popping through the exhaust at 4500 rpm or so.. HELP!

[30 Ounce]

Don't jump directly to 20wt, try 5wt fork oil or ATF (they are about the same), then 10wt then 15wt. There is a huge difference between each one. SU oil is about 12.5wt.

[motorman7]

I'm running 10W30

[steve91tt]

For what it's worth I found that both 10W30 and 20wt worked well in my SU's.

[240260280z]

Try a slow acceleration from 1000rpm on a flat stretch in 1st then 2nd then 3rd gear to see exactly where the popping occurs.

The damping oil should only affect:

1. throttle transitions due to pedal

2. engine vibration/road bumps

3. engine rpm changes from gear changes

If you do the test above you will rule out the oil viscosity.

[Zedyone_kenobi]

Good ideas guys.

Blue, the reason I am on the oil right now is I sort of half way ran that test yesterday with the Marvel Mystery oil (about 3 wt according to interwebz sources) and any gradual roll on to the throttle seemed okay, but any drastic change at all the engine just about stopped running. THe more I moved the butterflies the more drastic it became.

So here is my logic. Flawed as it may be.

Right now my pistons drop perfectly in tune with each other. With or without the top of the carb screwed in. So I know there is no hang up in the needle or the piston. CHECK!

With SU oil (12.5 wt, thanks 30 Ounce), it popped and sputtered at about 4500 rpm

With Marvel Mystery oil (3 wt ), it popped far worse at about 2500 rpm

I am now running straight 20 wt. We will see when I get her on the road today, if the trend continues, this SHOULD get me on the right track...

But I will not rule out a 15wt oil if this works as well.

Like a bad sit com... The saga continues...

[Captain Obvious]

I did notice that when I first looked at the front float, that the wrist pin was dislodged on one side. Now I do not know if I did that when I removed it or not, but it was the only thing noteworthy at all in inspecting the floats.

The bowl and lid were designed such that they cannot be assembled properly if the pin is out of place. If the pin were so badly out of place that it had slipped completely out of one hole, then the pin would hit the bowl during assembly and you wouldn't be able to get the lid on flush.

As for damper oil... The 72 manual suggests 10w-30 and the 74 manual suggests 20 weight or 10W-30.

Like a bad sit com... The saga continues...

No kidding!

I know I'm reaching, but I wonder if there's something wrong with the PCV system somehow. The PCV is supposed to allow a pre-determined "unfueled" amount of air around the carbs, but it takes manifold vacuum in order to make that happen. Because of the way the PCV is designed, if there's not enough vacuum (like maybe when you are at part throttle), the PCV will pass too much air.

Maybe you could cap of the PCV for a test run just to see what happens? Capping it will richen your idle mixture, but once you get off idle the change should be small enough that it won't matter much.

[Zedyone_kenobi]

I will put that on the list after I try this 20 wt.

I wonder what the 71 manual says to put in it. I cannot imagine it would be anything different. I have been running the SU oil for a while now as MSA says ..."this is the perfect oil for your SU's".... If it is 12 wt, I cannot imagine why they said that. When I first got my SU's from Z therapy, I put 20wt in them and that was as good as they ever ran. Then I remember switching to the MSA SU oil because... well because it was SU oil! I somehow thought it would work better...

[Captain Obvious]

Yeah, I'm with you. I would expect that "SU oil" would be perfect for the SU's!!

I haven't downloaded all the different years of FSM's. I got 72, 73, and 74, so I don't know what 71 calls for.

Another straw... I just noticed in your sig that you've done some changes to your evap system? What can you tell us about that? I can imagine scenarios where the evap system could allow too much air under certain conditions but still appear to be working correctly at idle.

[Zedyone_kenobi]

I made my evap system back to 100% stock. All hoses that are suppose to run to the tank, run to the tank. All hoses that are suppose to run to the engine and filler neck, run to the engine and filler neck.

[madkaw]

FSM states #20 weight Mobil oil. It says not to uses #30 weight or higher.

I will add that the float dimensions worked for my carbs, but I did wind up richening the float setting for max power. What I am saying is that the engine would rev up, but there wasn't much power at the upper end. You still may need to adjust them in the end. Good to see your making chances that actually effect the engine.

[Zedyone_kenobi]

Thanks for your advice madkaw! I certainly appreciate it.

[Captain Obvious]

FSM states #20 weight Mobil oil. It says not to uses #30 weight or higher.

72 FSM (EF-26) says to use 10W-30, but not SAE 30. They must have expected that SAE 30 would be too thick when cold, but the multigrade would be suitable.

As for the evap system, I just refreshed my memory on the early (flow guide style) systems and after further review, my mistake... I cannot come up with any reasonable explanations how the (early) evap system could cause any extra air around the carbs into the engine. Later carbon canister style, yes. But the flow guide style like what you have? No.