Surging on Acceleration (1 Viewer)

[bgray]

Hi All,

I have an 11/72 FJ40 with stock F-engine and Asian carb. Rig has been running great for the past year+ until the other day when I picked up what I thought was a vacuum leak. The engine runs smooth at idle (vacuum at 16+), but when I apply throttle and hold it steady at a given RPM, the engine will surge where it starts to die and then a second later revs up again, and then repeats this cycle continuously. All smog was removed years back, with the only connections into the carb being the hose from the PCV valve and vacuum hose from the carb base to the Idle Solenoid. Distributor is vacuum retard and is disconnected. I then have the vacuum line from the intake manifold to the brake booster and a few old vacuum ports that are plugged on he intake manifold.

No luck finding any vacuum leaks using carb cleaner at all points on the carb, intake manifold & fittings, brake booster valve, etc. I also pulled the two vacuum hoses from carb and plugged them with my finger, but no change. I have a header on the engine, so I'm also able to see the bottom side of the intake manifold and there are no signs of cracks or leaks. I will note that if I spray carb cleaner at the carb throttle shaft outlet on the front side of the carb, there is a small leak as the engine RPMs will increase slightly; however, I think this may be normal and it doesn't seem to affect the idle. Further, the float bowl remains full throughout and I'm getting a good stream out of the accelerator pump when you look down the carb. I've tested that I have 12 volts to the Idle Solenoid switch and it seems to be working fine. I also jumped it to the battery temporarily to ensure I wasn't getting a voltage drop through the standard connection. Further, if I draw air on the solenoid, it triggers and holds vacuum.

So at this stage, would folks suggest I inspect the various jets to see if there is any sediment blocking the flow? I rebuilt the carb a few years back and I have a new gas tank with both the stock fuel filter and another inline filter, so I wouldn't think any junk would make it through, but who knows. I have done a number of searches on this topic and most point to a vacuum issue.

Happy to send pics if it helps.

Brian

 

[edwjmcgrath]

The idle solenoid doesn't have a vacuum connection. Maybe you mean something else or maybe that's your problem. Do you have a pic?
Yes, a small vac leak at the throttle shaft is normal.

 

[sggoat]

Wow! You have covered more symptoms than I can think of at this point!
A surge at steady throttle seems like it would be vac related, but since you have checked this, I would go to fuel. The 72 should have the slow jets in the carb(like my 75 carb), and could potentially be a culprit. Not sure if the carb has the power valve setup,, but if so, it might be reacting to small changes in vac, causing pulsating fuel delivery.
my .02

 

[bgray]

edwjmcgrath: My apology as I meant to refer to diaphragm assembly on the secondary side of the carb. I didn't have a pic of my carb handy, but attached a similar pic courtesy of SOR.

sggoat: It seems like vacuum to me as well, but I haven't had any success in finding it at idle using carb cleaner. Since the symptoms only appear when I have the engine at higher RPMs and its surging, I can't tell from using the carb-cleaner technique if a vacuum leak is occurring. I'll take another run at trying to find a vacuum leak before I tear into the carb. My thinking is that any vacuum leak significant enough to cause stumbling and surging at higher speeds, should reveal itself at idle, right?

 

[bgray]

So I took another pass at trying to locate a vacuum leak, but no success. For anyone interested, I uploaded a short video where you can somewhat hear the engine stumble the minute you increase the throttle, with the vacuum bouncing between 10-12 while holding the throttle steady.

Looking down the carb, when you accelerate, you see the stream from the accelerator pump, then it stumbles for a few seconds and then revs ups (seemingly once the vacuum picks up), which is the exact moment you see the fuel spray out of the primary venturi. I'm probably missing something obvious, but I'm having a hard time envisioning a vacuum leak that would only occur off-idle and be so severe to cause the engine to stumble to the point of dying.

I also rechecked the idle solenoid and confirmed that it is getting constant voltage to ensure it wasn't cycling on and off.

So unless anyone has other suggestions, my next step will be to pull the carb and look for any debris or obstruction. Outside of a vacuum or carb issue, is there any other component of the engine that could trigger this issue on acceleration only.


thanks,
Brian

 

[edwjmcgrath]

Ok, I get it now. You mean stumble on acceleration, not surge on acceleration.
From your video it sounds like you have a rough transition from idle to medium throttle. This can happen from one of a few reasons.
One reason is if the accelerator pump is not working. You've ruled that out.
Another reason is if the transition slot within the carb is clogged. This transition slot provides some extra fuel between idle and when the primary is active.
The third (related) reason is if the idle speed is set so high that the transition slot is open at idle. Then the transition slot doesn't do what it's supposed to do. This is usually the case when there's a vacuum leak.

 

[sggoat]

I'm leaning towards edwjmcgrath's transition, esp since you have checked vac--

 

[bgray]

Thanks for the insight. edwjmcgrath, when you noted the "transition slot" within the carb, are you referring to the slot in the primary circuit picture below. I pulled this from great write-up at http://www.risingsun4x4club.org/forum2/showthread.php?t=18991 The author mentioned mud, so I suspect he's a member here as well.

 

[bgray]

Coming back to this thread, I was able to correct the surging issue. I opened up the carb to check the various passage ways and discovered that the bolts between the lower throttle base and the carb body had loosened up to the point where leaks seemed obvious. At the time, I also swapped the carb base with a ported version from SOR, so I changed two things, but I suspect the issue was with the loose bolts. Unfortunately, I now have a slight stumble / flat spot when I engage the secondary linkage during hard acceleration, but I suspect this is unrelated and has something to do with the new lower throttle body, for which I'll start a separate thread.

Thanks for everyone's help.