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?

I have quite the collection of jets anymore, so if you want to try something different, I can lend you some before you buy

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?

How do you know if and where the intake is leaking?

I've tried flowing propane, carb cleaner and water around the flanges with the engine running to no effect. I'm not sure how to find the leaks.

Sounds like you need to recheck your floats

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.

AFR? Are you good after your pilot screw adjustment?

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 May 12, 201311 yr by LeonV

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.

So, do you think that the porting caused a lean condition by allowing the motor to flow more air?

I know you said timing looks normal...have you tried advancing the timing a few degrees to see if it smooths out?

Mikuni 40phh intake runners - Fuel Delivery - HybridZ