Ailing Crossflow

[bball7754]

My Crossflow is not a happy camper these days. Symptoms are that cylinders 3 + 4 are running lean. I get spitback through the carb right up to 6K rpm, ceramics on both plugs are white. Came on more or less suddenly at a trackday.

 

Valve springs seem ok, and all the valves open and close. Compression check on the cylinders was good - they actually were the best of the 4, #1 the worst.

 

I've pulled the carb, but before I did checked to see if they were in synch, and they were. No apparent leaking around the O rings between the carb and manifold, but that's only based on a visual check. Didn't check by spraying starter around the carb while running.

 

Was hoping something would jump out at me when I got the carb off and apart, but everything appears ok. One other possible issue is that the O rings for the Pump Jet covers are disintegrating. Replaced them when I rebuilt the carbs about a year ago, and clearly the O rings provided in the rebuild kit weren't up to the task. But hard to imagine they would allow enough air in, particularly when the carb is on the main circuit.

 

I'm about to the point where I'm just going to put the carb back together (with the old O rings on the Pump Jet cover - yes, I did save the old ones) and hope for a miracle.

 

Any thoughts or suggestions?

 

Thanks

 

Steve

[scannon]

Is it possible to swap the carbs front to rear to see if the problem moves to the front cylinders?

[Ian7]

Is it possible to swap the carbs front to rear to see if the problem moves to the front cylinders?

 

assuming Weber dcoe's, yes, but lots of grief switching throttle linkage...

[TheDingo8MyBaby]

Have you checked your timing including advance? Fuel Pump?

[bball7754]

They are Weber 40 DCOE's, and agree that switching the linkage would not be fun.

 

Fuel pump is mechanical, and the front carb has plenty of fuel. Wondered about timing (I haven't checked it since this happened), but wouldn't it affect all 4 cylinders in the same way? Front 2 are definitely not running lean.

[xflow7]

Maybe check that the float level is correct and the needle valve isn't sticking?

 

Also, maybe make sure you don't have a big exhaust leak? I had a big leak on 2/3 and replacing the exhaust gasket made it run a lot better.

 

I think I have read tale of the screws in the bottoms of the barrels that secure the Venturis coming loose wreaking all kinds of havoc.

[bball7754]

No exhaust leak, and the venturi screws were in place. Looking more and more like put 'er all back together, and keep my fingers crossed.

[WestTexasS2K]

I would check float level in the rear barb make sure the needle set are clear.

[MHKflyer52]

Check your compression as that also effects how much suction you have through the carbs. Should all be within a couple of pounds I would suspect but if not then you need to adjust the carbs with a synchronizer to get the correct air flow through the ventures.

Also check the base of the carb to intake manifold and manifold to head for leaks as that can also cause a lean or rich condition if there is a leak there.

Just a though and hope it helps.