Harms Rebuilt '71 Thermoquad - Wondering Idle, Stalls when Put in Gear

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It is not a carb issue. Before doing anything else, do the 5 min PCV test in post #41.

Yes, there may be other problems, but they will take more time to trouble shoot & may require special tools [ compression tester ] to check.
The low vacuum may well be 'normal' if the engine has had a bigger cam fitted. That will THEN require some fine tuning of the ign system & carb to make every thing work in unisyn.
 
Dartnut said:
How old is the gas in the car?
You need good fresh gas in the system before you attempt to tune the engine.
I've seen people try to tune an engine with old gas and are pulling their hair out by the roots.
A change to fresh gas and bingo!
What a difference!
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Gas is fresh. Good point though.
 
SLOPAR72 said:
Low vacuum. I would do a compression and if that shows good and your willing to put in the work do a leak down. If they show good then you need to make sure everything is in order on the timing side and "then" I would look at the intake. If it has a power brake booster cap that off FYI and recheck vacuum.....

JW
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JW - Everything was capped or plugged when tuning including Booster and PCV. When you say timing, do you mean ignition timing or cam to crank timing? I assume ignition. I'm running 17 degrees initial with 35 all in by 3000rpms but I tried everything from 6 degrees initial to 22 degrees initial. 17 degrees gave me best vacuum and throttle response.
 
Bewy said:
It is not a carb issue. Before doing anything else, do the 5 min PCV test in post #41.

Yes, there may be other problems, but they will take more time to trouble shoot & may require special tools [ compression tester ] to check.
The low vacuum may well be 'normal' if the engine has had a bigger cam fitted. That will THEN require some fine tuning of the ign system & carb to make every thing work in unisyn.
Click to expand...
Yep. Plugged PCV and unplugged. Same vacuum numbers either way.
 
A stock 340 with 16 initial should be pulling 15" of vacuum or really close. Something is leaking somewhere. A stock 340 shouldn't be hunting at idle rpm. Clean idle at 750 should be good with minimal drop in gear.

If it idles at 800 and drops to 350, it's likely dropping timing when that happens or has a bad vacuum leak. Just my guess. Do all this stuff with the vacuum advance line plugged.
 
I believe waaaaaay back in this thread someone mentioned a vacuum leak(idle went up when u covered carb). Has this been addressed?
 
crackedback said:
A stock 340 with 16 initial should be pulling 15" of vacuum or really close. Something is leaking somewhere. A stock 340 shouldn't be hunting at idle rpm. Clean idle at 750 should be good with minimal drop in gear.

If it idles at 800 and drops to 350, it's likely dropping timing when that happens or has a bad vacuum leak. Just my guess. Do all this stuff with the vacuum advance line plugged.
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I agree.
But with smog carb and an unknown smog? distributor the idle rpm is critical and what happens with timing when it goes into gear and at the other rpms is critical. Without mapping the timing its pin the tail on donkey.
 
crackedback said:
A stock 340 with 16 initial should be pulling 15" of vacuum or really close. Something is leaking somewhere. A stock 340 shouldn't be hunting at idle rpm. Clean idle at 750 should be good with minimal drop in gear.

If it idles at 800 and drops to 350, it's likely dropping timing when that happens or has a bad vacuum leak. Just my guess. Do all this stuff with the vacuum advance line plugged.
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Timing port was plugged or T-ed off with another gauge to make sure advance wasn't pulled in while f-ing with everything else.
 
4spdragtop said:
Vacuum leak. If you use a spray bottle with water it should stumble at the leak.
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Unless leak is below intake, bad gasket on lower part of port, pulling vacuum from crankcase.
that would require a smoke test, i know of no other way to determine if its leaking there.
like shaved heads and intake wasnt matched.
 
[1] WD-40 is flammable in aerosol form, so if there was air leakage where it is sprayed, one would expect a change in idle.
[2] The increase in rpm from holding a hand over the carb is most likely due to the low idle vacuum. Not necessarily an indication of a faulty carb.
[3] The key to this strange problem lies in the low vacuum. I believe the carb & the ign timing, as the cause, can be ruled out. Enough tests have been done to eliminate these suspects.
[4] So back to low vacuum. Is it low because it has a performance cam? Or is there a mechanical problem with the engine: worn cam lobes, leaking valves, broken rings etc, causing low vacuum. A compression test might help, if it could isolate particular cylinder.
 
Bewy said:
Something not right here, getting drip fed info....
Now you say the Edel wandered at idle.
And the engine is 'bone' stock?
Stock engines will have 16-19" of vac at idle, not 10-11".
A stock engine should idle at 5-600 rpm, not 900....
Since this problem has occurred with TWO carbs, & from what you have done so far, I suspect the engine has had a larger cam fitted. That would also explain the 16* init timing [ which is good, not a problem ].
The high idle rpm could be activating the centri curve in the dist, especially if the springs are stretched. This could explain the varying rpm & needs to be fixed first. Check by monitoring the ign timing with a timing light.
One last thing to try: initial timing is at 16*. Loosen dist, engine idling, turn dist slowly CCW; keep going until rpm stops increasing & THEN see what the timing is.
I suggest you try the above & report back.
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A stock 340 is supposed to idle at 900 rpm. Check it out and report back. Kim
 
Bewy said:
[1] WD-40 is flammable in aerosol form, so if there was air leakage where it is sprayed, one would expect a change in idle.
[2] The increase in rpm from holding a hand over the carb is most likely due to the low idle vacuum. Not necessarily an indication of a faulty carb.
[3] The key to this strange problem lies in the low vacuum. I believe the carb & the ign timing, as the cause, can be ruled out. Enough tests have been done to eliminate these suspects.
[4] So back to low vacuum. Is it low because it has a performance cam? Or is there a mechanical problem with the engine: worn cam lobes, leaking valves, broken rings etc, causing low vacuum. A compression test might help, if it could isolate particular cylinder.
Click to expand...
It could also have too much preload on the valves. Kim
 
AJ/FormS said:
at idle the rods have to stay down.
in Neutral the rods should stay down with gently increasing rpm, to over 2000rpm or more, so long as the manifold vacuum continues to increase.
If the needles pop up when you put it into gear, the engine will flood and stall.

Here's a trick; Just take the springs out. At idle and in neutral, this should cure both problems. If it does, then you know to install weaker springs.
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There is only a spring under the plunger in the center of the carb. Kim
 
oldkimmer said:
A stock 340 is supposed to idle at 900 rpm. Check it out and report back. Kim
Click to expand...

Maybe a smog spec.

68 4 speed was 750 IIRC.

With 16 initial which way beyond the factory ever suggested (10 IIRC), it should idle really nice at 750rpm.
 
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