REALLY bad flat spot off idle

[1974jh5]

Okay, my 1980 has been running great until recently. It's basically stock with a stock bore big fin cylinder, a Mike's 2 into 1, BS34's and pods, stock TCI ignition with Iridium plugs. I'm pretty familiar with motorcycle carbs, messed with a bunch of them over the years. These BS34's are almost exactly like Stromberg CD's as used on British cars and I'm pretty familiar with those, but this problem is whipping my ass.

While riding recently, I noticed at WOT the engine would start breaking up. It was definitely throttle opening related, the engine would pull smoothly all the way to redline unless the throttle was wide open. But off the bottom everything was fine. Then on a short hop I noticed it ran like crap just off the bottom for a few seconds, then cleared up and ran great. The next time I fired it up, it ran perfect at idle but would die as soon as the throttle was cracked. This was (is) so bad that it's unrideable.

Pulling the choke out to the first notch gets rid of the flat spot and makes it rideable so that tells me that for some reason the pilot and progression phase is lean. So I figured the progression holes in the carbs were probably clogged with something or other. I've been through the carbs twice and verified that the progression holes, the idle hole and everything else are clear. Still the same problem. (BTW, it's possible to kill the engine by screwing the idle scews all the way in but cranking them way out doesn't seem to have much effect on how it runs.)

Here's the strange part: if the engine idles for 30 seconds or so and you open the throttle the first time it revs right up, then after that each time you try to open the throttle the flat spot comes back worse and worse until it just dies. If the engine idles for 30 seconds or so, again the first time the throttle is opened it's okay then each successive throttle opening gets worse, like the progression and pilot phase are running out of fuel.

Valve adjustments are correct, the plugs are clean, it has the correct type pilot jets, I've been through the carbs twice to clean everything out, fuel flows freely from the petcock, I've raised the wet fuel level as a test, all to no avail. The carb boots are not leaking vacuum but the throttle shaft seals are. Could this be the problem or am I missing something else equally simple?

 

[jedi_]

to me it seems like you are running rich pilot wise. as the engine gets hotter it is revving up worse

 

[1974jh5]

But in that case wouldn't using the choke make it run a lot worse? Using the first choke 'click' helps tremendously. (I shouldn't call it a choke, it's actually an enrichment device with an air bypass that raises the idle.)

 

[CDNTX650]

what shape are your throttle shaft seals.

edit, read it all. change them. doing my bs34's now....actually, every o ring/seal/gasket is getting replaced in the carbs. figure if i have to go that far, might as well strip em right down to nothing.

Buy the right screw drivers for the butterfly screws.

 

[1974jh5]

The shaft seals and a set of screws are already on the way since I know for sure they are leaking. I'm just wondering if there is anything else I should look for. This progression system is a lot like a Weber or Dellorto sidedraft (DCOE, DHLA) and those will idle crappy if the seals are bad but won't hesitate. Maybe that's because they have accelerator pumps...

 

[CDNTX650]

more than likely. unsteady idle gave mine away, werent leaking bad....but enough.

 

[jedi_]

did you spray something on the leak and the bike ran better temporarily?

 

[glennpd]

Please check your float seat screen. After idling your float bowls are as full as they are going to get, when you goose it you drop the fuel level and it takes a while to refill.

Glenn

 

[1974jh5]

Shooting a little carb cleaner at the shafts where they come out made it run worse at idle. It's pretty well impossible to get the throttle to where the problem is and then be able to shoot at the shafts, I'm just not that coordinated. The carb cleaner got sucked through almost immediately which indicates it must be a pretty sizeable leak. It's worst on the left side carb which means it would be almost impossible to properly balance the carbs.

Oh, after the throttle is blipped a few times and all this business starts when the engine goes back to idle it will occasionally pop back gently through the carbs and the exhaust has that splashy 'lean' sound. If I wait a short time those symptoms go away until the next time it's blipped. I kept thinking big vacuum leak but the only one I can uncover with the carb cleaner is the throttle shafts. At one point I wondered if the pilot jets weren't fully seated but AFAIK they are.

I pulled both the float screens and cleaned them, then cleaned the carb passages to the needles as well. Nothing weird came out. FWIW, these 34's have threaded needle valves.

 

[CDNTX650]

did you spray something on the leak and the bike ran better temporarily?

bogs it out almost to where it stalls. Never will i buy an older carbed bike and not do the seals. First on the list of carb to does while they are off of the bike.

 

[1974jh5]

I suppose the throttle shaft vacuum leak could cause all this. The progression holes are at the TOP of the throttle bore not the bottom, meaning engine vacuum has its work cut out for it drawing fuel 2" up from the bowl to the top of the carb. If the holes were at the bottom it would be a different story. With the shaft vacuum leak, the throttle plates would be almost completely closed which would cut off vacuum to that part of the circuit completely, i.e. most of its idle air is coming in around the shafts not around the throttle plate.

 

[Zombis]

I found my throttle shaft seals were leaking with a propane torch with a pen flame off. worked fantastic to find vacuum and air leaks. Carb cleaner isn't so friendly to rubber.

Did you just tune it and it started to do this; or was it running fine and then all of a sudden this started?
Im trying to dial in my Carbs right now for MPG and the condition you described sounds like what was happening to me when i went up to a 45 pilot and hadn't adjusted the needle yet (it also had a shim under it so it was a bit too rich).

 

[dwain61]

It could be bad/torn diaphragms. If they're torn you can notice acceptable performance under light to moderate throttle and a lean condition when you crank on it because the slides arent opening. Often times diaphragms can turn to mush if they get hit with carb spray and sometimes they are just old and they rip. Pull off the top of your carbs and take a look.

 

[mouser]

a quick check of the diaphragms is to lift the slide with your finger then blocking the oblong hole in the throat. It should take 15 sec or so to drop when you let it go!

 

[I am Carbon]

maybe check the tank vent. it might be clogged.preventing good fuel flow

 

[1974jh5]

The carb diaphagms are in good shape. The tank vent... that's a good question. The cap is a new one from Mike's. That of course doesn't necessarily mean it's good.

The bike's been running fine, I didn't tinker with anything, this started all on its own. Many moons ago, a buddy had a Pinto with a Holley/Weber DD carb that started doing something similar to this and it turned out to be rust particles in the progression holes of the primary venturi, that experience is why I went straight to the progression holes on these. I sealed the tank with POR15 last year and was afraid that stuff had gotten into the carbs. But the holes are clear, carb cleaner etc flows freely both ways and the air tube is clear as well. Weird.

EDIT: Even though I could not find any holes in them, I went back and tested the diaphragms like mouser suggested. After 30 seconds they still hadn't dropped all the way, when I took my finger off they fell the rest of the way with a soft 'thunk'.

 

[1974jh5]

Stuck a set of BP7ES plugs in it today, no change. The only way it's rideable is if I turn the idle speed screw up to about 2k, which moves the throttle plate over the first progression hole. I'm tired of messing with it, I'm going to leave it alone until I get the shaft seals.

 

[Dusty]

Could this be a TCI problem? Some guys on here found this problem to be ign. Just sayin?

 

[1974jh5]

At this point, anything is possible. But I would think (and I've been wrong before!) that if the TCI pooped I'd lose everything over idle. If it makes it past the flat spot, it runs good. If the shaft seals don't fix it, I'll pull the circuit board out and start peering carefully.

 

[dwain61]

It seems to me that you either have an unwanted vacuum leak somewhere or, and maybe someone with a little more knowledge than me can jump in here, your spark isn't being advanced properly (pickup coil, bad pickup coil connection, advance mechanism).

I would tend to lean toward the vacuum leak because it seems that your bike needs more fuel to compensate for the amount of air it is receiving. Once the excess air is compensated for the bike runs fine. I would double check the Carb manifolds, vacuum caps, possibly throttle shaft seal.

A good way to diagnose a possible vacuum problem is by restricting the intake temporarily and seeing if the bike runs better. Put some tape over about 50-75% of your pods to restrict the air flow, see if anything improves. If it does then it's definitely a vacuum problem.

Hope this helps. Good luck man and dont give up on her, remember the good times