65 396

My 65/L78 engine runs great. The only issue I have with it is erratic idle. The engine is basically stock except for a Lunati Voodoo roller cam and roller rockers. The cr was verified at 10.5 to 1 with speedpro hc pistons and heads and block cut slightly to insure a positive seal. The carb is a new Holley 750 df. The idle is set at 950. For the first ~ 1/2 hour of driving, it stays at 950. Then, when stopping, the idle goes to 750. The next stoplight, the idle may go back to 950. The engine is a new build and we used Hondabond on both sides of the correct Fel-Pro intake gasket around all the ports and the front and rear sections so I don't believe I have a vacuum leak. The carb was tuned/jetted on the dyno for HP and the correct AFR. There are no other issues with the engine. It does not ping/rattle under load or diesel when turned off. I could not ask for better performance as the car is a rocket. Initial ignition timing is set at 25 degrees and the distributor was setup by Lars. Original ignition was set at 30 but the engine wanted to surge in 4th gear at low rpm in traffic so I backed it off.

rongold

Do you have the vacuum advance connected ??? I had the same problem with my 67/435HP engine. It turned out that I had a B1 vacuum advance unit that needed way more vacuum to pull in fully than the engine produced at idle. Hook up a timing light and watch the timing mark at idle---See if it moves up and down excessively. A stock 375/425/435 HP engine develops about 14-15 inches of vacuum at idle, so it needs a vac unit that pulls in at no more than 12 inches of vacuum---I think that's the B26 unit. I switched mine, and now the idle is right on---no more hunting (varying idle).


RON

65 396

When we were dynoing the engine, we noticed the vacuum can was pulling in and out at idle like it was too weak. We placed a small steel ball in the vacuum advance line to prevent this. We proceeded with the dyno process and the engine produced 468 HP, so I did nothing else with the vacuum advance. Could this be causing the idle issue?

MikeM

Yes. See post #2

lars

If the vacuum advance is varying the timing at idle, it will cause the idle speed to be erratic. If your engine is pulling numerically less than 12 inches of vacuum at idle, you need to change out your B26 vacuum advance with a B28.

Lars

tbarb

One other thing to consider is the Holley carburetor secondary float level. When you stop the fuel can spill from the braking action and create a over rich condition.

Easy to check and correct just by lowering the fuel level about four flats on the secondary adjustment nut.

65GGvert

Well, if you blocked the vacuum advance line, you know the vacuum advance isn't pulling the timing.
My L79 Holley was changing idle from stop light to stop light and I moved the vacuum advance to intake manifold vacuum and it has been steady since. I haven't driven it a lot since, but it was definitely better when I did. That ball bearing in your line does away with vacuum advance completely, which I don't think is a good idea for your idle, and possibly idle temperature rising.

65 396

Remember, the vacuum advance is disabled with a steel ball inside the vacuum line from the carb to the distributor so I'm thinking it shouldn't be a vacuum advance issue...……..right?

JohnZ

Why on earth would you disable the vacuum advance (the steel ball you put in the hose) and expect the engine to run or idle decently? You said "initial timing was set at 25 degrees..." How was this measured? It's supposed to be set with the vacuum advance disconnected and plugged - can't imagine it even starting with 25 degrees static timing.

SWCDuke

You need to measure and report manifold vacuum at idle speed (the actual number) then use the Two-Inch Rule to select the proper VAC. As originally configured L-78 had full time vacuum advance, but the original 201 15 VAC did not meet the Two-Inch Rule, which could cause idle instability, so also tell us the ID number on the VAC mounting bracket in addition to the idle behavior data. Also verify whether the installed carburetor is set up for ported or full time vacuum advance.

Ditto what John said.

Duke

MikeM

I read the steel ball had the vacuum blocked only during they dyno runs. Not after the engine was installed in the car. It sounds like his engine is running in the car just like it did on the dyno before they inserted the steel ball temporarily.

Maybe I missed something here?

alexandervdr

This may be stupid, but I experienced it: my (new) acc pedal was too stiff on the hinge, so the (double) spring did not always pull the butterfly tight against the idle set screw. At idle it does not need a lot to go from 750 to 950 or vice versa. It took me many drives before I understood that was the cause of varying idle...I cut some of the pedal's rubber hinge and that solved it

65GGvert

He intentionally left the vacuum line blocked. Can't imagine why, but a friend just bought a 67 L79 and asked me to help him check time and dwell. No vacuum, pulled hose and it had been blocked with a nail. Some "mechanics" do things differently I guess.

65 396

Sorry if I offended anyone on here or upset them. The engine builder who dynoed the motor told me he disabled the vacuum advance. I witnessed the dyno pull when the engine produced 468 horsepower. The builder said he was very satisfied with the engine's performance. I too was amazed that it ran that well. He also told me he added a rear power valve which the carb did not have. He also disconnected the rear pump because he said the mixture at wot was too rich. I can't defend anything he did because I trusted him to do the engine build and the dyno tuning. Maybe I just picked the wrong guy to do the work for me...…...sorry about that.

MikeM

No reason I see to apologize to anyone here. But you have left a big mystery. Is that ball bearing still in your vacuum advance hose or was it removed after the dyno run? If it's still plugged, you need to find a Vacuum can that will pull all the way in at idle, manifold vacuum.

I don't see anything wrong your engine guy did unless he told you to leave the BB in the vacuum line.

SWCDuke

If you spend most of your driving time at WOT, like a racing engine, you don't need vacuum advance. As far as the dyno pulls are concerned the vacuum advance can stay active because when you open the throttle it goes zero, so there is no need to disconnect it for either lab dyno or chassis dyno sessions. In any event, the operator should have unblocked the vacuum advance signal line before delivering the engine to you. Most of these guys don't have a clue why road engines have vacuum advance, nor do they understand the concept of torque bandwidth. All they understand is WOT peak power in the upper end of the rev range, and since that's all most of their customers understand, that's the way it, unfortunately, is.

Duke

65 396

thank you for your response. I believe the bearing is still in the vacuum line. If I remove the bearing and the vacuum is pulling in and out at idle, which way do I need to go with a vacuum can replacement?

DansYellow66

See LARS post #5 above. He has the answer.

alexandervdr

There are many very experienced people here (and I am not one of them), on this topic especially Lars, JohnZ and SWCDuke are the guys to listen too. They helped me out numerous times or with their replies or with their write-ups. Lars gave a recommendation for a can in a post earlier, and SWCDuke's 2" rule is gold. That is a quick thing to test, but given all the mods your workshop has done to the carb, it may be more complicated than than. But frankly, if you're new to the topic of tuning a carb & distributor (but maybe you are not?), then there is no other way than go through the whole learning curve yourself (it's possible, but takes time and patience) or find a real competent workshop to help you out. Prepping an engine for lots of power at WOT is actually way easier than tuning for smooth transitions in a wide range op RPM and load.

SWCDuke

AMEN!!! You sure got that right. The easiest engine system engineering work I've done is Bonneville engines. All that counts is top end power. Road racing engines are a bit more complicated because you need to maximize average power in about the top 30 percent of the rev range depending on gear spacing and the lowest RPM the engine will see in the slowest turn. Road engines are more complicated because you have to achieve acceptable idle behavior, at least 80 percent of peak torque at 2000 so the engine is responsive in normal road driving , good throttle response from any speed/load condition, and, finally, decent top end power, but for a road engine that's the least important task and the easiest to achieve as long as you have budget/time for massaging OE heads or buying higher flowing aftermarket heads. It's head flow that makes top end power. Sure you can do it with a big cam, but that usually kills the low end torque and makes for a soggy and cantankerous engine in normal road driving, like the L-88.

Duke