Here's a question for the Gurus...

[trbig]

My carbs have very little vaccum at idle... 2-3 pounds when trying to sync them. I didn't realize this was out of the norm until I was at John's house and he showed me that his were getting @ 14-15 pounds. If you put your finger over the vaccum tube, every compression stroke sends a puff of pressure out the tube against your finger.

That's what's happening, but before you decide on a cause, everything is fresh on the motor, including a 3 angle grind on the valves. The valves are not burned and do not leak. The valves are all shimmed correctly. The timing on the cams versus the crank is dead-on, and the cam chain has less than 5k miles on it. I have run many motors in this bike and every one of them has gotten similar results with these carbs. The only common element that they all had, was that I have always run these 78 2H7 cams in the bike. What appears to be happening is that they are opening the intake too soon. I just can't understand why? The vaccum line attached to carb 2 can't be getting a good signal with these low readings. It pops and backfires out the carbs at idle... which it won't do on it's own right now. Once the rpm's get up above 3k though, it pulls hard and strong well up into red-line without a pop or miss. I did a high speed pass this weekend past John and Kat. Normally you would feel the HP dropping close to 8k rpm or so. I looked down while passing and it was at @ 9.5k and still pulling strong.

I have a couple sets of the 3H5 cams, so I think this week, I'll try installing a set of them. I have always wondered which would be best anyway. It just makes me wonder though. In the heart of this mechanically, this motor isn't any different than a '78's... why would it be doing this? I realize the valves and the combustion chamber are bigger, but ultimately you have the same pistons and crank spinning the same cams. Any thoughts on this?

Tod

[prometheus578]

"Sounds like something's wrong with your head, TRBIG."

What's wrong with the bike, I haven't a clue.

[trbig]

Well....

So much for the ""X"traordinaire" in your title....

As for my head... it fits perfectly bewteen my ears, so I'll just stick with it... as unsightly as it may be

This may be a stupid question... as many of mine seem to be... but has anyone experimented with different cams? Such as 3H5 on the intake and 2H7 on the exhaust side? It doesn't really sound like a smart move, but I don't have a clue about the lift/duration/ etc of these cams. I'll swap them as a set... but I was just wondering. Where's Dan Hodges when you need him?? (Yeah, I know... St Louis)


Tod

P.S. Thanks for moving the post Pro... if nothing else.

[prometheus578]

"Odd...."

(Firstly, TRBIG, Gurus don't post their troubles in open forums.
Guru's know all the answers... and if they don't, they at least remain silent about it... or quietly consult with other Gurus. Get with the program.
Sadly TRBIG, now there has to be a secret GURU meeting about this to decide your continuing status as a GURU.
Truthfully... I have to admit, that when the sacred, gem-encrusted voting beer stein is passed in front of me, I will be casting in a black marble instead of a white one)

Anyway...
I haven't a clue.
Low vacuum could mean a leak... but what of the compression pulses you feel?
Now then... If I 'member correctly, did you put on a YICS head?
Could the pulse you feel, say from cylinder #1's vacuum port be coming from cylinder #2?
Yes, as you know, valves opening too soon would cause low vacuum and a pulse, but your timing appears to be right on, and then there's the performance you state, which would have suffered had the valve timing been an issue.
Hhmm....
It may be the difference cams... but then again, this would mean that those cams, in their original engine would be doing the same?

Shall have to think on this a while. Too early for me.


Hhmm... could the cams be cut to compensate for YICS heads?

[wa407mpp]

try this!

Up your idle a bit, if you play with the cams, you have to play with the settings too. Everything is yeng-yang.

[TopCatGr58]

Hey Tod,

I don't think it's the cams totally, but probably are contributing to this. Let's postulate! One piece of info you forgot to include for the folks with CRS is that you have an XJ head on your XS Engine..hence what you said about the larger combustion chambers and larger valves.

I "THINK" what is happening is like the PLENUM affect with the homemade carb synching tool! Without the plenum, the vacuum pulse is stronger and more instantaneous at a gauge because there is less air moving between the carb and gauge, so the needles bounce, etc.!

Now, increase the amount of volume of air between the carb and gauge via a plenum and the gauge is slower to respond but doesn't bounce, because the total vacuum affect is dampened due to the reduced vacuum affect since it's spread out over a larger total volume of air in the plenum.

Now....we apply this to the engine. Larger combustion chamber means more air volume present before the piston drops to start creating the vacuum. The piston has to travel a bit farther before enough volume is created to raise the level/amount of vacuum since more air was already present with the larger chamber.

I've experienced the same thing with just a simple syringe. With the piston/plunger just barely away from the tip before you pull on it to create vacuum, you get more vacuum sooner in the plunger's travel. Now, position the plunger farther away from the tip before capping off the tip, you now have more air already in the syringe/vacuum chamber, and it takes more plunger travel before it creates the same amount of vacuum pressure at the tip!

This doesn't seem to affect it's hi rpm performance, but might be more evident in the low rpms, but I'm not a fluid/air dynamicist, or whatever you call it!?

Just saw while I was postulating, that Prom and you had a few more posts....I DO remember you saying that the XJ's head YICS ports were ALL PLUGGED, so the cross cylinder pulsing should not be an issue!
T.C.

[prometheus578]

"This is why I don't share needles with T.C."

I've experienced the same thing with just a simple syringe

Never know where they've been or what he's used them for.

[mro]

Did you and John use the same gage set?

Assuming...... .......
no leaks (intake/exhaust)
Carbs are dialed in etc. etc...

sorta leaves ignition/timing and cams.


mro
I've seen where there were gas actually "pulsed" out and back into a car carb. Engine barely ran at low rpm and ran somewhat better at higher rpm. Was caused by too much fuel, but not quite enough to flood it out.

[trbig]

As TC stated, the YICS ports are all plugged off. The rest of the motor is XJ XSecpt the cylinders (No YICS tube is only difference) and the cams. John used the same set of gauges. Intake boots have no leaks... nearly new and new gaskets between them and head.

I'm not ruling out that it may be running rich... but we can't synch the carbs very well with so low of readings. The pipes stay shiney and the plugs look good. I have only gotten this bike to idle well on a couple of occasions, but these readings have been the same when I had a '78 engine and head in, as well as the later model.

So this leads me to believe that either the intakes are staying open too long and aren't closing in time for the compression stroke, or they are opening too soon and catching some of the exhaust gasses. I am leaning towards the latter since it will backfire through the carbs sometimes. But if this was the case... why does it purr so perfectly at cruising speeds without the slightest hint of stumble or pop? If a valve is opening too early, it should be opening too early at any rpm, correct?


Pro, I did think about PMing you to ask in private. Even John suggested this being the wise Guru he is... But alas, I got the response from you that I assumed I would.... This is why I chose to post it publicly for all to see. Even though a guy can fluff his way to Guru-ship... it doesn't raise the I.Q once he arrives. I am living proof of this.

Tod

[trbig]

Guru's know all the answers... and if they don't, they at least remain silent about it

What's wrong with the bike, I haven't a clue.

Tod

[prometheus578]

Oh, shut up!

[wa407mpp]

Does it lope at idle? Assuming all the parts are in spec, did you go back and double check your Assembly to make sure nothings off, causing it to be more radical them intended. To me the fact that it runs good at higher rpm sounds like it is finally catching up to itself. If the cam is to radical it will cause low vac., that,s why you have to run an electric vac. pump for your power breaks on cars with built engines.

[red bandit]

Mine is the same. 8-10 lbs. of vacuum. I've checked with other gauges. I have new carb. boots. Sprayed the carbs and surroundings. no leaks. Thought the cam timing or valve lash was off. Nope. I have 173-175 psi compression between all cyl. It idles nice and smooth and runs great. I haven't done anything to the engine. I thought it was stock but I have no idea if someone else has done something.

[trbig]

I don't know how radical the cams can be just being stock 78's?? Everything else is installed correctly though. Since I bent valves the first time I tried to do this (Replace cams) I am super cautious about it now.


I guess later in the week, I'll post the results of the official "Seat of the pants dyno" when I get the 3H5 cams in.

Pro... If you could figure out how to shut me up... SWMBO would love to hear about it!


Tod

[81xsproject]

I doubt it is the cams. When I had 70's cams on my 80's head, I don't recall seeing any major difference in vac numbers than with the stock cams. Personally, my bet is something to do with the yics system. I don't know much about it aside from its purpose and how it functions. Where is the system blocked off? Even if the system is blocked, it may still be creating a larger volume between the intake valve and the butterfly. A larger volume would be affected less... (ALL BS AND SPECULATION)