Causes of tach reading different on 1 cylinder?

[grog337]

I recently posted about my 1997 Yamaha T50TLRV (4cyl, 4 stroke) losing RPM (from around 5000 down to around 4000) after it was running awhile for a few minutes. I synched the carbs and went through most of the adjustments noted in the service manual in the fuel system. I took it out today, and while it seemed to be running OK, it would top out at about 4100 (right away this time). I was going to make sure I didn't mis-adjust the throttle and was double checking the idle with my external tach (the kind you wrap the wire around the spark plug wire), when I noticed that on cylinder #4 the tach was reading much lower than it sounded (the dash was showing about double) I then checked the tach on the other wires and they agreed with the dash. Re checked # 4, and got the same thing (even moved it around on the wire to be sure it wasn't getting interference from something else. When I connect a spark tester, it sparks right up, and the type with the light seems to be flashing consistently.

Is this an ignition coil going bad? If it is the carbs or the fuel pump, I would think that it would be still sparking whether starved of fuel or not.

 

[gm280]

Re: Causes of tach reading different on 1 cylinder?

I recently posted about my 1997 Yamaha T50TLRV (4cyl, 4 stroke) losing RPM (from around 5000 down to around 4000) after it was running awhile for a few minutes. I synched the carbs and went through most of the adjustments noted in the service manual in the fuel system. I took it out today, and while it seemed to be running OK, it would top out at about 4100 (right away this time). I was going to make sure I didn't mis-adjust the throttle and was double checking the idle with my external tach (the kind you wrap the wire around the spark plug wire), when I noticed that on cylinder #4 the tach was reading much lower than it sounded (the dash was showing about double) I then checked the tach on the other wires and they agreed with the dash. Re checked # 4, and got the same thing (even moved it around on the wire to be sure it wasn't getting interference from something else. When I connect a spark tester, it sparks right up, and the type with the light seems to be flashing consistently.

Is this an ignition coil going bad? If it is the carbs or the fuel pump, I would think that it would be still sparking whether starved of fuel or not.

How about a bad spark plug? Swap the plugs around and recheck. If the problem still stays on that #4 cylinder, change the coils around and recheck. If that doesn't work, try removing each plug wire one at a time individually and see if the engine changes the RPMs. The one that doesn't change with the plug wire removed is the problem. After isolate the culprit, replace and happy boating... You have three other cylinders that work properly to swap parts from... Just my 1 1/2 cents worth.

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

I had already checked the plug, but rechecking can't hurt. Great idea on the swapping coils. I'll give that a shot.

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

Swapped plugs. Still #4 doing the same thing. Swapped coils, still #4 doing the same thing. Don't see how, but could that cylinder be grounding out somehow? Could it be something upstream of the coils?

 

[gm280]

Re: Causes of tach reading different on 1 cylinder?

Swapped plugs. Still #4 doing the same thing. Swapped coils, still #4 doing the same thing. Don't see how, but could that cylinder be grounding out somehow? Could it be something upstream of the coils?

Yes that would be my guess. Not knowing how your OB is setup, if the ignition is a single power pack type setup, that would be my next part to look at... Either way I'd look at what was feeding the coil now!

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

I was digging and thinking on this a little more...and becoming more confused. The same ignition coil assembly services cyl #1 and cyl #4. Cyl #1 reads full rpms, but #4 shows half of the others (and the dash). There are only 2 wires going into the coil unit (1 ground, 1 "hot"). How can it be sparking one cylinder all the time and the other only half the time? Would an inadequate input voltage cause this?

The service manual outlines a "system check" for the ignition system and gives voltages at cranking, 1500 rpm, and 3500 rpm on the charge coil, pulser coil, and cdi output. The locations they show to test are at connectors. it says to "connect test leads between connections" (which I assume is the test leads from the multi-tester), How do you check the voltages and keep the motor running? Is it as simple as shoving the lead into the connector, or do I need to rig up some kind of pigtail between the connector ends?

Has anyone done this test before?

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

Oh, I also tried just swapping the wires for #1 and #4, (the part seems to have no up or down noted)... same thing. Still the 1/2 rpm reading on #4.

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

Just replaced the CDI with a used one. Exact same thing.... Low rpm reading on cyl #4. I am really stumped on this. Played around with the position of the tach and noticed if I place it toward the coil side of the big disc around the plug wire (don't know what it's called). The reading is a good bit higher, but didn't seem to come up with the other cylinders. Maybe it's just interference from the other wires somehow interfering.

I also put a voltmeter on the hot wire input to either ignition coil and the other end to ground. Showed no voltage. the manual said that at idle you should read 128v at cranking, 134V at 1500 rpm. I checked with 2 digital meters and 1 analog on both DC and AC, but the indicator didn't budge. Something's got to be going through, as there is a good spark being delivered (shows on spark tester).

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

Oh, it appears that to measure the voltage, I need a DVA peak adapter. Missed that in the manual.

 

[gm280]

Re: Causes of tach reading different on 1 cylinder?

Oh, it appears that to measure the voltage, I need a DVA peak adapter. Missed that in the manual.

Okay a tach reading is one thing, but have you tried to remove each spark plug wire one at a time and see if that makes any difference in the rpms? If that still points to number 4 cylinder, then they may be something else going on other then the ignition...

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

I think I figured it out... the 2 and 3 wires both cross over the #1 wire. The tach must be picking something up from them (and when checking 2 and 3, the interference from the 1. The 4 doesn't cross the 2 or 3.

After not being able to find anything else wrong with the ignition, I opened the carbs and noticed some grit in the bowl. I went ahead and rebuilt them last weekend and that seems to have fixed my initial problem (the rpm drop). It amazes me that it could pick up so much trash in them since I cleaned them late last season.

 

[gm280]

Re: Causes of tach reading different on 1 cylinder?

I think I figured it out... the 2 and 3 wires both cross over the #1 wire. The tach must be picking something up from them (and when checking 2 and 3, the interference from the 1. The 4 doesn't cross the 2 or 3.

After not being able to find anything else wrong with the ignition, I opened the carbs and noticed some grit in the bowl. I went ahead and rebuilt them last weekend and that seems to have fixed my initial problem (the rpm drop). It amazes me that it could pick up so much trash in them since I cleaned them late last season.

So are all you problems now solved? Cross talk from one high voltage spark plug wire to the other can be a problem with a tach pickup. Sounds like you have it all worked out now though. Good deal...

 

[grog337]

Re: Causes of tach reading different on 1 cylinder?

So are all you problems now solved? Cross talk from one high voltage spark plug wire to the other can be a problem with a tach pickup. Sounds like you have it all worked out now though. Good deal...

Seems to be (finding a big block of wood to hit on). I just have a few adjustments on the idle that I need to make (mostly related to the "fast idle" adjustment) now that I have a bit more understanding what that is.