HELP! CR80 carb/fuel problem

Honda CR80 with mikuni / pump around setup runs fine on the stand but under load with me in the seat it runs for about a minute or so and then Peters out and wants to die. The carb has been cleaned the fuel pump was recently rebuilt and appears to be pumping fine. As far as I can tell everything is tight and right but clearly I’m missing something. Anyone got any ideas?

What’s your spark plug says? Exactly after the problem appears?

His plug will say he’s not getting enough fuel; that should be obvious given it runs fine unloaded and dies under load.

Assuming this engine ran fine at some point with the current setup and this isn’t a new install, and you haven’t screwed with the jet and needle recently, try replacing the fuel filter and make sure the pulse line isn’t cracked. Also regarding the carb cleaning… I’ve found that the best way to 100% ensure the carb is cleaned properly is to use Berryman Chem-Dip. That and ultrasonic cleaning are the only ways to be positive the carb is completely cleaned. I usually soak the carb overnight. If you cleaned the carb by any other means, go grab a can of Berryman at any auto parts store, take the carb completely apart, throw all the parts (rubber and plastic won’t be harmed) in the basket and let it soak over night. Rinse them off with water and blow dry, then reassemble.

As a side note, Berryman can also restore fouled plugs and will decarbonize parts such as cylinders and heads when you’re doing a top end.

Are you running a pump-around setup?

It was cleaned ultrasonically twice the pulse line is new and you’re correct it ran fine before the 4 months of rain we had and nothing was touched or altered in any way from the way it’s always ran.

Thanks \m/

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Generally speaking, if it ran fine before you worked on it, it’s probably something you did. I’m assuming you’ve gone back and triple checked your work, so grasping at straws here… Any cracks in the boot between the carb and reed block? Remove it and flex it by hand to check. Any bubbles in the fuel line between the pump and the carb when it falls on its face?

I did a new top end since the last time it ran. Something I’ve done a dozen times and nothing in the fuel system was touched. As far as I can tell there is no air in the supply line coming from the pump

Thanks \m/

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Well, I’m out of guesses. If it’s falling on its face under load but idles fine, it’s either not getting enough fuel, or it’s getting too much air. If you are absolutely 100% sure of your work, and have checked everything that could possibly restrict fuel flow or allow the ingress of air downstream of the slide, then I guess it’s just time for a new engine. Sorry, man.

It’s not time for a new engine. It has about 5 hours on it. Its a fuel issue that so far nobody can pinpoint I

Gonna try a different carb and see what happens but I gotta wait a week to do that because it’s about 60 miles away

Thanks \m/

And unless you want to ship the engine to one of us, nobody here is ever going to be able to pin point it. All anyone can ever do online is provide general ballpark suggestions. It’s up to you, the guy with the engine in front of you, to pinpoint it by using the suggestions offered online.

Partially plugged pulse hole, can’t keep up under load?

Ya I’m familiar with the concept but thanks

Thanks \m/

Long long way from having to replace the engine nor out of options online. Do you have any video yet Mark?

“Falling on its face under load” can be a rich, lean or ignition issue.

I think a carb is worth a shot, although I’d open up the fuel pump and clean it out. Even though it was done recently. If it’s a pumparound it’s not uncommon for oil to accumulate on the return side and cause flooding such that the engine will fall flat on its face.

Check inside the silencer and make sure the baffle and packing hasn’t self-destructed. That will make it run poor to put it mildly.

When the engine dies, does the RPM readout on your rev counter fall in line with the engine RPM. If it’s fluctuating (or reading zero) while the engine is rotating then needless to say you have an ignition issue.

That a bit premature. There are a bunch of causes for this kind of behavior, it’s rarely as simple as it not getting enough fuel under load.

Asking the condition of the plug is pretty reasonable. I’ve had engines that have flooded to a stop from over fueling under load.

When took the pumps apart they other day they were spotless. I can’t answer the question about the readout because I’ve been busy trying to keep it running enough to get off the line and out of the way but I’ve since swapped out the cdi to be sure and it didn’t make the problem go away I like the silencer suggestion I hadn’t thought of that so I’ll be on that tomorrow

Thanks \m/