Twisting the throttle kills it

[cthulhu]

I recently put a used engine into my '90 Honda PC800, and it fired right up, I was stunned. Then I tried to put it into gear: any throttle at all makes it die. Actually a very light twist only makes it try to die, then it revs a bit. I can make it rev hard by repeatedly almost killing it with little twists. This is true when it's warming up with the choke on or when it's fully warmed up, no choke.

The fuel has no water in it, I just replaced the sleeves connecting the carbs to the cylinder heads because I was sure that they were leaking air (they were torn up a bit), the spark plugs are new...

Any guesses are welcome.

 

[joespeedfast]

ALL guesses are welcome.

Fixed. :|

Fuel starvation.

 

[cthulhu]

Praise be that it doesn't make the ass any smaller.

I forgot to mention that the fuel pump is working, the filter seems to be fine.

 

[masameet]

I'll play this guessing game!

Your main jet could be plugged. Or maybe the fuel strainer in the gas tank needs to be cleaned. Or maybe you need to connect or unkink some hose between the carb and the tank. And maybe drain the float bowl (in case some water's there).

 

[2strokeYardSale]

Torn carb vaccum boot dealie-o.

 

[cthulhu]

Thanks for playing, all!
I've drained the float bowls, had the carbs back off, in fact, to replace the torn sleeve(s) holding it to the engine.
There doesn't seem to be any vacuum deal here, except for a smog control bit that broke and left one hose with nothing to do but suck air. The consensus among experts I've consulted so far is that smog control devices don't matter, plug it and forget about it; plugging the little hose doesn't affect performance, but my next move is to try and repair the plastic thing (evaporative emission carburetor air vent control valve) with some epoxy putty.
I'll feel pretty stupid if it works, but I'll feel like a stupid person with two working bikes, so.

 

[07chuck]

Have you cleaned both carbs(don't forget to check the vacume diaphrams)? If not do so. When they are off remove the boots and check them for cracks.

If that doesn't work then test the fuel pump output.

 

[Ogier le Danois]

plugged carb jet.

 

[cthulhu]

Thanks to you both, as well:
When I was removing the carbs to clean them and found the cracked sleeves I was so sure that they were the problem that I lost my motivation to tear into the carbs to clean them. Actually I did one side and it was so nice and shiny in there it really felt like a waste of time. So now I'm in denial about having to yank them again and clean the other side (or both sides if I can't figure out which one I already did).

Thanks again: I'll post here with the eventual answer if I ever do get it working.

edit: The fuel supply really seems ample, but I'll have to look at that one more time before I part it out (heh).

 

[hayabusafiend]

Clogged jets or idle circuit. Tear down the carbs, unscrew and clean the jets and unscrew the idle air screws. Remove the float bowls. Remove the main jet diaphram and needle. Soak the entire carb in kerosene or gasoline or use an ultrasonic bath.

 

[Zerox]

Actually I did one side and it was so nice and shiny in there it really felt like a waste of time. So now I'm in denial about having to yank them again and clean the other side (or both sides if I can't figure out which one I already did).

Cleaning the float bowls doesn't accomplish much, you need to remove the pilot jets and clean them, remove the main jets and clean them too. No way would I have installed those carbs without cleaning the jets first, you missd a golden opportunity there while youd the carbs in your hands. I'm guessing your carb jets have varnish gunk in them left by evaporating fuel, happens to any carb that sits for awhile with fuel in it (unless the fuel is treated for storage with a product like Sta-Bil).

When the carb jets are blocked, a bike will probably start, but soon as you try to give it any gas, it will die. Does this scenario sound familiar?

You were smart to replace the carb manifolds, not only do they crack and leak but those things harden with age which makes it very difficult to install/remove with the carbs.

You also still may have left a vacuum hose disconnected, or pinched it under the edge of the fuel tank.

Hope it's one of these things, all are simple fixes. Electrical snafus are more puzzling.

 

[masameet]

Regarding ultrasonic baths, recently I learned that heated Simple Green, agitated gently over time (even a mere 15 minutes), will help to remove grease and hardened gunk from small metal parts.

But I guess the easy way is to use some carb cleaner.

 

[hayabusafiend]

Regarding ultrasonic baths, recently I learned that heated Simple Green, agitated gently over time (even a mere 15 minutes), will help to remove grease and hardened gunk from small metal parts.

But I guess the easy way is to use some carb cleaner.

I never thought of using Simple Green on carbs. I use SG all the time on my scuba gear. Good tip! Thanks!

 

[cthulhu]

RESOLVED!!!

Okay this wasn't a fair test for anyone.

I haven't been working on the bike: preserving hope by procrastination, but good weather and long evenings forced my hand.

I borrowed a device to synch the carbs and in the process looked at the diagrams for the California and [other] carbs again. I thought to convert mine, which just means pulling hoses that lead to the little plastic valves (one of which valves I broke months back). The first hose that I pulled I plugged, and the bike died. I opened it and it started working*.

Thanks again for your cerebral cycles. Tomorrow I get to ride my first bike again (after spoiling myself with the beemer).

Good times.

 

[ST Guy]

If the carbs are the one's that came with the used engine, they're most likely gummed up and clogged up with old/bad gas. If that's the case, I wouldn't have even bothered trying to start and run the engine until I'd completely gone through the carbs. If your old engine was running fine, AND if those carbs haven't sat around for a long time with deteriorating fuel in them, then you might use those carbs instead. But if they also have sat a long time with fuel in them, they'll need the same thorough cleaning. Good luck.