Low RPM Hesitation Problem

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Crab_Cake

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Hey guys, new to the forum. I just bought a 1980 XS650.

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I am loving the bike but there have been some problems.

There is some hesitation between about 1000 - 2500 RPM. After that the bike runs just fine. The previous owner installed a 140 main jet and an adjustable needle in the carbs when he got the new pipes.

The bike hesitates and will either die or accelerate. Other than that the bike runs great, starts on the first kick and is a blast to ride.

What do I need to do? The carbs have been very thoroughly cleaned out recently by the previous owner (soaked in carb cleaner).

Anyways a manual for the bike is in the mail but I was hoping for some advice right now. It is pretty embarrassing to keep killing the thing at stop lights.
 
You need larger pilot jets thats what the bike runs on at that low RPM. I use 47.5's with open exhaust and pods. You may not need to go the large. Also mixture screws open about 3 - 3.5 turns try this first as there is no cost or labor involved.

Nice looking Special II
 
Thanks for the advice.

Where are the mixture screws on these carbs? I only see the idle screw and then the screw that is facing down between the two carbs which I think is for syncing right?
 
If you have BS34 carbs with the chrome tops the mixture screws are at the inlet side of the carbs where the air cleaner boot attaches. They are facing straight up one on each carb. If they are still capped you need to drill a small 1/8" hole in the center and inserta sheet metal screw and pop them out with a pair of pliers.

If they are BS38 carbs with aluminium tops the scres are on the side of the carb body. Cap procedure the same. I would think if the PO did cleaning and jetting he surely removed the caps.
 
This is stock jetting for the BS34's: Main jet 132.5, Pilot jet 42.5
I went 137.5 mains and 47.5 pilot. If you are using stock air filters and it doesn't look like your exhaust is freee flowing I'd go with 44 - 45 pilots whatever is available in that range. Let me know if the mixture screws 3.5 turns out solve the problem.
 
NJ1639 brought to my attention the I mislead you. The mixture scresw are on the engine side of the carbs not the air inlet. Sorry for the misteak. Have resolved your problem yet?
 
My manual should be here in a few days. I want to wait for that in case I screw something up.

So my bike has the british style pipes and the sport air filters. Should I start with a stock pilot jet or go to a #45? I am not sure what is on the bike right now, I am assuming it is stock.

The PO said it has #140 main jets.

Also I was wondering if out of sync carbs could cause this. I pulled the spark plugs and the right side had a lot more carbon on it.
 
Try just opening up the mixture screws first. Gently close to they stop and turn counterclockise 3 1/2 turns. Go have anice ride. If it still hesitates from idle to mid range go for a larger pilot jet 45 is a godd starting point. If it is not snap crackling and poppoing your diapframs are OK. 142 mains are good with your set up for top end or high speed. As I understand your problem it is sthe transisiton from idle to midrangd RPM.

My bike runs great and the left plug looks sootyier than the right. might be float height adjustment, but hey it doesn't foul and runs stron so I'm leaving it well enough alone. No sense chasing shadows.
 
Ok so I tore the carbs off and opened the mixture screws. I turned them exactly 3 times.

The bike idles much better now and the problem seems to be fixed. It is nice and strong off idle. Unfortunately the bike has no power anywhere else in the rev range. From 1800rpm on it just won't go. I can't really get past 10 - 15 mph.
 
just to add...I read somewhere here that an air leak can cause the hesitation at low rpms. I had that problem when i bought my bike last year. The bike had no carb holder gaskets and the new gaskets solved the problem. spray starter fluid around carb holders to see if rpms go up and you'll know theres a leak.
 
Ok so today I sprayed the intake boots with ether on the engine and carb side, nothing happened. Though when I sprayed it on the air filter the bike died.

I also turned the screws 2 stops out instead of 3. Same thing, idled fine but no power whatsoever off idle. Couldn't get the bike to move, it would just die. Actually it seemed a bit worse than yesterday and I had the screws at 3 turns.

Please help!
 
Did you get your bigger pilots installed? Have you thoroughly cleaned your carbs? Small pieces of debris can clog your pilot circuit and starve the bike right off of idle.
 
Did the po remove the diaphragms before he soaked them in carb cleaner? If he didn't then that might be your problem.
 
Ok so let me explain again what happen. I think I confused everyone. The bike was rideable and ran really well except for some hesitation from 1000 - 2500 RPM. So I pulled the carbs, drilled the mix screws out, and turned the screws out 3 complete turns.

The bike started up fine, it ran really nicely off of idle. Unfortunately now everything after 2500 RPM is dead. Trying to accelerate the bike goes "puh puh puh" and won't give enough power to get past 15 mph.

Today I turned the screws 2 turns from the bottom and tried to ride the bike. This time it didn't even have enough power to get the bike into motion. Every time I let the clutch go it would die.

I sprayed starting fluid around the intake manifolds and nothing happened. I also sprayed it onto the air cleaner and it killed the bike.


Any ideas? I don't really know what to do.
 
Crab_Cake said:
My manual should be here in a few days. I want to wait for that in case I screw something up.

So my bike has the british style pipes and the sport air filters. Should I start with a stock pilot jet or go to a #45? I am not sure what is on the bike right now, I am assuming it is stock.

The PO said it has #140 main jets.

Also I was wondering if out of sync carbs could cause this. I pulled the spark plugs and the right side had a lot more carbon on it.
Click to expand...

When I first got my bike-
PO had put 2-1 exhaust on, no change in jetting, stock air boxes. It ran ok but not great, little flat on takeoff. I picked up some stock exhaust and it made a world of difference. Sometimes you have to start at the beginning.
Now when I make changes or adjustments I have a base to compare to.
Hang in there, you'll get it sorted.
 
A change of mixture setting shouldn't do that. I would get everything back to the way you had it, see if you still have the problem and if so figure out what has changed.
 
Another option: Strip the carbs, dip them (Chem Dip), install all new diaphragm and gaskets. Then you KNOW what you have.

Don't leave any plastic or rubber parts in the Chem Dip bath.
 
Ok I think I am going to clean them really good. What parts do I need? Will the mikesxs carb rebuild kit have everything I need?

Let me know and I will update when I figure it out. Thanks a ton for the advice, I really appreciate it.
 

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