Carb Hesitation

Sparky, I hope you don't cosider this a hijack. I know the vacuum advance works off of ported vacuum. I also know that all of the books say to plug the vacuum line while setting the timing. So if there is no advance at idle due to the ported vacuum, then why the need to plug the vacuum line?

 

1. Just guessing but yes, advance devices are probably not the problem, however, base timing may be a problem. Seemed to get better with more degrees of base timing which seems backwards to me.
2. Some of the mag guys seem way out there to me. However, Pewe is the real deal. I was on the two trail rides he went on while here in Fairbanks. He's an off road guy first and a mag guy later.
3. Carb was in the tank for two days. However, based on the lack of response from turning the idle mixture screws I'm thinking (hoping?) that maybe there's a blockage there. I think I'll remove the carb again and remove the mixture screws and go after it with carb cleaner and compressed air. Won't be until the weekend at the earliest.
4. Yes, looking down the carb throat with motor running is possible and maybe useful for checking accel pump operation, however, put on a good face shield first or hold a piece of plexiglas or lexan between your face and the carb. The flame that shot out of there was enough to make you need a clean pair of Fruit of the Looms. The stream of gas that I'm getting (motor off) seems ok, however, I don't have much to compare it with. Could be too weak but I'm not really sure. Someone who has done this before would be able to tell. I'll be seeking help from someone more experienced eventually if I can't get it working.

 

Check and check. I made sure the ball was there and in position two or three times. The linkage seems very tight each time I check it. I may try and bend it to get more immediate action.

 

So it sounds like backing off the base timing helped this problem? I will try that also - maybe go down to 5 degrees instead of 8.

 

the vacume advance is for fuel milage you should disconnect the line and plug it to make sure you get the right base timming.
You should be able to disconnect it and not have any poformance chane other then lower fuel milage.
thing I would check.
1. vacume leaks (I know you have allready checked) your vacume gauge should have a stedy neadle = no leak
2. base timming at the correct Idle rpm 650 to 700 rpm.Also ck the points dwell or point gap.
3. Idle mixture bench set should be all the way in the backed out 1 1/2 turn. then turn to reach the higest rpm. if you have more then one adj screw they should be within 1/4 turn of each other.
Now I rebuilt a carb once and didnt get it clean enough and one screw did nothing. some thing to check.
3. is this a odd fire if yes the Distributor has to be put in correct I mean ther is a #1 postion on the Distributor.
4. does the acclator pump work? (I know you checked)some carbs have different setting on the linkage to adj how much fuel is squrted in each time. is it set right?
5.how is the mechanical advance you may need to check the advance curve. this meanes how quickly the advance goes to full advance.
6. the last thing I would check and only after checking everything else a hole lot of times is the cam timming.I realy dought it is off.

good luck I hope this helps and please excuse my spelling.R)

 

Did you remember to compensate for altitude?

Our vehicles ran different over at the U of A campus and up at DogPatch than they did when we went down to Anchorage for the salmon season.

 

i sincerely would NOT recommend this with the engine running unless you want to either make life even shorter or spend the rest of it in real pain. i like grilled meat but i just want that meat to be me or a friend. it's also hard to offroad blind...

i might as well put my $.02 in about this.

see if your rebuild kit had the usual little paper/cardboard gauge to set the float level and the accel pump linkage.

i used to skip the accel pump linkage setting by measuring but found that it actually helps quite a bit since i discovered that a late pump will starve it (obviously) and at least in mine a pump too early will stumble as well. (don't know why except perhaps it floods a bit if the vaccum doesn't suck enough air through to mix and has to catch up?)

my v6 OF is set at 5*btdc according to specs for the normal dist. and i found it works best there with hei as well.

if it's stumbling off idle i still think it's an accel pump problem. i don't have the feeling that it's timing.fwiw

 

Yes, I really suspect the carb because it was the most recent thing to be worked on. However, it is possible that before rebuilding, the carb was masking another problem that only now shows up. Also, there was a similer hesitation (not as bad) years ago when I first got this rig and it eventually went - slowly over the years. Maybe as something got dirty in the carb? I'm grasping.

Copper wire trick. Thanks. I may need all the help I can get.

 

Everyone, thanks for the input and keep posting if you think of something. I'll be working on this again this weekend - there are a few things to try such as cleaning the idle mixture passages and using my new vacuum gauge to search for vacuum leaks and tune the idle.

I'll also: confirm idle speed, reset base timing, and recheck the accel pump and linkage again.

As mentioned before it's an odd fire. Also, I did observe the (mechancal) timing advance by speeding up the engine while watching with the timing light - seemed very smooth. I do not suspect timing advance is a problem.

Cam Timing - I replaced the timing chain a couple years ago. I think it's ok.

Altitude - don't have much here, about 450 feet.

Accel Pump - the accelerator pump is definately squirting fuel. There may be a micro second at the beginning of the stroke where it's weak but I just am not sure how to judge if that's significant.

 

Thanks jeepen. The choke is manual. Yes, I get the engine warm before testing.

Something else: if the hesitation is due to a lean condition I should try running with the choke partially on to see if things improve. Just another data point.

I already changed the plugs - the old ones were carboned up some, probably from too much idling.

 

usually....an accelerator pump problem lessens as the engine warms up
in other words, worse when cold
maybe a little better when warmed up
FWIW
I'm more bothered by your mixture screws having little/no effect.
I wonder if you have the wrong gasket bewteen the base and the body ?
(I also don't know if that's possible, but kits can be generic sometimes)

 

Took the carb apart Saturday. I was able to clearly see where the gaskets are hitting and they're all good - correct gaskets, good fit, sealing well. Also checked the idle circuits and double cleaned them - everythings good there.
Reset base timing to 5 degrees. Reset idle speed to 680 rpm.
With the vacuum gauge tee'd into the vac line the PCV valve I get 11 inches of very steady vacuum. I think steady is good but is 11" too low? With the vacuum gauge and digital tach on I sprayed around all areas of the carb and got zero movement on either the tach or vacuum gauge - again, no vacuum leak.
Also, when I installed the tee I think it may have restricted the flow from the PCV valve - the hesitation improved slightly with the tee installed. The tee is the small (about 1/8") one that came with the vacuum gauge and I had to adapt it to the larger PVC hose.

So I put in a new PCV valve thinking the old one may be letting too much flow into the manifold but no change.

One more thing. I tried pinching the rubber hose to the PCV valve while the vacuum gauge was installed. I got more vacuum and the gauge started bouncing. Not sure what this means if anything.

 

OK. I've been avoiding saying it. Do I have a flat cam? Seems like if it was just one lobe it would wiggle the vacuum gauge but it was very steady @ 11".

I assume if it was a burnt or bent valve that would show up as a not steady vacuum reading?

The compression was 134 PSI plus or minus 3 PSI.

What would cause it to have 11 instead of 20 inches?

 

I would cap off the vac port at the rear of the carb, then take another reading; 11" of vac is way too low.
is the fitting loose/leaking or the hose ?
maybe the new PCV valve is bad
pinching the hose off is your indicator something is not right there

 

most likely backfire thru the carb on accel would be indicative of bad cam lobe
you could remove the valve covers, run the engine to watch and check for this