high rpm miss

[Winterhawk]

I think TC meant that the wrong pod filters could block a vent on the Carb by being the wrong size.

You could put a vacuum pump on it and it should measure x amount of degrees at so many inches. The info is in the book somewhere.

[edgo]

Never thought about a blocked vent. I'll check that. The new pods that I got are from MikeXS for the 650. They should be better since the XS650 uses the same 34mm CV carb, right?

What is the purpose of that big oval vent on the top of the back of the carb inlet?

Ed

[John]

The oval vent is the air passage to the diaphrams that lift the slides/needles.

[edgo]

What is the sequence of events that actually lifts the throttle slides? Doesn't the vacume hose suck them up with suction supplied by the advance under the left side cover? This is the first bike that I've owned that had one of these systems.

You guys are a great help,

Ed

[randy]

Ed,

The vacuum through each carb throat is applied, internally through a passage in the carb, to the tops of each of the slides, lifting them in concert with the throttle opening. (thus the name (CV) constant velocity carbs).

The oval port at the intake allows atmospheric pressure to the underside of the slide diaphram. If that port is blocked, the slides won't raise no matter how much vacuum you've got. make sense?

The advance mechanism gets a vacuum signal from the #2 carb port that's a little brass tube machined into the carb body.

The vacuum ports next to the head on the rubber intake tubes:

two of them are used for the fuel petcocks and two of them are plugged. All four used when synchronizing the carbs.

[edgo]

Thanks Randy. Yes it makes sense now. If the problem persists after I'm sure that my carbs are getting the proper amount of fuel delivered then I'll have to check out the vacume. Should I just get a regular automotive vacume gauge to check it?

Ed

[randy]

Ed,

You won't be able to check the vacuum signal at the slides, but you can view them through the carb and watch them raise as you work the throttle. Try this. engine off, raise each one with a finger and pencil mark the farthest travel. Then run it and compare the real world operation. You may find one (or more) does not go all the way up. That one has a leak either at the top seal or the slide itself or carb bore is worn out.

If they're unequal, you may need to synch the carbs.

A vacuum guage will work on the advance port on the #2 carbs, but a better way would be to use your timing light, on the timing plate, raise the throttle and you should have full advance at (?) 5,000 RPM. If not, you may have a leaky advance diaphram or hose, or stuck / binding mechanism.

[TopCatGr58]

Hey Edgo,

The bad pod filters DO obstruct that arced inlet on the top of the carbs. If you compare the mounting shoulder of the previous pods with the newer ones from MikesXS, you'll probably see that this inner lip sticks out/inward much farther, which puts it protruding well over this lip.

You can check for leaks in the carb diaphragms by pushing the slides all the way up, and then put your thumb across the top arced port, and let go of the slide, it should stay up or barely slowly slide downward. If it slides down fast, then you've got holes in your rubbers!

I was doing some work on my carbs, and had the filters off, and was running the engine, and was quit surprised when I slowly rolled on the throttle, that the slides barely raised even though I was able to get it up to 7krpm, of course there was NO LOAD on the engine either. Then after lettting it back down to idle, I then cracked the throttle wide open, the slides jumped up briskly, and pulsed quite a bit before the engine caught up to the throttle position, and then the vacuum equalized, and the slides came back down. Later, I mounted my camcorder down in front of my battery focused on a couple of carbs, and drove it around a few minutes and it's interesting to see the rise and fall action of the slides under different loads and throttle positions.

You can manually advance/rotate the timing plate thru it's full travel range(with the engine OFF), and then release it and it should return to it's original postion SMOOTHLY with no catches or hangups! IF not, then you will need to clean it and then relube it! Without a vacuum gauge, you can also just take the vacuum line from the #2 carb and apply suction any way you can, either my mouth, or syringe, and watch the timing plate rotate. Make sure it moves thru the full range that the vacuum piston can move, and while keeping the vacuum on, it shouldn't move or return, but if it slowly returns, then you may also have a leak in the vacuum advance unit!?
Keep at it, and keep us posted!
T.C.

[edgo]

My vacuume unit just passed the suction test. I applied suction and it rotated fully to a stop then I pinched the hose and it held until I released the pinch. The vacuume unit feels smooth when it is rotated my hand. I

When I cleaned my carbs I inspected the diaphrams and found no holes. The slides move freely up and down with no binding.

This weekend I'll take the petcocks apart and do the second half of the timing test for the vacuume advance. I also have tach wobble so holding the rpms at 5000 will be a bit of a guess. The wobble doesn't happen at 1000 rpms so I can set the timing to the mark there.

Ed

[edgo]

I just checked the advance. I removed the tube and plugged the canister and hit it with the timing light. It advances up to between 30 and 40 on the plate as the rpms rise. That's what it's supposed to do, right?

I guess I'll explore fuel delivery this weekend. Until it breaks up it is running really crisp. It's gonna haul butt when it get this figured out.

Ed

[thumperjsa]

I had a very similiar issue in the past where it would accelerate like a Bat out of Hell until 7500k and would not go a bit further. One issue was a leaky intake manifold which kind of help but not completely.
What did fix it is one out of the 4 carbs were set to 22 mm.

Three things:

Verify you have no leaky intake manifolds by spraying some wd40 or starting fluid at the intakes between the cylinders and carbs.

Verify all 4 cylinders are firing and heating up by spraying water on them.

Check the color of your plugs. I would bet at least one is white.

One other thing. When you are measuring the float height, you are going from the bottom of the carb where the float bowl rests and you don't have a gasket in the way, correct?

[edgo]

I have brand new carb manifolds. This week I went out and bought a metric caliper. I'll reset the floats this afternoon and I have been messuring height from the body of the carb. I do believe that all of the cylinders are running. I'm going to drive it to work today and do a chop test on the plugs to see where they stand now. I put new plugs in it last night and made a beer run to the 7-11 and the problem was still there.

Ed

[randy]

Ed,

Your advance seems to be working properly


After you re-check the floats, if the problem is still there, I think you may have a spark problem. Weak spark can do funny things under load. Like when your plugs are fouled and the motor will idle fine, but won't drive worth a damn.


If you're good with a soldering iron, re-solder the pins inside the TCI. it's a quick job and you won't have to remove the TCI to do it. Also clean the 2 harness connectors on the TCI body. For some pictures and instructions, visit my website here:

Randy's XS Page

After you're done with that, unscrew each plug cap, cut 1/4" off each plug wire and screw the cap back on.

If you're still stuck and really feeling ambitious, you can re-set the pickup coil gap. Yes, it's adjustable. When you're ready to tackle that, let me know, i'll give you the instructions.

Hope this helps.

[randy]

Here's something else that can affect spark, the resistors inside the caps!

Photo one is looking down the business end of a plug cap. See the slot in the brass connector?




Photo two - You can unscrew it to extract the resistor.



Photo trhee - clean out all the rust and crap.

[petejw]

could it b that the jets are still not big enuff,
with pod filters arent u suppose to go up 2 - 3 sizes
for the main jets. 140 from 137 is only 1 size up
could that make a difference?
also been suggested to check for air leaks
have u checked around the throttle bodies,
mine were leaking pretty badly, im in the process of
replacing them at the moment.


pete