My Mikuni Carbs Don’t Like Ethanol
August 10, 2014This morning I rode my Bonnie to the North side of Chicago to visit an Anglican church with some friends. My gas was low, so I filled up at the local Thornton’s before hitting the road. I made it to the destination OK, and on the way home noticed it seemed to be running rougher than usual. I had plans for the afternoon, and when I got home I decided to see if I could figure out what the issue was.
First I checked the sync of the carbs… and they seemed to be a bit off. So, in the process of adjusting them, I noticed that when I would rev the engine up to 2500 RPM, it would cough, sputter and sometime die completely! I called my good friend Bill Gately of Bonneville Performance and he walked me through some troubleshooting steps. I checked the spark plugs and they seemed fine.
Given the way it was acting we suspected maybe the choke (enricher) was stuck. So, I removed the carbs and started looking at them more closely. I got the choke (enricher) cables freed up and working well. Then, I noticed the throttle rod seemed to be sticking and the more I tried to loosen them the harder they were to move.
I called Bill again and after eliminating some additional potential items, decided I needed to remove the slides and inspect them. Here’s what I found.
Mikuni HSR42 Carburetor Slides Removed – Both rubber seals were damaged.
As you can see, the rubber seals that help the slides “float” in their groves were damaged and stretched. When Bill saw this he asked if the gas I had used had ethanol in it… and I had to say that I thought it did. He said that I needed to avoid this in the future because Ethanol and rubber/plastic don’t go well together!
The rule of thumb he told me is that if the 87 octane gas has no ethanol, you can use it. If it doesn’t, it’s better to put in the next grade higher (89 or more) which usually does not have ethanol.
Bill is shipping me new seals tomorrow!
So, a valuable lesson is learned, and I’m happy that if it was going to happen, it happened when and how it did!
Update AUG 14, 2014
I received the new seals from Bonneville Performance and was able to install them and reassemble the carbs without any trouble. I did some research and am ordering backup seal sets from Sudco International and will regularly change them each year – just to be safe.
TM42/10 throttle seals
Order# 007-670
Price: $10.93
Rebuild Kit HSR42/45
Order# 002-989
Price: $45.00
I am working with Sonny over at Sudco… [email protected]
Hi Randy,
Have you had any additional issues with your carbs after this one.
I’ve got serious overflow leaking from the carbs and the worst thing is that they’re only three months old.
First they were leaking from the bowl overflow tube onto the pavment and after the service/cleaning, including a new pair of needles, they’re leaking from the top onto the engine, which is a serious fire hazard.
I’m really disapointed with these Mikuni HSR42 and to be honest the extra 10 horsepower measued on the dyno is not worth the lack of reliability. I had the OEM carbs for four years+ using the cheap petrol without any sort of issues with having the bike in storage during winter, these Mikuni just lasted THREE MONTHS, even the mechanic was amazed with the state the carbs were after three months.
Nuno, No I have not, and for me they have been more reliable than the factory carbs. Its sounds to me like you may have some faulty seals or gaskets, perhaps they were that way from the factory (it happens). Have you tried purchasing the rebuild kit and replacing all the gaskets and seals?
Thank so for the feedback Randy. They seems to be fine now, I had to basically fully rebuild them to ensure they were working fine. Even did a small experiment with them, which was disassemble everything and reassemble using the parts from another set of fully working Mikuni hsr42 carbs from an Harley that was on the shop and…they still leaked, a lot less and only if the bike was shaken or on the side stand. The solution was go down one size on the needles diameter and the floats level had to be adjusted because they seemed to get stuck randomly if the bike was on the side stand. It’s working at least for now with plenty of torque and I’ve also added a Fire Starter Performance Igniter Unit from Triumph Twin Power and also took the bikes performace to a hole different level. Out of curiosity what’s your average mpg on a full tank?
I, too, installed the Mikuni hsr42 carbs on my ’05 Speedmaster, along with D&D pipes, and airbox removal kit. I have not done a dyno, but it definitely boosted the power a fair amount. The one problem I have had on a couple of occasions is that the engine will run just fine for an hour or so, when suddenly the power drops dramatically. The engine keeps running, but even with full throttle it acts like it is being choked off. It will continue that way for several minutes, then suddenly jump back to life again! It has done that twice now. I plan to take it to my local bike shop and have them check into it.
Peter, I am pretty sure the problem is the rubber rings on each slide in each carb. Your symptoms sound like mine. They wear out by ethanal gas use as described in this article, and need to be replaced I would say yearly, to be safe. I have purchased replacements from Sudco International – sudco.com