While waiting for my new Denso plugs to arrive Ive goth the carbs off and on the bench again! Every jet and fuel gallery is clean and flowing aerosol cleaner. However, I’ve noticed that while the slides are free moving in the carb body’s when I couple them up to the diaphragm plate with the diaphragm placed in the carb they tend to hang up even before I put the carb top on? could this be causing my problems? I know the vacuum and suction when the motor is running will move them but with the carbs not having a return spring surely they should drop under their own weight?
Your thoughts please gentlemen
I noticed this exactly phenomena on a set of Honda Constant velocity carbs. The carbs worked fine on the bike so my theory is that gasoline actually lubricates the slides when the motor is running (i.e. so there's no 'sticking').
Ok, further “none” progress” checked the carbs AGAIN! all jets and passageway’s are clear and I’ve now got the slides dropping under their own weight. I’ve set the fuel level at 7mm below the float bowl seating surface with the carbs clamped (lightly) level in my machine vice. Put the carbs back on the bike and connected everything up. Fitted new Denso plugs and started her up unfortunately there is no difference? I bought a set of ignition lighters and they showed a constant spark at a very lumpy tickover and a clean spark as the revs build up so I’m guessing the ignition side is ok although I haven’t checked the timing yet. If I pull the choke on full she seems a LOT better which would indicate a weak mixture plus when I knock the choke off the revs rise then she settles back to the lumpy tickover. It’s like a slow speed misfire? I took it out for a short run and as before above 2k it runs perfectly but refuses to tickover and I have to really slip the clutch to pull away from stationary. I checked the plugs after the run and they didn’t show any signs of a really lean mixture. Yes I know all this points to blocked pilots but they are definitely clear.
I think the next step is to check the timing and then alter the fuel level to richen the mixture. Any thoughts people?
I hate being a parrot and constantly repeating myself, but everything points to an idle (pilot) circuit blockage. Adding a little choke and it runs a bit better plus the mention of a slow return to idle from higher rpms definitely points to a lean idle circuit.
Now that being said, there is a possibility that maybe the intake vacuum at idle is really low. There's lots of reports of guys trying to sync their carbs and the intake vacuum at idle reported down at 7-8 in mercury. Scotch can chime in here. He just rebuilt his engine and I think his intake vacuum at idle jumped up to 10 In HG ?? With a 270 duration intake cam and a 50 deg overlap intake/exhaust and open exhaust pipes, I would think that if the carbs aren't perfect, it's going to be tough to get a smooth idle. But you reported that up until a week ago? it was running fine then something happened.
Did you check the cam timing yet? It might have jumped a tooth when you stopped the bike on the last good ride. A loose cam chain and a little bit of a backfire when shutting down would do it. Dodges and Chryslers back in the 70's were well known for this to happen. Car comes in and the customers says "I don't know what happened. It was running perfectly yesterday and today it won't start". How many times I heard that one.......
A one tooth jump on the cam sprocket is equivalent to 20 degrees crankshaft timing and a 270 degree intake cam if not timed "in the zone" won't run worth a shit.
The other one could be the ignition/water pump drive chain jumped a tooth. Not much discussion on the site about that chain or tensioner but the tensioner is the same set up as the camshaft drive chain so I guess it too could act up on a shutdown backfire.
Last but not least, someone ( I think it was Scotch) found a larvae shell planted in the pilot circuit air supply port at the bell of the intake bell on the carb. The pic he had up in the post showed the half larvae shell and it looked like maybe a hornet planted it.
Throwing it out there for consideration.
Post Note Found it. Check this out (and it was Scotch 5years, 8 months ago) The shit I remember but don't ask me what I had for breakfast today !@#@!#$@!#
Mystery Blockage idle port
Thanks KB, I’ve not checked to see if the camchain has jumped a tooth yet but that’s on the “to do” list. I’m going to raise the fuel level next and see if that changes anything. You mentioned about the vacuum? How is it possible to sync at a higher vacuum? surely the vacuum “is what it is” at idle? am I missing something (I usually do) but how can I adjust the vacuum I sync the carbs at?
again thanks for your patience and time taken to reply, it’s very much appreciated
p.s it’s definitely not the ports in the intake that’s blocked
You can't adjust the intake vacuum. It's a result of cam timing and rpm. Ignition timing will affect it marginally and I think that is a result of when the mixture is burned and expelled causing a draw on the intake at valve overlap to initiate the flow of the intake. A 220 deg-240 deg camshaft will have a very positive intake and I would expect intake vacuum up in the 18-21 in HG range. a 284 - 310 intake has so much duration that intake vacuum is hardly achievable below 1500 rpm.
So when the intake vacuum is really low, it's really difficult to tune the carb at idle. You need a positive vacuum signal to get the fuel to flow and the higher the vacuum draw, the easier it is to dial in the jetting.
When I asked about the intake vacuum, it was because it tells me things about the state of the engine mechanically. I always like to go back to basics and confirm all that information before trying to understand drivability issues. If the initial ignition timing is off or the valve timing is off, or the compression is off, the engine won't run at idle. and that's where we are trying to get you going. As an example, maybe the ignition timing at idle is advanced 20 degrees from where it's supposed to be. First thing I would look at is a seized ignition timing advancer that got stuck at a high rpm setting and didn't spring back to initial timing. That's why as an example, we need to do all the basic testing first before figuring out the issue. Confirm the basics first. It's a 40 year old machine with maintenance issues and lack of use. (not attributable to you)
Thanks again KB, being honest I’ve never taken notice of the numbers when balancing carbs I just even them all up at tickover. I’ll redo the balancing again at higher revs maybe 2k where she runs sweet and report back. I’ll also check ignition timing and lastly remove the cam box cover and check to see if the chain has jumped a tooth. I’ve been thinking about the fuel levels and as the plugs show no signs of running lean I’ll leave them alone for the time being. No sense in changing too many things at once, I’ll just end up chasing my tail.
ive just been reading through some of the carburettor related posts and in one of them it mentioned three transition ports/holes in the carb throat near the butterfly, I’ve only seen two in mine? I’m wondering if my third port is completely blocked which is why I can’t see it? can you confirm if there are two or three ports please?