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XL600R Ignition or Carburation issue?

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Hi all,

I have a 1993 XL600R which has a snag or maybe 2. The engine has recently been fully rebuilt, but the problem existed beforehand and was not the reason for the rebuild. I mention this so that we can discount any mechanical issues.

Fault symptom#1, The bike will start and run no problems whether hot or cold. It performs really quite well apart from a couple of issues.  I have noticed that if I really snap the throttle open quickly, the engine will bog momentarily, (and would probably die if I didn't close the throttle), so for example if I was riding along in 1st or 2nd and wanted to get the front wheel up to pop it over a log or something, unless the bike was up at mid range RPM, the engine will falter and die, and I have to close the throttle and then open it more progressively. It will even hesitate at mid range revs, but not significantly. Not much use off road! Carbs have also been fully rebuilt and jets sorted etc. It will also do this with the choke on, so I don't think it is a lean cut or carb problem, also doing a plug chop and the plug is a perfect colour. I'm thinking ignition and not enough advance? 

Fault symptom #2, The bike doesn't like revving out in top gear, although sometimes it will! It will hesitate and I have to back off the throttle and maybe then I can get it to rev out. However sometimes it just revs cleanly right through...

I have measured the stator resistance and that is within 80 ohms of where it should be. The pulse generator on the other hand is down at 380 Ohms cold, and 450 hot, the Honda book says it should be 510 - 570 but doesn't say hot or cold. Is mine showing too much of a difference? I have no way of testing the CDI box and it is an early one with the circular plug. How many £££ to replace it :-/

Anyone shed any light on the problem? 

Thanks in advance,

Mark.

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Symptom #1 sounds normal , as the carbs have no accelerator pump.           Symptom # 2 sounds like debris in the float bowls intermittently getting sucked into a main jet and blocking it.                                                                                

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Thanks for your reply - and that's the challenge, #1 doesn't seem like that kind of stalling, and playing with the mixture etc hasn't changed it at all, in my mind if it was mixture related it shouldn't do it when on choke, and it does. Failure of the ign timing to advance will produce similar symptoms. 

I had the float bowls off yesterday without draining the fuel, they are spotless, no dirt in there. In fact the carbs were totally rebuilt less than a week ago with new jets. holders etc. (the problem was there before) but I agree it could be that so I'll have a look at the filter screen in the fuel tap to make sure it's intact. 

In the back of my head I'm thinking the CDI unit is shot and not advancing, but if that was duff it would most likely do it all the time.  It's an interesting one!  

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try taking out the air screw on the side of carb number 1. there is a cap on it that you have to pop off or grind the little tab. before taking it out count the number of  turns it takes to turn it all the way in this will give you a starting point after cleaning. i had some debris in mine and it caused the same kind of problems

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Hi there, thanks for the advice. Can you remember where the 'debris' was exactly and how you removed it? There is no cap on my air screw (long gone) and whilst cleaning and setting the carbs up I did remove the air screw, but I may have missed something. 

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Update:

Checked float bowls - spotless.
Checked Fuel Filter bowl - spotless.
Removed Air Screw - blew out bleed holes with carb cleaner and compressed air. 

Checked resistance of:

Ignition Coil - within limits.
Stator - withing limits.

Cleaned and checked all electrical connections that might affect the ignition circuit.

Took it for a blast = same! Pretty sure it is ignition. 
Seems worse when engine is fully warmed up. 

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Pulse generator.

2 hours ago, x34Fred said:

Update:

Checked float bowls - spotless.
Checked Fuel Filter bowl - spotless.
Removed Air Screw - blew out bleed holes with carb cleaner and compressed air. 

Checked resistance of:

Ignition Coil - within limits.
Stator - withing limits.

Cleaned and checked all electrical connections that might affect the ignition circuit.

Took it for a blast = same! Pretty sure it is ignition. 
Seems worse when engine is fully warmed up. 

Pulse generator.

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So I've done a little bit of reading and apparently the input voltage from the Pulse Gen can actually alter the ignition timing. So if the resistance is down, I'm guessing that a different voltage to spec will happen when the electrical induction (pulse) happens and this will cause the ignition timing to be slightly out...I suppose I should just check it with a strobe...But how do you do that without the strobe getting covered in oil? :facepalm:

Edited by x34Fred
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What jets do you have? The hesitation on throttle snap is common. You can jet around it somewhat and change your technique, but the only cure is a pumper carb or EFI.

The pulse generator resistance spec varied over the years and models. My '88-90 factory manual says 360-440Ω at room temp. The earlier engines' resistance range was 510-560Ω, so I don't think there is anything with yours. Besides, bad pulse generators usually cause either problems at low RPM like hard starting or work fine when cold and start failing when hot, not tip-in stumbles.

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Hi and thanks. 

125 in LH carb, 120 in secondary. I think the pilot is a 56.

My problem only seems to happen when fully hot. The idle also goes to pot when hot. 

Might be jetting but I don't think so. 

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Have you figured it out yet? Please post your results here. I am going to watch this thread - I have some similar issues. 

I didn't have success checking resistances - i think my wire colors don't match the manual. I decided to get a new CDI, since it was inexpensive and easy to replace. The new CDI didn't improve things, nor did a new spark plug, valve adjustment, new air cleaner, and a carb inspection. I got a carb rebuild kit, but haven't bothered with it yet, as it seemed fine inside.

I tried a timing light, but also was frustrated with the oil spitting out the inspection hole. I've not seen anyone else mention that before, is that normal? Also, do i need a different timing light? Mine fires twice and indicates double the RPMs - i realize this is due to the spark firing again on the exhaust stroke, but are there timing lights better suited for this? I have the Innova 3568.

Thanks!

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Ok so I up jetted again, now running main carb on 135 - second carb on 125 and there is still a hesitation through the rev range. 

I haven't changed the pulse genny yet but things have changed as a result of altering the jets so it may just be a fuelling issue. 

I'll keep experimenting but I'm due to put the bike away for the winter so maybe next season until you hear back from me.

I think oil spitting is normal ;-) 

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The main jets have absolutely nothing to do with idle , you could leave them OUT and the bike should idle normally.

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The main jets have absolutely nothing to do with idle , you could leave them OUT and the bike should idle normally.

Joe's point is spot on, although I have experienced a noticeable difference to the idle mixture in these dual carbs when the main jet in the primary carb is significantly changed in size. Basically, I have found that at proper idle settings, the needle is open just a tiny bit and allows the main jet to add fuel to the pilot circuit. In my experience, this has only been noticeable when the main jet is changed by multiple sizes at once, like for example jumping from a 125 to a 135...... And before the flames start, I have only noticed this on these older dual carbs. I fully realize that main jet SHOULD not affect idle mixture, it's just what I've found.

In short, I think you are lean on the pilot causing an erratic idle when hot, and you saw a slight improvement by going crazy rich on your main jet.

Go back to 125 or 128 on the main and try a 58 or 60 pilot and it should put you in the ballpark.
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Thanks for your comments, and to get back on track and so you actually understand what I'm trying to achieve, which I don't think you do.

I know exactly how carbs work and what jet does what!!! 

The idle is fine, just a bit uneven when hot. The idle was never the problem - and still isn't. Idle circuit is stock. 

The issue is the flat spot / hesitation at high RPM in higher gears (probably there in lower gears but can't feel it). The bike doesn't rev out clean. I actually think it is an ignition problem BUT am playing with the jetting to eliminate the chance of it being fuelling. Also playing with the jetting to minimise the occasional hesitation caused by a lack of accelerator pump on the carbs when snapping the throttle open. My bet is still on a faulty pulse generator. 

By the way - a 125 to 135 isn't much of a jump in jet size....

 

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Thanks for your comments, and to get back on track and so you actually understand what I'm trying to achieve, which I don't think you do.
I know exactly how carbs work and what jet does what!!! 
The idle is fine, just a bit uneven when hot. The idle was never the problem - and still isn't. Idle circuit is stock. 
The issue is the flat spot / hesitation at high RPM in higher gears (probably there in lower gears but can't feel it). The bike doesn't rev out clean. I actually think it is an ignition problem BUT am playing with the jetting to eliminate the chance of it being fuelling. Also playing with the jetting to minimise the occasional hesitation caused by a lack of accelerator pump on the carbs when snapping the throttle open. My bet is still on a faulty pulse generator. 
By the way - a 125 to 135 isn't much of a jump in jet size....
 

Sorry I tried to help, I must not be familiar with the mysterious inner workings of the "1993 xl600r", a bike that was never built.
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