The Thunder series carb does not have the counter-weighted air valves, that's the Performer carb. Mine has the air flap. Owners manual covers both carbs but shows the Performer in the description. I see where it says both primary/secondary blades arrive at the WOT stop at the same time but didn't see where it says they are mechanical. If mine aren't opening all the way, I don't know how to adjust them. Only thing manual talks about adjusting is the air flap on top, which I've already done.

 

Their directions, as I took it, refer to when the engine isn't running. There is no vacuum with engine not running, which means it would be a mechanical movement. I'd have to have one in front of me, or a good video, to see what the linkage is doing, or not.

 

i used the same full syn on mine an had no problem shifting ,it was a question i asked the .builder ,he said great will work fine atf type .f maxlife ,said was great .for rebuild th350 ,4000 mile later shifts just fine an dandy

 

Check out these videos:

If the 1.5 turn preload pressure is not there, then the flap might be opening too soon, and causing the backfire.

 

Thanks for those Vall, I've watched those several times sir. I think I might have located the issue. Hard to explain, even harder to get pics showing what I mean, but here goes. When trying to manually open the secondaries last night I noticed that the small linkage on the throttle side that operates the secondary blades was hanging up on the piece behind the fast idle cam. The piece is supposed to fall out of the way after the choke opens from what I can tell. The choke was opening when it warmed up, but wasn't opening quite far enough in order for that linkage/tab to clear it for the secondaries. That's why when I said before that they were opening but didn't look like they were opening all the way. Here's a couple pics attempting to show what I mean. I adjusted the choke again so it would open all the way and the secondary blades began to open all the way. I'm uncertain whether or not the choke has to be fully opened before the secondaries become operational?? The air flap is adjusted the proper way. It was really late when I was working on it so I didn't do any WOT tests to see. Terrible storms up here yesterday and today also so there won't be any road tests unfortunately. I'm going to check it out today, at least in the garage and will get a clip showing more specifically what I mean. The throttle bracket is in the way from showing on the pics but I took an older pic of the 600 carb with the same type of linkage to show what I mean.

 

Lol, previous page post #1377
You found at least one of your issues.

 

Previous post 1378.



Again, forgive my ignorance. If that's what the secondary interlock is, then I just learned something. Thanks! I thought that had something to do with the choke flap being linked to the air flap over the secondaries. I did look down the throat of the carb, that's when I had stated it didn't look like they were opening all the way.


So, the choke DOES need to be fully opened in order for the secondaries to function?

 

K, I thought you may have known how the choke interlock works, as it's not specific to your model carb. The secondaries are locked out until the engine warms up. Linkage arms against the secondaries on some models/brands won't clear out of the way till the choke is fully open, other's can with a partial choke. This is to help prevent the pilot from beating on a cold engine.

 

That's makes sense to me sir, thank you for the insight. I'm going to keep an eye on the choke plate for a couple of starts/warm-ups to ensure that it is opening fully. The choke does open, but it was that last 1/8" that it wasn't opening that was keeping the secondaries from being able to open fully. Might need to adjust it again or maybe bend the rod a little bit if it keeps doing the same thing. I'm going to head out shortly, after I bundle up. It's a sweltering 59 degrees today, hope the choke works!

 

If the lockout linkage doesn't want to play nice where you like the choke setting, you could always fine-tune it; wire or bend arm out of the way, or slide/remove the split pin if that's how yours is. It's that way to make cold engine lower rpm operation numbskull proof. lol