DT75 bogs on throttle up

[surf]

have an 87 motor seems to run only on 2 cyl.I have checked for spark.and plug is wet so asume I have fuel .Will run only slightly above idle. after runnig only cyl. 1&2 are warm.I havnt checked comp. yet but assuming that is good what else should I check ? thanks

 

[jim dozier]

Re: DT75 bogs on throttle up

You said you checked the spark, so I assume you found that you had a good strong blue spark on all cylinders. You need to isolate which cylinder(s) are not firing properly. Unplug 1 cylinder at a time (it is recommended to ground the unplugged spark plug wire) and see which cylinder is not operating correctly. When you have isolated the offending cylinder take a look at the plug and carburetor for that cylinder. Is the plug wet (sign of a rich cylinder) or dry. If its dry, try squirting some premix in that cylinder and see if it picks up. If you have good spark at the right time, good compression and the cylinder doesn't fire it is either too rich or too lean.<br /><br />Causes of too rich mixture.<br />1. Choke stuck closed<br />2. Carb float valve stuck open, causing flooding in that cylinder.<br />3. Hole in inner fuel pump diaphragm injecting fuel into the cylinder (bottom cylinder only).<br />4. Somebody replaced jets in carb with too large jet (not likely).<br /><br />Causes of too lean mixture.<br /><br />1. Restriction in fuel pickup, fuel lines, fuel bulb.<br />2. Air leak in fuel lines/connectors<br />3. Carb float valve stuck closed.<br />4. Gum/varnish deposits in carb orifices/jets (see 2. above)<br />5. Hole in outer fuel pump diaphragm sucking air into fuel line.

 

[surf]

Re: DT75 bogs on throttle up

Jimd thanks for your time,good advice.cyl 3, the botom one is the one not firing. When I checked the spark it was in bright sun so cant say how strong it was. I will try it out in the shade.The plug on #3 was definately wet so looks to be to rich. Would a rich cyl.not fire at all? as it did not get warm at all. Thanks again for your time.

 

[jim dozier]

Re: DT75 bogs on throttle up

Something similar happened to my DT85. It started running poorly at low speeds like it was only running on 2 cylinders but at WOT would eventually catch and run fine. There was evidence of running rich, wet bottom plug, gas in the water, and the fuel bulb would not get firm. I traced it to the bottom cylinder and found that the inner diaphragm in the fuel pump was delaminating and injecting fuel directly into the bottom cylinder (to which it is connected in the intake port). Too much fuel not enough air = no fire. $22 and a new set of diaphragms and it runs fine.<br /><br />Try disconnecting the fuel line and letting the gas run out of the carbs and see if the engine picks up any.