio would raise the absolute pressure of the gas in the cylinder from 14.7psia to 147psia, where "psia" is absolute pressure in pounds per square inch.

The compression gauge can't measure absolute pressure, though, only the difference between your internal cylinder pressure and the outside air pressure. So if you made a perfect measurement of a perfect 10:1 cylinder, you would read 147psia - 14.7psai = 132.3psig (gauge pressure).

Since you were reading ~145 psig, you are getting about 11:1 compression.

Sounds even better than before!

Jim

 

Well, I just got back from the machine shop, and they weren't able (or maybe willing) to help me out. Basically they just said I was out of luck. So here is my next follow-up question. Does anyone know if the XJ700 and the XJ700X use the same carbs?

Thanks for any more help guys, I really appreciate it.
-Josh

 

No, the 700-X (water cooled) models use Mikuni carbs, while the XJ700 air-cooled engines use Hitachi carbs, and unless you want a nightmare on your hands, don't try to swap them!!

 

Can you post a picture of this screw situation? It must be something horrifying, to scare off a machine shop.

 

I've attached pictures of the screw. I'm not sure if the machine shop guys were really scared off, or if they were just not willing to take the time to help me out. I'm going to keep working on it though. Hopefully I van get this thing going again.

 

Try a different machine shop; or look for a matching set of HSC33's.

Doing it yourself will likely make it worse; swapping to a different set of (mismatched) carbs will be counterproductive.

OR, get a single, correct replacement carb body from chacal.

 

Nice pictures, thanks.

If you live in San Diego, come on over. I have a Bridgeport milling machine in the garage, and we can take a carbide end mill and go right through the broken screw remover.

Then some careful work with a new screw remover and a propane torch and it should come right out.

Jim

 

I actually lived in LA about a month ago, but now I'm in Ohio, so I'm not quite close enough. I appreciate the offer though. I'll keep working on that thing, hopefully I'll learn some new techniques along the way.

 

But that's only if you use an Ideal Gas (which doesn't exist).
Real air has molecules that can only be compressed so far, and the air heats up as it is compressed.

But what throws off guages the most is - the volume of the hose !!
Not so significant in a car engine.
Plus, the intake valve doesn't close at the very bottom. Welcome to the site. 8)

 

Josh - where're you at in Ohio. I'm in LaGrange 44050. I can help you.

 

Thanks for the welcome, TIMEtoRIDE

I hadn't thought about the valves closing partway into the compression cycle, that will certainly affect the measurement. I wonder how much...

But you are wrong about the hose volume -- that shouldn't contribute to any error. At least not if you measure after multiple rotations of the crankshaft. That's because a compression tester has a check valve at the end of the hose.

If you tried to measure the cylinder compression with only one compression cycle, the hose volume would completely screw up the measurement; I completely agree with you there. But if you turn the crank multiple times, air pressure is retained by the check valve in the hose. The effect is the same as if you multiplied the volume of the cylinder you're testing by 1/2 the number of crankshaft revolutions you are measuring over, thereby making the comparative volume of the hose insignificant.

Crank a 4-stroke engine through 20 rotations and the hose volume looks 1/10 as big (10 compression cycles at work). Keep cranking, and the measurement asymptotically approaches the result you'd get if the hose volume were zero. And your battery goes dead.

(This analysis assumes an ideal check valve that opens with the slightest positive pressure differential, which is never true, but now we're nitpicking.)

On another topic, how many posts do I need to move from "Biker wannabe" to something more impressive?