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Drive train, fuel, suspension, steering & brakes / Re: M129 idling issues
« Last post by Heckflosse on Today at 03:17:24 »Yes, I followed the factory procedure for setting up the carbs. Yes, the pump shot is good and even aimed right. Vacuum leaks have been eliminated, and I don't think the centre screw issue is at play here, I didn't torque up the covers and the gasket wear looks uniform. Bowl vents were taken apart and cleared, and the springs tested. The return line is clear and it all works as it should.
Using the mixtures screws does absolutely nothing. I even tried both completely screwed in, and again screwed out to the point of falling out, one in and one out, and so on, no change at all. It will not run on the idle circuit at all, period. It's like turning the key off, not even a cough, just instant death.
Normally, I would agree with everything that has been said about carburation issues, because this looks like classic operator incompetence. But there is one very import aspect that has been completely ignored here, the fact that the engine has very low vacuum. In order to get 10 inches of vacuum, I have to run the engine at over 1000 rpm. the moment I cut it back the vacuum falls completely off, as in instantaneously.
According to Pierre Hedary, these carbs need about 15 inches to function properly, anything less than that and they don't work. And that's part of the problem, I can't do a good syncro on these things because I can't even get it to run right. For all I know the secondaries might be trying to dump fuel because they are seeing low vacuum. (that might explain the rich burning and the acrid smell) Throttle response is good, so I know the accelerator pumps work, it starts like it should, so I know it's getting gas. With full choke the running is no better or worse than with no choke, and if there was a vacuum leak, it would run better on full choke. There is absolutely no difference how it runs, it flat out refuses to run at anything less than over 1000 rpm. Yes, TDC was verified, and there is little to no stretch in the chain, and the cam lines up as it should.
At higher speeds; the spitting back through the rear carb is more pronounced and can actually be timed. (there's flame too) Same with the exhaust note, I count 3 misses. Previously it only had one. Changing anything with timing or fuel makes absolutely no difference, none at all, no increase in vacuum, no change in the popping and farting, nothing. This is why I'm leaning to a mechanical issue, I do believe the cam is worn out, I don't know how well the upper cylinder area is oiling, and this engine is rumored to have sat motionless for close to 20 years. Apparently the oiling tube has a tendency to plug up, and the previous owner didn't do regular oil changes. (Judging by the colour of the crap I dumped out of there, I don't know if one was done at all since the motor was installed, and the filter was a discontinued old make) The cam lobes themselves look very worn and polished on the contact surfaces which appear flatter than the spare cams I have, and yes, the numbers on all cams are the same. So the lift is a minimum of a millimeter lower and the lobe peak is flatter on all lobes, not just one. So yes, the motor runs, and with the valves set, it tries to run "round", but the idle at 1000 rpm is not steady nor is it stable, it wants to die. Max vacuum gunning the engine gets to about 18, with a very shaky needle, (indicative of valvetrain failure) - it should peg over 25, and then it instantaneously falls off to nothing.
So I agree, it should and feels a lot like a carb issue, but any carb issues I've ever dealt with there is usually a change apparent whenever something is changed. In this case there no change whatsoever, and the carbs have been on and off and apart multiple times now.
So that's why I'm leaning towards a cam failure or a couple of burnt valves. The next test has to be a leakdown test if I can get the equipment.
Using the mixtures screws does absolutely nothing. I even tried both completely screwed in, and again screwed out to the point of falling out, one in and one out, and so on, no change at all. It will not run on the idle circuit at all, period. It's like turning the key off, not even a cough, just instant death.
Normally, I would agree with everything that has been said about carburation issues, because this looks like classic operator incompetence. But there is one very import aspect that has been completely ignored here, the fact that the engine has very low vacuum. In order to get 10 inches of vacuum, I have to run the engine at over 1000 rpm. the moment I cut it back the vacuum falls completely off, as in instantaneously.
According to Pierre Hedary, these carbs need about 15 inches to function properly, anything less than that and they don't work. And that's part of the problem, I can't do a good syncro on these things because I can't even get it to run right. For all I know the secondaries might be trying to dump fuel because they are seeing low vacuum. (that might explain the rich burning and the acrid smell) Throttle response is good, so I know the accelerator pumps work, it starts like it should, so I know it's getting gas. With full choke the running is no better or worse than with no choke, and if there was a vacuum leak, it would run better on full choke. There is absolutely no difference how it runs, it flat out refuses to run at anything less than over 1000 rpm. Yes, TDC was verified, and there is little to no stretch in the chain, and the cam lines up as it should.
At higher speeds; the spitting back through the rear carb is more pronounced and can actually be timed. (there's flame too) Same with the exhaust note, I count 3 misses. Previously it only had one. Changing anything with timing or fuel makes absolutely no difference, none at all, no increase in vacuum, no change in the popping and farting, nothing. This is why I'm leaning to a mechanical issue, I do believe the cam is worn out, I don't know how well the upper cylinder area is oiling, and this engine is rumored to have sat motionless for close to 20 years. Apparently the oiling tube has a tendency to plug up, and the previous owner didn't do regular oil changes. (Judging by the colour of the crap I dumped out of there, I don't know if one was done at all since the motor was installed, and the filter was a discontinued old make) The cam lobes themselves look very worn and polished on the contact surfaces which appear flatter than the spare cams I have, and yes, the numbers on all cams are the same. So the lift is a minimum of a millimeter lower and the lobe peak is flatter on all lobes, not just one. So yes, the motor runs, and with the valves set, it tries to run "round", but the idle at 1000 rpm is not steady nor is it stable, it wants to die. Max vacuum gunning the engine gets to about 18, with a very shaky needle, (indicative of valvetrain failure) - it should peg over 25, and then it instantaneously falls off to nothing.
So I agree, it should and feels a lot like a carb issue, but any carb issues I've ever dealt with there is usually a change apparent whenever something is changed. In this case there no change whatsoever, and the carbs have been on and off and apart multiple times now.
So that's why I'm leaning towards a cam failure or a couple of burnt valves. The next test has to be a leakdown test if I can get the equipment.