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Why are my mikuni's backfiring?


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I took my Mikuni 44's apart this weekend to gasket match the cannon intake. The car now backfires quite badly out of both the carbs and the tailpipe while idling, downshifting and cruising. The AFR's are 11-12 while backfiring and the car pulls hard at WOT where the AFR is in the 12's . The plugs seem normal, timing looks normal, float bowl levels look normal and compression is 170+/-5 across all cylinders. The engine is a Rebello L30. The carbs have 39mm chokes, 150 fuels, 180 airs and deleted balance tube on the intake.

Any advice as to how I should diagnose the problem? Could this be caused by a vacuum leak?

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I took the carbs off the intake, the intake off the head and I took the top off the float chamber. I smoothed out the transition from the carbs to the manifold. I did not do anything with the head side of the manifold.

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Yes and I replaced the o rings with new.

The previous owner had done a hack job on the carb side of the intake. The head side is still stock dimensions.

This is a photo from before porting the carb side of the intake...


90% done...


I didn't take a photo of the finished product but it made for a nice smooth transition from the carb to the plastic spacer to the manifold if you look down the throat of the carb.

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I re-tightened the manifold/exhaust bolts last night to no effect. Last night the car was backfiring though 1, 4 and 6. This morning I removed the manifold and exhaust and replaced the gasket with a felpro unit applied dry. I had a new set of exhaust and intake studs laying around so I replaced them while I was there. The car now seems to idle more roughly than it did before the gasket replacement and it pops quite badly though #4 and #6. The backfires are now more intense than they were. I plan to let the car cool and re torque the gasket. Any advice for getting the intake to seal?

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Sounds like you need to recheck your floats

I checked the float levels by removing a jet block from each carb. The float levels look normal and even between carbs. I would also expect that if the floats were to blame that pairs of cylinders would misbehave as one float adjustment services two cylinders.

I re-torqued all the bolts and the car still backfired. I then started turning the idle mixture screws to richen up the cylinders that were popping and lean out the ones that weren't. With this adjustment the popping/backfiring has stopped. I ended up with the following mixture adjustments per cylinder.

1 = 1.75 turns out

2 = 1.25 turns out

3 = 1.25 turns out

4 = 2.0 turns out

5 = 2.0 turns out

6 = 2.5 turns out

I assume that the extra fuel requirements of some cylinders is a function of a specific vacuum leak in that cylinder and potentially carb to cab differences in the idle fuel system.

This is a bandaide solution for some underlieing problem(s). The search continues.

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Have you checked valve lash just to be sure it's a carb problem?

By cleaning and resealing the carbs and intake, you could've revealed some issues that were masked before.

Edited by LeonV
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I checked the valve lash a couple of months ago. All was good no adjustment needed. AFR is in the 11's at idle. I can't drive the car right now as I took the rear stub axel off this morning to replace a grumbly bearing. Should have it back on the road for a test drive on Wednesday.

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