WOT hesitation/stall

[jsweet]

I'll start by saying I'm new to this site and to the Blazer "club". I bought a 72 Blazer a few weeks ago and I've already used this site for the wealth of knowledge you guys have.
The question I have is that the Blazer has developed an issue with full throttle. When you stand on the pedal, it'll either hesitate and then take off or just stall all together until you let off the pedal. It seems weird to me that it runs great at idle, half, or even 3/4 throttle but will have issues when it's all the way to the floor. Since this is new to me and I didn't put carb/motor in it, I'm at a loss on where to start. I thought of fuel filter but seems like it would sputter at WOT and not completely stall. Vacuum lines look good but I haven't checked pressures.
Any ideas?!

[old man jimmy]

need a little more info.. Like what carb you're running. If you still have the Q-jet. I'd make sure your back butterflys are opening freely. With engine OFF open throttle and you should be able to open your secondary butterfly easy,with your finger, and it should spring shut on its own. And does it do it if you go to the floor slow, instead of jumping on it?

[Tom Vogel]

If you have a Quadrajet, it is probably the accelerator pump.

[jsweet]

Originally Posted by old man jimmy

need a little more info.. Like what carb you're running. If you still have the Q-jet. I'd make sure your back butterflys are opening freely. With engine OFF open throttle and you should be able to open your secondary butterfly easy,with your finger, and it should spring shut on its own. And does it do it if you go to the floor slow, instead of jumping on it?

Its a 650 CFM Eldelbrock carb going to Eldelbrock intake and 383 motor. It doesn't do it when slowly applying throttle and seems to just have problems when you mash it down.

[fxdxharley]

i would start by adjusting the air mixture screws. to adjusts screws:
1. warm the engine to normal temp
2. turn screw in until it starts to stumble then back out 3/4 -1 turn.
3. do this for both screws, doing one at a time.
this will set your air/fuel mixture.
also check for vacuum leaks.

[cleszkie]

Accelerator pump diaphragm needs to be replaced.

[DustinU]

Originally Posted by cleszkie

Accelerator pump diaphragm needs to be replaced.

I agree. But before doing that you may want to try moving the accelerator pump "arm" to the top hole, increasing the squirt when you mash the pedal. It is located on the driver side, upper front of the carb... there are three holes, yours is probably in the middle, move it to the top.

[DustinU]

Also, with the truck not running , look down the carb to make sure you have an even spray from both the squirts. I recently had this issue and a good cleaning/rebuild solved the problem.

[Ole Beater]

My experience with edelbrock and quadrabogs is the accelerator pump diaphragm doesnt fit snug in the bore and therefore doesnt give a strong squirt off gas. The old quadrajets had diaphragms made of leather and i think they were better.

[jsweet]

Thanks for the help. I've ordered a rebuild kit online (nothing was available locally) so hopefully that'll sort out the problems...

[72K5NTX]

I would agree with the previous posts, besides from experience, this forum and its members are unsurpassed in reliability and good advice. I would offer one more piece of advice though even though it sounds goofy, check the rubber sections (if there are any) of your fuel lines. I had a PO who replaced a small 6" section of fuel line with vacuum tubing which held up fine and didn't leak but formed a balloon inside the line which caused it act as you have described above. Maybe save you some trouble, or someone else who reads this some. Hard to detect and never would have thought of it.

[jsweet]

Well I finally got the parts and time to rebuild the carburetor and replace accelerator pump and went for a test drive. It still does the exact same thing as before and totally dies when mashing the gas pedal. Any other ideas? Could the timing be off? Something wrong with distributor?

[jsweet]

Originally Posted by DustinU

I agree. But before doing that you may want to try moving the accelerator pump "arm" to the top hole, increasing the squirt when you mash the pedal. It is located on the driver side, upper front of the carb... there are three holes, yours is probably in the middle, move it to the top.

There is actually only one hole.

[DustinU]

Originally Posted by jsweet

There is actually only one hole.

Really??? Both my 1406 and 1407 have the three hole adjustment. It looks like this (mine is set in the middle on this picture):



I also read your other post. I have had to swap distributors in the past to put an end to my frustrations with this problem on my 65 Malibu SS 383. Unless yours is a all out race vehicle, I would run the vacuum advance (just my opinion).
Does your timing mark hold steady or bounce around? Distributor gear worn out or new cap and rotor? Could have the wrong one depending on cam.

I am just spit balling to help trouble shoot your problem because I know the frustration... the whole new distributor was my last straw before lighting the match... and it worked. If the PO put a brass gear on the dizzy the teeth may be worn out and you will need a composite gear depending on cam.

[jsweet]

Ah! Thanks. I thought you were referring to the actual shaft of the pump. I also have 3 holes on the linkage so I'll give that a try! I haven't messed with the distributor yet but I'll give it a shot. Thank you!

Originally Posted by DustinU

Really??? Both my 1406 and 1407 have the three hole adjustment. It looks like this (mine is set in the middle on this picture):


I also read your other post. I have had to swap distributors in the past to put an end to my frustrations with this problem on my 65 Malibu SS 383. Unless yours is a all out race vehicle, I would run the vacuum advance (just my opinion).
Does your timing mark hold steady or bounce around? Distributor gear worn out or new cap and rotor? Could have the wrong one depending on cam.

I am just spit balling to help trouble shoot your problem because I know the frustration... the whole new distributor was my last straw before lighting the match... and it worked. If the PO put a brass gear on the dizzy the teeth may be worn out and you will need a composite gear depending on cam.

[cleszkie]

I agree with DustinU in that it could also be a timing/distributor issue. Even if you don't have any play in the distributor gear/cam gear mesh, too much advance can result in poor manners under load. What distributor are you running (HEI, points, or Unilite points conversion)? What is your base timing (with vacuum advance plugged at idle)? What is your total timing (vacuum advance unplugged, revs up)? Based on my experience, base timing should be around 10 to 12 degrees, and I think total timing is like 34 degrees or so.

Also, note that a carb can stumble if it gets too small an acceration pump shot, AND if it gets too big a shot. Where is your pump linkage set currently (which hole)? Try all three holes on the pump arm to see if it makes a difference. Good luck.

[psuedopimp]

i had the exact same issue. ended up taking it to a mechanic and for 85 bucks got it running as good as the old tired 307 will.

turned out that the dwell wasn't adjusted correctly! then i had drove it like that for a few months so the spark plugs were fouled from city driving. then he fine tuned my carb.

i bet if you messed with your dwell/points adjustment you would solve your problem

[Ole Beater]

You should go through all the checks
1. vacuum leak
2. a timing issue (too retarded, or too advanced)
3. fuel starvation
4. too rich, Look for black smoke out the exhaust.
5. carb jetted wrong according to edelbrocks instruction chart for your carb.
6. if you still have points check dwell. Change to electronic, either HEI or conversion kit.
7. Distributor weights sticking.
8. Check your vacuum with a guage and install the correct springs for the carb.
http://www.edelbrock.com/automotive_...alibrate.shtml
9. Is the exhaust restricted. Does it still have the exhaust valve thing in it on the passenger side.

[Bull72]

bookmarking thread

[Ole Beater]

I thought of another thing to check is make sure your choke valve isnt closing.