RPM drop in 72 Vette

[L88 crazy]

Whats up Vette Owners!

I have a hestitation problem with my 72 T top. When cruising at 1500 rpm when I want a little more speed, the engine gets to 2000 rpm than drops 500 rpms to 1500 AND THAN it accelerates smoothly. Above 2000 rpms the car hesitates a little bit before gettin up and goin. I checked the timing and its perfect and I use the right octane gas, Premium. The Small Block Chevy in my Vette doesn't miss or burn a thing. Whats wrong?

 

[The73vetteman]

What carb do you have?

 

[L88 crazy]

Q-Jet

I have a rebuilt Q-jet.

 

[drags1998]

Sounds Lean, Check the accelerator pump in the carb!!!!

 

[Macgyver]

Yeah my Vette did the same thing when that #&$%! inline fuel regulator was quitting on me. It wasnt getting enough fuel at that point but wouldnt smooth out over 2200rpms. It just wouldnt go any faster than 55mph. But when the fuel pump went out it would get to about 2krpms and then drop to 1500 and after that it would smooth out. After one day of that i was stranded, hope that doesnt happen to you.

 

[cruzer99]

Go down the list...........

1. Fuel filter (all)
2. Fuel regulator
3. Timing chain
4. Good luck

 

[GTR1999]

Check the fuel inlet hose into the pump. It should be a molded piece,if someone changed it to regular fuel line it may be crimped or binding under load.

 

[tomtom72]

Hi L88crazy, check the gap on the Acc. pump linkage. If it is not correct you could get flat spots as you wanna acclerate, and not just from idle. The other place you could check is the metering rods. I'm not much use on Qjets, but sounds like your problem is when your carb is changing circuits. If there are no problems like Vac leaks, vac adv, mech adv, vac adv diaphram, dirt in the fuel bowl, I would say something is stopping enough fuel getting thru at a small time interval. That could be Acc pump adjustment, and or metering rod problems. You are not getting a lean backfire thru the carb when this symptom happens? If you are eliminate any major vac leaks first, but since you didn't mention it I think it is a subtle problem. Rebuilds seldom run right on straight out of the box, new ones also, without some tweeking. Hope this helps get you down the right road.tt72

 

[The73vetteman]

You are not getting a lean backfire thru the carb when this symptom happens?

This struck a chord for me. What is a "lean backfire"? I have a suspicion I'm suffering from one of these. Just occasionally I get a spit back thru the carb when I give it some gas from idle. I've checked and rechecked the valve timing and all seems OK. Maybe it's an accel pump issue?

'73

 

[tomtom72]

Lean Backfire

Hi 73vetteman, a lean 'pop' or backfire is what happens when you get a momentary lack of fuel in the A/F ratio(mixture). The dead give away on this is to clean the underside of the air cleaner lid, look 1st to see any smudges of anything, white smudges or black smudges. Put it back and see if you can get the motor to do it again, if it does stop immediately and look to see the color of the smudge.Black smudge=too rich, white=too lean. Usually you only get this 'pop' when you are building RPMs quickly(lean or rich), however if things are really messed up it might occur even at idle or slightly above idle, but things gotta be bad for that. Mostly it is more subtle like what L88 is having, just a loss in rpm momentarily. If you force the issue, more throttle, you can induce it. You don't want to on a regular basis, just once should be enough for diagnostic pourposes. That's one way carb fires start. Remember this is only a symptom, the disease is what is causing the lean or rich condition. Usually if you get black or brown smudges it indicates FO or In valve problem. If the smudge is white or very light in color you got lean problems. Kinda just like reading spark plugs. Hope this helps, tt72

 

[tomtom72]

Lean Backfire

Hi 73vm, I have to apologise I didn't answer with anything useful. A senior moment, brainlock. If you think you are getting a lean pop check your idle mixture 1st. I'd use a Vac gauge to set lean best idle then add 1/2 turn rich. See I think faster than I type, I forgot the tach, I use both to set idle mixture. Then see how the motor reacts to that adjustment under no load. If you have trouble getting a stable idle speed and mixture check for Vac leaks. Okay no Vac leaks then I set the Acc pump cam gap according to spec. Retest motor on no load and see if rpm progression is smooth from idle up to 2000-2500 rpm,if that is smooth rpm rise(no flat spots or hesitation) then you should be okay through the Idle-run circuits transitions. Next you do a few no load blasts to activate the secondaries. If you get hesitation upon initial call in on the secondaries then you got to try to close the Acc pump cam gap to get more initial so you get more fuel from the Acc pump. You should be able to see the pump shot upon secondary opening. Now if you get enough shot and it still hesitates then I'm lost with a Qjet. A holley has power valves that's the next suspect. Oh , you went too far on the pump gap if it sounds like it is drowning- too much gas in the shot too soon. I never played with a Qjet so I don't know much about them, sorry. Obivously float level must be right, fuel pump must pass PSI,volume vs time, and leak down tests, and no junk in the bowls to clog up things. Now I hope I gave something useful. tt72 I made a mistake. I have no idea if a Qjet has secondary pumps like a 4550 series holley. Then that means secondary hesitation = secondary vacume diaphram linkage adjustment and or trying different diaphram spring tension by changing the spring in the vac. secondary diaphram housing. If the transition points thru each circuit seem to be okay and all the mechanical adjustments don't solve the issue then you have to start looking for Vac leaks. Remember the sooner the trouble starts in the rpms the lower on the carb you should look for the Vac leak, the higher in the rpms = the higher up on the carb the leak might be. Now I hope I gave something useful. tt72

 

[tomtom72]

I'll get it together eventually!

Hi 73vm, I will get this right eventually. Does your car have headers? I know strange question. I've been away from the hobby since 84. Back then the bad thing about headers on a SB chevy was that mostly all of them would lean out Nos. 6 & 8 and Nos. 5 & 7. Reason given was the scavenging effect due to SB firing order. I never had headers on my vette,wanted to keep the A/C. Ya know check the burn on the plugs in those holes and the fix for holley runners was two steps up on all the jets as a starting point for the fix, I don't know on a Qjet I guess changing metering rods is the equivalent fix??? Heck I don't know if the header mfgs ever did anything about that problem. I was told that by a mechanic, and did some research and found it in print somewhere, one of the books devoted to SB chevys I think. I'm sorry I probably gave you all a headache by now. tt72

 

[The73vetteman]

tt72,

Great, very full answer to my question. It's going to take me a while to digest everything you said and carry out the tests. Two points tho':
1. I do have headers on the car (had 'em when I bought it), and the carb is a Holley.
2. I have a dark smudge on the inside of the top of the air cleaner - too rich? I pulled plugs 3 & 4 last weekend and both looked good. Guess I shouldn't be lazty and should look at all of them.

Once more, thanks for all the advice. I'll be in touch.
'73

 

[tomtom72]

Hi 73vm, No problem that's why I came and joined this club, for everyone's knowledge. I recommend Holley's web site and look to see what they have in the way of tech help. I remember I bought a book from them long ago on tuning their stuff. It was a big plus for me as I am not a mechanic and am not formally trained in that skill. In fact my mechanic friend told me to buy it after he gave me a short how to lesson. It's funny the booklet that used to come with their rebuilding kits is also a great source of info on tuning. Glad I could give something that might help. I remember a bit about the 4550 series,as I've used them. You could have a momentary rich condition from too much pump shot, and or too little Vac Sec Dia spring, or the power valve in the secondary block is too 'big'. Get holley's book cause it isn't hard it just will take some of your time. Once you get it right you will be amazed. I don't want to be guilty of hi-jacking a thread, I offer my apology to all at CAC. If you got a question you can e-mail me I'll give it my best shot. Good Luck tt72.