Here's a thinker for ya!

[Anonymous]

As some of you may know, I reciently upgraded my head to a M code head and adapted a YF off a 300. I timed it to the best vac and it idled great. Almost no misses. Purred like a kitten! But when I was cruising with RPMs around 2000-3000 it bucked and missed but only when holding at a constant throttle position. I went up a steep grade with engine up to temp and there was no pinging. So I timed it with a timing light and it was WAY TOO HIGH!! About 20* or so! So I brought it back down to 14* and the hesitation went away but the idle was not so good anymore. What gives?! Any thoughts?

I love this carb and head so far. It really gets up there and goes. I can now do aroung 5500rpm and it will pull all the way through!

Later,
Paul

 

[Anonymous]

Timing it by vacuum alone usually isn't the best way to go. You want to get as much advance as you can as long as it doesn't ping/knock/detonate. Work on getting the idle fuel mixture squared away with the vacuum guage. Sounds like you may have a main circuit fuel mixture problem with that YF... possibly too large or small of a main jet.

Your timing was probably ok if it wasn't pinging - you may have even been able to go with more. You can't put a whole lot of confidence in the balancer markings. You really need to check them and make new marks if needed.

 

[Anonymous]

The marks on the balancer are good. I've check many times. As for the carb, I think that It may be jetted a little too rich but my plugs don't show the usual signs of over-richness? There is no bogging down. Maybe I'll go change them for the heck of it and see what happens. What does a plug look like if it is running too lean?

Thanks,
Paul

 

[66200i6]

Do you have a healthy ignition? Good quick recovery coil, low resistance wire wrapped silicon wires and nice sharp edged plugs? Once your system starts to demand spark over 2500, without a good coil the spark at the plugs drops off dramatically.

Good luck, Ric.

 

[Anonymous]

Do you have a healthy ignition? Good quick recovery coil, low resistance wire wrapped silicon wires and nice sharp edged plugs? Once your system starts to demand spark over 2500, without a good coil the spark at the plugs drops off dramatically.

Good luck, Ric.

i agree. an msd 6a spark box might help too. it creates 7-12 sparks per plug per stroke @ or under 3,500 rpms. above 3,500 rpms there isn't enough time to set off more than one spark. still, this is much better than 1 spark per plug per stroke or even 2 sparks per plug per stroke on a dual points distributor. the spark box will also make for a smother idle and better gas mileage. you might experience a slight increase in power too. they are around $140 depending on where you get it.

 

[Anonymous]

Yep, I got MSD 6A, the MSD coil to go along with it, 8.8mm accel wires, Duraspark 2, and fresh Autolite Platinum plugs. So I know it isn't the ignition!

 

[Anonymous]

The MSD box will mask some of the plug reading ability. I have a dual quad setup on an old Vette that always fouled out the plugs. When I installed the MSD box, it keeps the plugs from fouling. It also runs and idles better with the MSD box. But, having not changed anything on the carb, the higher energy ignition system hides the fact that the carbs are running rich.

 

[66200i6]

What plug gap have you ended up with? With a health system you can and should start opening up your plug gap until you have a problem with ignition. With that system I would be running at least between .060 and .070 gap. I have seen the high output Jacobs systems actually run poorly because the gap had not been increased after installation of the system.

If the plugs look good I don’t know what else to do other than double check everything starting with the primary circuit. From the 12V needed for the coil to the resistance and continuity of the plugs and the wires.

Good luck, Ric.

 

[Anonymous]

Did you unplug the vacuum advance when you checked the timing? The thing about reading plugs is that they can change condition very quickly. You may be running lean or rich at cruise (when you are having a problem) but once you get the car off the road into your driveway and shut it down, they will be reading what the mixture was like at idle. Take some tools with you, run it down a road where you can pull over and pull some plugs, get it to miss like you describe, shut the motor off and pull over. If it's too rich they plugs will be black and suity. If lean they will be clean with a white insultor - remember, temps in the chamber are going to be a lot hotter if it's missing because it's lean.

I'd crank the timing back up to where it was and listen for any pinging. Get the timing right - then mess with the jetting.

 

[Anonymous]

My plugs are a .05 right now. I will have to give that a try Inliner. That sounds like a good place to start. I know that all the electrical is fine and the ignition is up to par. The only thing that has me pondering is the carb. I'll go check the plugs at the point where it is missing at. Other than that, I have checked the plugs may times and they all show a slight brown color. According to my book, that is the color to shoot for? tan to greyish tan?

Thanks for all the help guys. Without you, I would not have this great car today and the amount of knowledge I have gathered from you over the 3 years I have been around here.

Later,
Paul

 

[rbohm]

8) cut your plug gap ack to .040" as .050" is a bit large.

 

[Anonymous]

I'll do that!