There’s always a reason… find it! CL175 2.0 to completion.

Welcome to MrHonda’s mystery hour, where we delve into the unexplained behaviors of vintage Honda motorcycles, after normal repairs have been completed. In this edition, the subject in question is a 1969 Honda CL175K3, which had an engine overhaul and carburetor cleaning. The engine starts up easily but then loads up the spark plugs quickly with excess fuel.

Spark plug fouling at idle is generally attributed to either the wrong-sized pilot jets, high float levels, blocked air bleed passages at the carburetor inlet ports, incorrect ignition timing, high resistance spark plug caps, defective spark plugs (fake reproductions), low spark voltage due to failing coils, weak condenser, incorrect cam timing or aftermarket parts which are not made to OEM specifications.

Background information: The engine was brought to me in pieces for a rebuild, including the replacement of the kickstarter shaft. The cylinders were still on STD bore sizes, so the bores were honed and new rings were installed. The original valves were re-seated and all new seals and gaskets installed. It was pretty much a standard engine rebuild.

Once installed, the engine was started up, sounding rough as if it was running very rich at idle and just off-idle. It fuel-fouled the spark plugs in just a matter of minutes. The carburetors had been disassembled and cleaned, noting that one of the slide needles were marked D2, which indicates that they are from Keyster carb kit. The idle jets were also from the kit and marked #38, which is correct for this engine series. The main jets were marked #92, which is stock, but I have had issues with the accuracy of the kit parts in many cases. The bike is a Scrambler, with left-side high pipes, which have to be removed or at least loosened up so that the left-side cover, air filter and carburetor can be removed for service. The carburetors were inspected several times after each start-up with no favorable response to any adjustments done to the needle height or float levels. There was an odd situation where the left side carburetor fuel levels were greater than the right side, despite careful float adjustments. The petcock was checked as it was dripping, even in the OFF position.

The petcock had been installed without a sealing washer for the attachment screw that holds the petcock to the bottom of the tank. Additionally, the back side of the fuel lever was uneven allowing fuel to pass the barriers of the 4-hole petcock gasket. Once the fuel lever was flattened out and a sealing washer installed, the petcock functioned normally. Both fuel lines were matched in length to alleviate any issues with fuel flow to each carburetor.

Having removed, cleaned and inspected the OEM carburetors three or four times, I had to start looking at other associated problems that might have been overlooked or assembled in error. Starting with the basics, the compression readings were about 140 psi, which is a bit lower than Honda’s spec for a normal engine. Low compression readings, when both sides are the same, can be indications of incorrect cam timing. This engine had new rings, which were not seated yet, so some leakage was to be expected. A leak-down test showed nothing abnormal, apart from some air leaking into the crankcase past the new rings.

All indications were that the cam timing was correct, although Honda’s tune up book showed the intake valve opening at 10 degrees before top dead center, and actual valve opening appeared to be starting at about 20 degrees. This might look like the cam timing was incorrectly set, however most cam timing readings are taken at .040” so with that in mind the cam timing was correct. I actually removed the top cylinder head cover to verify that the marks were correct, which they were. So, take that off the list of possibilities.

I observed that the aftermarket replacement ignition points were contacting at the very edge of the contact faces, rather than in a more central location. A bit of bending of the stationary contact brought the contact back towards the middle. The gap was rechecked and timing adjusted to suit. The spark advance was checked for proper function in advance and full retard positions. The ignition coil was an aftermarket replacement with the original spark plug caps installed. The caps were measuring about 10k ohms, instead of the preferred 5k ohms, so were replaced. So far, no “smoking gun” was found to cause the rich running of what appeared to be stock OEM or replacement parts.

A set of aftermarket, Chinese-made carburetors had been brought down originally, so in desperation, they were installed. These replacement parts had their own issues. The “set” was actually two right-side carburetor bodies, so that both had the carb adjustment screws on the right side and fuel feed fittings on the left side. Both carbs had the same slides with the same cutaways, as that is what a right-side carburetor would be equipped with.

The carburetors were fitted to the stock intake manifolds which had been flattened off with my little belt sander to ensure flat contact surfaces. Once fitted, the bike was started up and immediately ran cleaner and idled down comfortably without the rich running that was occurring with the “stock” carburetors. At this point I was truly baffled about why the stock carbs failed to function properly, and a set of Chinese carburetors, which had no markings on any of the jets, were working normally.

However, the Chinese carburetors had a different issue. The next day, the bike was hard starting and only ran on almost full choke. As it warmed up, I slowly lowered the choke lever until it would start to stall out again, which indicated that the needles needed to be raised up a bit. The right side needle was a 5 notch type with the clip all the way at the top.. so super lean. I moved the clip to the middle groove. The left side needle came out and was a completely different type with only 3 notches! I went for the full rich option and restarted the engine again. Like magic, the bike took throttle, off idle, and was smooth and responsive in a 5-minute test ride.

I reported this limited success to the owner of the bike project, but could not give a cause for what had happened… until I went to put the old carbs into a box for storage.

Looking carefully at the carb needles, I finally realized that they were of different material. One was a brass D2 Keyster needle, but the other silver-colored needle was not marked at all. Comparing the two, the profiles of the needle tapers were different, as measured with a caliper. Ah, ha! Maybe I was on to something important, after all. Then, I flipped over both of the carb slides, which appeared to be the same, as far as the material and coating was concerned, but then was shocked to see that one slide was an OEM Honda part and the other one was not marked at all! Sitting them side-by-side, I could see that the slide cutaways were a bit different and then looking even more closely, the whole slide bottom, where the needles are located were at a different height! The aftermarket slide was a completely different part, as far as dimensions go for the needle and the cutaway. Eureka, I found it!

I was able to track down a set of OEM needles from an eBay seller, then went to cmsnl.com for the other needle, plus a new OEM slide, plus a pair of OEM idle jets. I now have confidence that with the correct OEM parts installed, the stock OEM carburetors should work normally, as designed. It’s been an arduous task with many hours of removing, inspecting, cleaning and installing the stock carbs over and over again, expecting a better result. I think that has been described as “insanity,” isn’t it?

I thought that this was the end of the story, however the bike had other ideas. When the fueling issue seemed to be solved, a long downhill run, a part of my testing process ended when the bike slowed down for a corner and then went completely DEAD! It had blown a 20 amp fuse. I had no other fuses with the bike and I was stuck a mile away on an uphill return. I called a couple of neighbors and neighbor Paul picked up the phone during his grocery shopping and said he would come and pick me up shortly.

I pushed the bike up an angled cement corner, made for wheelchair access, and left it under the shade of a palm tree. Paul came by, picked me up and dropped me at home. I loaded up the Tacoma and headed back down the hill to where the bike awaited my return. Loading a 280 lb. bike, even with my long ramp into the Tacoma can be challenging, but when I arrived at the bike a somewhat scruffy man was sitting next to it, accompanied by his dog. We spoke about the bike and what had happened. I could tell that he was looking for signs that this was my bike and I was the rightful recipient of it. He happily helped me load the bike into the bed of the Tacoma and walked back to his waiting Honda Accord, with the dog in tow. How amazing that someone came out of nowhere to help out just when I needed it the most!

I unloaded the bike and set to work finding the cause of the short. The bike only has one fuse, so when it is blown you are completely inoperable. I have a resetable relay connected to a set of wires with clips on the end. You can put them on the ends of the fuse holder and it will click on and off, when a short is present. Of course, it seemed perfectly fine all of a sudden. I did track down some worn wire for the rear brake light, which was repaired. Also, the fuse, in the fuse holder, should have been a short 15A fuse, but was a longer 20A fuse. The length of the fuse, caused the ends of the wires that attach to the fuse holder to extend out beyond the plastic fuse holder. That made them vulnerable to contacting a grounded frame area and create a dead short. I put a shorter fuse in the holder and pushed the wire ends into the plastic holder out of harm’s way. Nothing else was evident, so I turned my attention back to the carburetors.

On the next short run, partially down the hill, the bike ran well for about a minute, then went off on one cylinder. I slipped the clutch and revved the engine sky-high to try to get it back home again, just a quarter mile away. After it sat for a moment, it suddenly caught on again and I nursed it back home again. The problem was that the right side carburetor was running out of fuel, while the left side was doing fine. I rechecked the float level with the carb still installed and looked for any signs that perhaps the float was somehow contacting something inside and shutting off the fuel flow. I did another around-the-block test and it started to run on one cylinder again. No matter what I did, it wouldn’t keep fuel in the right carb bowl.

I had already bought another aftermarket petcock, in case there were problems related to the one that came with the bike. I drained the tank and removed the petcock. On more careful inspection I noticed that the two holes that feed the fuel lines to the carburetors were not on the same level. Wow! Could this be a factor in the fuel feed issue? The new petcock had a little screen on the reserve port whereas the original did not. Grasping at straws, I installed the petcock and hooked everything back up. Again, the fuel wouldn’t flow into the carb unless the bowl was dropped down and the float allowed to open the float valve.  The replacement petcock had its own problems. The space where the attachment screw is narrowed more than stock so the proper screw gasket (not included with the petcock) doesn't fit! I had to slowly grind down the OD of the gasket so it would fit snugly down and seal the screw head.

A friend suggested that I try another float bowl. The OEM carb bowls fit right up to the Chinese copies, but the result was the same. I removed the air filter and the carb from the studs to have another look at the bowl venting system and rechecked the float level with the carb held on the side, with just the weight of the plastic floats touching the needle. The measurement was different in that angle, so was re-adjusted again to 21mm.

Comparing the float bowls, I noticed that the brass overflow tube on the aftermarket carburetor had a really tiny hole. The carb had flooded over previously, so I knew that the passage was opened. But when I compared the OEM bowl to the A/M version, the original brass tube had about a 1.5mm hole vs about half of that on the copy. I opened the tip of the tube up with my 1.5mm drill bit and put it all back together once again. I turned on the petcock, waited a minute and then shut it off and removed the float bowl. It was FULL! 

I re-installed the air filter and side cover and went for a short ride just to test the performance. It was greatly improved and only hinted at a big of richness just off-idle. I removed the mismatched aftermarket needles… yes they were not a match on the same set of carbs, too! I compared all needles from both carbs and discovered that the D3 needle was match for the other brass needle and the two silver needles were also about the same. I dropped the needles down to the #2 groove and reinstalled them again. Suddenly, the bike felt and sounded like a almost normal CL175 Honda twin again. I brought my fuse tester with me, just in case, but made the full downhill test run back to the place where the fuse had blown before and then turned back around for the return leg. It was running well; as well as it could on aftermarket carbs and a mix of non-OEM carb bits.

Bill M. was on his way down to pick up the CL175 and leave me an XR75, plus the chassis for the SL100 engine that I had finished rebuilding. Obviously, I was very pleased to have finally conquered the worst of the bike’s problems and felt it was safe to drive regularly. I would feel even better once the OEM carb parts arrived from Holland. At some point the original carbs can be returned to the engine, for which they were intended.

I lost track of the actual hours of work devoted to analyzing and developing some strategies for a successful outcome. In the end there is a sense satisfaction that perhaps I learned something new, when facing similar challenges in the future.

Well, it certainly had me going round and round, unfortunately overlooking what was obvious in hindsight. It’s difficult enough to repair these old bikes given the lack of original parts, here in the 21^st Century. While the aftermarket vendors try to supply the needs of owners, unfortunately, the lack of quality control and design detail can cause a world of frustration, in cases such as this.

In situations, such as this, when nothing makes sense, even though you think that everything has been done correctly, you have to backtrack and dig deep into the minutiae of the smallest details. A small discrepancy can derail your best efforts to troubleshoot and diagnose the problem. In the end, you will find success.

Bill Silver

aka MrHonda

www.vintagehonda.com

08/2024