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Discussion Starter #1
First post here, great forum, lots of great info, thanks in advance for any replies!

Bike: Inherited a somewhat complete 1974 Cb360 (front drum) that had been stored for about 20 years, however it only had 3000 original miles and the bike appeared in decent condition so I dove in.

Issue: I still cannot get the bike to idle properly after it warms us. The bike will start right up cold, idle at first, run well above 2k RPM, but will progressively idle worse and worse as it heats up until it will not hold an idle after about 15 minutes of riding. At idle you can hear the bike start to miss more and more until it dies.At this point I know you're thing carb issues, but please read below.

Condition: Compression 150 on each side, no change in compression hot to cold. Bike had no airboxes or mufflers, added K&N RU-1822 filters and a set of shorty mufflers. Looking back I wish I had not gone that route, but lesson learned.


Work Performed (in somewhat chronological order):
- Oil change
- Cam chain adjusted
- Valves adjusted
- Advance serviced (and installed in the correct position, not 180 deg. out)
- New points and plate, static timed
- New condensers
- New coils, caps, and properly gapped plugs (tried B8ES and B7ES)
- New regulator / rectifier combo unit (Sparck Moto)
- Lithium ion battery
- New intake boots (Common Motor)
- New Petcock, fuel filters, and fuel lines
- New gas cap
- Carbs completely disassembled, cleaned all passages. Believe these were 745B bodies, screw in air jets (cleaned and reassembled in correct placement)
- Completely inspected diaphragms, no holes or rips, used flashlight etc. Looked good.
- Synced carbs
- Rebuilt carbs using original cleaned jets / parts at first, have since made a collection of jet combinations up to 115 main, 35-38 slow / idle, etc. Problem remains.
- Tried float heights of 16-21mm, no change to issue.
- Ordered a second set of carb bodies on ebay (press in air jets, cant remember number) that had been ultrasonically cleaned, thinking I had a clogged circuit still. Problem remains.
- In my desperation / frustration, ordered a set of VM28 carbs, intake boots, and Uni Filters, jetted by speedmotoco. Bike pulls nicely / smooth to redline, Still will not idle once hot, almost identical issue as with the CV carbs. Put original 745B carbs and K&N filters back on.

At this point I'm just reading threads and trying to think of where to look at next. I have not timed the bike with a light, and I did not test the impedance of the new coils before I installed them, which I will try to do in the next few days, however something tells me that I will still be where I am now.

Let me close with, this is my first vintage honda and between young kids, a wife, and work its entirely possible I am just missing something incredibly basic / idiotic on my part. Any comments are appreciated!
 

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Here are a few things you can do to eliminate some possibilities. Install new plugs, use an alternate fuel source (auxiliary fuel tank), unhook coils from the harness and run a wire directly to the coils from a known good battery (you can run another wire direct to starter to fire it up).
 

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Did you sync the carbs with a vacuum gauge or are you referring to the bench sync method? Since I am assuming you used a gauge, did you sync the carbs when the bike was warmed up as opposed to just started and cold? I prefer to sync after I take the bike on say a 5-10 minute ride, then park with a fan blowing on it.

Two things to try are:
1. Test the resistance of the ignition coils when cold and hot, see what the difference is there. When measuring across the blue and yellow wires (key off) you should get between 4 and 6 ohms. There is potential that when they get hot they malfunction.
2. (This comes from someone else not me) open the gas cap on the bike, the vent may be clogged which creates a vacuum in the fuel system making fuel not flow properly.
 

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Discussion Starter #4 (Edited)
Ran a 12v straight to coils from a known good battery, tried an aux fuel tank, and a new set of plugs. No luck.

The coils ohm out at 5 both hot and cold. The gas cap on the bike is new, but I still opened it up while the bike was floundering at idle with no help to the problem.

I also tried to again swap the CV carbs (which I had previously vacuum sync'd after bike was up to temp) for the VM28's and was able to replicate the problem equally with either set of carbs on the bike.

It seems like an electrical issue, but feeding 12v straight from a good battery to the coils gave it no help / change.

Could this be an issue with the new regulator / rectifier unit?

After taking a look at this thread: https://www.hondatwins.net/forums/50-electrical-discussion/128158-ironing-out-electrical-issues-misses-when-hot-6.html I think I will go through all my ground connections to make sure I have good contact.
 

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The Rubber Mounting boots that connect the carbs to the intake are notorious for leaking once warmed up.
It seems they soften and then the mirco cracks in the 30-40 year old rubber allows a vacum leak.

If yours were not repalced by you and have ANY indication of being more then 10 years old I'd replace them.
They aren't that expensive and are easy to remove and replace.
 

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he says he has new intake boots.

Hmm, head scratcher for sure. you took the reg/rec out of the equation once you hotwired the coils, so it's not that or the kill switch. you sure have a good seal on the exhaust gaskets? get a light to check timing while running and check your cam chain tension. have you pulled the top off and looked at your cam to make sure it's not wiped? and yeah, check those grounds. A leakdown test would tell you if your valves or rings are leaking. Did you get those ****ty aftermarket points or Honda ones?
 

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With the same hot idle stumble displayed using two sets of carbs and a new fuel system, I would then look into the electrical and ignition system. You say the coils, points, and plugs are new. Have you replaced the advance mechanism springs? When those springs are old and hot their tension will be reduced and allow timing to advance. Is the idle spark timing at the same mark when cold and when hot?
Second possibility is a weak charging system that will lead to lower voltage at idle and an erratic spark with wires that have more resistance when hot. Check the voltage at the black wire that leads to the coils, to see if the voltage drops when at idle and hot.
 

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Discussion Starter #8
Just wanted to say thanks for the replies and provide an update. Went through the entire electrical system, checking connections and grounds, swapped the battery and it seems the issue has been resolved. Not sure if it was the battery or some connection I serviced, but all is well that ends well I suppose.
 
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