oamhmad

iTrader: (1)

I've been trying to tune my Holley 4160 using a wide band gauge for quite a while now.

I've got the idle set to 13.1 afr however, I seem to have some heat soak issues as the engine misfires and stumbles really bad If I let the car idle too long while stationary.

After about 5 minutes of driving around it idles fine again. The richer I set the idle mixture the less I notice the problem but fuel economy suffers.

Is this vapor lock?

I've got a carb heat shield which helped a little.

Suspecting vapor lock, the last time this happened, I checked the fuel lines and the mechanical pump. Neither felt excessively hot. The float bowl also felt normal and not the slightest bit warm, yet the engine was stumbling very bad and I could see lean spikes on the AFR gauge corresponding with misfires.

I pulled the sight plug on the float bowl and fuel dribbled out a little. The fuel felt cool and the bowl was definitely full.

So why is this happening if the carb is full of fuel that is not vapor? The problem only manifests itself after the car has been idling stationary for a while or if it's restarted after sitting for 20-30 minutes. It does seem like a heat related issue, but where?

Damon

It's not vapor lock. Vapor lock happens on the suction side of a mechanical fuel pump when heat causes it to flash into vaport (boil) when combined with the suction of the pump. Like all pumps, it is incapable of pumping vapor, only liquid fuel. It just can't pump anything up to the carb.

The other issue that is heat-related is what you are trying to describe. It's call fuel PERCOLATION. That's when you get so much heat into the carb it boils the fuel IN THE FUEL BOWLS AND/OR FUEL LINES, causing it to bubble out of various passages and into the engine (flooding it). But that's not your problem either, since you aren't experiencing symptoms like that.

If I had to guess I'd say you have an ignition-related issue. Possibly the plugs just being fouled out (which will clear up a bit if you drive it around, but never really fix itself short of replacing the plugs).

How much advance are you running at idle? If you're running say, 12* initial and no vacuum advance on top of that (at curb idle) it will eventually foul the plugs no matter what you do- they just don't get enugh heat in them to burn off combustion contaminants with so little idle ignition advance.

If you have a lumpy cam in the motor don't expect to be able to keep the plugs clean under extended idling conditions with a carb no matter what you do. You're just never going to get it to idle clean for long periods of time with a carb and a lumpy cam.

oamhmad

iTrader: (1)

Advance at idle is about 32 degrees (12 static + 24 vacuum, connected to full man vacuum) This setting gives me about 36 degrees of mechanical advance at around 4000 rpm.

The plugs might be fouling up, I'm not sure. They look fine every time I've pulled them.

The cam is tiny. It's the stock cam in a 260 (claimed) hp Goodwrench 350.

sofakingdom

iTrader: (1)

That's too high.

Probably, the bowl overflows into the motor when idling for extended periods.

Try lowering the float level to where it's below the sight plug on the primary side, and you have to jiggle the car pretty good to get it to run out; and on the secondaries, where you have to really shake the car hard.

oamhmad

iTrader: (1)

I pulled the primary site plug on a running motor and fuel dribbled out. Should I set the float so this does not happen when the engine is running or stopped?

oamhmad

iTrader: (1)

I set the floats. They were both too high. Especially the primary side. It seems to have helped a little. But the problem still persists.

sofakingdom

iTrader: (1)

Check the temp of the fuel line WHILE IT'S IN THE ACT OF HAPPENING; while the malfunction is actually occurring. Just feel it with your hand and see what it's like.

oamhmad

iTrader: (1)

The fuel lines did not feel warm at all while this was happening. I've included pictures to show how they're routed. The heat wrap goes all the way to the back past where the header gets the closest to the fuel line.

I've also attached a video link while this was happening. You can see the engine surge on both the AFR gauge as well as the vacuum gauge.

Then I enrich the idle needles 1/4 turn each and the idle is rock steady with the AFR showing 12.7. Problem is, if i drive around with this setting for any length of time, the AFR will go rich to around 10.9 - 11 and waste gas until of course the car sits in traffic around 15 minutes or so and then the AFR will lean out again. So i have to set the AFR rich to compensate for this.

oamhmad

iTrader: (1)

No body?

I'm beginning to think the problem is with ethanol fuel, so baring a thick enough carb spacer that's not going to fit under a Firebird hood anyway, the only real solution is EFI.

skinny z

Despite what anyone may tell you, try the ported vacuum for your vacuum advance.
If your engine runs better when connected to full vacuum, then you have issues elsewhere.
Think about it...

skirkland1980

iTrader: (2)

You're running E85? That would have been helpful to know in the beginning.
You're afrs should be in the 9-10:1 range. E85 is also very corrosive and requires a completety special fuel system.

Zane M

iTrader: (1)

We should get clarification from the OP before jumping to this conclusion.
In many areas Ethanol is added(Up to 10%) to the regular fuel supply, but is not E85.

oamhmad

iTrader: (1)

Yes I did have an issue else where at that time. The carb base plate was cracked and I had a massive vacuum leak. Ive replaced that and now it runs the same on both ported and intake vacuum, but I like intake vacuum better because the greater advance at idle keeps the engine cooler and I get better throttle response.

oamhmad

iTrader: (1)

I'm running regular pump gas which here in NJ is 10% ethanol, not E-85. I wish I could find 100% gasoline somewhere.

skinny z

Regarding the full manifold vacuum for your vacuum advance: If you think about what's going on then it can be seen how this can produce an unreliable or unstable idle condition. You are changing one of the parameters that are needed to produce a smooth idling engine.
With manifold vacuum pulling the vacuum can to full advance at idle you're relying on that vacuum being constant so that the timing doesn't change. As your engine idles a small variation in vacuum (such as what results from the engine gaining a little temperature as it sits) the inital timing changes. Idle speed then changes as the timing changes, the AFRs become unstable and the cycle feeds on itself. The result is very much what you describe.
For what it's worth, I've tuned several Holleys over the decades (the wide band O2 sensor is possibly the greatest tool to use here) and have had reasonable and repeatable results. While my current ride is no rocketship, I do trap 110 mph in the quarter, am well into the 20 mpg range on the hiway and idle all day long at 650 rpm pulling 11-12" of vacuum. Initial timing is 14-16 degrees, 10 degrees of ported vacuum advance which is all in above 10" of vacuum (part throttle not idle) and 18 degrees in the distributor.
If you want to try an experiment, hook up two vacuum gauges, one to each type of port and observe how differently they behave under varying driving conditions.
Another experiment is to observe your igniton timing with the vacuum connected to a full manifold source and see what happens as you open the throttle slowly and then quickly. Your spark timing will be all over the place.
Another thing you may want to try is to disconnect your vacuum source altogether and set your intial spark lead to 15 degrees or so. Try your carb setup this way. If your carb is in good conditon and there are no vacuum leaks anywhere (which absolutely kill any chance of idle stability) you may find better results.
As for ethanol in the fuel, we have 15% ethanol in regular fuel and 10% in mid grade fuel here. I've seen no impact on idle quality with this blend. AFRs at WOT are a different story.
Anyway, that's my two cents. Good luck and keep us posted.

oamhmad

iTrader: (1)

I understand what you are saying about a manifold vacuum advance induced feedback loop causing the unsteady idle. I'm sure switching to ported vacuum for advance or no vacuum advance will help somewhat, but I don't think it's the root cause of the problem which is what I really want to get to.

I think the main problem is excessive heat soak in the carb in conjunction with the lower boiling point of 10% ethanol fuel resulting in unsteady idle, misfires and lean AFR spikes on the wide band, until there is enough airflow over the motor to cool things down again.

Yesterday while this was happening, I immediately shut down and pulled the carb off. While the floats were cool, the base plate was hot and so was the intake manifold. I dropped a bit of fuel directly into the intake and it started boiling off almost immediately. I don't know if this is normal or not, but it didn't seem right.

The reason I like manifold vacuum is bcause I get much better throttle response with the engine at 36 + degrees of total advance at idle. If I switch to ported vacuum it would drop to 15 or 16, and run hotter. I understand that manifold vacuum drops to zero when the throttle is snapped open which gets rid of all the vacuum advance, but in slower every day throttle applications, there is still vacuum advance present during the time it takes the throttle blade to uncover the ported advance hole, which would not be the case if I were using ported vacuum. The engine seems to like this additional vacuum advance and responds better.

P.S. you mentioned getting 20 mpg on your holleys on the high way. What do you get in the city?

skinny z

The city mileage is widely variable as you can imagine. What I can say is that after 7 tanks of fuel with driving conditions ranging from extended idling to (very little) high way cruising (and everything in between), the average is 14.3 miles/U.S. gallon.

camarito

I have the same problem, the AFR drops to 19:1 and it just dies the solution so far was to open the secondary throttle blades some more, this raised the AFR to 12:1 and then close the idle fuel screws to obtain a 13.5:1 +- in park.
I believe metal expansion due to heat is to bleme you need the 0.020 transition slot exposed primary and secondary for proper idle, try that. I'm using ported vacuum, it idles better and doesn't stall with low afr as soon as I put it in drive like with manifold vacuum

skirkland1980

iTrader: (2)

Is the carburetor percolating?

camarito

Hard to tell what's causing it, it's like all of a suden there is a vacuum leak or the fuel stops flowing through the idle circuits

oamhmad

iTrader: (1)

Does your problem go away after you have been driving at speed for say 10-15 minutes?

The secondaries in my carb are set to expose a square in the expansion slot.

skinny z

Have either of you done a compression test?

InfernalVortex

iTrader: (4)

Just wanted to say taht on hot summer days when my car sits after getting warm, the fuel line gets almost too hot to touch over the engine. As soon as the pump goes on and fresh fuel starts going in it cools back down to normal temperature within seconds.... after flooding the engine.

You've probably seen me complaining about it before, but I just wanted to emphasize that these lines can get VERY hot and also cool down VERY fast when fresh fuel is going through them.

camarito

I did recently got 200-220 psi, I ordered a 1" plastic spacer for the carb and next time it happens I'll check the pcv valve it might get stuck open and cause a vacuum leak

camarito

It does go away after driving for a bit thats why I have to check the PCV valve next time it happens

oamhmad

iTrader: (1)

Yeah same here....I didn't think about checking the PCV. I'll check and report back.

In the meantime I've ordered an L88 drop base air cleaner and a 1/2" wood carb spacer to keep as much heat as possible out of the carb.

oamhmad

iTrader: (1)

So I checked the PCV valve while the engine was doing the whole loping, heat soak rough idle thing and I noticed that it was rattling/buzzing away. Pulling the valve out of the valve cover and blocking it with my finger immediately smoothed out the idle!

From my understanding of how PCV systems work, the valve should remain closed at idle; it seems that mine is not.

Could this be the problem all along?

skinny z

That's not an unusual spot for a vacuum leak. There are different PVC valves with varying spring rates. Looking back through your posts I don't see a vacuum reading.
Do you have a gauge?

oamhmad

iTrader: (1)

Yes I have a gauge. The vacuum is 21" at idle. Which pcv valve should I be using?

Damon

They're all fairly similar. If a regular old replacement for your car doesn't work (or worse yet, buzzes at idle) stick in a PCV from a 91 V6 Camaro. Don't know why it works but it's cured a "buzzing PCV" valve issue every time I've used it.

skinny z

21" of vacuum! Wow. That's amazing. I'm assuming you're running a short duration camshaft . That fits in with the healthy cranking pressure you have too. With that amount of vacuum it shouldn't matter what pcv valve you're using provided it's at least spec'd for your car.
We were trying to dial in engines that were producing 6"-8" of idle vacuum and the "buzzing" pcv valve was a real problem. I've put in a call to my racing friend that was collectoing the information I had mentioned (about differing valve specs) although I doubt that is going to make any difference in your case.
It's starting to sound like you may have issues regarding vacuum in general. You had mentioned earlier that you had been chasing a leak but had the issue resolved although with that much idle vacuum I'm beginning to doubt that.
I still doubt that heat soak is your issue. It's not the most common problem and although New Jersey can be pretty toasty in the summer months it's not Arizona.
How old is your carb and when was the lst time it had been rebuilt?
My apologies for not being a lot of help.

oamhmad

iTrader: (1)

Yes it is a lot of vacuum. The engine is a Goodwrench 350 with a stock cam and low compression heads. The carb was a brand new out of the box Holley. I have about 7-8K miles on it miles on it and is about a year and a half old. I did previously have a vacuum leak and idle problems because of a crack in the base plate at one of the large vacuum ports. I have a new base plate on it now.

I replaced the PCV with a new one for a 1996 v6 Camaro. Although it does not buzz at idle any more, it didn't make too much of a difference.

The PCV valve opens at idle and I can feel a lot of suction at its tip. Blocking it off greatly improves the idle. Isn't it supposed to be closed at idle in the absence of crank case pressure?

oamhmad

iTrader: (1)

Also forgot to mention...I'm seeing puffs of oily mist/blow-by being pushed out of the PCV hole in the valve cover when I pull the PCV out at idle. Not sure if that is supposed to happen or if it means my 10,000 mile Goodwrench already has worn rings.

skinny z

Closed at idle yes but more because of the strong vacuum drawing the valve closed. There will be some suction though as the restriction at the valve is not 100%. With your 21" of vacuum you're sure to feel something.


With 200 psi of cranking pressure it sounds as though you're engine is still mechanically sound however that doesn't neccesarily eliminate above normal amounts of blow by. You're bound to see something in any case.
Are you still running your vacuum advance by way of full manifold vacuum? You had mentioned that you had experiented with one and the other .

oamhmad

iTrader: (1)

I haven't done a compression test yet. The 200 PSI was from another guy also posting on here. I'm running vacuum advance on full manifold vacuum.

skinny z

I re-read the thread. My mistake regarding the compression test. Makes sense too as I doubt very much that a Goodwrech 350 will produce much beyond 150-160 psi (which is not very good). It also makes more sense regarding your poor idle and the need to supply the additional advance from the manifold vacuum.
If I were pursuing this on my own, I would do a compression test. I've been trained to go big on data and testing and make changes based on the results rather than do what a lot of people do and that's to stab away until they stumble across a solution. With your having a wide band O2 sensor, you have the tools available (along with your vacuum gauge) so your not guessing in that department. (Reading spark plugs gets really old in a hurry).
Further to the "if it were me" theme I'd also make the switch to ported vacuum and crank the initial spark lead to something that the engine likes. There's nothing wrong with supplying 18-20 degrees of initial timing. Even more if your cranking pressure is very low. (There's the need for the compression test again).

camarito

In my case its the PCV valve, with manifold vacuum advance at idle I have to close t he throttle blades all the way for it to idle below 1k rpm all the idle air comes off the PCV valve if I block it with my finger the engine dies, any fluctuation on the vacuum starts a chain reaction of ups and downs, it's less frequent with advance on ported vacuum as the engine is not so sensitive to vacuum drop

skinny z

Vacuum advance connected to manifold vacuum is just plain wrong.
Whatever either of you are trying to do regarding your idle and tune, keep the vacuum advance on ported vacuum, adjust the base timing to what the engine wants (and set this with the vacuum advance disconnected), set your idle mixture and speed using a vacuum gauge, tachometer and wide band O2 gauge. Make the sure the PCV valve is connected to full manifold vacuum and has a functional breather on the opposite valve cover.
Be doubly sure that there are no vacuum leaks either through lines, intake manifold or carb.
The rest should come together unless you have an inherent problem with the engine. Poor or uneven compression, worn timing chain, poorly adjusted lifters, etc.

oamhmad

iTrader: (1)

Your advice goes counter to an article posted on a lot of different carb/muscle car forums, supposedly written by a GM engineer. I'm sure you've seen it.

InfernalVortex

iTrader: (4)

Yeah everything I've researched has suggested ported vacuum was merely a compromise to appease the pollution fascists, not so much the ideal method in general.

TreeFiddy

Ditto agreed, that's what my reading found too. I have run for extended periods and done much mucking about with timing using both sources. In my case not a lot of difference performance-wise between the two once it had it dialed in, but noticeably lower temps with full vac. So I stick with that now.

skinny z

I've read about it and heard arguements supporting it's use but I find the application just doesn't work in any of the vehicles I've worked on or tuned. It's always inidicated to me that there's an underlying problem and the full vacuum is a band-aid rather than a fix.
To each his own I guess.
If anyone can direct me to any of the articles that have been written regarding this, I'd like to read them.
I'm not so interested in what Joe the hot rodder has found works for his car but anything from an engineering standpoint would be appreciated.
Thanks.

skinny z

I've investigated this further and contrary to what I've been describing, there appears to some real merit to the manifold vacuum being the source for the vacuum advance.
I stand corrected...at least at this point in the conversation.
I reference this article:
http://www.classicinlines.com/SparkTech.asp
Interesting stuff and it's something I'll experiment with this season.
As for the OP's problem....I guess it's still a mystery.

TreeFiddy

Fwiw I think this may be the 'GM engineer' article referred to above, copied here on TGO - post 16 onwards:

https://www.thirdgen.org/forums/carb...m-vs-full.html

With my own empirical mucking around, note that I have ended up with full vac, but using an adjustable can set to about 3/4 of full on iirc to eliminate pinging.

My 2c on the OP's issue - float level and/or percolation sound most likely to me,as noted by others above. Also, I've had fuel sloshing out of the secondary vent under braking, cornering, or potholes/speed humps - causing surging which I mistook for vapor lock.

oamhmad

iTrader: (1)

Yes thank you. That was the GM engineer article i mentioned earlier. I would repost it here but its lengthy. The link above should suffice.

skinny z

I've read that before, I think quite a while ago.
I also remembered why I don't like the full manifold vacuum for my advance.
Unless you can nail down the vacuum advance curve, (start/stop and max) the fluctuating vacuum at idle on a cam with any significant overlap makes it difficult to get a steady idle. I don't have any isssues with the way my system is set up at the moment but once I've dialed in this seasons new carb (Demon 750 vac sec) I may experiement with different vacuum ports. I don't hold much faith though.
Having said that, the article did raise an interesting point regarding max available timing vs fuel economy. With my current set up, I'm below the rule of thumb 50 -52 of cruise advance. My emechanical isn't all in until 3000 rpm and with the overdrive, I'm cruising at 2500 or so.
I've got it a little easier in that I have distributor machine that I use to spin up whatever distributor I'm going to tune with. A little experimenting might mean a few extra mpg are in the works this summer.

camarito

I don't care what the article says manifold vacuum doesn't work right on my setup I have to close the throttle blades all the way for it to idle right and then I get off idle hesitation. With ported vacuum I can have the .020 transition slots exposed and it's instant throttle response off idle it also has a strong loppy idle the cam is supposed to have. No overheating problems and gas mileage doesn't change at all

skinny z

While I don't have a back to back comparisons for my latest engine, despite the arguements for the use of manifold vacuum to supply the advance, I can't see how the transition from idle to just off idle would be smooth.
What you're saying supports my thinking. How can going from, for example, 8 degrees initial (it's suggested that the initial be dialed back to offset the vacuum advance supplied) plus 20 more from the vacuum can (10 distributor degrees) for a total of 28 degrees of idle timing, to 8 degrees when the throttle is opened and the vacuum drops out allow for a smooth transition?
I haven't tried full amnifold vacuum arrangement yet (I'm still waiting for the spring temperatures to arrive) but I'll definitely get into it. To complicate things, I've got a new carb to dial in, one that I've modified to be fully adjustable for idle air bleeds/high speed air bleeds and will be spending a lot of time with the data logger sorting things out. Then I'll experiment.
In the meantime, I know what works for my application (apparently not so much for the OP) so I'll stick with it.

skinny z

While I'm thinking about, I want to thank oamhmad for the loan of his thread.
I feel bad in that I haven't been able to provide any insight into his problem but there's some pretty good dialogue going on here and in the other thread.
Keep us posted with your efforts oamhmad.
And thanks again.

oamhmad

iTrader: (1)

No problem, I'm glad I could help. I'm going to continue running my dist of manifold vacuum for the time being. I'm also going to focus on keeping the carb as cool as possible.

I just received an L88 drop base air cleaner which will allow me to install a 0.5" wood spacer under the carb. I'll post back with results as soon as I can get the air cleaner to clear the throttle bracket.