View Full Version : 1F issues...ideas welcome
04-13-2010, 04:07 PM
The F engine in my 1972 FJ40 has some relatively minor, but annoying,
issues that I can't seem to solve. Here's the story:
-F engine. P.O installed headers. I had carb rebuilt by Jim C., installed
an HEI distributor, replaced all fluids/wires/hoses/filters. Head rebuilt
-Runs well when warmed up, tho hesitates at low RPMs. Pulls very
strongly at higher RPMs. No problem maintaining highway speeds.
-Starts right up when cold. Runs poorly tho - just getting moving in first gear requires slipping the clutch to keep the RPMs up. Will hesitate and buck until warmed up.
-The header install required removal of the heat riser, which certainly didn't help with cold running. Seems tho that this problem has gotten worse, which suggests that it's not just the lack of heat riser.
04-13-2010, 04:08 PM
04-13-2010, 04:35 PM
It sounds like you're running too lean. That condition could be caused by a vacuum leak or could just be the carb/choke/timing need adjusted. Perform the basic tests to check for a vacuum leak around the base of the carb and where the intake manifold bolts to the head.
If you confirm no vacuum leaks then I'm betting it's just running too lean at idle since you say it has good power at high rpm. That means your high speed jets are probably the right size. If you told Jim C what elevation you live at he's very good about jetting the carb properly for you.
Running rough when cold is a symptom of the idle mixture being too lean as is the hesitation when transitioning from idle to high speed jets.
Also, you can get a fluid heat riser from Man-a-fre. I have one on my 40 and it works great. It bolts onto the bottom of your intake manifold where the stock exhaust manifold used to be. You T into your heater hoses so it circulates hot water to the bottom of the manifold. You would be surprised how much quicker it warms up with this and retains heat if you go shopping, etc. Another benefit is it prevents vapr lock in the summer when it's hot because you're liquid cooling the base of the carb. Go to the MAF website, go to parts and look under "H" for "heat riser fluid".
If you need a hand adjusting your carb let me know, I'm glad to help and there are many others that can help you too.
04-13-2010, 04:35 PM
I'd track the vacuum under cold, warm, and hot engine conditions. It's possible you have a vacuum leak opening up on the intake manifold when it's hot. Check for cracks in the manifold under the carb (pull the carb and look straight down). Watch the float level in the carb and perhaps check fuel pressure (real time) when it's running. Spray carb cleaner/WD40/whatever around the carb base and intake gasket when it's running to see if it speeds up. You can also use propane from an unlit torch.
Check your base ignition timing - it could be too far advanced, which could cause bogging on launch. Be sure your vacuum advance isn't stuck in one position or the other (stuck advanced or retarded). Also be sure the vac advance works at all (by applying vacuum or sucking on the hose while the engine is idling).
04-13-2010, 08:53 PM
Thanks all for the suggestions. The vacuum leak idea sounds particularly
promising, so I'll check that out first. As far as I can tell, the carb
does not have an overall mixture adjustment, but only an 'idle
mixture' screw. So I don't see a way to adjust the mixture.
(If I'm wrong about that, let me know). The carb is, AFAIK,
an AISAN correct for the year (1972).
Another bit of data: the choke works, in that it opens and closes
as it should, but does not raise the idle when cold as it should.
Instead, it has an almost random effect - sometimes helping when
cold, sometimes making it worse. More evidence for a vacuum
leak when cold?
04-13-2010, 09:32 PM
Could very well be a vacuum leak. Yes, there is only an idle mixture screw but you may need to richen that up if you confirm there are no vacuum leaks. There is some adjustability to the choke linkage depending on the model of the carb. Seems like your high idle cam isn't engaging consistently.
04-14-2010, 09:58 AM
The choke (without a choke breaker) should almost kill the engine - so I think this points again to a vacuum leak or internal carb problem (if not leaks are found). I've seen carbs that were reassembled wrong and the vehicle will run under some circumstances, but for all the wrong reasons. Heck - I've been the one doing the 'mis-reassembling' in two cases ;-)
04-14-2010, 02:41 PM
with it running take a UNLIT propane torch and fill the heat riser cavity on the bottom of the intake manifold. If the idle changes you found your leak. Early intakes are prone to cracking. If this is your leak I have a permanent solution for you that I think you will like...
vBulletin® v3.7.1, Copyright ©2000-2013, Jelsoft Enterprises Ltd.