I have a 69 ford 302 running a mild street cam and using 1969 351W heads on it . I am using a Pertronixs ignitor 2 distributor and one of those Summit 4010 knock off carbs they sell. Engine temp around 180.My problem is the truck will run fine until I get into stop and go traffic. Then the idle at each progressive traffic light starts to get rougher and will go down about 200 rpm and eventually not want to idle at all. Another problem is it will run fine if I am highway driving. But if I pull in for gas and say it's not running for a few minutes, it will start ok, but again the idle is extremely bad until I get it back out to the road again and drive it for a little bit. I have tried two different carbs and distributors and have the same issue. My question is could this be an engine running too lean with the idle circuit in the carb? Or could it be a form of percolation or vapor lock? Seems to be better if I give it more idle mixture. Like 2 and a quarter turns out....

 

Check to be sure the float level is not too high..
Check for vacuum leaks on all lines around the base gasket of the carburetor, pull the carb at look at the base gasket to be sure it is making proper seal with carb and manifold..

 

You have your answer right here, this is generally an idle mixture problem. The differences are in the cause of the idle mixture problem.


What manifold?
The carb has the power valve in the base of the bowl?
Auto or manual?
How are you setting idle mixture?

 

Hi Gman,
The float is set just a tad low. Using some brake cleaner spray, I can not seem to pin point any vac leaks. I have had this carb on and off many times, but use new gaskets everytime. In fact I ditched the special gasket Summit supplied with it, it would cause a whistle under part throttle.

 

I am running a stock Ford 4V intake manifold C50E-9425. There is the stock Ford spacer with PCV port under the carb. I am using a very thick gasket between the spacer and the carb. (thinking it might of been percolation)
The carb is using a 6.5 power valve, and yes it's in the base of the bowl (sort of reminds me how a bath tub drains)
Using FMX auto transmission.
I have been experimenting around with various settings of the idle mixture, I usually start by setting the idle mixture screws 1.5 turns out then I set the idle stop screw to around 1000 RPM (for a good reference point) from there using a vacuum gauge I slowly adjust each mixture screw for the highest vacuum reading/ idle RPM ending up around 1200 rpm, then I lower the idle to around 1000 again using the idle speed stop screw. And then go for a drive.

 

definitely leads back to A/F mixture screws, i am old school and always check to be sure the advance mechanism is working correctly and the timing is set before adjust the carb but if you think that's all good then warm up the engine to operating temp and turn the idle mixture screws in 1/4 turn at a time and listen for idle speed to drop, stop, back out 1 full turn, drive it, see what it does, keep us posted.

 

Then I wouldn't worry about it being a heat issue.
That last part in ( ) is why I ask. I haven't played much with those summit carbs but the autolite 2100/4100 series carbs have an issue with the power valve draining causing flooding. Though this is mostly a short time off/restart issue. It usually goes, park, shut off engine, engine off = no vacuum = open power valve, fuel drains into off engine, come back 5 minutes later, won't start cause flooded. I mention it though cause if you have a power valve issue those carbs are more susceptible to small power valve issues. For example not blown but just a leaking gasket.
I lean towards this being the root of the problem, setting it like that you're likley lean. First idle adjustments need to be made in gear when you have an automatic(1000RPM I assume you aren't in gear). The speed/vacuum/load etc. is all different in gear vs. park, this effects idle mixture.

Idle setting steps W/auto
1. Warm up engine, ideally go for drive.
2. Set brake, chock tires, get buddy behind wheel etc.
3. Put in gear, drive or reverse, doesn't matter, whatever is safer in case it moves, usually this is reverse cause you are in the front.
4. Leave fuel screws where they are, adjust throttle idle screw for idle speed.
5. Adjust one fuel adjuster at a time, first richen(turn out) 1 turn past fastest idle/highest vacuum, next it lean(turn in) until RPM drops, vacuum drops, or engine stumbles. Turn 1/4 turn out from there, for general setting.
6. Repeat 5 on other side.
7. If idle speed adjustment is needed start again at step 4.
8. Drive paying special attention to light throttle tip in and general throttle response. If stumbles turn each fuel screw out 1/8th turn, drive, repeat if the 1/8th turn improved the stumble, if not stop stumble has other cause.

Revving the engine IN PARK a bit at times between steps can help get better settings, most of all when leaning from rich in step 5 to clear out excess fuel in the manifold.

 

Yes sir. I have a new Pertronixs II distributor which I really like and have been playing around with advance curves so I know we are good to go there. Timing is about 6 top dead center. But I think with the cam I am running maybe another 4 to 6 degrees would not hurt. I appreciate your recommendations and will give it a try.

 

Hi Brute! Thank you for your very detailed explanations and suggestions and your idle mix setting proceedure. What you are saying makes a lot of sense. I will try it, put it to work for me and see what happens. You have given me a lot of good information and I appreciate it very much!

 

You're welcome




Let's do one thing at a time to keep things simple and know what caused what, however from what you say here I think you need some timing tweaks also, start with the idle mixture though. Once you get down the steps I set out you'll find you do it automatically and quickly after each other adjustment.


As for timing 6 BTC is very low, but before I make any specific suggestions I would like more info.


Distributor model, most Pertronix distributors are GM based when it comes to the advance mechanisms?


Vacuum advance, using manifold or ported?


Centrifugal advance characteristics; Advance starts at what RPM? Total advance amount and RPM it stops advancing?

 

Hi Brute, the distributor model is a Pertronix Ignitor II for a Ford 289-302, and it does have the GM based mechanical advance weights. Right now using the lightest springs.

I am now using ported vacuum. Originally I tried manifold vacuum, loved the smooth idle it gave me, but did not like the "hole" it gave me upon acceleration from a dead stop.

The advance curve looks like: (using the lightest springs)

600 - 6 degrees
700 - 7
850 - 15
1000 - 15
1500 - 27
2000+ all in at 32 degrees.

The cam I am running is a Comp Cams 31-218-2, 268H grind, 218/218 duration at .050"

I am using Autolight 42 plugs, gapping them at .034", and a new set of Moroso wires. The coil is also a Pertronixs Ignitor II.

Hopefully that fills in some of the gaps. But let me know if there is more information I can get you!

Many Thanks again!

 

Ok, first just to say it again, do the idle mixture thing first to see if it solves the problem. That said after each timing change that effects idle you'll want to adjust the idle again.

I'm a Ford guy but I have to say I prefer the GM advance mechanism, much more tunable.

I'm not a fan of Moroso wires, when/if they need replacing I suggest the Taylor Cable Thundervolts.

OK timing, as I said before 6 at idle is very low, as is 32 all in, I suggest an immediate bump to 10 at idle, this should put total at 36, still quite conservative/low but better and very safe numbers.

Next this will sound unusual as timing advance advice goes but I'd suggesting finding two useful numbers. I call then ideal starting, and ideal idle. After warning it up good and disconnecting the vacuum advance the first step is finding ideal idle, this is simple, advance the timing by turning the distributor and until vacuum no longer climbs. When you do this the idle will speed up, re-adjust it down, then re-find the max vacuum point for the new idle adjustment. After take a timing reading and write it down, you won't be leaving it here but snug it down for now. This is ideal idle, it's likely this number will be well over 20, don't panic if that seams really high. Next find ideal starting, shut it off and try and start it, it's very likely that it will not start, that it won't crank well. Back the timing off in small increments until it starts easy then take a timing reading and write it down.

Then post those numbers.

A last observation is that this;
600 - 6 degrees
700 - 7
850 - 15
1000 - 15
Is this with the vacuum advance attached at ported? The 15 at both 85 and 1000 makes me think so, if so do this again with the vacuum disconnected.

Edit, one more question, do you know what spark knock sounds like and if so do you have a decent ear for it?


Edit two, I forgot something, do you have an adjustable vacuum advance and did it come with a vacuum adcance limiting cam?

 

Hey Brute,

Looks like I'm going to spend some quality time in the garage. Hopefully I can eek out some time this weekend as in here in North Iowa it looks like we're going to close to 40 degrees. May not be able to get road time though as there is much snow, salt and general corrosive chemicals on the road right now. But for what I need to do right now, we don't need to drive it (even though I know it would help!) much of this is just under the hood, get it adjusted, and see how the engine acts. So I am going to follow your plan:

1. Adjust the idle speeds screws using your plan:

Idle setting steps W/auto
1. Warm up engine, ideally go for drive.
2. Set brake, chock tires, get buddy behind wheel etc.
3. Put in gear, drive or reverse, doesn't matter, whatever is safer in case it moves, usually this is reverse cause you are in the front.
4. Leave fuel screws where they are, adjust throttle idle screw for idle speed.
5. Adjust one fuel adjuster at a time, first richen(turn out) 1 turn past fastest idle/highest vacuum, next it lean(turn in) until RPM drops, vacuum drops, or engine stumbles. Turn 1/4 turn out from there, for general setting.
6. Repeat 5 on other side.
7. If idle speed adjustment is needed start again at step 4.
8. Drive paying special attention to light throttle tip in and general throttle response. If stumbles turn each fuel screw out 1/8th turn, drive, repeat if the 1/8th turn improved the stumble, if not stop stumble has other cause.

2. After the idle has been adjusted, and the engine idle still starts to drift down and stumble, I will play with the timing, bumping it up to around 10 degress BTDC. Then re-adjusting the idle mix again, while in gear with wheels chocked.) If still no progress or even if there is, going to the next step:

3. I'd suggesting finding two useful numbers. I call then ideal starting, and ideal idle. After warning it up good and disconnecting the vacuum advance the first step is finding ideal idle, this is simple, advance the timing by turning the distributor and until vacuum no longer climbs. When you do this the idle will speed up, re-adjust it down, then re-find the max vacuum point for the new idle adjustment. After take a timing reading and write it down, you won't be leaving it here but snug it down for now. This is ideal idle, it's likely this number will be well over 20, don't panic if that seams really high. Next find ideal starting, shut it off and try and start it, it's very likely that it will not start, that it won't crank well. Back the timing off in small increments until it starts easy then take a timing reading and write it down. I will then keep notes on the timing numbers.

To answer some of your questions:

The distributor curving numbers I shared with you earlier were taken with no vacuum advance. I am using the lightest springs so the curve comes in very early. Before I start the above. I am probably going to "base line" it and switch the springs back to the standard springs that were factory installed, then maybe later after we have some answers going back to the lightest springs if we decide they are needed. The standard "silver" springs brings 24 degree total timing all in at 3500, verses my total now at 2000.

As far as knowing what spark knock is, I have have heard severe spark knock before, not with this engine though. I am some-what hard of hearing so minor spark knock might escape me.

About the distributor advance. It does come with an adjustable advance can, and also comes with a advance limiter kit, basically some small metal strips that I think go under the springs on the spring posts, to keep the weights from moving too far out. Pertornix says with this type of advance system, mixing and matching springs, and advance limiters, there are some 70+ curving options you can come up with.

I'll let you know how I coming on this. Once again I appreciate your guidance on this!

 

You're welcome.

The centrifugal advance needs to be addressed then, it is likely a contributor to your idle problem. You can't have the centrifugal advance advancing during in gear idle. You need to put in heavier springs, the timing should be very stable at idle, should be able to adjust the idle a couple hundred RPM without the timing changing at all.

Likely your a little lean at idle, this is slowing the engine after it burns off any fuel residue left in the intake manifold. However in most cases this wouldn't be as drastic of an issue as your having. But in your case, as soon as it slows a little you lose timing advance cause the centrifugal is advanced at idle, this causes a larger rpm drop, and it snowballs from there until it gets to an RPM where the centrifugal advance is bottomed at which point it's pulling much less vacuum and is thus even leaner.

Vacuum advance limiter, doesn't sound like you have one of the ones I like, I suggest you pick one up, they're cheap. Here's a link to what I'm talking about. http://www.summitracing.com/parts/cr...PoMhoCX_Pw_wcB This sucker goes under the inside mounting screw for the vacuum advance and limits the travel of the rod. Easy to adjust, loosen screw and turn.

You have those flat "L" shaped limiters right? Those can be handy when the time comes.

Biggest thing is to make sure that the weights are tightly held in by the springs when stopped and that it's not advancing at idle.

 

Well Brute, got some time to work on the old truck today...here's what happened...

Started the engine. Checked the timing, it was 6 degrees at around 600.
Decided to baseline the distributor by pulling the light springs out of the distributor and put the factory installed heavier ones in. I set the timing again to 6 degrees at 600, and then checked the idle screws, adjusted them for the best idle, highest vacuum, I was chasing my tail for awhile then finally tried to screw one idle mix all the way in, it did not kill the engine, though it was rough, tried the other side, and it was the same. Tried to turn down the primary idle screw all the way, and the engine is still running at about 400 rpm! Ended up I found that the secondary throttle stop was open too far, I closed it some, about 1/8 of a turn open now, and now I find I am having more control with my idle mix screws! I am now getting the highest idle I could around -17 out of gear at 800 RPM, -11 at around 600 in gear. Adjusted idle mix screws again, in gear, at around 600 and gained a little more vacuum at about now -12. Engine seems to be running nice and smooth. Screws ended up being around 2 1/8 to around 2 1/4 out. After many times shutting down and restarting the engine, it seems the hot start stumbling issue is gone, and the engine idle is not drifting down anymore while running in gear and the rear wheels chocked. And it was running for a long time, longer than it would be at any stop light. Next I decided to bump up the timing a little more to about 10 BTDC, engine now runs smoother, and I am liking the exhaust note.
Next I look at the ideal start and ideal idle you asked me to check. Advanced the distributor to max idle, ended up at 56 degrees/1480 rpm/at -23 Hg. The start max advance was interesting, it labored just a tiny bit at 56 degrees, but the smoothest start was found around 40 degrees/1200 rpm/at -22 Hg. Put the distributor back to idle at 800 rpm/ but when the transmission engages, the idle drops to about 550, and the engine is sounding labored. I try to trim the idle mix screws a little bit more, but it seemed not to make any difference.
So I ended up with an curb idle of around 900, and the" in gear" idle at around 700. (Maybe the street cam interacting with the stock torque converter).
I wanted to check the distributor curving now since I switched in heavier springs before starting all of this, and with timing set at 10 degrees, there is no advance now til 1500, a little past 15, I am seeing 18 degrees, and it appears all in at 24 degrees at 2000. I drive around the parking lot and I notice that even though the engine seems much smoother, starts better, idle now much more stable, it lacks the punch it had. I am guessing next working on the distributor advance more?

All in all a productive night. With you help I am making progress I think!