Untold Posted February 4, 2011 Share Posted February 4, 2011 Can anyone help with a really annoying problem on my project XJ 600? I have done my best to refurbish it after 10 years storage, including a full carburetor cleanup and replacement float valves. I have stripped, reburbished and tested the fuel tap (petcock) and all seems fine. She has a new battery and ignition seems fine. She starts well and warms up gradually, even after several days standing. But: She does not like to run without full choke. Running off choke is possible with a squirt of cold start fluid, but remains very sensitive to throttle.When hot or cold, she won't respond properly to the throttle. Response is sluggish and engine tends to rev too high then try to cut out. Can be kept running by precise throttle control. I thought it was fuel starvation causing it to lean out under higher rpm, but have tested the petcock and fuel comes out under gravity when vacuum is applied to the valve. The vacuum tubes seem ok. Could this be caused by cracked inlet rubbers causing a poor vacuum when running? It is difficult to see if theres a problem there and I don't know whether they could be to blame? I think I need to apply a vacuum using another means while it is running to see if that makes a difference, and block off the existing tubes. Maybe I have answered my own question, but I would be really grateful for any advice? Actually there is another problem where the starter motor spins without being asked to while the engine is running. (Disengages fine, but re starts while engine running). But I think this is unrelated and due to electrical leakage through the starter switch.... Many thanks!!! Link to comment Share on other sites More sharing options...
JimR Posted February 4, 2011 Share Posted February 4, 2011 Can anyone help with a really annoying problem on my project XJ 600? I have done my best to refurbish it after 10 years storage, including a full carburetor cleanup and replacement float valves. I have stripped, reburbished and tested the fuel tap (petcock) and all seems fine. She has a new battery and ignition seems fine. She starts well and warms up gradually, even after several days standing. But: She does not like to run without full choke. Running off choke is possible with a squirt of cold start fluid, but remains very sensitive to throttle.When hot or cold, she won't respond properly to the throttle. Response is sluggish and engine tends to rev too high then try to cut out. Can be kept running by precise throttle control. I thought it was fuel starvation causing it to lean out under higher rpm, but have tested the petcock and fuel comes out under gravity when vacuum is applied to the valve. The vacuum tubes seem ok. Could this be caused by cracked inlet rubbers causing a poor vacuum when running? It is difficult to see if theres a problem there and I don't know whether they could be to blame? I think I need to apply a vacuum using another means while it is running to see if that makes a difference, and block off the existing tubes. Maybe I have answered my own question, but I would be really grateful for any advice? Actually there is another problem where the starter motor spins without being asked to while the engine is running. (Disengages fine, but re starts while engine running). But I think this is unrelated and due to electrical leakage through the starter switch.... Many thanks!!! Hi you still have a carb issue, I would guess, but the motor may also lack compression ..... can you give me an idea on how long the motor has run after it's 10 year lay off this will help on which problem you have Link to comment Share on other sites More sharing options...
Untold Posted February 4, 2011 Author Share Posted February 4, 2011 Thanks for getting back to me so quickly! Before the 10 year layoff the bike had done 26,000 miles. Since coming out of storage I must have run the engine for more than 3 hours through various experiments. It can be made to tick over ok and sounds alright. I have ridden it across the car park outside my flat and the engine seemed to be in good shape, except for the almost uncontrollable throttle. I had to carefully coax it into a fast idle and slip the clutch, then it took off smoothly. But it was very hard to get a stable fast idle without the choke. With the choke it will idle quite fast anyway when hot. I have had the carbs apart several times and everything seems right, but the finger still points there. Could sticking piston valves (ones that move the jet needles) cause the problem? I am not sure if they move freely enough. I really appreciate the help! Link to comment Share on other sites More sharing options...
wild foamy Posted February 5, 2011 Share Posted February 5, 2011 check your inlet rubbers, if they are showing signs of perishing its best to replace, dont ask me where to find them though as i just covered mine in speedtape Link to comment Share on other sites More sharing options...
Untold Posted February 5, 2011 Author Share Posted February 5, 2011 check your inlet rubbers, if they are showing signs of perishing its best to replace, dont ask me where to find them though as i just covered mine in speedtape I was wondering about those. They are not perfect by any means. I was thinking a coating of silicone sealant might keep them going a bit longer? I could test the theory by greasing them up really well and see if that helps. (I don't have any silicone at the moment) I tried connecting the vacuum hoses to the other cylinders to see if that made a difference. One of the rubbers it was previously connected to looked rather dodgy. But this made no difference. If they are generally affecting the running of the engine, the air leak must be very bad as the throttle control barely does anything. I also have to check the choke passages in the carbs. At the moment, only full choke seems to do anything, partial choke is useless even when hot. This doesn't seem right, so maybe something inside is blocked? I realised this is the only part of the carbs I have not taken apart and cleaned. Silly to overlook this I know. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 5, 2011 Moderator Share Posted February 5, 2011 as a starting point and from cold...feel (with caution) each exhaust header as you start the bike and note if any remain colder than the others to indicate which carb you may have issues with Link to comment Share on other sites More sharing options...
Untold Posted February 6, 2011 Author Share Posted February 6, 2011 as a starting point and from cold...feel (with caution) each exhaust header as you start the bike and note if any remain colder than the others to indicate which carb you may have issues with Thats a good idea. I have taken the carbs off now to look at the choke problem, so will try this after the rebuild. The inlet rubbers are cracked on the outside in places, but look in good condition on the inside. I am not sure if these have caused an air leak. The worst crack (outside) is shown in the first photo, the inside (hard to photograph) is in the second photo. It looks smooth and the rubber seems ok. I'm sure I could seal them with something just to be sure. Worst of the cracks: And the interior: Inconclusive. Carbs are still the prime suspects. At least there were similar petrol traces in each of the rubbers after running, so it looks like all carbs are working to some extent. Link to comment Share on other sites More sharing options...
oldduke Posted February 23, 2012 Share Posted February 23, 2012 Hi Untold I've just completed a similar project and to get the manifold/inlet rubbers and carbs checked I did the following. I removed all carburettors removed all the diaphragm covers found one slide to be sticking so cleared this ok and found one of the diaphragms to be missplaced allowing the slide to drop/not rise in a controlled fashion with vacuum. Next reassembled and using a single vacuum gauge check each inlet rubber in turn off its vacuum hose whilst ensuring the other three were bunged up (using the priming position on the fuel tap so no vacuum was needed here. This will do two things it will show if each carb is balanced and in your case more importantly (if there is a very low vaccuum) show you have an air leak which could well present the symptoms you describe. If there is a low vacuum then its a case of seeing whether or not that manifold rubber is to blame or the seal with the cylinder head or carburettor joint is at fault. Hope this is of use. Link to comment Share on other sites More sharing options...
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