Loss of Throttle Responsiveness

So, I've done a little work to the old girl and now I need a little help. I've rebuilt the carbs, checked the plugs, changed the fuel filter, lubricated throttle cables, synchronized carbs to with 1/2 in of each other, cleaned the air filter, replaced the sub-air filter, replaced the boots in between the carbs and engine, removed all the rust from my gas tank and sealed it and replaced the fuel pet cock. So, after all that work, I fire her up and take her for a spin. I decide to take it easy, nice easy accelerations. After about 1/2 an hour of riding, I decide to get on it a little. I go to get on the highway and when I give it full throttle (or any position in between), my speed just creeps up. I try it in every gear and it's about the same. I don't have any problems when I accelerate nice and easy. But, when I get on the throttle, there's no power. Any suggestions?

 

I'll double check, but before I put everything back together, I made sure that I was getting spark at all the plugs. Thanks for the advice.

 

Problems like that can be tricky. It sounds air/fuel related. Try applying a little 'choke' when the problem occurs to determine rich or lean. I would guess your carb slides aren't working as they should. Maybe one of the diaphragms is tore...or not seated properly in its groove in the top of the carb? Did you check or replace the air cut valves? Are you sure the air bleed circuit to the main jet emulsion tube is open in every carb? There's alot of stuff to miss in carbs unless you're VERY thorough...and even then stuff can happen.

If you think the problem is ignition related it's possible to 'rig' a timiing light to the handlebars and watch the light. Find a low traffic area if you try that one...

 

It's probably not ignition related. I don't seem to be having any difficulties with that. Since I've recently cleaned the carbs, it's probably there. The diaphragms are definitely not torn. I checked and rechecked them to make sure. Not only a visual, but holding a light up to the back and looking for holes, nothing. Could be that one is not seated properly. It seems difficult to believe though, everything went together so smooth. Even when it didn't, I stopped, slowed down and worked it into place. When it finally did go, it felt right. That might be my next step, take everything back off and check the diaphragms again. As far as the choke goes, with the engine running, if I apply the choke, the engine dies out. This is what should happen, right? Anything else to check, or look for?

 

That sounds like a good plan. Thanks, I'll give it a try.

 

Leaving the gas gap loose you can eliminate the possibility of a tank venting problem (as long as the tanks not full to where it will slosh out).

 

check your fuel pump.

 

Not at all. The choke is an enrichener circuit plain and simple. If you apply the choke all you are doing is pushing more fuel through the carbs, and on idle you'll be revving upward of 3K rpms, in gear and moving it'll push you along just a hair more than the throttle normally would. If it's too much fuel when your moving the engine will compensate and simply dump the unburned fuel out the pipes and you'll get the occasional backfire pop.

I've got some experience on this one cause I accidentally ran my Rebel 250 on a 25 mile trip during the winter with the Choke on full and noticed that she was a bit more perky than normal but when I got off the ramp and released the throttle it would do a deceleration pop. Ooops!

Also, with the VFR not being sync'd I had to run with the choke on 25% in order to prevent her from stalling, so you putting the choke on should NOT cause the engine to stall.

I'm not sure what the problem is, and not sure what direction to point you in, but a flag popped up when you said she stalled on choke application. Hope this helps.

 

What I meant about the choke was too allow the bike get in the situation where it 'feels' slow, and GRADUALLY apply the choke. If for example there's a vacuum leak...you will probably notice the bike runs a bit better, that is...it'll feel like a cylinder that wasn't there kicked in briefly. If the problem is a slide not operating and blocking the venturi...it'll run worse or make no difference. The concept is to determine if the situation is caused by lean mixture.

On a normal running bike like this at operating temperature, at idle the choke will likely make it go 'blup' and die. Riding around if you abruptly apply the choke full blast it may not die, but it'll run like crap.

 

So I topped off the gas tank, threw in some Seafoam and took her out today for about a 100 mile ride. The same throttle responsiveness I noticed at the beginning is still happening. After I brought her back, I tried applying the choke slowly. She ended up 'blurping', and then died. I started her up again and tried it even slower and she 'blurped' alot, and after about 1/2 choke, she died. Here in the next few days I plan on doing Tink's suggestion (post #6) with checking the slides. It seems like this is the next logical step.

Betarace - checked, and yes the fuel pump is working.

tinkerinWstuff - I double checked the float heights before I put the carbs back together. They is no doubt in my mind that they are definitely set right.

captb - Haven't tried that yet, but will try something to make sure it's clear.

Thank you for all the suggestions. If there is any more, please let me know.

Adam

 

My two pennies in the ring, if you have or can borrow a heat gun and check the pipes for equal heat. If it just one being a dog it will stick out like a sore thumb and if they are all equal that is a clue as well.Or you can get your drunk friends or brother in law to lick the pipes, the one that can still talk afterwards licked the bad hole.

 

tinkerinWstuff - It is not difficult to get her restarted after she dies out. Also, sorry if I misled, it doesn't pop, she just glubs along and then slowly dies out. After I take the choke off, she fires right back up. I'll try a couple of more things today and let you all know. Thanks for the help.

Adam

 

So if I'm understanding correctly, the bike runs fine unless large throttle demands are made at highway speeds...in which case it responds slowly? When it does this is it jerky or just picks up speed really slowly? Is it so bad you lose speed and have to pull over briefly? From your experimentation with the choke it doesn't seem like a fuel jet is clogged as this indicates the bike is plenty, maybe too rich. Suppose it's a good idea to make sure the bike runs on all four via a temp test on the headpipes, but if I'm understanding the symptoms don't know how illuminating that will be without being able to monitor all four on a roadtest.

Tinkerin...Of course you can disagree but that doesn't change the facts. Yes...it's an enrichener on this bike but it still 'dumps' fuel in after idle. It doesn't have as much effect as at idle because of the much larger air flow, but the perceptive ear and throttle hand can tell. I've been using that little trick to help narrow down diagnosis for twenty five years now...and I do ALOT of carb work...matter of fact I have a bank on my bench right now and a bank sitting on the shelf waiting.

Now you've met someone who has seen and diagnosed not one but MANY fuel tank venting problems. In the mid 80's I use to replace fuel tank caps on ATV's 'left and right'...very common. And the newer tanks that vent through an internal pipe and external hose that runs underneath the bike are so tight, that if obstructed you'd be lucky to get five miles down the road, if the bikes fuel pump doesn't 'suck' the tank into the shape of a 'crushed' pop can...seen it.

I'm not saying AF's bike has this going on...but when you don't know what's wrong...you do the simple things to eliminate possibilities and narrow it down as much as you can.

 

Dizzy - yes, the bike runs fine unless a large throttle demand. It's all all speeds, and all gears, the throttle responds slowly. It's almost like you're in 6th gear at 45mph and then give it full throttle. That's about the only way I can describe it. It doesn't jerk at all, it's very smooth and picks up speed really slowly, no matter what gear. In the lower gears it does it a little faster but still slow. Imagine babying the throttle all the time, that's what it's like for me no matter how I manipulate the throttle.

 

The way you described the symptoms this time makes it sound more like you are just missing a cylinder. I concur with Toe and Tinkerin...check for a cold head pipe. You may want to remove and inspect the spark plugs also, If you have a dead cylinder it should show up pretty obviously that one spark plug looks wet and dark while the others are OK. If that's the case, try a new spark plug in that hole...it could be your whole problem. Plugs foul on occasion, even new ones.