Project Honda CB450 Cafe Racer: How To Sort Idling Issues

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Although the bulk of this bike is complete, I’ve been working on getting the bike to run smoothly.

What does smoothly mean? Well, it means that it idles steadily and evenly and has predictable throttle response.

After I sorted out my electrical issues, by replacing the 55W H4 bulb I upgraded to back down a 35W, my battery holds a charge. That’s checked off. It starts most of the time, and if not, I just leave it on the Shorai BMS for an hour to top off the battery.

But I’ve found that the bike idles very poorly, no matter how much I fiddle with the mixture and idle screws. To diagnose the problem, I followed these steps, which finally got me the answer. If you’re struggling with these sorts of issues, read on:

Problem

The bike starts with a little throttle and variable choke adjustment. Sometimes it starts with half choke, full choke, or no choke. When it starts, I’ve got to hold open the throttle for about 30 seconds to really let the bike warmed up and the oil moving. If I don’t, the bike will idle at 1,400 rpm and slowly drop down before it cuts out.

On idle, one cylinder feels to be running rhythmically and with good pressure pulses, while the other has very weak pressure. An assessment of header temperature (holding my hand close but not on the pipe), tells me that one side feels warmer than the other.

When I ride the motorcycle and snap back the throttle to accelerate, the bike bogs for one or two seconds and then takes off. One exhaust still sounds louder than the other up until it starts pulling. You can hear the motor bog down, as well as feel that there’s laggy throttle response.

Troubleshooting Poor Idle

It took me awhile to determine that one of my cylinders wasn’t idling. I thought maybe the whole cylinder wasn’t firing due to lack of compression or lack of spark.

The first cheap test was swap the spark plugs. Still happening.

I fiddled with the air mixture screws on both sides, on one side adjustment caused an audible effect on the motor performance. On the other, all the screwing in the world didn’t cause a thing.

I fiddled with the idle mixture screws. If I adjusted one side, it had a big effect on the idle RPM’s. If I adjusted the other, the effect was less.

Check for air leaks in the carb and intake manifolds by using a hand held propane torch. You just let the propane gas flow out (NOT LIT) with the bike running and run it over and around areas of your manifolds. A noticeable spike in RPM’s indicates an air leak. If your carbs are dry and cracked, either replace them or refinish them with some Rubber Renue and some Black RTV Sealant like I did. And make sure your intake gaskets are still good.

I ran a compression test earlier and knew that one side was lower than other but not drastically, it should still function.

I checked my fuel lines to make sure they were getting enough fuel. To make sure nothing was restricting flow, I even pulled off the fuel filter that went to the side in suspect (the left side in my case). Okay full flow, still a problem.

Pulled the bowls, checked if there was enough gas. Pulled the other bowl to make sure that there was an equal amount of gas. Emptied both bowls, hooked them back up and let them fill again. They filled up equally. Started the bike, let it run crappily. Cut the fuel at the petcock, and turned the bike off immediately. Pulled the bowls, and found the left side had more fuel than the right. Okay, now we are on to something.

This told me that the left side carb just wasn’t taking in fuel at the same rate as the right. If this were another issue, I’d say my right was taking up too much fuel. That could be the case, but I’m considering my right side to running properly, since it’s actually running. Baby steps here, people.

I pulled the pilot jet on the left side and inspected it. Looked clear. I then pulled out the idle jet tucked up inside and behind the pilot jet. Looking through the jet at a bright light, you should see a pinhole of light peeking out. I got nothing, meaning it was clogged. Since this thing is super tiny, I sprayed it really well with some carb cleaner and then used a single strand of stripped electrical wire to chase the hole out. I repeated this for the side ports too. I then blew out the holes from the backside outward. Screwed them back in and now I have good idle on both sides.

Adjustment of the idle and air screws had reasonably the same effect on RPM level, and when I pulled each bowl after idling the bike, they were about equal.

If the problem persisted and I wasn’t getting a good idle on that side, I would then confirm that the points timing was correct. If that didn’t solve it, I would then assume the worst and consider I had blown piston rings.

Troubleshooting Poor Throttle Response

So it idles but I still had laggy throttle response. I made sure my carbs were synced and I was getting equal and dynamic action from the my throttle cables to the bodies. I also dropped some lube down the cable line to make sure it wasn’t sticking up in the line (Yes, brand new cables need to be lubed!).

Since I had swapped to Uni Pod Filters and kept on the slightly-baffled-basically-straight slash cut mufflers, I knew the jetting might be off. More flow means more gas. I checked my main jets and they were 145’s. These were the original jets from the PO, that I opted not to replace with the smaller main’s that came with the rebuild kit. But I had a feeling they weren’t enough now that I pulled the airbox for pod filters.

I ordered up some 150 mains and 155 mains from Dime City Cycles. When these came in, I popped in the 150’s and saw an immediate and HUGE improvement to the throttle response. Now it pulls when I say pull, with some slight bogginess. I jumped up to the 155’s and saw it respond even better. Not perfect, but better. But I know from research, that with pod filters, you’ll never get perfect response since you won’t have equal vacuum across both carbs. I’m okay with this, because I prefer the look of the pod filters. Gotta make sacrifices.

I hope some of you guys found it useful. Yes, it was long winded to just say “clean your jets and size up”. I read that a lot but its often useful to find the actual troubleshooting and diagnosis to getting there. It helps make a better decision and rule out variables (I’m a scientist after all).

Cheers!

4 Comments

  • Nicely done. Carbs and electrical are usually the source of problems. I have a 73 CL450 and run velocity stacks from DCC with great success. No rejetting was necessary but if I really wanted to tweak it I could play with the jetting.

    • Haha thanks! I’m glad you didn’t have to sort out issues with your 450. I find that most of the old timer’s or Honda Twin gurus love that “I told you so” feeling when all of us newbies have issues running pods and pipes. I don’t blame them though.

      cheers

  • With motor hot and at idle, turn the screw inward (clockwise) SLOWLY until the motor starts to falter. If the motor will not idle on its own when you begin this procedure, bump up the idle set screw until it will. Throughout this procedure try to keep the RPMs at or below 1000. Now having turned the screw inward until the bike falters, back it out slowly, making a mental note of the position on the clock. Turn outwards until the motor begins to run smoothly, then adjust idle stop screw as necessary to bring RPMs down, but not TOO low. Evo oiling systems need better than 700 RPM to work properly. Blip the throttle a time or two and observe the results. If the motor responds with a gratifying blast without backfiring through the carburetor, you have your idle mixture right. If it backfires through the carburetor (“carburetor farts”) you will want to back the idle mixture screw out another 1/4 turn. Do not go too far, as too rich an idle mixture will certainly cause you many headaches and poor gas mileage.

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