spinadog

Howdy all,

I have searched all the threads before posting this but can't seem to easily solve the problem I have. Took my car for a ride the other day and it's been running really well. Started her up a day or two later, and I now have a bad hesitation when I rev from idle, it's like a 'cough' before she gets up there. No problems throughout the powerband, checked the timing, disconnected vacuum advance and put the heaviest guage springs in my distributor but the same problem exists.

I'm running a 454 .60 over, mild cam, with an MSD distributor and 6AL box. Rebuilt carb through "The Carb Shop" here in LA. 900 miles on the motor and carb. Pulled plugs and they are perfect, dusted brown where it counts. CHecked timing at idle and I am at 8 degrees. Gave the carb a pump when stopped, and a steady stream throughout the whole range. Sprayed carb cleaner around the intake and carb to check for leaks - none found that I can hear.

Any suggestions? Just want to get her running nice for the weekend.

Lastly, I noticed what sounds like an exhaust leak coming from the manifold to exhaust flange, but it's quiet and tends to go away when at operating temperature. New exhaust, engine, gaskets the whole lot.

Just seems strange that it started right after a run....

Lars, any suggestions would be helpful. I would rather suspect timing than the carb rebuild. The Carb Shop are very experienced with QJ, and like I said, it 'suddenly' started exhibiting this symptom.

Also, when giving her a rev, i had a carb backfire and afterwards, things were great! That soon ended.

Could I have bad fuel? I have only filled with Chevron 91 - best I can get in CA.

Cheers to all.

lars

Check your accel pump shot - it's common for the pump cup to go bad on a Q-Jet, even on a new rebuild. Verify you have a strong, solid, instant fuel stream shooting out the discharge nozzles upon any movement of the throttle. If not, replace the pump.

Lars

spinadog

Thanks Lars, after some more research last night after posting I was also leaning towards this but had dismissed it, due to it being a new rebuild. That would also explain the suddeness of the change.

When I opened the throttle manually, I got 2 streams but they are a little anemic, more like a guy taking a leak than a powerful stream.

I'll check it tonight.

I appreciate you responding.

7T1vette

What's the condition of your heat riser valve (outlet of exhaust manifold on right side)? If it hadn't opened yet...or is sticky/binding...the engine won't run very well because of right-side back pressure.


P.S. Really like your paint job!

Rich's'78

I think you are making a mistake if you don't take a closer look at the accel pump, as Lars suggested.
It should pee strong right from the first movement of the throttle from hot idle. Especially for a big block, I suspect.

One other possibility I had on a friend's QJ years ago. The shot looked good and strong but it had poor-shot symptoms. It would move a bit and then sag, and then clear and go. Based on my limited experience (relative to Lars!) I had seen similar problems with a bit of water in the fuel bowl. Just enougt that the high surface tension would obstruct the jet temporarily, but not get sucked thru because of the sag. Told him to put gas line antifreze in it (methyl alcohol) and the problem quickly disappeared. Seen that twice before on other carbs years ago but since you probably have ethanol in your gasoline already it doesn't likely apply.

spinadog

7T1, no heat riser - removed that and replaced with a spacer. New exhaust manifold, exhaust pipe, donuts, etc. And I love my color too ;-), but it's nothing like the quality of shine on your paint job, friend.


And the problem just 'suddenly' started...

Rich - yeah - I am down to the accel pump. it's not a powerful stream as I recall and it used to be. Guess I blew the pump - can a carb backfire cause that, I wonder? I've only had two.

Also, would a poor accell pump lead to a potential carb backfire? As mentioned before, I could not find any vacuum leaks around the intake or carb, and had blocked off most lines to isolate any other leaks. I gather I can buy the parts from an O'Reillys or the such?

Cheers to all

7T1vette

You could replace the accel. pump only; or you could do a carb rebuild if it has been many years since prior one. You might be able to purchase only the accel. pump 'kit' at NAPA or other good parts store. If you have to buy the entire rebuild kit, you can always finish the job at a later time. But, it's not that much trouble to do the whole 'shebang' at once.

P.S. The 'shine' on my paint is primarily due to Zaino! My paint is more than 15 years old.

Rich's'78

Carb backfire is a result of lean mixture (ie: not enuf pump action). You should be able to buy just the pump cup w/o buying a whole kit. I would contact Oles carbs and ask. Should be cheap. The rebuilder probably accidently put a smal tear in the inner edge as he installed the cup and it went from there. You may need a stiffer spring as well. Lars may be able to help you too, including the proper setup for the pump overall.

hugie82

You may have lined up all the timing marks perfect but many balancers, timing chains, ect.. shift over time or are just off from the start. Even if the engine is rebuilt. I would try bumping the timing up 2+degrees until it starts pinging then back off until it stops.

Paul L

Use a Viton accelerator pump.

Vettekid1992

i currently have a similar problem wit my vette. when u floor the car @ an idle when in drive it hesitates then it goes. the problem is your car is not getting enough fuel fast enough. 3 things can be done to fix this.

change the size of the squirter.
change the cam to a longer or larger shot.
change the power valve to a higher rating(use a vacuum gauge to determine your vacuum @ idle and get a power valve slightly less then that)

i have 15in vac at idle and a 10.5 power valve and the car still stutters. so im planning to get a 12.5 power valve.

hope this helps

spinadog

Thanks to all for your posts - appreciate the support and help - believe me.

7T1: it's a new rebuild at a good shop here in LA - he's a 70-year old dude who has been rebuilding carbs since the '60's - I tend to lean towards the accell pump failing. I checked it again tonight after warming her up and yeah - i guess it's a feeble squirt - it should be strong and constant and I am sure that I remember it being far more powerful before. I'll look into Zaino - I have a 18+ y.o paint job too.

hugie82: the problem I have is that it suddenly happened. The engine rebuild was good and I did it right with some good help and didn't skimp on the parts - and had some great speed shops machine and spec what I needed. It ran like an insane big block until this issue and still does, just off-idle I get the 'cough'. It's a standard rebuild with about $5k in good stock parts. I took a lot of pride in getting her right. I had the timing right up until Ping City, then backed her down, so I am confident it's close as can be. I also backed it down when testing in case advance was an initial problem and had no joy there either.

Paul74: good suggestion and thanks for the photo - that helps me a lot since I never rebuilt the carb, just every other nut and bolt on the car!

Rich: yep, went to NAPA and they only offer kits but suggested another carb shop around the corner where I can get the cup only. Tomorrow night I will pull the carb off and start the diagnosis - and post photos for all you Visual Junkies....

Vettekid: I'll wait until the pump gets replaced before diving into other stuff but at some point I need to look at metering rods, but only in the pursuit of perfection power.

AND, I found my exhaust leak - it's the spacer gasket has blown out for some reason.

Thanks again all.

Peterbuilt

Here's a how to with some pictures:
http://www.4wheelnoffroad.com/qjet.html

spinadog

thanks Pete - but again - the carb was rebuilt. BUT if there's stuff in there about the accell pump, well, thanks for the info. All bits will help when I go to replace.

lars

No, a carb backfire will not damage the accel pump on a Q-Jet. Rather, the alchohol additives in modern fuels causes many accel pumps to swell up and seize in the pump bore. This locks the pump up or restricts is movement. You can't tell this from the outside, since the pump shaft will still go up and down as the pump shaft spring simply compresses. The solution is to install an alchohol-resistant accel pump, and NAPA sells them under part number 2-4034.

Yes, a weak or missing pump shot will cause a backfire - you will get backfire from a lean condition.

Lars

spinadog

Thanks for the info and the part number - I'll hit up my NAPA store today. And I gather that when you say a 'strong' squrt, it should be that, like a trigger spray pumped hard rather than a soft squirt?

hugie82

Here's a question, will it happen with a quick 1/2 pedal or only when stomped to the floor? If it only happens when stomped to the floor I found the spring tention that controls the secondaries are often adjusted to loose and open too soon causing a bog.
Pull the aircleaner and watch the secondaries, they should barely open when you crack the throttle wide open in park.

spinadog

It happens when 'stabbing' the pedal 1/2 or full. And I have a 4-speed M21. Tension on secondaries feels good and they have a gradual opening pattern as RPM build.

hugie82

Let us know how the accel pump works out. They are a PITA but doable.
Good Luck

spinadog

OK - now I have some pics for you all and would really appreciate some feedback.

In this first photo, I found some things that seem wrong. Note the comments I have made. I found one of the four brass tubes loose in the body. I marked where I found it, and where it came from. Any help here? Could this have caused my problem? Definitely not good. Also, there was a collection of oil where I indicate. I do have a PCV connected but the passenger breather is not fed into the air filter - I just have a breather pushed into the valve cover. Also, the choke butterfly rod came out when I took the top off, and there's 2 positions (holes in the arm down in the carb body) it can rest in - which one is correct? the one at the end of the arm or the one in the middle?



you can see in this photo that I have the vacuum line plugged and where the brass tube came from:



In this one, I'm showing my primary metering rods, that seemed a little crusty but are they OK?



Lastly, this is the state of my accel pump - and to me it seems absolutely fine - no tears or damage:



Any help here? please!!! I really want to get her running for the weekend. If all looks OK, and if that brass tube was not causing my problem, do y'all have some pointers for me?

Cheers

Simon.