General Category > Quadrajet Carb Talk and Tips
Hesitation when taking off
Kavesh:
Thanks Kenth. My mate will have a good look this weekend and hopefully find something else.
He mentioned that he believed the play on the primary shaft was alot. As the carb worked fine before I did not pay too much attention to the shaft.
My mate offered his theory here.....prior to motor being overhauled, vacuum may have been low which was not noticeable at the shaft. My motor is now rebuilt and vacuum should be higher which is now showing itself at the shaft and causing an idle issue. His logic makes sense but my vacuum is pretty low at 13 inches steady at idle. Perhaps i need a bit more timing to increase vacuum.
Cliff Ruggles:
You mentioned that the engine was "rebuilt".
13" vacuum is pretty low for a "stock" rebuild using a stock camshaft. Did you use something other than a stock cam during the rebuild?
Another BIG mistake folks make when "rebuilding" engines is not paying attention to quench.
Many oversize aftermarket pistons are shorter than stock ones for pin height placing them deeper in the bores at TDC. This LOWERS the compression ratio when it should actually raise it with the larger CID.
Most head gaskets found in "builder" gasket sets and THICKER than the stock ones which raises the cylinder heads off the engine and LOWERS the compression ratio.
Most engine "builders" either don't know these things or don't address them during the rebuild, so the engines compression ratio is typically lower than it was before the rebuild.
It's also common to replace the camshaft with an aftermarket cam of some sort instead of using a cam with stock specs. If a larger cam (more duration) is used this LOWERS dynamic compression, which hurts idle vacuum, idle quality and throttle response right off idle, etc.
Other mistakes I see with engine builds are not degreeing the camshaft and using "double roller" timing sets in them. Most of the roller sets being sold are JUNK!!! They are "weak" compared to the factory Morse chain set-ups, they do NOT offer constant tooth contact (smoother), not as accurate and the chains can stretch out very quickly.
Adding a high pressure and/or high volume oil pump is another BAD decision I see made with a lot of these engines. Stock oil pumps are more than adequate for all stock and most performance engine builds. Larger pumps rob power and put a LOT more load on the oil pump drive and distributor/cam gear, often wearing them out quickly.
I could add at least a half dozen more items to this list but I'll stop here.
Just throwing a few things out there that may be contributing to your problems........Cliff
Kavesh:
Thanks for the indepth reply Cliff.
Let me update on my carb first. My mate said that when he stripped it down he noticed that the seat for the needle and seat was not screwed in tight. With me having opened it up so many times i did not remove the seat each time so maybe this is my own doing. Perhaps the reason for the idle issues!
What impact does a not fully tightened needle and seat have on the carb? Rookie mistake I have made.
The carb was assembled by my mate and it is running fine on his test engine which is the same engine I have on my truck. I should have it back in a couple of weeks.
With regards to the low vacuum, I also was not happy with it.
The cam as best I know shoudl be a stock aftermarket replacement.
With the Holden 308 motor the replacement pistons are Chev 327 pistons. All rebuilders seem to use these as the Holden brands are just too expensive, if you can find stock.
My head gaskets are Felpro which as far as I understand are a little thicker.
My timing chain is double row like the original
Also using a standard oil pump.
Once I get my carb back I will try to determine why my vacuum is on the low side.
Cliff Ruggles:
No direct experience with the Holden engines.
A 327 Chevy, or even a 350 would have came from the factory with a camshaft that made at least 18-21" vacuum at idle speed with very little timing in it.
They NEVER used "double roller" timing sets in any of those engines, they were all the Morse type.
Contrary to popular belief the vast majority of the double roller timing sets being sold are NOT an "upgrade" over a stock type set. Most have cheap chains (poorly made or not "true roller") and cast iron sprockets. The "high end" varieties use billet sprockets and a real heavy duty true roller chain, but even those aren't nearly as strong as a factory style chain and probably why the factory used that variety instead.
Morse chains only got a bad reputation because early on they used a nylon coated top sprocket to minimize noise and absorb harmonics between the crank and cam. Those were actually excellent sets but over time heating/cooling cycles caused the plastic to crack and teeth would start falling off of them and eventually fail.
In my own engines here I will only use factory wide link type (Morse) timing sets and have had zero issues with any of them dating back nearly 50 years.
I tried a "high end" Rollmaster timing set once on my first 455 built in 1999 and it failed after 4 years of pretty hard use. I loved the ease of moving the cam around (9 keyway variety) and thought it would be a nice set-up for my engine back then. Sadly the chain got so loose it started "slapping" and retarded the cam so far the car slowed WAY down at the track.
Anyhow, a loose fuel inlet seat if leaking slowly will not provide full control of the fuel level in the carb and issues with fuel delivery as the fuel level has a direct impact on metering. I see quite a few leaking fuel inlet seats due to damage to the gasket surface. When the leak is really slow they car typically drives fine, but may "load up" when you stop if/when the leak is faster than the fuel the engine is consuming at idle.
It may also cause flooding after shut-down if there is still fuel pressure in the line between the pump and carb. For sure it's often difficult to diagnose but more common that folks think........Cliff
Kavesh:
Hello
Here is a further development on my carb issue. As mentioned before my buddy has leant me his spare quadrajet to use on my truck. For a few weeks now it has been working perfectly.
Yesterday I wanted to go for a drive and noticed that it was not idling like it should. The truck was last used a week ago. It is displaying the same issue I have had with my carb with the erratic idle.
Thinking that some dirt has got in I removed the airhorn and could not find any dirt inside the carb.
Nevertheless I used wire to make sure all the orifices are clear and they were.
Closing the primary flaps did smooth out the idle and the RPM increased. I am really confused guys, am I chasing a carb issue when in fact my issue may be elsewhere, or is this a lack a fuel on the idle circuit. If it is a lack of fuel why would it be fine for weeks and now display these idling issues.
Just to confirm I have not interfered with anything on the motor since last operated.
I will try and upload a video
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