1986-87 DT40 running issues

[n1zmo86]

Hi i have a 86 or 87 Suzuki Dt40.
Last season ran perfectly never missed a beat. This season it wouldn't stay running and discovered that the jets were blocked. I have cleaned it all and put it back together and now it runs very good. I took it out for a run and it went as expected good, until. I backed off the throttle. It then died and was very hard to start and keep running. I chocked it a few times but would start and wouldn't stay running for more than about 7-10 seconds. But if I got it started, put it in gear and give it full throttle it would jump into life and planeing would be fine. Again until I backed off the throttle. I again pulled down carburetor and it's clean. Tried again in the harbour same problem. Soon as I get it home and put it on the hose it is perfect. No missing, nothing. Lost! I'm pretty mechanically minded and any help would be much appreciated. I have also replaced all fuel lines but not fuel pump. Cheers
Aaron

[Solarman]

dt40, 87, idle issues

Sounds like you did a good job cleaning carb, but, something still isn't just right. It sounds like the low speed idle passage or the vent passage still may have some blockage/ old gas/ or trash. You may have to clean carb again, and use compressed air to blow through the idle jet /air mixture passages, carb cleaner will work too.
What did you adjust the air mixture, low speed idle screw at?? Tell me that before rebuilding carb again.
Good luck let us know.

[n1zmo86]

I haven't adjusted anything yet just pulled apart and cleaned it. I'll pull it apart and clean the idle jet again. I just found it weird it will run with hose fine but not in the water once warmed up

[dj_movers]

First, I would continue to look for something gummed-up affecting idle performance.

Second, this reminds me of a flaky problem I had on a 1986 DT-115. When I would pull back on the throttle, it would stall. I replaced the diaphragm in the fuel pump and never had the problem again...

I think I have been fooled before thinking everything was OK running on the ear muffs.

Good luck!

[Solarman]

dt40, 87, running issues

I thought you said you cleaned the carb twice? Did you remove the idle-mixture adjustment screw when you cleaned the carb the other times?
Let us know if you did?

[n1zmo86]

Thanks I did look at replacing fuel pump as I can get one quite cheap off ebay and I think I will wreck the diaphragm pulling it apart. Also I did clean carb twice but didn't make a difference. The pilot valve has a hole in the top of the flat head screw but it doesn't join to the 4 small holes in the base of the jet. Is this correct or is it blocked? It's currently soaking in carb cleaner. Also there is pilot air screw that seems to be very stuck and can only turn it half a turn either way. Could I possibly have blocked the air or something? If I try turn it too far I will break the flat head screw off of it.

[Solarman]

dt40, running issues

If your air-mixture screw won't unscrew, you can not hope to clean the idle passage, and you can not adjust it to the proper setting? It has to be removed and replaced if bad. If someone tightened it too tight, they may have ruined the seat inside, and you may have to replace that part/ section of your carb anyway. It should NEVER be tightened EVER, only lightly seated then backed out about 1.5 turns.
Let me know what you do.

[n1zmo86]

I don't think it's done right up but more it's seized or got corrosion on the thread stopping it from coming out. I'll try work it out today

[n1zmo86]

So I managed to get the idle (pilot) screw out without damage. I cleaned it thoroughly and used compressed air to blow it all out. I have put it right back in all the way and wound it out 1.5 turns. Is that right? Or is that something I need to adjust when it's playing up.? Thanks heaps guys.

[Solarman]

dt 40, 87, idling issues

I gave you the 1.5 out adjustment as a start. Your manual might give more of an exact number. But always, it still may require a little fine tuning.
If your mixture screw, air passage block screw (flat with the 4 holes with plug in top), and the seat are good, and if you've cleaned out your air-mixture passage well enough, you should have good idling again.
Let us know how it goes.

[n1zmo86]

I have cleaned it all out best I can. It all Looks good. I have had the thermostat out and it was jammed open and have now replaced it. Before engine would run cold and I could touch motor without a problem now it's nice and hot to touch. I am going to try water test it today but just curious on the pilot screw. Is it a air fuel mix screw? I just don't want to lean it out. Also does that screw control oil mix? I notice if it's too far in the idle comes right down and smokes. Cheers

[Solarman]

dt40, running issues

The mixture screw adjusts the air flow that mixes at idle, the fuel flow is preset - no adjustment. Turning the screw CCW - allows more air into the fuel flow, making the idle leaner. Turning the screw CW - allows less air into the fuel flow, making idle richer. If your motor has oil injection, this mixture screw has nothing to do with the oil mixture.

[n1zmo86]

Ok cool so how do I know how much to adjust the air/fuel screw? I can't find it in manual

[Solarman]

I would adjust at 1.5 turns open to start with. Then while impeller has water to it, start motor. Once motor is warmed up, adjust mixture screw in, or out 1/8th of a turn, allowing motor to adjust for each setting, for best/smoothest idle. Adjustment of idle stop screw may also need to be adjusted, until fine tuning of mixture screw is finished. After some running time (about an hour, or two), inspect plugs for proper coloring. A tan to light chocolate coloring is a good color. If too light, motor may be running too lean, if too dark it may be too rich. Adjust accordingly, for best long term operation.
Good luck, happy motoring.

[n1zmo86]

Success! I test ran it yesterday and I didn't get the dying as I back off the throttle. I went right up to full. Throttle and it actually died under load I think because I have next to no fuel left In tank and primer bulb was soft. Primed it up and away it went again. But now the exhaust is steaming a bit. Will it be running too lean?