Why are my mikuni's backfiring?

[madkaw]

Bad float adjustment?

Why did you adjust them?

[madkaw]

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

[steve91tt]

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?

[madkaw]

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

[steve91tt]

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.

[madkaw]

Sounds like you need to recheck your floats

[steve91tt]

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.

[Travel'n Man]

AFR? Are you good after your pilot screw adjustment?

[LeonV]

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, 2013 by LeonV

[steve91tt]

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.

[tlorber]

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

[TBone028]

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