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Terrible to start and idle - great on throttle - M750 1997

2K views 8 replies 4 participants last post by  Dan Boy 
#1 ·
So my 1997 M750 has started to start really badly, nothing I am aware of has changed, in fact I have not done anything to the bike to make this happen. It takes a while to start and will back pop quite violently. When it does finally start it will only run on throttle and even when warm if left to idle it will not run on both cylinders and will stop quite quickly.

On throttle is it great, runs smooth and revs well. I have no problems here.

I was wondering if it could be a blocked idle jet but just wanted some advice before I took the carbs apart.

Thanks, Dan.
 
#2 ·
Your diagnosis is correct. You don't need to pull the carbs. Just count the turns clockwise till your idle air screws bottom, then remove them . Welding tip cleaners are ideal for cleaning the passage through the screw hole, followed by spray cleaner. Install and Readjust your air screws and you're done.
 
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#3 ·
So I cleaned out the idle jets. THey looked clean but I gave them a good clean anyway and I cleaned their seating too.

Sadly this made no difference to the problem. It still will not idle and back pops badly when trying to start.

If I am sitting at lights or junction then I need to hold the throttle open a little to keep the engine running. It back pops all the time when doing this.

Any more suggestions because the first idea I had was the only one! :frown

Thanks, Dan.
 
#4 ·
Sounds like the passages and not just the jets are clogged. Sounds like you're going to need to tear the carbs all the way down. Either soak the carb bodies in cleaner or have them ultrasonically cleaned.

You might get lucky and run something like Cheveron's Techron and get them to open up, but it's a long shot
 
#5 ·
So I'm going to take the carbs off to get them cleaned but I just thought I would check the balance as a mate had some gauges. The vacuum seems to be pretty much the same on tickover but the needles are in the red zone. Should I be worried about this? Never used these before and I wasn't expecting there to be so much information on the dial.

Thanks, Dan.



http://vid834.photobucket.com/albums/zz263/danielhowden/Monster_zps0yaqwyis.mp4
 
#6 ·
The only way to clean the pilot jets is to remove the carbs and remove the float bowls from the carbs to get access to the jets.
As for the vacuum gauge you have chosen, I absolutely hate those gauges as on a engine of this type they are extremely difficult to adjust due to the engines intake pulse fluctuates.
For this engine type you must have a line restrictor in the gauge vacuum hoses to eliminate the pulsing effect to steady the needle - as for the gauges reading in the red - the gauge bleeds are most likely open to much for the engine.
 
#7 ·
So I have the carbs off and they are being cleaned. While I was working on the bike this evening I thought I would take a check at the valve clearances.

The first thing I noticed when I removed the cam belt covers to see the timing marks on the pulleys was that there are what I would call some considerable adjustment needed on the belts. This will not help my situation.

The second thing I notices was that when I checked the opening rocker clearance was that the intake rocker had a clearance of .1mm. I have a guide that says it should be between .05 and .12 so that looked good. When I checked the clearance on the exhaust rocker I found that it was less than .05mm. With a tolerance of .05 to .15 this is out of tolerance and I will need to replace this.

I didn't know the best way to check the closing rocker clearance but I have seen a tutorial on you tube and it is easy to do so I will check that next on both intake and exhaust.

Is what I am doing and finding all right with what I have said here? If my thoughts are wrong please let me know.

Thanks, Dan.
 
#8 ·
There are three ways to check the valve shim clearance. For the closing shims , If the engine is in the frame then the method I prefer is to use the measurement of the opening shim clearance + the measured closing shim clearance and add the two together for the result.
Example - with a feeler gauge the open rocker shim clearance is at 0.10 - now use a suitable tool to push down on the closing rocker at either side of the shim and hold it down while you enter a thicker feeler gauge at the open rocker to check the new clearance measurement ( in this manner both the closing and open measurements are taken at the open rocker) The first measurement plus the second measurement together equals the total for the closing rocker.
 
#9 ·
This is great information, thank you. I also found a you tube video from ducati tech that said the same.

So here I am again being in need of advice.

I checked the intake opening and closing shims for the front cylinder and they are both within tolerance.

However, when I checked the exhaust shims on the front cylinder, I get the opening rocker clearance at between .05 and .10 so that is all fine but when I check both the opening and closing shims clearance together as described above I can get a .30mm gauge in there.

This will give me a reading for the closing shim clearance of between .20 and .25 right? So just over top tolerance right? Could this cause a problem and if so should I get a closing shim that is .10 bigger?

Thanks for the help here, Dan.
 
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