17085586 on a 396 with a bit of a cam....

[Dtrue2]

Hi all, new member and first post.

Engine is:
10.25-1ish comp 396
Dual plane Weiand intake
Stock exh manifolds
Cam is 226/234@50 on 110 lobe centers.

I've been thinking about trying a Q-jet on my car versus the Holley's I've had on since forever so I asked a few pals if they had any collecting dust they didn't mind me ruining.

I got a 17085586 and took it apart, cleaned it up, had a pump cup to replace petrified one and tried carb on engine.

It ran badly as expected with main dribbling with throttle blades being way to open and bad stumble at low speed with mix screws not really responding.

Took it apart and made a few mods:

Idle tubes were small probably .033 so I drilled to .037
Down restriction was at .046 so I drilled to .052
Mixture screw holes were small probably .065 so I drilled to .093

Tried again and it is better and going the right way.

Float is at 5/16"
Mix screws are at 3 3/4 out
850rpm idle is smoother and vacuum is more steady and not hunting from the dripping mains.

Still pretty close to having main dribble because throttle blades are pretty far up trans slot so a slight opening of blades and it starts dribbling.

Still surging a bit in the lower rpms at cruise.

Hoping to not go too far with drilling but wanting advice on where to go from here.

Because it seems lean still I was thinking idle tubes to .039
down restriction to .055
Perhaps add some idle bypass also to move blades further down trans slots??
Thx,
D



 

[77cruiser]

How's your timing set up?

[Dtrue2]

18 initial
+8-10 manifold vac at idle
46 timing total by 2800rpm
Seems like a bit too much but engine likes it and starts right away with touch of key fully heat soaked
only get a small short bit of rattle/ping in 4th at low speed lugging real slow in 4th like you'd never drive like anyway.
Cold cranking compression is 160-165psi.
D

[Cliff Ruggles]

If you can run 18 degrees initial timing and the engine doesn't just about knock the nose off the starter "bucking it" on a hot restart then the cam isn't well chosen for the CID and compression ratio of the engine. 

Also liking well over 20 degrees initial timing at idle tells you the same thing.

The 110LSA cam is the culprit here, plus that carb is pretty lean everyplace and needs recalibrated for what you are doing.  I'd also install one of my rebuild kits in it with a high flow N/S assembly to keep up with the big block engine, and jets, metering rods, PP spring to get some more fuel to it.  It may need other mods to the idle system including bypass air, etc.....

[Dtrue2]

Thanks Cliff,
yes cam is a bit of a pig and It's what we did way back when, but it is still has all it's lobes. There may be a 468cu in bbc in the near future with roller cam anyway.

I am not thinking this exact carb will be the one I'm gonna finish with but would like to see how good I can get it to work, seems to be going the right way.

I will drill out idle tubes to .039
Drill down restrictions to .055
And open up idle bypass a bit.

Thx,
D

[Cliff Ruggles]

Yep, "tight" LSA is still alive and well and "pushed" by the cam grinders and quite a few resident "experts" and "guru's" on the Forums.

Here's a good example of how that works.  Attached is a pic of overlapping dyno charts from a customer who though he had Q-jet issues.  The root issue was the piss-poor cam choice.  Idled like crap, stinky exhaust and not making chit for power on the dyno.

Instead of offering tuning advice I offered specs for the replacement cam.  They followed my directions and were nicely rewarded with much better idle quality and vacuum at idle, plus just a "wee" bit more power at every RPM......FWIW....

[77cruiser]

My custom ground cam on 107 LSA does OK, gets 19 mpg on the road too.

[77cruiser]

Tried to PM you back but it said your inbox was full.

[Cliff Ruggles]

"My custom ground cam on 107 LSA does OK, gets 19 mpg on the road too."

Well, I stayed at a Holiday Inn Express last night and it was "OK".

Seriously, what are you suggesting here?  That the OP buy another 226/234 @ .050" cam and have it ground on a 107LSA with the ICL clear down at 103 to help with their issues?......