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Discussion Starter #1 (Edited)
Trying to restore a 1981 SP500. The bike will start with one kick and only when I hold the throttle open. Once it's running I have to hold the throttle open for it to continue running. If I let off the throttle the bike dies. If I open the throttle all the way up the bike maintains the same RPM and very slightly increases but does not go past 2000-3000 RPM. I have taken apart the carburetor a dozen times and checked and rechecked everything making sure it is to spec and working. What would cause this issue and what are some possible fixes? I live at 4500' and everything is stock: jets, airbox, air filter, carburetor, etc.
 

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Oddly, you didn't mention the choke......and that is critical for starting cold.

So is a stock air filter; without that it will tend to run lean.
 

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@EasyRider The bike won't start at all without the choke. I did fail to mention that I use the choke everytime I try starting though. Any ideas on what I can try to get more throttle response? There is little to no fluctuation of the RPM despite opening the throttle all the way
 

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Check for vacuum leaks and give it a healthy dose of a good carb cleaner.

This isn't a 2-stroke, is it ??
 

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Discussion Starter #5
It is not a 2-stroke. I will check the boots between the airbox and the cylinder. I've deep cleaned the carb 4 times. I will check back after I check for any leaks.
 

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Does it have a CV carb and if so is the rubber diaphragm holed...or the return spring tangled up preventing the slide rising. If the engine doesnt splutter while this is happening it has to be a failure of the slide to rise for some reason.

Just had another thought...is the muffler/exhaust free and clear....no animal nests in there?
 

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Discussion Starter #7
@Aussie Steve, yes a CV carb. The diaphragm, when the carb is outside of the bike, seems to move up and down pretty well. I even blew compressed air through the air intake side of the carb and watched the slide move freely up and down (up with air, down when no air was blowing). No visible tears or holes. As far as the exhaust goes, there is quite a bit of air coming out of the tail pipe directly, but then there is a bit of white smoke coming out of the airbox as well. I can stick a wire hanger all through the pipe and see whether there is anything blocking airflow. Will update you in 12 hours
 

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So it appears to be a failure to provide sufficient vacuum from the motor. Either a catastophic leak from the intake boot or a leaking valve.....would have to be very bad to not provide sufficent vacuum so unlikely. The only other thing I can think of is blocked vacuum ports....You may need to remove the carb take all of the rubber parts off and boil the metal parts in salt water for about an hour. Then flush with clean water and pop in the oven to dry. I would give a good dose of quality carb cleaner through first....i know you cleaned the carb four times already but there isnt much else it could be.....

Make certain there is a rubber cap on the vauum port if it has one. A leak here will defeat any vacuum from the engine to provide a decent fuel supply. Also check any vacuum line to the fuel valve. Kinked lines as well as damaged ones.

Finally is sufficient fuel getting into the float bowel and is it getting from there to the engine?...things to check would be the main jet...the emulsion tube....if one is fitted it'll be under the main jet. Has the needle got a clip in it....the slide may rise but the needle may stay put....just about out of idea's here...will give it some though overnight.

good luck
 

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Discussion Starter #10
Okay time for an update. I am confident there are not any vacuum leaks. I sprayed carb cleaner into all the ports and engine did not change or go up in rpms or anything. There is something rattling in the exhaust and i tried getting it out, but it seems small and the exhaust comes out normal so I'm not sure that's worth my time. I can only assume the problem lies in the carburetor or the cylinder. The upper cylinder leaks a small amount of oil out the side. Maybe a drop for every 10 minutes the bike is running. I'm going to check and see if i have compression. Am i right that the rpm problem I'm having could be affected by the cylinder?
 

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An engine with a leaking cyclinder would run but it would be rough and lack power. The rpms failing to rise is another thing altogether. I did a little research and found that your bike has PEI ignition....CDI in other words, and should never need timing. The page I found did have some info relating to the stator being removed and the timing needing to be reset. I copied the specific page. Albeit that this is for a 250 Suzuki I would suspect yours will be the same. If the page doesnt show i will try to copy the relevent link. It could be the original owner removed the stator and never bothered to re set the ignition timing...or the CDI unit is faulty and the timing isnt advancing.

It could also be a failing spark due to any of the ignition components...this would also cause the engine to not rev...although I would expect a lot of backfiring and misfiring too.



The P.E.I. system is just the opposite to the old points system. There is nothing to physically wear out. The Owner's Manuals for the 1974 RL250L and the 1975 RL250M said the following under the section titled 'Ignition Timing':

"Ignition timing is adjusted at the factory and should normally require not adjustment (sic). However, if the stator is removed or tampered with, adjustment may be necessary."

Suzuki's perspective was that they set it up correctly during the manufacturing process and, as long as the owner didn't tamper with it, the system required no routine maintenance. After 40 years and counting, these systems still don't need maintenance. Amazing.
So, how do you set the ignition timing on an RL250? Well, first off, congratulations on getting the flywheel off. That task itself has been known to make grown men cry. Since you probably didn't go to all this trouble just to tinker with the ignition system, you must have had the engine apart and now it's time to, well,...time the thing.

Right up there with the P.E.I. system as a feat of engineering is the ease in which the system is set.
You've got the center cases together and the next task is to install the stator plate. The stator plate is a round aluminum disk with two coils installed; the bigger Exciter coil creates the voltage for the spark and the smaller Pulsar coil that provides the spark trigger. It is installed to the left center case with three 5mm Phillips head screws (use #3 Phillips screwdriver) with large flat washers. The holes for the three screws are slotted allowing the stator to rotate a few degrees. Position the stator against the center case with the coils at about 10:00. Thread the two lower screws and leave loose. Don't install the top screw yet.

Referring back to the 'Ignition Timing' sections of the Owner's Manuals, the following sentence sums it up:
"In this case the top mounting screw should be aligned with the mark stamped on the stator plate."

This is only slightly misleading, because you can't see the 'mark' with the screw and flat washer installed. So, what they mean is to align the top mounting screw 'hole' with the 'mark stamped on the stator plate'.
Refer to the two images of the stator plate installation. The image on the left is the plate with arrows pointing to the three stator plate screws. The image on the right shows the top screw removed and one arrow pointing to the 'mark'. The mark is a line cast into the stator plate that bisects the slot.

The goal, according to Suzuki, is to rotate the stator plate until the mark bisects the screw hole. Once accomplished, the two lower screws may be tightened to hold the setting. Install the top screw and you're done.
That is, at least, what Suzuki wants you to know about setting the timing. Frankly, I don't know why they didn't mention that big pointy thing that looks like an arrow that is cast into the left center case just above the stator plate that just happens to bisect the top screw hole and, if the mark is aligned with that pointy arrow thing, the same timing setting is achieved with less guesswork. But, what do I know?


Good luck
 

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My 1981 DR500S splutters between 2 & 3 000rpm. If I choke it, it revs smoothly from 1 200 to 7 000 rpm. cleaned carb lots of times. Adjusted float, revs and pilot screw but am now baffled. No intake leaks. Any solutions?
 

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Do you have an actual stock air box and filter on it ?
Stock muffler too ??
Making modifications for "high flow" tends to make it run lean.
(Applies to almost all gas engines.)

And just for grins, give it a healthy dose of Berrymans B12 Chemtool in the gas and run it a few times with the choke partly closed. Very often the low speed jets are near to impossible to clean manually.

And......if this machine is new to you, you do know that some choke being required until it warms up is normal......right ??
 
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