Honda Twins banner

1 - 20 of 22 Posts

·
Registered
Joined
·
202 Posts
Discussion Starter #1
I have a persistent high idle issue after my ‘68 CB450 heats up. The bike locks in on 1400 RPM at stops regardless of carb settings.

The bike has stock carbs, jetting, and air filters. The ignition is a CP electronic unit and the coils are new.

The bike runs strong, starts immediately, and idles perfectly from start.

I have been doing some tuning so the carbs are clean with new seals and I can’t find any air leaks by spraying carb cleaner around boots or throttle shafts. Carb needles are operating well and tested in all orientations with compressed air/straw method.

As I’ve been tuning, I’m checking the spark plugs after every ride and they’re looking pretty good if not slightly dark, but those results change with adjustments to float height. I was running rich with the float at 20mm, so I adjusted the floats and cured the issues aside from slightly too dark plugs.

Thanks for any insight.
IMG_2079.jpg



Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
2,520 Posts
Have you checked the timing with a light? If the springs on the advancer are a little loose, the timing can advance relative to what you set statically.
 

·
Registered
Joined
·
202 Posts
Discussion Starter #3
I have checked the timing with a light and it’s good. I actually tightened up the advancer springs a bit when I installed the E-ignition because I was having the issue prior to the install and the timing light seems to show the timing return from the advanced position. Do you think heat could change the advancer function?

I was having problems with the carb needle getting stuck at the top of its travel, but I believe I’ve worked that out.

I’ve also had the idle fluctuate a bit up and down when hot, but usually it just sticks at 1400 RPM. Adjusting float height seems to have calmed the fluctuation.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
2,520 Posts
OK, next step. Try oiling the felt seals on the throttle shafts.
 

·
Registered
Joined
·
202 Posts
Discussion Starter #5
I’ll do that. Just use motor oil?


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
2,520 Posts
Motor oil may be sufficient, but may pull through after a while. Some use 90wt, others force grease in. For a test, though, motor oil will do. If there is no overall effect, you will have to look elsewhere for a small air leak, like the gaskets between the carb boots and cylinder head. Thinking of that, you could check the torque on those bolts.
 

·
Registered
Joined
·
202 Posts
Discussion Starter #7
Okay, I have all sorts of grease too. I’ve sprayed solvent all around those areas with no noticeable change, but I’ll look at those shaft seals, I’ve never oiled them in all the carb cleans.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
2,520 Posts
They normally don't require any kind of lubrication, but as they age, adding oil or grease can improve the seal. Some have even replaced them with new felt.

Have you tried a vacuum sync? I, and others, have made vacuum port adapters from spare boots; others have drilled into the cylinder heads to make permanent vacuum ports. In any case, an attempt at a vacuum sync will reveal a leaking intake, because the sync will wander around with throttle position.

If the leak is at the boot to cylinder seal, though, it may not show up. Spraying a solvent at the seal may reveal a leak there. A sticking CV piston will be hard to find, too, and can be thermally caused.

Another thought: when it does this, have you tried putting your fingers on the throttles and pushing them down against the idle screws? One of the throttle controls between the carbs can rub against the clutch cable, and make it stick open just a bit.
 

·
Registered
Joined
·
341 Posts
Leaking valves can cause high idle. Because the idle gets adjusted up to get it to idle. They will idle fine until warm then blip the throttle and it hangs up high

Sent from my SM-J320V using Tapatalk
 

·
Registered
Joined
·
202 Posts
Discussion Starter #10 (Edited)
I don’t appear to have any throttle shaft seals. It looks like they would go in the recess on the throttle cable side and maybe against the idle stop arm on the idle stop side. The plastic washer with the shoulder looks like it could hold a felt washer. My parts catalog doesn’t show the felt seals, am I correct on placement?

Thanks


Sent from my iPhone using Tapatalk
314262
314263
 

·
Registered
Joined
·
2,520 Posts
If you look in the hole for the throttle shaft on the carb body, you should be able to see a thin felt washer; in that photo, I'm pretty sure you can see it, between the outer surface and that internal edge. Yours looks like it is black. They are not on the parts lists, nor are the throttle plates or shafts, as they aren't serviceable under normal conditions. Notice the four dimples around the outside of the hole that are peen marks that hold that cup-like part that forms the outside bearing, and holds the seal in that gap.
 

·
Registered
Joined
·
202 Posts
Discussion Starter #13
I don’t have a felt seal on the other side or on the shaft. Should it love in the shaft recess or on the shaft.

I have not tried a vacuum sync.
314282
 

·
Registered
Joined
·
2,520 Posts
There was no seal in the groove between the fiber washer and what looks like a welded-on washer in the first pic? Looks like there should be a seal there, to me. There seems so be a mark in the casting where a seal may have been, too.
 

·
Registered
Joined
·
202 Posts
Discussion Starter #15 (Edited)
Yes, I believe there is a felt seal where you pointed out, there’s a thin black ring there. I’ve seen replacements on EBay and perceived the seal to be bigger, so I missed that little sliver of a seal. I didn’t try to remove it, just greased it, but there appears to be nothing on the other side and I didn’t notice the same black ring or a recess where it would go.

This last photo is the opposite side of the carb, the idle stop side.

Should a seal be in the carb casting or on the shaft?


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
2,520 Posts
I thought the seal goes in the groove on the shaft; it's been so long since I had one open I really don't recall, but I'm not sure where else it would go.
 

·
Registered
Joined
·
202 Posts
Discussion Starter #17
Thanks for the help. I found a video online, the other seal goes on the shaft between the brown plastic shouldered washer and the metal washer. I’m definitely missing the seal. I’ve ordered some from eBay, hope they fit. Video link below.
 

·
Registered
Joined
·
2,520 Posts
I don't know if mother Honda did this, but I would consider pre-soaking the seals in a heavy oil; may help getting them over that metal washer without tearing. Good luck, and it should run better with new seals.
 

·
Registered
Joined
·
202 Posts
Discussion Starter #19
The c-clip comes off and the new seal just slides on. The one on the other side looks tricky.


Sent from my iPhone using Tapatalk
 
1 - 20 of 22 Posts
Top