The fix for the carb that just won't idle

[wonderboy]

So what do these two parts have in common?


Its the anti-friction coating. The engine piston skirts get this to prevent scuffing in the cylinder bore.

The carb slide valve has it to build the dimensions back up to offset what is lost due to years of wear.

Here is the story: I had posted a while back about an experiment I was going to try regarding the carb for my 84 200ES Big Red. The condensed version of the story: replaced every jet and servicable part with new Honda OEM parts, but it just wouldn't idle correctly. Read on some other forums that the slides wear and air leaks past the slide valve and causes the idle problems (hanging idle, surging, all around crappy running)

I found a place local that offers the service to coat pistons. http://www.dartheads.com/products/services/coatings . I called them and asked about my idea, and the guy on the line said he's done carb slides before. He went as far as asking me how much buildup I needed, and that he could tailor the application of the coating and give me very close to the buildup I needed. Measurements showed I had about a 0.003" difference between the bore diameter of the carb, and the outside diameter of the slide. We wanted to make sure to close it up as close as possible. He went a bit over and said that I could use some fine sandpaper to work it down to the proper size.

After getting the slide back, it measured the same diamter as the carb bore... it was line-to-line, and the slide didn't really want to slide in. This is exactly what we were shooting for. After about 5 minutes with some 1000 grit wet/dry, I had the slide moving perfectly in the carb. The final measurements show that the dimensions are still line-to-line, but that is most likely due to the cheap measuring equipment I'm using (telescoping gauge and micrometer setup). But, my equipment clearly showed a 0.003" inch difference before, which is clearly too much.

I had the chance to install it this weekend and try it out. After dumping tons of money into all the jets and other OEM parts trying to get this thing running to no avail, FINALLY this $20 investment has fixed my big red carb. The thing starts, idles, and runs just like I remember it doing way back when it was new.

I don't think there is anything special about the coating or shop that I had do this work, other than a guy who is comfortable and knowledgeable in the product to get you the proper buildup of material. If you are having trouble at idle and have exhausted all other options, give this a try.

[pipeline triker]

sounds great, thanks for the tip.

[coolpool]

That sounds like the fix I need for my 84 200es. That carb pisses me off to no avail. It only runs good when hot, which makes sense now if the clearences tighten up with heat. Thanks for the tip.

[fabiodriven]

This is something I've wondered about for years. I had the theory that the slide or slide bore could be worn out, but I never took any steps to fixing it. It's like you say wonderboy, you just keep replacing stuff and the thing runs like crap. I started to ask myself what parts in a carburetor could possibly wear out with time and usage. The only thing that I could come up with was the slide. I'm glad to see that you figured this out and it does in fact work.

[dougspcs]

Excellent deductive reasoning & beautifully explained....well done!!

A really credit to the group!!

[El Camexican]

Great story. That explains at least three carb nightmares I’ve had, but it also makes me wonder about a couple I’ve owned that have had scored and worn slides but ran like Swiss watches. Any time I’ve read up on carb maintenance the authors are very adamant about checking that the slides aren’t worn. I always figured that if the cap, needle and cable connection were well sealed it didn’t matter. I wonder if that is true and that the carbs we are having problems with are breathing past the needle clip, or cable coupler? Wonderboy, what condition is your cap and cable hardware in on that unit? Just curious.

[wonderboy]

I thought about the cable and cap at the top of the carb. For these to be a leak source though, the air has to first come in through the cable, but then it has to make its way to the engine side of the carb venturi. I think all the air leak paths that originate from the top side of the carb slide have much longer distances to travel relative to the distance air must move to go from the airbox side of the carb slide, to the engine side. If you study the carb without the slide installed, you'll see how very short the leakage path is.

Now, my cap and cable, and boot around the cable at the top of the slide is in very nice shape, but I don't think it is the path of least resistance if they are bad, meaning, I don't think they are a significant contributor to the problem. Of course, I could be wrong...

[El Camexican]

I thought about the cable and cap at the top of the carb. For these to be a leak source though, the air has to first come in through the cable, but then it has to make its way to the engine side of the carb venturi. I think all the air leak paths that originate from the top side of the carb slide have much longer distances to travel relative to the distance air must move to go from the airbox side of the carb slide, to the engine side. If you study the carb without the slide installed, you'll see how very short the leakage path is.

Now, my cap and cable, and boot around the cable at the top of the slide is in very nice shape, but I don't think it is the path of least resistance if they are bad, meaning, I don't think they are a significant contributor to the problem. Of course, I could be wrong...

Hi there Wonderboy. I agree with everything you say except the part about the air having to travel the length of the cable. I agree that particulate source is too far to matter and is likely sealed sufficiently with oil, or grease, but the leak I’m thinking of could occur where the cable housing end usually drops into an unthreaded fitting and SHOULD be protected with a rubber seal. If the cable is poorly routed, or the rubber is not sealing it would make sense in theory that the engine could draw air in, especially on the round slid Mikuni carb style with a slot cut for the cable to pass through. Being that that leak would only be a couple inches above the main jet orifice I’m guessing the vacuum would be pretty strong there. Thanks for your posts.