Mixture screw has no effect.. What does it mean?

[darrell3001]

[Backstory] After nearly 3 months of of fiddling with my newly rebuilt carbs on my '83 11E, i finally made a breakthrough last week. I will document this journey in a separate post once i get this last mystery solved.

I doing the final, fine adjustments and ive run across something strange. Everything is dialed in. Full, mid-range and idle all working to near perfection. Just doing a bit of final adjustments on the idle with the mixture screw. Nothing really major. The factory setting of 2 and 1/2 turns was just about right on the money. As so many folks have pointed out, go with factory settings.

One oddity i found is with number 2 cylinder. The mixture screw has NO effect. The other three work as advertised, however, number 2 has absolutely ZERO effect. Even if i screw it all the way it, it does not effect the running engine in the least. On the other three, screwing the mixture screw all the way in stalls the engine, but not on number 2.

Im running with Pods and factory exhaust. Factory pilot jets, DynoJet carb kit with 132 main and Dynojet supplied needle. Needle e-clip set to number 3 position (as defined by DynoJet ket). Carbs sync'd and valve clearance checked.

Number 2 plug is ash white (no fouling). I even have a ColorTune that ive put on number 2 just to make sure its firing. It is. Nice bluish-white flame.

Im going to dig into this a bit more over the weekend, but i though i would put this out there and see if anyone can offer any explanation/guidance.

Many Thanks and Best Regards,

Darrell

[Who Dat?]

One possibility is that the tip of the screw is broken off and is plugging the port. You can check that easily by removing the screw and examining the tip. If it's a nice, sharp point, the screw is likely not your problem.

One other possibility is that something is blocking the pilot circuit. It is not unheard of to have something block the pilot jet after a rebuild job. If you have compressed air available, remove the mixture screw and blow some air into the hole it came out of. If something was blocking the jet, it might have gotten blown loose. NOTE: this does not mean that it's fixed. You still need to open up the carb to verify.

[Nessism]

The reality of the situation is that a small mixture adjustment on each and every carburetor can't be expected to change the idle rpm of the engine. Some do, and that's well and good, but not every cylinder will have that much influence over the engine. Especially, if the engine is near optimal to begin with.

[zed1015]

White plug means lean and the pilot mixture screw on these carbs is a FUEL metering screw so no change when adjusting coupled with the lean plug means that there is no fuel exiting the pilot orifice.
Broken screw tip,blocked pilot jet or blocked fuel passage to the screw etc will be the cause.

[Suzukian]

Or someone else broke off the tip and it's wedged in there. Some Tig welding rod pushed up the known working hole compared to the non working hole will tell you if something is wedged. It might help remove any blockage. How's the bike running? Does it idle well? Does it ride good? If it's running fine, set the screw, pour some Sea Foam and see if it cleans up.

[tom203]

Remove and plug the petcock vacuum fitting at this carb and see if mixture screw still has no effect.

[darrell3001]

One possibility is that the tip of the screw is broken off and is plugging the port. You can check that easily by removing the screw and examining the tip. If it's a nice, sharp point, the screw is likely not your problem.

One other possibility is that something is blocking the pilot circuit. It is not unheard of to have something block the pilot jet after a rebuild job. If you have compressed air available, remove the mixture screw and blow some air into the hole it came out of. If something was blocking the jet, it might have gotten blown loose. NOTE: this does not mean that it's fixed. You still need to open up the carb to verify.

So i removed the stack last night and removed all 4 mixture screws. No broken tips. I even went so far as to measure all 4 with a micrometer and they are all same length. I verified that i can see daylight down each hole so no blockage at the tip anyway. I put carb cleaner down the hole and blew each of them out with compressed air. All 4 blew out the pilot air fitting. I also removed the pilot air jet and sprayed carb cleaner there and blew back the other direction with compressed air. No obstructions there either. I even went as far as to spray carb cleaner into the pilot air fitting, plug up the top and bottom of the mixture screw fitting, thus forcing everything back thru the pilot jet and main jet. Again, as expected, i got good flow out the main jet. Anyway, i feel pretty confident that there is no blockage.

The thing that is really strange about this is that the bike is running absolutely perfectly. I did make one minor adjustment today as i moved the e-clip on the needle from number 3 position to number 2 (from top). But other than that, i could not ask for anything more. Ive got a great idle, great mid-range (now with latest adjustment) and awesome full throttle response and pull. Im inclined to take Ed's advise and just not worry about one mixture screw. I just dont like unexplained mysteries.

[darrell3001]

Or someone else broke off the tip and it's wedged in there. Some Tig welding rod pushed up the known working hole compared to the non working hole will tell you if something is wedged. It might help remove any blockage. How's the bike running? Does it idle well? Does it ride good? If it's running fine, set the screw, pour some Sea Foam and see if it cleans up.

re: how is bike running?? This is the really odd part. Its running perfectly. Idle, mid-range and full all doing great. Its certainly not causing any problems that I can tell, but just really strange that one is not working like all the rest. I just saw that someone also suggested to plug the vacuum line to the petcock and see if that has any effect on mixture screw. I think im going to spend just a bit more time on this and then move move on. I can (probably) live with knowing that one of the mixture screws not functioning on an, otherwise, perfectly running bike.

[darrell3001]

Remove and plug the petcock vacuum fitting at this carb and see if mixture screw still has no effect.

Humm.. Interesting idea. Are you thinking that the petcock is not forming a good vacuum? Its obviously doing enough vacuum to open the petcock valve, but you could be on to something about it bleeding off too much vacuum thus looking like a vacuum leak. Im going to try that tomorrow morn. Great idea. Thanks for the tip.

[darrell3001]

Remove and plug the petcock vacuum fitting at this carb and see if mixture screw still has no effect.

tom203 WOW!!! You are correct!!!! plugging the petcock vacuum fitting magically causes the mixture screw to magically work again. Amazing!!! Now, what does this mean? Does it mean that i have faulty vacuum seal in my petcock??

Many, many thanks for the tip.

Regards,

[Sandy]

tom203 WOW!!! You are correct!!!! plugging the petcock vacuum fitting magically causes the mixture screw to magically work again. Amazing!!! Now, what does this mean? Does it mean that i have faulty vacuum seal in my petcock??

Many, many thanks for the tip.

Regards,

Check the vacuum hose for split in it. Enough vacuum to open the petcock but sucks some air through the split.

[sacruickshank]

tom203 WOW!!! You are correct!!!! plugging the petcock vacuum fitting magically causes the mixture screw to magically work again. Amazing!!! Now, what does this mean? Does it mean that i have faulty vacuum seal in my petcock??

Many, many thanks for the tip.

Regards,

It likely means that the cyl was getting too much air (due to the leaky vacuum hose) and not firing at all. I had a similar experience where I switched to a gravity fed petcock but initially forgot to plug the vacuum port on the intake manifold. Once I did, the cyl fired right to life.

[darrell3001]

So im definitely on to something with the vacuum on number 2 and the petcock. However, i do not believe there is anything wrong with the vacuum hose itself. Its a brand new hose and its genuine Suzuki parts so its the right size. Also, to further confirm this, i have disconnected the hosw from the petcock and pugged it at the petcock end (and just left it dangling). Then i turned the petcock to the full right position (ie pointing toward the back of the bike). I took it for a test ride and WOW!!!!!!!! Its back to running the way it should. All minor problem are gone. So i think that this test is pretty definitive that is not the hose. I am going to do one other test but putting a clamp on the vacuum hose at the petcock.

So, if its something related to the petcock, what do i do next? Do i need to rebuild or source a new petcock? This seems rather strange that the petcock obviously works as the vacuum is opening the valve, but it appear to have a leak. Is this the correct assumption? I just want to confirm this before i go further.

Regards,

[dorkburger]

No personal experience, but from reading here rebuilding petcock with kits has a very high failure rate.

[darrell3001]

So i ordered a petcock rebuild kit from Z1Enterprises. We will see how it goes. In the meantime, i installed a solid piece of rubber where membrane was in the petcock. I also put an inline fuel cutoff. This way i can still ride the bike while waiting on the rebuild kit.

Another thing that i found while debugging the faulty petcock was that the vacuum hose was not sucking excess air (ie vacuum leak). It was indeed sucking fuel. I did a check on this by testing the vacuum while the hose was connected to the petcock. Lets just say i spent the better part of yesterday afternoon trying to get the taste of gasoline out of my mouth. So what this tells me is that the reason that the mixture screw had no effect on the number 2 carb was not as a result of a vacuum leak, it was actually an excessive rich condition as a result of the vacuum hose sucking fuel from the petcock. Also, after figuring this out, now this explains something else.. I had noticed that after i came back from a ride i could always hear air "escaping" from the tank at the cap. I had always just thought that this was the fuel forming vapor that was leaking off. In reality, this was not air "escaping" from the tank, it was air being "sucked" into the tank.

A couple of conclusions that i have reached based on this revelation. Please feel free to correct me if my conclusion is faulty.

1) The mixture screw on number 2 having no effect was actually an excessive rich condition rather than an excessive lean condition

2) If i forgot and left my petcock in the prime position, i always ended up with a bunch of fuel on the floor. I just equated this to the bike being old and never thought more about it. Now i realize that this situation was actually being caused by the faulty petcock and the fuel was actually leaking out the vacuum hose and through the number 2 carb only. Now that i have temporarily turned my petcock into a gravity feed and disconnected the vacuum line, boom, no more leakage when in the prime position.

Bottom line: If you have fuel leaking out when you are in the PRI position, you probably have a faulty petcock like me.

Best Regards,