Honda Twins banner

1 - 20 of 41 Posts

·
Registered
Joined
·
12 Posts
Discussion Starter #1 (Edited)
Hi there!

A bit of background: My 1973 CL350K5 was running well and a few weeks ago I stripped lots of parts off to clean, replace old gaskets, rebuild brakes etc. During this process I didn't touch the engine but installed new stock air filters, replaced the rectifier and regulator with a new combo unit and rebuilt the carburetors with a kit from 4into1. Everything went well and I put it all back together without a problem. The only parts I changed in the carburetors were the small jets which I switched from 68 to 70. The large jet is 105. Float height is factory 26mm.

Now the engine idles very well, battery is fully charged, timing and points gap set, carbs are synced and fuel mixture is 3/4 turn out. BUT when I am riding and I accelerate around 6000rpm in any gear the engine stops producing power, sounds strained, lurches in and out, backfires a bit and won’t go above 7000rpm. Seems like it might be starved for fuel. Below 6k the power seems normal and it rides very well. Spark plugs looks decent. Dry, charcoal grey with a bit of brown/tan on the tops.

My first thought was that this has something to do with my new regulator/ rectifier combo unit not producing enough voltage back into the charging system. I put the old units back on but the problem persisted so I have ruled that out for now. I think that this isolates the problem to the carburetors since I didn't mess with anything else. I have taken the carburetors off twice now to check that everything is properly installed and it all looks good. They are clean, and the diaphragms are in good condition.

Because the problem starts at around 6000rpm, it leads my to believe that the problem might have something to do with the main (larger) jet which to my limited knowledge kicks in at about that point. Should I try going up a size? I am not sure what the next steps are for a diagnosis. It has occurred to me to rebuild the carburetors again with all the old parts and see if that brings it back to normal. One note: to make it easier to get the tank on and off for the time being I looped fuel hose from the left side over frame to the right side. It has made my life so much easier...could this be causing the problem by somehow restricting fuel flow?

I hope someone can shed some light on this! I am new to working on the carburetors and I am sort of at a loss. I feel a bit dumb because it was working well before I made any changes but I guess this is how you learn...

Look forward to hearing back!
Jake
 

·
Registered
Joined
·
703 Posts
I don't think its a fuel starvation issue - if it were, you'd get one or other cylinder cutting out as the fuel level in the float bowl drops, which wouldn't happen at consistent revs. Re-routing the fuel-link pipe might cause problems if it has an airlock in the middle, but you wouldn't get problems until the fuel level gets low, and you end up running on fumes on one side of the tank, with fuel stuck in the other and not able to get through the link pipe. Certainly won't happen at a specific point in the rev range either.

Could be fuelling, but does sound more like ignition/advance issues to me - notwithstanding you haven't done much to the engine.

For peace of mind - and a simple diagnostic check - I'd switch the old carb parts back in and see whether the symptoms change or disappear. Also if they're anything like the 450 - might just be worth double-checking the fuel hose routing from the fuel-taps to the carbs, to remove one other variable from the equation, just in case :)
 

·
Registered
Joined
·
12 Posts
Discussion Starter #3
Thank you for the reply!

Tomorrow I will make sure that the carbs are getting proper fuel and then move on to the "re-rebuild" of the carburetors if I don't find any problems. :)

If it is an ignition issue, how do I go about diagnosing the source of the problem? And what would cause something rpm specific like this? I had someone else tell me to try a new condenser unit. As I said before timing seems to be spot on after I adjusted it using a test light.

I should mention that originally I had the floats set at 23mm (suggested from a video) and the problem seemed to start at a lower rpm range...more like 4k. Once I changed the float height to factory spec 26mm is when the problem started around the 6k range. I don't know if this is related but it might help with the diagnosis.
 

·
Registered
Joined
·
12 Posts
Discussion Starter #7
Interesting! I didn't know any better... Thankfully the original parts are in pretty good condition except for the jets which a PO mutilated. You think it's ok to use the new jets?

I'll donate the rest of my brass for your hammer :)
 

·
Registered
Joined
·
346 Posts
If it's your last resort there's not much you can do other than use those unless you can find OEM on eBay, but I don't think you'd find HONDA OEM since they didn't offer them as parts I think. Don't worry, I bought 3 different kits before I found out.
 

·
Administrator
Joined
·
13,089 Posts
Unfortunately the jets are the focal point of most aftermarket kit mistakes, and float needles are probably second
 

·
Registered
Joined
·
12 Posts
Discussion Starter #10
Hmm. I might be able to use the old ones. I'll inspect them again. It just looks like someone tried too hard to pry them out with pliers. I'll switch it all out tomorrow and let you know how it goes.

Thanks for the all the info!
 

·
Registered
Joined
·
698 Posts
I haven't had issues with the 4 into 1 kits, after you rebuild so many carbs you do run out of usable parts.
Couple of things ... how did you set the float height ... that seems to be a problem area and can cause the sort of issues you are having if it wasn't done properly.
How clean is the petcock filter?
I chased a similar issue and it turned out to be a voltage problem at the points. Using a volt meter compare your battery voltage to the voltage at the points or the coils if the tank is off. It should be close. If you can do this while riding even better but you need to do some solid connections. The ignition voltage goes through the key switch, up to the kill switch and back to the coils.
On the bike I had there were voltage drops at the switches and the worst was a wire that was almost broken in the wire harness at the headstock.
 

·
Registered
Joined
·
12 Posts
Discussion Starter #12
I set the float height by bending the little tab that rests on the float needle. I had the carbs upside down sitting flat with the tops taken off and measured using a ruler from the edge of the float bowl to the highest point of the float as it rested on the float needle. How could the way the float height is set cause my problem?

This morning I checked the voltage at the condensers and the left and right points. Voltage at the condensers with engine off (ignition on) was 11.9v and around 12.3v-12.6v with the engine on and would get close to 13v when engine revved around 5k rpm. The points both measured around 9v at idle to 13v at 6k rpm. Seems normal?

The fuel flow seems fine from the petcock (I rebuilt that around a year ago) so I imagine that it is still clean. I also sealed the tank so there is no rust.

Right now I am rebuilding the carbs with the old parts and I should be able to try them out by the end of the day.
 

·
Administrator
Joined
·
13,089 Posts
The issue with setting the float level with the carb upside down is the weight of the float depressing the spring-loaded pin in the float needle and not giving you an accurate level setting. You're supposed to hold the carb at an angle so the pin in the float needle just touches the tang of the float, but with no weight on it
 

·
Registered
Joined
·
12 Posts
Discussion Starter #14
Just opened up my left carburetor and the diaphragm was loose and overlapping slightly on the top of the slide. Oh boy. I carefully pushed the edges back into the slide with a small screwdriver and it looks normal now. I don't know if I missed this the first time around or if it happened in the interim but I think that could definitely be the source of my problem. I'm gonna set the float heights the correct way and then put them back on the bike without changing out all the parts and see if my problem is solved.
 

·
Registered
Joined
·
346 Posts
Haha. That could definitely be the issue and I wanted to suggest you check your slide needle to make sure it wasn't broken, but didn't since you had been in there so much. I bet you fixed a big part of your issue.
 

·
Registered
Joined
·
12 Posts
Discussion Starter #16
No luck. The problem persists. Next I will be putting all the old parts back into the carburetors.

Interesting note: while I was riding, I decided to open the choke (restrict air) and I was able to accelerate past 6k rpm even though it was a bit rough. What could this mean?
 

·
Registered
Joined
·
346 Posts
Probably means you need to properly jet the carbs. If you can't go past 6k it sounds like your slides are sticking or your needle isn't right. I had an issue where i was perfect idling, perfect up to 4500, then it would sputter and act like I was pulling a trailer, then randomly pop to 7k and run fine. I changed out needles to OEM, my float valve for one with less tension (went from the 4into1 to OEM) and changed from 105s to 110s. Now I get the front wheel off the ground just shifting into second.

try getting compressed air or your mouth and blow onto the back of the carbs. If you can't raise the slide with your lungs, the carbs can't raise them either. The passages are the two smaller holes at the top inside the mouth.
 

·
Administrator
Joined
·
13,089 Posts
No luck. The problem persists. Next I will be putting all the old parts back into the carburetors.

Interesting note: while I was riding, I decided to open the choke (restrict air) and I was able to accelerate past 6k rpm even though it was a bit rough. What could this mean?
Just so we're all on the same terminology, you closed the choke - that's what restricts the air flow to help it start when cold. Good way to find out if it's running lean or rich, and since it seemed to improve it a bit then it points to a bit lean
 

·
Registered
Joined
·
12 Posts
Discussion Starter #19
I was confused about the correct way to refer to the choke since when it's closed the lever is "open". Thanks. :)

I don't believe the slides are sticking. I have tried blowing into them and they seem to be operating properly. I'll be changing out the parts on the carbs tomorrow and we'll see if bringing them back to stock makes a difference. I may try going up a size with the primary jet as well.
 

·
Premium Member
Joined
·
39 Posts
I had a similar issue and after some trial an error new spark advancer and new points did the work. Now the bike revs all the way to red line both with and without load
 
1 - 20 of 41 Posts
Top