After the head rebuild, I the slant in for a bit of professional maintenance. Timing was dialed in, compression (although I don't have the numbers from the test) came back in good health, and a tune-up was made. Something the matter, though, is that the carb (an early Holley 1920) is acting up. I rebuilt it a few years ago and it was running just fine before. It had a mild idle that occasionally had a hitch somewhere, but it ran well. Now, it doesn't want to hold idle and its running lean. I've rebuilt it again to make sure there wasn't something wrong internally. I have more adjustment response now that I have done it, but it idles better with my hand restricting half the airflow. Is this a choke issue of some sort? I went ahead and bought the electronic choke that is a popular upgrade, I've replaced the fuel line from fuel pump to carb and a new filter was installed. Still struggling to hold idle in park, much less idle in-gear at a stop light. Any ideas, maybe?
Runs a bit lean...
- Thread starter zmarty27
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Kendog 170
Let the boy go !
Vacuum leak?
Not to my knowledge. The mechanic I go to is quite thorough and a vacuum leak wasn't even a concern. He assumed it was a fuel problem, hence the reason I replaced the upstream lines and filter (as well as a carb rebuild).
Kendog 170
Let the boy go !
Bad PCV stealing too much vacuum? Block it off to test. Spray water from a squirt bottle around intake gaskets, carb base, and any other potential areas while idling and see if idle drops. Some use carb cleaner or starting fluid which I would NOT do. I had a rubber cap on one of my carb vacuum ports that dry rotted off. Maybe fuel pick up sock collapsed. If it's a bad vac. leak you might hear it. I'm no expert but I check that stuff easy enough. Or Post #4
I think you guys were right about the jet flow. I took the carb off and gave it another once-over with the carb/choke cleaner. Any hole on that thing got a good spray. Its now holding idle in park and at the light. Naturally, the nature of the carb will function differently with a temp change (for example, it was 78 during the day yesterday and 52 at night when I took it for another spin around the block). At night, the carb wanted to stall out. I did not change a thing, and I know that happens occasionally if not regularly. Any pro-tips before I start finding the sweet spot in my adjustments? Thanks for the help so far. You are all a wealth of knowledge.
Ant
Well-Known Member
Check the metering block, something may be coming loose or falling apart internally. I had a similar running issue, one of the brass orfice tubes things or whatever it's called kept moving in and out.
The 1920 is notorious for the 'economizer' (power)valve seizing/hanging-up, & related passages plugging. If it's running well, You're wise to keep it clean, & seek out alcohol-free fuel if it sits for any lengthy duration.
AJ/FormS
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The Transfer slots are your Low-speed fuel delivery system.
The Idle mixture screws are just there for Idle enrichment.
If the ignition timing is NOT factory, ie is now advanced, then to keep the idle-speed down, this will require the throttle-blade to be closed, which then closes the Transfers, which then makes idle-issues and usually an accompanying tip-in sag/aka a hesitation on take-off.
To compensate for the loss of transfer fuel, one/I will usually find the idle-mixture screws set wide-open.
This works at idle, but makes the entire low-speed system go rich.
The only cure is to put the idle timing back to stock, and with it, the mixture screws adjustment.
Having said that, slanty intakes are notorious for sucking air at the head interface.
Someone mentioned the PCV, but I have never found a PCV make trouble, unless it was an old dried out and cracked hose, or a hose was installed that was not designed for the high-vacuum purpose, which got sucked to near flat at idle....... but this makes the idle speed go slow, and so the throttle has to be opened to get more air, which opens the transfer ports too far, and the whole program goes rich.
In your case, covering/restricting the air intake is proof of one of three things;
1) the engine is getting too much air, or
2) the carb is not delivering the correct amount of fuel, or
3) the fuel droplets are not being properly mixed into the airstream, which occasionally points to the under-carb heater.
If the problem is ONLY at idle, I would go straight to the transfer-port exposure.
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The Idle mixture screws are just there for Idle enrichment.
If the ignition timing is NOT factory, ie is now advanced, then to keep the idle-speed down, this will require the throttle-blade to be closed, which then closes the Transfers, which then makes idle-issues and usually an accompanying tip-in sag/aka a hesitation on take-off.
To compensate for the loss of transfer fuel, one/I will usually find the idle-mixture screws set wide-open.
This works at idle, but makes the entire low-speed system go rich.
The only cure is to put the idle timing back to stock, and with it, the mixture screws adjustment.
Having said that, slanty intakes are notorious for sucking air at the head interface.
Someone mentioned the PCV, but I have never found a PCV make trouble, unless it was an old dried out and cracked hose, or a hose was installed that was not designed for the high-vacuum purpose, which got sucked to near flat at idle....... but this makes the idle speed go slow, and so the throttle has to be opened to get more air, which opens the transfer ports too far, and the whole program goes rich.
In your case, covering/restricting the air intake is proof of one of three things;
1) the engine is getting too much air, or
2) the carb is not delivering the correct amount of fuel, or
3) the fuel droplets are not being properly mixed into the airstream, which occasionally points to the under-carb heater.
If the problem is ONLY at idle, I would go straight to the transfer-port exposure.
.
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