40 DCOE issue, maybe?

GregS

GregS

ProjectX
I have just refitted the 1500 engine in my track X after a year and a half doing body repairs. The engine hasn't been touched in that time, I had the headers ceramic coated before refitting, and on start up the pipes from cylinders 3 & 4 are running hotter than 1& 2, so much so that the white ceramic coating on 3 & 4 has turned a noticeabley darker colour than 1 & 2. I am thinking 3 & 4 may be running a bit lean, but I have checked everything I can think of that may be causing this, float levels, all jets in both carbs, progression holes, fuel passages as much as possible without stripping completely. Can't find anything wrong. Car is not registered, so only running between 1200 and 5,000 rpm in the shed, no load. Any suggestions would be appreciated.
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Hi Greg,

In the picture, the fuel hose and the linkage aren't in place, but I guess you did them before starting the engine. I also guess you already checked if the fuel is coming to both carb. After that I would check the linkage. If you return it by hand, is the idle slowing down. This may sound silly, but maybe there is a restriction in the linkage and it would not return to "no fuel" position.

After that, I would check if there is no air leak at the carb/intake, and intake/engine. Did you test the air pressure with a synchronizer at each horn?
 
Have you checked your timing (with a timing light) on cylinders 3 & 4, just to see if the plug leads are firing at the same rate as cylinders 1 & 2? How do the spark plugs look on all four cylinders?
 
I would work on the carb setup being the source although I can't for the life of me figure out what it could be. How is the synchronization of the two carbs to each other?
 
I haven't got to the bottom of it yet. I pulled the manifolds off to put some sealant on the gasket for the inlet ports incase there was an air leak. And found no. 1 exhaust port all wet and oily or soaked in fuel, not sure which, and no. 1 plug. Did a compression test and all are between 145 to 150 psi, so not bad. Checked out the no. 1 carb for possible excess fueling, float level is fine, needle and seat not leaking, cold fuel enrichening (choke) circuit is not stuck or leaking. Possibly a valve stem seal? I put the sealant on the inlet gasket and put it all back together, ran out of time to start it and check things, will try again tomorrow. And I broke the electrical connection to the temp sensor trying to fit the compression tester, now I have to fix that.
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Number 1 header pipe not getting as hot as the others.
 
Sounds and looks like #1 plug is not firing. 3 & 4 plugs look normal. 2 looks in between fouled and not fouled. Maybe your timing is a bit advanced and a separate issue may be that energy is not being supplied to #1 & intermittent to #2?

It also could be that the butterflies on all carb circuits are not synchronized by the linkage tying the throttle shaft movement together. (It may be allowing more fuel to 1 & 2 and less to 3 & 4.)
 
As I recall, DCOEs have a little view port where you can see if the throttle plates are all synchronized. At least verify they are set correctly but I would think unsynchronized carbs would give a nasty idle with the engine rocking.
 
I think I may have fixed the issue. One carby was not closing properly, bit of an issue with linkages. Made a few changes and paid more attention to the balance of the carbs, as suggested. This time when I stared it I only used the choke for a few seconds, to not foul the plugs. Ran it till fully warm at around 2,000 revs, number 1 header pipe changed colour to the same as the others, and the plug on no. 1 no longer wet. Still black from running rich, but that's ok, it's a track car so the important thing is at WOT. Thanks for all the suggestions.
 
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