'89 DT75 dropping a cyl at speed

[mphelle8vld]

It doesn't detect flow, only if the oil filter in the sensor is dirty. Let us know about the 10 sec, if you don't think the problem is limp mode related I'll defer to Solarman. 4700 sounds low for wot, overproped?

[Flatsboy]

Protection/Limp/Fail Safe modes - Like to understand exactly how this works. I've read the OEM Service manual and the Seloc 1600 Repair Manual but I can't find a rundown on how the ECU processes the sensor inputs and how it reduces RPM. It drops a cyl (in the DT75) but how does it do this and which cyl? Does it block the fire signal from the ignition timing coil in the magneto or does it accept it and block the signal from the ECU to a particular ignition coil?

May get some test time in tomorrow and update. Going to install the 3 inline spark testers to see which cyl cuts out when misfiring and going into 3,000 rpm mode. Will also document how may secs after going above 3,400 mark it takes to misfire and drop into 3,000 limp mode. Will also get more exact readings on TPS.

The '88-'03 Seloc 1600 manual (as with Mercs) is more detailed in some areas than the OEM but is frustrating as it documents readings for most engines from 115HP and up, not much for the 3 cyl and below units.

[mphelle8vld]

Sounds like a good test plan, I'll knock the dust off of my old manuals.

[Flatsboy]

Great, took the rig out today, got all readings (within spec) again on the TPS, checked spark, counted secs, took videos. Will post results later today if I have time. It was way more than 10 secs though before it limped and when it did all 3 plugs were firing, no alarm.

[mphelle8vld]

Carnauba wax is recommended for the flywheel, awaiting your results.

[Flatsboy]

Tried several Carnubas and settled on Joe Gibbs Racing's Driven Race Wax. According to the formula EK ⋅ mass ⋅ radius2 ⋅ RPM2 the flywheel edge is spinning about 379mph so I may need to upgrade ;-)

[mphelle8vld]

I don't see your motor out on Craigslist, what's the latest?

[Flatsboy]

Good one Ok, here's the goods, hope it sheds some light:

1. After engine cuts back I have good spark on all 3 cyls
2. Notice no alarm in videos
3. If I hadn't rebuilt the fuel pump I'd be guessing it was the culprit.

TPS settings smooth increase in readings all the way up
https://www.flickr.com/gp/12676294@N02/11w42L

Video 1st run, wide open then it cuts back after 20 secs, surges (no surging on subsequent runs)
https://youtu.be/Z8TDvnYkjms

Video 2nd run cruise at 3,700, cuts back to 3,200 and stays steady
https://youtu.be/o2qNCepLySg

Video 3rd run wide open, cuts back to 3,200 and stays steady (excuse the cavitation at throttle up, forgot to trim it down)
https://youtu.be/y24BeSnZIgs

[mphelle8vld]

Thanks for posting the videos, it sounds good... I've been doing some reading, fun fact - Suzuki triples don't drop a cylinder for limp mode, they cut every other spark to all cylinders to keep from fouling a plug. I think it would be a worthwhile test to disconnect all sensor inputs to the cdi (at the cdi) including the 6 pin black plastic connector (only 5 used) which would eliminate the harness also. Let us know if you get a speeding ticket.

[Flatsboy]

Thanks but I already tried disconnecting all sensors incl TPS with no change. Tried disconnecting the black 6 pin a few weeks ago and it wouldn't run over 2,000.

[mphelle8vld]

I'll have to look at the schematic again, I thought the black connector only had sensors and the warning horn. Did you try that test with the known good cdi?

[Solarman]

Your videos sound like carb(s) may be running low on fuel at higher rpms, this could indicate float levels set too low in at least one carb, possibly more than one. This could explain why it takes time before it starts to surge. Or, you could have trash blocking high speed jets.

Good luck, post back when able to let us know.

[Flatsboy]

Jets are clean, float levels ok. That 1st video is the only time it surged like that it of maybe 50. Thinking fuel pump or reeds. Pump has new diaphram, acted the same after replacement, possibly the spring or pinhole air leak in housing? Sourcing used fuel pump, maybe a reed set. Thanks for the brainstorming, I'll figure it out and hopefully it'll help someone later.

[Solarman]

FB, i understood you rebuilt fuel pump. Did you inspect the check - valves in the lower section (clear discs)? These check - valves only come with new pumps (not in rebuild kits). If these discs fracture/breakup, fuel pump will not provide enough fuel. These check-valves cause the diaphragm movement by the pressure and vacuum in the crank case through that small hole in back of the fuel pump. Likewise, if the small hole is blocked up, movement will also be limited, a new diaphragm kit does not fix these problems. Inspect these areas closely, to see if any problems.

Good luck, post back when able to let us know how it's going.

[Flatsboy]

I did not check these but I will. Thanks for the tip.