Honda Twins banner

1 - 13 of 13 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter #1
Hey gang,

Recently got my first bike, 1970 cb350 and it started right up and seemed to run decently for a week. It was idling high in neutral but once I was riding and it warmed up, it would stall in neutral. It also wouldn't get past 5k rpms and would start to backfire around there. After a few more rides, it wouldn't hold an idle and then wouldn't turn on at all- no electric.

Being 50 years old, I decided to replace the coils, condenser, spark plugs, boots, voltage regulator/rectifier and checked all the connections to solve that issue. Definitely not the cheapest solution but I figured it would work. Electric seems fine now.

I then rebuilt the carbs, set the floats to 26mm, cleaned the heck out of them. The air filters look great being the bike only has 5800 miles on it. I adjusted the cam chain, the valve tappets and set the points. I synced the carbs to open simultaneously and put the air mixture screw out two turns.

It is now running but idling around 3k rpm and jumps sporadically higher. If I give it some gas it wants to stay revved up even higher.

The valve tappets are making a little noise. I watched a video on CMC that said to use a feeler gauge and get them about .002-.003 inches on the intake side and .003-.004 on the exhaust side (hopefully I'm describing this correctly).

I reset the points again using a light and they look spot on. A problem I noticed however, that I have not heard anyone else talk about anywhere, is that the right side point produces a tiny spark constantly. Here's a video of the spark problem: video

Not sure what my next steps are. Might try to cruise around the block tomorrow and see how she runs but I'm not too confident.

Thanks in advance you legends!
 

·
Registered
Joined
·
738 Posts
Did you keep the original brass parts for the carbs or use a replacement kit? If possible always use original brass pieces and new rubber pieces. Honda offers an OE quality gasket set as they call it.
If you reuse the brass pieces you must make sure the idle jet is clear. It is the one that always is plugged up. It is a small hole and to be sure it is clear I shine an LED light through it.
Set your floats to 27mm, set them according to the Factory service manual, it's available on another web site. The original setting was 26 but fuel today is a different density than the 70's.
Make sure the slide moves smoothly. I clean and polish the barrel with liquid bar keepers friend. make sure no fuel residue is on any parts.
Idle mixture screw spec is 1 1/4 .
Check for air leaks around the mounts to the engine and carb. 90% of the clamps I see need to be straightened and repaired.
Point arcing. Lots of crap condensors being sold these days. Also crap points. Did you set the timing according to the FSM? Check your grounds and check to see you have close to battery voltage at the coils when the engine is running.
The advance mechanism for the points is also probably in need to cleaning and lubrication. It should operate very smoothly. I lube the weight pivots as well. Be sure to observe how it fits together. It can be assemble 180 degrees out of phase.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #3
Did you keep the original brass parts for the carbs or use a replacement kit? If possible always use original brass pieces and new rubber pieces. Honda offers an OE quality gasket set as they call it.
If you reuse the brass pieces you must make sure the idle jet is clear. It is the one that always is plugged up. It is a small hole and to be sure it is clear I shine an LED light through it.
Set your floats to 27mm, set them according to the Factory service manual, it's available on another web site. The original setting was 26 but fuel today is a different density than the 70's.
Make sure the slide moves smoothly. I clean and polish the barrel with liquid bar keepers friend. make sure no fuel residue is on any parts.
Idle mixture screw spec is 1 1/4 .
Check for air leaks around the mounts to the engine and carb. 90% of the clamps I see need to be straightened and repaired.
Point arcing. Lots of crap condensors being sold these days. Also crap points. Did you set the timing according to the FSM? Check your grounds and check to see you have close to battery voltage at the coils when the engine is running.
The advance mechanism for the points is also probably in need to cleaning and lubrication. It should operate very smoothly. I lube the weight pivots as well. Be sure to observe how it fits together. It can be assemble 180 degrees out of phase.
Someone had rebuilt these carbs right before I got the bike and they did not have the original brass parts. I used a kit I bought from CMC. I'll reset the floats and clean the advance mechanism, check the coils too. What is the FSM that I should be checking the timing against?
 

·
Registered
Joined
·
738 Posts
FSM, factory service manual. Common Motors doesn't follow it very often with their videos and it causes issues.

If aftermarket brass pieces are in your carbs they will probably be running rich.

I can't tell you the website to go to for the FSM download because this website will block the link.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #5
I set the floats to 27mm and disassembled and cleaned the advance unit, made sure it was reassembled correctly. Started and kept revving uncontrollably up to 6k an then I'd hit the kill switch. When I gave it some gas and released, it continue to go up in rpm. I watch the line connecting to the carbs and it wasn't hanging up. After a few times of that, it won't start at all. Spark plugs are brand new and a little black. I have no idea where to go from here. This doesn't seem to add up.
 

·
Registered
Joined
·
2,866 Posts
A small amount of sparking at the points is normal. In the video it didn't look like the left point set was doing any thing. New points normally have a preservative coating on the contacts when they come out of the package, did you clean them before installing ? Roll the engine over to where the left side points are closed, put a piece of paper between the right side points(to save the battery and keep the right side coil from overheating). With the switch on, use a piece of wood or plastic(non metallic) to pry the points apart, you should see/hear a spark at the end of the plug wire. If you don't get a spark try shorting the points arm to ground, if that makes a spark the points need cleaned so they make contact.
 

·
Registered
Joined
·
2,316 Posts
You are fighting a vacuum leak. It could be the carb boots, the crapy poorly machined brass parts along with the cheap Chinese rubber parts that come in the crapy aftermarket carb kits, or the throttle shaft seals. I have encountered vacuum leaks with all of these items.
TOOLS
 

·
Premium Member
Joined
·
2,963 Posts
When you cleaned your carbs - Did you soak the entire Carb Body - Throttle Shaft still installed in Carb Cleaner?
There are FELT Sealing Washers on the Ends of the Throttle Shafts that SEAL them to prevent air (Vacuum) leaks.
Soaking the entire body will wash out any oil on them and cause them to dry out and NOT Seal.
I would suggest thinking about going back and oiling these felts seals or replacing them if they are in bad shape.

If/When you do this make sure you use ONLY a JIS Screw Driver on the Screws.
Work them back and forth as you remove them.
The ends were STAKED to prevent them from vibrating loose.
USE RED Locktite to secure them when you re-install.
Make sure the Throttle Plates are centered and the bevel installed correctly.

Also be 100% Sure you have SLACK in the Throttle Cables.
And Adjust them to pull in sync.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #9
A small amount of sparking at the points is normal. In the video it didn't look like the left point set was doing any thing. New points normally have a preservative coating on the contacts when they come out of the package, did you clean them before installing ? Roll the engine over to where the left side points are closed, put a piece of paper between the right side points(to save the battery and keep the right side coil from overheating). With the switch on, use a piece of wood or plastic(non metallic) to pry the points apart, you should see/hear a spark at the end of the plug wire. If you don't get a spark try shorting the points arm to ground, if that makes a spark the points need cleaned so they make contact.
I did clean them before installing but I will try what you have recommended for sure.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #10
You are fighting a vacuum leak. It could be the carb boots, the crapy poorly machined brass parts along with the cheap Chinese rubber parts that come in the crapy aftermarket carb kits, or the throttle shaft seals. I have encountered vacuum leaks with all of these items.
TOOLS
I was unaware of the throttle shaft seals, I'll take a look at them. Any way you recommend to narrow down the leak?
 

·
Registered
Joined
·
7 Posts
Discussion Starter #11
When you cleaned your carbs - Did you soak the entire Carb Body - Throttle Shaft still installed in Carb Cleaner?
There are FELT Sealing Washers on the Ends of the Throttle Shafts that SEAL them to prevent air (Vacuum) leaks.
Soaking the entire body will wash out any oil on them and cause them to dry out and NOT Seal.
I would suggest thinking about going back and oiling these felts seals or replacing them if they are in bad shape.

If/When you do this make sure you use ONLY a JIS Screw Driver on the Screws.
Work them back and forth as you remove them.
The ends were STAKED to prevent them from vibrating loose.
USE RED Locktite to secure them when you re-install.
Make sure the Throttle Plates are centered and the bevel installed correctly.

Also be 100% Sure you have SLACK in the Throttle Cables.
And Adjust them to pull in sync.
I did not soak the entire carb body because they were in great shape, had recently been done. I think you and TOOLS are both on the right track with a leak, potentially the throttle shaft. I'll look into this. Thanks.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #12
It may be worth noting that the exhaust on the bike (how I recieved it) is with a 2 into 1 system. Could this exhaust be a problem in the mix?
 
1 - 13 of 13 Posts
Top