Keihin CV34 Throttle Advance Adjustment

The carbs were actually pretty clean, each jet was cleaned individually also. They've been fully flushed and all of the channels have been blasted with carb cleaner. In fact, I even tried running with a new set of slightly larger jets to see if things were just too lean and the performance was similar but smokier and stinkier. The old jets are now back in after I came to my senses and realized that I shouldn't be changing jets on a stock bike instead of finding out what's actually wrong. So the carbs have been pulled apart and cleaned four or five times now, I just haven't pulled out the diaphragms yet because unless I know I've got a problem with them I don't want to touch em because I have yet to find a source for replacements, and I have fat clumsy fingers But they're likely next on the list of things to check.

I left out some history that might also be relevant. The reason for the valve adjustment and carb syncing was because the year prior the top end of the motor was off to replace a valve. I had a low oil situation one summer and it resulted in a valve getting bent. Prior to the top end work this bike would start up like clockwork. I could follow exactly the same procedure every time; set full choke, press starter button for 3 seconds, wait ~20 seconds, set choke to 1/2 on, wait 2 minutes, ride away. I'm confident I could have started it with my eyes and ears covered, it was that consistent. Now it's a 5 minute wrestle with throttle and choke positions before it starts to behave.

One thing that might be staring me in the face is that although my vacuum gauges say they carbs are all well balanced the vacuum on all of them is quite low. The manual says they should be around 22cmHg, but mine read around 5cmHg when the bikes cold and go up to about 13cmHg when the bike's hot, and they all change the same way, I'd think they were all connected to the same carb if I didn't know better..... I wonder if my cam timing is wrong..... but I've put over 5000km on this bike since the last time the cams were out.

So next two things to check are cam timing and diaphragms.

Glad to hear you found the advancer problem and the difference that made.

As to the cold starting - no offence intended - but I wonder if a slightly dirrerent technique might work.

I have similar carbs, Keihin CV34 and the 1980 E1 model

Cold start I set choke on full, pull clutch lever and hit start button. It will fire right up and I might on occasion blip the throttle at this point. Within maybe 15 seconds the motor will start to stumble. Does yours do the same ? At this point I turn the choke down a tad. Maybe a quarter and the idle will smooth out. I think full choke is rich enough to start but to rich to run. By the time I have jacket helmet and gloves on I can set the choke at half and at this point I ride ( Others will say let it warm fully but I dissagree). If I`m riding in town stop light to stop light it might be another 10 minutes before choke lever goes to zero.

If this does not work then I`d suggest going back to the carbs and thoroughly cleaning the jets, pasage ways etc and then setting the fuel service levels (search clear tube method here). Having the carb fuel levels right on spec makes a huge difference to starting and running.

Good luck

Edit. I guess I was typing while you were replying above so you can disregard this

gottcha. I hear ya. But... i guess what I am saying is, if you did not pull the diaphragms from the bodies... then you have not soaked the carbs in a carb dip, or pine sol / lemon juice or whatever. I prefer the paint can of carb dip that you buy at the auto parts store.

Your choke passageways are located on the top there along with other passageways. I dont mean to sound like a stickler. But when i pull mine apart... i really take em down to the nuts/bolts and soak everything. It takes me about 4 days to get it done, As I leave each body in the solution for 24 hours. (i should pick up another can of dip) but im cheap.

Anyways. If yours are good. - you said you completed a valve adjustment. If all your valves are in tolerance, and you did not mess with the cam chain... I would go on a limb and say your cam timing is in tact. I think the problem is somewhere in the carbs. Whether they are dirty, or something in the equation isn't right.

If all your valves are in tolerance, and you did not mess with the cam chain... I would go on a limb and say your cam timing is in tact. I think the problem is somewhere in the carbs. Whether they are dirty, or something in the equation isn't right.

Exactly !

I've seen 10 cmHg across all 4 carbs (Morgan Carbtune) as a result of one carb being so far out it was dragging the other 3 down
Dip, spray (patton has posted some great diagrams), poke with guitar string or nylon bristle, blow with compressed air, examine diaphragms, set air mix screws all same, set service fuel levels and try synch again when fully warmed up and idling 1100 rpm minimum

So I should clarify, that I haven't taken the diaphragms out in these last few times but I have taken them out for cleaning in the past. Since then every time I've taken the carbs apart they've been spic 'n span,

Your quote actually leads me to believe even more I have a cam timing issue. Prior to having the valve replaced this bike started up exactly the way you described yours does, what you said even matches when we get our gear on Since I reinstalled the head and reassembled the bike It's been acting like this, and the vacuum has been low.

Nothing on the carbs was touched during the rebuild, they sat in a box, dry, for a few months while it was being worked on but other than that they were the same, but it's been lousy starting ever since.

Recently I've gone down the carb path to try and solve it but with no changes to speak of. So far I've:
- Cleaned the bowls (although there wasn't really anything to clean)
- Cleared the jets with a tip cleaner
- Removed and cleaned the pilot screws and blown out the passage with carb cleaner.
- Cleaned the float valve seats
- Bench synchronised the throttle butterflies
- Set pilot screws back to factory setting (2 turns out) Note: the idle screws haven't seemed to do anything, ever, in all of this experimenting
- Set float levels with clear tube method.
- Tested with one jet size up.
- Synchronised carburettors using gauges
- Tested for leaks around both sets of boots with carb cleaner, propane, and ether... (not all at the same time )
- Felt the exhaust headers to see if any warm up faster/slower. No cold ones, all seemed to heat up at the same rate

It's also worth pointing out that it's been like this for more than a year now. I've put over 5000km on this bike since the rebuild, when it started acting this way. All the plugs look the same, none look hot or fouled.

I'm not ruling out a contaminated carb yet, but I think I need to verify the cam timing just to reassure myself at this point.

In your carb synchronizing experiences, have you known a bike who's vacuum increases substantially as it warms up? Mine pulls 10-12cmHg when it's hot, but when it's stone cold it usually pulls around 3-5cmHg.... which when I think about it is absurdly low, no wonder it barely starts....

I wouldn't bother synching carbs when cold.. best when fully warmed after a good ride min 30 minutes.
If the cam timing shows to be fine then my suggestion would be to go back to the synch and be prepared to fiddle around. A few years back I had what appeared to be low vacuum 10 cm Hg showing and it took a while fiddling until one came up and then the others followed.
Sorry i can't be more specific than that

I knew the carbs were good but the vacuum was wrong and I just kept at the synch and ended with all in the region of 21 cm having started at approx 10 cm. I did have a big fan blowing on the front of the motor.

Checked it tonight and sure enough, both cams were one link behind where they should have been.

Set it right and then fired it up with the vacuum gagues attached and it settled down around 19cmH, everything stayed pretty balanced. But most importantly it idled without me having to help it along! I don't have to try to put my helmet and jacket on with one hand anymore!!

Glad to know you finally solved it. So in the end it was cams timing only right?

Seems that way. Nothing else was making any difference but once the cams were set right it fired right up with the choke on and idled easily. The weather's cooled down again here, so it might be a little while before I can get out for a test run, but it certainly feels more right.

So I finally got out for a test ride and almost everything is well. It ran well for about three hours of riding. Smooth power from idle all the way up to 7k, I'm real happy about that.

Then the bad news, after putting about 130 km on the bike, I left it parked in a parking garage while I ran an errand. When I came back it started right up and idled smoothly, but when I tried to pull away it died abruptly. It was as if someone flipped the kill switch. I was able to start it up again immediately and it idled fine but died again once I tried to pull away. After a great deal of immense frustration I was able to get moving if I revved the engine super high before pulling off, but if I eased back on the throttle enough it would start choughing and sputtering and eventually die, or drop back to idle if I pulled in the clutch. When I was up to speed it seemed ok, but a soon as I slowed down it seemed to get into this zone where it wanted to die.

The fact that it seemed ok at idle, and ok at speed, but bad going between the two is made me suspect that I have a pilot screw problem. I took the carbs off yesterday and had a look at both the diaphragms and the pilot screw o-rings. The diaphragms look fine and clean, but the pilot o-rings look pretty beat up, likely from all of the previous cleaning. I reassembled them and tried blowing air around the pilot screw from the top (atmosphere) side to see if I could feel any air leaking past and on at least one carb I could tell a little was getting through.

For brevity other things checked were:
- Fuel service level (clear tube test) - OK
- Fuel flow rate from the bowl drains (both PRI and ON) - OK
- Fuel flow rate from the tank (both PRI and ON) - OK
- Vacuum petcock operation. - OK

I've ordered new pilot screw o-rings, but I'm wondering if anyone has had experience with leaky pilot screws before and if they'd cause that cutting out off idle situation like I described?

Well, the o-rings didn't help, it was dying exactly the same way. But new thing happened, one of the times it died it wouldn't start cranking to start up again. Turns out that if I made right turns in just the right way it would quit.
Well frig.....