Drilling Jets

I decided to get myself setup to drill out the main jet on the Mikuni mounted on Salty. As it happens, I also have been dialing in the tuning on the same model of carb mounted on the 140cc chinese engine I have in my Trail 55 rat bike project.

I came across this fantastic page, which saved me the time and effort of converting the numbers and matching up the drill sizes:

http://www.affordablegokarts.com/Drilling%20Main%20Jets.php

As luck would have it, I had picked up a cheap drill bit set from Harbor Freight last month, just to fill in some gaps I had with other drill bit sets.

Salty's carb had a 95 main jet, and is acting as if it too lean. I've had to move the needle clip to the bottom most position to get it to pull at part throttle, yet it totally falls flat at wide open throttle. My spark plug is also too light in color after I did a few spark cuts.

Ratty's carb came with a 97.5 main jet, and has always acted very lean at part throttle application. Almost like a lean surge you would feel with a weber carb on a car when it was too lean.

So I made up a little aluminum block fixture to mount the main jet in, which fits nicely in my bench vise. I made a one jet size increase at a time (or as close as I can get) so I used a #60 drill bit for Salty, and a #59 for Ratty. The process was very easy and quick. I can remove the carb, take off the bottom half and have the main jet in my fixture in less than 5 minutes.

Unfortunately, I've had no daylight hours to run either bike since I drilled the jets. And the weather has started to turn a bit rainy this evening. Hopefully I can get out on the road this weekend and try to dial things in.

C'

More Jetting Notes

So...I was completely wrong in thinking I was running rich in the mid range. After adjusting the needle clip up two grooves (making it leaner), I went for a ride. It was terrible. It wouldn't take throttle at lower rpm, and would lose power at 3/4 throttle. I was able to clear it up by adding some choke, which adds fuel.

I rode home, and adjusted the needle clip to the lowest of the five groove positions, making it full rich. It responded instantly. It now runs great through the rpm range, even capable of pulling from a stop in 2nd gear. It is amazing how much of an effect such a small change can have.

As a reminder, here is the diagram of the needle clip arrangement:

The only issue I'm seeing now is I'm puking a bit of oil out both the main crankcase vent, and around the dipstick. Now I see why they made aftermarket breathers that screwed into the dip stick opening. I'll have to scan ebay for such a device.

C'

Mikuni Carb Adjustments

I've been tinkering with the jetting on Salty the past couple days, and have had a couple old Honda enthusiasts show interest in how this larger 24mm Mikuni will work on a small 50cc Honda. I'm curious too, and have only begun to start messing with things.

First, I wanted to link everyone to this page: http://www.iwt.com.au/mikunicarb.htm

This is a fantastic primer on how a typical Mikuni motorcycle carb functions.

The first two times I've run Salty, it was very boggy at lower rpm. This was compounded by the fact I have it geared very tall, using a 16-tooth front sprocket, a stock 40-tooth rear sprocket, and the taller 2.75"x17" rear tire. I'm also running a 1/4-turn throttle mechanism, which reacts much quicker than a typical slide throttle.

Pulling away from a stop wasn't easy. It would bog very badly unless I gave it a little Fred Flinstone action to get rolling. It also ran very poorly at low rpm in second gear, when trying to pull away from slower turns.

So I started by adjusting the pilot circuit, as this has the biggest effect on low speed, and low throttle opening running. Usually this screw is set at 1.5 to 2 turns out from all the way in. In my case, it was set at 1.5 turns out. After getting the bike warmed up, and with a decent idle, you simply turn the adjustment screw in or out, and you'll be able to hear/feel a change in idle speed. In my case, it was rather dramatic, as turning the screw out (making it leaner) raised the rpm quite a bit. If you go too far, the rpm will start to lower. So you get a feel for where to set it based on rpm. In my case, I settled on 2.5 turns out. I then adjusted the idle speed back to where I wanted it.

It immediately improved throttle response. Even with the bike just sitting in neutral at an idle, it was very boggy before. Now it just revs clean with no hesitation. I then went for a short ride. It was dramatically improved, especially pulling away from slow turns in 2nd gear at lower rpm.

I now need to turn my attention to full throttle use, as it isn't quite right. I have a feeling I'm quite rich overall, especially since I needed to lean it out so much even at small throttle openings. When I'm cruising at mid to upper rpm in second gear, it pulls very nicely as I open the throttle. But I notice the last 10-15% of the throttle opening makes zero difference to acceleration. If anything, it feels like it stumbles a bit. A check of the plug indicates it is running rich. Going from 3/4 to full throttle opening is right in the transition range between the needle and main jet circuits.

I don't have any spare main jets, so I'll need to order some. In the meantime, I thought I would play with the needle jet position (1/4 to 3/4 throttle mixture) by adjusting the needle clip position leaner. I'm currently set in the middle of 5 clip positions. so I'll move it up a clip and see what that feels like.

If it isn't too cold tomorrow, I might go for a short ride.

C'

Riding Salty for the First Time!

After many months of little issues, combined with the fact I built up another complete bike (the Trials bike) I finally got back to Salty the past couple days. I had it running, but not very well many months ago. It was running very strangely, and sporadically. Sometimes it would kick over first try, other times I would wear my leg out trying to kick it over. If I got it running, I could never get it to hold an idle without needing the choke fueling circuit on, which of course raised the revs up high enough. So basically, I've been troubleshooting little issues and not having a lot of luck isolating the main problem. It wasn't fun dealing with the same problems everytime I turned my attention to it, so I moved on to other projects.

Yesterday I decided to take off the complete carb/manifold/fuel line and do a thorough cleaning. I drained the tank and didn't see any sediment. I built up a new fuel line with a small fuel filter, which it didn't have before. I also disassembled the carb and gave it a douche, even though it looked spotless inside. I put everything back on, and it fired up fairly easily. It still hunted a lot while trying to fast idle. Then, as I was glancing down at the ground, I hurt it sputter and I saw a little puff of moisture shoot out around the base of the intake manifold I had built. It kept sputtering, and shooting out. It was obvious there was a little crack in the intake manifold where I had welded it together. For a refresher, here is the manifold I built:



The "left" side of the manifold is where it bolts to the cylinder head. It is a very tight fit between the pipe and the nuts that secure it, so I had to file away a bit of meat. I also apparently didn't do a fantastic job welding it in that area, as the crack formed right there.

Guess what? I did a shitty job welding two intake manifolds. I had built a test manifold before this one, to get all my angles and dimensions correct. So I grabbed it out of the bin, and bolted everything back up. Salty fired on the first kick, and ran great for about 2 minutes, holding a perfect idle and taking throttle. Then I heard it stumble and saw a puff from the same exact location! What the hell. I think I'm just cracking what little weld is there as I tighten the nuts to secure it to the head. Two manifolds down. But at least I maybe found my mystery problem?

Instead of trying to fix both manifolds, or make a new one, I thought of some better solutions. One of which uses a stock C110 intake manifold, which I happen to now own three of. The two C110 parts bikes I grabbed from Ashland for $50 each had one. The problem is, this manifold is designed to use a small section of rubber hose that clamps to a round section of the stock C110 carb. The Mikuni I'm using has a bolt-on flange. So I have a few options. I can make up a new flange and have it welded to the stock C110 item. Which may be the best long term solution. I don't weld aluminum, however, so I'll need to find someone to do that for me though. I'll likely have two built.

The short term solution was to hack off the carb side of one of the manifolds I built. This side has a much wider hole spacing for the two mounting bolts, so I have a very good welded connection there. I then connected this piece to the stock c110 intake with a piece of 7/8" ID fuel hose I had. It is a very snug fit, and with a few ugly hose clamps it fits and feels secure.

It worked too, as the bike fired right up, held an idle, and even held a nice low rpm idle with no choke after it warmed up. It takes throttle well too. So of course, I grabbed my helmet and realized I was going to actually ride Salty for the first time. I was never able to get it running well enough to want to try.

So I did. I blasted up and down the road a few times, going through all the gears. It actually runs on the road now! I was very excited. It wants to bog a bit right off idle, but if I'm smooth with the throttle and give it a quick paddle or two to get it rolling, I can accelerate off. The engine in the bike is a 3-spd auto clutch, with a large front sprocket. So first gear is a bit tall, and I think the 1/4-turn throttle I have on it is a just a bit too quick reacting. I'm sure I'll get a bit smoother as I ride it more.

So there you have it, Salty not only seems to run, but it goes down the road. It isn't the most comfortable bike in the world, but it is actually doable on the road. I just fold up my long legs, hunch over, and go for it. I'm going to fiddle with a few more things this afternoon, and hopefully take him around the block more before the rain comes this weekend.



C'