JDX8 Engine Missing When Warm

DLDeere · Post by **DLDeere** » Thu Jan 22, 2009 12:09 am

I've searched all over this forum for answers and can't find any that might apply. I've even read most of Danzig's 15 pages of "X8 still broke" and still don't have an answer.

I have a 1973 JDX8, CCW 440/21. Starts fine and will run great for a few minutes. 50+mph. After it warms up for a few minutes or has been driven for 300 yards it doesn't have enough power to move itself. I have determined it is missing on #2 cylinder (PTO side) by pulling the #2 plug wire and there is no change in engine sound or performance. When I pull the #1 wire, the engine dies immediately. After standing and cooling for about 30 minutes, it will start and run fine and then repeats the above.

There is still good spark on both sides, both cold and warm. (Accidently got zapped by #2!) When cold there is 140 & 150 psi compression. I haven't checked this when hot. During the rebuild process we put in a new carb kit, fuel lines & pulse line, filter, pickup, and cleaned the fuel tank. I am using a 50:1 mix with premium. It has new external coil on #2, new plug wires and new NGK B8HS plugs. #2 cylinder head had stripped plug threads. Put in a heli-coil to fix that. Checked and cleaned reed valves. Checked, filed and adjusted the points and gap.

I can't figure out what why the #2 cylinder quits firing after it is warmed up for a few minutes. I'm stumped!

Help!

Howard

Post by **Matt** » Thu Jan 22, 2009 12:27 am

Maybe bad internal coil? Coils can fail when they heat up, and work again when they cool back down.

DLDeere · Post by **DLDeere** » Thu Jan 22, 2009 1:15 am

That was my first thought, so I double checked for spark when it was missing and I still had good spark.

JDFanPa · Post by **JDFanPa** » Thu Jan 22, 2009 2:13 am

You should check compresion when this happens could be a leaking head gasket. Ive had it happen. How is your fan belt fan etc. Does the engine have all the tin work inplace? If your engine is not cooling properly no2 will get hottest 1st if it gets to hot the fuel will actually vaporize and burnat the wrong time. What does the plug look like on the dead cyl when this happens dry or wet?

JDT · Post by **JDT** » Thu Jan 22, 2009 9:57 am

Howard what about the cap and wire on #2?

WatertownBob · Post by **WatertownBob** » Thu Jan 22, 2009 10:46 am

It must be one of the coils. Did you put new spark plugs in?

I am not familer with this series of sleds are there seperate external coils one for each cylinder?

Could you ohm the coils after there warm.

Had a liquifire act this way changed the externial then it ran good.

I test airplane spark plugs at work. I can turn up airpressure and the spark will change under pressure... Get fatter and blue on a good plug or quit firing on a bad plug.

Not shure if a plug fires while laying ontop of the head if it still will fire installed under the compression of a running engine on a weak ignigtion system.

Bob

tmartin · Post by **tmartin** » Thu Jan 22, 2009 12:43 pm

Condensors will act this way too. I have had it to many times on my antique tractors where it will run fine until it gets hot and then it will loose power and develope a miss. It usually turns out to be a condensor which is often overlooked. This would be a cheap place to start as well. I believe dennis kirk or like companies can supply you with the correct parts.

Rodimus Prime · Post by **Rodimus Prime** » Thu Jan 22, 2009 5:19 pm

tmartin wrote:Condensors will act this way too. I have had it to many times on my antique tractors where it will run fine until it gets hot and then it will loose power and develope a miss. It usually turns out to be a condensor which is often overlooked. This would be a cheap place to start as well. I believe dennis kirk or like companies can supply you with the correct parts.

there is two types used in jd unfortantly niether of the condensors work in the other.

if your condensors have the prong for cam felt then you can order them from dk or whatever. if no tab for cam felt not so easy to find.

One is nippon denso other is kokusan i forget which is which.

DLDeere · Post by **DLDeere** » Thu Jan 22, 2009 6:07 pm

Andy, good suggestion. I'll check compression when it's hot. Fan, belt and tin is all in place and adjusted properly, so it should be cooling, but I'll look at the plug again and try to get a better idea.

Bob, interesting about the spark plugs. When I pull the plug, it still sparks, but I wasn't aware that it can change under pressure as you said. I put on a new external coil. So next step is points and condensor as tmartin said or the internal coil as Matt suggested.

Todd, new cap and wire, but it doesn't click on to the plug real well. Still there is spark on the plug so it should be OK.

I'll start with another look at the plug after it's warmed up and missing and do the compression test. If compression is good and plug is wet, then it's into the ignition.

Thanks for all your suggestions! I'll keep you posted.

Howard