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Adjusting Q-jets, SBC, need help please.

B

Ben Malinski

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I bought a 454 some time back. Ran fine for awhile and threw a rod. Reinstalled the 305, original radiator, etc.. while the 454 gets rebuilt. Here's my question(s).
This 305 is in good shape, as far as 305's go.
I can't seem to get the adjustments on the carb correct. Truck has a manual trans. When trying to drive off from a stand still, I get some hesitation it seems before it "catches." A lag sort of. I am use to letting the clutch out and the truck moves! I set the timing at 4 degrees, then changed it to 6,8 10 and 12. No difference. Someone mentioned tweking the Q-jet so that the secondaries open more quickly. If this helps at least. Or tweaking something so that I get no hesitation. Any ideas or suggestions would be appreciated! Thanks! M4ME carb. I'm using a different carb for the 454.
 
Oooo, been there. Don't worry about the secondaries, they only open past 2/3 throttle. 12-16 is a good initial timing setting. Don't know why so many people are scared of it that high. A few GMs in the day had a 16 intial, as do I. Anyways, where is your vac advance hooked to? If its on full manifold vac, its gonna stumble and bumble off-idle. I believe your M4ME should have a port for timed manifold vacuum on the bottom left of the front of the carb. No vac will be present at idle. Its just like full vacuum, only the port in the primary is covered by the throttle blade until the throttle is open far enough. With that big 16 inital 8-12 degrees of vacuum advance should suffice. Adjustable units are great for that. Just use the allen wrench that comes with them to set it. All have the same numbers stamped on them, like AS-20 (probably same manufacturer), but Crane Cams has some BS about theirs having adjustable rate and they give you a limiting plate to set the amount, unlike Accel and other companies. I checked it all over with a vac gauge/pump and there really is no affect on rate, just amount. Again, anyways, have your mechanical advance in by at least 3,000 RPM.

That should do it for you, if not..... Look down the primaries (engine off) and give the throttle a pull. You should see two squirts of gas, thats your accelator pump system. Beyond this would require removing your air horn and having a look at some other things that we can get into later.
 
Sorry if these seem stupid, but..

Is your choke staying open?
Are you getting a good squirt of gas from your accelerator pump? You can test this by holding your choke butterfly open and then giving the throttle a steady twist. There should be two streams about the size of a pencil lead.
 
Yep, the choke is fully open and the carb is getting gas. Replaced the fuel pump also and the fuel filters. Thanks for the suggestion!
 
So the accelerator pump is working right, correct?

I'm beginning to wonder if you mean your secondaries aren't performing like you want. If that's the case then you can drill a bigger hole in the vacuum can (motor, whatever you want to call it) so it opens faster or buy one that opens faster (around 1 second to open instead of the stock 3-4).
 
Blazer Boy. I've got the vacuum hose connected to the driver's side port of the carb. It was on the passenger side and I just swapped places. I notice no difference no matter which side it's on! I did re-adjust the timming to 12 degrees.
Does it matter which side the hose is connected to on the carb? Thanks!
 
If u give it more secondarie it will "bog" even more !!
U should check if the front vacuum break is still good,(the one that as a rod that go to the secondarie) to do that take the vacuum hose off and push on the rod then place ur finger on the end(were the vac.hose should be) and see if the rod stay there .the rod should pull a bit but not all the way. if it does,2 thing can be wrong .
1 the vacuum brake is bad
2 the vacuum brake has a little hole in it . look to see if ur vac.break has it (suposed to be under a rubber thingny in the front )them plug it and try again .If it still pull out get a new one!!
If u buy a new one make sure u bring the carb.number , they will aske for it (driver side right above the sec.shaft start whit 170.....or 70.... depending of the year of ur carb.
hope it helped a bit!!
 
wrathorc::. Tell me more about how to get the secondarys to open quicker..Pictures would be wonderfull...Thanks

Also i am having kind of the same trouble (i think) I have a 350 backed by an th350 auto. And at a stand still, if you Punch the gas and hold it open it kinda stumbles and falls on it self (so to speak) but if you wait till about 2000 rpm you can punch it and it wont do that.....also if itpicks itself back up after stumbling the secondarys jump open and it hauls major ASS!!!! but if you do that 2000 rpm WOT it wont hit the secondarys for a long long time..................Any ideas????? /forums/images/icons/confused.gif
 
I believe that may be for your torque convertor lock-up, if you have one. I've got a M4ME from around '85 laying in my lap right now and the port on the far right is also in the throttle body (base), but is NOT covered at idle. Are you by chance setting your timing with vac advance connected? If you are and say you have a 20 degrees vac can, you could have your timing at 8 degrees AFTER TDC. Funny, huh?

I don't like going beyond stock timing settings much unless the original vac line routing is used. They got crap in there that takes out vac advance on decel, and in lower speeds, etc. The full 20 degree amount of vac advance probably doesn't get used a lot. 8-12 degrees vac advance is a much better deal when you start uping the initial timing and only have one line going to the vac advance.

Plug it into the bottom left port. Time it 12-16 with the vac advance off. Then start the adjustable vac advance at 8 degrees and work up one turn at a time until performance falls off (you can hook it to full vac at idle and watch how the timing jumps when its plugged in to get an idea of the amount). I think you'd also see a big help if your mechanical advance came in at or a little under 3,000 RPM. No need to buy new kit springs, as stock ones can do this. I've got "090" weights and one super fat stock spring and another medium stock spring from an old HEI and its all in my 3,000.

(Check that vac can you have now, it will have a number stamped on the end of the plate like "20" or "15" for the amount)
 
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