Carb Diaphragms ( ST1100 ) *

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Carb Diaphragms

Original article on Mike Martin's web page here: http://home.insightbb.com/~mmartin36/CarbDiaphragm.htm

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Help - My Bike is a Slug!

Well, as some of you know, my bike wasn't running properly at WesTOC XI, with the complaint being a lack of power. The bike started and ran normally, but when I needed passing power, there was nothing there. There was no noticeable misfire or bucking, but the exhaust note did sound different at wide throttle openings. As I had entered Colorado from Kansas, I determined that top speed was about 20 mph lower than normal. I had a big trip ahead of me, so I discussed the symptoms with my friends when I arrived at the rally site in Golden, CO. With a lot of help from a number of interested parties, I tried to fix the problem in the parking lot of the hotel.

First: I cleaned and re-oiled the K&N air filter. No improvement.

Second: I took the vacuum fuel shut-off valve out of the system. No improvement.

Third: I removed the ignition coil wiring and cleaned all the connections. No improvement.

Fourth: I installed new CR8EH9 spark plugs. No improvement.

Fifth: I added Berryman's carb cleaner to the fuel tank. No improvement.

Since the bike was running OK otherwise, I decided to tolerate the problem until I got home. The consensus was that there was a carb problem. I didn't want to risk working on the carbs until I was in my own garage. Sue and I put almost 8000 more miles on the bike on the remainder of the trip. The bike was still a slug (I had to downshift to even make it over some of the mountain passes), although it started and ran OK (as long as I didn't try to go too fast), and fuel economy seemed close to normal.

I pulled the carbs off the bike when I got home. I feared tearing them apart and not find an obvious culprit. The float bowls had a little dirt in them, but only a little. I couldn't see anything plugging the jets. The screens for the needle jets were clean. So I decided to pull the throttle slides to see if the diaphragms were OK or the pistons were binding.

Voila!!! Three of the diaphragms were holed. Pictures (click to enlarge):





Considering the size of the holes, it's no wonder the bike was down on power. One cylinder had to be doing most of the work!

The fourth diaphragm wasn't holed - yet. It does show some surface cracking, though. That evening I ordered four new ones - $72.44 each at Zanotti. The Honda part number is 16111-MT3-610.

Sue gave me a hug and said, "Merry Christmas".

So a great big "Thank you" to everybody who pitched in to try to solve the problem at Golden.

Here's a couple of photos Steve Wendelboe took for posterity:




Shortly before I left for WeSTOC XI, I had found the small foam sub filter had disintegrated. There's a write-up about that problem
here. As a result of that, coupled with the split diaphragms, there was a noticeable amount of dirt in the carbs below each split diaphragm. The sub air filter may have failed partly due to the extra air that was being pulled through it because of the failed diaphragms, who knows?

So now the bike is back to normal again. Yippee!!!

Last updated on December 27, 2007     © 2007 M. E. Martin, all rights reserved.

Thank You again for your contribution Mike Martin

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Comment by Rob Parker:

If your ST1100 is getting up there in mileage and you think the performance is not quite what it used to be, you may want to inspect the carburetor diaphragms. At 178,000 miles, all 4 of mine were torn. I had experienced a decrease in power across the rev range and top speed was down to around 100 mph. Four new diaphragms (PISTON COMP., VACUUM 16111-MT3-610) restored things back to normal. Attached is a photo of what you might expect to find. IMHO eventually all ST1100s will be needing this service. I think it would be a good idea to check them whenever you have the carbs out. It pays to shop around because I found prices on line ranging from $79 to $145 each.



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Comment by George Catt:

I just had two sets of hi-mileage carbs apart last week. No tears on either set of diaphragms. Ya gotta get in there to check, tho, so Rob's advise is good.

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Comment by Michael Martin:

FWIW, I'm told that using spray carb cleaner will damage the rubber parts.