DT85 loses power

[RHafford]

I have viewed tons of threads which contain similar problems to what I am experiencing. My 1985 Sunkist DT85 starts great, idles awesome, and runs perfectly unless I run at full throttle for more than 30 seconds. It does not misfire or spit and sputter but just runs as if I had pulled it to 1/4 throttle. I have had someone squeeze the primer bulb and this still occurs. If I continue to pull the throttle back to a sweet spot (about 3/4) throttle and give full throttle for a short amount of time, the engine does fine.
When it occurs I have to turn the motor off. Once I restart, the problem is gone. Some talk about a limp mode. Is this for my model year? I don't believe so.
I am going to check cylinders for fire while this occurs where I believe it is caused by a bad coil.
Can anyone give me more help please.

[artdf175]

I am not the expert but an observation
It sounds like it is starving for gas at the WOT
Have you done anything with the carb?
It is a almost 30 year old motor
I would start with cleaning the carb and making sure all the gas filters are clean if you have not already do so..
Art

[keakar]

I agree with art but also, when was the last time you replaced the water pump?

if it is old it is fairly common to cool fine up to 3/4 throttle but overheat at full throttle so it may be worn and not pumping enough water at full speed letting the engine run hot and go into limp mode due to overheating.

other things to check are blocked or restricted fuel line or filter screen in the gas tank or blocked restricted vent in tank cap so try running with the cap off and if its still acting up try it with a different tank and fuel line. also replace the fuel filter on the engine.

[RHafford]

Thank you for any help. It is very tough to explain just all the things that I have chard out. But here it goes.

I feel that I have exhausted the fuel issues. I ran on an external can. I ran with fill cap off and checked the vent. I cleaned the carbuerators. I replaced the fuel line. I rebuilt the fuel pump.

Even if it it was fuel related, why does it not spit and sputter? Why does it not come back when I bring the throttle back down? Why must I shut down and restart? As soon as I restart it will run fine. But not until I restart.

I have replaced the water pump. It has an overheat alarm. The alarm works. The alarm does not sound when this occurs. I would believe the overheat issue if the alarm was triggering.

Can anyone tell if this model truly does have a limp mode? Can it be bypassed?

As for the coils. I found bad plug caps to be more concerning. I have a new coil for cylinder 1 and a new plug cap. Going to tre her out tonight.

I appreciate all the help and I hope you understand that I am frustrated with this issue.

Thanks.

[Solarman]

running issues, at higher speeds

I might check the gear counting coil, if slightly too far from teeth it might (blur the count) cause timing issues at higher throttle then reset if turned off and back on. And it could be one of the pulse coils also related to timing issues. Pulse coils under the flywheel are not adjustable, but the gear counting coil can be bumped and change timing. Just a thought, let us know if you find out.

[keakar]

I know you said you replaced the fuel line but i have also seen rare cases where the engine gets fuel starved only at full throttle because its hooked to a 5/16" fuel line instead of a 3/8" one and along the same train of thought there was a guy whose fuel line swelled on the inside reducing the size so he was being fuel starved at high rpms. in but cases the fix was new fuel lines from tank to carb including under the cowling which gets overlooked many times even when all other fuel lines get replaced.

[RHafford]

Thanks for the help.

Update on the ignition coil. Number 1 failed the other night. While replacing it I noticed that it was only the spark plug cap that was faulty. I replaced all 3 plug caps. There is no change in my original issue.

I will check these fuel lines out better. I understand the fuel concept. However can someone explain a test that I can do? It really feels as though I am losing a cylinder. If it was starving for fuel should it come out of it by throttling down and not having to shut the engine off and restarting.? Is there a test while it is limping along?

I will check these other coils out and test. It just feels to me that it is an electrical problem.

Thanks again. I am impressed that I can get help with an old motor.

[keakar]

If it was starving for fuel should it come out of it by throttling down and not having to shut the engine off and restarting.? Is there a test while it is limping along?

what is happening, assuming its a gas restriction, is the float bowl is not filling up as fast as the gas is going out at full throttle so it runs dry starving for fuel. even a slight decrease in throttle may be enough to equal or keep up with the gas going out at that speed so the bowl can refill and the engine runs normal again.

this may not be whats going on but that's how a gas restriction works to make strange things happen, its not always a big enough restriction to be obvious and easy to spot.

the very nature that it resolves itself quickly says its a temporary situation going on only at full throttle and anything mechanical or electrical will usually not act that way to return to normal so easily and regularly.

the only test I can think of would be to immediately shut off gas flow and pressure to it and quickly pull the carb float bowl off or drain it into something when its acting up and see how full it is. this is not easy and would require a shut off valve you can close just before the carbs fuel inlet so the pressure in the fuel lines even on a disconnected line will have enough pressure in it to immediately start refilling the bowl.

if you have a new 3/8" fuel line connected straight to the fuel pump then that would eliminate a restriction up until that point and you can concentrate on the rest of the path like perhaps the diaphrams you installed in the low pressure fuel pump may have an issue causing them not to be supplying enough gas pressure.

all that being said if it was a fuel starvation problem caused by the gas supply I think pumping the primer bulb would force the gas to fill the bowl and should have resolved the issue so I think your problem may be a restriction in the fuel pump screen not letting gas through even when you pump the bulb.

I agree with your concerns that this issue doesn't seam to be following the logical way the symptoms should be to fit the possible causes we are discussing so it may be more then one problem working together to confuse the situation.

you may have 2 carbs working fine and just one has an issue of running low on fuel so your only running on 2 cylinders, it may also be a coil cutting out when it overheats only at full throttle but that would be rare.

try this, next time its cutting out on you (wearing gloves so you don't get zapped) unplug one plug at a time and see if you find one that doesn't result in a change in how it runs, this would at least tell you which cylinder is the one cutting out then you can swap ignition coils or even the carb for that cylinder with another and see if the problem moves with it.

[Solarman]

dt85 running issues

Another way to test as keakar suggested is with a timing light. When the problem occurs at wot clip the light to each of the plug wires.if there is an electrical issue with one cylinder one wire would flash differently than the others. If all flash the same, then put your hand over each carb throat and see which carb is causing the problem, it could be a needle valve sticking partially closed, or piece of trash in that carb slowing the fuel flow. But should be able to identify which carb is causing the problem, if fuel related.
Good luck, keep us informed.

[13eggs]

DT85 loses power

R Hafford..... You can check your floats in carb's sticking till you run the bowl dry,,or needle opening to much and your pulling all fuel out faster then coming in to the bowl..? then when you back off throttle you pull even ,,on both in and out flows ..just thinking could be your problem.. I have similar problem on my DT-8 runs great for a bit then spits and miss-fires and loses power LoL I could use some help too ......Good luck R Hafford hope my thinking helps..to my problem I think bad coil/to lean full/or like you bad plug or just cylinder just wore out ..could use some suggestions
people...thx 13eggs