2,500-3,000 RPM Idle After Warm-Up (with choke off)

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

Hi All --
I'm having an idle problem with a '72 XS2 that I'm hoping someone might be able to help me with.

Specifically, the problem is that after the bike warms up (~5 minutes), the idle speed jumps from its normal 1,200 RPM to between 2,500 - 3,000 RPM (choke OFF) & stays there, which makes it difficult (and sometimes nearly impossible) to shift gears.

The 2 throttle cables are new (& sliding freely) & the throttle levers on the sides of the carbs both return to their normal positions when I release the throttle. So I know the problem isn't related to the throttle cables hanging up somewhere.

The problem began immediately after I had the carbs rebuilt by a Yamaha-trained tech, so I'm guessing it's related to something with the carbs.
However, I had a similar problem with the bike (though not quite as bad as it is now) about 20 years ago (I've owned the bike since '75) and, if I recall correctly, the shop replaced the 2 advance unit springs, which fixed the idle problem. So I just replaced the 2 springs myself, but that had no effect on the idle.

I don't know if this is related, but when I got the bike back from the shop that rebuilt the carbs, I also noted (and complained) that the idle speed when the choke is on is about 3,000 RPM & the choked idle speed has always been around 1,800 for the bike's entire life.

Anyone know what the problem is or what I should check?

Retiredgentleman

Here's a list of things to check:
Air leaking at the intake manifolds?
Is the advancer rod and weights working freely. If you don't know the last time the rod was lubed, you should remove, clean and relube with some moly based grease.
Points gap and timing check using a timing light.
Adjust the air/fuel mixture (pilot screws) using the dead cylinder method.
Adjust the carbs synchronization.

sure · Full Member Joined: May 07, 2008 Posts: 195

also could have clogged jet...

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

Retiredgentleman wrote:

Here's a list of things to check:
Air leaking at the intake manifolds?
Is the advancer rod and weights working freely. If you don't know the last time the rod was lubed, you should remove, clean and relube with some moly based grease.
Points gap and timing check using a timing light.
Adjust the air/fuel mixture (pilot screws) using the dead cylinder method.
Adjust the carbs synchronization.

Thank you for the quick reply.
Two of the first things I checked/adjusted were the air/fuel mixture screws & carb synchronization, but both are fine.
I'll check the points and timing next.

I was debating whether to replace the advance rod & weights, but when I replaced the advance springs, I lubed the posts the weights pivot on (with moly based grease) and the assembly seemed to be moving freely (by hand) and the weights do move outward when I run the engine (with the advance unit cover removed). Should I still remove the advance rod & re-grease it? I'm sure the rod hasn't been removed or greased for a long time (at least since the last shop replaced the advance springs 25 years ago).

Re the intake manifold: Are you talking about where the carburetor mates with the engine? I've actually been kind of thinking that the carbs don't quite look like they're installed exactly perpendicular to the engine since I got the bike back from the shop. So, maybe, they failed to seat the carbs all the way down onto the manifold? Does that make sense?

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

sure wrote:

also could have clogged jet...

I was wondering that, too. But if a jet was clogged, wouldn't the (unchoked) idle be too fast all the time, not just after I've ridden for awhile?

The confusing thing is that when I drove away from the shop that did the carb rebuilds (rebuild kits purchased from mikesxs, if that makes any difference), the idle was slightly fast (about 1,300 RPM) and I told the tech about that because the idle had always been between 1,100 and 1,200 for the bike's entire life. He just said it's an old bike and that was the best he could do.
By the time I'd driven half way home, the idle had creeped up to the 2,500-3,000 range. I supposed I should have returned the bike to the shop, but I had other issues with them and didn't want to go back (long story).

Anyway, the next day, I reset the air/fuel mixture screws & synchronized the carbs (one carb was WAY off, so I initially thought I'd discovered & fixed the problem when I re-synchronized the carbs). Then I readjusted the idle speed back to around 1,200 (a bit high, but within the reasonable range, so I left it there) and put the bike away without riding it.

The next day, the idle was fine when I started the bike, but it went right back up to between 2,500-3,000 within a few minutes of gentle riding.

Retiredgentleman

Yes, you should remove the advance rod. I take off the nut on the advancer side. Remove the weights and springs and remove the slotted disk and withdraw the rod from the points side. Just be careful of the pin on the advancer shaft, that it does not fall out.
Clean out any dried grease in the grooved bushings, and relube with a moly based grease.
The intake manifolds are the short connections from the carbs to the head. You don't want any air leaks there, with the engine idling at 1200 RPM try spraying around the manifolds with some carb cleaner................if the RPM rises it would indicate an air leak.

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

Retiredgentleman wrote:

Yes, you should remove the advance rod. I take off the nut on the advancer side. Remove the weights and springs and remove the slotted disk and withdraw the rod from the points side. Just be careful of the pin on the advancer shaft, that it does not fall out.
Clean out any dried grease in the grooved bushings, and relube with a moly based grease.
The intake manifolds are the short connections from the carbs to the head. You don't want any air leaks there, with the engine idling at 1200 RPM try spraying around the manifolds with some carb cleaner................if the RPM rises it would indicate an air leak.

Ok, that sounds like the plan.
Since it's easiest, this weekend, I'll try spraying carb cleaner around the carbs. If that proves unsuccessful (i.e., no leaks), I'll try removing, cleaning & reinstalling the advance rod. I'll report back with the results.
Wish me luck and thanks again!

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

I don't know if anyone will still be following this thread, but, over the past couple of months, I:
1) cleaned & lubed (with moly grease) the advance unit. It actually looked surprisingly clean when I disassembled it, but I cleaned/re-lubed everything and replaced the weight springs anyway. Unfortunately, that didn't affect the problem. The idle was still too high after the bike warmed up.

2) I tried spraying carb cleaner around the intake manifolds (the 2 rubber components that fit in between the carbs & engine body). The idle speed did increase ever so slightly when I sprayed around the right-hand carb manifold, so I figured that must be the problem. So I ordered another pair of carb rebuild kits along with new manifold gaskets (which, as I found out after removing the carbs, the shop that had just rebuilt the carbs apparently failed to install the manifold gaskets when the did the carb rebuilds). So, thought I, BINGO, there's my air leak....and I ought to be able to fix it with a pair of gaskets.

Last weekend, when I removed the carbs, I found that the shop had left off the carb manifold gaskets along with the 2 drain lines that connect to the bottom of the carb float bowls (they also left off the battery drain line, which then leaked battery acid all over my new head pipes (bought just weeks before taking the bike into the shop, which took off some of the chrome).
So, seeing all of these things, I figured it was probably best to rebuild the carbs again.

So, today, I finally got both rebuilt carbs back on the bike and started it up for the first time since I posted the last message.

Once I got the engine started (very hard to start!), the engine was running really rough and had no power. It sounded/felt like it was running on only 1 cylinder. And, as I found out, it was (running on only 1 cylinder). So I increased the idle speed and tried removing the left side spark plug wire. No change in the idle. When I disconnected the other side, the engine stopped immediately.

So it was obvious that the left cylinder isn't firing. So I removed both air cleaners to see if the carb pistons were lifting properly. They were. However, with the engine running (both spark plugs connected), if I lift the right side carb's piston, the idle changes. But when I lift the left side carb piston, nothing happens.

I don't know if fuel is getting from the left carb into the engine, but I do know for sure that fuel is at least getting into the carb. I disconnected the fuel inlet hose (from the petcock) going to the right side carb & (with the left carb getting fuel) immediately, fuel started pouring out through the right side fuel inlet hose where it was disconnected at the right side fuel petcock. So fuel's definitely flowing into the left side float bowl, then out through the balancing tube into the right side carb & up and out through its (disconnected) fuel inlet tube.

So, is the left cylinder dead because of an electrical problem or because I screwed something up in the carb rebuild????

I've got those yellow high performance spark plug wires with high performance NGK spark plug caps, which make it impossible to see if there's a spark between the spark plug wires and the spark plugs.

So, now I'm not sure what to do???
Again, the problem (now) is that the engine's running on only 1 cylinder (the right side) & the left isn't firing.
Can anyone recommend where to go from here?

oldbikenut

I would definately check to see if you have spark. Do a search on this site as there are some fancy ways of doing it, though I just take the plug out and reconnect it to the plug wire and hold the spark end near the cylnder head and give the bike a few kicks. Make sure have the key on and to take the other plug wire off when doing this so the bike does not try to start on the one cylinder.

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

I'll try that and report back.
Thank you!

Retiredgentleman

I agree with oldbikenut. First thing is to check the quality of the left spark.

You could swap over the coils and see if the left cylinder starts firing,and the right stops firing.

If these are the original coils, I would just replace them with a new single (dual output) coil. If you don't like fiddling with points you could go with a Pamco ignition.

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

Thanks, but I want to keep the bike as close to OEM/stock as possible.
I should've thought of swapping the spark plug wires over to the opposite coil myself, but I'll try that after checking for a spark at the plug.

However, since the problem with the dead cylinder only began after I rebuilt the carbs & I didn't do any work on (or near) the electrics, I have a feeling the problem's related to something I screwed-up on the carb. I replaced all of the jets and valves with the standard parts that come with rebuild kits, and didn't change any of the settings. The only thing that stuck out when I took the carbs apart was that the clip that adjusts the height of the center needle in the main jet was set at the last notch in both carbs. This was done by the shop that originally screwed up my idle (from the original post), so I just replaced the clips on the new needles at the same position.

Also, when I installed the new main nozzle & o-ring, I had to lightly tap it into the hole to get it to seat all the way down. I know I didn't damage it, but I don't know why it didn't slide in and out as easily as the old nozzle did (the shop apparently didn't replace the old nozzle when they rebuilt the carbs).

Retiredgentleman

Th clip nominal position for the jet needle is the 4 position from the top of the needle.

Joseph · Full Member Joined: May 22, 2007 Posts: 767 Location: NY

I didnt read this whole thered but after it warms up to that high idle, adjust the idle down and keep it there. You will have to use the choke for starups. I had a similar problem on my 80, and that was it just a missadjusted idle.

schtipp

that's like speaking without hearing the conversation, just bad policy. i am suspect of that left carb. see if you can pick up a spare set somewhere to try. if they don't help you can resell them if they do you've proved it's the carb.

Joseph · Full Member Joined: May 22, 2007 Posts: 767 Location: NY

True, I did read the thread now. I am not sure about the new problem but I think the original high idle was due to it being set to high. Once the bike reaches operating tems set your idle to 11-1200, done.

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

Joseph wrote:

True, I did read the thread now. I am not sure about the new problem but I think the original high idle was due to it being set to high. Once the bike reaches operating tems set your idle to 11-1200, done.

Thanks, but that definitely wasn't the (original) problem. I only posted here after failing to adjust/get a proper idle speed (at operating temperature).
Initially, after receiving the bike back from the shop (which rebuilt the carbs), the bike ran fine when cold (with or without choke). But, within a few minutes of driving, the idle speed would creep up to ~3,000~3,500 and hover (i.e., when I released the throttle, the idle speed would remain high for several seconds before coming down to ~2,500, where it remained. Often, the high idle would make gear changing so difficult that I'd have to pull over to the side of the road and shut off the engine before I could change gear! This, effectively, disabled the bike.

After 3 return trips to the shop that rebuilt the carbs (and caused the high idle problems), each resulting in more problems or damage to my bike, I decided to cut my losses and try to fix it myself.

I won't have time today, but, hopefully, tomorrow or Wednesday I'll report back re the spark and if the dead left cylinder shifts over to the right cylinder when I swap the left-and-right coils.

tacoswild · Last edited by tacoswild on July 27, 2009, 7:22 pm; edited 1 time in total

You'll have to tell us on the new dead cylinder problem, but on the old high idle issue if the timing and advance are fine then you have to think you're running lean for some reason. Air leak at the maifold could surely cause that, but so could a lot of other air leaks or carb problems

Joseph · Full Member Joined: May 22, 2007 Posts: 767 Location: NY

I dont understand why you cant get it to operating temps at the idle that is too high and at that point adjust the idle down to 11-1200 and ride. The next morning you might have to use the choke a bit longer on warm up but why cant this happen? I am learning here myself so i am verry interested in this outcome now.

tacoswild

Joseph, you certainly do want to set the idle at operating temp but such a big difference between cold and hot idle speeds suggests something isn't right. If you need to use the choke to start the bike when temps are over 65-70 degrees then something probably isn't right. (I got that temperature from a mikuni VM guide but I think it's a reasonable rule of thumb)

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

UPDATE:

Okay, both spark plugs are getting a good spark, so I think that rules out the electrics as the cause of the non-firing left cylinder and points the finger back at the carb (which I just rebuilt).

I've got the carb disassembled in front of me and I can't see anything wrong. I've cleaned the entire body out with carb cleaner again; I've blown air through all of the journals and passages, including the 2 tiny jet openings at the top of the exhaust opening where the butterfly valve closes between them, and they all appear completely clear.

Just in case the rebuild kit I got from MikesXS contains a wrong part, here's the numbering on the jets:

ain Nozzle - Z-6
Pilot Jet - 42.5
ain Jet - 130
Needle - Y22 (clip installed on 2nd indent (out of 5) from the bottom (both carbs).

Just before I removed the carb just now, I ran the bike again. I can start it and it idles...more or less normally. The pistons in both carbs rise the same amount when I turn the throttle, but if I manually raise the piston in the left carb, it has no affect on the idle (lifting the right carb's piston does affect the idle).

But if I turn the throttle the bike sounds like it's running on 1 cylinder, which, when I disconnect the left spark plug wire, is confirmed since it runs the same with or without the left spark plug wire connected (if I remove the right side spark plug wire, the bike dies immediately).

Again, other than the fast idling problem, the bike was running great prior to my rebuilding the carbs. So I know the current "dead cylinder" has to be due to something in this left side carb.

I know that the Solex carbs are designed with journals and sections devoted to idling, mid-range speed and high-range speed. So I'm guessing that the idle section is OK and there's a problem with the mid-range speed section, but that's as far as I can reckon it out.

Can anyone spot the problem or recommend my next move?

grant

I can say only that I had similar problems with my '76. I rebuilt carbs, tried another set of carbs, different jets, all the time thinking carb trouble. Put in new points 8-9 months ago and problem solved. Recently the idle would drop off or backfire at 1500 rpm's or less and after warming up sometimes would climb to 2000 or more. Reset my point gap (which had gone out of spec) and problem solved once again. I was also getting what I presumed a "good spark" but the whole problem was with points. Hope this helps.

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

Thanks, Grant.
I think I checked/cleaned the points early on (before I posted here) and they looked fine. On the other hand, the shop did replace the points and I guess, considering the sad state the carbs were in after they "rebuilt" them, it's possible they bullocks those up as well. So, once I get the left cylinder running again, I'll definitely have a closer look at the points. I'm pretty sure I've got an extra set around that I can swap if need-be.

Another UPDATE:
As I was fiddling with the carb parts just now, it occurred to me that the fuel that exits the left carb via the balancing fuel line (that runs between the 2 carbs) does so BEFORE entering the float bowl. Previously, I thought that the line running between the 2 carbs came out of the float bowl. This is relevant because when I first installed the carbs after my initial rebuild, I attached the left side fuel line to the fuel tank and turned on the fuel, which immediately started pouring out the right carb's fuel line (which I hadn't yet attached to the fuel tank). Because of this, I assumed (incorrectly) that both carb's float bowls must've been full. And, therefore, I assumed (also incorrectly) that the dead left cylinder couldn't have been a fuel problem. But now that I see that the fuel can flow to the other carb without ever entering the first carb's float bowl, I decided to recheck the float's clearance in the left carb.....

Wouldn't you know it, the floats, which are supposed to be 24mm's above the ceiling of the float chamber (measured upside-down), were actually about 31mm. So I'm guessing that, when the carb's in its normal position, the floats were probably bottoming out against the inside of the bottom of the float bowl, which caused them to press against the main jet, closing it and keeping it closed. That, of course, would mean that the left cylinder's combustion chamber was getting nothing but air (i.e., no fuel).

So I readjusted the float's clearance to 24mm and reassembled the carb. Then, with the carb rightside-up (so the floats were at the bottom of their travel and the main jet was open) I blew air (by mouth) into the fuel line and made sure I it was entering the float bowl chamber. OK! Then, with the carb turned upside-down (so the floats were at the top of their travel and the main jet was closed), I was unable to blow any air into the carb.

Unfortunately, it's too late now to put the carb back on the bike, but I'll try it tomorrow and report back.

freddy333 · Full Member Joined: Nov 02, 2008 Posts: 12

UPDATE:
I just reinstalled the left carb (after sorting out the float adjustment issues) and the bike started right up. That's the good news.
The bad news is that I seem to be back at square one as far as the original problem (fast idling) goes. All was fine for the first 2-3 minutes of driving. But, slowly, the idle speed increased as I drove so that after about 10 minutes, my previously set 1,200 idle speed was hovering around 2,500 and wavering towards even higher rpms.

Unfortunately, it got too dark for me to check the points, so I'll have to do that this weekend. But if anyone can think of anything else to check (I think it's safe to say that there're absolutely no air leaks between the carbs and engine at this point), please post it here.
And thank you to all who got me over this last hurdle of the dead cylinder.

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