Strange Start/Idle Behavior By ellpee (Read 710 times)

[ellpee]

I posted about this in the trike classroom, but it's really more of a generic question, so thought I'd post it here as well.

My 2012 Magnum, 1400 miles, has been hard to start sometimes, planning to take it to a shop for tweaking. But I have been able to get it started most of the time recently. It needs a little bit of throttle at first, otherwise the idle is too low and it dies. Odd thing is, if I give it a little throttle and lock the throttle in that position, as it warms up the idle fluctuates up and down. I would expect a fluctuation at the point where the enricher closes and cuts off the extra gas for cold starting, but I don't understand what would cause it to fluctuate in both directions, up and down. If I have it running at around 1500 rpm for warmup, it'll suddenly accelerate to maybe 2500 for awhile, then drop off and idle normally for a bit, then maybe drop down below 1000 for a bit, etc., etc. Anybody have any thoughts on what could cause this strange behavior?

BTW, yes, I know how to set the idle up for starting purposes, but then once it warms up it idles way too fast. Haven't found a happy medium yet.

[bones113]

The only things that come to mind is the obvious such as a air leak (which you probably checked already) or the enricher could be malfunctioning (Happened to me), Clogged air filter etc. Check the air box connection where it connects to the carb. It can work loose and create one big air leak ( Again happened to me) in fact that was the reason for my last breakdown, the airbox hose worked off and caused me all kinds of grief. At 1400 miles it is doubtful the valves need adjusting. You might also check your fuel filter. It might be clogged. Again you probably already knew these things. Just trying to lend a hand.

[cyborg]

my money is on an airleak

[poppawelee]

bones113 I realize your expertise is far greater than mine; I'm just confused by a comment. I use a how-to guide written/posted online by towingdelivery.co.uk which focuses on repair and maintenance of 50cc Chinese scoots. It recommends valve adjustment around the mileage (1400, I think ellpee wrote) as they 'seat' with use. Is there a general difference between a 50cc scoot and a larger engine for mileage / valve adjustment?

[bones113]

I probably have no more expertise than the next guy. I am going from experience. The 50 cc scoots may be different but on my 2 300 cc scoots I had to adjust the valves around 3600 miles on the first one and 9000 miles on the 2nd one. Of course others may think different. His problem I think is air leak related primarily as it exhibits symptoms of that.

[ellpee]

Peeked into the engine compartment today and the airbox and its hose appear to be well attached. Are there other places to check for an air leak?

[bones113]

Intake manifold is a sure fire suspect. They wear out quite often and can develop cracks. I just replace one on one of my scoots.

[ellpee]

OK, will look at that too.

LATER: pulled off the manifold and examined it closely, no holes or cracks. It's made of very thick, sturdy hard rubber, so not likely prone to cracking at only 1400 miles. So not sure where else air could be leaking in.

LATER STILL: hard starting again this morning, checked my re-installation of the manifold, discovered by accident that if I removed the little vacuum line from the manifold and blocked the vacuum port intermittently with my finger, it helped the engine to start. Once started, reattaching the vacuum hose caused no problems, and the engine ran fine. Have no idea what that tells me, hoping someone here will translate it into terms I can understand.

BY THE WAY, though, removing, examining, and re-installing the manifold APPEARS to have resolved the original problem, that revving up and down at idle. Once the scooter started, it ran steadily with just a wee bit of throttle, and not even that once fully warmed up. Perhaps that means that one or the other of the manifold connections wasn't fully tightened?

[bones113]

Those manifolds can be a pain for sure. Most of them aren't very well made. I know from experience about these things failing and just because they appear good don't mean that they are.

p.s.
You can run a small wire up into the opening of that vacuum port to make sure there isn't any blockage in there. You can check the seal of the fuel pump (assuming you have the vacuum style pump) by tightening the screws that bind it together. The can work loose also. Should be 4 screws. Just re tighten them.