stumble/hesitation

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mellow_utah

mellow_utah

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Hello all, I have a 360 in a 73 scamp. I pulled the motor from a 1978 1-ton passenger van along with the 727. I installed an air gap intake, headman headers, MSD 6AL, new distributor, cap, rotor, plugs, plug wires, and a new quick fuel 600cfm carb with vacuum secondaries. The timing is set at 12 degrees at an idle, not sure what it advances to but its a stock distributor. It runs great when cruising around. Off of an idle, or when RPMs are low (when driving highway speeds) and i floor it there is a stumble and hesitation that I cant seem to figure out. It had the exact same issue with the 600 holley 4160 i took off, I also replaced the distributor, plugs, cap, rotor, and tried both MSD 6als I had laying around (one is the non digital and the one thats on there now is the new digital style). The van I pulled the engine from seemed to have the same issue and I thought going from the stock 2 barrel to the new airgap and carb would fix it but it's still happening. Im lost!
 
try a little more initial timing before getting to carried away with making other changes.
 
Ok, ill give that a shot, is there anything special about the cams in the vans? Standard or RV maybe? For a low compression motor, once it gets past the stumble it moves pretty well.
 
Weak accelerator pump,, do a "search" , lotsa threads on that,..

A vacuum leak,, diss-connect all vacuum attachments,- plug em,, and see if it's any better.. put a vac guage on it, a lotta info will show up there..

failing that, "reading the plugs", and a compression test would seem in order,, and look closely at all the sp plugs as you take them out, keep them in order, looking for one that is "different"..

hope it helps
 
Ill look up the weak accelerator pump (although it is a brand new carb), i do have vacuum hooked to the dizzy, power brakes and climate controls. if I find a leak, should the idle drop (less air for the air/ fuel mix), or is there any other indications that there was a leak? I havnt had the plugs out since the new carb and MSD, they were fouled before that but it was running like garbage, far better since the new msd and carb. Checking the plugs and compression check, think theres a bad cylinder? shouldnt there be smoke from the exhaust if a cylinder is bad? Maybe not if its a valve?
 
Could be several things, but as mentioned, keep bumping the initial timing to see how it reacts. If things get better, you can always restrict the total timing when you find out what it needs down low. One other thing is, are you running it with the vacuum advance? If so, hopefully you timed with no vacuum showing at the dizzy.

I'm not sure of the "as delivered" specs on the Quick Fuel carb, but I doubt it's a lean condition for the mild engine you're running.
 
Not nec any smoke if a rocker, pushrod or lobe are not as they should be, vac guage should tell that, and will likely show on a plug..

But check your accel pump operation first,,

hope it helps
 
Ok, gotcha I will check the plugs too. I did pull the vacuum advance on the distributor when I set the timing and plugged the vacuum port on the carb. I just read another thread that said that the idle mixture needed to be checked in gear? Also said that his quickfuel needed like 1 and 1/4 turns out on the idle compared to his 1 turn on his holley so maybe thats part of it? I am new to tuning so it seems like every adjustment I make I just end up chasing my tail lol. Although I did hook up a vacuum gauge to the dizzy port on the carb and at idle i didnt have anything, is that normal? I assume so because that should be reading above the throttle blades, correct? I will see about doing a compression test if the adjustments dont work. hoping its not part of the valve train lol. I have a new xe274 with lifters and springs but figured that would be too big until i can afford heads.
 
Yes you should adjust the air mixture screws to highest rpm/vacuum. Check the vacuum below throttle blades . you were using ported vacuum before. In gear is a good idea. If all this doesn't cure it, you'll need to go larger on your accelerator pump squirter.
 
so just hook up a vacuum gauge and adjust the idle mixture screws moving the vacuum up until it drops off and move em back to the spot with the highest vacuum?
 
mellow_utah said:
so just hook up a vacuum gauge and adjust the idle mixture screws moving the vacuum up until it drops off and move em back to the spot with the highest vacuum?
Click to expand...

Basically yes,, but the vacuum guage can tell so much more ...

Check out this link, and clic on the green tabs to see how many diff vac readings can guide you..

http://www.secondchancegarage.com/public/186.cfm

hope it helps..
 
Thats an awesome link, thank you! Now I gotta go buy a vacuum gauge:) Maybe this will help with my issues with carbs lol. been so frustrated Ive been thinking of converting to fuel injection lol
 
JoJo said:
My vote is for the timing as well.
Click to expand...


yep. may not be it, but it only takes 5 minutes to try it, and you don't have to go out to buy a gauge.

you need a vac gauge anyway though.
 
Anybody know what the correct initial timing should be? I tried 10 degrees and 12 degrees btdc?
 
71gtdart said:
yep. may not be it, but it only takes 5 minutes to try it, and you don't have to go out to buy a gauge.

you need a vac gauge anyway though.
Click to expand...

True. Bumping up the timing helped when I was in the same situation.


mellow_utah said:
Anybody know what the correct initial timing should be? I tried 10 degrees and 12 degrees btdc?
Click to expand...

I do believe 10-12 is about right. Somebody else may want to confirm this.
 
IIRC, 73, depending on the dizzy,, 5, 71/2, or 10..

As others have mentioned, bumping up the timing may make it "perkier", but at the 12* you had it at, should have been sufficient to run decent, without a stumble, or hesitation.

The accel pump is commonly the culprit, and the hardest to get people to check,,
Again,, I would strongly suggest you eliminate that as a possibilty, by reading threads on "accel pump', hesitation, and stumble'' ...

If it's not the pump,, then next is perhaps a cylinder not performing as it should..

Now it's a process of elimination,, starting easiest first,, like firing order,, these engines run exactly as you describe, reasonably well actually, with # 6 & 8 wires crossed,, no backfires, just shitty.. double, triple check the firing order..

Then go on to eliminate the other stuff with the tests suggested..

What happens after the stumble,.. does it go like hell ??

hope it helps..
 
inertia said:
What happens after the stumble,.. does it go like hell ??

hope it helps..
Click to expand...

I have to agree with the accelerator pump being the problem because 12 degrees at his geographical location and approximate elevation should be pretty darn close (unless the elevation is above 6000 feet or so.)

One problem though, is that one man's "moves pretty well" could be another man's "go like hell" :)
 
lol, the elevation here is about 2200', i say goes pretty well in comparisson to my built big block ford (high compression, big cam etc.). The big block goes like hell, this is just peppier than I expected for a low compression smog motor. Im on nights so i did not get a chance to check the excellerator pump today however, I did replace the cap and the terminals on the cap side of the plug wires because I found some cracks in the boots (thanks to a tweaker that worked on the car) and it did get better but now the stumble is a little speratic (only did it twice on my way to work). It does however sound a little different now (as if theres a random misfire on the drivers side, sounds kinda ping'y)and has some vibration to it that wasnt there before. I will check the firing order again tomorrow but now im wondering if a plug wasnt firing correcly before (as a result of the plug wires) and is now but maybe fouled? I will check the plugs tomorrow as well.
 
While checking the plugs I found this....even if I fixed the terminals on the cap end this would still cause the misfire and stumble. The plug hadn't fired at all....the heat shield didn't do its job. That back header tube is a pain in the a**!
 

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Congrats on the early find,, the process of elimination.. lol

hoping you have lotsa miles with lotsa smiles..

cheers..
 
Gonna get new boots for it tomorrow hopefully that will be the only problem lol. Thanks for all the help!
 
I wish lol, however I did make some progress today. I changed all the burnt plug wire ends and the misfire went away but the stumble was still there. I started messing with the carb again and found that the rear float level was way high (don't get that because I had already set it) so I lowered it back down and voala it's like a whole new car. I couldn't even pore brake before now I don't even need to when I want to lay rubber
 
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