Thermoquad help
Working on a 70 model 318. It is in a project car I acquired. It ran, but ran poorly. I spent the day tuning and had it running so good. Idled well, took gas good. Then after I finished, put up tools, I started it, let it idle then reved it up. It immediately died returning to idle. Now when you start it, it surges at idle, runs rough and smells very rich. I am not the best thermoquad guy. Any suggestions on where to start looking? The car sat for a while. Think I could have stirred up som trash in the carb? I am currently running off a jug so I know the gas I fed it was clean.
It is a 318 with a 268ish cam.
I was taking a video on the run where it died and also have a video of it surging now. I don’t think I can post video here but I can email it.
Thanks
@Vaanth is our TQ guru. Perhaps he will stop in to assist.
The described problem is not necessarily a Thermo-Quad specific issue, but could be more general, depending on what has occurred. Given that after tuning, the engine ran well, idled well, and revved off-idle well, and presumably with a decent idle vacuum, the carburetor and engine sounded generally healthy. After restart, idling, then revving, the engine died at return to idle so the issue appears rather suddenly with no warning conditions. Now the engine surges at idle, runs rough, and apparently is over-rich. The gasoline is clean.
Due to the sudden, and potentially transient, problem, it does sound like debris in the system. The sudden dying after revving the engine could have been due to a heavy over-rich condition from too much raw fuel. The running rough now and the gas smell could be from too much fuel flooding the engine from debris holding a needle off the seat and throwing the float(s) level off. The surging though sounds more like debris in a primary jet, holding a metering rod up, or somewhere in one of the feeds in the low speed circuit.
When idling the step-up piston should be pulled fully down in its bore, stopped by the step up rod. If the piston is fluttering up and down, it can be a vacuum issue due to the poor running, or an issue in the step up system. To potentially clear a jet, while idling, pull the piston up by the arms. The carburetor will go rich, so rev the engine, varying a bit. This may allow the debris if present in the jet to flush through. Unfortunately, it may lodge elsewhere, but if it clears fully, the situation may return to normal if this was an issue.
The step up piston can bind in it bore, as mentioned, but if it moves fully down cleanly, and lifts when initially revved, then eases back as higher RPM vacuum builds, it should be okay. If it sticks while up, the engine will run and idle rich and erratically. If it sticks while down, which is not likely if operating okay recently, the step up lever will bring the throttle when actuating.
To help with cleaning the carburetor internals without pulling the carburetor, use a mix of a can of Berryman B-12 to a gallon of gasoline. Seafoam can be used, but is not as strong or effective as the B-12 in this manner. This may help clean up potential light blockages in the carburetor.
Because this was noted after the engine died after a short run, then displayed issues after restart, ensure that the choke vacuum pull off did not dies (ruptured diaphragm) allowing the choke plate to close, killing the engine, and over-choking it at restart. If the engine was, or is, fully warm and choke fully open, this is not the issue though.
When it was revved up then died, if the secondaries were opened, ensure the throttle plates are not sticking open. This would generally just cause more of a high idle or erratic idle, but depending on the opening, it might behave badly otherwise.
The jet wells can leak, but the issue typically builds over time, and light leaks can be controlled by the throttle body to bowl gasket. Heavy leakage that overpowers the gasket results in harder starting, especially hot, faster fuel loss while sitting, richer idle and low speed condition which can clear for a time while driving or as the engine runs at higher RPM. The sudden onset of the issue mostly rules jet well leakage out, unless one came off completely and even then running at high RPM should temporarily clear the issue as the engine cleans itself.
The o-ring seal, if leaking badly, or missing, will cause similar issues with harder starting, especially hot, faster fuel loss while sitting, richer idle and low speed condition which can also clear for a time while driving or as the engine runs at higher RPM. The sudden onset of the issue mostly rules this out too, unless one split badly, and even then too, running at high RPM should momentarily clear the issue as the engine cleans itself.
Because it smells rich, this seemingly rules out other issues in the ignition, etc. causing erratic issues that appear to be carburetor related.
If the foregoing checks do not clear up the situation, or point to something definite, it could be something else, but carburetor removal, disassembly, inspection and cleaning, or maybe rebuild is likely in order.
For additional information about the Thermo-Quad carburetor, see the
Vaanth Thermo-Quad Guide at the
Carter Thermo-Quad Guide Site.
Also, see
Tips For Rebuilding a Thermo-Quad Carburetor.