Team Camaro Tech banner
Cancel
Create thread New Garage Forums

My theory on quadrajet starting issues

1 reading
Jump to Latest
24K views 21 replies 12 participants last post by  97Z4C  
F
#1 ·
would appreciate some feedback on this theory

my 69 has a quadrajet, when it cold starts it runs and dies a second or two later unless i feather the gas for a few seconds then its fine, and car runs fine,
hot start after sitting more than a half hour it starts acting like its flooded a little but will start then run ok.

my theory is i have a medium size gas leak into the manifold , when it leaks over night enough is gone to cause a flow problem seconds after starting and when it sits for an hour or so the fuel is flooding the manifold and hasnt all dried up yet

i will be rebuilding my carb next week so i thought id just post this in case it was possible that someone else has had these symptoms. Im going to look for some videos on the fix to the common quadrajet leak senerios in the meantime
 
#2 ·
Re: my theory on quadrajet starting issues

It sounds like your carb doesn't have a choke or the choke isn't working. Any carb'd engine will require working the gas to keep it running until it warms up a bit if there is no choke to richen the mixture.

What is your technique for warm starting? Try turning the key without stepping on the gas. If it doesn't fire give it a half pedal as you turn the key. I see a lot of folks use the 2 full pumps before they turn the key every time they try to start the engine and it often causes starting issues!
 
S
#4 ·
Re: my theory on quadrajet starting issues

I see a lot of folks use the 2 full pumps before they turn the key every time they try to start the engine and it often causes starting issues!
Click to expand...
Yeah left over habits from the 1920s and 1930s.
I agree symtoms are of a choke not working or mis adjusted...
Thu if the carb is old seen a few 100K miles, worn butterfly bushes stuff like that will cause leaner starts to..just like a choke mis adjusted when rather worn..
 
F
#5 ·
Re: my theory on quadrajet starting issues

choke works and pull off works right.
starts fine warm except if it sits for over a half hour.
i never have to pump the gas before starting except when cold to engage the choke and high idle cam. yes it has return line.

it starts cold then dies a few seconds later if i dont feather pump the gas for 5 seconds.
my theory is a leak because if i let it die and start it , it will act like it is in perfect working order and i am guessing the cranking at that point is enough to fill the dry spot in the flow
 
#6 · (Edited)
Are you talking electric choke or divorced choke? Sounds like it is either opening too far or not far enough on cold start. For a test have some one else start it while you observe the choke operating. Tip the butterfly by hand to see what will keep it from stalling out then adjust the choke to accomplish the same result.


Sent from my iPhone using Autoguide
 
9
#7 ·
Re: my theory on quadrajet starting issues

the plugs are leaking under the bowl of carb --remove carb -remove base plate -u will see the 2 plugs on bottom side of bowl--it has been perty common for those to leak after long terms of use and JB weld around the plugs has worked in past -just a thought
 
L
#11 ·
Re: my theory on quadrajet starting issues

97Z4C said:
the plugs are leaking under the bowl of carb --remove carb -remove base plate -u will see the 2 plugs on bottom side of bowl--it has been perty common for those to leak after long terms of use and JB weld around the plugs has worked in past -just a thought
Click to expand...
Yes, they mention that in the "Rochester Carburetors" book by HP Books, only they use epoxy instead of JB Weld.
 
F
#8 · (Edited)
Re: my theory on quadrajet starting issues

it is a divorced choke , original style. The main reason im guessing it is a leak of some kind is that after it starts cold and dies (if i dont give it gas by pedaling it lightly) it has only run for about 3 seconds. When i start it again after that it runs perfectly choked and high idled and i dont think the engine gets much warmer in 3 seconds meaning the choke seems to works ok and i have observed it and played with it to see if that was the problem . as mentioned it is perfect warm except if it sits longer than a half hour then acts like a mild flooded condition feel to the starts (but it always starts) so in my mind the two senerios together point towards a mild leak of some kind out of the carb into the manifold.
i think the next step is to take the carb off and analyze the bottom plug situation to determine if that is leaking like 97z4c mentioned. It is a possibility but not a sure thing. Im not ruling out the choke 100% but I have looked into that and tinkered with it and it seems ok to me.
 
#10 ·
Re: my theory on quadrajet starting issues

Is the Accelerator pump injecting any fuel when you manually push it? If not yes time for a rebuild kit.

Could be low float, or stuck float not filling the bowl. Also could be dirty filter low fuel flow due to dirty filter or just fuel boil off.

Q-Jet intake manifolds had the "Hot-Slot" underneath the front bowls and required a tin gasket on on the manifold then the fiber gasket with the slot on top then the carb in order to prevent fuel boil off. If the slot is still open to exhaust gas or if it has been sealed you should still use the tin heat spacer.

I had a similar issue with hard warm - hot starts until I noticed the boiling sound and did some research here. It triggered a memory of me installing a gasket kit in the early 80's and me tossing the tin spacer in the trash not knowing what it was. The original gasket I pulled off the spacer was sandwiched into the fiber so I did not notice until long after but figured I didn't need it. Guess I was 20 years wrong.

I ordered the correct gasket and tin plate spacer on-line at rock-auto.com with our member discount and problem gone.
 
R
#14 ·
Re: my theory on quadrajet starting issues

I have built so Many Q/jets in the last 35 years I dont care to remember, 44 years as a mechanic will do that. As stated above the starting problem is with the welch plugs leaking that are under the baseplate, there are 2 front ones and also 2 bigger ones at the rear, also 1 other on the rite hand side of the body, and also 1 external plug by the fuel inlet.
I allways clean the area with scotch brite or a wire wheel, then Lacquer thinner and use JB weld. Dont use epoxy or the 5 min JB weld, use the original JB weld that takes overnite to harden. I usually mix it up and let it sit for about 5-10 mins to let it start to catalyse, then smear it over the welch plugs, letting it start to harden stops it from running or drooping. 5 min Epoxy wont work, fuels eat it. Never had JB weld come off, I have a Q jet on my truck I did 25+ years ago and recently rekitted it, it was as solid as when I put it on. Never had a comeback from the JB weld failing. I usually let the carb sit for 24 hrs before putting it back on, this lets the JB weld cure completly.
 
Save
Reply Quote Like Like
J
#15 ·
Re: my theory on quadrajet starting issues

This will sound a little strange but back in the day we always used nail polish on those Welch plugs and they never leaked. Different fuel and maybe different nail polish back then. Nail polish was some tough stuff. That choke pull off has to be right and once you have the touch it works well. Off a little and it will be to lean or to rich. Rochester built a great carb and when adjusted correctly it worked well.
 
S
#17 · (Edited)
Re: my theory on quadrajet starting issues

I too have rebuilt more q-jets than I care to remember, and I'm convinced the Welch plugs get blamed for everything from the budget crisis to global warming. The only (unlikely) problem a leaking welsh plug(s) would present, is they'll slowly drip an insignificant amount of fuel which can empty the fuel bowl over time, which will cause hard cold starts.

Run a simple test. When the carb is apart, fill the fuel bowl with fuel and see how much fuel, if any, is leaking past the plugs below the main body. Most cases, none.

If your fuel bowl dries out between starts (2-3 days for instance), I'd first blame today's fuel which has much higher evaporation rate than fuels of yesteryear (ethanol blends have a very quick evaporation rate). From here, for instance:

http://e85.whipnet.net/outlook/subsidation.html

...is this:

"There's another problem: Ethanol, when mixed with gasoline, results in a higher evaporation rate."

Evaporation is not much of a problem on modern cars because the fuel system is never exposed to the atmosphere as is a carburetor. I would bet the cold start problem the OP is having is related to fuel evaporating (not leaking) from the fuel bowl. This problem is less prevalent on a Holley because a Holley's fuel bowl(s) are much bigger, but it still happens, at least in my garage.

The hard "hot starts" could, as stated above, very likely be caused be the heat-crossover in the intake which routes extremely hot exhaust gases directly to the underside of the carb. If you don't have the correct 'gasket sandwich' between the intake and carb, fuel will percolate in the fuel bowl. You can actually hear the fuel boiling.

Here's the integral part of the "sandwich" (the heat baffle):

http://www.rickscamaros.com/camaro-carburetor-heat-baffle-rochester-model-4661-1967-1969.html

FWIW
 
F
#19 ·
Re: my theory on quadrajet starting issues

sometimes it is the slow leak too and it can develop over time creating a problem that had not always existed. ive been analyzing this problem for a year now and will report back on the condition of the plugs underneath.
I totally undrerstand the skepticism about the leak and the choke setting and pull off. ive been studying these for a while now and will be doing two rebuilds. one to analyze the leak theory and the seccond to try new/different jets and rods. the pump pumps fine and i have exprience with all the settings so i am just going to have some fun and get to the bottom of this, I have mainly worked on carters and holeys on 440's in the past but a few years ago converted to camaros. ive done a few quadrajet rebuilds before on my personal cars but nevr really got into understanding them fully because they always worked well.
i wil get back to the thread and let you know if it had a leak or not.( dont know how soon though)
 
F
#20 · (Edited)
Re: my theory on quadrajet starting issues

plain as day wow
one plug is leaking a bit more than the other but both are leaking.
so easy to check this and i should have sooner.
I may drill them out and crew in plugs havent decided yet.
Its a beautiful thing when you discover the cause of a problem that was annoying you.
Im going to have fun and play around with quadrajets from now on so all ive heard about this flaw, in this case it was factually what was causing bad starting conditions hot and cold and its so simple and obvious when you take it off.
Its fun to learn about quadrajets and there is a lot on the internet about them
I began liking them when i nailed it and heard a lion roar, but i didnt know enough abiut them and now i am starting to understand them

update: 5 leakes found . The po or someone in the past used a little bit of some crap on the four underneath. there was the remnants of one of those pads too that they stick in the center well . It was the fronts that were really bad though
the crap chipped right off from using my nails and is very brittle. I can see now how this can be a progressive problem before or after a repair attempt.

So, the worst leaks were repaired leaks that didnt last. I cant tell what it was but it was brittle and brown.
Clearly what was doing the most damage were the two plugs under the jets toward the front. The larger middle plugs were only leaking into the well and the gas cant get into the manifold from there.
Before I drill them out thread and seal them I tapped the two main culprits with a hammer to try and re-seal it that way a little and expoxied them with jb kwik. I dont think this is the fix but I didnt have time to cut some theads and tap it and do all that and Im anxious to test it out
without the leaks so I'll report back in a couple days. I didnt put a lot of the epoxy on, so it wont be hard to get it off.
Im going to reinstall tomorrow or the next day and drive it for awhile and see what happens.
 
R
#21 ·
Re: my theory on quadrajet starting issues

firstrs said:
update: 5 leakes found . The po or someone in the past used a little bit of some crap on the four underneath. there was the remnants of one of those pads too that they stick in the center well . It was the fronts that were really bad though
the crap chipped right off from using my nails and is very brittle. I can see now how this can be a progressive problem before or after a repair attempt.

So, the worst leaks were repaired leaks that didnt last. I cant tell what it was but it was brittle and brown.
Clearly what was doing the most damage were the two plugs under the jets toward the front. The larger middle plugs were only leaking into the well and the gas cant get into the manifold from there.
Before I drill them out thread and seal them I tapped the two main culprits with a hammer to try and re-seal it that way a little and expoxied them with jb kwik. I dont think this is the fix but I didnt have time to cut some theads and tap it and do all that and Im anxious to test it out
without the leaks so I'll report back in a couple days. I didnt put a lot of the epoxy on, so it wont be hard to get it off.
Im going to reinstall tomorrow or the next day and drive it for awhile and see what happens.
Click to expand...
Only stuff I EVER got to stick was the original JB weld as I posted before. JB kwik is just like 5 min Epoxy IMO. Keep us informed and Good Luck.
 
Save
Reply Quote Like Like
Team Camaro Tech
posts
2.4M
members
90K
Since
1998
A forum community dedicated to 1st generation Chevy Camaros owners and enthusiasts. Come join the discussion about performance, builds, restoration, modifications, classifieds, troubleshooting, reviews, and more!
Full Forum Listing
Explore Our Forums
Bench Racing Engine Body Shop Performance Brakes, Suspension & Steering