idles fine but dies when throttle turned


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I have a pretty much stock 1981 XS650 that I recently picked up, it starts great and idles fine but the engine dies as soon as I give it some throttle. The bike is stock with the exception of a two into one exhaust and reverse megaphone muffler. The carbs were cleaned and rebuilt by the previous owner using one of Mikes rebuild kits, they otherwise remain at the stock settings. I trust the previous owner on this that he did the carbs correctly. He thinks the problem may be with the ignition or electrical system. Where should I start and/or what could be the cause of this problem?

Thank you,
That's a classic very lean symptom. Something about the carb and or exhaust combo is making you too lean. I bet it does better if you throttle up slowly. Once you fix it you'll find it starts even easier than now.
Sounds like the diaphram's are not lifting, for some reason. Read about them in the carb section via tech section.
As a rule of thumb, you should NEVER trust what the previous owner claims. You are having carb problems and they need to be adjusted, cleaned, and/or repaired.

When the engine will not rev up from idle, it probably means the pilot circuit is partially blocked.

Don't trust anything PO's tell you. He may have done a proper job in the carbs or he may not have. Assuming he did a proper job, you may have had rust/dirt come down from the tank (is the tank rusty?)

Pilot circuit blockage:
-Pilot jet partially blocked
-Mixture screw not adjusted correctly
-passageways from mixture screw to the carb throat not clear
Ok, so the consensus is that it's a carb issue.

I just came in from switching out a different set of the same carbs I had on a 1980 that was running fine with no carb issues. With a different set of carbs, the same thing happened, it idled good but died when given some throttle. This tells me it's not the carbs but something else :shrug: ?

Is it possible for the problem to be something other than a carb issue?

Also, no the tank is not rusty, he has a new nonvacuum petcock, and is running a paper filter on the fuel line. Could it have something to do with the petcock, the fuel line, or the filter? The filter looks clean.
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Does that different set have stock jetting as well? If it does then you really haven't changed anything. If you do have an off idle leaness problem because of the exhaust, it will be present with both carb sets if they're both jetted stock.
Before removing the carbs from the bike and taking them apart I would do 3 things.

1) check charge on battery
2) confirm that the gasoline has an unimpaired flow to the carburetor
3) check the float level

All of these things are quick to do and one of them could potential be causing this problem.

I've had a similar problems.
Tore through the carbs several times until i bypassed the brand new inline fuel filter, then it fired right up.

For some reason the filter did not work, it was an standard automotive fuel filter I bought at a gas station, pic below.


  • bränslefilter_81.jpg
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I just realised that I've also had similar symptoms when my charging system started acting up.
(Ground wire to regulator vibrated off and one of the diodes on my home made rectifier blew. Refitted the ground cable and switched to the unused diode and rode it home. Victory...)

Make sure your battery holds a good charge. Compare results with ignition on and off.
Measure voltage at coils.

Good luck!
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Hey everyone, thank you all for the help on this. I have a set of BS38's I'm going to have rebuilt and set up for this bike, that should solve any carb problems. Electricaly speaking, battery and everything seems good.

Thanks again, I'll report back with any updates. This is a side project I picked up for $450, it's a 1981, with mikes 2 into 1 exhaust, bronze swing arm bushings, new tapered neck bearings, an extra set of carbs, and many new spare parts. I think just with the new parts it is worth more than what I paid. But the guy just wanted to get rid of it. Now I have 3 XS 650's, 1979 (working on a bobber build, should be done by next summer), 1980 that runs but needs the wheels trued and some fine tuning, and the 1981 that I've been talking about here.

Thanks again everyone.