Mild miss until around 3K....I am thinking it's the carbs....now what?

[Gymbo]

I am on my 4th ST1100 now and this most recent one I picked up has a mild miss in the lower RPM range...up until around 3K. Then, it either goes away or is just less noticeable. It's a 1992 ST1100 with 53K on it. The previous owner says he replaced all the carb jets.

I have never removed or torn into the carbs on any of the ST's I've owned...count myself lucky, I guess. I used a STE synchrometer to synch the carbs yesterday and it didn't help. I've run a tank of Seafoam and next I have some Berrymans to run through it. I've only put one tank through it so far...I've only owned it for 2 weeks.

I am thinking I have a vacuum leak. I have a Honda manual....and, forgive my ignorance....which are the vacuum lines to check? Do I have to pull the carbs to check them? I know it could be the carb boots...I am not sure if the PO replaced them when he pulled the carbs or not...going to try spraying WD-40 at the boots and see if it changes the stumble. Also thought about the fuel cutoff valve....but haven't yet bypassed it...don't want to change too many things at once for fear of not being able to isolate what the problem is.

If I were skilled enough, I'd just pull the carbs and check the jets and do the idle drop test, etc....but after reading how to do those...I am feeling a little intimidated to tear into that myself...and I live a long way from any other ST-Owners or liSTers. So...is there anything I can do without removing the carbs?

I am ordering new spark plugs too...and going to check the plug wires. So trying to cover a lot of bases...but I still think it's probably the carbs.

Any suggestions?

Thanks!

 

[vinny]

It's possible with the age of the bike, the carb boots may be the culprit. However, the description sounds more like dirty low speed jets. If the bike has set a lot, it may take more than one tank of seafoam to clean them. When I acquired the 1999 1100 I have, it had set quite a bit and the jets were dirty. Ran several cans of seafoam and berrymans through it and only had minor improvement. It wasn't until I pulled the carbs and physically cleaned the jets that it started performing right. Also replaced the carb boots at that time. It really is not that difficult to pull the carbs - just remove the shelter top, the air box, the fuel shut off valve, the sub filter assy, disconnect the throttle cables. Loosen the clamps around the boots, shoot a little silicone spray or WD-40 at the carb boots and let them sit for a few minutes. Take a cut of wooden broom handle or like instrument and pry up gently and they should come off.
Once the carbs are off, pull the large dome shaped covers off and inspect the vacuum pistons for any rips or holes. Then turn them over and pull the bowls and you can pull the needles and clean the jets. Just be sure to reassemble in reverse order and watch all the iddy biddy parts. Good luck!

 

[Gymbo]

After a bunch more reading, I am feeling like I could pull the carbs and have a look though them. It would require most of a weekend and a little patience. But I have a couple questions...the one that springs to mind right now is:

Do I need to replace the float chamber o-rings or can they be re-used?

 

[dwalby]

I am on my 4th ST1100 now and this most recent one I picked up has a mild miss in the lower RPM range...up until around 3K. Then, it either goes away or is just less noticeable.

I am ordering new spark plugs too...and going to check the plug wires. So trying to cover a lot of bases...but I still think it's probably the carbs.

Any suggestions?

Thanks!

When you say 'mild miss' what exactly does that mean? I've seen ignition problems that are RPM dependent, kind of like what you describe, is the missing you're feeling similar to an ignition misfire? I'm no expert on carb issues, but I'd suspect that carb problems are going to be more like it won't run well at all at a particular engine speed/load, rather than intermittently run good/bad. If what you're feeling is intermittent power loss that lasts for a fraction of a second and recovers, I'd be more inclined to look at ignition causes first. A good way to rule that out is to monitor each plug wire with an oscilloscope, it will detect these types of intermittent firing problems. Not sure if the average bike shop has a scope, but most automotive repair places would have one.

 

[Gymbo]

When you say 'mild miss' what exactly does that mean? I've seen ignition problems that are RPM dependent, kind of like what you describe, is the missing you're feeling similar to an ignition misfire? I'm no expert on carb issues, but I'd suspect that carb problems are going to be more like it won't run well at all at a particular engine speed/load, rather than intermittently run good/bad. If what you're feeling is intermittent power loss that lasts for a fraction of a second and recovers, I'd be more inclined to look at ignition causes first. A good way to rule that out is to monitor each plug wire with an oscilloscope, it will detect these types of intermittent firing problems. Not sure if the average bike shop has a scope, but most automotive repair places would have one.

When you say ignition miss...do you mean spark plug, wires, coil?

It misses from idle on up to around 3K....then either gets better or is less noticeable above that. It has what I call a "lumpy" idle. It has an idle exhaust note that sounds like lub-lub-lub....that coincides with the feeling of a miss. It could be an ignition miss. I am going to pull the plugs and inspect them when I have some time this week.

 

[Mark]

up to around 3k...

That sounds like clogged low speed jets to me...

 

[dwalby]

When you say ignition miss...do you mean spark plug, wires, coil?

It misses from idle on up to around 3K....then either gets better or is less noticeable above that. It has what I call a "lumpy" idle. It has an idle exhaust note that sounds like lub-lub-lub....that coincides with the feeling of a miss. It could be an ignition miss. I am going to pull the plugs and inspect them when I have some time this week.

Its kind of hard to describe in words, but an ignition miss is very abrupt, a quick transition on/off in power that is very noticeable. An ignition miss could be caused by a plug, a wire, or the coil itself. Its often RPM dependent because of varying pressure in the cylinder requiring more/less voltage to jump the plug gap at different RPM. When the pressure is at some critical level, the ignition misses, but at other RPM it fires just fine, that's why its often intermittent across the RPM range.

I've never had a jet get clogged, so I can't say what kind of symptom that produces, but others here have said that it might also explain your description.

 

[Blrfl]

I'm going to throw in my vote on a dirty pilot jet and possibly the main jet's needle. It could be an ignition problem, but I kind of doubt it since it runs fine at higher speeds.

Before you take the carbs apart, run some more Sea Foam at a slightly-higher-than-recommended concentration (i.e., put an extra ounce in the tank). Then run as much of the tank as you can in the RPM range where you have problems. This will make sure the dirty parts get as much treated fuel past them as possible.

It might also be worth "priming" the process from the top with a bit of carb cleaner, but I'll leave giving advice on that to those who know these carbs better than I do.

--Mark

 

[Coucours Grad]

Having had experience with C-10 Connies with the same malady I'd suggest pulling the tupperware and starting it in a a dark garage or shed (well ventilated of course). crank it and look at the spark plug wires and coils and see if you see any arching. I had a coil fail on the Connie several years back and after pulling/cleaning carbs twice I discovered that a coil was arching over to the frame.


But, If its sat more then 3 monthes without being run I'd suspect fuel related problem first.


IMHO YMMV

 

[George]

Dirty low speed jets until prove otherwise.

Pulling the carbs is not really an issue. Agree with Vinny, anything north of 50,000 miles and/or 10 years old, replace the boots too.

In the case of torn diaphragms (yes, there is a "g" in there), search this forum for how to fix them using a British piece or send your slieds with the torn one(s) to them for repair, much cheaper than OEM replacement and they claim a better material...

If it's running okay at speed, consider attending a west-coast tech-day...

 

[Gymbo]

I got an email from the PO last night....he says he cleaned all the jets...but didn't say when. He didn't change the carb boots.

So here's my plan. I am going to inspect the spark plugs tonight. I am going to order some parts and then pull and inspect the carbs. And if I am pulling the carbs, it looks like I should replace the coolant hoses under the carbs....at the very least inspect/tighten hose clamps.

So if I was going to pull the carbs...in addition to the "insulator" boots, what parts would I want to order in advance of tearing into the carbs. Float bowl O-rings?

Where/when is there going to be a west coast tech day?

I will search the for the British diaphragm fix......I am aware it could also be a torn diaphragm.

Thanks for all the suggestions guys! I feeling more confident about pulling the carbs the more I read.

 

[vinny]

At a minimum, order the float bowl O-rings in addition to the insulator boots.

 

[Falkirk]

At a minimum, order the float bowl O-rings in addition to the insulator boots.

Vinny,

Where can one order just the float bowl o-rings? From Honda, the carb o-ring kits have to be ordered at $30+ a kit. I can't see spending over $120 for 4 o-rings.

 

[Gymbo]

Vinny,

Where can one order just the float bowl o-rings? From Honda, the carb o-ring kits have to be ordered at $30+ a kit. I can't see spending over $120 for 4 o-rings.

Exactly my thought....is there an aftermarket supplier of float-bowl o-rings?

 

[Tom P]

Vinny,

Where can one order just the float bowl o-rings? From Honda, the carb o-ring kits have to be ordered at $30+ a kit. I can't see spending over $120 for 4 o-rings.

Yes there is a after market kit that works great and is about 15.00 for all four. I have used them. Company is in Canada I think. It is in a forum. Also remember, the primary jets that are probably causing the problem (mine were) are not inside the bowl, they are accessed from out side and are behind the needles with the "D" head adjustment. I cleaned all the stuff inside the bowl (which should be done) but I did not know where the primary jet and needles were. Put it all back together and it still missed. Took apart again, shot cleaner through primaries after removing needles and cleaned with single wire brush bristle, re-installed needles to 2 5/8 turns (Calif.) and no more miss.
I also replaced my diaphragms with the British ones. That process, re-installing carbs the easy way and alot of other great info is explained in detail on a recent forum. https://www.st-owners.com/forums/showthread.php?114430-lt-heavy-sigh-gt-Replaced-my-carb-diaphrams.-Again.

 

[Gymbo]

Minor update....

As I think I said in the original post....I had only owned the bike for 2 weeks when I made my original post...I've now had it for a month and had some decent weather to ride a little. I changed the spark plugs and it helped some, but didn't completely fix it. When I start the bike cold and the choke is on, there is no miss...it runs and sounds great. I can actually smooth out the miss in the lower RPM range by applying a little choke.

So I am fairly sure I have a carburation problem. It has actually cleaned up a little bit with some more fuel treatment and fresh plugs. The bike is dead smooth above 3K up to 6K with lots of power....that's as high as I've rolled it up to. So it appears I have a date with the innards of my carbs in the not-so-distant future. After doing a lot more reading, I am almost looking forward to it.

And thanks for the info Tom P...I planned on checking both the pilot and main jets...but thanks for reminding me that the mains are "hidden." In the midst of tearing into the carbs for the first time, I may not have remembered that.

The bike has a K&N filter, which I am going to replace with an OEM Honda paper element...I never noticed any performance difference with the K&N in any of my ST1100s, and the paper does a better job of filtration.

If anyone has the link for the Canadian source for the float bowl o-rings....post it up, eh!

 

[Tom P]

Minor update....

As I think I said in the original post....I had only owned the bike for 2 weeks when I made my original post...I've now had it for a month and had some decent weather to ride a little. I changed the spark plugs and it helped some, but didn't completely fix it. When I start the bike cold and the choke is on, there is no miss...it runs and sounds great. I can actually smooth out the miss in the lower RPM range by applying a little choke.

So I am fairly sure I have a carburation problem. It has actually cleaned up a little bit with some more fuel treatment and fresh plugs. The bike is dead smooth above 3K up to 6K with lots of power....that's as high as I've rolled it up to. So it appears I have a date with the innards of my carbs in the not-so-distant future. After doing a lot more reading, I am almost looking forward to it.

And thanks for the info Tom P...I planned on checking both the pilot and main jets...but thanks for reminding me that the mains are "hidden." In the midst of tearing into the carbs for the first time, I may not have remembered that.

The bike has a K&N filter, which I am going to replace with an OEM Honda paper element...I never noticed any performance difference with the K&N in any of my ST1100s, and the paper does a better job of filtration.

If anyone has the link for the Canadian source for the float bowl o-rings....post it up, eh!

Here is the link though it says they are out of stock. Probably have to call and talk to them. http://www.siriusconinc.com/pro-detail.php?pid=&product_id=3089

 

[Gymbo]

Here is the link though it says they are out of stock. Probably have to call and talk to them. http://www.siriusconinc.com/pro-detail.php?pid=&product_id=3089

Thanks Tom!

 

[Gymbo]

The folks from the link Tom provided above said they should have the float bowl o-rings sets back in stock in 6-8 weeks.

 

[Brant]

Minor update....

As I think I said in the original post....I had only owned the bike for 2 weeks when I made my original post...I've now had it for a month and had some decent weather to ride a little. I changed the spark plugs and it helped some, but didn't completely fix it. When I start the bike cold and the choke is on, there is no miss...it runs and sounds great. I can actually smooth out the miss in the lower RPM range by applying a little choke.

So I am fairly sure I have a carburation problem. It has actually cleaned up a little bit with some more fuel treatment and fresh plugs. The bike is dead smooth above 3K up to 6K with lots of power....that's as high as I've rolled it up to. So it appears I have a date with the innards of my carbs in the not-so-distant future. After doing a lot more reading, I am almost looking forward to it.

And thanks for the info Tom P...I planned on checking both the pilot and main jets...but thanks for reminding me that the mains are "hidden." In the midst of tearing into the carbs for the first time, I may not have remembered that.

The bike has a K&N filter, which I am going to replace with an OEM Honda paper element...I never noticed any performance difference with the K&N in any of my ST1100s, and the paper does a better job of filtration.

If anyone has the link for the Canadian source for the float bowl o-rings....post it up, eh!

When you stated opening the enricher circuit smoothed it out a bit, your problem is pretty much related to the idle circuit and pilot jets as others have already stated.