'89 DT75 dropping a cyl at speed

[Sealfie]

I haven't timed it exactly but I seem to remember it's random, not exactly 10 secs. I'll get it on the lake and document. I noticed yesterday after warming it up idling about 5 minutes, I took off at full throttle (about 4,700 rpm) and it cut back well before it did when I ran it up from the same spot and took it to 3,800 or so several other times. That made me think it may be overheat related but I've replaced both heat sensors (puck and duckbill) with new.

I haven't used a Suzuki gauge. From what I see it warns of over-rev, temp and low oil, correct? Or does it also warn of oil flow? I can see where this might help direct a search but won't pinpoint failed components.

I haven't read all thread but this problem really sounds like a classic lack of fuel problem. You say you have rebuilt fuel pump, thats good. My concern is within the fuel lines and connections. I think you have an airleak somewhere even though you replaced the fuel lines. If you have an airleak the pump will partly pump air instead of fuel. I strongly recommend that you try with another portable fuel tank, preferably with a friends fuel tank including fuel lines and connections that you know is OK before you do anything else becuase this is so easy to check. You can also try to push fuel forward manually (with the bulb) while you drive, could make a difference, wont help enough if the leak is severe though.

[Solarman]

Can use a timing light on each plug lead to see if the light flashes normally, or is skipping during times it is missing.

Post back what you find out.

[Flatsboy]

Just tried a known working fuel pump from same year DT75, same result. Next step is to rebuilt carbs again, double check float levels, replace reeds. Need to get 2nd pair of hands to pull spark plug leads one at a time once it limps to isolate which cyl is misbehaving.

[Flatsboy]

Checked the parts diagram and I don't see the disks nor does the service manual mention them. Exactly where are they located? Thanks SM

[Flatsboy]

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

[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]

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]

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.

[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?

[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]

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]

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]

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

[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]

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